by Michael Meltzer National Aeronautics and Space Administration

Office of External Relations

NASA History Division

Washington, DC

2007

The Galileo mission to Jupiter explored 

an exciting new frontier, had a major impact 

on planetary science, and provided invaluable 

lessons for the design of spacecraft. This 

mission amassed so many scientific firsts and 

key discoveries that it can truly be called one of 

the most impressive feats of exploration of the 

20th century. In the words of John Casani, the 

original project manager of the mission, “Galileo 

was a way of demonstrating . . . just what U.S. 

technology was capable of doing.” An engineer 

on the Galileo team expressed more personal 

sentiments when she said, “I had never been a 

part of something with such great scope ... . To 

know that the whole world was watching and 

hoping with us that this would work. We were 

doing something for all mankind.” 

When Galileo lifted off from Kennedy 

Space Center on 18 October 1989, it began an 

interplanetary voyage that took it to Venus, to 

two asteroids, back to Earth, and finally on to 

Jupiter. The craft’s instruments studied Jupiter’s 

enormous magnetosphere and its belts of intense 

radiation. The spacecraft also sent off a planetary 

probe that accomplished the most difficult 

atmospheric entry ever attempted. After this, 

the craft spent years visiting Jupiter’s moons and 

delving into their structures and properties. 

This book attempts to convey the creativity, 

leadership, and vision that were necessary for the 

mission’s success. It is a book about dedicated people 

and their scientific and engineering achievements. 

The Galileo mission faced many significant problems. 

Some of the most brilliant accomplishments and 

“work-arounds” of the Galileo staff occurred 

precisely when these challenges arose. Throughout 

the mission, engineers and scientists found ways to 

keep the spacecraft operational from a distance of 

nearly half a billion miles, enabling one of the most 

impressive voyages of scientific discovery. 

 * * * * *

Michael Meltzer is an environmental 

scientist who has been writing about science 

and technology for nearly 30 years. His books 

and articles have investigated topics that include 

designing solar houses, preventing pollution in 

electroplating shops, catching salmon with sonar and 

radar, and developing a sensor for examining Space 

Shuttle engines. He recently retired from Lawrence 

Livermore National Laboratory, where he helped 

start a pollution prevention program. Michael lives 

in Oakland, California, with his wife and five-yearold daughter.

Mission To Jupiter: A History of the Galileo Project NASA SP-2007-4231 Michael Meltzer
by Michael Meltzer 

National Aeronautics and Space Administration

NASA History Division

Washington, DC

2007

NASA SP-2007-4231

MISSION 

TO JUPITER:

A History of the

Galileo Project
Library of Congress Cataloging-in-Publication Data

Meltzer, Michael, 1946-

A Mission to Jupiter: A History of the Galileo Project / by Michael Meltzer.

 p. cm.

1. Galileo Project. 2. Jupiter (Planet)—Exploration. 3. Jupiter probes. 4. Space flight to Jupiter. I. Title. 

QB661.M48 2005

629.43’545—dc22

2005027890

 

(SP-2007-4231)

Contents

 ..................................................................................

..................................................................................

...................................................................................

	..................................................................................

..................................................................................		

..................................................................................

The Importance of the Galileo Project...................

	 From Conception to

Congressional Approval.......................................

The Struggle To Launch Galileo: Technical	

	Difficulties and Political Opposition..........................

The Challenger Accident and Its Impact

	on the Galileo Mission......................................................

The Galileo Spacecraft.....................................................

Galileo Deployment, the Inner Solar

 System Tour, and the Asteroid Belt....................................

The High-Gain Antenna Failure:	

	 A Disappointment and a Challenge..........................................

Jupiter Approach and Arrival............................................................

 The Orbiter Tour......................................................................................

Profiles of Selected People Important to the Mission....................................

Conclusion.........................................................................................................

Acronyms and Abbreviations...........................................................................................

Index.........................................................................................................................................

	 	 NASA History Series.......................................................................................................................

311 

list of tables

list of figures

PREFACE

foreword

acknowledgments

INTRODUCTION

v

xiii

xv

xvii

xix

xxi

1

9

37

71

107

149

171

187

223

283

299

	

301

305

1

2

3

4

5

6

7

8

9

10

11
List of Figures

 

Cover Page The planet Jupiter. (PIA03451)

Table of Another view of Jupiter. (PIA02873) 

Contents iii

Introduction Galileo the man and Galileo the spacecraft.

(NASA GPN-2000-000672), (Library of Congress 3a10555u) xxi

Chapter 1 Sources of volcanic plumes near Prometheus. (PIA02505) 1

Chapter 2 The United States Capitol. (5a50935u) 9

Chapter 3 Two scientists working on Galileo in its early stages. 37

3.1 The ∆V-EGA trajectory employed an Earth gravity-assist to place

 Galileo on a Jupiter trajectory. (Folder 23, Box 2 of 6, JPL 14) 56

Chapter 4 The crew of Challenger STS 51-L. (S85-44253) 71

4.1 Booster rocket breach. At 58.778 seconds into powered flight, a large flame 

 plume was seen just above the right-hand booster engine exhaust nozzle. 

 (Johnson Space Center image number S8725373, 28 January 1986) 74

4.2 Liquid-oxygen tank rupture. The luminous glow at the top is attributed to the 

 rupture of the liquid-oxygen tank. Liquid oxygen and hydrogen mixed and 

 engulfed Challenger in a fiery flow of escaping liquid propellant. (Johnson 

 Space Center number S87-25390, 28 January 1986) 76

4.3 The VEEGA trajectory. Galileo saved fuel by getting gravity-assists from Venus 

 and Earth. (Image number P-48182) 84

4.4 Joint Environment Simulator test. Booster motor sections needed to be 

 rigorously tested under conditions approximating actual use. (Image number 

 8772121) 89

4.5 Launch of Galileo: liftoff of the Space Shuttle Atlantis, carrying the Galileo 

 spacecraft and its Inertial Upper Stage. (STS34(S)025) 105

FIGURE pAGE
vi	 MISSION TO JUPITER: A History of the Galileo Project List of figures vii

 between the region of space influenced by the planet’s magnetic field and

 the region of the undisturbed, interplanetary solar wind. (Figure drawn by 

 NASA History Office from the Web site: 

 http://www2.jpl.nasa.gov/galileo/messenger/oldmess/Earth3.html) 134

5.10 The energetic particles detector (EPD) included two telescope systems: the 

 low-energy magnetospheric measurements system (LEMMS) and the 

 composition measurement system. These determined particle charges, 

 energies, and species. (JPL image no. 230-1027A) 134

5.11 The plasma subsystem measured energy and mass per unit charge of 

 plasma particles and was able to identify particular chemical species. 138

5.12 NIMS was especially useful for studying gaseous chemical species and 

 reflections from solid matter. The cone on the left was a radiative cooler. 

 (JPL image 382-2165A) 143

5.13 Photopolarimeter radiometer. (Image number 23-1158A) 145

5.14 The Orbiter’s SSI. (JPL image number 352-8284) 148

5.15 Cutaway schematic of the SSI. Shows principal optical components, 

 charge-coupled device detector, particle radiation shield, and front aperture. 

 (JPL image 230-537) 148

Chapter 6 The Galileo spacecraft about to be released from the bay of Atlantis. 149

6.1 The deployed Galileo spacecraft and Inertial Upper Stage booster rocket. 

ote the two circular Sun shields installed to protect the spacecraft’s 

 sensitive equipment. (Image number S89-42940) 150

6.2 Solar wind–bow shock–magnetosphere interaction for a planet such as 

 Earth. Venus’s bow shock occurs nearly on the planet’s surface. 155

6.3 The Gaspra asteroid as seen by the approaching Galileo spacecraft. The 

 images show Gaspra growing progressively larger in the field of view of 

 Galileo’s SSI camera. The earliest view (upper left) was taken 164,000 

 kilometers (102,000 miles) from Gaspra, while the latest (lower right) was 

 taken 16,000 kilometers (10,000 miles) away. (NASA image number PIA00079) 162

6.4 Highest resolution picture of Gaspra, constructed as a mosaic of two images 

 taken by the Galileo spacecraft from 5,300 kilometers (3,300 miles) away, 10 

 minutes before closest approach on 29 October 1991. The large concavity on 

Chapter 5 The Galileo spacecraft. (S85-44253) 107

5.1 The Galileo spacecraft consisted of three major parts: the Atmospheric Probe

 and the spun and despun sections of the Orbiter, each of which had specific 

 tasks to perform. (Adapted from JPL image no. 230-1235) 109

5.2 Galileo’s despun section receives its cabling. 112

5.3 Cross section of Galileo’s spin bearing assembly showing slip-ring and rotary 

 transformer assemblies. (JPL image 230-957Bc) 113

5.4 The Probe deceleration and descent modules. (JSC digital image collection, 

ASA photo number S89-44175) 116

5.5 Probe heatshield ablation. The forward heatshield, which was made of carbon-

 phenolic, accounted for about one-half of the Galileo Probe’s total mass before 

 entry (as shown in the left half of the figure). More than one-half of the heat

 shield’s mass was ablated during the entry period. The right half of the figure 

 shows the final heatshield shape (in black) as determined from sensors within 

 the heatshield. The ablated material is shown in grey. (NASA Ames photo 

 number ACD96-0313-13) 117

5.6 Probe science instruments. (Adapted from JPL image number 230-900) 123

5.7 Jupiter’s net radiation fluxes. The net flux radiometer experiment measured 

 the difference between upward and downward energy flow (“net flux”) at the 

 wavelengths of visible sunlight, as well as at the wavelengths of thermal infra-

 red radiation, as a function of depth in Jupiter’s atmosphere. The sudden rise 

 in solar net flux followed by an abrupt dropoff is probably caused by an 

 ammonia ice cloud layer. (NASA Ames photo number ACD96-0313-6) 128

5.8 The Galileo Orbiter. Fields-and-particles instruments were mounted on the 

 spun section (including the long science boom); remote sensing experiments 

 were mounted on the Orbiter’s stable despun section (at the bottom of the 

 spacecraft drawing). (Adapted from JPL image number P-31284) 130

5.9a Jupiter’s magnetosphere, a region of intense magnetic fields and rapidly 

 moving charged particles. The small black circle at the center shows the 

 relative size of Jupiter. Also depicted are lines of Jupiter’s magnetic field. 

 (Adapted from JPL image number PIA-03476) 133

5.9b Parts of a planet’s magnetosphere. Deflected solar-wind particles flow 

 around a magnetosphere. Note the magnetosphere’s typically elongated,

 rather than spherical, shape. The planet’s bow shock is the interface

FIGURE pAGE FIGURE pAGE
viii	 MISSION TO JUPITER: A History of the Galileo Project List of figures ix

 the lower right limb is about 6 kilometers (3.7 miles) across, while the prominent 

 crater on the left is about 1.5 kilometers (1 mile) in diameter. A striking feature 

 of Gaspra’s surface is the presence of more than 600 small craters, 100 to 500 

 meters (330 to 1,650 feet) in diameter. (NASA image number PIA00118) 163

6.5 The geocorona, shown here in a photograph taken with an ultraviolet camera, 

 is a halo of low-density hydrogen around Earth that absorbs sunlight and emits

 “Lyman- ” radiation. (NASA image number AS16-123-19650) 165

6.6 The geocorona and its geotail. As the atomic hydrogen in the geocorona is 

 exposed to sunlight, the hydrogen emits radiation. This process results in a 

 force that sweeps much of the hydrogen in the direction opposite to that of 

 the Sun, creating a geotail. 165

6.7 The asteroids Ida (left) and Gaspra (right). Gaspra was imaged on 29 October 

 1991 at a range of 5,300 kilometers (3,300 miles). Ida was imaged on 28 

 August 1993 from a range of 3,000 to 3,800 kilometers (1,900 to 2,400 miles). 

 The surfaces of Ida and Gaspra contain many small craters, evidence of 

 numerous collisions. The fact that craters are more abundant on Ida sug- 

 gests that it is older than Gaspra. (NASA image number PIA00332) 167

6.8 Dactyl, satellite of the asteroid Ida. The little moon is egg-shaped and only 

 about 1.6 kilometers (1 mile) across. The large crater is about 300 meters 

 (1,000 feet) across. This image was taken on 28 August 1993 at a distance of 

 3,900 kilometers (2,400 miles) from the moon. (NASA image number PIA0029) 168

Chapter 7 An artist’s conception of the Galileo spacecraft approaching Jupiter. 171

7.1 The unfurled high-gain antenna. (Adapted from JPL image number 230-893B) 172

7.2 HGA’s supporting ribs did not fully deploy. The HGA was not able to send 

 usable data in its partially opened state. 174

7.3 A blowup view of the HGA in its closed, or “stowed,” configuration illustrates 

 the suspected misalignment of its restraint pins. Rather than fitting into the 

 centers of their housings, the pins might have been bent so as to hang up on 

 the edges of the housings, thus preventing deployment of the antenna. Note: 

 “LGA Assembly” at the top of the figure refers to the low-gain antenna 

 assembly that was eventually used in place of the HGA. 176

Chapter 8 An image of Jupiter and one of its moons captured by the Galileo spacecraft as 

 it neared the planet. 187

8.1 The Galileo trajectory from launch to Jupiter arrival. This drawing charts 

 Galileo’s flightpath to Jupiter through its three planetary gravity-assists and 

FIGURE pAGE

two asteroid encounters. Gravity-assists included those from Venus 

 (February 1990) and from Earth (December 1990 and December 1992). 

 Encounters with the asteroids Gaspra and Ida are also depicted. Galileo 

 arrived at Jupiter on 7 December 1995. (NASA photo number S89-44173) 188

8.2 Comet Shoemaker-Levy 9 fragment W impact with Jupiter. These four 

 images of Jupiter and the luminous night-side impact of fragment W of comet 

Shoemaker-Levy 9 were taken by the Galileo spacecraft on 22 July 1994, 

 when it was 238 million kilometers (148 million miles) from Jupiter. The images 

 were taken at intervals of 21/3 seconds using a green filter (visible light). The first 

 image, taken at an equivalent time to 8:06:10 Greenwich mean time (1:06 a.m. 

 Pacific daylight time, or PDT), shows no impact. In the next three images, a 

 point of light appears, brightens so much as to saturate its picture element, 

 and then fades again, 7 seconds after the first picture. (NASA image number 

 PIA00139) 191

8.3 An artist’s concept of the Galileo Probe inside its aeroshell being released 

 from the mother ship, the Galileo Orbiter, prior to arrival at Jupiter. (NASA 

 photo number ACD95-0229) 194

8.4 Arrival Day events. Notice the two different trajectories of the Probe and 

 Orbiter. (Adapted from image number P-45516A) 203

8.5 Probe entry and descent into the Jovian atmosphere. (NASA photo number 

 ACD96-0313-4) 204

8.6 Atmospheric Probe parachute deployment sequence and separation of 

 deceleration module components from descent module. After the high-speed 

 entry phase during which the atmosphere slowed down the Probe’s speed, 

 a mortar deployed a small drogue parachute, the drogue chute opened, the 

 aft cover of the deceleration module was released, the drogue parachute

 pulled the aft cover off and deployed the main parachute, and the forward 

 heatshield was dropped. (NASA photo number AC89-0146-2) 205

8.7 Orbiter’s first 11 orbits around Jupiter. Notice that the apojove of the first orbit 

 is over 250 Jupiter radii (RJ) away from the planet. (Adapted from JPL image 

 number P45516B) 209

8.8 Jupiter’s expected cloud layers, along with the clouds actually observed at 

 the Probe entry site. (NASA image number ACD96-0313-7) 212

8.9 The Jovian atmospheric composition provides a “tracer” for planetary history. 

 (NASA photo number ACD96-0313-9) 214

FIGURE pAGE
x	 MISSION TO JUPITER: A History of the Galileo Project List of figures xi

9.4 A comparison of the size of Europa’s iceberglike surface structures with 

 features of Earth’s San Francisco Bay area. Both images show areas of equal 

 size, 34 by 42 kilometers (21 by 26 miles), and resolution, 54 meters (59 yards). 

orth is to the top of the picture. Europa’s crustal plates, ranging up to 13 

 kilometers (8 miles) across, have been broken apart and “rafted” into new 

 positions, superficially resembling the disruption of pack ice on polar seas 

 during spring thaws on Earth. The size and geometry of Europa’s features 

 suggest that motion was enabled by water underneath the ice rafts. (JPL 

 image number PIA00597) 260

9.5 Jupiter’s inner satellites and ring components. (NASA image number PIA01627) 277

9.6 Thebe, Amalthea, and Metis (left to right), taken in January 2000 by Galileo’s 

 solid state imaging camera. The images resolve surface features as small as 2 

 kilometers (1.2 miles). The prominent impact crater on Thebe is about 40 

 kilometers across. The large white region near the south pole of Amalthea is 

 the brightest patch of surface material seen anywhere on these three moons. 

 (JPL image number PIA02531) 279

9.7 Bright streak on Amalthea. Galileo’s solid state imaging camera obtained the 

 left image in August 1999 and the right in November 1999. The images show 

 features as small as 3.8 kilometers (2.4 miles) across. The bright linear streak 

 in the top left of the images is about 50 kilometers (30 miles) long. The large 

 impact crater near the right-hand edge of the images is about 40 kilometers 

 (25 miles) across. Two ridges, tall enough to cast shadows, extend from the 

 top of the crater in a V shape resembling two rabbit ears. (JPL image 

 number PIA02532) 280

Chapter 10 Project Galileo mission control. 283

Chapter 11 Artist’s concept of the Galileo spacecraft burning up in Jupiter’s atmosphere. 299

11.1 The demise of Galileo. 300

8.10 A mosaic of Jupiter’s equatorial region, photographed in the near-infrared and 

 showing Jupiter’s main visible cloud deck. The smallest resolved features are 

 tens of kilometers in size. The dark hotspot near the center is a hole in the 

 equatorial cloud layer and resembles the Probe’s entry site. Circulation 

 patterns and composition measurements suggest that dry air is converging 

 and sinking over these holes, maintaining their cloud-free appearance. The 

 bright oval in the upper right of the mosaic and the other, smaller, bright 

 features are examples of moist air upwellings resulting in condensation. 

 (NASA photo number PIA01198) 219

Chapter 9 Lava flowing from a volcano on the surface of Jupiter. 223

9.1 “Flower petal” plot of the Prime Mission Orbiter tour, depicting close 

encounters with satellites. Note the eccentricity of the first orbit as 

 compared to the others. Due to a solar conjunction, the spacecraft did 

 not have a close encounter with any satellites on its fifth orbit. 

(Adapted from JPL image number P45516B) 232

9.2 Solar conjunction occurs during the Galileo mission when the Sun lies directly 

 between Jupiter and Earth. The Sun is a strong source of electromagnetic 

 activity, and it wreaks havoc with the spacecraft’s radio signal, essentially 

 reducing the spacecraft’s data-transmission rate to Earth to almost zero for 

 the two and a half weeks spanning the conjunction. Mission planners and 

 telemetry engineers define this problem time as occurring when the Sun-

 Earth-Craft (SEC) angle is less than 7 degrees, although a relatively “quiet” 

 Sun can allow data to be successfully transmitted to Earth at SEC angles as 

 small as 3 to 5 degrees. During Galileo’s primary mission, solar conjunction 

 periods occurred from 11 to 28 December 1995 and 11 to 28 January 1997. 

 During the Galileo Europa Mission (GEM), solar conjunctions occurred from 

 14 February 1998 to 4 March 1998 and from 22 March 1999 to 10 April 1999. 

 A Galileo Millennium Mission (GMM) solar conjunction period ran from 

 28 April 2000 to 17 May 2000. 233

9.3 Europa’s surface is covered with water ice. Straight and curved low ridges 

form the boundaries of ice fragments that resemble those seen in Earth’s polar 

 regions during springtime thaws. The white and blue colors outline areas that 

 have been blanketed by a fine dust of ice particles. The unblanketed surface 

 has a reddish-brown color that has been painted by mineral contaminants 

 carried and spread by water vapor released from below the crust when it was 

 disrupted. The original color of the icy surface probably was a deep blue seen 

 in large areas elsewhere on the Moon. The colors in this picture have been 

 enhanced for visibility. (NASA image number PIA01127) 254

FIGURE pAGE FIGURE pAGE
List of Tables

2.1 Milestones leading to Galileo mission approval. 10

3.1 Planned Probe investigations of Jupiter’s atmosphere. 39

3.2 Planned Orbiter investigations of Jupiter and its satellites. 39

3.3 Chronology of Galileo development: 1978 to 1986. 43

4.1 Impacts of Challenger on the Space Shuttle Program and the Galileo 

 spacecraft. 91

4.2 Potential asteroid opportunities. 94

4.3 Impacts of Challenger on the Galileo mission and spacecraft. 103 

5.1 Atmospheric Probe instrumentation. 122 

5.2 Orbiter fields-and-particles instruments. 131 

5.3 Orbiter remote sensing equipment. 142 

8.1 Timeline of Atmospheric Probe events. 206

9.1 Overview of Galileo’s Prime Mission. 231

9.2 Prime Mission satellite tour. Includes the naming convention for Jupiter 

 orbits and dates of closest approach to the designated satellite on 

 each orbit. 233

9.3 Galileo Europa Mission key events. 235

9.4 GMM activities. During GMM, the spacecraft conducted a study of the 

Jovian magnetosphere–solar wind interaction in conjunction with the 

 Cassini spacecraft (which was passing by Jupiter on its way to Saturn) 

 and with Earth-based telescopes. 238

9.5 Io encounters. 240

9.6 Io statistics. 243

9.7 Europa data. 253

9.8 GEM Europa encounters. 257

9.9 Ganymede statistics. 267

9.10 Ganymede encounters. 269

9.11 Callisto encounters. 274

9.12 Callisto data. 275

9.13 Jupiter’s inner moons. 278

10.1 Galileo project managers. 285

TABLE pAGE
Preface

The Galileo Project: Commitment, Struggle, and Ultimate Success

I am glad to see that someone has followed through and done a really comprehensive 

and workman-like job capturing the history of the Galileo project. This book goes all 

the way back to the initiation of the project, when it was more or less just a thought in 

a few peoples’ minds, and traces the whole evolution from there. I got a great deal out 

of reading it. 

I have a vested interest in the Galileo project—I was deeply involved in it for 

over a decade. A lot of people know about the mission and its terrific science return, 

but they don’t know about the struggle putting the project together, getting it started, 

and keeping it going through all of the reprogramming and restructuring. One of the 

arguments that we used with people on Capitol Hill to keep the program alive through 

delays in the congressional budgeting process was that Galileo would be a nonthreatening 

manifestation of our country’s technological capabilities, and this would send a powerful 

message to the rest of the world. It did just that. 

Galileo meant a lot to the United States, but it also meant a lot to our space 

science community, because at the time that we were going through the development 

of Galileo, it was the only major deep space project. There were Earth satellite launches 

going on, but nothing to the planets. It was Galileo that really helped NASA and the U.S. 

space science community maintain viability during a period of extreme drought in program development. A lot of capability would have disappeared over the course of the 10 

years that Galileo was in development. 

The commitment that individuals made to Galileo was extraordinary. Many individuals committed a third or more of their professional lifetimes to executing this project. 

Over the years, situations developed so many times where it looked like there was just no 

way out for the project, but we always managed to come up with a solution. The number 

of times we managed to pull the fat out of the fire was truly remarkable. 

The Galileo project was complex in that it required funding for science instrument 

and spacecraft development from numerous sources, including NASA and its Centers, the 

Department of Energy, U.S. universities, and the Europeans (especially the Germans). Just 

how the pieces of the fabric were woven together into what turned out to be a very successful program—one that required an investment of almost two decades of preparatory 

and execution work to bring about—is an interesting story. Revisiting the program from a 

historical point of view is what motivated me to read this book. I think that people who are 

interested in the space program, its science achievements, and its contribution to technology in general will really appreciate this history. It’s comprehensive, it’s complete, and it 

seems to me to be pretty even-handed. I’m very appreciative of what Michael has done.

 —John Casani, First Galileo Project Manager
Foreword

We Are All Standing on the Bridge of Starship Enterprise

This book details the history of the Galileo mission. Galileo had political ups and downs, 

technical challenges and hurdles, and was a multigenerational task. We had people starting 

the mission on advisory committees and senior management jobs who have now passed on. 

Many people went through parts of Galileo in stages of their careers. I have friends who still 

mark their anniversaries and the birthdates of their children in terms of, “That was when we 

were on the beginning stages of prelaunch preparation,” or, “It was during our first Europa 

encounter when that happened.” The Galileo team felt much more as a family than a pure 

professional enterprise. People worked together for long periods of time, through good 

times and bad, to accomplish this thing not just for them, but for everybody. 

The Galileo mission to Jupiter was part of the grand sweep of solar system exploration. 

You can view planetary exploration as a human endeavor—a wave sweeping outward from Earth. 

We went to the close-in places first—the Moon, Venus, and Mars. The outer solar system was the 

next big frontier, and it was an order of magnitude more difficult to explore. The distances are 

truly staggering, and the problems of developing spacecraft that could survive on their own for 

long periods of time were major challenges. 

The early explorations of the outer solar system were performed by relatively fasttrajectory spaceships, like the Pioneers and Voyagers. They and Galileo were major steps 

forward in being able to develop reliable craft that would operate for decades, continue 

to send data back without failing, and be smart enough to take care of themselves out of 

communication with Earth. At Jupiter, the time available to send a radio signal is typically 

45 minutes, and another 45 minutes before you get an answer back. You are well beyond 

being able to do the types of things that can be done in Earth orbit. 

NASA’s outer solar system missions helped turn that region into a known place, 

rather than just the realm of astronomers. The outer solar system became someplace that 

can be talked about and thought about in geological and geophysical terms. The person on 

the street and kids in school can say, “Hey, I saw a picture of a moon of Jupiter the other 

day and it had volcanoes on it.”

We planned the Galileo mission in that context. The scientific advisory committees 

to NASA and the U.S. government laid out an exploration strategy of fast reconnaissance 

missions followed by missions such as Galileo, in which we orbited planets and did more 

detailed studies. There was a leapfrogging characteristic to this type of exploration. The 

missions take so long to plan and execute that the next wave of exploration must be prepared even before the current one can be launched. We began work on Galileo in 1972 in 

its infant form, and we really got to work on it in 1974 with detailed studies, even before 

Voyager was launched in 1977. 
On Galileo, we combined orbital and in situ exploration strategies. We believed 

that if we were going through so much effort and so many resources to get there, we ought 

to not only study the planet, its satellites, and magnetic fields, but that we also should 

understand the chemistry of the atmosphere in detail with an entry probe that actually went 

into the atmosphere, grabbed a sample, analyzed it, and sent the data back to the mother 

ship before burning up in the atmosphere. This was very ambitious.

We had a strategic plan that said we were going to go orbit Jupiter, get into its 

atmosphere, and then inform that plan with tactics derived from Voyager results. It is a good 

example of the way exploration progresses. You learn things that lead to new questions 

you want to answer, and so forth. We were preparing to follow up and understand Jupiter’s 

miniature planetary system at a very detailed level, even as Voyager was continuing on from 

Jupiter to Saturn, opening up the rest of the outer solar system. 

People frequently ask, “Why should the average person be interested in what’s 

going on in the Jupiter system?” There are answers to that on all levels, ranging from the visibility of high technology to developing new things that have spinoffs to enhancing national 

prestige to satisfying pure curiosity. But really, it is all about changing the way we look at 

the universe and the world. We want to know how planets tick and understand the processes that control us here on Earth—everything from geophysics to climatology to global 

warming. The universe is effectively a laboratory waiting for us to study these things. 

Most of the people who have worked on Galileo over the years probably regard 

their biggest contribution as having changed the textbooks. Kids today learn about such 

things as the moons of Jupiter, and they know what they’re talking about. To them, the 

planets become not just dots in the sky that you can barely see with a telescope. The planets 

become real places, and kids know their characteristics.

There is always a tension in the national debate about how much robotic exploration (such as Galileo) we should do versus so-called human exploration (such as Apollo). 

This misses the point! What we call robotic exploration is in fact human exploration. The 

crews sitting in the control room at Jet Propulsion Laboratory as well as everyone out there 

who can log on to the Internet can take a look at what’s going on. So, in effect, we are all 

standing on the bridge of Starship Enterprise.

It is important to note that Galileo was an international mission. Science is, by its 

nature, international. We had people contribute from countries all over the European community and other countries as well. This was one of the first missions in which the analysis 

of data more or less continued around the clock, around the globe. We’d wake up in the 

morning and hear that one of our colleagues in Berlin had processed some images overnight and brought new data to the table—and we could look at it immediately, while they 

had a chance to sleep. 

The intellectual children, grandchildren, and great-grandchildren of the people 

who worked on Galileo have now spread through the crews operating the Cassini spacecraft at Saturn and the MER Rovers, Spirit and Opportunity, on the surface of Mars. It is an 

ongoing spirit of exploration and, to me, that’s really the bottom line of what Galileo was 

all about. It is important to record the history of these types of things, both because of the 

intrinsic interest that the public has and because there are always lessons to be learned. I 

sure hope people will read this book and get that feeling from it. 

 —Torrence V. Johnson, Galileo Chief Scientist

Acknowledgments

So many people gave me valuable input for this book. I want to thank them for their time 

and interest in this project. Some of these people include: 

Torrence V. Johnson, who related with eloquence how decisions were made and 

operations were carried out on the mission.

John Casani, whose project leader perspectives on the mission’s importance 

and on the political battles that had to be fought were hugely important for my book. 

Also, thank you to the mission’s other project managers: Dick Spehalski, Bill O’Neil, 

Bob Mitchell, Jim Erickson, Eilene Theilig, and Claudia Alexander, all of whom gave me 

valuable insights on what Galileo was all about. 

Thanks go out to the mission engineers and technicians at Jet Propulsion Laboratory 

(JPL), especially Nagin Cox, who is indeed a poet when she talks about Galileo, Gerry 

Snyder, Duane Bindschadler, Brad Compton, Gregory C. Levanas, Theodore Iskendarian, 

and various members of the Galileo mission support crew; experts from Ames on the 

Probe, including Charlie Sobeck, Ed Tischler, and Joel Sperans; and Krishan K. Khurana 

of UCLA. Also, thanks to Dick Malow, a former congressional committee staff director; 

Allan J. McDonald of Thiokol Propulsion; Carl Fromm for his help and encouragement; 

Julie Cooper for patient help in finding graphics for the book; and Craig Waff for sharing 

his extensive research on the Galileo mission. In addition, a special thanks goes to 

the staff of JPL Archives, especially Russell Castonguay, who was incredibly helpful in 

locating long-buried letters and documents. 

At NASA Headquarters, the History Division staff, in particular, former NASA 

Chief Historian Roger Launius and the current NASA Chief Historian Steven Dick, deserve 

much credit for their support and oversight. Archivists Colin Fries and Jane Odom helped 

a great deal by finding and organizing essential archival material. Steve Garber oversaw 

the production process. Interns Liz Suckow, Jennifer Chu, Giny Cheong, and Gabriel 

Okolski all helped tremendously in obtaining and organizing images. 

Also at NASA Headquarters, the talented professionals in the Office of Printing 

and Design deserve much credit. Lisa Jirousek and Dyana Weis carefully copyedited the 

manuscript, designers Cathy Wilson and Smahan Upson laid out the manuscript in a very 

attractive manner, printing specialists Jeffrey McLean and Henry Spencer handled this crucial 

last step, and supervisors Steven Johnson and Gregory Treese managed the whole process. 

And finally, a thank you to my wife, Naisa Kaufman, for her talented and tough 

manuscript editing. 
In anaddress to the U.S. Senate Subcommittee on

Science, Technology, and Space, author James Michener 

asserted that “it is extremely difficult to keep a human life 

or the life of a nation moving forward with enough energy and commitment to lift it into 

the next cycle of experience . . . . There are moments in history when challenges occur 

of such a compelling nature that to miss them is to miss the whole meaning of an epoch. 

Space is such a challenge.”1

The Galileo mission to Jupiter successfully explored a vast new frontier, had a 

major impact on planetary science, and provided invaluable lessons for the design of subsequent space vehicles. In accomplishing these things, Galileo met the challenge of “such a 

compelling nature” that Michener envisioned. The impact of the mission was felt by those 

who worked on it, the country that supported it, and the people from other parts of the 

world who were deeply impressed by it. In the words of John Casani, the original Project 

Manager of the mission, “Galileo was a way of demonstrating . . . just what U.S. technology 

was capable of doing.”2

 An engineer on the Galileo team expressed more personal sentiments when she said, “I had never been a part of something with such great scope . . . . 

To know that the whole world was watching and hoping with us that this would work. We 

were doing something for all mankind . . . I’d walk outside at night and look up at Jupiter, 

and think, my ship’s up there.”3

Introduction

Meeting the 

Grand Challenge

1 James A. Michener, “Space Exploration: Military and Non-Military Advantages” (speech delivered before the U.S. 

Senate Subcommittee on Science, Technology, and Space, Washington, DC, 1 February 1979). Published in Vital 

Speeches of the Day (Southold, NY: City News Publishing Company, 15 July 1979). 

2 John Casani interview, tape-recorded telephone conversation, 29 May 2001.

3 Nagin Cox interview, tape-recorded telephone conversation, 15 May 2001.

“All truths are easy to understand once they are discovered. 

The point is to discover them.”

—Galileo Galilei
xxii	 MISSION TO JUPITER: A History of the Galileo Project

Like other grand voyages of discovery, Galileo altered the way we view our surroundings (in this case, our planetary surroundings). It is thus fitting that this mission to 

the Jovian system was named after a man whose own astronomical observations radically 

challenged the way that people of his time viewed their universe. The discoveries of both 

Galileo the man and Galileo the spacecraft brought us new perceptions of our planetary 

system, made our lives richer and more interesting, and breathed new vitality into our quest 

to understand ourselves and our universe. 
Whenthe Galileo spacecraft launched on 18 

October 1989, it began an interplanetary voyage that 

took it to Venus, two asteroids, back to Earth (twice), 

and finally on to Jupiter. It studied the planet’s intense magnetic fields, belts of radiation, 

and high-energy particles, and it sent off a planetary probe that accomplished the most 

difficult atmospheric entry ever attempted. The Galileo spacecraft repeatedly swooped 

by Jupiter’s Galilean moons, using a suite of scientific instruments to delve into their 

structures and properties. The mission amassed so many scientific “firsts” and key discoveries that it can truly be called one of the most impressive feats of exploration of the 

20th century. The Galileo mission added dramatically to our understanding of the Jovian 

system and our entire solar system. Moreover, the mission was a triumph of teamwork 

and ingenuity under exceedingly difficult conditions. 

The Lure of Jupiter for People on Earth

For centuries, the Jovian system has stirred our imaginations and been the frequent 

subject of our observations. The astronomer Galileo Galilei, using an early telescope that 

he himself made, discovered that Jupiter was not only a planet, but also the center of its 

own system of moons. This discovery called into question the Aristotelian-Ptolemaic view 

1	 Statement by Torrence V. Johnson, Galileo chief scientist. Reported in “Galileo’s Bounty,” San Francisco Chronicle

(31 July 2000): A4.

It’s continuing to be a marvelous voyage of discovery.1

—Torrence V. Johnson, Galileo Chief Scientist

Chapter 1

The Importance of the

Galileo Project
	 MISSION TO JUPITER: A History of the Galileo Project Chapter 1 I The Importance of the Galileo Project 

our solar system. Being able to perform such an examination was important because it 

helped untangle the “bewildering array of processes and phenomena which have affected 

the evolution of the planets and which control their environments and futures.”5 

Galileo: A New Phase in the Study of the Outer Planets

The two Voyager spacecraft, which launched in 1977, completed visits to all of the planets 

known to ancient astronomers. The Galileo mission began a different phase in the study 

of the outer planets: an era of more careful, systematic study, featuring close flybys and 

in-depth analyses of planetary system characteristics.6

Galileo was not a Jupiter mission in the traditional sense of the term, which 

typically refers to a project focusing on a single target. Galileo’s objectives were much 

broader, encompassing a holistic analysis of the entire Jovian system of satellites, primary 

planet, magnetic fields, and particle distributions. 

Galileo’s Impact on Future Deep Space Missions

The Galileo project team’s approach to exploration, spacecraft technical design, and operational problems will all strongly affect the manner in which other projects are conducted 

in the decades to come. Among the Galileo mission’s memorable achievements were 

the repeated successes of its staff in solving serious technical problems, even though the 

spacecraft was hundreds of millions of miles from home. Examples include the responses 

of the team to the high-gain antenna deployment failure and to the jamming of the 

spacecraft’s data tape recorder, both of which will be discussed in detail in later chapters. 

In each instance, the team attacked potentially mission-ending problems and found ingenious ways to keep the spacecraft operational and productive. Results of these efforts, 

such as enhancement of the Deep Space Network (DSN) and development of new datacompression software, constituted technological improvements that will benefit future 

missions as well. 

The Many Firsts of Galileo

Galileo was the first deep space mission designed to launch on an expendable launch 

vehicle from the Space Shuttle’s payload bay rather than from Earth. This was not a trivial 

achievement. Designing enough thrust into the craft to reach Jupiter while carrying its 

large, sophisticated payload took a great deal of creativity, especially after the Challenger

disaster precluded the use of liquid fuel due to safety considerations. 

The Galileo mission was also the first to make a close flyby of an asteroid 

(Gaspra), discover an asteroid moon (Ida’s satellite Dactyl), orbit a gas giant planet 

5	 T. V. Johnson, C. M. Yeates, and R. Young, “Space Science Reviews Volume on Galileo Mission Overview,” Space 

Science Reviews 60.1–4 (1992): 5–6.

6	 Eric G. Chipman, Donald L. De Vincenzi, Bevan M. French, David Gilman, Stephen P. Maran, and Paul C. Rambaut,

“The Worlds That Wait,” chapter 7-2 in A Meeting with the Universe: Science Discoveries from the Space Program

(Washington, DC: NASA Publication EP-177, 1981), http://www.hq.nasa.gov/office/pao/History/EP-177/ch7-2.html

(accessed 20 March 2000).

of the universe, which was accepted at the time by the Catholic Church, that all things 

revolve around Earth and, by extension, us. Thus, there could be only one center of 

revolution, and yet, Jupiter did not appear to obey that law.2

The Aristotelian-Ptolemaic model held that Earth was fundamentally different 

from the other planets in that it was “corruptible,” “alterable,” and “naturally dark and 

devoid of light.”3 Earth, it was thought, was also the only planet accompanied by a moon. 

Galileo’s discovery showed that this was not so. Jupiter was more like Earth than had 

been previously imagined. Using this new knowledge, Galileo argued that Earth indeed 

might not be fundamentally different from the other planets in additional ways. In his 

1632 treatise entitled A Dialogue Concerning the Two Chief World Systems, Galileo used 

his discovery of the Jovian moons, as well as many other observations of Jupiter, the other 

planets, and Earth’s Moon, to defend the Copernican model of our planetary system. In 

Copernicus’s view, Earth was not immovable. All of the planets, including Earth, revolved 

around the Sun. Such a concept challenged the anthropocentric ideas of the time and flew 

in the face of official Church doctrine.4

Jupiter’s sheer size and energy have added to our interest in the planet. It is one 

of the brightest objects in the night sky. Unlike other planets, it emits more thermal radiation than it absorbs from the Sun. It is also a radio-wave source. A storm bigger than Earth 

(the Great Red Spot) has been raging on its surface for centuries. 

Jupiter contains 75 percent of the total nonsolar matter in the solar system. The 

planet is so big that it constitutes a transitional object between a terrestrial-type planet and 

a star. Its composition—mainly hydrogen and helium—is closer to the Sun’s composition 

than to that of Earth. 

Another important aspect of Jupiter is the effect that its enormous gravity has on 

its satellites. Nowhere else in the solar system do planets interact so strongly with their 

moons. In particular, Jovian gravity causes mammoth tides and extensive volcanism on 

the satellite Io, the most tectonically active body in the solar system. Geological changes 

occur so rapidly on Io that in the space of only five months, Galileo was able to observe 

significant alterations in a large section of the moon’s surface (see the Io section in chapter 9 for a fuller discussion of these observations). 

The Galileo mission was the first to provide extended observations of Jupiter’s 

dynamic magnetic fields. The Jovian magnetosphere is the largest of any planet’s, so 

expansive that it could envelop the Sun and much of its corona. If our eyes could see 

magnetic field lines, Jupiter’s magnetosphere would appear larger than our Moon, in spite 

of its distance from us.

Finally, Jupiter is a window into our own past. Many scientists believe that the 

composition of the planet is little altered from that of the original solar nebula. As such, 

the Galileo spacecraft’s observations provided a look back in time to the early stages of 

2	 Office of the Vice President of Computing, Rice University, “Jupiter and Her Moons: One Planet’s Quest to Defy

Aristotle,” The Galileo Project, http://es.rice.edu/ES/humsoc/Galileo/Student_Work/Astronomy95/jupiter.html 

(accessed 25 July 2003).

3	 Galileo Galilei, A Dialogue Concerning the Two Chief World Systems, 1632, available online at http://webexhibits.

org/calendars/year-text-Galileo.html (accessed 24 July 2003).

4	 Roger D. Launius and Steve Garber, “Galileo Galilei,” Biographies of Aerospace Officials and Policymakers, E–J,

National Aeronautics and Space Administration (NASA) Office of Policy and Plans, http://www.hq.nasa.gov/office/

pao/History/biose-j.html (accessed 20 November 2001); “Nicolaus Copernicus,” JOC/EFR, University of St. Andrews,

Scotland, November 2002, http://www-history.mcs.st-andrews.ac.uk/history/Mathematicians/Copernicus.html;

A. Mark Smith, “Galileo,” World Book Online Americas Edition, http://www.worldbookonline.com/ar?/na/ar/co/

ar215300.htm (accessed 20 March 2003); Galileo Galilei, A Dialogue.
	 MISSION TO JUPITER: A History of the Galileo Project Chapter 1 I The Importance of the Galileo Project 

The Multiteam Cooperation Required To Develop the Galileo Spacecraft

The success of the Galileo mission was partly due to the efficient management of an incredibly 

complex and diverse network of specialist teams. Such a network was required to develop 

the sophisticated spacecraft that could perform the many mission functions required.

The Galileo spacecraft was characterized by its highly integrated design. Its 

diverse subsystems, developed by different project teams, needed to interface flawlessly 

with each other over many years. Subsystem dependencies needed to be fully understood 

and controlled because the environment experienced by a typical subsystem was strongly 

affected by the behavior of other connected subsystems. For example, the limited electric 

power that was available aboard Galileo had to be shared by temperature control and 

many other spacecraft subsystems in a manner that minimized energy use. Large temperature swings inside the spacecraft could result from this situation, and this placed stringent 

design requirements on the subsystems that had to withstand those swings.12 

The task that had been laid before Galileo project management13 was to design, 

fabricate, test, modify as needed, and ultimately fly the highly integrated, complex 

spacecraft. To accomplish this, communications among a large number of different organizations had to be smooth and effective. The lead NASA Field Center for the project was 

the Jet Propulsion Laboratory (JPL) of the California Institute of Technology, working for 

NASA’s Solar System Exploration Division. Galileo’s Orbiter, which conducted extensive 

flybys of Jupiter and its moons, was developed by JPL, operating in close coordination 

with a range of university, industry, and government laboratory groups. 

Galileo’s Jupiter Probe, which entered the planet’s atmosphere, was designed 

by NASA’s Ames Research Center and manufactured by Hughes Aircraft Company under 

contract to Ames. The U.S. Department of Energy provided the plutonium-powered electrical generators that ran the various spacecraft functions. NASA’s Lewis Research Center 

(now known as Glenn Research Center) helped develop the propulsion system and integration of the payload with the Space Shuttle. The Shuttle itself was supplied by Johnson 

Space Center, while Kennedy Space Center made available launch and landing facilities. 

The Galileo Project also involved international cooperation with the Federal 

Republic of Germany, which provided instrumentation for both the Orbiter and the 

Probe, a propulsion system for the Orbiter, and telemetry and command support from 

the German Space Operations Center.14

The agreements forged in this project established a context for future space missions 

and the activities and complexities that surround them. Important management lessons were 

learned from the international nature of the mission. For instance, when mission-critical 

equipment is being designed, manufactured, and tested by other countries, NASA needs 

to create parallel organizations in both the managing country and “subcontractor” countries to handle the day-to-day coordination issues that arise. 

During the Galileo Project, parallel organizations in the United States and Germany 

developed a contractual instrument called an “Interface Requirements Document” (IRD) 

that contained all the technical specifications for both the propulsion system being built 

12	 R. J. Spehalski, “Galileo Spacecraft Integration: International Cooperation on a Planetary Mission in the Shuttle Era,”

Earth-Orient. Applic. Space Technol. 4.3 (1984): 139.

13	 	John R. Casani of JPL was the Project Manager during most of Galileo’s development.

14	 	Spehalski, “Galileo Spacecraft Integration,” pp. 139–140.

(Jupiter), send a probe down into the Jovian atmosphere, and make direct observations 

of a comet (Shoemaker-Levy) smashing into a planet.7

Other Milestone Accomplishments

The Galileo spacecraft made two flybys of Earth before leaving the inner solar system, 

using our planet’s gravity to alter its trajectory in critical ways. During these flybys, the 

spacecraft made important observations of our Earth-Moon system. For instance, the spacecraft confirmed the existence of a huge impact basin on our Moon’s far side that had been 

inferred, but never observed, from Apollo data. Galileo also provided evidence of more 

extensive volcanism on the Moon than previously thought.8

To reach Jupiter, Galileo traveled 2.4 billion miles at an average speed of 44,000 

miles per hour (mph). After leaving Earth, it required only 67 gallons of fuel to control 

its flightpath and orientation. So accurate was Galileo’s trajectory that the craft missed its 

target point near the Jovian moon Io by a mere 67 miles. This feat has been likened to 

shooting an arrow from Los Angeles to New York and missing a bull’s-eye by 6 inches.9

On its way to Jupiter, Galileo discovered the most intense interplanetary dust 

storm ever observed. It found a very strong radiation belt above Jupiter’s cloud tops 

and Jovian wind speeds exceeding 720 kilometers per hour (450 miles per hour, or 

mph). These speeds remained fairly constant to depths far below the clouds, unlike 

the jetstreams on Earth. Probe measurements within the Jovian atmosphere revealed a 

surprisingly small amount of water compared to what had been expected from Voyager 

observations, as well as far less frequent lightning than predicted. The Probe observed 

only one-tenth the lightning activity per unit area of that found on Earth, although the 

strength of individual events exceeded those on Earth by an order of magnitude.10

The spacecraft observed that Jovian helium abundance (24 percent) is very 

nearly that of the Sun (25 percent)—one reason why Jupiter can be considered a transitional object between a planet and a star. From magnetic data collected by Galileo, 

scientists suspect that Ganymede, the largest Jovian satellite, creates its own magnetic 

field by means of an Earth-like dynamo mechanism within its core. This was the first 

moon found to exhibit such magnetic field characteristics. But the most dramatic and 

important discovery, according to many on the Galileo team, was the strong evidence 

of a deep saltwater ocean beneath the icy crust of the moon Europa. Such an ocean 

could conceivably harbor simple forms of life.11

7	 National Space Science Data Center, “Galileo Project Information,” http://nssdc.gsfc.nasa.gov/planetary/galileo.html,	

p. 2 (accessed 11 March 2000).

8	 “Galileo Project Information,” pp. 2–3.

9	 Everett Booth, “Galileo: The Jupiter Orbiter/Space Probe Mission Report,” December 1999, p. 19, JPL internal

document, Galileo—Meltzer Sources, folder 18522, NASA Historical Reference Collection, Washington, DC.

10	 “Galileo Project Information,” p. 3; Donald J. Williams, “Jupiter—At Last!” Johns Hopkins APL Technical Digest 17:4 

(1996): 347, 354; “The Probe Story: Secrets and Surprises from Jupiter,” Galileo Messenger (April 1996): 3.

11	 “Galileo Project Information,” p.3; Booth, p.9.
	 MISSION TO JUPITER: A History of the Galileo Project Chapter 1 I The Importance of the Galileo Project 

In its 1981 budget authorization, the U.S. Senate’s Committee on Commerce, 

Science and Transportation recognized the importance of the Galileo mission, as well 

as that of the rest of the planetary exploration program. The Committee noted the 

“spectacular accomplishments” of the exploration program, particularly with respect 

to the investigation of Saturn, Jupiter, Venus, and Mars. The Senate Committee was 

concerned, however, about the four-year gap that would occur between Voyager 2’s 

1981 encounter with Saturn and its encounter with Uranus, as well as the dearth of 

planetary imaging data that would result during this time. The Committee expressed 

its strong support for our country’s world leadership position in planetary exploration 

but was quite apprehensive about other nations’ aggressive programs in this area. In 

particular, the Committee regretted the lack of planned U.S. representation in missions 

to Halley’s Comet when it made its closest approach to the Sun in 1986. The Soviet 

Union, France, and other European nations, as well as Japan, were all developing 

missions to investigate the comet.19

The U.S. House of Representatives Committee on Science and Technology 

concurred with the Senate’s concerns, commenting that the House Committee would 

“not preside silently over this abandonment of U.S. leadership in planetary exploration.” 

The House Committee went on to point out the need to “nurture planetary science with 

new data,” expressing the fear that “where there is no new information to work with, a 

vital science becomes arid academic speculation.”20

The Committee also commented that it considered NASA’s civilian programs 

a “national resource” that has contributed substantially to the U.S. economy and our 

scientific preeminence. Results from these programs have, in the Committee’s view, 

added to our understanding of the universe and to key questions regarding life, 

matter, and energy.21

How Important Is Our Need To Continually Pioneer New Frontiers?

Throughout human history, we have repeatedly wondered about the nature of our 

universe and have sought to understand it better. Ancient Greek, Roman, Chinese, Arabic, 

Egyptian, Mayan, and many other cultures expended considerable resources trying to 

fathom what lay beyond their immediate lines of sight. These early scientific quests 

resulted in bodies of knowledge that continue to inspire today’s space scientists.22 

Plato sensed the significance of these types of inquiries. In Republic VII, Socrates 

asked, “Shall we make astronomy the next study?”23 Glaucon replied that certainly they 

should, listing the practical benefits that a knowledge of seasons and lunar and solar 

cycles have for everyone, from military commanders to sailors and farmers. But Socrates 

19	 Senate Authorization Report No. 97-100 (p. 37), Committee on Commerce, Science and Transportation, John Casani

Collection, Galileo Correspondence 11/81–12/81, folder 24, box 3 of 6, JPL 14, JPL Archives.

20	 	House of Representative Report No. 97-32 (pp. 11-8 and -9, 12-10, 16-20), Committee on Science and Technology, John

Casani Collection, Galileo Correspondence 11/81–12/81, folder 24, box 3 of 6, JPL 14, JPL Archives.

21	Senate Authorization.

22	Roger Launius, ed., The U.S. Space Program and American Society (Carlisle, MA: Discovery Enterprises, Ltd., 1998), p. 4.

23	Martin J. Collins and Sylvia D. Fries, eds., A Spacefaring Nation: Perspectives on American Space History and Policy

(Washington, DC: Smithsonian Institution Press, 1991), p. 29, quoting from Plato, The Republic: Book VII, circa 370 to

375 BCE.

in Germany and its interface with the U.S.-built Orbiter. The IRD gave the United States 

a vital measure of control over the work performed by Germany. Contractual arrangements, however, necessitated the development of the IRD early in the project, while the 

spacecraft design was still in a period of flux. A great deal of time, more than originally 

planned, was expended by both countries in negotiating acceptable designs and verification procedures for the propulsion system development. The unplanned time and effort, 

which were deemed necessary due to the mission-critical nature of the propulsion system, 

resulted in unforeseen costs and delays. 

NASA might have avoided the above cost overruns and schedule delays if it 

had not placed responsibility for mission-critical equipment in one country and project 

management in another. Lessons learned from the Galileo experience were implemented 

in planning the International Solar Polar Mission, in which NASA and the European Space 

Agency each took on the responsibility of designing and fabricating functionally independent spacecraft. A similar management model could be applied to an international 

Orbiter/Probe effort by assigning one nation the job of developing the Orbiter and another 

nation the job of delivering the Probe.15

Importance of Galileo for the U.S. Space Program

John Casani, Galileo’s first Project Manager, believed that our country’s space exploration 

program has had strong benefits. It has elicited worldwide respect and admiration for the 

United States and has effectively demonstrated, in a nonthreatening way, our country’s 

technological capabilities. Moreover, our space program has been a source of pride and 

inspiration for millions of Americans, as well as a motivation for young people to pursue 

careers in science and technology. Our ventures into space have demonstrated, loud and 

clear, that the U.S. holds a position of unparalleled preeminence in the fields of space 

science and space technology.16

The Galileo mission was critical for preserving NASA’s planetary exploration 

capability and for advancing the technology and survivability of robotic spacecraft. Galileo 

was an international cooperative program that strongly demonstrated to the European 

community the dependability of U.S. space exploration commitments. Such a demonstration was especially pertinent because it came in the wake of the International Solar Polar 

Mission’s termination.17

Galileo was a cornerstone of NASA’s planetary exploration program and achieved 

the highest scientific objectives for the outer solar system. The project maintained high 

visibility in a very positive manner for the United States at what can be considered a 

modest cost. During Galileo’s development, the entire planetary exploration program 

constituted only 5 percent of NASA’s budget, which was in turn only 1 percent of the 

federal budget.18

15	 Ibid., pp. 148–150.

16	John R. Casani to Thomas G. Pownall, President, Martin Marietta Corporation, “Why Galileo?” 8 December 1981,

John Casani Collection, Galileo Correspondence 11/81–12/81, folder 24, box 3 of 6, JPL 14, JPL Archives.

17	Casani, “Why Galileo?” p. 2.

18	 John R. Casani to Fred Osborn, 4 December 1981, John Casani Collection, Galileo Correspondence 11/81–12/81,

folder 24, box 3 of 6, JPL 14, JPL Archives.
	 MISSION TO JUPITER: A History of the Galileo Project

NASAstruggled for many years to develop a mission 

that would intensively study Jupiter and its satellites. Two 

decades of analyses, debates, and political maneuvering 

elapsed before Congress finally gave its stamp of approval to such a mission in 1977. The 

tortuous path to a congressional “yes” reflected not only the technical difficulties and uncertainties of outer planet missions, but also the dramatic social upheaval of the 1960s and 

1970s. Space exploration expenditures had to be balanced against the country’s new commitment to social reforms, as well as to the tremendous cost of the Vietnam War. Also debated 

were the types of space exploration most appropriate to the country’s goals and needs. Table 

2.1 gives an overview of the many steps involved in creating the Galileo project.

Early Concepts of Outer Planet Missions

Four Types of Missions

Although serious planning for the Galileo project did not begin until the 1970s, seeds were 

planted as early as 1959, when JPL scientist Ray Newburn, Jr., developed several types of 

mission concepts for exploring the outer solar system. Deep space flights would pass only 

through interplanetary space, making measurements of the regions encountered. Flyby

missions would briefly encounter planets on their way to other parts of the solar system. 

The data collected would be limited by the encounter time, but flyby missions had the 

advantage that several planets could be studied during one trip. Flyby missions might be 

compared to “windshield tours” of a city’s major landmarks. Orbiter missions would place 

24	Collins and Fries, Spacefaring Nation, p. 29.

25	Launius, The U.S. Space Program, p. 18.

chided Glaucon for his eminently sensible view, hinting that astronomy may be important to study even if it is not immediately profitable. Former NASA Administrator James C. 

Fletcher spoke much more directly to this point when he linked the study of space to the 

highest of human endeavors—the “salvation of the world.”24

The benefits of space exploration, as well as scientific research in general, have 

never been easy to quantify in terms of dollars and cents, although we have identified 

various ways in which such activities enrich us. Reaching out into space and developing 

the technology to make this possible have added to our understanding of Earth, our solar 

system, and the universe that surrounds us. Increasing our knowledge of these things is 

satisfying in its own right—we feel wiser and better knowing more about our universe. 

But our quest is also highly practical. The technology we develop can help improve our 

lives. Furthermore, we must make sure that control of space by other nations is not used 

to endanger our national security. Our explorations help prepare us to use our access to 

space to defend ourselves if necessary.

We enhance our country’s prestige among the peoples of the world by successful endeavors beyond our atmosphere, while at the same time adding to our scientific, 

technological, industrial, and military strength. Last but not least, we satisfy the compelling urge of our species to explore and discover, to set foot where no one else has. Our 

unstoppable curiosity distinguishes us from other species on our planet. Since Earth’s 

surface has largely been explored, we must now turn our gaze upward and outward.25

	 MISSION TO JUPITER: A History of the Galileo Project

Chapter 2

From Conception to

Congressional Approval
10	 MISSION TO JUPITER: A History of the Galileo Project Chapter 2 I From Conception to Congressional Approval 11

Table 2.1. Milestones leading to Galileo mission approval. (continued)

1966–1970s Popular and government support for post-Apollo space exploration wanes. Budgets

decline. Nevertheless, outer planet mission conceptual studies continue.

February 1968 NASA initiates serious planning for two Pioneer missions to Jupiter and beyond.

December 1968 NASA authorizes its Lunar and Planetary Missions Board to suggest space

exploration strategies. The board does not form a clear strategy on outer planet

exploration but recommends more study.

1969 NASA forms an Outer Planets Working Group of scientists and planners from

its laboratories. The group develops an outer planet trip schedule for 1970s and

early 1980s that includes flybys, orbiters, and probes. The recommended flybys

collectively constitute a Grand Tour.

June 1969 NASA convenes a “summer-study panel” of experts from the U.S. space science

committee. The panel recommends five missions, all of which include atmospheric

entry probes. Like the Working Group, the panel recommends outer planet flybys

that collectively constitute a Grand Tour.

Summer 1969 NASA begins formulating a budget request to Congress for starting up a Grand

Tour program.

Summer 1969 The Nixon administration issues budget guidelines that discourage NASA from

seeking Grand Tour funding for FY 1971. The budget request is delayed for a year.

1969 The ASC-IITRI (Illinois) study recommends the use of two types of spacecraft

(spin-stabilized and three-axis-stabilized) to meet different outer planet exploration

objectives.

July–August

1970

The National Academy of Sciences convenes an Academy Review Board to

recommend how NASA’s reduced budget should be apportioned among its seven

scientific disciplines. No clear consensus is achieved on outer planet exploration.

1970 The Office of Management and Budget (OMB) tries to delay startup of Grand Tour

program from FY 1972 to FY 1973 but fails.

January 1971 NASA announces that it will choose an outer planet program direction by November.

JPL and Ames Research Center are authorized to evaluate costs and benefits of

different mission concepts.

Spring 1971 Outer Planet Missions funding is reduced by the Senate. OMB requests that NASA

conduct simpler, less costly endeavors. The House and Senate agree.

July 1971 To build unified support in the U.S. scientific community for outer planet objectives,

NASA appoints members of the community to a Science Advisory Group (SAG). JPL

and Ames report their findings to SAG. But SAG cannot reach consensus on the

best mix of Grand Tour flybys versus orbiter/probe missions.

August 1971 The National Academy of Sciences Space Science Board also reviews the JPL and

Ames findings but, like SAG, fails to establish a unified position on outer planet

mission strategies.

Summer 1971 NASA Administrator James Fletcher asks Congress for Grand Tour funding for FY

1973, but his position is weakened by the scientific community’s divided opinions.

The Grand Tour startup is funded, but with a greatly reduced budget.

Table 2.1. Milestones leading to Galileo mission approval.

1959–1961 JPL identifies the importance of outer planet exploration and envisions experiments

for Jupiter flybys and orbiters.

1960 The Army Ballistic Mission Agency (ABMA) investigates propulsion system

requirements for a Jupiter mission.

Early 1960s Nuclear-electric propulsion concepts are analyzed by JPL’s Advanced Propulsion

Engineering Section. Problems are identified, and NASA focuses on chemical propulsion.

1964 Lockheed studies payload design concepts and launch vehicle requirements for

asteroid belt and Jupiter missions.

Mid-1960s Work on outer planet programs moves from conceptual studies to initial mission

planning. JPL, Goddard Space Flight Center, and Marshall Space Flight Center all

agree that the first outer planet missions should be Jupiter flybys.

Summer 1965 The National Academy of Sciences panel lays out plan for outer solar system

exploration. A progression of Earth-based studies, flybys, orbiters, atmospheric

probes, and landers is suggested.

Late Summer

1965

The possibility of a single spacecraft’s Grand Tour of Jupiter, Saturn, Uranus, and

Neptune (made possible by a rare configuration of the planets) is identified.

1	 R. L. Newburn, “Scientific Considerations,” in Exploration of the Moon, the Planets, and Interplanetary Space, ed. Albert

R. Hibbs, JPL Report No. 30-1 (Pasadena, CA: JPL, 30 April 1959); Craig B. Waff, “A History of Project Galileo. Part 1: The

Evolution of NASA’s Early Outer-Planet Exploration Strategy, 1959–1972,” Quest 5.1 (1996): 4.

2	 JPL, Ten-Year Plan, JPL Planning Report No. 35-1 (Pasadena, CA: JPL, 1 September 1960), p. 11.

3	 Newburn, “Scientific Considerations,” pp. 60, 76, 89; Waff, “A History of Project Galileo,” p. 5.

spacecraft in long-term trajectories around planets, providing opportunities for intensive 

study and data collection. Planetary entry and lander missions would involve penetration of the planetary atmosphere, allowing observations that spacecraft flying above the 

atmosphere could not obtain.1

In a 1960 planning report, JPL staff envisioned a planetary exploration program 

that incorporated some of Newburn’s concepts and that, by 1970, would have “demonstrated and exploited” spacecraft capable of planetary orbiter and lander missions.2

Scientific Analyses Proposed

Ray Newburn’s section in Albert Hibbs’s 1959 report on the feasibility of solar system exploration envisioned particular types of experiments that could be performed by spacecraft 

visiting other planets. For Jupiter flyby and orbiter missions, he identified photographic, 

radar, and spectral investigations that would yield data on the planet’s atmosphere, meteorology, and thermal structure. He imagined ways to study Jupiter’s gravitational and 

magnetic fields and outlined what was known about the planet’s radio emissions. He also 

mentioned pressure, density, temperature, and opacity analyses that could be performed 

during missions that penetrated the Jovian atmosphere. Newburn sensed the importance 

of outer planet exploration and understood the impact that new knowledge about Jupiter 

and its satellites would have on theories of the solar system’s origin and evolution.3 
12	 MISSION TO JUPITER: A History of the Galileo Project Chapter 2 I From Conception to Congressional Approval 13

Table 2.1. Milestones leading to Galileo mission approval. (continued)

May–June 1977 The U.S. space science community organizes a massive lobbying effort for

JOP. The community is successful in obtaining the Senate HUD–Independent

Agencies Appropriations Subcommittee’s approval for JOP (as well as the

Hubble Space Telescope).

Summer 1977 A vote on the House floor supports JOP two to one. The JOP project officially

begins on 1 October 1977.

Engineering Challenges

Newburn foresaw the types of scientific data that would be vital to collect on outer 

planet missions, as well as the technologies that would be needed to make the necessary 

observations. During this same time, other visionaries were establishing the engineering 

requirements necessary to transport a spacecraft. Warren H. Straly and Robert G. Voss 

of the Army Ballistic Missile Agency (ABMA) examined questions of trajectory; flight 

environment; and spacecraft design, guidance, control, and data transmission. They also 

examined the energy required versus payload for an initial mission to Jupiter and its 

satellites. They anticipated possible hazards in the flight environment that could end the 

mission, such as the following: 

•	 Collisions with large chunks of rock in the asteroid belt between Mars and 

Jupiter. Such collisions were thought to be probable, although this later 

proved not to be the case. 

•	 Effects from Jupiter’s radiation belts, which were feared to be intense enough 

to damage spacecraft systems. Years later, a Pioneer mission showed these 

belts to be less hazardous than suspected. 

•	 The difficulty of keeping electronic and other equipment operational through 

years of transfer time from Earth to Jupiter, during which the equipment 

would be subjected to dramatic extremes of temperature. 

•	 The great distances involved in a Jupiter mission, which would make accurate navigation and dependable spacecraft-to-Earth communication and data 

transmission very difficult.4

Propulsion Systems

Straly and Voss identified many of the hurdles that would have to be overcome in order 

for a scientific payload to reach Jupiter in operative condition. It was not at all clear in 

these early planning stages, however, what propulsion system would best transport the 

payload to its destination. While Straly and Voss imagined that it would be a large chemical 

4	 Warren M. Straly and Robert G. Voss, Basic Requirements for the Exploration of Jupiter and Its Moons, preprint no.

60-57 (American Astronautical Society, Sixth Annual Meeting, 18–21 January 1960, obtained from JPL Archives);

Waff, “A History of Project Galileo,” p. 6.

Table 2.1. Milestones leading to Galileo mission approval. (continued)

December 1971 President Nixon strongly supports the startup of the Space Shuttle program but

does not want also to fund a Grand Tour program that relies on developing the

expensive new Thermoelectric Outer Planet Spacecraft (TOPS).

January 1972 NASA proposes two less ambitious Jupiter-Saturn flybys using already-developed

Mariner spacecraft.

Spring 1972 Congress approves the Mariner missions (eventually called Voyagers 1 and 2).

March 1972 Pioneer 10 launches, bound for a Jupiter flyby.

1972 NASA’s SAG, concerned about the lack of adequate heatshield technology and

radiation exposure protection, recommends that Jupiter orbiter/probe missions not

launch until the early 1980s.

1973 NASA studies the possibility of launching planetary missions from the Space Shuttle.

The Jovian “satellite tour” concept is born.

April 1973 Pioneer 11 launches, bound for a Jupiter flyby.

December 1973 Pioneer 10 discovers that Jupiter’s radiation is less severe than was feared. This

finding, coupled with advances in heatshields, makes a Jovian orbiter/probe mission

more feasible.

May 1974 NASA considers teaming with the European Space Research Organization (ESRO) to

launch a Jupiter orbiter/probe in 1980.

Summer 1974 The National Academy of Sciences’ Committee on Planetary and Lunar Exploration

(COMPLEX) recommends Jupiter orbiter missions in 1980 and 1985, then changes

its position, urging more study and, due to the budget situation, a decision between

flyby and orbiter missions.

February 1975 NASA conducts further studies and recommends that a Pioneer Jupiter orbiter/probe

mission be conducted, as well as outer planet flybys. The National Academy of Sciences also backs a Pioneer Jupiter orbiter/probe mission. NASA authorizes Ames to

initiate “Phase B” development efforts for such a mission.

Fall 1975 Management of the Jupiter orbiter/probe development is transferred to JPL from

Ames, which will only develop the atmospheric probe. The mission is officially

named the “Jupiter Orbiter Probe,” or JOP.

Fall 1975 The Air Force chooses a solid-fueled rocket design to launch JOP from the Space

Shuttle. A Centaur liquid-fueled rocket was rejected because of fire danger during

transport in the Space Shuttle’s cargo bay.

Late 1975 NASA develops a marketing plan for convincing Congress to fund an FY 1978 start

for JOP.

January 1977 A proposed project budget of $270 million is approved by NASA’s top management

and OMB and is submitted to Congress.

May 1977 The House Housing and Urban Development (HUD)–Independent Agencies Appropriations Subcommittee votes to eliminate all funding for JOP from the budget. The

subcommittee is opposed to funding both JOP and the Hubble Space Telescope.
14	 MISSION TO JUPITER: A History of the Galileo Project Chapter 2 I From Conception to Congressional Approval 15

The missions of the 1960s were driven by such launch vehicles as Atlas-Agena and 

Atlas-Centaur combinations, Saturns, and Titans. These used such chemical fuels as liquid 

hydrogen and oxygen (Centaur D-1A), modified kerosene and liquid oxygen (Atlas SLV 

series), fuming nitric acid and dimethylhydrazine (Agena B), or powdered aluminum and 

ammonium perchlorate (solid-fueled rocket motors).8

The Lockheed Study 

In July 1964, NASA contracted with Lockheed Missiles & Space Company for a study 

that would help the Agency move forward on missions exploring the asteroid belt and 

Jupiter. The Lockheed team reported various findings regarding appropriate propulsion 

systems. The team noted that using Atlas-Agena or Atlas-Centaur vehicles for launching 

outer solar system missions had certain advantages in that they were smaller and more 

economical than, for instance, a Saturn-series vehicle. But it would take two to three 

years to reach Jupiter, depending on the particular launch time selected, and the team felt 

that this would tax the reliability of spacecraft systems. This difficulty could be avoided 

by switching to more powerful launch vehicles such as the Saturn 1B or Titan IIIC. Such 

a change would decrease trip time to as low as 450 days and could also increase the 

maximum payload size. 

It was the Lockheed team’s opinion that before detailed outer planet missions 

could be conducted, the asteroid belt between Mars and Jupiter had to be properly characterized. Asteroids were of considerable scientific interest in themselves, but in addition, 

the team thought that the asteroid belt might pose a threat to spacecraft passing through. 

Existing sensing equipment was not adequate for studying this region of the solar system, 

and the team thus stressed the need to develop meteoroid detection equipment. 

The Lockheed team had much to say about the most appropriate type of power 

source for keeping spacecraft systems operational during a Jupiter mission. Solar panels 

were ruled out for generating power, due in part to the decreasing insolation (incident 

solar radiation) as the craft journeyed away from the Sun. Large areas of photovoltaic 

panels would also present a considerable weight penalty and might be damaged by the 

high particle fluxes that were anticipated. Because of the distributed mass, it would be 

more complex to control spacecraft orientation, and the panel array might at times block 

some of the sensing equipment. Many engineers recommended using radioisotope thermal generators (RTGs) instead, with the hope that the steep costs of such systems would 

decrease and availability of their isotope fuel would increase by the time construction of 

the spacecraft commenced. One side benefit of the RTG systems was that the heat they 

generated could be used to help maintain spacecraft operating temperatures. 

Adequate spacecraft-to-Earth communications could be maintained, according to Lockheed, using a 7-foot-diameter antenna dish operating at 10 watts of power. 

Two possible payload scenarios were envisioned. One involved video observations and 

required fairly complex data handling. The other employed an x-ray detector, radio 

receiver, sounding system, and visual spectrometer. The data from this latter system would 

have been simpler to handle, but a heavier instrument payload would have resulted.9

8	 “Appendix E: Launch Vehicles for Mars Missions.”

9	 	Waff, “A History of Project Galileo,” pp. 8–10.

rocket, their colleague at ABMA, Ernst Stuhlinger, advocated nuclear-electric drives for a 

range of space programs, including sending a craft to the outskirts of the solar system. 

He suggested that nuclear-electric propulsion was a good technology to deliver robotic 

probes into Jupiter and Saturn orbits, for it could reduce the travel from about three 

years (for a chemically driven craft) to a little over one year. The propulsion system he 

envisioned would be capable of delivering continual thrust from Earth to Jupiter. Once 

Jupiter orbit was attained, the nuclear-electric system would also supply abundant power 

for maintaining spacecraft operating temperatures, communications, antenna orientation, 

attitude control, and data transmission functions.5

In-depth analyses of nuclear-electric and other propulsion designs were performed 

at JPL in the early 1960s by the newly formed Advanced Propulsion Engineering Section, 

headed by John J. Paulson. At first, nuclear-electric systems looked like an excellent choice. 

A detailed conceptual design of a Jupiter mission envisioned that a 45,000-pound spacecraft would be boosted to Earth orbit by conventional rocket stages and then propelled by 

a 1-megawatt nuclear-electric system. One-third of the spacecraft’s weight (15,000 pounds 

of propellant, possibly cesium) would be consumed by the time the spacecraft reached 

Jupiter. The trip to Jupiter would include 70 days of powered flight to escape Earth orbit, 

an additional 145 days of powered flight to adjust the trajectory, 400 days of coasting, and 

60 days of reverse thrust to slow down during the final approach. 

Spacecraft propulsion appeared to constitute an ideal use of nuclear power. In 

1963, the JPL group expressed its strong conviction that it was necessary to develop such 

propulsion systems for exploring the solar system. The group considered this technology 

as the logical next step to chemical rockets, which, in its view, would have to be exceedingly large to deliver only modest payloads. 

Although nuclear-electric systems initially held tremendous promise, many practical difficulties were identified in more thorough analyses. Belief in the inevitability of 

nuclear-electric propulsion decreased severely when these difficulties, and the inherent 

costs of working through them, were examined. Nuclear reactors required large amounts 

of heavy shielding materials, and this was problematic for a spacecraft with an extremely 

high cost for transporting each pound of its weight out of Earth orbit. The necessity of 

physically separating the reactor from spacecraft electronics and payload also presented 

difficulties, as did the requirement for a large radiator area to control reactor temperatures. It was thought in the early 1960s that such a radiator, which might have to be fairly 

flimsy due to spacecraft weight restrictions, would be quite vulnerable to micrometeorite 

threats. Furthermore, the long duration of outer planet missions required that nuclearelectric system lifetimes approach or even exceed 20,000 hours. In the early 1960s, most 

engineers considered that to be a major difficulty.6

As a result of the perceived difficulties with nuclear-powered space vehicles, 

NASA focused its efforts on chemical means of propulsion. The Agency continued to 

“redirect” ballistic missile weapon systems into launch vehicles for the space program.7 

5	 Straly and Voss, Basic Requirements for the Exploration of Jupiter and Its Moons, pp. 31–35; Waff, “A History of

Project Galileo,” p. 7.

6	 “Appendix E: Launch Vehicles for Mars Missions,” On Mars: Exploration of the Red Planet, 1958–1978, http://www.

hq.nasa.gov/office/pao/History/SP-4212/app-e.html (accessed 29 May 2000).

7	 LieutenantColonelC. L.Gandy, Jr., andMajorI.B.Hanson, “Mercury-Atlas Launch-Vehicle Development andPerformance,”

chap. 5 in Mercury Project Summary, Including Results of the Fourth Manned Orbital Flight (Washington, DC: NASA SP-45,

October 1963), online at http://www.hq.nasa.gov/office/pao/History/SP-45/ch5.htm (accessed 29 May 2000).
16	 MISSION TO JUPITER: A History of the Galileo Project Chapter 2 I From Conception to Congressional Approval 17

that each outer planet first be analyzed in depth with studies performed by telescopes 

on Earth, as well as with observations taken from Earth-based balloons, rockets, and 

satellites. Once these baseline data were obtained, each outer planet should then be 

explored sequentially by flybys, orbiters, atmospheric probes, and eventually landers.12 

 Flybys, the panel noted, would provide valuable data upon which subsequent 

missions could build. In particular, flybys could assess magnetic fields, charged particle densities, particulate matter quantities, and infrared emissions. Subsequent planetary 

orbiters would then carry out many more measurements, including high-resolution 

photography; infrared surveys as functions of local time and latitude; laser reflection 

determination of cloud heights; ionospheric sounding; and detailed mapping of fields, 

particles, and radio sources over space and time.

 Specialized satellites ejected from the planetary orbiter were envisioned. Lowaltitude satellites would seek to make precise measurements of a planet’s surface features; 

satellites with eccentric orbits would encounter a planet’s moons; and close, inclined satellites would study areas of high interest such as Jupiter’s Great Red Spot. Data collected by 

the specialized satellites could be relayed to Earth by the planetary orbiter. 

 The panel also underlined the importance of an entry probe (or, in the panel’s 

terminology, a “drop sonde”), ejected from the planetary orbiter, that would plunge into 

Jupiter’s atmosphere, radioing back temperature, pressure, and other data as it descended. 

Such a probe could obtain decisive information about pressure-density relationships deep 

in the atmosphere that would be available in no other way. In addition, the point at which 

the probe was destroyed could possibly identify the location of a fluid-solid interface. 

Acoustic sounding might also be able to identify interfaces.13

The panel recognized that an outer planet lander would be the most ambitious 

type of mission, requiring formidable reserves of fuel to land successfully once it reached 

its target planet. Also noted was the difficulty that would be encountered in transmitting 

data to Earth from vehicles of any kind among the outer planets. The panel observed that 

transmitting data to Earth from Jupiter, Saturn, or Uranus would be 12, 40, or 160 times 

more difficult, respectively, than transmitting from Mars. To attain a sufficiently high rate 

of information transfer, space vehicle power supplies would need to be improved and 

Earth-based receiving antennas would need greater sensitivity.14

The efforts of the National Academy of Sciences panel were pivotal in laying out 

the U.S. space program’s course of further development. This was the first major analysis 

by the U.S. scientific community of how outer planet exploration might be conducted. 

From the panel sprang a basic vision for outer planet mission structures that provided 

a foundation for Galileo and other programs. 

The Grand Tour Concept

Shortly after the National Academy of Sciences panel met, there was a discovery that 

greatly enhanced both public and government interest in an outer planet mission. In 

12	Space Research, p. 50; Waff, “A History of Project Galileo,” p. 11; Butrica, “Voyager: The Grand Tour of Big Science,”	

p. 253.

13	Space Research, pp. 47–51; Waff, “A History of Project Galileo,” p. 11.

14	Space Research, pp. 51–52, 59–60.

The various conceptual studies of the late 1950s and first half of the 1960s 

identified a range of possible design scenarios and tradeoffs for the spacecraft, such as 

minimizing propulsion system costs at the risk of compromising spacecraft reliability, or 

minimizing payload weight while having to deal with difficult data-handling requirements. 

In the following years, selections would have to be made from the spectrum of what was 

possible, narrowing down system options to the best choice, given the budgetary and 

technical constraints that were being imposed on the space program. Such choices were 

to be made more difficult because the fiscal policies and social environment under which 

NASA operated were about to undergo a period of rapid change. 

Jupiter Mission Planning During the Apollo and Mariner Years 

The 1960s were golden years for NASA. Its momentous Apollo lunar program and 

the Mariner investigations of Venus and Mars conveyed the image of an agency that 

could accomplish great things and send spacecraft anywhere in the solar system. 

Never was morale higher. NASA Administrators radiated confidence and a cando attitude toward the future. Apollo had the strong support of Presidents John 

F. Kennedy and Lyndon B. Johnson. President Johnson, in fact, saw the quest to 

capture outer space from the evil hands of the Soviets as a 20th-century equivalent to 

the Roman Empire’s road system or the British Empire’s mighty 19th-century navy.10 

 It was during the heady time of the mid-1960s that work on outer planet 

programs moved from conceptual studies to initial mission planning activities. The 

staff at JPL, Goddard Space Flight Center, and Marshall Space Flight Center agreed 

that the first outer planet efforts should be flyby missions of Jupiter. The spacecraft 

would conduct particle and field research and also evaluate dangers that other missions 

would encounter, such as those posed by asteroids and Jupiter’s intense radiation. 

 NASA staff recognized the necessity of flyby Jovian reconnaissance missions, but 

they were not in consensus regarding subsequent programs. This question was addressed in 

some detail when the Space Science Board of the National Academy of Sciences convened a 

study panel to analyze key areas of space research. The Space Science Board was established 

in 1958 and served as the scientific community’s chief agency for advising NASA on space 

programs. The study panel that the Board organized met in Woods Hole, Massachusetts, 

during the summer of 1965. A working group of 33 scientists examined planetary and lunar 

exploration issues, and three of the panelists, G. B. Field of the University of California at 

Berkeley, Raymond Hide of the Massachusetts Institute of Technology, and J. C. Jamieson 

of the University of Chicago, served on a Panel on Jupiter and Other Major Planets.11

 The working group identified planetary exploration as “the most rewarding scientific objective for the 1970–1985 period,” and, among other accomplishments, 

the group laid out a plan for exploring the outer solar system. The group suggested 

10	Joan Hoff, “The Presidency, Congress, and the Deceleration of the U.S. Space Program in the 1970s,” in Spaceflight 

and the Myth of Presidential Leadership, ed. Roger D. Launius and Howard E. McCurdy (Urbana and Chicago:

University of Illinois Press, 1997), p. 93.

11	Space Science Board, Space Research: Directions for the Future, Part One (Washington, DC: National Academy of

Sciences, December 1965), pp. iii, 142; Andrew J. Butrica, “Voyager: The Grand Tour of Big Science,” chap. 11 in

From Engineering Science to Big Science: The NACA and NASA Collier Trophy Research Project Winners, ed. Pamela

E. Mack (Washington DC: NASA SP-4219, 1998), p. 253, online at http://history.nasa.gov/SP-4219/Chapter11.html;

Waff, “A History of Project Galileo,” p. 11.
18	 MISSION TO JUPITER: A History of the Galileo Project Chapter 2 I From Conception to Congressional Approval 19

Programs Office, viewed the Grand Tour as a mission with “incredible sex appeal,” as 

well as a wonderful way to carry out an ambitious multiplanet reconnaissance project. 

A Grand Tour was alluring not only because of what it could accomplish, but also because 

of how rare this opportunity truly was. Such a planetary configuration as was about to 

occur only happened approximately once every 175 years. In marketing the Grand Tour 

concept, Hearth noted that the last time the planets had been so configured was during 

Thomas Jefferson’s presidency. Federal budget personnel always asked NASA why a 

mission had to be done at this time rather than later. The rarity of the Grand Tour opportunity provided a compelling answer to this question. 

In 1968, Hearth funded JPL and the Illinois Institute of Technology Research 

Institute (IITRI) to carry out studies of the Grand Tour mission concept. While recognizing its potential as an initial reconnaissance mission, Hearth did not lose sight of its 

limitations. Other mission concepts might be more appropriate for providing the type of 

in-depth planetary data needed in order to more fully understand the natures of the outer 

planets. With this in mind, Hearth also allocated funds for advanced studies of orbiter and 

atmospheric-entry probe missions to Jupiter.19

Developing New Missions During a Time of Decreasing Budgets

Planning for post-Apollo missions took place in a context of declining support, both fiscal 

and political, for NASA’s programs. The country’s euphoric excitement over space exploration began to lessen after 1965, along with NASA’s budget. In the words of Bruce Murray, 

who became the Director of JPL in 1976, there was “less willingness to gamble on the future 

as did John Kennedy when he created the Apollo program in 1961.” The U.S. became “a 

divided country in terms of presidency versus Congress, since that period of time.”20

Although President Johnson continued to recognize the space program’s importance, political pressures forced reductions in its budget. The escalating Vietnam War 

and Johnson’s “Great Society” social reform programs were both experiencing spiraling 

costs.21 While Johnson never reneged on supporting the Apollo Program’s goal of landing 

a man on the Moon by the end of the decade, he resisted making firm commitments to 

post-Apollo missions.22

Johnson’s position was no doubt made easier by eroding public and congressional support for the space program. By the summer of 1965, a third of the nation favored 

cuts in the space budget, while only 16 percent wanted to increase it. A White House 

survey of congressional leaders at the end of 1966 revealed pronounced sentiment for 

“skimping on post-Apollo outlays.” Confidence in NASA was further damaged by the 

19	Butrica, “Voyager: The Grand Tour of Big Science,” pp. 254–255; Flandro, “Utilization of Energy,” p. 22; Waff, “The

Struggle for the Outer Planets,” p. 45.

20	Robert Dallek, “Johnson, Project Apollo, and the Politics of Space Program Planning,” in Spaceflight and the Myth of 

Presidential Leadership, pp. 80, 83–84; “Biography: Dr. Bruce C. Murray,” 13 July 1976, Bruce C. Murray Collection,

1975–1982, folder 1, box 1, JPL 216, JPL Archives; Bruce Murray, “Planetary Exploration and the U.S.: What’s the Future?

(Some Private Views for the JPL Advisory Council),” 15 August 1976, Bruce C. Murray Collection, 1975–1982, folder 3,

box 1, JPL 216, JPL Archives.

21	 	Hoff, “The Presidency, Congress, and the Deceleration of the U.S. Space Program in the 1970s,” p. 92.

22	Dallek, “Johnson, Project Apollo, and the Politics of Space Program Planning,” pp. 80, 83–84.

late summer of 1965, a Caltech graduate student demonstrated that Jupiter, Saturn, 

Uranus, and Neptune would soon be positioned just right for a single spacecraft to 

visit all of them in a relatively short amount of time. The student, Gary Flandro, was 

working at JPL at the time and used the work of another graduate student, Michael 

Minovitch, in his demonstration. 

Minovitch, a University of California (UCLA) student also working at JPL, had 

been examining a spacecraft orbital strategy called “gravity-assist,” in which a planet’s 

gravitational field is used to modify a craft’s trajectory. Gravity-assists could, Minovitch 

believed, increase a spacecraft’s velocity and alter its trajectory without violating the physics principle of conservation of energy. The planet giving the gravity-assist would lose 

precisely the amount of energy that it gave to the spacecraft. If the gravity-assist concept 

could be successfully applied to an interplanetary flight, the amount of fuel and flight time 

that a spacecraft would require to reach its destination might be significantly reduced. A

spacecraft could take advantage of the gravitational influence of a planet, Minovitch wrote 

in a 1963 JPL report, if it passed the planet “on a precisely calculated trajectory that will 

place it on an intercept trajectory with another planet.”15

Gary Flandro called a potential mission to four outer planets of the solar system 

the “Grand Tour.” Michael Minovitch, in his 1963 JPL report, had recognized the value of 

finding a trajectory that would guide one spacecraft to flybys of several planets by using 

multiple gravity-assists. Minovitch also noted in a 1965 JPL report that such windows of 

opportunity do not occur that often, but mentioned that a 1976 or 1977 mission could 

employ planetary gravity-assists to reach all of the outer planets.16 Flandro demonstrated 

that a gravity-assisted Grand Tour of Jupiter, Saturn, Uranus, and Neptune would optimally launch at the beginning of November 1979 and would take less than eight years 

to complete if the spacecraft was given a launch energy of 140 kilometers per second 

squared. By means of comparison, a direct, non-gravity-assisted flight with the same 

launch energy would take nearly as much time to reach only Jupiter and Neptune.17

Public attention was drawn to the Grand Tour idea after it was described in a 

1966 Astronomy and Aeronautics article, written by JPL’s Homer Joe Stewart, that was 

then referenced in a range of national newspaper articles. Rousing such public interest 

was rare indeed for a robotic mission idea. The name given to the mission definitely 

helped—it harkened back to 1920s ocean liners setting sail for a “grand tour” of the 

wonders of Europe. Capitalizing on this interest, JPL included in its advertisements for 

new employees this promise: “We’re going to give Jupiter, Saturn, Uranus and Neptune 

the run-around. Join us on Project Grand Tour.”18

The Grand Tour concept also elicited excitement at NASA Headquarters. Donald 

Hearth, the program manager for advanced programs and technology in the Planetary 

15	Michael A. Minovitch, The Determination and Characteristics of Ballistic Interplanetary Trajectories Under the 

Influence of Multiple Planetary Attractions, Technical Report No. 32-464 (California: JPL, 31 October 1963), p. viii;

Craig B. Waff, “The Struggle for the Outer Planets,” Astronomy 17.9 (1989): 44–45; Bob Mitchell, former Galileo

Project Manager, interview, tape-recorded telephone conversation, 10 May 2001.

16	Minovitch, Utilizing Large Planetary Perturbations for the Design of Deep-Space, Solar-Probe, and Out-of-Ecliptic 

Trajectories, JPL Technical Report No. 32-849 (California: JPL, 15 December 1965), p. 67.

17	Flandro, “Utilization of Energy Derived from the Gravitational Field of Jupiter for Reducing Flight Time to the Outer Solar

System,” JPL Space Programs Summary 4, no. 37-35 (31 October 1965): 22.

18	Minovitch, Determination and Characteristics of Ballistic Interplanetary Trajectories, p. viii; Butrica, “Voyager: The Grand

Tour of Big Science,” pp. 254–255; Waff, “The Struggle for the Outer Planets,” pp. 44–45.
20	 MISSION TO JUPITER: A History of the Galileo Project Chapter 2 I From Conception to Congressional Approval 21

in the decade. The Agency’s reduced budget was largely focused on achieving piloted 

lunar landings and other Apollo objectives. The staff resources and funding committed to 

Project Apollo made a focused effort to develop outer planet missions very difficult.28

The Pioneer Missions

In January 1968, an important step toward outer planet exploration was taken when NASA

initiated planning for two missions using Pioneer-series spacecraft that would eventually reach Jupiter and points beyond. The spacecraft were to function in some ways as 

scouts for future, more intensive outer planet exploration missions (such as Galileo). 

One objective of the Pioneer missions was to study the asteroid belt between Mars and 

Jupiter. This region was considered a potential hazard for spacecraft traveling to the outer 

solar system. Also of interest was “the gradient of the Sun’s influence on interplanetary 

space,”29 as well as the changes in cosmic radiation intensity as the craft moved out of the 

solar system and into galactic space.30

Ames Research Center was given responsibility for preproject planning (prior to 

congressional approval) and sought a spacecraft design derived from Pioneers 6 through 

9 (also known as Pioneers A through D). This subseries of the spacecraft was intended 

to obtain continuous measurements of interplanetary phenomena from widely separated 

points in space.31 These four spacecraft had been launched during the period from 1965 

through 1968, with missions to investigate the solar wind; cosmic rays; and interplanetary 

electron density, dust, and magnetic fields. The spacecraft were all spin-stabilized, with 

the spin axis perpendicular to the ecliptic plane. 

The Pioneer series originated in 1958, when the first three spacecraft were launched 

to attempt lunar orbits or flybys. None reached their targets due to failures of the launch 

vehicles, although Pioneer 4, which took off the following year, did make a successful lunar 

flyby. It was the first U.S. spacecraft to escape Earth’s gravitational field.32

For a mission to Jupiter, the basic design of Pioneers 6 through 9 needed significant modifications, one of which was in the mode of on-board power. The photovoltaic 

panels that generated power for inner solar system trips could not perform nearly as well 

at Jupiter because of the low solar energy density at that distance from the Sun. RTGs, 

which employed nuclear power to supply electricity, were considered as possible alternatives. They had a distinct weight advantage over the large solar panel array that would be 

needed for a Jupiter mission. 

The Pioneer missions to Jupiter were officially approved in 1969, and Ames 

was given management responsibilities for the project. The initial spacecraft designs still 

called for solar power, due to an apparent reluctance to commit to RTGs. This position 

28	Waff, “The Struggle For the Outer Planets,” p. 1; Butrica, “Voyager: The Grand Tour of Big Science,” p. 256.

29	Waff, “A History of Project Galileo,” p. 12.

30	“Pioneer Flights Revised/Two More Missions Added,” Space Daily (9 February 1968): 226.

31	 “Pioneer 6,” National Space Science Data Center Master Catalog, Spacecraft ID: 65-105A, 20 January 1999, http:// 

nssdc.gsfc.nasa.gov/cgi-bin/database/www-nmc?65-105A.

32	“Pioneer 1,” http://nssdc.gsfc.nasa.gov/database/MasterCatalog?sc=1958-007A; “Pioneer 2,” http://nssdc.gsfc.nasa. 

gov/database/MasterCatalog?sc=PION2; “Pioneer 3,” http://nssdc.gsfc.nasa.gov/database/MasterCatalog?sc=1958-

008A; “Pioneer 4,” http://nssdc.gsfc.nasa.gov/cgi-bin/database/www-nmc?59-013A (sites were reached via National 

Space Science Data Center Master Catalog, http://nssdc.gsfc.nasa.gov/nmc/sc-query.html, accessed 3 June 2003).

tragic Apollo command module fire in January 1967, which killed three astronauts and 

portrayed the Agency as careless in its attempts to achieve a Moon landing too quickly.23

A federal budget crisis brought on in the summer of 1967 by the Vietnam War 

motivated Johnson to ask Congress for a 10-percent increase in income taxes. In order 

to persuade Congress, Johnson offered to make cuts in his programs, starting in October 

1967. As a result, NASA’s budget was targeted for a half-billion-dollar reduction. This was 

followed in September 1968 by a White House–proposed quarter-billion-dollar cut. After 

congressional appropriations committees recommended only a $3.99-billion NASA budget 

for 1969 (down from a high of $5.25 billion in 1965), James Webb resigned as the head 

of the Agency. Webb was a Washington insider who had been very successful since the 

early 1960s in garnering political and economic support for NASA’s programs. He was 

considered a “master at bureaucratic politics” with “a seamless web of political liaisons.”24 

Nevertheless, Webb was not able to change the reality of declining budgets and a lack of 

government enthusiasm for NASA’s post-Apollo space exploration plans.25

Although government budget cuts made it difficult to start ambitious new exploration 

programs, such programs were being studied in depth during the latter years of the decade. 

IITRI and JPL were analyzing Jupiter orbiter mission concepts. IITRI’s J. E. Gilligan also considered various designs for Jupiter atmospheric entry probes. Such probes, he believed, could 

provide data on the Jovian atmosphere’s composition and structure that could not be obtained 

from Earth-based measurements, flybys, or orbiters. But there were severe technical obstacles 

to overcome. The most challenging would be to protect the probe from the high temperatures 

it would experience due to its high entry velocity. Gilligan estimated the entry velocity to be 

between 30 and 37 miles (48 to 60 kilometers) per second. Gas surrounding the probe would 

soon heat to temperatures 10 times greater than those that were currently being experienced 

by spacecraft reentering Earth’s atmosphere. The danger was that a substantial part of the 

probe would burn up before much information could be obtained.26

During 1968–1969, an important study was conducted by the Astro Sciences 

Center (ASC) of IITRI on the types of spacecraft that would best accomplish various 

mission objectives. An ASC team led by John Niehoff indicated that scientific returns could 

be maximized if two differently designed spacecraft were employed. A spin-stabilized 

craft such as Ames Research Center’s Pioneer, which revolved at about 60 revolutions per 

minute (rpm), had certain advantages for making particles-and-fields measurements. If 

placed in a highly elliptical orbit around Jupiter with an apoapsis, or most distant point, of 

100 Jupiter radii, it would have the chance to measure various field regions surrounding 

the planet, including its magnetosphere and bow shock. A “three-axis-stabilized” craft such 

as JPL’s Mariner, which used a series of jets and gyroscopes for precise attitude control, 

offered an excellent platform for planetary and satellite photography and remote sensing 

measurements.27 The ASC study had a significant effect on outer planet mission planning, 

which often included the two types of spacecraft in various program scenarios. 

Although a range of outer planet mission conceptual studies took place throughout the 1960s, NASA did not move from such efforts to serious mission planning until late 

23	 Ibid.

24	James E. Webb,” NASA History Office Biographical and Other Personnel Information, http://www.hq.nasa.gov/office/

pao/History/Biographies/webb.html (accessed 9 June 2003).

25	 Dallek, “Johnson, Project Apollo, and the Politics of Space Program Planning,” p. 85.

26	 Waff, “The Struggle for the Outer Planets,” p. 46.

27	 	Waff, “A History of Project Galileo,” p. 14.
22	 MISSION TO JUPITER: A History of the Galileo Project Chapter 2 I From Conception to Congressional Approval 23

By May 1969, the Working Group had folded its ideas into an ambitious outer 

planet trip schedule with launch dates throughout the 1970s and early 1980s. All three 

mission types—flybys, orbiters, and probes—were included in the schedule:

• 1972–73: Two Jupiter flybys. These missions were already approved by this 

time and would become Pioneers 10 and 11.

• 1974: Jupiter flyby/solar system escape. The mission trajectory would allow 

study of the unexplored region between the solar system and the surrounding interstellar neighborhood.

• 1976: Jupiter orbiter.

• 1977: Jupiter-Saturn-Pluto flybys. 

• 1978: Multiple Jupiter atmospheric entry probes.

• 1978: Saturn orbiter and probes.

• 1979: Jupiter-Uranus-Neptune flybys.

NASA’s summer-study panel convened in June 1969, shortly after the Working 

Group issued its recommendations. The summer-study panel consisted of 23 members 

who were asked to make recommendations for outer planet exploration from 1972 

through 1980. Five missions with possible launch dates were selected by the summerstudy panel and prioritized in order of scientific importance:

•	 1974 or 1975: Jupiter deep-entry probe. A trajectory out of the solar system’s 

ecliptic plane would be used.

• 1976: Jupiter orbiter combined with deep-entry probe.

• 1977: Jupiter-Saturn-Pluto mission combined with an atmospheric-entry 

probe for one of the planets.

• 1979: Jupiter-Uranus-Neptune mission that also would include a Neptune 

atmospheric probe.

• Early 1980s: Jupiter-Uranus mission with a Uranus probe.

As shown above, there was considerable overlap between the summer-study 

panel’s recommendations and those of the Working Group. The summer-study panel, 

however, included atmospheric-entry probes in all of its suggested missions. The panel 

believed that data obtained from probes actually entering a planetary atmosphere would 

help greatly in interpreting subsequent atmospheric data from flybys and orbiters.35

35	 	Waff, “The Struggle for the Outer Planets,” pp. 47–48; Butrica, “Voyager: The Grand Tour of Big Science,” pp. 255–256.

was reversed after RTG designs were studied in more detail. Pioneer 10 was scheduled to 

launch in February or March of 1972; Pioneer 11, 13 months later.33

An Ambitious Plan for the Future

In December of 1968, NASA became more serious about post-Apollo planning when its 

Associate Administrator for Space Science and Applications, John Naugle, was tasked 

with examining which outer solar system missions were the most appropriate to focus 

on. Naugle asked a NASA-appointed advisory group, the Lunar and Planetary Missions 

Board, to suggest strategies and scientific objectives for outer planet exploration. 

The Board was chaired by James Van Allen, of the University of Iowa, and 

included Thomas Gold of Cornell and Von R. Eshleman of Stanford. These men had 

already spent considerable effort in conceptualizing a Pioneer mission to Jupiter and 

returned a report within two months. In its early 1969 report, the Board said much that 

was complimentary about a Grand Tour mission, mentioning that it would constitute “a 

technological and scientific tour de force of heroic dimensions.” The amount of outer 

planet data that could come out of such a flyby mission for a relatively modest cost 

particularly impressed the Board. 

The principal alternative to the Grand Tour that the Board studied was a series 

of intensive orbiter missions, one to each of the four major outer planets (Jupiter, 

Saturn, Uranus, and Neptune). A Jupiter orbiter would probably be the first objective. 

The Board recommended that such orbiter missions be included as a major NASA goal 

for the 1970s. 

Although the Board identified many positive aspects of both a Grand Tour 

and a series of orbiter missions, it did not clearly recommend one over the other. 

Instead, it suggested that NASA conduct a more exhaustive examination of outer planet 

mission options. 

Responding to the Board’s suggestion, NASA formed an Outer Planets Working 

Group that began meeting almost immediately, drawing scientists and advanced mission 

planners from JPL, Ames Research Center, Goddard Space Flight Center, and Marshall 

Space Flight Center. In addition, NASA planned an intensive study that would take place 

in June 1969. 

Although the Outer Planets Working Group recognized that a Jupiter orbiter 

project would have “greater scientific potential” than a series of multiplanet flybys such 

as the Grand Tour, it favored the latter because of the chance to accomplish many objectives at a relatively low cost. But instead of one four-planet Grand Tour, the Working 

Group envisioned two truncated flyby missions—one to Jupiter, Saturn, and Pluto, and 

the other to Jupiter, Uranus, and Neptune—which collectively would constitute a more 

ambitious Grand Tour than was envisioned by Gary Flandro and would allow more to 

be accomplished because all five outer planets would be visited.34

33	“Pioneer 10,” http://nssdc.gsfc.nasa.gov/database/MasterCatalog?sc=1972-012A; “Pioneer 11,” http://nssdc.gsfc.nasa. 

gov/database/MasterCatalog?sc=1973-019A (sites were reached via National Space Science Data Center Master Catalog,

http://nssdc.gsfc.nasa.gov/nmc/sc-query.html, accessed 3 June 2003); Waff, “A History of Project Galileo,” pp. 12–13.

34	Waff, “The Struggle for the Outer Planets,” pp. 47–48; Butrica, “Voyager: The Grand Tour of Big Science,” pp. 255–256.
24	 MISSION TO JUPITER: A History of the Galileo Project Chapter 2 I From Conception to Congressional Approval 25

Though Nixon portrayed astronauts as American heroes and used them to further his 

political aims, he never displayed “the personal enthusiasm for or expansive commitment 

to the space program that [Lyndon] Johnson and John F. Kennedy had shown.” Nixon did 

not need the space program to prove himself able to deal with the Soviets, and this may 

have contributed to his attitude. Or perhaps he had so many staggering economic issues 

to deal with that he was forced to retrench and cut costs wherever he could.38

Several other factors contributed to the space program’s downsizing in the 

1970s. First, none of Nixon’s close advisors promoted the space program. Neither of 

NASA’s Administrators during that time, Thomas O. Paine and James C. Fletcher, had 

the ear of the President or of his inner circle of advisors. Second, NASA’s highly visible 

budget overruns hurt its reputation. This situation was exacerbated because NASA upper 

management did not develop as close a working relationship with the Bureau of the 

Budget and, later, the Office of Management and Budget as it might have. Such a relationship was necessary in order for NASA to receive the highest priority consideration for its 

space exploration and other projects. 

Third, since NASA’s healthy budget of the early 1960s had been created partly 

because of Cold War pressures and perceptions, that same budget suffered when U.S.-

Soviet relations improved. Beating the Soviets to outer space seemed less important 

than it had under Johnson and Kennedy. Finally, by 1969, the political tide had turned 

against increased NASA funding. Liberals and conservatives in both parties were loathe 

to add more dollars to the space program when severe domestic problems such as 

poverty, crime, urban renewal, racism, and the deteriorating environment were staring 

them in the face.39

Living with Reduced Means

The budget guidelines that President Nixon issued in the summer of 1969 were of great 

concern to NASA. The Agency had been planning to ask for $33 million in startup funding for its Grand Tour missions and $3 million for advanced studies of outer planet orbiter and 

probe missions. However, it became clear that Nixon was planning to cut $75 million, or almost 

20 percent, from NASA’s planetary programs budget. With the ongoing Viking Mars program 

costs escalating, NASA did not think that it would receive sufficient funds for initiating a Grand 

Tour program. The Agency delayed its request for startup funding for another year, which meant 

that the Grand Tour test flight would not take place until 1975 at the earliest.40

Seeing the severe budget cuts that NASA would soon be forced to endure, 

National Academy of Sciences president Philip Handler suggested that a Space Science 

Board review panel be convened to develop priorities for spending. The review panel 

would evaluate how to balance resources among the seven different scientific disciplines 

that NASA supported: planetary exploration, lunar exploration, astronomy, gravitational 

physics, solar-terrestrial physics, Earth environmental sciences, and life sciences. 

NASA Administrator Thomas Paine agreed to the convening of a Space Science 

Board review panel. Before the panel met, however, Paine submitted his resignation to 

38	Ibid.

39	 Ibid., pp. 94-95.

40	Butrica, “Voyager: The Grand Tour of Big Science,” pp. 256–258; Waff, “A History of Project Galileo,” p. 15.

In a later meeting, the Working Group expressed doubt that entry probes 

could be designed and built in time for the mission schedule that the summer-study 

panel envisioned. That schedule required a probe to be ready for launch by 1974. The 

Working Group recommended instead that orbiter missions be sent off first and that 

NASA wait until 1978 to launch missions involving probes. 

The area in which the Working Group and summer-study panel most closely 

agreed was on schedules for Grand Tour missions. Both recommended a 1977 JupiterSaturn-Pluto flyby and a 1979 Jupiter-Uranus-Neptune flyby (although the summer-study 

panel also wanted to add probes to both of these missions). Perhaps because of the 

two groups’ agreement on Grand Tour schedules, and certainly because of the narrow 

window for launching such missions (1976 through 1980), NASA decided to focus on 

Grand Tour missions throughout the 1970s. During the summer of 1969, the Agency 

began formulating Fiscal Year 1971 funding requests for starting up a Grand Tour 

program. Initial funding was sought for a 1974 flyby test flight to Jupiter, followed by the 

1977 and 1979 multiplanet missions that would collectively form a Grand Tour. 

NASA planned to submit the funding request to the federal Bureau of the Budget 

in September 1969. But before it could, the administration of newly inaugurated President 

Richard Nixon issued budget guidelines that compelled NASA to reexamine its priorities.36

Changing of the Presidential Guard and Its Effect on Outer Planet Exploration

The year 1969 was not an easy time to nurture new space projects. The costs of the Vietnam 

War were spiraling upward, as were those of the Great Society social reform programs 

that Nixon had inherited from Johnson. American society was becoming disillusioned 

with and mistrustful of everything that the federal government did. Government priorities and funding allocations were frequently perceived as inappropriate, if not criminal. 

This lack of confidence in all things federal extended to the space program. In addition, 

Richard Nixon, who became president in January 1969, instituted very conservative fiscal 

policies that took their toll on NASA and its grand plans for the future. Today’s outer 

planet exploration program grew from the tensions of these times. 

NASA , touting its impressive success record, battled continuously against evergrowing budget constraints, an unsympathetic administration, and a mistrustful populace. 

There were many in the country who perceived NASA as arrogant and elitist. This negative 

image was furthered by astronauts promoting personal business endeavors and marketing 

space trinkets. Project Apollo launches were becoming chic media events, complete with 

high-visibility celebrities and the country’s “beautiful people” in attendance. The lunar 

landings in 1969 elicited tremendous excitement, but they were also viewed as part 

of NASA’s “three ring circus” and may have added to the public’s disillusionment with 

Project Apollo. In short, the U.S. government and NASA in particular were perceived as 

doing a poor job of responding to the American people’s needs and desires.37

President Nixon’s emphasis on frugality led to a decelerated rate of space exploration in the 1970s and a philosophy of maximum scientific return for minimum cost. 

36	Waff, “The Struggle for the Outer Planets,” pp. 46–48; Hoff, “The Presidency, Congress, and the Deceleration of the

U.S. Space Program in the 1970s,” pp. 92–94; Butrica, “Voyager: The Grand Tour of Big Science,” pp. 256–257.

37	Hoff, “The Presidency, Congress, and the Deceleration of the U.S. Space Program in the 1970s,” pp.92–94.
26	 MISSION TO JUPITER: A History of the Galileo Project Chapter 2 I From Conception to Congressional Approval 27

spacecraft for the Outer Planet Missions.”45 (The name “Outer Planet Missions” had been 

given by NASA to a program that encompassed both Grand Tour flybys and Jupiter orbiters.) In June 1971, a Senate budget committee voted to slash NASA’s Outer Planets Missions 

funding from $30 million to $10 million. Although this amount was raised to $20 million 

by a combined House and Senate conference committee, the conferees maintained the 

need for NASA to develop cheaper, less sophisticated spacecraft and recommended that 

NASA reexamine whether developing the expensive TOPS was appropriate.

NASA strongly wanted to build an outer planet program that would have the 

unified support of the scientific community, for it recognized the importance of this in 

securing congressional funding. Toward this end, NASA organized a Science Advisory 

Group (SAG) in July 1971 and appointed members of the U.S. space science community. 

JPL and Ames presented their findings on various mission concepts to SAG in July 1971. 

JPL cautioned that substituting orbiter missions for Grand Tour flybys would add considerably to the cost, especially if TOPS was the spacecraft used. By means of comparison, 

four TOPS multiplanet flyby missions would cost approximately $750 million, while two 

such missions and two TOPS Jupiter orbiter trips would raise that to $925 million. The 

orbiter missions were more expensive because, among other reasons, they required a new 

propulsion system to be developed for inserting the space vehicles into Jovian orbit. 

JPL presented a fallback plan that was cheaper. The above mission concepts 

assumed that sophisticated TOPS spacecraft would be designed and constructed for the 

endeavors. If this was not possible due to budget constraints, then already-available 

Mariner-class spacecraft could be employed instead, at a savings of several hundred 

million dollars. Ames had a similar backup plan but suggested using the even less expensive Pioneer-class craft for orbiting Jupiter. 

Unfortunately, NASA’s SAG did not reach consensus on which outer planet 

plan should be pursued. Four Grand Tour flyby missions would have “broad exploratory 

appeal,” it said, but two Grand Tours combined with two Jupiter orbiters would better 

serve those who thought that “more intensive investigation of Jupiter and its satellites will 

yield greater physical insight.”46

The National Academy of Sciences Space Science Board reviewed the JPL/Ames 

findings in August 1971, but like NASA’s SAG, the Board could not establish a unified 

position on the flyby versus orbiter debate. Shortly thereafter, NASA Administrator James 

Fletcher argued with Congress for inclusion of the Grand Tour in NASA’s FY 1973 budget, 

claiming that it would have “great popular interest as mankind’s farthest reach out into 

space,”47 but his position was weakened by the lack of clear support from the scientific 

community. Fletcher only managed to get a reduced-budget Grand Tour project approved, 

with its funding level slashed from the planned $100 million to $29 million. 

Prospects for outer planet missions continued to worsen. The Space Shuttle 

Program was then under consideration for funding and had the strong support of President 

Nixon. In December 1971, NASA learned that he was ready to approve it. Unfortunately, 

the President was of the mind that both the Shuttle and a Grand Tour program relying on 

the development of an expensive TOPS spacecraft could not simultaneously be funded. 

45	Waff, “The Struggle for the Outer Planets,” p. 50.

46	Waff, “The Struggle for the Outer Planets,” pp. 49–51; Butrica, “Voyager: The Grand Tour of Big Science,” pp. 260–

261. Note: Quotations are from the Waff article.

47	Waff, “The Struggle for the Outer Planets,” p. 51.

President Nixon. His recent NASA budget request of about $3.3 billion was “down more 

than $500 million from the previous fiscal year,” and Congress appeared to be “in a mood 

to cut space funding even more.” But according to one history of the Apollo program, 

Paine probably quit not just because of the erosion of NASA’s budget, but also because 

he saw no prospect of the situation’s improving.41

The Space Science Board review panel met at Woods Hole, Massachusetts, from 

26 July to 15 August 1970. Nearly 90 scientists participated, divided among the seven 

scientific disciplines and an executive committee that was to integrate the various proposals and recommendations.42 The scientists agreed on some issues, for instance, that a large 

program such as the Grand Tour should not displace smaller but nonetheless important 

missions such as the exploration of Venus. But in certain key areas, consensus was not 

achieved, and this would eventually have consequences for the space program. 

In particular, the participants did not reach agreement on what the highest priority for outer planet exploration should be. One of the scientific discipline groups, the 

Planetary Exploration Working Group, held that the Grand Tour opportunity was so 

important and rare that it should not be missed. The executive committee was more interested in the scientific data that could be generated from orbiter and probe missions to 

Jupiter. It thought that such intensive investigations would generate more useful information of the type needed to launch other successful outer planet missions. Furthermore, the 

executive committee viewed Grand Tour projects as quite risky. During such long flights, 

the various spacecraft systems might not continue to operate reliably. Also, the need for 

gravity-assists would constrain the mission trajectory so much, the committee believed, 

that studies of the outer planets’ many satellites would be hindered.43

OMB, which the Nixon administration had recently formed to replace the Bureau 

of the Budget, tried to postpone the startup of Grand Tour development from fiscal year 

(FY) 1972 to FY 1973, which would have further delayed launch dates. NASA fought 

successfully to keep the startup in FY 1972 but began considering the executive committee’s negative views on the mission. NASA was worried that once Congress learned of the 

executive committee’s position, Congress would wonder why a space exploration program 

was moving forward when heavy hitters in the U.S. scientific community opposed it. 

After discussions with the Space Science Board, NASA decided to keep several 

outer planet options open for the time being. At its budget briefing in January 1971, the 

Agency announced that by November, it would choose to develop a Jupiter-Saturn-Pluto 

mission, a Jupiter orbiter program, or a combination of both. The Agency also decided 

to investigate the costs and benefits of using two different spacecraft in a Jupiter orbiter 

program—a spin-stabilized Pioneer and a sophisticated three-axis-stabilized vehicle that 

NASA wanted to build, the Thermoelectric Outer Planet Spacecraft (TOPS).44

During the spring of 1971, JPL and Ames studied Jupiter orbiter concepts based 

on TOPS and Pioneer spacecraft. The study results were to have been presented to NASA

and National Academy of Sciences panels during the summer. But before the panels 

even met, OMB requested NASA to study “simpler, less costly alternatives to the TOPS

41	W.DavidCompton,“FirstPhaseofLunarExplorationCompleted:CutbacksandProgramChanges,”chap.12-2inWhere 

No Man Has Gone Before: A History of Apollo Lunar Exploration Missions (Washington, DC: NASA SP-4214, 1989),	

http://www.hq.nasa.gov/office/pao/History/SP-4214/ch12-2.html.

42	Waff, “A History of Project Galileo,” p. 15.

43	Waff, “A History of Project Galileo,” p. 15; Butrica, “Voyager: The Grand Tour of Big Science,” p. 258.

44	Waff, “A History of Project Galileo,” p. 16; Butrica, “Voyager: The Grand Tour of Big Science,” p. 260.
28	 MISSION TO JUPITER: A History of the Galileo Project Chapter 2 I From Conception to Congressional Approval 29

experiments studying magnetic fields, solar wind characteristics, cosmic rays, the heliosphere, hydrogen abundance, dust particles, the Jovian atmosphere, aurorae and radio 

waves, and the planet’s satellites, especially Io. Pioneer 11 launched in April 1973 and 

passed within 21,000 miles (34,000 kilometers) of Jupiter’s cloud tops in December 1974. 

It went on to Saturn, reaching the planet in 1979.51

Pioneers 10 and 11 were important scouts for future, more intensive investigations. In particular, they charted Jupiter’s radiation belts. This was critical for determining 

design requirements that future spacecraft would need in order to survive in the planet’s 

radiation environment for visits that might last years. One of Pioneer 10’s most useful finds 

was that Jovian radiation levels were not as severe as previously feared. 

The Mariner Voyager Missions

The two Mariner Jupiter-Saturn flyby missions, which would eventually become Voyagers 1 

and 2, launched in 1977, and both flew by Jupiter in 1979. JPL engineers designed Voyager 1’s 

path so that the spacecraft would also pass near the Saturnian satellite Titan, which at the time 

was the only moon known to have an atmosphere. Voyager 2’s trajectory was designed so 

the spacecraft could potentially undertake the original “Grand Tour,” that is, a Jupiter-SaturnUranus-Neptune mission, should the spacecraft continue to function that long and should 

additional mission funding become available. Congress eventually became more receptive to 

this idea and provided funds for extending the mission to both Uranus and Neptune.52

Voyagers 1 and 2, the last of the Mariner series spacecraft, discovered three new 

Jovian satellites, as well as a set of rings around the planet. Voyager 1 went on to Saturn, 

arriving the following year; Voyager 2 flew by Saturn a year after that and then turned 

toward Uranus and Neptune, reaching those gas giants in 1986 and 1989, respectively. 

Gravity-assists were used to modify the spacecraft’s trajectories. When Voyager 2 reached 

Neptune, it had covered all four of the original Grand Tour planets envisioned by Gary 

Flandro 24 years earlier.53

Planning for the Jupiter Probe and Orbiter 

Following the approval in the spring of 1972 of what would become Voyagers 1 and 2, 

the NASA-appointed SAG for Outer Solar System Missions examined additional outer planet 

mission concepts. In particular, scientific returns and design requirements of Jupiter atmospheric entry probes were analyzed. It was concluded that the heatshield technology likely to 

be available in the next several years would not protect a probe during a descent deep into 

51 “Pioneer 10,” National Space Science Data Center Master Catalog, Spacecraft ID: 72-012A, 21 April 2000,	

http://nssdc.gsfc.nasa.gov/cgi-bin/database/www-nmc?72-012A; “Pioneer 11,” National Space Science Data Center 

Master Catalog, Spacecraft ID: 73-019A, 29 January 1998, http://nssdc.gsfc.nasa.gov/cgi-bin/database/wwwnmc?73-019A.

52 Craig B. Waff, former JPL contract historian, telephone conversation with author, 22 May 2000; JPL, “Planetary

Voyage,” Voyager—Celebrating 25 Years of Discovery, http://www.jpl.nasa.gov/voyager/science/planetary.html

(accessed 1 December 2003).

53 “Voyager Project Information,” National Space Science Data Center, 19 May 1999, http://nssdc.gsfc.nasa.gov/

planetary/voyager.html; Waff, “The Struggle for the Outer Planets,” pp. 49–51.

The chances of getting the Grand Tour program funded with the development of a TOPS

spacecraft looked dismal indeed. 

As a result, in late December 1971 and early January 1972, NASA management decided to propose that less sophisticated Mariner-class spacecraft perform two 

less ambitious Jupiter-Saturn flyby missions. These reduced missions were approved by 

Congress in 1972. 

At a NASA budget briefing and news conference, Fletcher pointed to the “less 

than enthusiastic response from certain elements of the scientific community” as pivotal 

in foiling a more extensive outer planet effort. His deputy, George Low, added that “the 

simple truth was that there wasn’t unity among scientists regarding the value of the Grand 

Tour, and this made it an easy target for cancellation.”48

A few months later, Bruce Murray, a scientist who later became JPL’s Director, 

expressed the disappointment of those who’d believed in the value of a Grand Tour. Its 

cancellation, he felt, would have a serious impact on the U.S. space program. In a testimony to Congress, he stated:

The Grand Tour cancellation was a self-inflicted setback of unprecedented magnitude. Those are strong words, but that’s how I feel. Our 

prospects for future achievements suddenly were narrowed. We did it 

to ourselves. What had been established as a major commitment with 

widespread popular support and unique scientific promise was allowed 

to tremble and collapse. As a consequence, I feel the credibility of our 

commitment to outer planet exploration is brought into question.49

It is interesting that one factor that made the Grand Tour opportunity so marketable, its rarity (occurring only once every 175 years), also worked against the mission’s 

implementation. The inflexible constraints on mission schedules and on the research and 

development that had to take place to meet those schedules ultimately presented a formidable barrier to carrying out the Grand Tour.

50

A Reduced but Still Fruitful Outer Planet Program

NASA did not get its sophisticated TOPS spacecraft built, and it looked doubtful in 1972 that 

a Grand Tour would be accomplished. Nevertheless, outer planet efforts did go on during the 

1970s using tried and true Pioneer- and Mariner-series spacecraft, and the information from 

these was vital for designing a Jupiter orbiter mission that would eventually become Galileo. 

The Pioneer Missions

Pioneer 10, the first outer solar system mission, launched in March 1972 and passed 

within 120,000 miles (200,000 kilometers) of Jupiter on 3 December 1973. It conducted 15 

48	Butrica, “Voyager: The Grand Tour of Big Science,” pp. 261–262; Waff, “The Struggle for the Outer Planets,”	

pp. 51–52.

49	Waff, “The Struggle for the Outer Planets,” p. 52.

50	 Ibid., p. 51.
30	 MISSION TO JUPITER: A History of the Galileo Project Chapter 2 I From Conception to Congressional Approval 31

mounted on the spacecraft to track the position of Canopus, while two primary and four 

secondary sensors recorded the Sun’s position. Attitude information was also provided 

by an inertial reference unit and an accelerometer. The attitude stability that the Mariner 

craft offered was valuable, but its price was more weight and a higher economic cost than 

the Pioneer spacecraft. There were also more systems on the Mariner that could fail and 

end a mission.56

In some of its studies, Ames Research Center had recommended Pioneer craft 

for outer solar system work. JPL, on the other hand, was a strong advocate of three-axisstabilized craft, and this position would ultimately affect space vehicle design for the 

Galileo mission.57

In summer 1974, the National Academy of Sciences Space Science Board’s new 

Committee on Planetary and Lunar Exploration (COMPLEX) recommended a Mariner 

Jupiter-Uranus project, the 1980 Pioneer Jupiter orbiter and probe mission, and the 1985 

Jupiter orbiter endeavor. Two months later, however, COMPLEX changed its position, 

stating that the current budget environment did not allow two large projects such as the 

Mariner Jupiter-Uranus and Pioneer Jupiter orbiter/probe missions to start simultaneously. 

A reassessment of outer planet exploration priorities was urged. 

As a result, NASA organized working groups to study the various mission concepts 

and present recommendations to a Strategy for Outer Space Exploration (SOPE) advisory 

panel. By February 1975, after studying the working group reports, SOPE recommended a 

Mariner Jupiter-Uranus mission that would possibly include a Uranus atmospheric probe. 

SOPE also advised that a Pioneer Jupiter orbiter/probe mission precede the probeless 

Mariner Jupiter orbiter because atmospheric probes would initiate a whole program of 

outer solar system atmosphere exploration.

During the same month, National Academy of Sciences’ COMPLEX also backed 

the Pioneer Jupiter orbiter/probe project, although it was divided over the Mariner 

Jupiter-Uranus mission. Acting quickly on the clear support for a Jovian orbiter and probe 

mission, NASA authorized Ames to initiate a “Phase B” development effort for the Pioneer 

project. By this time, ESRO had decided to drop out of the project. While the mission had 

the strong support of West Germany, other European countries were not so enthusiastic 

about participating.58

The major project that NASA was planning at this time was the Space Shuttle, 

which would have a human crew and would be partially reusable. It would have many 

uses and would eliminate NASA ’s need for most of its expendable launch vehicles such 

as the Titan/Centaur. Shuttle development costs were running higher than expected, 

and NASA activities were under budgetary pressures by the fiscally conscious Nixon 

and Ford administrations. 

These were compelling factors for NASA to cease production of the expensive Titan/

Centaur launch vehicles for missions beyond the already-approved Viking and Mariner 

Jupiter-Saturn (Voyager) projects in 1976 and 1977. But a Jupiter-Uranus mission using 

the Mariner spacecraft would need the Titan/Centaur. A Mariner spacecraft’s mass was 

far more than the 321 pounds (146 kilograms) of the Pioneer. The Mariner space vehicles 

56 “Mariner 6,” National Space Science Data Center Master Catalog, Spacecraft ID: 69-014A, 13 April 2000, http://nssdc.

gsfc.nasa.gov/cgi-bin/database/www-nmc?69-014A; “Mariner 9,” National Space Science Data Center Master Catalog,

Spacecraft ID: 71-051A, 19 April 2000, http://nssdc.gsfc.nasa.gov/cgi-bin/database/www-nmc?71-051A.

57 Torrance V. Johnson interview, tape-recorded telephone conversation, 31 July 2001; Waff, “Jupiter Orbiter Probe,” p.7.

58 Waff, “Jupiter Orbiter Probe,” pp. 4–6.

the Jovian atmosphere. In fact, the facilities to test an entry probe adequately under the heating conditions expected to be encountered at Jupiter would not be available until 1980. 

The SAG was also concerned about radiation exposure and its effect on the spacecraft’s operating systems and experiments. It recommended that not until the Pioneer 10 and 

11 missions determined the severity of Jupiter’s radiation environment should a Jupiter orbiter 

or probe mission be undertaken. With the above cautions in mind, the SAG recommended that other missions be undertaken in the 1970s, such as Jupiter flybys that then 

proceeded on to Saturn or out of the ecliptic plane. A 1979 atmospheric probe to Saturn 

was also recommended, because the conditions that the spacecraft would encounter 

there were presumed to be less severe than at Jupiter and within the bounds of what 

heatshield technology would be able to handle. The SAG also envisioned two Mariner 

Jupiter orbiter missions launching during 1981 and 1982.54

NASA authorized Ames, JPL, and their contractors to study the recommended mission 

concepts in more detail. In particular, JPL was directed to develop the concept of a Mariner 

Jupiter orbiter mission. Following up on a major new direction for a launching strategy, NASA

directed JPL to make the first study of launching a planetary mission from the Space Shuttle. 

An important idea that emerged from the study was of a “satellite tour,” in which 

the trajectory of each satellite flyby would be designed so as to receive a gravity-assist 

that aimed the spacecraft toward its next satellite encounter. This would minimize fuel 

requirements and could extend the lifetime of the mission.55

The December 1973 results from Pioneer 10’s Jupiter flyby indicated a less severe 

radiation environment than feared. Coupled with the progress being made in heatshield 

studies, it was thought that the atmospheric probes being designed for Saturn and Uranus 

might also be sufficiently durable for Jupiter. NASA considered adding an entry probe 

capability to a future Jupiter orbiter mission. The Agency was continuing to deal with 

tight budgets and, early in 1974, explored joint mission concepts with the European Space 

Research Organization (ESRO). Out of this study came the announcement in May 1974 

that a 1980 NASA-ESRO Jupiter orbiter/probe mission using a Pioneer H spacecraft was 

now being actively considered (along with one of the Mariner Jupiter orbiter missions, 

which had been delayed to 1985). 

Pioneer- and Mariner-series craft were both highly capable space vehicles that 

had been used for many years. One of the major differences between the two was in 

the method of attitude stabilization—a critical function if precise planetary measurements 

were to be taken. The Pioneer’s stabilization was achieved through spinning the craft at 

(typically) 60 rpm. This provided dependable stabilization without requiring a propulsion system to maintain attitude. It also kept the spacecraft’s weight and cost down. The 

Pioneer’s spin gave its instruments a 360-degree view of its surroundings and a good 

platform from which to make field-and-particle measurements.

The Mariner, on the other hand, had a sophisticated system of gyros and jets that 

maintained stabilization in all three axes, a feature that could help maximize photographic 

resolution and the returns from remote sensing experiments. Three gyroscopes plus two 

sets of six nitrogen jets mounted on the ends of its solar panels controlled Mariner’s attitude, which was measured relative to the Sun and the star Canopus. A special sensor was 

54 Craig B. Waff, “Jupiter Orbiter Probe: The Marketing of a NASA Planetary Spacecraft Mission” (paper presented at

American Astronomical Society meeting, session on “National Observatories: Origins and Functions (The American

Setting),” Washington, DC, 14 January 1990,) pp. 3–4.

55 Waff, “Jupiter Orbiter Probe,” p. 4.
32	 MISSION TO JUPITER: A History of the Galileo Project Chapter 2 I From Conception to Congressional Approval 33

slated to start in FY 1978. As a result of the JPL-Ames joint ventures, Murray thought that “both 

institutions are stronger, and NASA is much better off as a consequence.”63

Development of the Inertial Upper Stage

One of the necessary steps in launching JOP, as well as other spacecraft, from the Shuttle 

was the development of an adequate propulsion technology to send the vehicles on 

their way after the Shuttle lifted them to Earth orbit. In fall 1975, the Air Force chose to 

modify the solid-fueled Boeing Burner II rocket for this purpose. The two-stage propulsion system was originally given the name Interim Upper Stage (IUS), because it was 

to serve only until NASA found the funds to develop a Space Tug. Initiation of the Tug 

project slipped further and further into the future, however, and the propulsion system’s 

name was changed to Inertial Upper Stage (with the same IUS acronym).64

A Centaur rocket had also been under consideration, but it was rejected because 

of the increased fire danger that a liquid-hydrogen-fueled rocket carried in the Shuttle’s 

payload bay presented. This was a difficult decision; the Centaur was the more powerful 

rocket and would make planetary missions with larger payloads possible. 

The solid-fueled IUS rocket was reluctantly approved when it was shown to be 

barely powerful enough to launch JOP, which was the most demanding planetary mission 

on NASA’s books at that time, during the December 1981–January 1982 launch window. 

This was the best window in terms of launch energy requirements for several years. If 

the JOP spacecraft was not ready by that time, it would not be able to launch in its thencurrent configuration until 1987. Also, if mission requirements demanded a heavier space 

vehicle, or if IUS development was delayed or its performance did not meet expectations, 

it would be difficult or impossible to launch JOP during the 1981–82 time slot.65

Growing a JOP Project

NASA’s “Jupiter Orbiter with Probe Marketing Plan,” written by Dan Herman around the 

end of 1975, outlined the steps that would be taken to support the start of a JOP project 

in FY 1978. These steps included the following:

• Forming a science working group (JOPSWG) to define mission objectives and 

outline probe and orbiter payload requirements. JOPSWG was chaired by 

James Van Allen and began meeting in the first half of 1976. 

• Issuing a Request for Proposal (RFP) for Phase-B Probe development. After 

the RFP was issued in May 1976, two contractors were selected to prepare 

proposals: Hughes and McDonnell Douglas. Hughes, partnered with General 

Electric, ultimately won the Probe contract in June 1978.

63 Bruce Murray, First Annual “State of the Lab” Talk, 1 April 1977, Bruce C. Murray Collection, 1975–1982, folder 31, box

2, JPL 216, JPL Archives.

64 Waff, “Jupiter Orbiter Probe,” p. 8; The Boeing Company, “IUS Introduction,” Inertial Upper Stage, 2003, http://www.

boeing.com/defense-space/space/ius/ius_Introduction.htm.

65 Waff, “Jupiter Orbiter Probe,” p. 8.

used for the Voyager missions, for instance, each weighed 1,590 pounds (722 kilograms). 

The fact that a Mariner mission required a launch vehicle such as Titan/Centaur, as well 

as the lack of unified support from National Academy of Sciences’ COMPLEX for a 1979 

Mariner Jupiter-Uranus mission, may have contributed to the mission’s exclusion from the 

FY 1977 budget request that NASA submitted in September 1975.59

JPL Takes Over the Management of the Jupiter Orbiter Probe Effort

NASA management considered various factors in deciding which of NASA’s Centers would 

manage the Jupiter Orbiter Probe (JOP) project. According to John Casani, Galileo’s first 

Project Manager, all of the Centers had certain roles that represented their core mission 

responsibilities. “Some centers were designated as ‘research,’” said Casani, “and some as 

‘operational.’” JPL’s role was operational—to conduct planetary missions—while the role 

of Ames Research Center, which also wanted the JOP project, was just what its name 

implied, a facility focused more on technology development than on planetary exploration. The distinction between the two Centers was not black and white, however. Ames 

had managed Pioneers 10 and 11 (which both flew by Jupiter), as well as the Pioneer 

Venus mission (involving the Pioneer 12 and 13 spacecraft). “But I don’t think [Ames was] 

viewed as having the in-house capability to support a major planetary program activity,” 

said Casani, referring to Galileo. “Or the charter for it.”60

Restricted budgets were also factors that helped determine which Centers 

managed which NASA projects. NASA needed to consolidate its operations and cut costs. 

An Agencywide “roles and missions” study found that NASA could afford to maintain only 

one Center devoted to planetary mission development. The Center it recommended was 

JPL, which had originally fulfilled the assignment during NASA’s early years in the latter 

1950s. JPL was currently managing the Viking and Mariner Jupiter-Saturn (Voyager) projects, but these were scheduled to launch by 1977. Due to the elimination of the Mariner 

Jupiter-Uranus project, JPL would soon find its very talented staff without a major in-house 

development project on which to focus. Ames was not in this situation. It was conducting 

various large aerodynamics projects in fields such as astrobiology, flight simulation, and 

vertical takeoff and landing aircraft.61

In the fall of 1975, NASA chose JPL to manage the JOP mission. Ames was not cut 

out of the mission, however. JPL and Ames negotiated and committed to paper an agreement 

in which Ames was assigned the task of developing the atmospheric-entry probe for JOP. This 

assignment took advantage of one of Ames’s strengths—atmospheric studies.62 In his “State 

of the Lab” address on 1 April 1977, JPL Director Dr. Bruce Murray stressed that one of the 

major institutional accomplishments of the past year was to develop this working partnership 

with Ames that had not existed one year earlier. The JPL-Ames affiliation included synergistic 

efforts not only on JOP, but also on the Infrared Astronomy Satellite (IRAS) project, which was 

59	 Ibid., pp. 6–7.

60 John Casani interview, tape-recorded telephone conversation, 29 May 2001; First to Jupiter, Saturn, and Beyond 

(Pioneer project home page), 18 August 2001, http://spaceprojects.arc.nasa.gov/Space_Projects/pioneer/PNhome.

html.

61 Glen Bugos, Ames Research Center historian, telephone conversation, 1 June 2000; Waff, “Jupiter Orbiter Probe,” p. 7.

62 The Pioneer Missions, 20 April 2001, http://spaceprojects.arc.nasa.gov/Space_Projects/pioneer/PNhist.html; Waff,

“Jupiter Orbiter Probe,” p. 7.
34	 MISSION TO JUPITER: A History of the Galileo Project Chapter 2 I From Conception to Congressional Approval 35

Appropriations Subcommittee chairman Eddie Boland (D-Massachusetts) entered the 

picture. It was still January 1977 when Boland repeatedly asked NASA Administrator James 

Fletcher to prioritize between JOP and the Hubble Space Telescope. Fletcher refused to 

do this, wanting both programs to receive funding. 

It was quite unusual for NASA to request two new program starts in the same 

year, although the main factors that determined JOP’s fate in the subcommittee were 

fiscal constraints. Dick Malow remembered that the “general feeling on the subcommittee 

was that we should proceed with one [program], given the budget situation, and reserve 

judgment on the other. So Hubble went ahead, and then there was a major debate over 

JOP.” Malow further explained that “beginning in the mid ’70s, particularly in FY78, we 

were entering into a long string of government-wide budgetary problems that only grew 

worse . . . . These problems were also affecting NASA, because NASA was in the throes 

of trying to bring to fruition the Space Shuttle, and that was running into overruns . . . . 

NASA was not getting the same percentage of the budget as it was in the late ’60s or early 

’70s. So there was a squeeze everywhere.”69

Malow and other subcommittee members expected the initial budget estimates 

for Hubble and Galileo to go up, which was typical behavior for NASA programs. “Carrying 

two missions, especially if they were going to grow exponentially, which of course they 

both did . . . given the budget climate there may be some problems. In any case, it was 

Hubble that we chose to fund.”70 The subcommittee may also have assumed that NASA

would fight the strongest for the Hubble Space Telescope, which had been eliminated 

from the previous year’s budget. JOP likely appeared to the committee to be the easier 

target, with only a relatively weak and possibly divided planetary science community 

to defend it. The result was that Boland’s House subcommittee voted on 4 May 1977 to 

eliminate all funding for JOP.

71

Boland had radically underestimated the resolve of the planetary science 

community. It organized a massive lobbying effort that, only five weeks later, succeeded in 

obtaining the Senate HUD–Independent Agencies Appropriations Subcommittee’s approval 

for both the Space Telescope and JOP. This was done over the objection of Chairman 

William Proxmire.72 About the planetary community’s effort, Dick Malow said, “Rarely 

have I ever seen such a successful lobbying campaign . . . . It was masterfully done.”73

The House and Senate were now in disagreement over the JOP issue. A conference committee from the two congressional houses tried to resolve the matter on 12 

July 1977 but could not. At this point, the matter was returned to the House. As Malow 

explained, “There’s a very technical procedure you can go through to bring an item back 

. . . where the conference does not come to an agreement. And then you vote it up or 

down in the House, and that’s what happened.”74 Much to Boland’s displeasure, the 

House of Representatives approved JOP by a large margin. In the words of JPL Director 

69	Malow interview, 19 September 2001.

70 Ibid.

71	Waff, “Jupiter Orbiter Probe,” p. 10.

72 Ibid., p. 11.

73	Malow interview, 19 September 2001.

74	 Ibid.

• Initiating Phase-B Orbiter development. During the first half of 1976, JPL

investigated the use of a three-axis-stabilized space vehicle for the JOP

mission, and TRW conducted studies of spin-stabilized craft. JPL eventually 

chose a “dual-spin” design that combined features of both types of space 

vehicles. Hughes had employed the dual-spin concept in communication 

satellites, but it had never before been used for planetary missions. The 

dual-spin configuration accommodated the needs of both particles-and-fields 

scientists, who preferred spinning spacecraft for their experiments, and planetologists, who wanted a stable base from which to perform photography 

and remote sensing studies.66

It is important to note that performing the above tasks did not ensure an 

official JOP mission start in FY 1978. For that, congressional approval was needed. A

marketing strategy that was put together to accomplish this involved getting approval 

first from NASA’s top management, then OMB, and finally Congress. Regarding the 

first step, Dan Herman said that obtaining NASA’s strong commitment to a JOP

mission was necessary because convincing OMB of the importance of JOP would 

involve communicating the “keen interest in the program by science advocates for the 

mission, the Space Science Board, and the aerospace industrial community.” Similarly, 

the pitch to Congress would have to stress the “vital interest in the mission by the 

scientific community.”67 In other words, it was critical to build a unified backing for 

JOP by the scientific community and aerospace industry, which had not been done a 

few years earlier for the Grand Tour mission. 

One other condition that had to be met for an FY 1978 start, Herman emphasized, 

was to recognize the importance of not competing with the Hubble Space Telescope project for either FY 1978 budget dollars or total project funding. Herman recognized that 

Hubble would be the NASA Space Science Office’s highest priority new start for FY 1978, 

with a current total budget estimate of about $500 million. Hubble also had considerable 

support in Congress. According to Dick Malow, staff director of the House HUD–

Independent Agencies Appropriations Subcommittee at the time, “Hubble had been in 

the development phase for some time . . . . There had been a considerable amount of 

auxiliary work that had been done on it . . . and there was a lot of attraction to Hubble 

too.” To put Congress in an “either-or” position regarding the two programs might end 

up being fatal for JOP. To avoid this situation, Herman estimated that the maximum JOP

mission cost should not exceed $200 million.68

The Office of Space Science was not successful in keeping estimated JOP costs 

down to the recommended amount. A project budget of $270 million was developed. It 

did receive approval from NASA top management and also passed the OMB hurdle. After 

these events, it was included, along with the Space Telescope, in President Ford’s federal 

budget submittal to Congress in January 1977.

Congress’s first actions on JOP were favorable. The project was approved by both 

the House and Senate authorizing committees. But then House HUD–Independent Agencies 

66 Ibid., p. 9.

67	 Ibid., pp. 9–10.

68	Waff, “Jupiter Orbiter Probe,” p. 10; Dick Malow, former staff director of the House HUD–Independent Agencies

Appropriations Subcommittee, interview, tape-recorded telephone conversation, 19 September 2001.
36	 MISSION TO JUPITER: A History of the Galileo Project

Bruce Murray, “We won that one handily, 280 to 131. It was an extraordinary victory and 

quite unexpected.” The last preproject barrier had been crossed, and JOP officially kicked 

off on the first day of the new fiscal year, 1 October 1977.75

Critical to JOP approval was a planetary science community united in its backing 

of the project. JOP had been very astutely designed to offer value to all of the planetary 

science community’s subdisciplines: planetary meteorology, satellite geology, particlesand-fields studies, and so on. But offering so much required a commitment to build one 

of the most complex and sophisticated spacecraft ever conceived.76

Dr. Bruce Murray expressed both the strong support for JOP and the challenge 

that now lay before NASA when he said, in an address to JPL management:

So our job now is to organize and get off to a good start with this 

mission. It is credible and warrants the kind of endorsement we have 

had—not only from NASA, but from the entire country—as manifested 

in the Congressional vote.

I . . . believe that JOP demonstrates that there really is a clear constituency for planetary exploration in this country. It is not a constituency 

that will frequently support very large, Viking-class missions . . . . 

We were fortunate to discover this in time to affect the outcome of 

JOP . . . . However, our happiness over the good news is tempered by 

the fact that we do have to deliver on difficult commitments.”77

75	Bruce Murray, Semi-Annual Report to the Laboratory, 17 October 1977, Bruce C. Murray Collection, 1975–1982, folder

32, box 2, JPL 216, JPL Archives.

76	Johnson interview, 31 July 2001; Waff, “Jupiter Orbiter Probe,” p. 11.

77	Murray, Semi-Annual Report to the Laboratory, 17 October 1977.
Whenthe Jupiter Orbiter Probe (JOP) Project 

received congressional approval to begin operations 

on 1 October 1977, NASA planned to build, test, and 

launch the spacecraft in just over four years. JOP was to be transported up to Earth orbit 

in the Space Shuttle, then propelled onto a direct Earth-Jupiter trajectory by means of a 

solid-fueled Inertial Upper Stage (IUS) launch system.2 Using the direct trajectory, the 

spacecraft was supposed to reach Jupiter by November 1984.3

Initial Project Planning

NASA designed the JOP mission to conduct a comprehensive exploration of Jupiter, along 

with its atmosphere, physical environment, and satellites, employing an orbiting spacecraft and an atmospheric entry probe. The mission was to build upon Pioneer 10 and 11 

and Voyager 1 and 2 results, but to focus more on long-duration, in situ investigations. 

NASA decided that the mission’s primary objectives were to determine the following:

1	 “The National Value of Galileo,” in John R. Casani to K. Kaesmeier and Dr. K. O. Pfeiffer, “GLL 8th Quarterly Report

Agenda: February 26, 1981,” John Casani Collection, Galileo Correspondence 2/81, folder 30, JPL 14, JPL Archives.

2	 W. J. O’Neil, “Project Galileo” (paper no. AIAA 90-3854, presented at the American Institute of Aeronautics and

Astronautics (AIAA) Space Programs and Technologies Conference, Huntsville, AL, September 1990), p. 10.

3	 NASA Program Approval Document, Research and Development, Code Number 84-840-829, 19 November 1977,

Galileo Documentation, folder 5138, NASA Historical Reference Collection, Washington, DC.

Chapter 3

The Struggle To 

Launch Galileo: Technical 

Difficulties and Political 

Opposition

Galileo will be the primary source of information for the chapter on Jupiter 

and its moons in the atlas of the solar system read by our grandchildren.1

—John R. Casani
38	 MISSION TO JUPITER: A History of the Galileo Project Chapter 3 I The Struggle To Launch Galileo 39

• The chemical composition and physical state of Jupiter’s atmosphere.

• The chemical composition and physical state of Jupiter’s Galilean satellites.

• The topology and behavior of the Jovian system’s magnetic field and energetic particle fluxes.4

Early in his tenure as Galileo’s first project manager, John R. Cassani solicited a 

more inspirational title for the endeavor than “Jupiter Orbiter Probe.” Many people associated with the mission submitted suggestions, and the name “Galileo” received more votes 

than any other title. Casani commented that naming the mission after the 17th-century 

Italian astronomer was especially appropriate, for Galileo “was the first person to view 

the planet Jupiter through a telescope, and in so doing discovered the four largest moons 

of Jupiter, now known as the Galilean satellites.” One of Casani’s staff, Dave Smith, also 

pointed out that in the Star Trek series, Galileo was the name of the Enterprise’s shuttle 

craft that Spock piloted for planetary excursions.5

On 1 July 1976, before the mission had received congressional approval, NASA 

issued an “Announcement of Opportunity for Outer-Planets Orbiter/Probe (Jupiter),” in 

which proposals were solicited from 1) principal investigators and co-investigators for 

experiments to be performed by either the Orbiter or Probe, 2) individual investigators 

seeking membership on a NASA-formed team that would use subsystems such as the 

radio or imaging subsystems, and 3) individuals wanting to participate in the mission as 

interdisciplinary scientists or theorists. 

Proposals were reviewed by an ad hoc advisory subcommittee of the Space Science 

Steering Committee, which was appointed by NASA’s Associate Administrator for Space 

Science. The proposals considered to have the greatest scientific merit were further 

reviewed by the JPL project office (for Orbiter experiments) and Ames Research Center (for 

Probe experiments) using engineering, integration, management, cost, and safety criteria. 

Additional reviews were carried out by NASA Headquarters, the Agency’s Office of Space 

Science Program Office, and the Space Science Steering Committee. Based on these reviews, 

NASA’s Associate Administrator for Space Science appointed the investigators and selected 

the mission experiments. The Associate Administrator picked six science investigations for 

the Probe atmospheric entry vehicle (see table 3.1), with 29 scientists participating, including five from Germany and one from France. The Associate Administrator also selected 

11 science investigations for the Orbiter (see table 3.2). These would employ 82 scientists 

that included nine from Germany, five from the Netherlands, and one each from France, 

Sweden, and Canada. In addition, the Associate Administrator identified 14 interdisciplinary 

scientists to conduct analyses that drew data from two or more investigations.6

4	 NASA Program Approval Document.

5	 J. R. Casani to distribution, “A Name for Project Galileo,” 6 February 1978, JPL Interoffice Memo GLL-JRC-78-53,

folder 5139, NASA Historical Reference Collection, Washington, DC.

6	 John R. Casani, “Testimony for: Space Science and Applications Subcommittee, Committee on Science and

Technology, United States House of Representatives,” 1 March 1978, JPL Archives; “Announcement of Opportunity

for Outer-Planets Orbiter/Probe (Jupiter),” 1 July 1976, A. O. No. OSS-3, folder 18522, NASA Historical Reference

Collection, Washington, DC.

Table 3.1. Planned Probe investigations of Jupiter’s atmosphere. 

Study Objectives Instrument Principal Investigator and Organization

Determine abundance ratio of helium

to hydrogen.

Helium interferometer Ulf von Zahn, University of

Bonn

Obtain temperature, pressure, and

density profiles; atmospheric mean

molecular weight; wind velocities and

wind shear; and turbulence intensity

and scale.

Atmospheric structure

instrument

Alvin Sieff, NASA Ames

Research Center

Determine chemical composition

and physical state; measure vertical

variations.

Mass spectrometer Hasso Niemann, NASA

Goddard Space Flight Center

Measure vertical distribution of solar

energy and planetary emissions,

locate cloud layers, and use infrared

to study cloud and aerosol opacities.

Net flux radiometer Robert Boese, NASA Ames

Research Center

Determine vertical extent, structure,

and particle sizes of clouds.

Nephelometer Boris Ragent, NASA Ames

Research Center

Verify existence of lightning and measure basic physical characteristics;

measure scale of cloud turbulence;

study electrification; identify evidence

of precipitation, sources of heat, and

acoustic shock waves; and measure

radio-frequency (RF) noise levels and

magnetic fields.

Lightning instrument Louis Lanzerotti, Bell Labs

Table 3.2. Planned Orbiter investigations of Jupiter and its satellites. (continued)

Study Objectives Instrument Principal Investigator(s) and Institution(s)

Obtain high-resolution images

of Jupiter, satellites, and targets

of opportunity.

Imaging system

(remote sensing

instrument)

Michael Belton, Kitt Peak National

Observatories, Arizona

Satellites: Map and identify

chemical species; relate chemical

to geological regions. Jupiter:

Map atmosphere, record temporal

changes in cloud morphology and

vertical structure.

Near infrared

mapping

spectrometer

(remote sensing

instrument)

Robert Carlson, NASA JPL
40	 MISSION TO JUPITER: A History of the Galileo Project Chapter 3 I The Struggle To Launch Galileo 41

Table 3.2. Planned Orbiter investigations of Jupiter and its satellites. (continued)

Study Objectives Instrument Principal Investigator(s) and Institution(s)

Determine physical and dynamic

properties of small dust particles,

including their mass, flight

direction, and charge.

Dust detector

(fields-and-particles

instrument)

Eberhard Grun, Max-Planck-Institute

fur Kernphysik, Heidelberg, Germany

Using Galileo’s radio telecommunications system, investigate

gravitational fields and internal

structures of Jupiter and satellites,

as well as structures of their

atmospheres. Also, search for

Very Low Frequency (VLF)

gravitational radiation.

Spacecraft radio

system (fields-andparticles instrument)

John Anderson, NASA JPL

The First Design Changes and Delays

Galileo engineers identified the need for changes to the spacecraft design almost immediately after the mission began. The first project review in October 1977, as well as 

subsequent reviews, revealed that the combined Orbiter-Probe spacecraft required modifications that would make it significantly heavier than originally planned. For instance, 

project engineers decided that it was necessary to build a vented rather than pressurized 

Probe atmospheric entry vehicle in order to enhance reliability and reduce costs, even 

though this would add 100 kilograms to the craft’s planned 1,500 kilograms. NASA staff 

also identified the need for structural improvements that would add 165 kilograms. These 

weight changes would have ramifications for both the Space Shuttle and the Inertial 

Upper Stage (IUS). It was the Shuttle’s job to lift the Galileo spacecraft and IUS to lowEarth orbit; after that, the IUS would propel Galileo onto a Jupiter trajectory. Galileo’s 

projected weight gain would require the IUS to take on additional fuel—but the Shuttle’s 

maximum payload weight had severe limits as well. As a result, the IUS would not be able 

to carry sufficient fuel to launch the now-heavier Galileo space vehicle on a direct ballistic 

trajectory to Jupiter. A more fuel-conservative trajectory to Jupiter had to be found. JPL 

quickly identified a trajectory that would get Galileo to Jupiter through the use of a Mars 

gravity-assist, but it would add five months to the journey.7

The added weight issues were among the first of many problems that had to be 

addressed before the Galileo spacecraft was ready to fly. Table 3.3 provides a chronology 

of the steps taken to develop a launch-ready space vehicle.

Since the Galileo spacecraft depended on the Space Shuttle to carry it up to lowEarth orbit, delays in Shuttle development were threatening to postpone Galileo’s launch. 

In August 1979, Thomas O’Toole, a reporter for the Washington Post, published an article 

Table 3.2. Planned Orbiter investigations of Jupiter and its satellites. (continued)

Study Objectives Instrument Principal Investigator(s) and Institution(s)

Satellites: Study surfaces.

Jupiter: Study cloud and haze

properties, radiation budget, and

atmospheric dynamics.

Photopolarimeter

radiometer (remote

sensing instrument)

Andrew Lacis, NASA Goddard

Institute for Space Studies

Satellites: Study high neutral

atmospheres, determine loss

rates, and determine geometry of

extended atmospheres. Jupiter:

Study high neutral atmosphere,

mixing ratios of ammonia and

ultraviolet (UV)-active trace constituents, and auroral emissions.

Ultraviolet

spectrometer

(remote sensing

instrument)

Charles Hord, University of Colorado

Characterize magnetic fields

of Jupiter and satellites, map

magnetosphere and analyze its

dynamics, investigate magnetosphere-ionosphere coupling, and

measure magnetic fluctuations.

Magnetometer

(fields-and-particles

instrument)

Margaret Kivelson, UCLA

Identify sources of Jovian

plasmas and investigate their

interactions with satellites and

their role as sources for energetic

charged particles, characterize

equatorial current sheet, and

evaluate impact of rotational

forces and field-aligned currents

on Jovian magnetosphere.

Plasma subsystem

(fields-and-particles

instrument)

Louis Frank, University of Iowa

Collect data on electron density

and temperature, measure

electron saturation current

collected by spacecraft, and

study conduction current of

electromagnetic and electrostatic

waves.

Electron emitter

instrument (fieldsand-particles

instrument)

Rejean Grard, European Space

Research and Technology Centre

(ESTEC)

Study distribution and stability

of trapped radiation and its

interaction with satellites, solar

wind, and particles.

Energetic particles

detector (fieldsand-particles

instrument)

Donald Williams, National Oceanic

and Atmospheric Administration

(NOAA)

Study electromagnetic

wave phenomena generated

by magnetosphere, atmosphere,

and satellites.

Plasma wave subsystem (fields-and-	

particles instrument)

Donald Gurnett, University of Iowa

7	 U.S. General Accounting Office (GAO), “Fact Sheet for the Chairman, Subcommittee on Science, Technology, and

Space, Committee on Commerce, Science and Transportation, U.S. Senate,” in Space Exploration: Cost, Schedule, 

and Performance of NASA’s Galileo Mission to Jupiter, GAO/NSIAD-88-138FS (GAO, May 1988), p. 23; Craig Waff,

“Jovian Odyssey: A History of Project Galileo,” unpublished outline, 9 December 1987 revision, folder 18522, NASA

Historical Reference Collection, Washington, DC.
42	 MISSION TO JUPITER: A History of the Galileo Project Chapter 3 I The Struggle To Launch Galileo 43

expressing serious concern over whether a Shuttle engine that could lift the combined 

weight of Galileo and the IUS into Earth orbit would be ready in time. O’Toole noted that 

the Shuttle engine-testing program was way behind schedule, and development had not 

even begun on the special advanced Shuttle engine needed for Galileo and its IUS. The 

advanced engine was termed the “109-percent” engine because during takeoff, it would 

burn at a higher temperature than normal, delivering 109 percent of its rated thrust. To 

protect the engine against the higher temperatures, a more elaborate cooling system 

needed to be developed.8

The Galileo mission was shooting for a 1982 launch window in which Earth, 

Mars, and Jupiter were in a favorable orientation that allowed the spacecraft to fly by 

Mars and use the planet for a gravity-assist to Jupiter. If the 109-percent Shuttle engine 

was not ready, launch might have to be delayed until 1983 or even later. This possibility 

introduced a serious problem: if the launch date was delayed and Galileo couldn’t get a 

Mars gravity-assist or had to use additional fuel to reach Mars, then there would be insufficient fuel for the spacecraft to carry out all of its planned activities. Mission objectives 

would have to be abridged. One abbreviated mission scenario called for Galileo to reduce 

the number of its orbits of Jupiter and its moons from 11 to only 5. Another approach, 

which would allow a full Jupiter mission, was to substitute a more powerful engine for the 

solid-fueled IUS and possibly not use the Shuttle at all. For instance, NASA might employ 

an expendable, liquid-fueled Titan lower stage launch vehicle and a liquid-fueled Centaur 

upper stage. Such a plan, however, would add at least $125 million to the mission cost 

because it would require a rebuilt launchpad at Kennedy Space Center.9

A simpler option that did not require rebuilding a launchpad was to use the 

liquid-fueled Centaur as the upper stage but continue to use the Shuttle as the launch 

vehicle. Centaur had proven itself to be a reliable “mainstay upper-stage engine for most 

of NASA’s large spacecraft and all of its planetary missions,”10 although it had always been 

launched as part of a ground-based, expendable propulsion system that included initial 

stages such as the Titan. Centaur used liquid-hydrogen fuel, which, pound for pound, 

delivered significantly more thrust than the IUS’s solid fuel. Centaur’s weight had been 

kept down by using thin-walled stainless-steel fuel tanks with a “balloon” design, which 

relied on internal pressurization rather than heavy reinforcing elements to give them their 

strength.11 The fueled Centaur upper stage was light enough for it and Galileo to be borne 

aloft by the Shuttle without need of its 109-percent engine. Use of the Centaur upper stage 

would increase the chances that the Shuttle could be used for a 1982 launch.12

NASA Associate Administrator Jesse Moore believed that the Centaur would 

become a “very integral, longtime part of the Space Shuttle program.” A Shuttle/Centaur 

would combine the newest human space exploration program with “the world’s most 

powerful upper stage rocket.” Shuttle/Centaur appeared to be an effective way to send 

payloads into interplanetary trajectories.13

There were downsides to the use of Centaur as the upper stage. It would have 

to be modified for transport in the Shuttle, and this modification would add to the cost of 

the mission. In addition, NASA engineers had concerns about the safety of such a venture. 

Centaur’s liquid-hydrogen fuel presented an explosion danger to the Shuttle and the astronauts flying it.14 The difficulty of safely handling such a low-density liquid, which boils 

at -253°C (-423°F) and can leak through minute cracks, made Galileo mission designers 

wary of mounting Centaur in the Shuttle cargo bay.15 Safety features could be added, such 

as a system to vent hydrogen fuel into space in case of emergency. But NASA engineers 

estimated that reliable safety systems would cost $100 million or more. In addition, such 

systems would take considerable time to develop and test. NASA’s Director of Planetary 

Programs, Angelo Guastaferro, projected that five years might be necessary to implement 

a venting system.16

Table 3.3. Chronology of Galileo development: 1978 to 1986. 

Date Event

1978 JOP project is renamed Galileo.

January 1978 Project Science Group meetings commence. Group chooses Galileo

science investigations.

1978 Need for heavier spacecraft is identified. NASA develops Mars gravity-assist

trajectory to conserve fuel.

August 1979 Concerns are raised over 109-percent Shuttle engine readiness by Galileo

launch date. Substitution of liquid-fueled Centaur upper stage for solidfueled IUS is considered.

November 1979 NASA Administrator Robert A. Frosch opposes Centaur option for solving

Galileo’s weight problem. Congressman Boland pressures NASA to initiate

Centaur development.

January 1980 NASA announces plans for separate Orbiter and Probe missions to solve

weight problem and sets 1984 launch dates.

1980 Boeing cost overrun for developing IUS exceeds $100 million. NASA is

forced to reconsider Centaur.

Late 1980 NASA cancels IUS development and initiates “wide-body” Centaur planning.

Orbiter and Probe are recombined into one mission. Launch is postponed

until April 1985.

8	 Thomas O’Toole, “More Hurdles Rise in Galileo Project To Probe Jupiter,” Washington Post (15 August 1979): A3.

9	 Ibid.

10	Thomas O’Toole, “Problems Stall Plans To Launch from Shuttle,” Washington Post (15 December 1980): A12.

11	 G. R. Richards and Joel W. Powell, “The Centaur Vehicle,” Journal of the British Interplanetary Society 42 (1989): 99.

12	O’Toole, “More Hurdles.”

13	Virginia P. Dawson and Mark D. Bowles, Taming Liquid Hydrogen: The Centaur Upper Stage Rocket, 1958–2002

(Washington, DC: NASA SP-4230, 2004), p. 226.

14	O’Toole, “More Hurdles.”

15	Richards and Powell, “The Centaur Vehicle,” p. 99; U.S. Department of Energy, “Hydrogen Fuel,” Energy Efficiency and 

Renewable Energy Network Web page, http://www.eren.doe.gov/consumerinfo/refbriefs/a109.html, February 2000.

16	Thomas O’Toole, “NASA Weighs Deferring 1982 Mission to Jupiter,” Washington Post (4 September 1979): A5.
44	 MISSION TO JUPITER: A History of the Galileo Project Chapter 3 I The Struggle To Launch Galileo 45

Table 3.3. Chronology of Galileo development: 1978 to 1986. 

Date Event

November 1980 President-elect Reagan’s OMB director, David Stockman, indicates that

NASA may be targeted for severe budget cuts.

February 1981 JPL learns of OMB “hit list” which includes Galileo cancellation. Casani

drafts and circulates “Galileo Urgent to America” statement and begins

campaign to keep Galileo alive.

February 1981 President Pro Tempore of Senate Strom Thurmond backs Galileo for its

important military benefits.

November 1981 OMB proposes budget cuts which, if approved by Congress, could force

NASA to eliminate most planetary exploration activities and shut down JPL.

December 1981 George Keyworth, head of White House Office of Science and Technology

Policy, recommends halt to all new planetary missions for at least a decade.

Space science community conducts campaign against the cuts.

December 1981 Department of Defense (DOD) and Aerospace Industries Association oppose

proposed OMB cuts to NASA, fearing effect on U.S. space and aviation

leadership and on American industry. President Reagan commits to support

Space Shuttle development, which aids NASA’s planetary exploration plans.

OMB agrees to reinstate Galileo in FY 1983, but without Centaur.

January 1982 NASA makes plans to use Boeing’s solid-fueled IUS, plus an additional

“Injection Module” for increased propulsion. Launch is still planned for 1985,

but reduced propulsion compared to that of the Centaur option will delay

Jupiter arrival from 1987 to 1990. The ∆V-EGA spacecraft trajectory, which

will use an Earth gravity-assist, will be implemented.

March–July 1982 Congress favors a return to Centaur upper stage propulsion system.

Bill is passed in July blocking funds for any other type of upper stage

development. Launch is eventually delayed until 1986, with Jupiter arrival

in 1988.

July 1982 Probe is subjected to drop test simulating Jupiter atmospheric entry.

Probe’s parachute deploys, but later than planned.

March 1983 Electrical and mechanical integration of Galileo’s components and scientific

instruments begins at JPL’s Spacecraft Assembly Facility.

April–July 1983 Probe’s redesigned parachute system passes quarter-scale, half-scale, and

full-scale tests.

September 1983 Integration of Probe and Orbiter begins at JPL.

September 1984 Full-scale structural model of Centaur tested at San Diego’s General

Dynamics Convair plant. System-level vibration and acoustic testing of

Galileo spacecraft are completed.

Table 3.3. Chronology of Galileo development: 1978 to 1986. 

Date Event

December 1984 John Casani proposes a close flyby option of the Amphitrite asteroid. NASA

Administrator James Beggs endorses this option. Visiting Amphitrite could

provide data on nature of the primordial nebula from which the Sun formed,

but it would delay Galileo’s Jupiter arrival from August to December 1988.

Decision on exercising the flyby option would be made postlaunch.

April 1985 Space Shuttle Atlantis, scheduled to carry Galileo into orbit, is “rolled out”

of Rockwell International’s Palmdale, California, facility. Rollout ceremony

denotes completion of the craft and readiness for flight.

August 1985 Centaur G-prime upper stage rolled out of General Dynamics Convair facility

in San Diego.

October 1985 Environmental and electronic compatibility testing are completed. Problems

arise with computer memory devices and with spin bearing assembly (SBA)

connecting spun and despun section of Galileo.

November 1985 Galileo development and testing phase is completed; launch and flight

operations phase begins.

December 1985 Galileo spacecraft is transported to Kennedy Space Center in Florida.

January 1986 First “tanking up” of the spacecraft with its hypergolic propellant takes

place.

28 January 1986 Launch and loss of the Space Shuttle Challenger.

NASA Administrator Robert A. Frosch did not favor the Centaur option for solving Galileo’s weight problem. In November 1979, he stated that integration of Centaur with 

the Shuttle would be unwise at that time. Instead, NASA was considering a mission strategy 

that would allow the IUS to be kept as the upper stage propulsion system. The mission 

could be split into two trips to Jupiter—one for the Orbiter and another for the Probe, 

thereby solving the weight problem but adding significant cost for the additional spacecraft 

that would have to be built. 

Representative Edward P. Boland (D-Massachusetts), whose opposition to Galileo 

in 1977 had nearly prevented approval of the mission, was strongly against Frosch’s option. 

Boland chaired the House HUD–Independent Agencies Appropriations Subcommittee of the 

House Appropriations Committee, and it was Boland’s subcommittee that handled NASA’s 

budget. Rather than allowing NASA to decide its own strategy, the Congressman influenced 

the House Appropriations Committee to order NASA to integrate the Centaur upper stage 

into the Galileo mission, should the spacecraft’s weight problems cause a slip in the launch 

schedule, and to fly a combined Orbiter-Probe mission rather than two separate missions.17

Boland made it clear that the House Appropriations Committee, in pushing 

Centaur development, was not advocating IUS termination altogether. The IUS could still 

Table 3.3. Chronology of Galileo development: 1978 to 1986. (continued) Table 3.3. Chronology of Galileo development: 1978 to 1986. (continued)

17	 “House Panel Orders Shuttle-Centaur Integration If Galileo Slips,” Aerospace Daily (30 November 1979): 142.
46	 MISSION TO JUPITER: A History of the Galileo Project Chapter 3 I The Struggle To Launch Galileo 47

be used, for instance, for putting military payloads into Earth orbit. But Boland considered 

the IUS underpowered for planetary exploration and favored instead the Centaur, with 

its impressive past performance. Besides “being a known quantity,” he said, “Centaur 

has 50% more payload capability for planetary missions than the IUS.” Boland was very 

concerned about making the best choice of upper stage from a cost-benefit point of view, 

and he opposed retaining the IUS for a delayed Galileo launch because it would be “chasing good money after bad.” Bringing IUS performance up to specification would involve 

costly improvements and necessitate “expensive and risky Shuttle weight reduction 

efforts.” Because Centaur had greater payload capacity, Boland saw its use as insurance 

against the typical increases in spacecraft payload weight that often occurred as mission 

development proceeded. In short, he saw a Centaur-driven Galileo spacecraft as much 

more likely to become a “long-term national resource.”18

Boland’s position on the appropriate upper stage for Galileo was influenced by 

information collected by Dick Malow, the staff director for the Congressman’s subcommittee. Malow made it his business to spend “a lot of time going around and talking to people” 

in the space science community about issues before Congress, and he regularly discussed 

his findings with Boland. Regarding the Galileo upper stage matter, Malow said, “It came to 

my attention that if we used the Shuttle/Centaur combination . . . you could launch to any 

of the outer planets, from launch date to arrival in two years, and that had a huge attraction 

. . . it gave you a lot of backup capability if you missed the launch opportunity. There’s 

always a launch opportunity practically sitting there for you . . . that seemed to me to be 

a very logical way to go.” Malow also mentioned that Boland was beginning to see the 

powerful Shuttle/Centaur combination as a potential asset for defense applications, and this 

influenced his backing of Centaur for use in the Galileo mission.19 

When NASA received the committee’s instructions to switch to Centaur in the 

event of another launch delay, the Agency questioned whether the orders had any legal 

standing. NASA could defy the orders by submitting an FY 1981 budget request that 

followed the original plan to use the IUS. Doing so could open up the issue for consideration by both houses of Congress. At a Senate hearing on Galileo several days later, 

Administrator Frosch said only that the Agency was considering its response to the Boland 

subcommittee’s “order.”20 The order raised questions about NASA’s autonomy in planning 

the details of its own missions and about whether Congress could or should micromanage 

the technical aspects of the space agency’s projects. 

At the end of 1979, NASA announced that delays in the Shuttle’s development 

would indeed necessitate a delay in the Galileo launch. In JPL Director Murray’s words, 

“The people who are in charge of the Shuttle have declared that they cannot confidently 

support the January ’82 launch date of Galileo. Thus, we must reconfigure the mission 

for a later launch date, in one way or another, and achieve the primary objectives. The 

process of working out how that will be done is still underway.”21

NASA released details of its new mission plan in January 1980. The plan did not 

follow Congressman Boland’s guidelines to use the Centaur upper stage alternative in 

order to work around the delayed development of the 109-percent Shuttle engine. NASA’s 

plan also did not call for a Titan-Centaur launch vehicle that would not require the use of 

the Shuttle. Instead, the Agency decided to split the Galileo mission into two parts—one 

for the Jupiter Orbiter and one for the Atmospheric Probe—and set the target launch dates 

for early 1984.22 

The planetary configuration during the originally planned 1982 mission would 

have permitted a very advantageous Mars gravity-assist. This would have reduced fuel 

requirements and allowed a single spacecraft, carrying both Probe and Orbiter and propelled 

by a solid-fueled IUS, to attain all of the project’s objectives. Although a Mars gravityassist was also possible on a mission launching in 1984, it would not have saved the 

spacecraft as much fuel, and this is why NASA planned to send the Probe and Orbiter 

into space as separate payloads on different Shuttle flights. NASA perceived this approach 

as safer than carrying the hydrogen-fueled Centaur aloft in the Shuttle’s cargo bay. A split 

mission would, however, introduce serious budgetary and technical implications. In a 

Probe-Orbiter combined mission, the Probe would ride “piggyback” on the Orbiter. But 

a split mission would require that a separate rocket and carrier structure be built for the 

Probe, at a cost of over $50 million.23

NASA mission planners scheduled the Orbiter launch for February 1984 and the 

Probe launch for March 1984. A 1983 launch was possible, but it would have been an 

expensive alternative, requiring a sole-source contract for part of the job. Delaying the 

launch until 1984 was perceived by NASA as more cost-efficient because it allowed time 

for a competitive bidding process for construction of the Probe rocket and carrier. 

Galileo’s Atmospheric Probe was light enough for the Shuttle plus the solidfueled IUS to launch it on a direct ballistic trajectory to Jupiter. Getting the much heavier 

Orbiter space vehicle to Jupiter, however, was more difficult. Although the Orbiter part 

of the split mission would use a Mars gravity-assist, the solid-fueled IUS still wouldn’t 

have enough power to get the spacecraft to Jupiter. As a result, Galileo would need an 

auxiliary upper stage in addition to the IUS to provide sufficient thrust.24

New problems arose with the mission. Boeing Company, the contractor 

charged with developing IUS versions for both the Air Force and NASA, experienced 

severe cost overruns and projected that completing the IUS would cost over $100 million more than previous estimates. Administrator Frosch considered this unacceptable. 

Boeing proposed scaling down the IUS in order to reduce the overrun, but Frosch 

worried about the consequences of such an action. He decided that the possibility of

“readying the IUS to send an orbiter and probe to Jupiter for its scheduled launch were 

remote.”25 The best course, Frosh concluded, was to support the propulsion system that 

NASA had rejected the year before and that Senator Boland had favored, the Centaur engine.26

18	 Ibid., p. 142.

19	Dick Malow, former staff director of the House HUD–Independent Agencies Appropriations Subcommittee, interview,

tape-recorded telephone conversation, 19 September 2001.

20	“House Panel,” p. 143.

21	O’Neil, “Project Galileo,” p. 10; Bruce Murray, “Mid-Year Review,” 5 October 1979, Bruce C. Murray Collection, 1975–

1982, folder 43, box 2, JPL 216, JPL Archives.

22	O’Neil, “Project Galileo,” p. 10.

23	Bruce Murray, “Third Annual ‘State of the Lab’ Report,” 26 March 1980, Bruce C. Murray Collection, 1975–1982, folder

44, box 2, JPL 216, JPL Archives; O’Toole, “NASA Weighs Deferring 1982 Mission.”

24	O’Toole, “NASA Weighs Deferring 1982 Mission”; GAO, p. 23.

25	Dawson and Bowles, Taming Liquid Hydrogen, p. 240.

26	O’Toole, “Problems,” p. A12.
48	 MISSION TO JUPITER: A History of the Galileo Project Chapter 3 I The Struggle To Launch Galileo 49

“No other alternative upper stage,” he stated, “is available on a reasonable schedule or 

with comparable costs.”27

In spite of its dangerous liquid-hydrogen fuel, Centaur had an impressive track 

record of reliability, with 53 operational flights and only two failures during 19 years of 

service.28 NASA planners had rejected Centaur the previous year because they had thought 

that adapting it for safe use in the Shuttle would be too expensive. Also, such a project 

would divert many engineers whose main mission had been to complete the Shuttle. But 

the IUS cost overruns altered this picture. Frosch reexamined the Centaur and concluded 

that it could be implemented more simply and cheaper than he had thought; plus, it might 

actually be the best alternative,29 for it would offer “both to commercial customers and to 

national security interests a highly capable launch vehicle with growth potential.”30

Many space scientists were concerned about the continued delays in Galileo’s 

launch and did not welcome yet another postponement resulting from a new design 

change. They worried that the White House, OMB, and Congress would refrain from supporting any Shuttle-launched solar system exploration after Galileo, such as missions to 

Venus, Saturn, and Halley’s Comet.31 Nevertheless, in late 1980, NASA decided to cancel 

development of the IUS and instead build a “wide-body” Centaur upper stage, modified to 

be carried aloft inside the Shuttle rather than on top of an expendable launch vehicle. The 

launch was postponed from 1984 until April 1985 in order to give NASA time to develop 

the wide-body Centaur, as well as an Orbiter-Centaur interface.32

By changing from the solid-fueled, limited-thrust IUS to a liquid-fueled, 

higher energy Centaur, JPL was able to recombine Orbiter and Probe into a single 

payload that could be launched in a direct, rather than Mars gravity-assisted, trajectory 

to Jupiter that would enable the mission to be carried out in less time. The Centaur 

had other technical advantages as well. It would deliver a gentler thrust than the IUS. 

Solid-fueled rockets such as the IUS typically had a “harsh initial thrust,” which could 

possibly damage delicate payloads. Liquid-fueled rockets such as Centaur developed 

thrust more slowly. In addition, liquid-fueled rockets were more controllable, in that 

they could be turned on and off as needed. Not so with a solid-fueled rocket: once 

ignited, it would burn until its fuel was used up.33

Planetary Exploration and Reagan’s Austerity Program

During the early 1980s, Galileo project management struggled not only with technical 

problems associated with the spacecraft, but also with the Reagan administration’s less 

than enthusiastic support for planetary exploration. Though Reagan voiced interest in 

revitalizing the space program, it never became a key national policy as had been the case 

in the 1960s. The new administration proposed dramatically reduced government spending 

as a means of combating the country’s growing economic problems that had begun in the 

1970s, and this policy greatly affected the scope of NASA’s activities.34 In November 1980, 

even before Ronald Reagan was inaugurated, his designated OMB Director, Dave Stockman, 

indicated that NASA might be targeted for severe budget cuts. In February 1981, JPL learned 

of a Stockman “hit list” for NASA that included possible cancellation of Galileo. OMB was 

seeking drastic funding reductions from the levels set in the Carter administration’s FY 

1982 budget, which OMB called “incompatible with a program of across-the-board (federal 

government) restraint.” OMB proposed an immediate $96-million cut for NASA during the 

remainder of FY 1981 and a massive $629-million reduction in the FY 1982 budget.35

Galileo Project Manager John Casani and his team responded immediately upon 

learning about the OMB “hit list,” drafting a “Galileo Urgent to America” statement containing seven strong reasons why the project had to continue:

• The science that Galileo will perform will be exceptional and has the 

strong support of the National Academy of Sciences and the general scientific community.

• Congress has given Galileo its strong support in each of its reviews for the 

past three years. 

• Public interest and support are unusually strong. 

• Galileo is the only U.S. planetary exploration project currently under 

development. 

• The project is multinational, involving commitments to the West German 

government.

• The $230 million already committed to Galileo would be lost if the project 

were canceled. 

• The U.S. industrial community has committed significant resources to its 

Galileo contracts. Cancellation of those contracts would cost the government $35 million.

The next day, a more formal JPL statement expanded on these points, underlining the outstanding success of the U.S. planetary exploration program since the “beginning 

of the space age” and the national pride it has engendered, as well as the innovative 

27	Dawson technologies it has fostered. JPL argued that cancellation of Galileo would effectively and Bowles, Taming Liquid Hydrogen, p. 240.

28	Craig B. Waff, “Jovian Odyssey: A History of NASA’s Project Galileo,” chap. 11, photocopy of unpublished draft, JPL,

17 March 1989, pp. 1, 43–44, folder 18522, NASA Historical Reference Collection, Washington, DC.

29	O’Toole, “Problems.”

30	Dawson and Bowles, Taming Liquid Hydrogen, p. 176.

31	O’Toole, “Problems.”

32	GAO, pp. 23–24; O’Neil, “Project Galileo,” p. 10.

33	Dawson and Bowles, Taming Liquid Hydrogen, p. 176.

34	Lyn Ragsdale, “Politics Not Science: The U.S. Space Program in the Reagan and Bush Years,” in Spaceflight and the 

Myth of Presidential Leadership, ed. Roger D. Launius and Howard E. McCurdy (Urbana and Chicago, IL: University

of Illinois Press, 1997), pp. 135–136.

35	Waff, “Jovian,” pp. 1, 9–10.
50	 MISSION TO JUPITER: A History of the Galileo Project Chapter 3 I The Struggle To Launch Galileo 51

terminate the U.S. planetary exploration program at a time when the Soviet Union, Europe, 

and Japan were all “vigorously pursuing” their planetary programs.36

JPL also issued statements demonstrating that Galileo was so far along in development that terminating it would cause damage far overshadowing the money saved. 

Al Wolfe, Galileo’s Deputy Project Manager, stressed that the mission was on schedule, 

within budget, and in the final stages of engineering after three years of mission and operations planning and spacecraft system design. Major problems had been resolved, Wolfe 

reported, including the development of reliable radiation-hardened microprocessors and 

peripherals. Ninety percent of the long-lead-time electronic components had been delivered to JPL. Issues with sensitivity of the imaging system and data transfer between the 

spinning and nonspinning sections of the Orbiter had been worked through, leaving no 

current Orbiter problems threatening the launch schedule. Probe development was also 

on schedule, Wolfe reported. The Probe had passed its preliminary design review, as well 

as an important parachute test. These statements were all incorporated into a document 

presenting arguments against the termination of Galileo.37

JPL took another tack as well in its campaign to save Galileo, and it may have 

been the critical one. JPL staff focused on the one government department that had 

received large budget increases rather than cuts—the Department of Defense (DOD)—

and portrayed Galileo as vital to military goals. It was the opinion of JPL Director Bruce 

Murray that the “silver bullet” that saved Galileo was a letter sent to David Stockman on 

6 February 1981 by Strom Thurmond (R-South Carolina), the new President Pro Tempore 

of the Senate and a member of the Senate Armed Services Committee. In this letter, 

which was actually drafted at JPL, Thurmond argued that the military applications that 

could result from Galileo made it unwise to cancel the program. For instance, the Air 

Force needed satellites with a “survivable autonomous capability” to remain operational 

during time of war. This was especially so because ground control stations were not 

“hardened” to survive nuclear attacks. Defense satellites had not achieved the autonomous capability of which Thurmond wrote, but he hoped that it would be derived from 

the technology “that will be developed and demonstrated as part of the Galileo project.” 

In fact, he noted, JPL had already undertaken the task of developing and applying such 

autonomous technology for the Air Force Space Division, in parallel with JPL’s development of the Galileo spacecraft.38

The Air Force had turned to JPL for such help because of the Laboratory’s 

expertise in developing highly autonomous spacecraft, according to David Evans, a 

manager in JPL’s work for the Air Force. JPL’s spacecraft needed to be autonomous 

because of the communication distances and round-trip communication times involved 

during planetary exploration. The craft needed to take care of themselves when they 

were not in close touch with ground stations. JPL was designing several autonomous 

features into the Galileo spacecraft that were of potential interest to the Air Force. The 

craft would be able to determine its attitude from any orientation, using only on-board 

systems. Typical spacecraft were only able to do this within a narrow range of angles; 

otherwise, they had to rely on ground-based data processing. Other subsystems and 

instruments aboard Galileo, such as its dust and plasma analysis instruments, also operated far more independently than in typical spacecraft. In addition, JPL was designing 

Galileo to withstand the intense radiation surrounding Jupiter, and such radiation hardening was also of high interest to the defense establishment.39

Strom Thurmond made additional statements during February 1981 that indicated 

his strong support of NASA’s space program, as well as of missions such as Galileo. He 

recognized that the U.S. space program was at a “critical turning point” and that our country 

was retreating from space during a time when “Russian and Soviet bloc cosmonauts come 

and go like weekend tourists . . . [and] the Russians, West Europeans and Japanese will 

visit Halley’s Comet while we sit home and watch.” As a result of this situation, he believed 

that the United States needed to “keep its flight manifest full with the military and scientific 

payloads that will help to make America first in the world again and keep America first.”40

The Attacks Continue

The Galileo program stayed alive, but the Reagan administration’s onslaught against 

NASA’s planetary and other space science programs went on. Projected cost growth 

in FY 1983 for the Space Shuttle was $300 million to $500 million, and this deeply 

troubled the administration. NASA managers and scientists grew very concerned that 

the price tag for adapting Centaur to Shuttle-launched solar system missions, which 

could run as high as $500 million, might convince the White House to oppose its use 

for planetary exploration. Hans M. Mark, NASA’s new Deputy Administrator in 1981, 

considered Centaur “very precarious politically,” especially since Galileo was the only 

planetary mission for which it was definitely scheduled to be used.41

Mark tried to convince other Shuttle customers to orient their designs toward 

using Centaur, in order to “spread support base” for the adaptation of the upper stage. 

At stake was the Galileo mission itself. Delay or abandonment of Centaur would in turn 

delay and possibly kill Galileo. To prepare for this eventuality, NASA began developing fallback mission options that did not rely on Centaur. But such scenarios could not 

ensure the continuance of NASA’s planetary program. Hans Mark saw a bleak future for 

Galileo and other solar system exploration missions under the Reagan administration, 

whose main priorities were revitalizing the country’s economy and making sure that 

the U.S. remained the foremost military power in the world. The Reagan administration would not clearly commit to completing the Shuttle, which was vital for launching 

planetary missions, and this deeply worried Mark.42

36	John Casani memo, “Galileo Urgent to America (paraphrased excerpts),” undated (circa 1981), Galileo—Meltzer Sources,

folder 18522, NASA Historical Reference Collection, Washington, DC; T. Johnson, “Cancellation of Galileo—Reclame

Statement,” 5 February 1981, in a set of John Casani Collection papers that begin with “GLL 8th Quarterly Report

Agenda,” Galileo Correspondence 2/81, folder 30, box JA370, JPL 14, JPL Archives; Waff, “Jovian,” pp. 11–13.

37	A. Wolfe, “Galileo Status Statement,” draft, 5 February 1981, in a set of John Casani Collection papers that begin with

“GLL 8th Quarterly Report Agenda,” Galileo Correspondence 2/81, folder 30, box JA370, JPL 14, JPL Archives; Waff,

“Jovian,” pp. 13–15.

38	Waff, “Jovian,” pp. 16–18.

39	Waff, “Jovian,” pp. 17–18; J. R. Casani to J. N. James, “Galileo and the ASP Connection,” Interoffice Memo GLLJRC-81-697, 10 July 1981, and two attachments to this memo: “A.F. Merchandising Plan for Galileo” and “Galileo

Improvements Over Voyager in Autonomous Technology,” Galileo Correspondence 6/81–7/81, folder 27, box 3 of 6,

JPL 14, John Casani Collection, JPL Archives.

40	All quotations and information in this paragraph come from the NASA Kennedy Space Center History Program,

Chronology of KSC and KSC Related Events for 1981, KHR-6, 1 September 1983, p. 65, http://www-lib.ksc.nasa.

gov/lib/archives/chronologies/1981CHRONO1.PDF (accessed 11 September 2005).

41	Craig Covault, “Shuttle Costs Threatening Science Programs,” Aviation Week & Space Technology (6 July 1981): 16.

42	 Ibid., p. 16.
52	 MISSION TO JUPITER: A History of the Galileo Project Chapter 3 I The Struggle To Launch Galileo 53

Over the latter months of 1981, NASA forged a new planetary exploration policy 

in an attempt to salvage a “viable but more limited” ability to study the solar system within 

an ever grimmer budget environment. Under the policy envisioned by the key U.S. scientists and mission planners on NASA’s Solar System Exploration Committee, exploration 

objectives would remain the same but would be spread over a larger number of more 

limited and less expensive missions. The exploration objectives would take possibly 

decades longer to be accomplished, and the total runout costs would go up because 

more spacecraft would have to be built. But peak-year funding would be reduced to 

within budget limitations.43

While envisioning more constrained future missions, all of the Committee members remained very concerned about preserving Galileo objectives in the face of Reagan 

budget cuts.44 After OMB announced its proposed cuts to NASA’s budget of $1 billion in 

both FY 1983 and FY 1984, however, it was not at all clear whether the Committee’s vision 

for planetary exploration could be attained. In October 1981, NASA Administrator James 

M. Beggs scheduled a meeting with Counselor to the President Edwin Meese to argue 

that the cuts proposed by OMB would kill broad areas of U.S. aerospace capability. NASA 

would have to consider closing JPL and terminating not only Galileo and Centaur, but also 

all other U.S. planetary spaceflight.45

In spite of Beggs’s meeting with Meese, OMB delivered proposed budget strictures to NASA on 24 November 1981 that would, if approved by Congress, force the 

Agency to virtually cease its planetary exploration activities as of FY 1983. Cancellation 

of Galileo would result. The Venus Orbiter Imaging Radar (VOIR) project (which became 

Magellan),46 had been penciled into NASA’s budget projections as a new start for 1984, 

but it would also have to be canceled. The only mission that would not be affected by the 

budget cuts was Voyager 2, already on its way to Uranus and Neptune. NASA had little 

time to appeal the OMB decision because its FY 1983 budget had to be ready for submittal 

to Congress in January 1982. OMB also leveled its sights on NASA’s aeronautics program, 

recommending that its FY 1983 budget be cut by 50 percent.47

In December 1981, George Keyworth, head of the White House Office of Science 

and Technology Policy, echoed OMB’s position by recommending a halt to “all new planetary space missions for at least the next decade.” He believed that the White House would 

support this position. Keyworth had worked closely with OMB in formulating a pared-down 

NASA budget and favored a shift away from planetary exploration and toward Shuttlelaunched experiments such as a space telescope. Although he thought that a great deal had 

been learned during NASA’s 12 years of solar system exploration, he also thought that new 

missions would be nothing more than “higher resolution experiments.”48

Keyworth’s statement enraged many members of the space science community. 

The chairman of National Academy of Sciences’ Subcommittee on Lunar and Planetary 

Exploration, Eugene Levy of the University of Arizona, said that such a position “does 

not stand up to rational scrutiny” and insisted that “there are fundamentally important 

objects, the comets and asteroids, that we haven’t even approached yet. They hold primitive, undisturbed material. Not only would [their exploration] enhance our understanding 

of the origin of the solar system, but of stars in general.” Levy’s point was that sending 

spacecraft to explore the solar system was very complementary to developing the space 

telescope experiments that Keyworth envisioned. It was “intellectually naïve” to try to 

separate one kind of research from the other.49

Galileo Project Manager John Casani expressed the “keen feeling of disappointment” that he would have “should Keyworth’s views indeed be adopted by the White House.” 

Casani went on to say, “It is difficult to accept that this country would abdicate by Presidential 

policy, leadership in a field of exploration where our accomplishments have been a source of 

pride and inspiration to people all over the world.”50 Casani responded to Keyworth’s attack 

on planetary exploration by organizing a campaign of Galileo supporters, urging them to register their views on solar system exploration directly with the President or with Keyworth.51

James Van Allen also joined the fray. He was an influential University of Iowa 

physicist famous for his discovery of Earth’s “Van Allen radiation belts,” and he was also 

a Galileo interdisciplinary investigator. Van Allen initiated a letter-writing campaign to 

Keyworth to save Galileo. In a speech to the National Academy of Sciences’ Space 

Science Board, he called Galileo’s research the most exciting physics that the U.S. was conducting in the solar system and said that its loss would be devastating.52

Galileo’s loss would hamper advances in space science, as well as the capability 

of U.S. scientists to remain the world leaders in solar system exploration. An editorial in 

Aviation Week & Space Technology expressed the effect that the loss of Galileo would have, 

in particular, on JPL:

Without Galileo, there is little for the 1,200 program scientists of the Jet 

Propulsion Laboratory to work on. The disappearance of Galileo would 

disperse U.S. planetary capability. There is no way to put Jet Propulsion 

Laboratory on hold for two to three years while Reagan’s budget

director, David Stockman, leads the country out of the economic wilderness with candor and off-the-record interviews.53

The future looked grim indeed at the end of 1981 for Galileo and other U.S. 

solar system missions. But in December, NASA received some much-needed support 

from DOD, American industry, and, surprisingly, the White House. In a new Space 

Shuttle policy directive, President Reagan voiced a commitment to NASA for Shuttle 

development support, an action that helped all of NASA’s planetary exploration plans. 

The Department of Defense strengthened its backing for the conversion of the Centaur 43	Craig Covault, “NASA Moves To Salvage Planetary Program,” Aviation Week & Space Technology (2 November

1981): 16.

44	 Ibid., p. 16.

45	Craig Covault, “NASA Assesses Impact of Budget Cut Proposal,” Aviation Week & Space Technology (12 October

1981): 26.

46	“Chapter 2: The Magellan Mission,” in The Magellan Venus Explorer’s Guide, JPL Publication 90-24, August 1990,

http://www.jpl.nasa.gov/magellan/guide2.html.

47	M. Mitchell Waldrop, “Planetary Science in Extremis,” Science 214 (18 December 1981): 1322; Philip J. Hilts, “Science

Board To Advise President Proposed,” Washington Post (2 December 1981): A25.

48	Hilts, “Science Board,” p. A25; Waldrop, “Planetary Science,” p. 1322.

49	Waldrop, “Planetary Science,” p. 1322.

50	John Casani to Melvin M. Payne, 4 December 1981, letter no. 230-JRC:db-778, John Casani Collection, Galileo

Correspondence 11/81–12/81, folder 24, box 3 of 6, JPL 14, JPL Archives.

51	 John R. Casani to multiple addresses, 4 December 1981, John Casani Collection, Galileo Correspondence 11/81–

12/81, folder 24, box 3 of 6, JPL 14, JPL Archives.

52	 “Washington Roundup,” Aviation Week & Space Technology (7 December 1981): 17; Craig B. Waff, untitled chronology

of Galileo events, 1979–82, JPL, undated, folder 18522, NASA Historical Reference Collection, Washington, DC.

53	William H. Gregory, “Bean-Counting the Solar System,” Aviation Week & Space Technology (14 December 1981): 15.
54	 MISSION TO JUPITER: A History of the Galileo Project Chapter 3 I The Struggle To Launch Galileo 55

into a Shuttle-transported upper stage propulsion system. Such a conversion would further DOD goals, as well as efforts to maintain a U.S. solar system mission capability. In 

addition, top officials of DOD and the Aerospace Industries Association attacked OMB’s 

proposed slashes to NASA’s budget, fearing that, if implemented, they could be a major 

factor in removing the U.S. from its position of space and aviation leadership. Secretary 

of Defense Caspar Weinberger and Air Force Research and Development Chief Richard 

DeLauer made their protests directly to David Stockman, Director of OMB. The Aerospace 

Industries Association met with President Reagan himself, claiming that such a “bare 

bones” NASA budget would put the industry’s important long-haul programs at risk.54

NASA also conducted meetings with critical Reagan administration staff. The 

Agency’s top managers sat down with White House chief of staff James A. Baker and 

OMB Director David Stockman in order to underline how crucial to the space agency a 

planetary exploration program was. NASA staff not only talked about the important scientific data that would be lost if planetary exploration were eliminated, but also stressed 

the impact that the loss of 1,200 JPL jobs and considerable international prestige would 

have on the country. 

Galileo Is Reinstated, but at a Cost

The NASA, aerospace industry, and DOD lobbying efforts against OMB’s proposed funding cuts had an effect. Days before the end of 1981, OMB agreed to reinstate Galileo into 

the FY 1983 budget but recommended that Centaur development be killed. OMB was 

firm on this position, in spite of the support that Centaur had from DOD.55 According to 

John Casani, “This change was driven solely by budget pressures and has resulted in a 

net reduction of about $150M in FY82 and 83 combined . . . .” He noted, however, that 

although the cost of developing and launching Galileo was reduced, total costs that would 

be charged to the mission over its entire lifetime would actually increase. Due to the 

elimination of Centaur, NASA had to foot the bill for design changes to the spacecraft and 

its propulsion system, as well as for two or more years of additional operating expenses, 

because the spacecraft would not be able to reach Jupiter and complete its mission as 

quickly. The additional operating expenses, however, would not have to be paid until 

years after the launch.56

NASA set to work immediately on restructuring Galileo for a mission without 

Centaur, making plans to use Boeing’s solid-fueled IUS, augmented with a solid-fueled 

Injection Module (also called a “kick stage”) for increased propulsion. Even with the

Injection Module, the spacecraft would not receive as much thrust as with Centaur, and 

this required JPL to plot out a different, gravity-assisted trajectory to get Galileo to Jupiter. 

The launch date would remain in 1985, but Galileo would not reach Jupiter until 1989 or 

1990—an increase of 24 to 30 months in trip time over what a Centaur-driven spacecraft 

could have achieved.57

Although cutting Centaur would save considerable funds through FY 1984, both 

NASA and the Air Force realized that upper stages more powerful than Boeing’s IUS 

would eventually need to be built. By 1987 or 1988, heavy military payloads currently 

under development would need to be boosted up to geosynchronous orbit. High lift capability might also be required to send components of a proposed military space platform 

into orbit.58 Opinions differed, even within agencies, as to the best way to prepare for the 

heavy lift requirements. Although loss of Centaur would reduce near-term capability of 

the Shuttle and upper stage, some NASA advanced planners thought this the best course 

to take. Retention of Centaur would, in their opinion, push the development of a more 

modern, possibly reusable upper stage further into the future. Others in NASA lobbied the 

Department of Defense to intensify its advocacy of Centaur, hoping that development of 

the liquid-fueled upper stage could eventually be salvaged.59

The Air Force was split on what to do. There was strong sentiment among its 

uniformed brass, with which the White House at this time concurred, to avoid putting 

development funds into the 20-year-old Centaur. Their belief was that this might “lead 

down a dead-end street on both cost and hardware utilization.” A new high-energy upper 

stage should instead be built from scratch, implemented into Shuttle applications in the 

late 1980s, and used through the 1990s. Top-level civilians in the Air Force, however, were 

considering a different plan—an early transition to Centaur as the Shuttle upper stage, 

cutting off development of the IUS once Centaur was available. The civilian Air Force 

view was driven by estimates that IUS operating costs would be higher than expected—as 

much as $70 million more per mission if the IUS was manufactured in small quantities. 

The Air Force and NASA began meeting in January 1982 to further discuss this issue and 

lay out possible courses of action. They reportedly focused their discussions on how to 

insert Centaur funding into the FY 1983 budget.60 Eventually, however, the Air Force and 

NASA decided against a joint development of Centaur due to funding constraints and 

a limited user base for the Centaur until the late 1980s. They did form a review team, 

though, to thoroughly “reexamine the entire Shuttle upper-stage issue.”61

JPL scheduled Galileo’s launch for August 1985, four months later than for the 

Centaur-driven craft, and adopted a “ΔV-EGA” (Earth gravity-assist) trajectory (see figure 

3.1). Under this plan, the Shuttle would attain Earth orbit, after which Galileo would be 

injected into a two-year elliptical orbit around the Sun. In mid-1986, near the spacecraft’s 

aphelion, or furthest point in the orbit from the Sun, the Orbiter’s propulsion system 

would fire and impart a change in velocity, or “ΔV,” of over 500 meters per second. This 

change would alter the orbit such that Galileo would cross inside Earth’s orbit on the 

spacecraft’s way to perihelion, the point of closest approach to the Sun. When Galileo 

54	“Washington Roundup,” Aviation Week & Space Technology (14 December 1981): 15.

55	Craig Covault, “Galileo Reinstated in the Budget,” Aviation Week & Space Technology (28 December 1981): 10; Waff,

untitled chronology of Galileo events, 1979–82.

56	J. Casani attachment GLL-JRC-82-810 to letter from Al Diaz to John R. Casani, 19 February 1982, John Casani

Collection, Galileo Correspondence 2/82–4/82, folder 21, box 2 of 6, JPL 14, JPL Archives.

57	John R. Casani to Galileo Project Science Group, “Galileo Baseline Change,” 5 January 1982, John Casani Collection,

Galileo Correspondence, 1/82, folder 23, box 2 of 6, JPL 14, JPL Archives; Covault, “Galileo Reinstated,” p. 10; Al Diaz

to John R. Casani, 12 February 1982, John Casani Collection, Galileo Correspondence 2/82–4/82, folder 21, box 2 of

6, JPL 14, JPL Archives.

58	“Washington Roundup,” Aviation Week & Space Technology (15 February 1982): 15.

59	Covault, “Galileo Reinstated,” pp. 10–11.

60	Craig Covault, “New Shuttle Stage Aims at Manned Use,” Aviation Week & Space Technology (28 June 1982): 137; “Air

Force Mulls IUS-to-Centaur Switch,” Aerospace Daily (13 January 1982); Waff, untitled chronology of Galileo events,

1979–82.

61	 “Washington Roundup,” Aviation Week & Space Technology (15 February 1982): 15.
56	 MISSION TO JUPITER: A History of the Galileo Project 

reencountered Earth in June 1987, the spacecraft would receive just the right EGA to 

accelerate it on the proper trajectory toward Jupiter.62

The June 1987 EGA would put Galileo on a trajectory to Jupiter that was nearly 

identical to one that would have been followed if a Centaur-driven craft had launched 

in June 1987. In other words, it would take an IUS-driven Galileo spacecraft almost two 

years (from launch in August 1985 to EGA in June 1987) to attain the trajectory that a 

Centaur-driven craft could have attained immediately because of its greater thrust.63

Figure 3.1. The ∆V-EGA trajectory employed an Earth gravity-assist to place Galileo on a Jupiter trajectory. 

(This figure was redrawn from a diagram found in JPL archives: Folder 23, box 2 of 6, JPL 14)

The IUS-driven Galileo would reach the Jovian system in January 1990. Because 

a significant fraction of the craft’s fuel would have been used for the near-aphelion ΔV 

maneuver, Galileo would arrive at Jupiter with less propulsive capacity than it would have 

with Centaur as an upper stage. As a result, only six Jovian satellite encounters might be 

attainable, compared to the 11 encounters that had been envisioned previously.64

Continued Changes to Galileo’s Propulsion System

In March 1982, Aerospace Daily reported that Congress was considering implementing yet 

another change in Galileo’s propulsion system. Many in Congress wanted to restart the 

62	GAO, p. 24; Thomas O’Toole, “Budget Squeeze Stretching Out Journey to Jupiter 6,” Washington Post 

(6 January 1982).

63	J. R. Casani to distribution, “∆V-EGA Plans,” 20 January 1982, John Casani Collection, Galileo Correspondence 1/82,

folder 22, box 2 of 6, JPL 14, JPL Archives.

64	Casani, “Delta VEGA Plans”; John R. Casani to Galileo Project Science Group, “Galileo Baseline Change.”

65	“Congress Considering Attempt To Save Centaur Upper Stage,” Aerospace Daily (1 March 1982); Waff, untitled

chronology of Galileo events, 1979–82.

66	Bruce Murray to James M. Beggs, 26 February 1982, Galileo History files, folder 5139, NASA Historical Reference

Collection, Washington, DC.

67	 Ibid.

68	“Use of Centaur Upper Stage Would Delay Galileo,” Aerospace Daily (26 May 1982); Waff, untitled chronology of

Galileo events, 1979–82.

69	Waff, untitled chronology of Galileo events, 1979–82.

development of Centaur for use in the Shuttle, even though Centaur had, just months 

before, been rejected by OMB as an inappropriate propulsion system for Galileo.65 These 

continual flip-flops in the Galileo mission plan upset NASA; each time the propulsion system 

was changed, the space agency’s engineers had to backtrack and redesign a large part of the 

project. Several entities outside NASA, including Congress, the White House, and OMB, were 

trying to micromanage NASA’s missions, down to the choice of propulsion systems used. 

NASA managers and engineers, who had the most intimate knowledge of mission needs, 

were often kept out of the decision-making. In a letter to NASA Administrator James Beggs, 

JPL Director Bruce Murray expressed his laboratory’s frustration with the repeated changes 

in direction and with the threat these changes posed to the mission’s continuance:

. . . The project has had to redo spacecraft designs and implementation plans, redirect contractors, redeploy people and generally 

reeducate and remotivate many people and organizations toward new 

plans and goals on three major occasions. Most seriously the project has 

been made vulnerable to and, in several instances, actually threatened 

by cancellation as a result of the program changes.

The present plan [to launch in 1985 on a ΔV-EGA trajectory, 

using a solid-fueled IUS plus an Injection Module], although non-optimum, 

is believed to be realistic, doable, and credible. It is very important for the 

success and viability of the project to permit it to settle down on and to 

proceed to carry out a realistic and stable implementation plan.66

Murray went on to enumerate the threats that the propulsion system changes 

posed to the mission. At the top of his list was that Congress might react to the continual 

delays and budget modifications by canceling the mission (even though Congress itself 

had been responsible for some of the changes). He also worried about the morale of 

the project’s staff, who perceived some of the OMB- or Congress-imposed modifications as totally unrealistic. He voiced concern about keeping his project team together 

and retaining subcontractor capabilities during the long delays. Losing members of his 

team or subcontractor capabilities would impair the “specific knowledge and competence 

required” for a successful launch and smooth in-flight operations. Finally, Murray feared 

that Centaur development would be so expensive that it would drain FY 1982 and FY 

1983 funds needed for Galileo spacecraft development.67

Others in the space science community shared Murray’s concerns regarding more 

propulsion system changes. Robert Allnut, NASA’s Deputy General Counsel for Policy 

Review, said in an interview with Aerospace Daily that a switch to Centaur would delay 

Galileo’s launch for at least a year.68 Air Force Major General Jasper A. Welch, Jr., cautioned at 

an American Institute of Aeronautics and Astronautics (AIAA) meeting that significant, costly 

modifications would be required to reconfigure the Galileo mission from IUS to Centaur.69

Chapter 3 I The Struggle To Launch Galileo 57

JUPITER

ARRIVAL

1/08/90

EARTH RE-ENCOUNTER

6/25/87

EARTH LAUNCH

8/15/85

SUN

V = 562 m/s

7/30/86

TICK MARKS 50 DAYS APART
58	 MISSION TO JUPITER: A History of the Galileo Project Chapter 3 I The Struggle To Launch Galileo 59

In spite of the concerns, Congress favored a return to Centaur. On 11 May 

1982, on the recommendation of Senator Harrison Schmitt, chairman of the Senate Space 

Subcommittee of the Commerce, Science and Transportation Committee, the U.S. Senate 

added $150 million to NASA’s FY 1983 budget to fund Centaur development. Days later, 

Representative Bill Lowery from San Diego, home of major aerospace contractor General 

Dynamics Convair, urged Centaur development during a House floor discussion of the FY 

1983 NASA budget. In July 1982, an Urgent Supplemental Appropriations Bill for FY 1982 

(H.R. 6685) was proposed, which included a provision directing NASA to restart Centaur 

development for Galileo as well as the International Solar Polar Mission. The language of 

the bill indicated that no more funds were to be obligated for any other upper stages. 

Both Secretary of the Air Force Orr and NASA Administrator Beggs urged 

Congress to eliminate the Centaur provision. The Air Force warned that switching to 

Centaur for the Galileo upper stage would involve significantly more expense than was 

generally recognized. Nevertheless, the House and Senate passed the bill containing the 

Centaur provision on 15 July 1982, and President Reagan signed it into law three days 

later. The bill allocated $80 million for the design, development, and procurement of the 

Centaur upper stage. Lewis Research Center in Ohio would manage NASA’s multi-Center 

Shuttle Centaur program. The Agency sent letters to Boeing and the Air Force informing 

them that they had to stop work on the IUS for Galileo and the Solar Mission.70

Why did Congress reinstate Centaur, especially considering all the objections? 

John Casani believed that there were several reasons: 

• Congress had high regard for NASA’s planetary exploration programs and 

was concerned that future missions would be damaged if they were limited 

to using underpowered launch vehicles. 

• While the Air Force would require a higher energy upper stage by the late 

1980s, Congress was reluctant to embark on a major new propulsion system 

development program, with its attendant cost risks and uncertainties, when 

the very reliable and proven Centaur was available. Centaur had a long history of performance, as well as development investment. It was a mature 

technology that could be counted on. So funding was better spent adapting 

Centaur to the Shuttle than developing a new launch vehicle from scratch.

• Congress wanted to give NASA responsibility for upper stage development. 

This might not have happened if, instead of reinstating Centaur, Congress 

had directed the Air Force to develop a new high-energy upper stage to be 

ready in the late 1980s. Such a launch vehicle’s development would be heavily controlled by Air Force requirements and needs. 

• Finally, Europe’s development of its own commercial space program worried Congress. It saw an “exodus of commercial customers to the European 

Ariane vehicle.” The vehicle’s parent company, Arianespace, had already 

captured nearly 30 percent of the market. Casani referred to estimates indicating that Centaur’s recurring costs would be only half those of the IUS. But 

Centaur had twice the launch capability. Congress, Casani believed, thought 

that using Centaur was the best way to keep or recapture the largest commercial market share.71

Casani believed that, all things considered, the change back to Centaur was a 

good one. Centaur could get Galileo from Earth orbit to Jupiter faster than the IUS and 

with more propellant left in its tanks, which meant, in Casani’s words, that there would 

be a “higher assurance of obtaining our science and mission objectives.” The task now at 

hand was to adapt to the changed mission plan, adhere to Orbiter’s and Probe’s development and test schedules, and make Galileo a success.72

On the Road to a Launch

True to Robert Allnut’s prediction, NASA delayed the new launch a year, to May 1986, 

to allow ample time to complete Centaur development, design and build the necessary 

Galileo-Centaur interfaces, and develop a new trajectory.73 As the months went by without 

more changes, it appeared that a mission plan to which all parties could subscribe had 

finally been found. NASA and its contractors set to work designing the modified mission 

and developing the spacecraft for its 1986 launch.

For the next several years, the challenges to project staff became more technical and management-oriented than political. Some of the tasks before the project team 

included the following:

• Modify the Galileo spacecraft design to interface with Centaur.

• Reestablish documentation, management, and personnel interfaces with 

NASA’s Lewis Research Center (now called Glenn Research Center), which 

would direct the development of Centaur for use in the Shuttle.

• Develop a direct trajectory to Jupiter for the 1986 launch because the extra 

power of Centaur would not require gravity-assists.

• Stop design modifications that had been required by the Earth gravity-assist 

in the ΔV-EGA trajectory.

• Revalidate the strategy for inserting Galileo’s Orbiter into a Jupiter orbit. 

70	 “Centaur for Shuttle Would Cost Too Much, Air Force Complains,” Aerospace Daily (14 July 1982); John Casani, “From

the Project Manager,” Galileo Messenger (August 1982): 1, 4; Waff, untitled chronology of Galileo events, 1979–82;

“NASA Directs Air Force, Boeing To Stop Work on IUS for Galileo and International Solar Polar Mission,” Aerospace 

Daily (22 July 1982); Craig B. Waff, “Searching for Options, Part 4: Centaur—To Be or Not To Be?” (outline of chap.

12 of unpublished history of Galileo project), folder 18522, NASA Historical Reference Collection, Washington, DC;

Dawson and Bowles, Taming Liquid Hydrogen, pp. 248–249.

71	John Casani, “From the Project Manager,” Galileo Messenger (August 1982): 4.

72	 Ibid., pp. 1, 4.

73	GAO, p. 24.
Chapter 3 I The Struggle To Launch Galileo 61

• Reassess the pre- and postlaunch development plan. 

• Close out all activity on Injection Module development because it would not 

be needed with Centaur.74

Other JPL tasks included modifying Galileo’s subsystems to decrease their sensitivity to cosmic radiation, refining flight software, fabricating memory components, and 

improving the craft’s spin bearing assembly, which would separate the rotating segment 

of Galileo from the nonrotating part.75 In addition, John Casani planned for a second 

spacecraft, termed a “proof-test model,” to be built—something that could serve as “a 

source of spares for the flight spacecraft.”76

The original Galileo mission design had envisioned a “limited-spares” concept, 

but that design had been done when an early-1982 launch had seemed possible. A JPL 

environment had been envisioned in which the skills and facilities necessary to maintain 

and repair flight hardware would be readily available to support Galileo and other new 

projects. No other new starts had materialized, however, and Galileo’s launch date had 

been delayed more than four years, with further delays possible if problems with Centaur 

development were encountered. As a result, Casani was concerned that JPL’s ability to 

support a limited-spares concept might be seriously eroded by the time of the launch.77

Casani planned to address the spares issue through a combination of upgrading 

engineering models of Galileo and fabricating new parts in order to construct a proof-test 

model spacecraft. His intent was to manufacture the new hardware during 1983 and 1984, 

while the flight spacecraft was undergoing system-level integration and testing. Much of 

the fabrication would be done at JPL in order to avoid paying various subcontractors to 

maintain their abilities to manufacture Galileo parts, and also to optimize the use of the 

mission’s engineering staff, who needed to be kept in place to provide support through 

Galileo’s launch. Also, the fabrication work would help preserve the JPL staff’s expertise 

in spacecraft maintenance and repair. 

Casani envisioned acceptance testing of the flight spacecraft to be completed in 

late 1984, after which it would be put on an extended “burn-in”78 schedule and the assembly of the proof-test model would be started using the same project team. Testing of the 

proof-test model would be completed by the end of 1985, and both spacecraft would be 

shipped to Kennedy Space Center in January 1986 to support the May 1986 launch.79

Atmospheric Entry Probe Testing

NASA engineers designed Galileo’s 4-foot-wide Probe to separate from Orbiter 150 days 

out from Jupiter and eventually plunge into the planet’s atmosphere. It would enter the 

atmosphere on a shallow trajectory of approximately 9 degrees below horizontal and 

gradually slow from a velocity of 107,000 mph (48 kilometers per second) to Mach 1. The 

Probe’s thermal heatshield would enclose and protect the scientific instruments during 

this time. Once Mach 1 was reached, a parachute system would separate from the forward 

heatshield and control the rate of descent. Meanwhile, the scientific instruments would 

collect data that the Probe’s transmitter would send to Orbiter, which would then relay 

the information to Earth.80

One of the critical tests in preparing the Probe was to confirm proper parachute 

operation. On 17 July 1982, NASA scientists placed the 460-pound Probe vehicle in a 

gondola attached to a 5-million-cubic-foot-capacity, helium-filled balloon and launched it 

from Roswell, New Mexico. A total of 830 pounds of ballast had been added to the Probe’s 

forward thermal shield to get the test vehicle to the required Mach number and dynamic 

pressure that it would experience in the Jovian atmosphere. After 4 hours of flight time 

and at an altitude of 97,000 feet, the Probe was dropped from the balloon above New 

Mexico’s White Sands Missile Range. The Probe’s pilot chute deployed, followed by the 

removal of the aft heatshield and the deployment of the large main parachute. The Probe’s 

Project Manager, Joel Sperans of Ames Research Center in Mountain View, California, 

reported that from on-board camera data, the entire descent sequence appeared to have 

been carried out successfully. Speeds of descent and dynamic pressures were virtually 

identical to those expected to be encountered in Jupiter’s atmosphere.81

Although Sperans reported that a major milestone in Probe development had 

been attained, the test did not go flawlessly. Some of the Probe’s ballast failed to separate. 

More importantly, the main parachute opened several seconds late. Its function was to 

slow the Probe and help separate its descent module, which contained the scientific package, from the deceleration module that performed a heat-shielding function. Instead, the 

main parachute did not fully open until the descent module had already separated. 

Although project staff did not think these were serious problems, the parachute 

delay was painstakingly analyzed during the following months. Pictures of the test suggested that the wake created as the Probe sped rapidly through the air may have delayed 

the chute’s opening. To fix this, project staff considered extending the length of line that 

attached the parachute canopy to the Probe. This would place the parachute farther from 

the Probe, where wake effects were less intense. The staff decided to delay the release 

of deceleration module components until Probe velocity decreased further, which would 

also reduce wake effects. Hughes Aircraft Company, the contractor developing the Probe, 

scheduled wind tunnel tests to examine the effects of these modifications.82

During the following months, Hughes integrated most of the descent module’s 

instruments and components into the Probe flight vehicle and conducted electrical and 

other tests. Project staff also scheduled a series of environmental stress tests and more 

74	A. E. Wolfe to J. R. Casani, “Project Impacts of 1986 Centaur Launch,” 25 May 1982, John Casani Collection, Galileo

Correspondence 5/82, folder 19, JPL 14, JPL Archives.

75	GAO, pp. 24–25.

76	John Casani interview, tape-recorded telephone conversation, 29 May 2001.

77	J. Casani, “Impact of Switching to Centaur and Delay to ’86” (draft), 25 May 1982, John Casani Collection, Galileo

Correspondence 5/82, folder 19, JPL 14, JPL Archives.

78	Years of empirical data have shown that electronic parts tend to fail early in their lifetimes if they contain defects.

A “burn-in” period consists of continuous operation of the electronic parts for as many hours as possible in order

to screen out the unreliable ones before actual use. Many of Galileo’s parts were new designs that had been built

using new manufacturing approaches, and so burn-in was a very essential part of spacecraft testing. “Q&A,” Galileo 

Messenger (July 1985): 5.

79	Casani, “Impact of Switching.”

80	Bruce A. Smith, “Deployment Problem Forces Galileo Test Reschedule,” Aviation Week & Space Technology (20

September 1982).

81	Peter Waller, “Galileo Probe Drop Test Successful,” NASA news release, Ames Research Center, 30 July 1982, pp.

1–2; Smith, “Deployment,” p. 106.

82	Waller, “Galileo Probe,” pp. 2–3; Smith, “Deployment,” pp. 106–107.

60	 MISSION TO JUPITER: A History of the Galileo Project 
62	 MISSION TO JUPITER: A History of the Galileo Project Chapter 3 I The Struggle To Launch Galileo 63

drop tests.83 In April 1983, Lewis Research Center used its Transonic Dynamics Facility 

to test a quarter-scale model of the Probe’s new parachute assembly in order to assess 

the impact of the parachute’s position and distance from the Probe body on its behavior. 

Lewis investigated parachute distances ranging from 5.5 to 11 body diameters behind the 

Probe; results showed that chute behavior was normal at all distances except the original design position of 5.5 diameters. Lewis performed additional tests to optimize chute 

performance that included varying the chute’s porosity, the Probe’s ballast configuration, 

and the Probe’s angle of attack when entering the Jovian atmosphere. At the completion 

of these tests, Lewis performed half-scale parachute tests.84

In late July 1983, NASA carried out a drop test of the redesigned full-scale Probe. 

The successful test verified that the parachute modifications avoided the slow-deployment 

problems of the year before.85

Component, Spacecraft, and Intersystem Testing

Exhaustively testing each component of the Probe and Orbiter and then integrating it into 

the spacecraft required almost as much time as Galileo’s primary mission did. Each component, as well as the complete spacecraft, had to be carefully examined and subjected to 

a rigorous series of functional and environmental tests to make sure everything met design 

requirements. Staff also tested “intersystem functions,” such as those between ground-based 

mission control and the spacecraft, or between the spacecraft and the Shuttle.86

Component Testing. Testing of the spacecraft’s many subsystems began at the various 

contractor facilities that developed and fabricated them. Before their delivery to JPL, Galileo’s 

components and scientific instruments had to be qualified and accepted for flight. 

Integrating Flight-Qualified Components. After the components arrived at JPL, project staff integrated them mechanically and electrically into a functioning spacecraft. These 

integration operations began in March 1983 at JPL’s Spacecraft Assembly Facility (SAF) in 

cleanrooms environmentally controlled to remain at 72°F and 50 percent humidity. Special 

procedures eliminated the possibility of electrostatic charging of personnel and equipment 

that could come in contact with and damage the spacecraft’s sensitive electronics.87

Integration operations necessitated mating the Probe to the Orbiter. Hughes, 

which had built the Probe, shipped it to JPL in September 1983, and these two parts of 

the Galileo spacecraft were connected for the first time. Project staff needed to verify the 

operation of the interfaces between each Probe and Orbiter component and the rest of 

the spacecraft, as well as testing all the components using Orbiter power, telemetry, and 

commands. The staff electrically coupled the Probe to the Orbiter, then set up a data 

link between them to verify overall data flow from the Probe to the Orbiter’s radio relay 

hardware, command and data system, modulation/demodulation system, and radio transmitter. The transmitter then sent test data to Earth-based tracking systems, to Orbiter and 

Probe ground data systems, and to scientists for analysis.88

Environmental and System Testing. Once all flight components had been integrated 

and tested, the Probe was sent back to Hughes for environmental testing. Orbiter system 

testing also began, including the following:

• Interference analysis to determine whether the operation of any component 

had adverse effects on other components. 

• Mission profile tests to simulate key mission phases such as launch, trajectory correction maneuvers, cruise operations, Probe release, and insertion 

into Jupiter orbit. During these simulations, operational capabilities were 

analyzed and areas for improvement identified.89

Probe-Orbiter System Testing. After Hughes completed Probe’s environmental 

tests, it returned the Probe to JPL and project staff carried out operational tests of the 

combined Probe-Orbiter system. These were followed by system environmental tests, 

which included vibration, acoustic, pyrotechnic, electromagnetic, vacuum, and solar radiation tests. Vibration analyses sought to verify that the spacecraft would operate properly 

after being shaken and jostled during liftoff, ascent, and injection into its interplanetary trajectory. Project staff bolted the spacecraft to a 30,000-pound shaker table in JPL’s 

Environmental Test Laboratory, then attached 29 accelerometers and strain gauges so that 

vibration levels could be monitored and controlled. To simulate flight conditions, vibrations ranged in frequency from 10 to 200 Hertz, producing accelerations of up to 1.5 g’s. 

Staff added an additional 100 sensors for monitoring during the test. Fifty-nine of these 

were part of an automatic shutdown system to protect the spacecraft in case vibrations 

exceeded specified limits.

Acoustic testing simulated the high noise levels to which Galileo would be 

subjected during liftoff in the Shuttle payload bay. Project staff placed Galileo in JPL’s 

10,000-cubic-foot acoustic chamber, a concrete room containing two 4-foot-square “feed 

horns” to generate the sound. Numerous microphones were placed around the spacecraft to monitor noise levels during the testing. Preparation for the testing took several 

days, although the test itself lasted only 1 minute, with noise levels of 142 decibels. By 

comparison, the noise level of a typical conversation is about 50 decibels, while heavy 

traffic generates about 75 decibels. Each 6-decibel increase indicates a doubling of 

sound pressure.90

After the actual launch, explosive devices were set off to deploy booms, release 

Centaur, separate the Orbiter’s spun and despun sections, and jettison instrument 

covers. Pyrotechnic testing assessed impacts on the spacecraft of concussions similar 

83	 to those that were generated by these small explosions. Electromagnetic compatibility Smith, “Deployment,” pp. 106–107.

84	“Galileo Probe Parachute Tests,” NASA Daily Activities Report, 28 April 1983.

85	“Galileo Probe and Orbiter To Be Mated in September,” Aerospace Daily (24 August 1983): 298.

86	“SAF Activities,” Galileo Messenger (June 1983): 1.

87	 “SAF Activities,” pp. 1–2; “Status/Highlight Summary,” in a set of John Casani Collection papers that begin with “GLL

8th Quarterly Report Agenda,” Galileo Correspondence 2/81, folder 30, box JA370, JPL 14, JPL Archives.

88	“Galileo Probe and Orbiter To Be Mated in September,” Aerospace Daily (24 August 1983); “SAF Activities,” p. 2.

89	“SAF Activities,” p. 2.

90	“Environmental Tests,” Galileo Messenger (July 1985): 2.
64	 MISSION TO JUPITER: A History of the Galileo Project Chapter 3 I The Struggle To Launch Galileo 65

tests examined impacts of the radio-frequency environment that surrounded the craft 

during and after launch.91

To assess the impact of vacuum on Galileo, project staff employed JPL’s large 

space simulator. This 25-foot-tall chamber was evacuated rapidly to simulate the pressure reduction Galileo would experience during the Shuttle’s ascent from the launchpad. 

Pressure in the simulated space environment was brought down as low as 1 x 10-5 torr.92 

Although testing at this level of vacuum was able to build a degree of confidence in 

Galileo’s ability to withstand actual vacuum during the mission, the test chamber could 

not attain the vacuum of deep space, which can be as low as 1 x 10-14 torr.93

Project engineers conducted the tests in hot and cold modes to represent both 

the inner solar system part of the mission, when Galileo would be closest to the Sun, and 

the times when the craft would be out at Jupiter and hidden behind the planet, receiving 

no warming solar radiation at all. Liquid nitrogen cooled the inner wall and floor of the 

room to temperatures as low as -196°C (-321°F) to simulate the frigid parts of the mission, 

and a focused array of 20- to 30-kilowatt arc lamps approximated the times when Galileo 

would receive large doses of solar radiation.

The testing needed to validate Galileo’s ability to function under the extreme 

conditions it would encounter. Spacecraft thermal controls had to manage heat from the 

Sun as well as from the craft’s internal radioisotope thermal generators (RTGs), various 

heater units, and complex electronic devices. Galileo was designed to maintain thermal 

equilibrium by minimizing variable solar input as much as possible, distributing heat to 

those areas of the craft needing to be maintained at higher temperatures, and controlling 

the loss of heat to space. The craft employed louvers similar to Venetian blinds. It also 

used multilayer thermal blankets, which gave some protection against micrometeorites 

and electrostatic discharge, as well as helping to retain heat.94

Galileo’s electric heaters could be turned on and off, but its RTGs always generated thermal energy (because their plutonium fuel was constantly decaying and emitting 

energy). Orbiter contained a total of about 70 heating devices, while Probe contained 34. 

For safety’s sake, the environmental testing did not use the actual RTGs, with their radioactive fuel, but instead employed electrically heated, simulated RT Gs.

Most of Galileo’s subsystems were designed to operate between 5 and 50°C but 

were tested at temperatures that ranged from -20 to 75°C (-4 to 167°F). Some components, 

however, had narrower restrictions, and it was important to determine whether these could 

be maintained during the environmental tests. The temperature of the craft’s propellant 

fuel, for instance, had to be strictly maintained because overpressures could cause its storage tank to rupture. In addition, fuel lines could never be allowed to freeze. The hydraulic 

system that deployed the Orbiter’s booms was also very sensitive to temperature. In order 

to maintain acceptable viscosities in the system’s hydraulic fluid, temperatures had to be 

kept between 20 and 30°C (68 and 86°F).95

After testing the Probe and Orbiter together, staff performed similar tests 

on Orbiter alone to assess its expected performance after Probe was released near 

Jupiter. During the final months of testing, spacecraft power was maintained 24 hours 

per day in order to log the maximum number of operating hours on Galileo’s electronics assemblies.96

Adapting Centaur to the Shuttle

Design studies integrating Centaur with the Shuttle date back to the start of the 

Shuttle program, although when Boeing’s solid-fueled IUS was the favored upper stage, 

prospects for a Centaur-driven Galileo spacecraft seemed very remote. When General 

Dynamics finally received the go-ahead in 1982 for full-scale Centaur adaptation to the 

Shuttle, it had to fulfill both NASA’s requirements and those of the Air Force. The result 

was two versions of the upper stage: the G type for the Air Force and the G-prime for 

NASA. Both were shorter and wider than the version used as an upper stage for Atlas, 

which was over 9 meters long and 3 meters in diameter (about 30 feet by 10 feet), but 

they also differed markedly from each other. The Air Force needed space in the Shuttle 

cargo bay for its larger satellites, so the Centaur G was made only 6 meters long but 4.3 

meters in diameter (roughly 20 feet by 14 feet). Its liquid-oxygen tanks and two engines 

were left unchanged, but its liquid-hydrogen tank was shortened and widened.97

In the design of NASA’s G-prime Centaur, the need for more thrust took priority 

over extra space in the Shuttle cargo bay. The G-prime was less than a foot shorter than 

the 9-meter Atlas upper stage and was as wide as the G version, or in other words, 40 percent wider than the Atlas upper stage. The G-prime’s liquid-hydrogen and liquid-oxygen 

tanks were longer than those of the G type, although its engines and electronic systems 

remained the same. The performance characteristics of a Shuttle/G-prime Centaur combination were very impressive when compared to other launch system combinations. The 

Atlas-Centaur combination could lift a payload of only 1,900 kilograms (4,200 pounds) to 

geostationary orbit. The Shuttle plus the Air Force’s G-type Centaur could move a satellite 

weighing 4,800 kilograms (10,600 pounds) to geostationary orbit, while the Shuttle plus 

NASA’s G-prime Centaur could transfer a 6,350-kilogram (14,000-pound) satellite into 

geostationary orbit.98

Inside the Shuttle’s payload bay, Centaur was attached to a support structure 

and tilt-table. The support structure also brought electrical connections to Centaur that were 

necessary for supplying power, transferring data, and controlling fueling and emergency fueljettisoning operations. Deployment of Galileo and Centaur would involve opening the Shuttle’s 

doors and rotating the tilt-table to swing the forward end of the spacecraft out of the payload 

bay, then using a shaped charge 

99 to sever the spacecraft’s connection to the Shuttle. After 

91	 Ibid., pp. 2–3.

92	One torr of pressure is equal to the pressure of one millimeter of mercury, or 1/760 of an atmosphere (the mean

pressure on the surface of Earth).

93	“Environmental Tests,” pp. 3–4.

94	 Ibid., pp. 4–5.

95	 Ibid., p. 5.

96	 Ibid., pp. 1, 4–5.

97	Curtis Peebles, “The New Centaur,” Spaceflight 26 (September/October 1984): 358; G. R. Richards and Joel W.

Powell, “The Centaur Vehicle,” Journal of the British Interplanetary Society 42 (1989): 115.

98	Peebles, “The New Centaur,” p. 358.

99	A shaped charge is an explosive charge shaped to focus the energy released during detonation along a single line,

making the charge’s delivery of energy very accurate and controllable. Lawrence Livermore National Laboratory,

“Shaped Charges Pierce the Toughest Targets,” Science & Technology, June 1998, http://www.llnl.gov/str/Baum.html.
66	 MISSION TO JUPITER: A History of the Galileo Project Chapter 3 I The Struggle To Launch Galileo 67

this, 12 steel springs would push Galileo-Centaur away from the Shuttle. Ignition would be 

delayed for approximately 45 minutes, until the Shuttle flew to a safe distance away.100

The long list of tests to be performed on Galileo before launch day included a 

very thorough examination of the Centaur rocket, which had never before been deployed 

from the Space Shuttle. The hydrogen-fueled Centaur had been used for years as an upper 

stage of robotic rockets, but during Galileo’s launch, it would be carried aloft inside the 

cargo bay of a craft with a human crew. Because Centaur’s liquid-hydrogen and liquidoxygen fuel was potentially explosive, elaborate safety features had to be developed 

before it was deemed safe for use in the Shuttle. These features included the capability 

to dump Centaur’s fuel overboard quickly in the event of an emergency. NASA staff had 

designed and installed a dump system that could jettison all of Centaur’s propellant in 250 

seconds in the event of a launch abort or failure to deploy in orbit. The staff had also built 

in the capability of waiting to fuel Centaur until the Shuttle was on its launchpad.101

The Amphitrite Asteroid Option

In December 1984, John Casani proposed that an additional task be added to the Galileo 

mission: to fly by the asteroid Amphitrite (the last part of its name rhymes with “flighty”). 

Because the spacecraft would traverse the solar system’s asteroid belt on its way to Jupiter, 

it would be in an excellent position to make close observations of the asteroids. Casani 

and other Galileo staff thought that such a study would add significantly to our understanding of the solar system and its origins and that it fell within the stated mission of 

NASA’s solar system exploration program. Primitive bodies such as asteroids and comets 

appeared to be “the best preserved remnants of the early solar system and most representative of the overall composition of protoplanetary/protosolar nebula material.” This was 

important, because evidence of early solar system processes is not easily extracted from 

larger planetary bodies.102

The narrow region between Mars and Jupiter known as the asteroid belt was also 

the boundary between the rocky inner planets of the solar system and the more volatile, gasrich outer planets. Over 3,000 asteroids have been identified in this belt. Amphitrite, whose 

name refers to a queen of the sea and one of Poseidon’s wives in Homer’s Odyssey, is a 

rocky little body 200 kilometers (120 miles) in diameter. It was the 29th asteroid discovered 

(and is often referred to as 29 Amphitrite). Albert Marth identified it on the night of 1 March 

1854 from William Bishop Observatory in London. Later work determined that the asteroid 

revolved in a nearly circular orbit around the Sun at an average distance of 230 million miles 

and rotated on its axis once every 5.4 hours. Beyond these basic facts, little was known 

about Amphitrite. It was chosen because it was in the right place at the right time.103

The idea of a Galileo flyby of Amphitrite arose at JPL in 1983 after engineers 

planning the spacecraft’s course through the asteroid belt finished a series of computer 

simulations looking for possible navigational hazards. The asteroids in the general area of 

Galileo’s path turned out to be far enough away to not present a collision danger. Other 

asteroids in this group besides Amphitrite, such as 1219 Britta or 1972 Yi Xing (the number 

indicates the order of discovery), were only a few kilometers in diameter. For safety reasons, Galileo’s planners did not want to send the spacecraft closer than 10,000 kilometers 

(about 6,000 miles) from any asteroid, because no one knew what the surrounding environment would be like. Space scientists considered it possible that an asteroid could be 

enveloped by a cloud of fine dust particles from numerous collisions with other asteroids. 

If Galileo were to fly through such a cloud, its optics could be severely damaged and the 

spacecraft itself might be destroyed. 

At the “safe encounter” distance, not much could be learned from Galileo’s cameras’ observation of a small asteroid only several kilometers in diameter. Amphitrite, on 

the other hand, was among the largest 1 percent of all asteroids and huge compared to 

Britta or Yi Xing. A flyby from even 20,000 kilometers, twice the safe encounter distance, 

could yield significant data.104

Because of the attractive opportunity, John Casani requested that NASA management approve a postlaunch flyby option as a secondary mission objective. Under this plan, 

a trajectory modification could be implemented such that Galileo would pass at a distance 

of 10,000 to 20,000 kilometers from Amphitrite and arrive at Jupiter about three months later 

than planned, in early December 1988. But if the spacecraft was not “healthy” when flying 

by Amphitrite—that is, if all systems did not fully check out—then no encounter operations 

would be performed and no pictures or measurements of the asteroid would be taken.105

A successful encounter with Amphitrite was viewed by NASA as quite significant 

because no U.S. mission to the asteroids was planned until at least a decade later. In 

addition, experts in the taxonomy of asteroids thought that Amphitrite would be especially valuable to study because it was an S-type asteroid. Such asteroids were believed 

to be either remnants of a larger planetary body (perhaps part of its stony iron core), or 

fragments of chondrites—bodies that coalesced directly from the primordial nebula out 

of which the Sun and planets formed. During the two (Earth) days of Galileo’s closest 

approach to Amphitrite, the asteroid would turn 10 times on its axis, allowing Galileo 

to repeatedly map its entire surface and discern features as small as 600 feet across. 

Photographs with such resolution of the Martian surface from Mariner 9 resulted in a radically changed understanding of the planet. In addition, with Galileo’s infrared, visual, and 

ultraviolet sensing systems, a detailed geological survey of Amphitrite could be carried out. 

Atmospheric instruments would also search for any signs of a tenuous atmosphere, and 

Galileo’s dust detector would look for evidence of debris orbiting the asteroid. Changes 

in Galileo’s velocity due to Amphitrite’s gravitational field would allow the body’s mass 

and average density to be calculated for the first time.106

On 6 December 1984, NASA Administrator James M. Beggs endorsed the 

Amphitrite option and supported a change in Galileo’s trajectory and in its Jupiter arrival 

date from 27 August 1988 to 10 December 1988, with the understanding that Amphitrite 

100	Richards and Powell, “The Centaur Vehicle,” p. 115.

101	William Harwood, “NASA Current News,” from United Press International (UPI), 31 December 1985; Peebles, “The

New Centaur,” p. 358.

102	J. R. Casani to W. E. Giberson, “Addendum to Project Plan 625-1, Rec. C, Section III,” 3 December 1984, John Casani

Collection, Galileo Correspondence, 11/84–12/84, folder 1, box 1 of 6, JPL 14, JPL Archives.

103	Casani, “Addendum”; Paul Stinson, “Why Amphitrite,” Space World (December 1985): 24.

104	Paul Stinson, “Why Amphitrite,” and Alan Stern, “Amphitrite, Maybe,” both from Space World (December 1985):

23–24.

105	J. R. Casani to W. E. Giberson, “Galileo Asteroid Flyby,” 11 September 1984, and R. J. Parks to Geoffrey A. Briggs, 20

September 1984, both in John Casani Collection, Galileo Correspondence, 11/84–12/84, folder 1, box 1 of 6, JPL 14,

JPL Archives; Casani, “Addendum”; Stern, “Amphitrite, Maybe,” p. 22.

106	Stern, “Amphitrite, Maybe,” pp. 22–23; JPL Public Information Office, press release no. 1062, 17 January 1985,

http://www.jpl.nasa.gov/releases/84-86/release_1985_1062.html.
68	 MISSION TO JUPITER: A History of the Galileo Project Chapter 3 I The Struggle To Launch Galileo 69

was to be a secondary objective and must in no way compromise the primary Jupiter 

objectives. Because no prelaunch funds were authorized for the Amphitrite option, and 

due to the political sensitivity of asking for funding to cover yet another Galileo mission 

change, Beggs stressed that no Amphitrite mission planning was to be undertaken until 

after launch. And no papers or articles were to be written on any aspect of the Amphitrite 

option until after the actual encounter.107

Down-to-the-Wire Problems

By October 1985, project staff had completed all electronic compatibility and characterization testing of the spacecraft, as well as environmental testing. But Galileo was still 

missing its spin bearing assembly, the mechanical and electrical interface between the 

spun and despun sections of the craft. The bearing assembly used a system of metal sliprings and brushes to transmit electrical signals from one section of the spacecraft to the 

other. Project staff had discovered contamination on the slip-rings and sent them and the 

brushes to be remanufactured.108

Analysis of the bad parts and their manufacturing process revealed that a whole 

chain of circumstances had led to the problem. These circumstances involved the coatings 

on the parts, the effect on the coatings of heat from soldering operations, the chemical 

used to clean the parts, and the impact of vacuum on the system. Heat from soldering had 

made the coating material more porous, and when the parts were dipped into a chlorofluorocarbon (CFC) solution for cleaning, some of the CFC was absorbed into the pores 

in the coating. During environmental testing, the low-vacuum conditions pulled the CFC

out of the pores, and it mixed with the fine, powdery debris generated as the brushes 

and slip-rings slowly wore down. This debris normally did not cause problems, but when 

mixed with the CFC, it clumped up, eventually accumulating under the surface of a brush 

and opening up the contact between it and the slip-ring. The result was an intermittent 

power transmission problem.109

A second problem involved Galileo’s command and data system. Project staff 

noticed that the performances of certain memory devices deteriorated in an electromagnetic 

radiation environment. The origin of this problem, too, was traced back to the manufacturing process. But after JPL acquired and installed several thousand remanufactured memory 

devices, they were found to have yet another problem called “read-disturb.” Reading the 

contents of a memory cell in a device could disturb the contents of adjacent cells. The 

project team determined that the processing changes made to repair the memory cells’ sensitivity to electromagnetic radiation also caused them to operate faster, and it was these parts 

that were susceptible to the read-disturb problem. Project staff had to construct a special 

screening test, and one-third of the memory devices had to be removed and changed. This 

was a very labor-intensive operation because of the parts’ small size and close spacing.110

107	J. R. Casani to distribution, “Amphitrite Option,” 17 December 1984, John Casani Collection, Galileo Correspondence,

11/84–12/84, folder 1, box 1 of 6, JPL 14, JPL Archives.

108	Bruce A. Smith, “Galileo Undergoes Final Tests Prior to Shipment to Kennedy,” Aviation Week & Space Technology 

(21 October 1985): 135.

109	Ibid.

110		Ibid.

In spite of the problems, Casani believed that Galileo had a good record for parts 

quality. Components were carefully examined and burned in at elevated temperatures. 

All spacecraft equipment and spare parts received at least 2,000 hours of operation prior 

to launch. NASA designed Galileo to operate for at least five years, including two and a 

half years of travel to Jupiter and sufficient time to carry out its experiments and observations in the Jovian system. This “design life” was dictated by the propellant capacity of the 

spacecraft, as well as expected degradation due to the radiation environment in which 

it would operate. Project officials believed that damage from radiation would gradually 

deteriorate electronic components and eventually bring about the end of the mission.111

The First 3,000-Mile Leg of a Half-Billion-Mile Journey

“Everything seemed on the home stretch, when at 3:00 a.m. on December 19, 1985, Galileo 

began the first 3,000-mile leg of its half-billion-mile journey,” announced the narrator of 

WGBH ’s NOVA television program on Galileo, “The Rocky Road to Jupiter.” JPL staff had 

been readying Galileo for a high-speed truck convoy from Pasadena, California, across 

America to Kennedy Space Center, Florida. When the convoy rolled out of JPL in the 

middle of the night, its drivers did not yet know the exact route they would be taking. They 

only knew that they would drive all night and all day, stopping only for food and fuel. 

Galileo management was concerned that the plutonium units that would power spacecraft 

functions would draw antinuclear protesters or, even worse, a terrorist attack.112

After being repeatedly criticized by Congress, the American press, and the antinuclear movement, Galileo finally appeared ready to take off. If it survived the truck 

journey to Florida and another battery of grueling tests, it would leave Earth in May 1986, 

unless some new, unforeseen problem arose. But if Galileo missed the May launch date, 

it would have to wait 13 months for another “window.” 

The physicist James Van Allen thought of Galileo’s history as the “Perils of 

Pauline.” He remembered four instances when the whole project had been formally 

canceled and many times when it was “just sort of hanging by its fingernails.” Time and 

again, as in the Perils of Pauline silent film serial,113 a hero would come by in the nick of 

time and rescue Galileo from oblivion.

James Fletcher, NASA Administrator, raised the question whether so much time 

had gone by without a launch that there might now be a better mission to carry out in 

Galileo’s place. But when NASA seriously considered this question, the answer that came 

back was, “No, Galileo is still the best thing we can do at this time.” 

Galileo would be the first planetary mission to launch from the Shuttle, and 

“routine emergencies” were expected in the weeks before launch. To deal with these 

incidents, JPL engineers would stay at Kennedy Space Center until Galileo was on its 

way to Jupiter. In NASA’s major past missions, such as Voyager, the Agency typically 

launched two similar spacecraft. But Galileo’s cost precluded this strategy, making it 

111		Ibid.

112	 Transcript, “The Rocky Road to Jupiter,” NOVA no. 1410 (Public Broadcasting System television program), 7 April

1987, WGBH Transcripts, Boston, MA.

113	 In the Perils of Pauline silent film episodic serial (circa 1914), the heroine would weekly evade attempts on her life

perpetrated by pirates, Indians, gypsies, rats, sharks, and her dastardly guardian. Her most well-known plight was

being tied to railroad tracks in front of a rapidly approaching train. From The Greatest Films Web site, http://www.

filmsite.org/peri.html (accessed 29 June 2003).
70	 MISSION TO JUPITER: A History of the Galileo Project 

essential that the launch take place without problems. Seventeen principal investigators from six nations would be flying experiments on board the spacecraft. Over one 

hundred scientists would be working closely with them. Some of the staff had been 

planning for this mission since the 1960s.

As the trucks rolled through the night on their way to Florida, some staff worried 

about the possibility of a road accident. Tom Shain, an electrical engineer on the project, 

commented, “Think of the repercussions—we get sideswiped, driving along here, something like that. And there’s an awful lot of people worked an awful lot of long hours on 

this thing, y’know, for something like that to happen . . . . Maybe that’s one of the reasons 

I’m having a hard time sleeping.”114

John Casani was worried about crises a little further down the line. For instance, 

could the spacecraft be brought back home for repairs if trouble arose soon after launch? 

He concluded, “We have looked at the problem of recovering the spacecraft from orbit. 

It is virtually an impossibility. There are no provisions on the spacecraft for mechanical 

attachment or recapture . . . we’re going to Jupiter or we’re not going anywhere.”115

For three and a half days, the truck convoy rolled. Spacecraft technicians became 

truckers and talked on the Citizens Band (CB), learning its special language. Escorted by 

police, state troopers, and other, less obvious guards, the caravan crossed America without incident or mishap, reaching Kennedy Space Center on 23 December. Once there, the 

staff breathed a large sigh of relief and took some vacation time over Christmas.116

Ahead of the team lay the integration of Galileo with Centaur and a host of final 

tests. One of the activities of late January 1986 that could be performed only at Kennedy 

was the first “tanking up” of the spacecraft. To power its course-correction jets, Galileo used 

hypergolic propellant, two different substances that ignited upon contact with each other. 

Loading such fuel into the spacecraft’s propulsion module could be very hazardous, and so 

project staff evacuated the area around Galileo for a distance of 200 yards. The crews took 

one full day to load each of the four tanks, working short, carefully monitored 80-minute 

shifts. Each crew member donned a special suit for protection that was equipped with selfcontained breathing equipment. In those suits, the crew resembled astronauts.117

On 24 January, NASA threw a large party to introduce JPL staff to their new 

cohorts, the Kennedy personnel who would integrate Galileo into the Shuttle Atlantis. 

The night of the party, the weather was growing noticeably chilly. A cold front was 

moving in. The cold front did not dampen the spirits at the NASA party, but it would have 

a dire effect on the Space Shuttle that was at that time sitting on a nearby launchpad. 

This was not the Shuttle Atlantis, which was scheduled to bear Galileo aloft. It was the 

Challenger, and it was due to launch on its next mission in four days. 

114			“Rocky Road,” pp. 2–5, 11.

115		Ibid., pp. 15, 16.

116 Ibid., pp. 16–17.

117 Ibid., pp. 17–18.

Preparations

for the la unch of

Shuttle mission 51-L

were not unusual, 

although they had been made more difficult by changes in the launch schedule. The 

sequence of complex, interrelated steps preceding the mission were being followed 

closely, as always. Flight 51-L of the Challenger had originally been scheduled for July 

1985, but it was postponed to late November to accommodate payload changes. The 

launch was subsequently delayed further and finally rescheduled for late January 1986. 

NASA had successfully launched the Shuttle 24 times before and expected this launch to 

be successful as well.1

Events Leading up to the Challenger Disaster

In spite of NASA’s confidence in the Shuttle, signs of malfunction had periodically arisen over the years. Morton Thiokol built the Shuttle’s solid-fueled booster engines, 

and years before flight 51-L, its personnel had seen evidence of problems with the O-rings 

that sealed the booster engines’ field joints. These were joints between sections of the 

engine that were assembled at the launch site rather than at the factory. As far back as the 

second flight of the Shuttle, high-pressure gas penetrating the joints had caused erosion 

1	 William P. Rogers, chairman, “Chapter II: Events Leading Up To the Challenger Mission,” in Report of the 

Presidential Commission on the Space Shuttle Challenger Accident, vol. 1 (Washington, DC: Government

Printing Office, June 1986).

Chapter 4

The Challenger Accident

and Its Impact on the 

Galileo Mission 
72	 MISSION TO JUPITER: A History of the Galileo Project Chapter 4 I The Challenger Accident and Its Impact on the Galileo Mission 73

to the O-rings. The thinking at the time, however, was that even with a small amount 

of erosion, the joints could reliably contain combustion gases.2

The primary and secondary O-rings in the field joints constituted the main 

barriers against a loss of pressure once the booster motor fired. These rings could not 

by themselves withstand the high temperatures generated during a burn, however. 

A substance called “vacuum putty,” containing asbestos and zinc chromate, was also 

packed into the joints. Occasionally, trapped air expanded so much during the burn 

that it blew a hole through the putty. This allowed hot gases to blow past the inner, primary O-ring and attack the field joint’s outer, secondary O-ring, the last line of defense 

against a pressure leak.3

After a January 1985 Shuttle flight, one year before flight 51-L, Morton Thiokol 

engineers inspected the solid rocket motors (SRMs) and found blow-by that was much 

worse than had been thought. The January 1985 launch had been the coldest in Shuttle 

history up to that point, with 53o

F air temperatures at Kennedy Space Center. Though 

this was chilly for Florida, the air temperature during the Challenger Shuttle flight 51-L 

launch a year later was even colder—36o

F.4

27 and 28 January 1986

Flight 51-L was ready to fly on 27 January 1986, but NASA project managers and contractor support personnel (including Morton Thiokol staff) worried about the high 

crosswinds at the launch site. Shortly after noon, NASA managers scrubbed the launch 

for that day and polled project personnel, asking if there were any “constraints” against 

launching the next morning, when temperatures were predicted to be very low.5

Morton Thiokol management expressed concern about field joint performance 

at low temperatures. The company’s vice president of engineering, Robert Lund, recommended waiting until O-ring temperatures reached at least 53oF. NASA challenged 

this recommendation. George Hardy, Deputy Director of NASA’s Marshall Space Flight 

Center, stated that he was “appalled” by Morton Thiokol’s proposal to delay the launch. 

According to Al McDonald, who was Morton Thiokol’s SRM project director at the time, 

NASA had been under considerable pressure to reduce the frequency of its launch 

delays. The American press had been extremely critical of the Agency’s apparent inability to launch Shuttles according to schedule.6

In response to NASA’s challenge, Morton Thiokol upper management revisited 

the launch issue. Four of the company’s vice presidents weighed various factors, then 

decided to go against the company’s previous recommendation and approve a launch 

the next morning. They said that the SRM’s primary O-ring could erode “by a factor of 

2	 Allan J. McDonald, Morton Thiokol’s solid rocket motor project director at the time of the Challenger launch, interview,

Thiokol Propulsion, Corinne, UT, 23 August 2000.

3	 Ibid.

4	 Ibid.; Mark A. Haisler and Robert Throop, The Challenger Accident: An Analysis of the Mechanical and Administrative 

Causes of the Accident and the Redesign Process That Followed, Mechanical Engineering Department, The University

of Texas at Austin, spring 1997, http://www.me.utexas.edu/~uer/challenger/challtoc.html.

5	 Rogers, p. 104.

6	 Ibid., pp. 105–108.

three times the worst previous case” before it would fail. And even if it did fail, they 

thought that the secondary O-ring would provide a backup seal.7

When Al McDonald was informed of his management’s decision, he argued 

for scrubbing the launch and asked how NASA could rationalize launching below the 

qualification temperature. He stressed that there were three factors whose impacts were 

unknown, and they should preclude Challenger from taking off the next morning:

• What would happen to the O-rings at subfreezing temperatures?

• The ships tasked to recover Morton Thiokol’s booster rockets after the launch 

would have to contend with 30-foot seas and high winds. Could they safely 

tow the boosters to shore under these conditions?

• There was ice everywhere around the launchpad. What effect would it have 

on Space Shuttle equipment? 8 

If anything happened to Challenger, McDonald told his bosses, he would 

not want to explain it to a board of inquiry. In response, Morton Thiokol management told McDonald that his comments would be passed on in an “advisory 

capacity” but that the decision to launch was not his concern.9

Between 7:00 and 9:00 the next morning, B. K. Davis, a member of the project’s “ice crew,” measured temperatures on the rocket boosters and found some of 

them to be even lower than predicted. The right-hand booster’s aft region measured 

only 8oF. But there was no Launch Commit Criterion on booster surface temperatures, and so the ice crew did not even report its measurements.10

Photos taken the morning of the launch showed thousands of foot-long 

icicles hanging in curtains from the structures around Challenger, as well as water 

troughs frozen solid, even though antifreeze had been added to the water to 

prevent icing. The lower section of Challenger’s left booster rocket had sheet-ice 

deposits on it. At 9 a.m. on 28 January, NASA management met with project managers and contractors to discuss the icy conditions, but, according to the Rogers 

Commission report, there was “no apparent discussion of temperature effects on 

(the) O-ring seal.” Never had the Shuttle taken off under such conditions. And yet, 

NASA’s and Morton Thiokol’s decision-makers were apparently so confident that 

the Shuttle had adequate safety margins built into its design that they still opposed 

another launch delay.11

At 11:38 a.m., 28 January 1986, Challenger’s three main engines and two 

booster engines were ignited. Al McDonald, who was watching from Kennedy Space 

Center’s control room, thought that if a booster rocket O-ring blew, it would do so 

on the launchpad during the first seconds after ignition, as pressure was building 

up in the engines. Indeed, less than a second after booster rocket ignition, a puff 

7	 Ibid., pp. 107–108.

8	 Ibid., p. 109; McDonald interview, 2000.

9	 Rogers, p. 109.

10	 Ibid.

11	 Ibid., pp. 109–113.
74	 MISSION TO JUPITER: A History of the Galileo Project Chapter 4 I The Challenger Accident and Its Impact on the Galileo Mission 75

of gray smoke was seen above one of the right-hand booster’s field joints. In the 

next couple of seconds, eight more puffs of increasingly blacker smoke were observed 

near the right-hand booster rocket. The black, dense smoke was a bad sign. It indicated 

that the grease, joint insulation, and rubber O-rings in the field joint were burning. 

But then the smoke stopped, Challenger lifted from the pad, and McDonald thought 

that Challenger might be in the clear. During the reconstruction of launch events that 

occurred later, investigators thought that aluminum oxide from the booster’s fuel had 

formed a temporary ceramic seal over the leak.12 

Figure 4.1. Booster rocket breach. At 58.778 seconds into powered flight, a large flame plume 

was seen just above the right-hand booster engine exhaust nozzle. (Johnson Space Center 

image number S87-25373, 28 January 1986)

A minute later, the right-hand booster motor sprouted a flame out of its side 

(see figure 4.1). The temporary ceramic seal had perhaps held only until pressure 

inside the engine rose and blew it out. Or possibly it was the high-altitude shear 

winds through which Challenger flew 37 seconds after launch that flexed the booster 

motor enough to break the ceramic seal. The pressure inside the right-hand booster 

started to diverge from that of the left-hand booster. This was potentially dangerous 

because radically different thrusts could drive the Shuttle onto a crash trajectory. That 

didn’t happen, however. The booster engines’ nozzles, which could swivel, were able 

to compensate for the different thrusts and keep Challenger on course.13

12	 “Sequence of Major Events of the Challenger Accident,” http://science.ksc.nasa.gov/shuttle/missions/51-l/docs/

events.txt (accessed 28 August 2000); Rogers, chap. 3; McDonald interview, 2000.

13	 “Sequence of Major Events”; McDonald interview, 2000.

The flame increased in size. Air racing past the ascending Challenger deflected 

the flame plume rearward. It hit a protruding structure that attached the booster motor 

to Challenger’s large external tank, which contained liquid hydrogen and liquid oxygen. 

The protrusion directed the flame onto the surface of the external fuel tank. At 65 seconds 

after launch, the shape and color of the flame abruptly changed. It was mixing with hydrogen leaking from the Shuttle’s external fuel tank. A “bright sustained glow” developed on 

the underside of the Challenger spacecraft, between it and the external tank.14

Challenger began to skew off course. Hydrogen fuel tank pressures dropped, 

first slowly and then precipitously. Seventy-two seconds after launch, the lower strut connecting booster engine to external tank pulled away from the weakened tank, perhaps 

burned through by the flame. The booster was now free to rotate around an upper strut. 

Seventy-three seconds after launch, white vapor emerged from the external tank bottom. 

The tank then failed catastrophically, its entire aft dome dropping away and releasing 

a massive amount of hydrogen. As this was happening, the loosened booster motor 

smashed into the fuel tank’s liquid-oxygen compartment. Hydrogen and oxygen mixed, 

burned explosively, and engulfed Challenger in a fireball (see figure 4.2).15

If the booster motor field joint alone had failed, but not the Shuttle’s external fuel 

tank, the disaster might have been averted. Al McDonald was watching the launch, and he 

could see that the swiveling booster motor nozzles were compensating for the sideways 

thrust from fuel spurting out through the field joint. But the field joint failure had the bad 

fortune to occur inboard, near the external fuel tank. The spurt of flame from the booster 

was directed onto the external tank wall, eventually burning through it and liberating 

the hydrogen within. Still, if hydrogen had not mixed with oxygen, the disaster might 

not have occurred. There was no explosive burning immediately after the hydrogen leak 

was detected. The explosive burn started around the time that the booster motor crashed 

through the top of the fuel tank’s liquid-oxygen compartment, and it was this that sealed 

Challenger’s fate.16

As Challenger broke apart, shock and disbelief swept through Kennedy 

Space Center’s control room. Some people started to cry; others could barely talk. Orders were shouted out: “No one leave the room!” “Disconnect all telephones.” “Don’t 

talk to the press.” The staff members were kept in the control room for hours, until events could be sorted out. Vice President George Bush flew down to Kennedy 

to talk with staff members and head the investigation into the cause of the disaster.17

Impact of the Disaster on Galileo

After the destruction of flight 51-L, NASA suspended all Shuttle flights until it could determine the root causes of the tragedy. Within a week of the crash, the White House formed 

a Presidential Commission, led by William Rogers, that had four months to submit a report 

to the President. Galileo would not be allowed to take off on its planned May 1986 launch 

14	Rogers, chap. 3.

15	 “Sequence of Major Events”; Rogers, chap. 3.

16	McDonald interview, 2000; Rogers, chap. 3.

17	McDonald interview, 2000; Carlos Byars, “Challenger Explodes, Shuttle Falls into Ocean, Crew Apparently Killed,”

Houston Chronicle (28 January 1986): 1.
76	 MISSION TO JUPITER: A History of the Galileo Project Chapter 4 I The Challenger Accident and Its Impact on the Galileo Mission 77

date. It would have to wait at least 13 months until the next suitable launch window, if not 

longer. The delay would eliminate the possibility of flying by the Amphitrite asteroid. This 

was especially disappointing to the project team because the chances of finding another 

asteroid as large as Amphitrite (125 miles in diameter) when Galileo finally did launch 

were very small.18

Figure 4.2. Liquid-oxygen tank rupture. The luminous glow at the top is attributed to the rupture 

of the liquid-oxygen tank. Liquid oxygen and hydrogen mixed and engulfed Challenger in a fiery 

flow of escaping liquid propellant. (Johnson Space Center number S87-25390, 28 January 1986)

No firm launch date could be set until after the Rogers Commission made its 

report to President Reagan. The Galileo and other project teams wanted answers to vital 

question such as these:

• When would Shuttles be allowed to fly again? 

• How many missions would be conducted each year? 

• Which missions would be placed at the top of NASA’s priority list?

Galileo’s managers worried that safety modifications that would probably be 

made to both the Shuttle and the Centaur upper stage might significantly increase their 

weight. This would reduce their payload lift capability and could compromise their ability to send Galileo on a direct trajectory to Jupiter. In this regard, Neil Ausman, Galileo’s 

mission operations and engineering manager, said, “If we lose too much performance, 

we have no mission.”19

18	Boyce Rensberger, “Space Shuttle Explodes, Killing Crew,” Washington Post (29 January 1986); Jay Malin, “NASA

Postpones Next Three Shuttle Missions,” Washington Times (11 February 1986).

19	 “On Hold: Probes to Jupiter and the Sun,” Science (28 April 1986): 20–21.

As of April 1986, Galileo remained in a processing facility at Kennedy Space 

Center, where it was being coupled to the Centaur upper stage. NASA’s plan was to 

complete the coupling operation, then put the craft into “active storage” at Kennedy until 

the new launch date.20

Concerns About Plutonium

As NASA and the Rogers Commission struggled to sort out the details of the Challenger

tragedy, a new concern arose about missions such as Galileo that would carry plutonium 

fuel to power on-board functions. A periodical called the Nation expressed the opinion 

that “far more people would have died” if the explosion that destroyed Challenger had 

occurred during Galileo’s launch, for in that case its plutonium fuel might have rained 

down throughout the area. 

The Nation made much of a draft NASA Environmental Impact Statement that 

said that a Galileo launch would involve a very small risk of a plutonium-238 release 

because of the possibility of a Space Shuttle malfunction. In closing, the Nation article 

expressed the opinion that the indefinite suspension of the Galileo project should be 

made permanent.21

The 1981 review draft of the Environmental Impact Statement for Project 

Galileo did address the risk of a plutonium release. Statistical calculations performed 

during the preparation of the draft Environmental Impact Statement had indicated that a 

Shuttle mishap resulting in a plutonium release was very unlikely. The total probability 

of failure of a radioisotope thermal generator, or RTG (the plutonium-bearing source of 

on-board power), resulting from an unsuccessful Shuttle launch was estimated at one 

in ten thousand. But even in the event of a plutonium release, it was not considered 

likely that “a person would accumulate enough plutonium to exceed current radiation 

exposure standards.” The risks of human fatality or injury from “accidental reentry of 

Galileo payload components” were estimated at one in a million and one in ten thousand, 

respectively.22

In spite of the low estimated probability of health damage from a plutonium 

release, NASA requested that JPL study, for a second time, the armoring of Galileo’s RTG. 

In a previous study, JPL had conducted shock tests on the RTG containment capsule, 

subjecting it to a pressure of 2,000 pounds per square inch (psi) without a failure in the 

capsule. These were pressures three times higher than those to which the RTG might be 

subjected during an explosion on the launchpad, according to a risk analysis. Because of 

such analyses, NASA had considered the RTG to be safe. Armoring, which would have 

added several thousand pounds to Galileo’s launch weight, had been rejected. But this 

decision had been made before the Challenger explosion.23

NASA now had reason to revisit its decision. Another of its statistical studies 

had estimated that the chance of losing the Shuttle and its crew was quite low, between 

20	 Ibid.

21	 “The Lethal Shuttle,” Nation (22 February 1986): 16.

22	R. W. Campbell, Environmental Impact Statement for Project Galileo, Review Draft, JPL, 24 July 1981.

23	Michael A. Dornheim, “Galileo Probe Managers Review Jupiter Mission Launch Options,” Aviation Week & Space 

Technology (3 March 1986): 20.
78	 MISSION TO JUPITER: A History of the Galileo Project Chapter 4 I The Challenger Accident and Its Impact on the Galileo Mission 79

one in one thousand and one in ten thousand. Since just such a tragedy had occurred on 

only the 25th Shuttle flight, NASA had little confidence in its previous statistical study and 

asked JPL to investigate the armoring issue again.24

Representative Edward Markey (D-Massachusetts) also requested a study, asking 

that the Department of Energy (DOE), which had built Galileo’s plutonium-bearing RTGs, 

estimate the impacts of the most probable Shuttle launch accident involving plutonium 

releases. Department of Energy analysts estimated that such an occurrence might result 

in over 200 cancer deaths and contaminate almost 370 square miles of land. The cancer 

estimate was based, however, on the somewhat unlikely case that people would remain 

in the contaminated launch area for a year with no decontamination effort and no protective actions conducted. Representative Markey, whose House Energy and Commerce 

energy conservation subcommittee was investigating the risks posed by Shuttles carrying 

radioactive materials, advised that no such missions be launched until public health and 

safety could be assured.25

New Launch Scenarios

By April 1986, NASA planners had come up with likely scenarios for resuming Shuttle 

flights. One possible schedule called for a 12-month “standdown,” with the first postChallenger flight taking off in February 1987, followed by only three more flights that 

year. Another schedule delayed the first post-Challenger launch until July 1987, followed 

by three more flights in 1987. The critical factor, Shuttle experts believed, was the length 

of time it would take to implement reliable design changes in the Shuttle’s solid-fueled 

rocket boosters. In the 18-month delay scenario, the spacecraft that would launch in July 

1987 would carry a Shuttle tracking and communications satellite to replace the one lost 

with Challenger, as well as an experiment to develop heat-rejection techniques for use in 

NASA’s planned space station. The Shuttles that would launch in September and October 

1987 would carry military surveillance experiments and classified cargo. Galileo would 

launch on 1 December 1987 aboard the Space Shuttle Atlantis.

26

This schedule was by no means final. Galileo was in competition with other 

missions for space on the Shuttle. National security was the first priority in assigning 

launch dates, and DOD had many payloads that it wanted to send into space. The next 

priority in line after defense, according to Chester Lee, NASA’s director of customer services, was to complete the satellite relay system needed for the space telescope. NASA 

was also deeply committed, after it recovered from the Challenger disaster, to pushing 

toward its long-term goal of building and running a space station, which would require 

many Shuttle missions.27

24	 Ibid.

25	 “Launch Hazards of Atomic Satellites Studied,” Washington Post (19 May 1986): A11.

26	 “Tentative Flight Plan Is Drafted for Shuttle,” New York Times (14 April 1986).

27	Douglas B. Feaver, “The Spreading Ripples of Challenger Disaster—Scientific Ventures Losing Momentum,”

Washington Post (16 June 1986).

Implementing the Rogers Commission Report

The Presidential Commission on the Space Shuttle Challenger Accident, led by William 

Rogers, issued its report on 6 June 1986. Much of the blame for the disaster was placed on 

NASA’s seriously flawed decision-making process. A better system would have “flagged 

the rising doubts about the Solid Rocket Booster joint seal” and the strong opinions

expressed by most of the Morton Thiokol engineers, and, in all likelihood, the launch 

would have been delayed under such a system. The Rogers Commission also sharply 

criticized the “propensity of management at Marshall [Space Flight Center] to contain 

potentially serious problems and to attempt to resolve them internally rather than communicate them forward.” This too contributed to the disaster. Although it was not revealed 

what Marshall Space Flight Center Shuttle managers said to Morton Thiokol managers on 

the night of 27 January, the Rogers Commission concluded that Morton Thiokol managers 

reversed their position in favor of delaying the launch and overrode the strong recommendations of their solid rocket motor project director because they wanted to keep a 

very major client happy.28

On 13 June 1986, President Reagan directed NASA to implement, as soon as possible, the Rogers Commission’s nine recommendations for returning the Space Shuttle to 

safe flight status. NASA was to report in 30 days or less how it would implement those recommendations; it would also and give milestones to measure progress. At the top of the list 

of recommendations was to reengineer the SRM design. Other recommendations targeted 

the Shuttle‘s management structure and procedures for communication among all levels of 

management. Also targeted were methods of hazard analysis and safety assurance.29

NASA’s view was that it would take at least until early 1988 to meet the Rogers 

Commission recommendations and prepare the Shuttle to fly again. Galileo would probably have to wait longer, depending on where it ended up on the flight priority list. It 

would have to compete with military payloads and high-priority civilian missions such as 

the Hubble Space Telescope and the Ulysses project to orbit and study the Sun.30

Centaur Is Canceled

On 19 June 1986, NASA Administrator James Fletcher delivered a huge blow to American 

space science and to Galileo in particular by canceling the $1-billion Shuttle/Centaur 

program. This decision ended the development of the hydrogen-powered satellite 

booster that was supposed to send Galileo and other spacecraft from low-Earth orbit 

into a planetary trajectory. The venerable Centaur, with its track record of reliability, 

was deemed unsafe for use in the Shuttle due to concerns regarding its volatile, highly 

flammable fuel. The final decision to cancel Centaur use on board the Shuttle was 

made on the basis that even implementing the modifications identified by the ongoing 

28	Rogers, chap. 5.

29	NASA’s Actions to Implement the Rogers Commission Recommendations After the Challenger Accident, http://

www.hq.nasa.gov/office/pao/History/actions.html (accessed 28 August 2000). Excerpted from executive summary

of Actions to Implement the Recommendations of the Presidential Commission on the Space Shuttle Challenger 

Accident (Washington, DC: NASA, 14 July 1986).

30	NASA’s Actions to Implement the Rogers Commission Recommendations; Kathy Sawyer, “Quickness, Safety Again

Conflict as NASA Builds New Shuttle,” Washington Post (29 November 1986): A3.
80	 MISSION TO JUPITER: A History of the Galileo Project Chapter 4 I The Challenger Accident and Its Impact on the Galileo Mission 81

reviews, Centaur would still not meet the safety criteria being applied to other cargo on 

the Space Shuttle.31

It is ironic that Representative Edward Boland, who, just a few short years before, 

had argued so vociferously for using Centaur in NASA’s planetary exploration program, 

now was one of the agents of its demise. An independent study conducted by the staff of 

Boland’s House Appropriations subcommittee led congressional leaders to pressure NASA 

to terminate the Shuttle/Centaur program.32

A major concern was the ability to jettison Centaur’s 22 tons of highly volatile fuel 

in the event of an emergency. If the Shuttle and its astronauts had to abort the mission and 

return to Earth for a forced landing with Centaur still in the cargo bay, could Centaur’s liquid hydrogen and oxygen be dumped quickly and safely? In the months before Challenger, 

NASA engineers had reported trouble with the spacecraft’s “dump valve,” and this raised 

serious concerns about carrying Centaur inside the Shuttle.33

There were other, less obvious reasons for canceling the Centaur program. As 

Dawson and Bowles point out in their book on the Centaur, NASA management realized 

that returning the Space Shuttle to flight would require a huge expenditure of money 

and personnel. The Agency did not think there would be sufficient resources both to 

accomplish this and to solve the remaining technical issues of the Centaur. Another reason for the cancellation was the lower tolerance for risk that many NASA personnel and 

the public had after the Challenger tragedy. Astronauts especially opposed flying with the 

hydrogen-filled Centaur inside the Shuttle’s cargo hold, considering it too hazardous. 

Before Challenger, the risks associated with a Shuttle/Centaur were known but 

considered acceptable in the face of the benefits to be derived from successful missions. 

These benefits included an enhanced U.S. capability to explore and exploit space for 

both civilian and national security purposes. The Shuttle/Centaur design had initially been

accepted based on extensive technical arguments, safety features, and social benefits. But 

after the devastating loss of life resulting from the Challenger explosion, strong emotional 

factors came into play as well. The result was “a new era of conservatism and a diminished acceptance of risk.”34

JPL’s Alternative Launch System Concepts

Jet Propulsion Laboratory staff had anticipated that Centaur might be scrapped for use in 

the Shuttle and had already initiated a series of alternative launch plans for Galileo, as 

well as for Ulysses, which would orbit and study the Sun, and Magellan, which would 

map Venus. The alternative plans included the following elements: 

Enlarge NASA’s Inertial Upper Stage. NASA’s Inertial Upper Stage (IUS), a solidfueled rocket that the Agency had developed for use with the Shuttle in launching 

31	NASA’s Actions to Implement the Rogers Commission Recommendations.

32	John Noble Wilford, “NASA Drops Plan to Launch Rocket from the Shuttle,” New York Times (20 June 1986): A1.

33 “Centaur Program Canceled by NASA Over Safety Issues,” Washington Times (20 June 1986): 3A; Wilford, “NASA

Drops Plan.”

34	Virginia P. Dawson and Mark D. Bowles, Taming Liquid Hydrogen: The Centaur Upper Stage Rocket, 1958–2002

(Washington, DC: NASA SP-2004-4230, 2004), pp. 215–219.

communications satellites, could be made powerful enough to launch planetary craft 

such as Galileo from the Shuttle.35

Rely on an Expendable Lower Stage. Planetary spacecraft could be launched 

using an expendable lower stage such as Titan 34D-7 in place of the Shuttle, with either a 

Centaur upper stage or the enlarged IUS.36 The Titan 34D-7 was a more massive version of 

the expendable launch vehicle that had successfully sent Viking and Voyager into space. 

Unfortunately, the 34D-7 was still under development in 1986 and was not expected to 

be ready until at least January 1991. The Titan 34D-7 alternative was also an expensive 

one. JPL estimated that building three of the vehicles to launch the Galileo, Ulysses, and 

Magellan missions, plus adapting the spacecraft to the new launch system, would cost up 

to $1.2 billion.37

Modify Orbital Science Corporation’s Upper Stage. Orbital Science Corporation was a 

company that had developed a solid-fueled rocket engine for commercial applications. JPL 

considered developing a more powerful version of it and implementing it as the upper stage 

in planetary missions. It could be used with either a Titan or Shuttle launch vehicle.38

Add Propulsion Stages to NASA’s Existing IUS. Rather than design and develop a 

more massive IUS vehicle, NASA could also achieve the additional power needed to send 

large spacecraft such as Galileo into planetary trajectories by adding two propulsion stages 

to the existing IUS. The combination vehicle could be launched from either a Titan or a 

Shuttle lower stage.39

Waiting for a New Launch Date

While waiting for a new launch date, the Galileo project team continued with spacecraft 

testing and integration procedures at Kennedy Space Center. The spacecraft’s software 

was verified, and an end-to-end data-flow analysis was conducted. RTGs were mated 

to Galileo. The Orbiter received a thorough electrical system exam. Several of Galileo’s 

spare subsystems had been transferred to the Magellan spacecraft, and new ones were 

fabricated. They included Galileo’s spare attitude and articulation control, as well as its 

power subsystem and command and data subsystem. Galileo’s retropropulsion module 

oxidizer tanks had been filled for the May 1986 launch and needed to be drained and 

decontaminated. Safety analyses also continued. Shields were fabricated for additional 

protection of the RTGs, in the event that this should be required. The Probe was sent to 

Hughes, where it had been built, for an upgrade of its scientific instruments.40

35	J. R. Casani, “From the Project Manager,” Galileo Messenger (July 1986); Wilford, “NASA Drops Plan”; “Centaur

Program Canceled.”

36	Centaur was only banned for use in the Shuttle, because of the danger to its crew. It was not banned for use with

robotic launch vehicles such as Titan or with robotic spacecraft such as Galileo.

37	Wilford, “NASA Drops Plan”; “Centaur Program Canceled.”

38	Wilford, “NASA Drops Plan.”

39	 Ibid.

40	Casani, “From the Project Manager,” Galileo Messenger (July 1986).
82	 MISSION TO JUPITER: A History of the Galileo Project Chapter 4 I The Challenger Accident and Its Impact on the Galileo Mission 83

The Galileo team hoped that during the fall of 1986, a new launch date would 

be selected. 

Selecting a New Trajectory

Without Centaur, a direct flight to Jupiter was impossible. The available solid-fueled 

upper stage options simply weren’t powerful enough to propel Galileo on a direct 

Earth-Jupiter trajectory. Gravity-assists were required for the spacecraft to attain a 

trajectory that would get it to Jupiter.41

Because of the need for gravity-assists, JPL analyzed new trajectory options 

in conjunction with its investigation of launch vehicle alternatives. Using planetary 

gravity fields to adjust Galileo’s velocity reduced on-board fuel requirements. But 

gravity-assists also resulted in longer travel times because the trajectory to Jupiter 

would be much less direct and would cover many more kilometers. Parts would age, 

and on-board power systems would be depleted more than initially estimated. Also, 

some trajectories would bring Galileo closer to the Sun, resulting in additional thermal stresses. The aim was to balance the benefits of various gravity-assist options 

against their deficits and select the trajectory–launch vehicle combination that would 

result in the highest possible scientific return.42

One of the promising trajectory strategies that NASA considered was termed 

∆V-EGA (and was discussed in chapter 3). It made use of an Earth gravity-assist 

(EGA) to change velocity and had been successfully employed in sending the Voyager 

spacecraft to the outer planets. If Galileo used ∆V-EGA and launched in December 

1989, one of the “windows” for the trajectory, it would take almost five years to reach 

Jupiter. If Centaur could have been used in conjunction with the Shuttle, Galileo could 

have reached Jupiter in only two years.43

Two discriminating factors in selecting the best trajectory–launch vehicle 

combination for Galileo were the “injection energy” needed and the “interplanetary 

∆V” required. Injection energy is the amount of kinetic energy a spacecraft will eventually have when it escapes Earth’s gravity field onto its interplanetary trajectory.44 A 

spacecraft’s injection energy needs to be supplied by its launch vehicle. The choice of 

interplanetary trajectory for Galileo was thus constrained by the injection energy that 

its launch vehicle could provide.45

Galileo’s “interplanetary ∆V requirements” were the changes in velocity it would 

need for trajectory-shaping maneuvers between Earth and Jupiter. The changes could not 

be so large that overall propellant requirements for the mission exceeded the capacity of 

Galileo’s on-board propulsion system, which contained 925 kilograms of propellant. This 

fuel would have to be used not only for ∆V maneuvers during the long flight to Jupiter, 

but also for the spacecraft’s attitude control, trajectory corrections, insertion into a Jupiter 

41	 J. R. Casani, Galileo Messenger (December 1986).

42	 Ibid.

43	Casani, “From the Project Manager,” Galileo Messenger (July 1986).

44	A more technical definition: a spacecraft’s injection energy is measured in terms of the square of its asymptotic speed

with respect to Earth on its departure hyperbola. This is excerpted from Louis D’Amario, Larry Bright, and Aron Wolf,

“Galileo Trajectory Design,” Space Science Reviews 60 (1992): 25.

45	 Louis A. D’Amario, Larry E. Bright, and Aron A. Wolf, “Galileo Trajectory Design,” Space Science Reviews 60, nos. 1–4 (1992): 25.

orbit, and tour of the planet’s satellites. The propellant needed for these other maneuvers 

was significant and severely restricted the amount of fuel available for interplanetary ∆V 

requirements. This constraint eliminated all but one of the trajectories that JPL considered for Galileo. Given the spacecraft’s propellant capacity, only the Venus-Earth-Earth 

(VEEGA) trajectory would work.46

In this trajectory, Galileo would swing toward the Sun after leaving 

Earth’s gravitational field. A launch in late 1989 (which Galileo staff were using as a 

target date even though NASA had not made its final decision) would be followed by a 

flyby of Venus about three and a half months later. Gravitational forces experienced 

during this and other flybys would alter Galileo’s velocity without the need for burning 

more fuel (see figure 4.3). Assisted by Venus’ gravity, Galileo would head back toward 

Earth, flying by it approximately 10 months later.47

Galileo’s first Earth flyby would send the craft on a two-year elliptical orbit 

around the Sun. Midway through this orbit, Galileo’s engine would fire and use 

much of the spacecraft’s ∆V fuel allotment for trajectory shaping. At the end of the 

two-year orbit, Galileo would once again intercept Earth, performing its second and 

last flyby. This time, interaction with Earth’s gravity would swing Galileo in a new 

direction, toward Jupiter. The total flight time from launch to arrival at Jupiter would 

be about six years.48

The new trajectory introduced new problems. Galileo had originally been 

designed to operate at distances between 1 and 5 Astronomical Units (AU)49 from the 

Sun. Within this range, Galileo’s temperature-control systems would be able to keep 

the spacecraft’s instruments and experiments from getting too hot or too cold. But 

on the VEEGA trajectory, Galileo would travel as close as about 0.7 AU from the Sun, 

encountering twice the solar radiation intensity as that found near Earth.50 VEEGA 

posed a severe thermal threat to the integrity of the spacecraft. To protect Galileo, 

blanket material and thermal shades and shields would have to be added.51 Changes 

to Galileo’s power management system would also be needed. Due to the extended 

trip time to Jupiter (roughly six years, versus the two years the trip would have 

taken if the Centaur upper stage had been used), the plutonium in the craft’s RTGs 

would decay more than had originally been planned, thus reducing the generators’ 

power output. 

The May 1986 launch plan had included a chance to fly close to the asteroid 

Amphitrite. The later launch date would not make that possible, but the VEEGA trajectory would carry Galileo through the asteroid belt twice, and the mission might be 

able to target a new asteroid.

46	 Ibid., p. 25.

47	Casani, “From the Project Manager,” Galileo Messenger (December 1986).

48	 Ibid.

49	One AU is the distance from Earth to the Sun, 150 million kilometers (93 million miles).

50	Solar radiation intensity is proportional to the inverse square of the distance to the Sun.

51	Casani, “From the Project Manager,” Galileo Messenger (December 1986).
84	 MISSION TO JUPITER: A History of the Galileo Project Chapter 4 I The Challenger Accident and Its Impact on the Galileo Mission 85

Figure 4.3. The VEEGA trajectory. Galileo saved fuel by getting gravity-assists from Venus and 

Earth. (Image number P-48182)

VEEGA was a very fuel-conservative trajectory. JPL staff calculated that after 

Galileo flew to Jupiter and completed all mission objectives, the craft would still have 80 

kilograms (176 pounds) of propellant aboard. This surplus fuel would prove to be enormously important during the actual mission.52

Making the Shuttle Safe To Fly: The Need for New Management 

One major step in getting the Shuttle safely back into space was to examine the management structures that had led to the Challenger disaster. The presidential investigating 

commission headed by William Rogers found that NASA managers at Marshall Space 

Flight Center had “repeatedly ignored warnings about the potential for the booster rupture 

that destroyed Challenger.” Marshall had isolated itself from the rest of NASA’s management structure, according to the panel. This attitude was nothing new. Marshall’s leaders, 

beginning after World War II with German rocket pioneer Wernher von Braun, had built 

a proud, close-to-the-vest tradition of doing business. For decades, Marshall’s attitude had 

52	 Ibid.

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been for Washington to give it a job and then go away. “Don’t worry,” they would say, 

“we’ll do an excellent job.” Such an approach was no longer acceptable.53

In the wake of Challenger, NASA Headquarters insisted that Marshall cease its 

insular tradition of project management. To ensure that this happened, NASA appointed 

James R. Thompson, the former head of the Shuttle’s main engine development program, 

as Marshall’s new Director. Thompson had spent 20 years at Marshall, shepherding new 

engine designs through years of frustrating explosions on test stands, delays, and cost 

overruns, eventually producing the world’s most advanced and powerful liquid-fueled 

rocket—the Space Shuttle, which first took flight in 1981.54

Thompson was made Marshall’s Director in part because he knew how to turn 

failures into successes, but also because he was “untainted” by the Challenger tragedy. 

He had left Marshall in 1983, taking a position at the Princeton Plasma Physics Laboratory. Thompson had also aided NASA in its internal investigation of the crash, serving 

as vice chairman of the task force and supervising day-to-day collection and analysis of 

accident data. As the new Director of Marshall, Thompson’s plans were to open up lines 

of communication among various management levels and return to the rigorous system 

of checks and balances that he felt had been missing over the last few years. Thompson 

saw the need for aggressive, top-down probing of NASA technical centers, programs, and

individuals. He also vowed to “beef up ground testing of high-risk hardware, and pay for 

it out of other programs if necessary.” Thompson appeared to be perfect for the directorship of Marshall Space Flight Center. He had not been involved in the mistakes leading 

to the Challenger disaster, and he was familiar enough with Marshall operations to implement a new Shuttle management system.55

Another action aimed at breaking the “parochialism” and autonomous decisionmaking of Marshall, as well as in other technical centers, was to shift overall authority 

for the Shuttle program from Houston to Washington. Establishing strong direction from 

NASA Headquarters made the Shuttle program’s new management approach more like 

the highly respected approach of the Apollo Moon landing program. NASA appointed

Arnold Aldrich director of the Shuttle program and gave him “full responsibility and 

authority” to run it. Now the Shuttle program was in the hands of someone whose only 

job was to lead the program. In appointing Aldrich, NASA gave him many of the responsibilities of NASA Associate Administrator for Space Flight Richard Truly. The Associate 

Administrator had many other issues on his plate besides the Shuttle and supported this 

management change, stating that the Shuttle program “deserves a full-time leader.”56

The Government’s Role in the Disaster

Initial probes of the Challenger disaster placed the blame largely on NASA and Morton 

Thiokol management. But lingering questions remained as to the role other parts of the 

U.S. government played in pressuring NASA to end its long string of delayed launches 

and get Challenger up into space. On 7 October 1986, the House Science and Technology 

53	Kathy Sawyer, “Old Hand Takes Up a Mandate to Break Space-Center Tradition,” Washington Post, The Federal Page

(30 September 1986): A13.

54	 Ibid.

55	 Ibid.

56	Kathy Sawyer, “Shuttle Management Shifted from Houston to Washington,” Washington Post (6 November 1986): A3.
86	 MISSION TO JUPITER: A History of the Galileo Project Chapter 4 I The Challenger Accident and Its Impact on the Galileo Mission 87

Committee issued a report that placed a share of the blame on both the White House 

and Congress for launch schedule pressures that led to the tragedy. The government 

had pushed NASA too hard to meet schedules, while at the same time starving the 

Agency of sufficient resources. The White House, with the support of Congress, had 

directed NASA to “achieve a launch schedule of 24 flights per year in order to make its 

costs competitive internationally.”57

The House committee report expressed concern that NASA managers still had 

not figured out how a fatal hardware flaw had been missed in the Agency’s elaborate 

testing procedures. The House committee questioned whether NASA’s level of technical 

expertise was adequate for the decisions it had to make. How, the House committee 

asked, would NASA be able to protect against similar breakdowns in its checks-andbalances system in the future? 58

Although the House committee generally concurred with the Rogers Commission 

findings, it reached a different conclusion as to the root cause of the disaster. The Rogers 

Commission thought that the cause was poor communication and inadequate quality-assurance procedures, while the House committee felt that the crash had been due to “poor technical decision-making over a period of several years by top NASA and contractor personnel.”59

Getting the Shuttle Back into Space

By the fall of 1986, NASA had set its sights on launching the first post-Challenger Shuttle 

in February 1988. Al McDonald of Morton Thiokol and other engineers thought that this 

goal was highly “success-oriented,” meaning that the only way it could be met was if no 

major equipment tests ended in failure and no major design changes were needed. Only 

an uninterrupted string of successes in the reengineering of the Shuttle launch system 

would allow a safe February 1988 launch. By March 1987, NASA and Morton Thiokol 

would need to have completed 90 percent of the solid rocket booster redesign, with 

ground tests continuing until the end of 1987.60

Some NASA watchdog groups were uneasy about the importance the Agency 

placed on meeting its new schedule versus observing safety considerations. The House 

Science and Technology Committee reported that pressure for an “unrealistic number of 

flights” continued to exist within NASA, a condition that jeopardized a “safety first” attitude.61

Members of the President’s Rogers Commission were angered to learn that NASA 

was proceeding with a redesign strategy chosen largely because it could save time in getting the Shuttle back into space. NASA’s strategy involved using 72 steel booster rocket 

casings that had been ordered back in 1985. Only one company, the Ladish Company in 

Cudahy, Wisconsin, was equipped to forge these large pieces of hardware, and Ladish 

needed one to two years to manufacture and deliver them. A new casing design would 

involve retooling costs and long delays.62

57	Kathy Sawyer, “New Challenger Report Faults Hill, White House,” Washington Post (8 October 1986): A12.

58	 Ibid.

59	 Ibid.

60	Kathy Sawyer, “Quickness, Safety Again Conflict as NASA Builds New Shuttle,” Washington Post (29 November 1986): A3.

61	Kathy Sawyer, “Redesign of Shuttle Combines Pressures for Care and Urgency,” Washington Post (10 November

1986): A1; Sawyer, “Quickness, Safety.”

62	Sawyer, “Redesign of Shuttle.”

NASA defended its decision to use existing designs of key hardware by saying 

that it was better to stick with the known in this case because radical changes would be 

complex; would require extensive analysis; and, most important, might introduce new, 

unforeseen risks along with their advantages. Morton Thiokol managers strongly supported using the current field joint design, saying that even though it failed, it did many 

things well, and with certain modifications, it would be as good as it needed to be. But 

the Rogers Commission thought that NASA should have more seriously considered new 

ways of thinking. A radical redesign of the booster rockets might have produced a safer, 

more reliable spacecraft in the long run.63

Disturbing opinions on the reliability of NASA’s new designs were voiced 

by some within the Agency itself. Marshall engineer Frank Ledbetter thought that if a 

redesigned joint near the booster rocket exhaust nozzle was not further altered, it carried 

a “potential for disaster.” John Thomas, director of the redesign effort at Marshall, summarized some of his engineers’ concerns as, “If you look at the numbers over 50 to 100 

flights, the possibility [of failure] may be higher than one would want to accept.”64

The Rogers Commission was seriously bothered by what NASA considered to 

be sufficient safety factors and backup systems to minimize the chances of technology 

failures, and it worried that NASA would continue to sacrifice safety to schedule and 

budgetary pressures. The Commission recommended that NASA’s Administrator request 

the National Research Council to form an independent solid rocket motor design oversight committee to implement the Commission’s design recommendations and oversee 

the design effort. After this oversight committee was formed, one of its criticisms was of 

Marshall’s review and approval structure for new designs. The same office that approved 

new designs also set the testing and analysis requirements for them and had the power to 

waive those requirements if the hardware failed to meet them. The oversight committee 

strongly urged NASA to separate these conflicting responsibilities.65

Once the Shuttle did launch, DOD estimated that it would have a backlog 

of about 20 military payloads waiting to go up. The first major science payload, the 

Hubble Space Telescope, was scheduled for the fifth post-Challenger flight, which was 

then planned for November 1988. If that date was met, this mission would be the first 

science-oriented flight in about three years.66

Al McDonald, one of the Morton Thiokol engineers who had tried to stop the 

Challenger launch, now led the task force charged with fixing the flawed booster rocket 

design. He was one of the many NASA and contractor staff walking the line between 

implementing better, safer designs and trying to meet mission schedule pressures. Two 

questions that McDonald had to ask constantly were 1) what aspects of the booster rocket 

design still need to be reexamined and possibly modified and 2) when have we done 

enough analysis and redesign? 67

The Morton Thiokol task force, along with the NASA managers overseeing it, were 

taking a high-stakes gamble on the field joint redesign. Rather than start from scratch and 

63	 Ibid.

64	Kathy Sawyer, “Launch Delays Darken Future of Space Science,” Washington Post (26 January 1987): A1; Sawyer,

“Quickness, Safety.”

65	Rogers, “Recommendations of the Presidential Commission”; Sawyer, “Quickness, Safety.”

66	Sawyer, “Quickness, Safety.”

67	Sawyer, “Redesign.”
88	 MISSION TO JUPITER: A History of the Galileo Project Chapter 4 I The Challenger Accident and Its Impact on the Galileo Mission 89

design a new type of field joint, they were betting that a complex modification of the old 

design that had failed on Challenger would now work. If their modified field joint ran 

into problems during ground testing, the task force would have to begin again, grounding the Shuttle for an additional one to two years and calling NASA’s engineering judgment and credibility into question once again.68

Morton Thiokol was working on its redesign under the critical eyes of watchdog 

panels, business competitors, and reporters. Top NASA officials said that they would 

not “be stampeded by schedule pressures,”69 which had been cited as a factor behind 

Challenger’s destruction. Nevertheless, pressures to get the Shuttle back into space 

weighed heavily in every design decision. It was ironic that the Shuttle was in a situation 

that NASA had always labored to avoid—grounded because of a single-point failure. 

NASA had a well-known policy of developing a backup technology for every missioncritical system. But it had built no backup for its flawed booster rocket field joints, and 

thus the Shuttle could not fly until a reliable joint was developed. In hindsight, NASA 

could also have developed backup launch vehicles such as expendable Titan stages, to 

be used in the event that the Shuttle ran into problems. Since the Agency had not done 

this, U.S. solar system exploration was now in the position of near-total dependence on 

the Shuttle. In the words of Torrence Johnson, Galileo project scientist, “We ain’t got no 

rockets! It’s humiliating.”70

NASA critics urged that one or more alternative field joint designs be developed, to be used in the event that the chosen joint design exhibited major problems 

during testing. Such a strategy would enable the Agency to better meet its mission commitments in the future.71

Business rivals of Morton Thiokol were in fact drawing up alternative joint designs 

for NASA’s consideration. But the enormous expense involved in bringing an alternative 

joint design from conception to production, an estimated $1 billion, would probably prevent 

NASA from funding such an effort—unless, of course, Thiokol’s redesigned joint did not 

work. To determine the joint’s reliability with a high degree of confidence, the National 

Research Council panel urged NASA to implement a greatly expanded ground-testing 

program. NASA also needed to ensure that safety requirements would not be compromised 

in order to accommodate unexpected test results.72

NASA and Morton Thiokol added several tests to their field joint program and 

were considering adding more, but a schedule deadline was fast approaching that would 

force them to decide how much testing was enough. In order to meet the February 1988 

target date for the first post-Challenger launch, flight hardware would have to be ordered 

before the ground-test program could be completed. Was the Agency’s faith in the new 

design sufficient to commit funds and order the hardware, or was it better to delay the 

launch until a higher degree of confidence was achieved?73

Ground testing was performed in the hills of Morton Thiokol’s rocket-manufacturing 

site, in a new facility called the Joint Environment Simulator (see figure 4.4). The simulator 

68	 Ibid.

69	 Ibid.

70	Sawyer, “Launch Delays Darken Future.”

71	Sawyer, “Redesign of Shuttle.”

72	 Ibid.

73	 Ibid.

duplicated a range of launch conditions, such as intense temperatures and pressures, 

using life-sized segments of the booster rocket. Powerful jacks on the simulator wall 

applied side stresses to the field joints while sleeves reaching out from the walls blew hot 

or cold air on the joints to simulate a range of weather conditions. During the test firing, 

150 channels of data were telemetered to nearby engineers.74

Figure 4.4. Joint Environment Simulator test. Booster motor sections needed to be rigorously 

tested under conditions approximating actual use. (Image number 8772121)

Near the simulator was Morton Thiokol’s full-scale test stand, where complete 

rocket motors, rather than just segments, were strapped down horizontally, fired, and 

analyzed. NASA had concluded that the horizontal test position of the motors better 

simulated the stresses experienced during launch. For safety, the motors were nosed up 

against 5,000 cubic yards of concrete and steel to make sure that they did not go anywhere when fired. 

In previous trials, the 2-minute “hot-fires” of booster motors had blackened 

slopes adjacent to the test stand for a distance of 500 yards. NASA had decided to build a 

second, more advanced horizontal test stand, fitted with upgraded monitoring and simulation equipment and costing $19 million. This second stand would be able to more closely 

duplicate actual launch stresses when the booster rockets were connected to the rest of 

the Shuttle stack (which included the large external hydrogen-oxygen fuel tank and the 

Shuttle spacecraft).75

By January 1987, it became apparent to the National Research Council oversight 

committee that NASA would not have time to determine and incorporate all the substantial 

74	 Ibid.

75	 Ibid.
90	 MISSION TO JUPITER: A History of the Galileo Project Chapter 4 I The Challenger Accident and Its Impact on the Galileo Mission 91

design changes that might be needed in safety-critical items in time to meet NASA’s 

February 1988 target for the first post-Challenger flight. Although the panel praised NASA’s 

reviews and critiques of hundreds of items and procedures that could lead to another 

disaster, it felt that the Agency needed to base its reviews not only on the judgment of its 

experienced engineers, but also on quantitative risk analysis techniques. To accomplish 

this, the panel recommended that NASA rank mission-critical items on their likelihood of 

failure and evaluate Shuttle redesign activities in the light of this ranking. NASA needed to 

use such a risk-related analysis in order to determine the adequacy of the changes it was 

making to Shuttle hardware, software, and operational procedures.76

Days after making the above report, the National Research Council panel found 

additional reason to doubt that the Shuttle could be safely launched by February 1988. 

NASA had been on such a tight schedule to make the launch date that it could do so only 

if the results of each major equipment test were “expected and understood.” There was 

little room in the schedule for modifying designs due to unexpected test outcomes. But 

unexpected outcomes were exactly what occurred in several tests. In one, the “aft skirt,” 

a cone-shaped covering at the base of the booster rocket, cracked under stress sooner 

than predicted by the original 1970s stress analysis. This result called into question how 

reliable the stress analyses were for the rest of the Shuttle. In particular, the panel worried about stresses on the struts attaching the booster rockets to the Shuttle’s external fuel 

tank. During the Challenger mission, a snapped strut had been involved in the series of 

events leading to the disaster.77

In another ground test, the booster’s new O-rings, which had been designed 

to better withstand cold weather, deteriorated after extended exposure to rust-inhibiting 

grease. As a result, engineers decided to use the old O-rings and to install heaters in the 

field joints to keep the O-rings warm and resilient. But could this new system be finished 

and adequately tested by the February 1988 launch date? The panel noted that the heaters 

might adversely affect nearby adhesive bonds, which were known to weaken at elevated 

temperatures, and suggested that engineers instead try using different greases with the 

new O-rings.78

The panel also questioned the integrity of the booster motor’s rear joint, where 

the rocket case met the exhaust nozzle. It bothered the panel that NASA was not developing a backup for the rear joint, which was critical for Shuttle safety. Few tests had been 

planned for the joint, and they would occur late in the test program. The panel believed 

that new designs would eventually have to be developed, not only for the rear joint, but 

for the entire booster. “The nation’s manned space flight program,” the panel said, “cannot afford to rely on older technology indefinitely into the future.”79 The panel thought 

that NASA should actively begin the design of future generations of rocket motors.

The February 1988 launch date grew increasingly doubtful, but it was not out 

of the question until new manufacturing mistakes were discovered. Morton Thiokol

had applied insulation materials to the inside of the booster in a sloppy manner. As 

a result, there was a two-month delay in the first full-scale firing test of the rocket. 

NASA management was furious. J. R. Thompson, the new Director of Marshall Space Flight 

76	Kathy Sawyer, “Panel Questions Shuttle Launch Plans,” Washington Post (14 January 1987): A8.

77	Kathy Sawyer, “February ’88 Shuttle Launch in Doubt,” Washington Post (16 January 1987): A4.

78	 Ibid.

79	 Ibid.

Center, publicly berated Morton Thiokol. Although NASA did not immediately announce 

a change in launch date, the Agency switched its budget schedule to support an August 

1988 return to space for the Shuttle, a six-month delay. The booster redesign team, in the 

view of some, had been working too hard and too fast. They were overburdened. A launch 

delay was definitely necessary.80

By the time the Space Shuttle was ready to fly again, approximately 200 safetyoriented changes had been implemented into its design. These included changes not only 

to its solid rocket motor, but also to other parts of the spacecraft. Modifications to NASA’s 

system of managing the Shuttle program had also been made. Many of the major changes 

to the Shuttle program and spacecraft are listed in table 4.1. 

Several of the changes addressed the number-one “necessary corrective action” 

on the Rogers Commission list, which stated that “the faulty Solid Rocket Motor joint and 

seal must be changed” and that the joint must be insensitive to environmental effects, 

flight loads, assembly procedures, and other conditions. Toward this end, the Solid Rocket 

Motor field joint was totally redesigned and included an additional O-ring as a thermal 

barrier, a heater for cold weather conditions, and a pressure-actuated J-seal considered 

far more fail-safe than the putty used in the old design. Case-to-nozzle joint modifications 

similar to those in the field joint were also implemented.81

Table 4.1. Impacts of Challenger on the Space Shuttle Program and the Galileo spacecraft.

Management Changes Comments

Managers that were held

responsible for Challenger disaster

were removed from jobs.

Both NASA and Morton Thiokol Managers were affected.

Shuttle program management

structure was improved.

Overall authority was moved from Houston to Washington,

DC, and NASA Headquarters.

Aggressive oversight of NASA technical centers, programs,

and personnel was instituted.

“Parochialism” of local spaceflight centers was attacked.

Testing Procedures Comments

Equipment-testing procedures were

improved.

Additional analysis of high-risk hardware was instituted.

Hazard-analysis procedures were

improved.

NASA implemented procedures to prevent safety’s being

sacrificed to budget and schedule pressures.

Hardware Modifications Comments

Changes were made to the Space

Shuttle orbiter.

Maneuvering system valves were fitted with “sniffers” to

detect contamination that could affect system operation.

80	Kathy Sawyer, “NASA Assuming Further Delay,” Washington Post (11 April 1987): A4.

81	 “Recommendations of the Presidential Commission,” in Report of the Presidential Commission on the Space 

Shuttle Challenger Accident, vol. 1 (Washington, DC: Government Printing Office, June 1986), http://www.panix.

com/~kingdon/space/rogers11.html.
92	 MISSION TO JUPITER: A History of the Galileo Project Chapter 4 I The Challenger Accident and Its Impact on the Galileo Mission 93

Hardware Modifications Comments

Fuel cell electric heaters were replaced with safer

temperature-control system.

Redundant pipes were added to potable water system to

prevent freeze-ups.

Redundant water-purity sensors were added.

Main landing gear was strengthened, and braking system’s

reliability was upgraded.

Thermal protection system was improved.

Wings were strengthened to increase safety margin.

Computer system was upgraded to incorporate more

capabilities with smaller size, lower weight, and less

power required.

Auxiliary power units with longer lifetimes were installed.

Inertial navigation system was upgraded.

In-flight crew escape system was installed.

Middeck emergency egress slide was installed.

Fuel valves between external tank and orbiter were

upgraded to prevent inadvertent closure, which could cause

catastrophic engine failure.

Main engine durability and combustion chamber and turbine

life were improved.

Nose wheel steering system was made safer.

Solid rocket motor was redesigned. Field joint was reengineered for greater reliability. New design

ensured positive compression at all times on O-rings. Heater

was added for cold-weather operation. Additional, inner Oring was installed as a thermal barrier in case joint insulation

was breached. Field joint internal insulation was now sealed

with a pressure-actuated flap called a J-seal, rather than with

putty, as in the old design.

Case-to-nozzle joint modifications similar to those in field joint

were implemented.

Internal joints in nozzle were redesigned to incorporate

redundant and verifiable O-rings at each joint.

Insulation was added to engine casing’s factory joints.

Ignition system case was strengthened.

Ground-support equipment was redesigned to minimize

distortion of SRM during handling.

Table 4.1. Impacts of Challenger on the Space Shuttle Program and the Galileo 

spacecraft. (continued)

Hardware Modifications Comments

SRM certification now included testing over full range of

operating environments and conditions. It included joint

and nozzle joint environment simulation, as well as transient

pressure tests in which SRM was subjected to ignition

and liftoff loads in addition to maximum dynamic pressure

structural loads.

A Date Is Set for Galileo

By April 1987, NASA had given the Galileo team an approximate launch date for its spacecraft—October or November of 1989. Galileo had been given priority over the Ulysses 

mission to the Sun, which would not be launched until October 1990. NASA based its 

decision on optimizing data return from the two missions. Launching Ulysses first would 

have resulted in too long a wait before Galileo reached Jupiter and began transmitting 

prime data from the Jovian system.82

New Spacecraft Requirements

JPL would send a very different spacecraft to Kennedy Space Center in April 1989 from 

what had been sent three-plus years before. Mission programmers were modifying the 

complex software to accommodate VEEGA’s new requirements. Because the VEEGA 

trajectory would carry Galileo so much closer to the Sun, the craft would require substantial thermal protection changes, which JPL would have to test exhaustively. The Galileo 

team was currently fabricating new Sun shields and blanketing. When Galileo traveled 

inside Earth’s orbit, the craft’s axis would need to be pointed toward the Sun in order to 

orient the new Sun shields so as to shade the craft’s high-gain antenna (HGA) and other 

delicate equipment. As a result, radio communications with Earth would not be maintained 

by the HGA, which would be pointed in the wrong direction for that. Two low-gain antennas (LGAs) would be used instead. The LGAs were far less sensitive than the HGA but did 

not require protection from the Sun. The original design called for only one LGA, but an

additional one had been added so that at any given time, at least one of them would usually be able to communicate with Earth. The LGAs could not transmit data back to Earth at 

the rate that the HGA could have, and as a result, much of the information collected during 

the Venus flyby would have to be stored temporarily on Galileo’s tape recorder. It would 

be transmitted months later during the first Earth flyby.83

Galileo would now use a solid-fueled Inertial Upper Stage instead of Centaur to 

boost it from low-Earth orbit into its VEEGA trajectory. Because of this change, the hardware that had been built to connect Galileo to Centaur needed to be redesigned.84

The VEEGA trajectory presented new scientific opportunities for Galileo. During 

the six-year flight to Jupiter, the spacecraft would pass close to Venus, Earth, and the Moon. 

82	J. R. Casani, “From the Project Manager,” Galileo Messenger (April 1987).

83	Casani, “From the Project Manager,” Galileo Messenger (April 1987): 1; “Galileo: Up to Date,” Galileo Messenger

(November 1989): 3.

84	Casani, “From the Project Manager,” Galileo Messenger (April 1987): 1.

Table 4.1. Impacts of Challenger on the Space Shuttle Program and the Galileo 

spacecraft. (continued)
94	 MISSION TO JUPITER: A History of the Galileo Project Chapter 4 I The Challenger Accident and Its Impact on the Galileo Mission 95

Galileo would also fly twice through the asteroid belt, which would give it the opportunity to be the first spacecraft to make a close observation of an asteroid. The project team 

studied encounter opportunities with three asteroids: Ausonia, Gaspra, and Ida, ranging in 

diameter from 6 to 92 kilometers (see table 4.2).85

Table 4.2. Potential asteroid opportunities.

Asteroid 

Name

Asteroid Diameter 

in Kilometers (Miles) Possible Encounter Date Relative Velocity in Kilometers per Second (Miles per Second)

Ausonia 92 (57) 9 April 1992 8.2 (5.1)

Gaspra 6 (3.7) 29 October 1991 8.0 (5.0)

Ida 13 (16.1) 27 August 1993 12.5 (7.8)

Source: J. R. Casani, “From the Project Manager,” Galileo Messenger (April 1987).

VEEGA also presented challenges in the area of equipment reliability. Shelf lives 

and aging of equipment took on increased importance in JPL’s testing program. Galileo’s 

previous mission schedule had had an end date of October 1990. But with the current 

launch set for late in 1989 and a longer flight time to Jupiter, Galileo would, scientists 

hoped, be collecting data into 1998. The JPL team had to find the parts and components 

that needed to be augmented or replaced in order to keep Galileo functioning through 

that time.86

JPL staff examined components for a range of possible aging effects, including 

the following:

• Degrading flexibility of cable coverings.

• Adhesion loss of conducting tape.

• Metal corrosion.

• Chemical changes and aging of paints and coatings.

• O-ring fracturing.

The project team worried that exposure to Earth’s atmosphere during the years the spacecraft 

was waiting to launch would corrode some of its magnesium alloys (once in space, the alloys 

would not corrode). Quality-assurance personnel needed to inspect all such surfaces before 

shipping Galileo back to Kennedy. Personnel also performed periodic visual inspections 

of painted surfaces in order to search for signs of peeling, flaking, or cracking. Specialized 

coatings such as uralane and humiseal could shrink over time, damaging the parts they 

were meant to protect. O-rings were subject to fracturing after long periods of disuse. To 

85	 Ibid.

86	Casani, “From the Project Manager,” Galileo Messenger (April 1987).

87	 “Aging: Its Effect on the Spacecraft,” Galileo Messenger (April 1987).

coatings such as uralane and humiseal could shrink over time, damaging the parts they 

were meant to protect. O-rings were subject to fracturing after long periods of disuse. To 

guard against this danger, project engineers made sure that all parts containing O-rings 

were “exercised” twice a year. Springs kept compressed for extended periods, such as those 

that would help deploy the spacecraft’s magnetometer boom, had to be regularly checked 

for signs of stress. Even the testing of parts could itself introduce damage. Personnel might 

deposit dirt on the components or overstress flexible parts.87

Attrition of Critical Project Staff

Galileo mission delays not only aged spacecraft components and parts; they also aged the 

project team. For scientists and engineers who joined Galileo at the time it received congressional approval, or even before, half their research careers would be over by the time 

the spacecraft reached the Jovian system. For those who had joined the project in their 40s 

or 50s, time was running out. Many scientists were nearing retirement and hoped that they 

would still be working when their Galileo experiments started to return data.88

Tom Donohue, a University of Michigan professor who had an experiment on 

Galileo, was very apprehensive about the years of delay between solar system exploration 

missions. The last major U.S. scientific space probe had been Pioneer to Venus, which had 

been launched in 1978. Galileo was not going to launch until over a decade later, when 

Donohue was in his late 60s. Running his Galileo experiment was to have been a major 

effort during his last years in the field. But now Galileo would not even reach Jupiter until 

he was in his mid-70s.89

The Galileo project needed new blood, as did other solar system missions, but 

young talent was hard to find. With the long gaps between missions, during which limited solar system study could be performed, many young scientists were not staying in 

the space science field, or even entering it. A young space scientist with a newly earned 

Ph.D. typically had five years to conduct sufficient research and publish enough respected 

papers to earn a permanent research or academic position. This postdoctoral period was 

a feverish time for most new graduates, and if meaty, important projects could not be 

found in space science, the graduates would be under great pressure to find them in 

other fields. Fifty science missions had been planned for the Shuttle during the years from 

1986 through 1992. Now, with the Shuttle’s reduced schedule, only 17 were planned, and 

due to Challenger, they were all facing an average three-year delay. This was yet another 

legacy of Challenger—the loss of top talent from the space science field, and the threat 

to the field’s future.90

The solar and space physics department of Baltimore’s Johns Hopkins University 

was one of many places hit hard by the aftermath of Challenger. The department 

had been involved in four space science missions scheduled to fly in 1986. But after 

Challenger, Stamatios Krimigis, the department chairman, received notice from NASA of 

a 50-percent cut in funding between 1987 and 1989. “We will have to let go our young 

research associates,” Krimigis said at the time, “and they will probably become computer 

88	Sawyer, “Launch Delays Darken Future.”

89	Douglas B. Feaver, “The Spreading Ripples of Challenger Disaster—Scientific Ventures Losing Momentum,”

Washington Post (16 June 1986).

90	Sawyer, “Launch Delays Darken Future.”
96	 MISSION TO JUPITER: A History of the Galileo Project Chapter 4 I The Challenger Accident and Its Impact on the Galileo Mission 97

scientists or join other fields where they are in great demand. You can’t do space science 

with no wages.”91

Nor would there be the flow of small science payloads that had in years past 

been carried aloft by balloons and modest sounding rockets. These missions, as well as 

larger ones such as Galileo, had been vital in training graduate students and establishing them on a career path. But the Shuttle, which had been expected to carry the small 

payloads more efficiently, effectively had phased out many other launch vehicles. Diverse 

launch options had been consolidated into just one, and when that went down, it threw 

not only the Galileo mission, but also the whole space science field, into crisis.92

Antinuclear Groups Try To Block the Launch

As Galileo’s scheduled October 1989 launch date approached, public concern about sending radioactive materials into space flared up into open opposition. Residents of the area 

surrounding Cape Kennedy, Florida, allied with environmental as well as disarmament 

groups from around the nation, focused their attention on stopping the launch. “Humans 

and technology can fail,” said Bruce Gagnon, coordinator of the Florida Coalition for 

Peace and Justice. “We don’t want Brevard County, the Space Coast or anyplace else made 

into a national sacrifice area. We don’t want to be guinea pigs.” Gagnon and thousands of 

others were worried about Galileo’s exploding and raining “radioactive death” over eastern Florida, the “Space Coast.” Gagnon didn’t want people endangered to further NASA’s 

planetary exploration program.93

NASA could not definitively say that this would not happen, any more than 

it could have predicted without a doubt whether Challenger was going to explode. 

However, NASA, other agencies, and the executive branch of the federal government 

could and did bring extensive analytical capabilities and oversight to bear in order 

to gauge the expected safety of the mission. Galileo’s launch approval process was 

defined by Presidential Directive PD/NSC-25, which required that the President’s Office 

of Science and Technology Policy approve the launch of any nuclear power source. 

The complete approval process involved approximately 100 personnel from federal 

agencies, private industry, universities, and the Executive Office of the President. A key 

part of the process was a series of independent safety evaluations by an Interagency 

Nuclear Safety Review Panel (INSRP) made up of representatives from the Department 

of Defense, the Department of Energy, and NASA, as well as observers from the Nuclear 

Regulatory Commission, the Environmental Protection Agency, and the National Oceanic 

and Atmospheric Administration.94

Three safety analysis reports (SARs) were developed during various stages of 

Galileo project development. The final safety analysis report (FSAR), the one most relevant 

91	 Ibid.

92	 Ibid.

93	Victoria Pidgeon Friedensen, ‘The Protest,’ chap. 4 in “Protest Space: A Study of Technology Choice, Perception of

Risk, and Space Exploration” (master of science thesis, Virginia Polytechnic Institute and State University, 11 October

1999); Michael Lafferty and Chris Reidy, “NASA Tries to Catch Up in the PR War Over Space Probe,” Orlando Sentinel 

(22 July 1989).

94	NASA, Mission Operation Report,Report Number E-829-34-89-89-01, NASA Office of Space Science and Applications

(1989), pp. 38–41.

to questions of RTG integrity and the radiological risks that it presented, was completed 

in December 1988 and reviewed by INSRP. After reviewing the FSAR, both Department of 

Defense and Department of Energy agency heads had to decide “whether to concur with 

or recommend against NASA’s request for launch.”95 If both agency heads concurred, then 

NASA had to request approval for the launch from the President’s Office of Science and 

Technology Policy, whose director could either give approval or refer the matter to the 

President. No launch could take place without approval from either the Office of Science 

and Technology Policy’s director or the President.96

NASA’s safety analyses did not do much to stop the protests. Perhaps this was 

because nuclear energy had become not only dangerous but also morally evil in many 

people’s minds. To activists all over the country, it was connected to the evils associated 

with the military and big corporations, such as waging unjust wars and greedily reaping 

excessive profits. Nuclear material was controlled by entities such as the Department of 

Energy, Department of Defense, and electric power megacompanies that many people 

didn’t trust. Bruce Gagnon, for instance, thought that the Department of Energy was forcing NASA to use plutonium aboard Galileo. He believed that the nuclear industry in turn 

controlled DOE and would stop at nothing to expand the market for radioactive fuels. 

When an accident involving nuclear fuel occurred, such as Three Mile Island or Chernobyl, 

or even an accident such as Challenger, in which nuclear materials had not been used but 

might have been used if it had been a later mission, it just confirmed many people’s worst 

fear: nuclear energy could not be made safe. Its use potentially threatened everyone.97

Larry Sinkin was an influential opponent of nuclear power who was based in 

Washington, DC. He was the director of the Christic Institute, an interfaith center for just 

laws and public policy. The Christic Institute had been involved in the Karen Silkwood 

case and in legal action against the Three Mile Island power plant in Pennsylvania. Like 

Bruce Gagnon, Sinkin mistrusted the Department of Energy, stating that its long history 

of lies, deceptions, and cover-ups gave it no credibility. “This is the agency telling us that 

plutonium is safe. We simply don’t believe them,” said Sinkin, who was perhaps referring 

to DOE’s controversial role in the management and cleanup of its radioactively contaminated sites such as Rocky Flats, Colorado; Savannah River, South Carolina; and Fernald, 

Ohio. Sinkin believed that most people were unaware of just how dangerous plutonium 

was. If the public was alerted to the threat that the material posed, popular opinion might 

demand that Galileo be kept on the ground.98

NASA stated that it used plutonium as a power source in its spacecraft for reasons of necessity, not because DOE insisted. Batteries could not supply enough power for 

Galileo’s needs without being prohibitively heavy, and the spacecraft would travel too 

far from the Sun to use solar power. Because of Jupiter’s distance from the Sun, 2,000 

square feet of solar panels weighing over 1,000 pounds would be required, which was 

not feasible.99

Not all activist groups opposed Galileo. The Committee to Bridge the Gap was 

an organization that frequently lobbied against space nuclear power projects, yet it gave 

95	 Ibid., p. 40.

96	 Ibid., p. 40.

97	Lafferty, “NASA Tries to Catch Up”; personal communications with nuclear power protesters at Diablo Canyon

nuclear reactor in California, 1980.

98	 Ibid.

99	 Ibid.; William Harwood, “NASA Current News,” from UPI, 28 June 1989.
98	 MISSION TO JUPITER: A History of the Galileo Project Chapter 4 I The Challenger Accident and Its Impact on the Galileo Mission 99

Galileo its blessing. The Committee was aware of the risks that the mission presented. In 

a paper analyzing Galileo’s safety, the Committee’s executive director, Steven Aftergood, 

identified several situations that could result in plutonium release. But Aftergood also

believed that the “unique benefits” that would be derived from exploring Jupiter outweighed the nuclear safety risks. The Committee’s view was not dissimilar to that of 

NASA’s management, who recognized that space travel had certain dangers associated 

with it, both from the use of nuclear materials and from other factors. NASA’s decisions 

to launch spacecraft in spite of known dangers said much about the Agency’s values. 

Sometimes attaining an ideal outweighed certain risks.100

The risk presented by the plutonium aboard Galileo was not that it would turn 

into a nuclear bomb and explode. The isotope used in Galileo’s RTGs was plutonium238, a material unsuitable for nuclear weapons. In contrast to nuclear weapons or nuclear 

reactors, no fission process is involved in the operation of RTGs, nor was a fission process 

possible, given the type of plutonium used by Galileo and the design of its RTGs. But 

plutonium-238 poses other dangers due to its radioactivity.101

Unlike radioactive materials with intense gamma radiation, plutonium’s alpha 

emissions are dangerous only at close range. Alpha rays do not generally pose much 

risk unless pieces of plutonium (even tiny pieces) enter the body. In that case, the alpha 

emissions can do intense damage. In the event of a Galileo accident, small fragments 

of the RTGs’ plutonium might be released into Earth’s environment. The largest human 

health risks would come from inhaling particles that could get into the lungs or ingesting 

plutonium-contaminated food.102

Although the RTG design included triple encapsulation to contain the plutonium 

fuel in case of an accident, NASA conceded that the safety records of previous missions

using RTGs were not perfect. In fact, 3 of the 22 missions launched with RTGs seriously 

malfunctioned. In 1968, a Nimbus weather satellite did not attain orbit due to a launch 

system malfunction, and its two RTGs fell into the Santa Barbara Channel. But the RTGs 

were recovered intact, without loss of fuel, from the ocean floor five months later. Their

plutonium fuel was reprocessed and used in a later mission. In 1970, the Apollo 13 spacecraft failed to land on the Moon, where it was going to leave its RTG. After the abort of 

the mission, the Lunar Module reentered Earth’s atmosphere, falling into the South Pacific 

Ocean in the area of the Tonga Trench, where it still remains. Extensive air and water samples taken in the vicinity of the reentry, however, found no evidence of fuel release.103

Radioactive fuel did disperse in an incident that occurred before NASA implemented its RTG triple-containment system. In 1964, the Transit 5BN-3 navigational satellite failed and released radioactive fuel over a wide area of the upper atmosphere. The 

release of radioactive material was extensively studied by scientists from the Atomic Energy 

Commission in air- and soil-sampling efforts. As a result of the assessment, U.S. RTG 

100	 Theresa Foley, “NASA Prepares for Protests over Nuclear System Launch in October,” Space Technology (26 June

1989): 23, 25.

101	 “Facts About RTG Misconceptions,” NuclearSpace.com Web site, http://www.nuclearspace.com/facts_about_rtg.

htm (accessed 6 June 2005).

102	 Foley, “NASA Prepares,” pp. 87–88.

103	 Ibid.; “Facts About RTG Misconceptions”; “RTG Fact Sheet: Past Releases of Radioactive Materials from US Nuclear

Power Sources,” NASA SpaceLink, 17 July 1991, http://spacelink.nasa.gov/NASA.Projects/Human.Exploration.and.

Development.of.Space/Human.Space.Flight/Shuttle/Shuttle.Missions/Flight.031.STS-34/Galileos.Power.Supply/

RTG.Fact.Sheet.

design philosophy changed to full fuel containment. The objective was that in the event 

of an abort during launch or the on-orbit phase of a mission, the RTGs would retain all 

their fuel.104

NASA had learned much from these past incidents and had confidence in its current triple-containment RTG design, which was used in the Galileo spacecraft. The plutonium was divided up into little spheres about the size of golf balls, and each of these was 

welded into iridium spheres, which were intended to seal off the plutonium even at high 

temperatures. The spheres were packed into extremely strong cylinders designed to withstand the impact of a crash, and these in turn were placed inside 2-by-4-inch bricks made 

of graphite, the same sort of material used for ballistic reentry nose cones. In fact, the 

bricks themselves were designed to serve as little reentry bodies. Department of Energy 

tests on the RTGs included subjecting them to explosion and fire environments of varying 

intensities, as well as high-velocity impacts—situations intended to recreate the conditions 

that might occur during a Shuttle accident. The RTGs withstood all their trials very well, 

including pressures that exceeded 2,200 pounds per square inch. In defending its RTG 

design, NASA cited the Nimbus and Apollo incidents, holding that the RTGs “performed 

precisely as designed and maintained full containment with absolutely no release of fuel.” 

NASA believed that these experiences demonstrated the RTGs’ fail-safe reliability.105, 106

A final environmental impact statement (FEIS) issued by NASA in cooperation 

with the Department of Energy on the Galileo mission concluded that the RTGs posed 

only small health or environmental risks. The risk analysis performed during the preparation of the FEIS considered three accident scenarios:

• Most Probable Case: The highest probability accident in a mission phase 

leading to a release of plutonium.

• Maximum Credible Case: The accident in a mission phase that leads to a 

release of plutonium with the most severe impact on human health.

• Expectation Case: The probability-weighted sum of all accidents in a mission phase.

The FEIS stated that the Most Probable Case resulting in a plutonium release was an IUS failure leading to spacecraft breakup, reentry of the RTG modules, and impact of the modules 

on hard rock. The FEIS set the probability of this occurrence at 1 in 2,500.107

The Maximum Credible Case involved an accidental reentry during one of Galileo’s 

Earth flybys (scheduled for 1990 and 1992); it was given a probability of occurrence of 

104	 Foley, “NASA Prepares”; “RTG Fact Sheet: Past Releases of Radioactive Materials from US Nuclear Power Sources”;

Regina Hagen, “Past Missions—a Chronology,” chapter 3 in Nuclear Powered Space Missions—Past and Future, on

the Web site of the Global Network Against Weapons and Nuclear Power in Space, 8 November 1998, http://www.

space4peace.org/ianus/npsm3.htm.

105	 John F. Murphy, NASA Assistant Administrator for Legislative Affairs, to the Honorable Alfonso D’Amato, United

States Senate, 30 April 1986, Galileo Documentation File, folder 5138, NASA Historical Reference Collection,

Washington, DC; “Radioisotope Thermal Generators (RTGs),” Galileo Messenger (April 1984); “RTGs—A Plutonium

Crap-Shoot?” Space World (June 1987): 24; Foley, “NASA Prepares,” p. 87.

106 William Sheehan, NASA Associate Administrator for Communications, to Officials-in-Charge of Headquarters

Offices; Directors, NASA Field Installations; and Director, Jet Propulsion Laboratory, “Galileo and Ulysses Mission

Information,” 17 November 1988, Galileo Documentation File, folder 5138, NASA Historical Reference Collection.

107 Final Environmental Impact Statement for the Galileo Mission (Tier 2), NASA No. N90-22147, May 1989, pp. iv–v.
100	 MISSION TO JUPITER: A History of the Galileo Project Chapter 4 I The Challenger Accident and Its Impact on the Galileo Mission 101

one in nine million. In this very unlikely scenario, the RTG modules were subjected to 

intense reentry heating and a collision with a hard rock surface, resulting in a plutonium 

release that would expose over 70,000 people. Nevertheless, the health effects predicted 

for even this extreme case were small. Over a 70-year period, the exposed population 

would receive an average dose of less than 20 percent of the natural background radiation 

level. Without this radiation exposure from the RTGs, roughly 14,000 of the 70,000 would 

be expected to die of cancer. With the radiation exposure from the RT Gs, only about nine 

more people would be expected to die of cancer. Thus, even for the Maximum Credible 

Case, the human health impact would be small. 

The Expectation Case analysis, which represented a probabilistic combination 

of all accident scenarios, reached a similar conclusion. It estimated the risk of fatality to 

an individual exposed to radiation from any Galileo accident to be about four in one billion, which was hundreds to thousands of times lower than from “ordinary” causes such 

as motor vehicle accidents, falls, fire, disease, or natural disasters (hurricanes, tornadoes, 

or lightning).108

The Committee to Bridge the Gap, as discussed above, supported the Galileo 

mission. But even this activist group thought that the FEIS’s numbers were surprisingly 

low, and it was not convinced that they really represented worst-case scenarios. “Since we 

have not launched, say, 100,000 missions, we cannot speak with high confidence about 

how many failures out of 100,000 launches can be expected,” the Committee said, and 

pointed out that NASA’s estimates were based on engineering judgments that contained 

a degree of subjectivity.109

As the Galileo launch date grew nearer, antinuclear activists stepped up their 

efforts. In September 1989, a group called the Maryland Safe Energy Coalition demonstrated outside NASA Headquarters in Washington, DC. The Florida Coalition for Peace 

and Justice began a 200-mile “peace walk” from Cape Canaveral, Florida, to Kings Bay,

Georgia, to protest both the Galileo and the Trident missiles, which were flight-tested in 

Kings Bay. The Florida group also planned to enter Kennedy Space Center and sit on

Galileo’s launchpad in order to block the launch, said Bruce Gagnon, the group’s leader.110

Because Galileo was going to carry nuclear material into orbit, its launch 

required an executive office decision. President George H. W. Bush was not expected to 

oppose the flight, according to Associated Press reports, and preparations for the launch 

continued as planned. On 12 September 1989, the crew that would fly the Shuttle Atlantis

and its Galileo payload into orbit arrived at Kennedy Space Center. Two days later, the 

Shuttle’s “Countdown Demonstration Test” began. The astronauts suited up and boarded 

Atlantis for the test.111

On 18 September 1989, NASA reported that the White House had approved the 

Galileo launch. In response, a coalition of antinuclear activists filed suit to stop the takeoff. The coalition argued that a catastrophic launch failure would endanger the health of 

people in central Florida and that using the Shuttle to launch Galileo had a higher probability of an accident than using a robotic rocket. “NASA is in effect playing ecological 

roulette with the people of Florida,” said Anthony Kimbrell, one of the lawyers asking 

108	 Ibid., pp. v, 4-16 to 4-28.

109	“NASA Says a Nuclear Generator on Satellite Will Pose Little Risk,” New York Times (7 January 1989): 9.

110		Associated Press, 12 September 1989.

111		Ibid.; J. Kukowski, NASA Headline News, Message EJIJ-2835-7747, 12 September 1989.

for a court restraining order on the launch. Attorneys for the coalition maintained that as 

many as 700,000 cancer deaths might result from an accident and that the chances of this 

happening were greater than NASA believed.112

Steve Frank, a lawyer arguing for the Justice Department, countered the coalition’s position by maintaining that all of its “tales of horror” were based on the fear that 

Galileo’s plutonium fuel would get pulverized to the extent that people up and down the 

Florida coast would inhale little particles of it. Frank insisted that with NASA’s triple layers 

of protection, the chance of pulverizing the plutonium pellets was virtually zero.113

Radically different numbers were thrown back and forth by both sides, but what 

the court focused on was not whether NASA’s or the coalition’s estimates of risk were more 

accurate, but whether NASA had broken the law. The coalition maintained that NASA had 

failed to follow National Environmental Protection Act requirements by filing an insufficient 

environmental impact statement that did not adequately address the launch dangers. It was 

this assertion that Judge Oliver Gasch of the U.S. District Court examined.114

In the midst of the court hearings, a petition was introduced that gave the hearings an international dimension. West Germany’s Green political party joined the American 

coalition of citizens’ groups in trying to get a restraining order on the launch. The Greens 

claimed that Galileo would violate a 1967 outer space treaty that both the United States and 

Germany had ratified, and they wanted the launch delayed so that the issue could be put 

before the International Court of Justice. A provision of the 1967 treaty stated that one country’s space exploration activities were not to harm any other country. The plutonium carried 

by Galileo might, in the Greens’ view, result in radioactive contamination of Germany. The 

Greens petitioned the U.S. District Court to issue a 10-day temporary restraining order on 

the launch. This would allow the Greens time to determine whether the world court would 

hear the case and whether to approach the German government to intervene formally.115

While the coalition and the Greens brought their suit to court, the Shuttle 

astronauts, having finished their countdown demonstration test, flew to Washington for a 

meeting with Vice President Quayle, who was the chairman of the National Space Council. 

Quayle threw his support behind Galileo by announcing that he would travel to Kennedy 

Space Center, where he planned to watch the launch from the control room.116

On 9 October 1989, as the final countdown began for a 12 October takeoff, protesters enacted a mock death scene at Kennedy Space Center and waited for the court’s 

decision on their suit to stop the mission. They vowed that if Judge Gasch did not stop 

the mission, then they would carry out the earlier threat made by the Florida Coalition for 

Peace and Justice to sit on the launchpad so that Galileo couldn’t fire its rockets.117 On 10 

October, Judge Gasch ruled that it was not the function of his court to decide whether the 

government’s decision to launch Galileo was a good one or whether the launch would 

endanger this or other countries. The judge said that his only job was to examine the 

coalition’s charge that NASA had violated the National Environmental Policy Act (NEPA) 

112	 NASA Headline News, 18 September 1989; Jay Malin, “Galileo Survives Suit, but Mishap Delays Liftoff,” Washington 

Times (11 October 1989): A3.

113		 Malin, “Galileo Survives Suit,” p. A3.

114		 Ibid.

115		 “West Germany’s Greens Want Shuttle Issue Before World Court,” Washington Times (13 October 1989): A7.

116	 Suelette Dreyfus and Julian Assange, Underground (Australia: Mandarin, a part of Reed Books, 1997), p. 15; “Shuttle

Countdown Begins Amid Protests,” Washington Times (10 October 1989): A2.

117	“Shuttle Countdown Begins,” p. A2; Associated Press, 12 September 1989.
102	 MISSION TO JUPITER: A History of the Galileo Project Chapter 4 I The Challenger Accident and Its Impact on the Galileo Mission 103

by filing an insufficient impact statement. The case record explained that the court needed 

to “ensure that the environmental impact statement contains a sufficient discussion . . . 

to allow the agency to take a hard look at the issues and make a reasoned decision on 

the matter.” The case record went on to explain, “The court will not substitute its own 

judgment regarding the merits of the proposed action for that of the government agencies. NEPA itself does not mandate particular results, but simply prescribes the necessary 

process.”118 The court concluded that NASA had complied with the provisions of the act 

and rejected the coalition’s request for a restraining order against the launch. Judge Gasch 

added that issuing such an order would have a “costly adverse” economic effect on the 

public interest if it caused Galileo to miss its present launch window, for it would cost 

NASA, and ultimately the public, $164 million just to maintain the Galileo program until 

the next launch opportunity 18 months later.119

Judge Gasch’s decision had removed the legal hurdles blocking Galileo’s planned 

liftoff on Thursday, 12 October, but an hour after the ruling was issued, a faulty computer 

in the Shuttle delayed the mission once again. Launch director Robert Sieck announced 

that the computer would have to be replaced, pushing liftoff forward by about a week. 

Meanwhile, lawyers for the three citizens’ groups that had brought the Galileo matter to 

court—the Florida Coalition for Peace and Justice, the Christic Institute, and the Foundation on Economic Trends—announced that they would appeal Gasch’s decision.120

If the protesters could succeed in delaying liftoff for just a few weeks, it would 

have a very significant impact on the mission. Galileo had to launch by 21 November 1989 

or wait until 1991 for the planets to align properly again for the mission. The possibility of another delay alarmed many on the Galileo team. Neil Ausman, who would direct 

Galileo in flight, was “very, very frustrated” by the delays thus far, saying “I’m looking 

at an investment of 20 years” by the time the spacecraft was expected to reach Jupiter 

in 1995. Some of the scientists who’d initiated the mission had already retired, died, or 

moved on to other projects out of frustration, and Galileo could lose more good people 

if it was delayed yet again.121

Anticipating demonstrations, Kennedy Space Center security officials beefed up 

the protection of the site with high-tech equipment and armed patrols. Gary Wistrand, 

deputy director of Kennedy’s security office, said that his force, which numbered 200 to 

225 guards, was armed with M-16 assault rifles and 9-millimeter semiautomatic pistols 

and was ready to arrest anyone illegally entering the 140,000-acre space center. While 

Wistrand expected the trespassing attempts to be made on foot, his force was also on the 

lookout for other modes of entry, including by boat or plane. He thought that the terrain 

helped keep the site secure against a land invasion. Much of the space center boundary 

was swampy and infested with alligators, wild hogs, and rattlesnakes. In the past, wouldbe trespassers had kept to the high, dry places and had been easy to locate.122

While security forces were preparing to stop protesters, technicians began the 

laborious task of replacing the faulty Shuttle computer that had given erroneous fuel pump 

118	Florida Coalition for Peace & Justice v. Bush, Civil Action No. 89-2682-OG, United States District Court for the District

of Columbia, 1989 U.S. Dist. Lexis 12003, decided and filed on 10 October 1989.

119		Ibid.; Malin, “Galileo Survives Suit”; “Galileo,” The Acorn—Newsletter of the American Nuclear Society 96-1 (February

1996): Oak Ridge/Knoxville Section, http://www.engr.utk.edu/org/ans/newsletter/1996/acorn96_1.html.

120	Kathy Sawyer, “Faulty Engine Computer Delays Launch of Shuttle,” Washington Post (11 October 1989): A3.

121	Ibid.

122	“Shuttle Guards Spare No Effort to Foil Foes,” Washington Times (12 October 1989): A6.

pressure readings and delayed the mission. The computer was a critical piece of equipment because it controlled all functions of one of the Shuttle’s three main engines during 

liftoff.123 By Sunday, 15 October, a spare Shuttle engine computer had been installed and 

tested. Workers began closing up the engine compartment, removing work platforms and 

reinstalling heatshields. Meanwhile, NASA closed large sections of the space center to 

the public and mobilized its security forces, including boat and helicopter patrols. NASA 

scheduled the resumption of the launch countdown to begin the next day at noon, with 

the goal of launching Galileo on Tuesday afternoon, 17 October. The weather on Tuesday 

looked favorable, although there was a 10-percent chance that winds from the southeast 

would exceed the 23-mph launch limit.124

On Monday, 16 October, the last-minute appeal of the U.S. District Court’s ruling 

on Galileo was rejected by the Washington, DC, Circuit Court of Appeals. Chief Justice 

Patricia Wald wrote in her concurrence that, contrary to the protesters’ main legal claim, 

she was not able to find that NASA had improperly compiled its environmental impact 

statement on the Shuttle launch risks. Also on 16 October, eight protesters were arrested 

at the space center for trespassing. The arrests were made at a gate near Kennedy’s headquarters. If the protesters had succeeded in approaching the Shuttle, they might have 

delayed the takeoff, because NASA guidelines prohibited a launch when anyone was 

within the “blast danger zone,” a 4,000-foot-radius circle around the launchpad.125

Finally, Galileo appeared ready to be launched. It had survived the Challenger

disaster, changes of upper stage and trajectory, and the protests of antinuclear activists. 

A summary of the impacts on the Galileo mission that followed the Challenger tragedy is 

given in table 4.3.

Table 4.3. Impacts of Challenger on the Galileo mission and spacecraft.

Change Comments

Centaur upper stage was eliminated. A liquid-hydrogen-fueled rocket engine was considered too

dangerous to carry in the Shuttle cargo bay.

Solid-fueled Inertial Upper Stage (IUS)

was used instead.

Perceived as safer, but its lower thrust would lengthen travel

time to Jupiter and necessitate a more complex trajectory.

VEEGA trajectory replaced direct

trajectory to Jupiter.

Venus and Earth gravity-assists were needed to minimize

fuel consumption.

Thermal blankets and Sun shades

were added.

Necessary because on the VEEGA trajectory, Galileo would

approach closer to the Sun.

Sun shields were added to shade

the high-gain antenna and other

equipment.

Galileo had to orient its axis toward the Sun so that Sun

shields could shade sensitive parts of the spacecraft. Radio

contact with Earth had to be maintained with two low-gain

antennas instead of a high-gain antenna. Data transmission

rates were affected.

123	Ibid.

124	“NASA Clears Atlantis for Liftoff Tuesday,” Washington Post (15 October 1989): A18; “NASA Deploys Guards as

Shuttle Launch Nears,” Washington Post (16 October 1989): A20.

125	Kathy Sawyer, “Galileo Launch Nears; Court Appeal rejected,” Washington Post (17 October 1989): A3.
104	 MISSION TO JUPITER: A History of the Galileo Project Chapter 4 I The Challenger Accident and Its Impact on the Galileo Mission 105

Change Comments

Galileo spacecraft/upper stage

interface was modified.

IUS connections required different hardware from that

needed for Centaur.

Due to launch delay, encounter with

Amphitrite asteroid was no longer

possible.

Opportunities for flying by the asteroids Ausonia, Gaspra,

and/or Ida could be exploited instead.

Attrition occurred among project staff. Staff members retired, died, or changed programs.

Nuclear material aboard Galileo

became a major public relations

issue.

Public confidence in the safety of NASA spacecraft had

already been damaged by the Challenger disaster. Wide-	

spread fear of plutonium releases that might occur if Galileo

crashed led to protests and lawsuits.

The Launch

The new liftoff date, 17 October 1989, looked like a go. The Shuttle appeared to be 

operating as it should, with Galileo in its belly and the astronaut crew aboard. Then 

cloud cover began to build up. Countdown continued, but finally the Launch Director scrubbed liftoff once again. Visibility was poor enough that if problems arose and 

the Shuttle had to return to Earth shortly after takeoff, it would not be able to land at 

Kennedy. Launch was delayed until the following day.126

As the Galileo team prepared for the next day’s launch, a magnitude 7.1 earthquake struck northern California, just miles south of the Inertial Upper Stage (IUS) control 

center, which was crucial to mission operations. The crew had to evacuate, and it looked 

like the mission would have to be pushed back once more. But the crew was able to

return later in the night and go back online. The next morning, however, ominous clouds 

at Kennedy Space Center put the launch in jeopardy one more time. Fortunately, the skies 

finally cleared enough for the Shuttle to launch safely. At 12:54 p.m. eastern daylight time 

(EDT) on 18 October 1989, the Shuttle Atlantis, carrying its Galileo payload, took off into a 

sky filled with puffy white cumulus clouds. Galileo was finally on its way (see figure 4.5).127

Six hours and 21 minutes after launch, mission specialist Shannon Lucid initiated the deployment procedure for Galileo, ejecting it and the IUS rocket to which it was 

attached from Atlantis’s cargo bay. After deployment was completed, Atlantis commander Donald Williams declared, “Galileo is on its way to another world. It’s in the 

hands of the best flight controllers in the world—fly safely.” One hour later, when the 

Shuttle had flown to a safe distance, the IUS rocket fired, speeding the spacecraft into 

interplanetary space and on to its first objective—Venus.128

126	“Launch to Landing,” Galileo Messenger (November 1989): 1–2.

127	Ibid.

128	Ibid.

Table 4.3. Impacts of Challenger on the Galileo mission and spacecraft. (continued)

Figure4.5.Launch of Galileo: liftoff of the Space Shuttle Atlantis, carrying the Galileo spacecraft and 

its Inertial Upper Stage. (STS34(S)025)

Figure 4.5. Launch of Galileo: liftoff of the Space Shuttle Atlantis, carrying the Galileo 

spacecraft and its Inertial Upper Stage. (STS34(S)025)

 The theme of the Galileo mission was accomplishment in the face of adversity. 

Even after the successful launch of Atlantis, deployment of Galileo, and firing of its IU S, 

there was trouble. Heavy winds were forecast for Atlantis’s landing site at Edwards Air 

Force Base in the California desert. To minimize the danger, two orbits of Earth, which 

would have taken 3 hours, were cut from the Shuttle’s flight plan so that it could get 

down to the ground as soon as possible. Even so, shortly before landing, the fog started 

rolling down off nearby hills into the dry lakebed where Atlantis was to touch down. 

Luckily, the skies cleared at the last moment, and the Shuttle made a perfect landing. In 

attendance at the landing site were reporters, Galileo flight team members, and other 

people, some of them now retired, who had helped to design and construct Galileo.129

R. J. Spehalski, who had recently taken over as Galileo Project Manager from 

John Casani, had this to say about the events of 18 October 1989: 

The Galileo launch, to many of us, represents a culmination of 

years of personal and team efforts and sacrifices. The emotions present 

within each of us at such an event defy description . . . I was fortunate 

to witness the coming together of such a diverse set of activities that 

it boggles the mind. The professionalism, teamwork, and dedication 

visible in this process was truly inspiring to behold.130

129	Ibid.

130R. J. Spehalski, “From the Project Manager,” Galileo Messenger (November 1989): 1.

The Galileo spacecraft was designed to spend years traveling through the inner solar system and asteroid belt before it 

reached its main objective: the Jovian planet-satellite system. NASA

intended the spacecraft to take some observations of inner planets and asteroids but to 

focus on investigating Jupiter, its moons, and the fields and particles of its magnetosphere 

at a level of detail never before attempted. The overall mission would last more than a 

decade. 

NASA intended that the Galileo project advance the goals of the Agency’s Lunar 

and Planetary Exploration Program, which were to understand 1) the origin and evolution 

of the solar system; 2) Earth, through comparisons with other planetary bodies; and 3) 

the origin and evolution of life. To make progress toward these goals, the Galileo mission 

was to conduct a comprehensive investigation of the entire Jovian system, including both 

in situ and remote observations of the planet, its environment, and its satellites. In order 

to make so many different types of measurements, the Galileo spacecraft needed to have 

a wide range of capabilities. NASA constructed the spacecraft in three segments, each of 

which had a specific objective: 

• Analyze the composition and physical properties of Jupiter’s atmosphere. To 

accomplish this, a pod of instruments called the Atmospheric Probe detached 

from the spacecraft and descended into the planet’s atmosphere. Its in situ 

measurements broadened our understanding of the events and processes 

that formed the planets approximately five billion years ago. 

Chapter 5

The Galileo Spacecraft
108 MISSION TO JUPITER: A Histor y of the Galileo Project Chapter 5 I The Galileo spacecraft 109

• Explore the vast region of magnetic fields and ionized gas (plasma) that surrounds Jupiter. To meet this objective, the main section of the Galileo Orbiter 

was given a spin, which allowed its instruments to sweep through and measure characteristics of fields and particles in all directions relative to the 

spacecraft. The spinning section of the Orbiter also carried communications 

antennas, the main computers, and most of Galileo’s support systems.

• Investigate the nature of the Jovian system’s principal satellites. Cameras and 

sensors capable of forming high-resolution images in the visible and other 

parts of the spectrum were needed to meet this objective, as well as a stable 

platform from which to take the pictures. Galileo’s designers included a 

“despun” (nonspinning) section in the Orbiter that could be pointed and 

maintained at a given orientation. From this platform, Galileo’s imaging 

system obtained pictures of Jovian satellites at resolutions from 20 to 1,000 

times better than Voyager’s best.1

The above objectives necessitated developing a spacecraft of great complexity 

and technological capability. Figure 5.1 provides an illustration of some of the Galileo 

spacecraft’s many parts and instruments. The spacecraft made it possible for scientists 

to conduct a long-term, close-range study of the Jovian system. The Atmospheric Probe, 

which weighed 340 kilograms (750 pounds), carried six scientific instruments. The Orbiter 

carried 10 instruments and weighed 2,200 kilograms (4,900 pounds), including about 900 

kilograms (2,000 pounds) of rocket propellant. The spacecraft radio link to Earth and 

the Probe-to-Orbiter radio link served as instruments for additional scientific investigations. Galileo communicated with its controllers and scientists through the Deep Space 

Network’s tracking stations in California, Spain, and Australia.2

The Political and Economic Reasons for a Three-Segment Spacecraft

The complex Galileo spacecraft design arose not only to satisfy the scientific objectives 

discussed above, but also because of politics and cost factors involved in creating the 

mission. In the 1970s, the space science community debated at great length what types 

of projects should follow the Pioneer and Voyager flyby missions to the outer planets. 

As discussed in chapter 2, the space science community formed many committees and 

conducted numerous studies to help answer this question. After the successful Pioneer 

encounters with Jupiter in 1973 and 1974, more difficult spacecraft maneuvers seemed 

possible. Pioneer collected enough data about the Jovian system, for instance, that NASA

engineers thought that they could now design a probe spacecraft to descend through 

Jupiter’s atmosphere and stay operable long enough to make valuable observations. 

1

	 John R. Casani, testimony for Space Science and Applications Subcommittee, Committee on Science and

Technology, United States House of Representatives, 1 March 1978, JPL Archives; “Galileo’s Science Instruments,”

Galileo: Journey to Jupiter, http://www.jpl.nasa.gov/galileo/instruments/ (accessed 11 April 2000); NASA Office of

Space Science and Applications, “Mission Operation Report,” Report No. E-829-34-89-89-01, 1989, p. 12.

2 “Galileo Overview,” Galileo: Journey to Jupiter, http://www.jpl.nasa.gov/galileo/overview.html (accessed 1 November

2000).

3	 James A. Van Allen, “Planetary Exploration,” section 11 in “What Is a Space Scientist? An Autobiographical Example,”

University of Iowa, Department of Physics and Astronomy, http://www-pi.physics.uiowa.edu/java/, 22 August 2002.

This article originally appeared in Annual Review of Earth and Planetary Sciences 18 (1990): 1–26.

Figure 5.1. The Galileo spacecraft consisted of three major parts: the Atmospheric Probe

and the spun and despun sections of the Orbiter, each of which had specific tasks to perform. 

(Adapted from JPL image no. 230-1235)

What made Jupiter mission planning complicated was that several different factions were involved in the debates, and each one envisioned a different kind of mission. 

Atmospheric scientists wanted a probe descent into the Jovian atmosphere. The “space 

physics” faction, which wanted to study the Jovian magnetosphere, favored a spinning 

spacecraft that would orbit Jupiter and take fields-and-particles data. And the “remote 

sensing” faction needed a stable, nonrotating orbiting platform on which to mount its 

imaging equipment. The trouble was, trying to do everything that the scientists wanted 

could require two or three trips to Jupiter, and this was economically untenable. 

A spacecraft needed to be designed that would satisfy at least some objectives of the different scientific disciplines. In 1976, James Van Allen of the University of 

Iowa headed a NASA committee known as the Jupiter Orbiter Probe Science Working 

Group (JOPSWG), whose purpose was to “develop a follow-on Jupiter mission of more 

advanced capability than the Pioneers and the Voyagers, with a deep atmospheric entry 

probe and an orbiter having a useful lifetime of at least two years.”3

 JOPSWG attempted 

to combine the best aspects of the different scientific factions’ visions into one spacecraft 

concept. According to Torrence V. Johnson, who served on Van Allen’s committee and 

was later appointed Galileo’s Project Scientist, “What Galileo became was basically a massive compromise in trying to get all the various scientific disciplines on board . . . . Van 

Allen was actually the godfather of the mission as it currently exists. He played the role 

High Gain Antenna

(Earth Communications

and Radio Science)

Spun Section

Plasma Science

Probe Relay Antenna

Dust Detector

Ultraviolet Spectrometer

Solid State Imaging

Plasma Wave

Search Coil Sensor

Magnetometer

Sensors

Plasma Wave

E-Field Sensor

Radiosotope Thermoelectric

Generator

Near Infrared

Mapping Spectrometer

Photopolarimeter

Radiometer

Atmospheric

Entry Probe

Retropropulsion

Module

Radioisotope

Thermoelectric

Generator

Despun

Section

Energetic

Particles

Detector
110 MISSION TO JUPITER: A Histor y of the Galileo Project Chapter 5 I The Galileo spacecraft 111

8

	 	James W. Stultz, “Thermal Design of the Galileo Spun and Despun Science,” J. Spacecraft 28 (March–April 1991):

139; “About JPL,” http://www.jpl.nasa.gov/about_JPL/about_JPL_index.html (accessed 20 November 2001); John

Casani interview, tape-recorded telephone conversation, 29 May 2001.

9

		“Galileo Frequently Asked Questions (FAQ)—General Spacecraft Anatomy,” http://www.jpl.nasa.gov/galileo/faqana.

html (accessed 4 December 2000); “Thermal Blanketing,” Galileo Messenger (March 1983): 3.

10	“Flight Cabling,” Galileo Messenger (March 1983): 2.

of bringing the [factions] together and getting them to agree to necessary compromises to 

get a mission that would do a good job for everybody. And he did it in a pretty effective 

and objective way.”4

Selecting Science Instrumentation To Meet Mission Objectives

In the process of developing the mission concept, JOPSWG had to consider the “number, 

type, and capabilities of instruments that might be carried on the mission to make a credible case the mission would achieve its primary scientific objectives.”5

 Torrence Johnson 

commented that this was

. . . always a somewhat incestuous process since the people on the 

committee inevitably have their own axes to grind—involving conflicts 

between discipline areas like remote sensing versus space physics, 

cameras versus spectrometers, whose instrument worked best on the 

last mission, etc. Nevertheless, the (JOP)SWG had to come to consensus 

on a package that would have credibility with the rest of the science 

community. This involved putting together lists of prioritized objectives 

and how precise measurements had to be if they were to be useful, and 

comparing those with the capabilities of previously flown instruments 

and instruments under development.6

Numerous researchers and instrument developers gave presentations to JOPSWG 

during this time, trying to sell their current instrument concepts. JOPSWG eventually 

developed a “straw-man” payload, or, in other words, a set of instruments that could meet 

the minimum objectives of the mission and which were credible in terms of their costs, 

masses, power requirements, and data-collection rates. 

In NASA’s “Announcement of Opportunity for Outer-Planets Orbiter/Probe 

(Jupiter),” the Agency solicited proposals for instrumentation to meet primary Jupiter mission objectives. Proposers were given information about the straw-man payload but were 

not required to limit themselves to just those instruments—if a group felt that it could 

meet the objectives better with another approach, it was free to propose it. The straw-man 

instrument types did “have an edge” over other instruments in the selection process, however, having already been examined and discussed by JOPSWG and identified as being 

capable of meeting mission objectives.7

The specific instruments selected are discussed later in this chapter in the 

“Atmospheric Probe” and “Orbiter Scientific Experiments” sections. 

Spacecraft Features

NASA’s lead Center for robotic exploration of the solar system, the Jet Propulsion 

Laboratory in Pasadena, California, fabricated the Galileo Orbiter and was given overall 

responsibility for the mission. NASA’s Ames Research Center, one of whose specialties was 

atmospheric research, oversaw the design and development of the Jupiter Atmospheric 

Probe. The retropropulsion module (RPM), which was part of the Orbiter’s spun section, was built by Messerschmitt-Bolkow-Blohm of the Federal Republic of Germany 

(West Germany). It contained the 400-newton engine used for trajectory maneuvers 

and Jupiter Orbit Insertion, and it also included two 10-newton thrusters for spin and 

altitude control.8

Figure 5.1 depicts a long extension jutting out from the spacecraft. This was 

Galileo’s 11-meter (36-foot) science boom, which was attached to the Orbiter’s spinning 

section and on which fields-and-particles instruments were mounted. These instruments 

were placed there because they happened to be extremely sensitive to local fields generated by Galileo’s other equipment, and the distance that the science boom provided 

helped minimize interference. For Galileo’s launch, the science boom was collapsed into 

a cylinder only 0.6 meter (2 feet) long. After Galileo separated from its Inertial Upper 

Stage, the boom was extended like a telescope to its full 11-meter length. 

Figure 5.1 also depicts the plutonium-bearing RTGs, which, like the fieldsand-particles instruments, were located a safe distance away from the main body of the 

Galileo spacecraft.9

Flight Cabling

Galileo was the most ambitious and complex interplanetary spacecraft that had yet been 

launched. If laid end to end, the electrical wire used in the Orbiter’s many cables would 

have stretched 7,500 meters (25,000 feet), or nearly 5 miles. Over 700 connectors joined 

these cables to one another and to Galileo equipment. Every connector and cable had 

to be electrically shielded to protect equipment from Jupiter’s strong magnetic fields and 

radio signals, as well as from electrostatic discharges on the spacecraft. Poorly shielded 

cables and connectors could have caused interference or damage to computers and sensitive instruments. Voyager, which spent only weeks in Jupiter’s hostile environment, 

experienced some electrical discharge problems. Galileo would spend years in the region 

and would get much closer to Jupiter.10

Project staff spent four months installing the myriad cables in the Orbiter’s despun 

section and half a year in the spun section. This was painstaking, labor-intensive work, 

sometimes performed with tweezers (see figure 5.2). The cabling team had to solder 

4

	 Torrence V. Johnson interview, tape-recorded telephone conversation, 31 July 2001; Craig B. Waff, “Jupiter Orbiter

Probe: The Marketing of a NASA Planetary Spacecraft Mission” (paper presented in the “National Observatories:

Origins and Functions (The American Setting)” session of the American Astronomical Society meeting, Washington,

DC, 14 January 1990).

5

	 Torrence V. Johnson, e-mail message, 24 October 2001.

6

	 Ibid.

7

	 Johnson e-mail message; “Announcement of Opportunity for Outer-Planets Orbiter/Probe (Jupiter),” 1 July 1976, A.O.

No. OSS-3, folder 18522, NASA Historical Reference Collection, Washington, DC.
112 MISSION TO JUPITER: A Histor y of the Galileo Project Chapter 5 I The Galileo spacecraft 113

tens of thousands of electrical contacts, and connectors had to be “potted”—filled with a 

molding compound that held individual wires in place and made the cables more durable 

and less likely to fail during the mission. Cables that would be directly exposed to space 

required fiberglass sleeving to protect them from micrometeoroids. The cabling crew consisted of 15 technicians who worked 60-hour weeks to deliver the completed cables on 

schedule and who had been intensively trained in cable fabrication, soldering, crimping 

(an alternate method to soldering of fastening wires or electrical shielding to connectors), 

potting, and various cleaning techniques. This training was critical—a carelessly soldered, 

“cold” joint or improperly crimped connection could impair an instrument’s functioning 

or separate during the mission and disrupt the gathering of key data.11

Figure 5.2. Galileo’s despun section receives its cabling. 

Galileo’s Dual-Spin Design

Unlike previous planetary spacecraft, Galileo was designed with a “dual-spin” feature. 

Part of its Orbiter rotated continuously at typically 3 rpm, which accommodated 

fields-and-particles experiments. The craft’s magnetometer, for instance, built up data 

on a magnetic field by sweeping through it. The despun section of the Orbiter remained 

stationary, providing a fixed orientation for imaging equipment and other sensors (see 

figure 5.1). 

During delicate propulsive maneuvers, as well as at the time of Probe release, 

spacecraft stability was critical. To maximize stability, the sections of the spacecraft were 

locked together and spun as one at a higher-than-usual rotation rate of 10 rpm. A stable 

spacecraft helped to precisely aim the Probe toward the Jovian atmosphere when it 

was released from Galileo. Galileo’s higher spin rate also helped direct the Probe on its 

proper course in much the same way as a gun barrel’s rifling spins a bullet to send it on 

a straighter line toward its target. Aiming the Probe accurately was absolutely essential 

Figure 5.3. Cross section of Galileo’s spin bearing assembly showing slip-ring and rotary 

transformer assemblies. (JPL image 230-957Bc) 

because the Probe had no propulsion system of its own to correct any mistakes. Once it 

was sent away from Galileo, its course could not be altered.12

Fabricating a dual-spin spacecraft required that a reliable electrical interface be 

developed to carry signals and power between its despun and spinning sections. Cables 

could not be used; they would soon have become twisted and wrapped around various 

parts of the spacecraft. Instead, the spun and despun sections of Galileo were mechanically connected by means of a “spin bearing assembly (SBA),” and it was via this that 

electricity was passed. Electrical power, as well as data at low rates, was sent across the 

SBA through the sliding contacts of 48 slip-rings. In these assemblies, a flexible brush contact rode over a large revolving ring that was electrically connected to the spun section of 

Galileo. Data were transmitted at high rates through 23 rotary transformers that consisted 

of pairs of coiled wires wrapped around a ferrite core. In each pair, one coiled wire was 

connected to and rotated with the spun section of Galileo; the other wire was connected 

to the despun section. The electric current passing through one coil created a magnetic 

field that induced current in the other coil. Data were transmitted without physical contact 

between the coils (see figure 5.3). 

It took project staff considerable effort to achieve reliable data transmission 

through the spin bearing assembly. During extensive testing begun in 1984, electrical 

noise interfered with signal transmission. After laborious disassembly and inspection of 

11	 Ibid. 12	“Galileo FAQ”; “WWWWWH,” Galileo Messenger (March 1983): 4.
114 MISSION TO JUPITER: A Histor y of the Galileo Project Chapter 5 I The Galileo spacecraft 115

the assembly, the staff determined that chemical contamination accumulated during the 

manufacturing process was probably the cause of the interference. Ultraclean components replaced questionable ones, and redundancies were built into the system to make 

it less sensitive to electrical noise.13

Multipurpose Blanketing for Galileo

Galileo staff covered most of the spacecraft, except for its radioisotope thermal generators, 

antennas, and certain radiating areas, with nearly 300 thermal blankets. The blanketing 

design process began with paper patterns fit onto models of various Galileo components. 

The blankets were then fabricated from these patterns and laboriously laced together into 

place on the spacecraft.14

The blanketing consisted of multiple layers of insulation. The stratum closest to 

the spacecraft was a sort of “thermal underwear” made up of 10 to 20 layers of aluminized 

Mylar and Dacron net. Not only did the blankets help vital parts of Galileo to retain heat, 

but they also protected against impacts from micrometeoroids. The blanketing on most of 

Galileo was mounted with a space between it and the surface in order to better disperse 

the energy of a micrometeoroid impact. 

A third function of the blanketing was to prevent electrical discharges. This was 

accomplished by covering Mylar reflecting layers with electrically conductive blanketing 

that was grounded to the rest of the spacecraft. In the past, spacecraft had been plagued 

by electrical arcing, which sometimes occurred when one surface amassed a large differential charge relative to another surface. Signals caused by this arcing had been wrongly 

interpreted as radio signals from a source external to the spacecraft. Galileo’s blanketing 

electrically connected different regions of the spacecraft surface with each other so that 

there would be no more than 10 volts of potential between the regions at any time. This 

equalization of potential prevented arcing and spurious radio signals.15

Galileo’s Main Power Source: Radioisotope Thermal Generators 

Electrical power for most of Galileo’s equipment was provided by its RTGs, in which heat 

produced by radioactive decay was converted to 500 watts of direct current. (The Probe’s 

more modest needs were supplied by long-life lithium batteries.)16

Several NASA spacecraft had employed RTGs in the past, including Viking Mars 

landers, Apollo Moon landers, and the Voyager and Pioneer craft on missions to the outer 

planets. NASA’s policy has been to use RTGs on robotic spacecraft traveling beyond the 

orbit of Mars, where the faint radiation from the Sun makes solar panel arrays impractical; they would have to be prohibitively large to collect enough energy for a spacecraft’s 

system. RTGs were used on Mars and lunar landers as well because the long nights would 

have reduced the effectiveness of solar panels.17

The Atmospheric Probe 

Attached to one end of the Galileo Orbiter was a pod of scientific instruments called the 

Jupiter Atmospheric Probe (or, as figure 5.1 calls it, the “Atmospheric Entry Probe”). Nearly 

five months before Galileo reached Jupiter, its two parts—Orbiter and Probe—separated 

from each other with the aid of explosive nuts. Three springs pushed the Probe away at 

just the right angle to send it on a collision course with the planet.18

Once the Probe was released, it drifted in freefall toward Jupiter, for it had no 

on-board propulsion system. The planet’s strong gravitational force reeled the Probe in 

at ever-increasing speeds. The Probe then conducted the most challenging atmospheric 

entry ever attempted by NASA. By the time the Probe entered the Jovian atmosphere 147 

days after release, it had attained a speed of 171,000 kilometers per hour (106,000 miles 

per hour) relative to the planet—a speed sufficient to fly from Washington, DC, to San 

Francisco in 100 seconds. When the fast-moving Probe hit the Jovian atmosphere, it experienced a braking force equal to 250 times the gravitational force at Earth’s surface. Then it 

fell through a region where atmospheric pressure ranged from one-tenth to over 20 times 

Earth’s surface pressure. The Probe had to be extremely durable to withstand such entry 

forces and pressures and still be able to collect and transmit useful data.19

Probe Components

In order to simulate the Probe’s expected response to atmospheric entry stresses, NASA’s 

Ames Research Center had built special equipment during Galileo’s design process that 

included arcjet and laser facilities. Ames, working with NASA Langley Research Center 

and outside contractors, also wrote a sophisticated modeling program that predicted the 

Probe’s ability to withstand expected entry temperatures. These simulations helped to 

identify the need for a Probe composed of two major segments, the deceleration module 

and the descent module (see figure 5.4). 

The deceleration module provided an extremely tough outer shell for the Probe; 

it surrounded and protected its payload during the first 2 minutes of atmospheric entry, 

when temperatures would be the highest. The deceleration module consisted of fore and 

13	 “Spin Bearing Assembly,” Galileo Messenger (September 1985): 2–3.

14	 “Thermal Blanketing,” Galileo Messenger (March 1983): 3.

15	 Ibid., p. 3.

16	 “Mission Operation Report,” p. 14.

17	 “Backgrounder: Shuttle Launches of Ulysses and Galileo,” December 1984, John Casani Collection, folder 48, box 6

of 6, JPL 14, Galileo Discreet Correspondence, JPL Archives.

18	Ames Research Center, “Probe Mission Time Line,” in Galileo Probe Background, no date given, folder 18522, NASA

Historical Reference Collection, Washington, DC.

19	Ames Research Center, “Galileo Probe To Look for Secrets of Jupiter” and “The Probe Spacecraft,” both in Galileo 

Probe Background, no date given, folder 18522, NASA Historical Reference Collection, Washington, DC; J. R. Casani,

“Galileo Fact Sheet,” 4 June 1981, JPL Interoffice Memo GLL-JRC-81, Galileo Correspondence 4/81–6/81, folder 28,

box 3 of 6, JPL Archives; Galileo to Jupiter: Probing the Planets and Mapping the Moons, JPL Document No. 400-15

7/79 (Washington, DC: Government Printing Office (GPO) No. 1979-691-547, 1979); “Probe Mission Events,” http://

spaceprojects.arc.nasa.gov/Space_Projects/galileo_probe/htmls/Probe_Mission.html (accessed 17 October 2004),

folder 18522, NASA Historical Reference Collection, Washington, DC.
116 MISSION TO JUPITER: A Histor y of the Galileo Project Chapter 5 I The Galileo spacecraft 117

Figure 5.4. The Probe deceleration and descent modules. (JSC digital image collection, NASA 

photo number S89-44175)

aft heatshields, the supporting structure for the heatshields, and thermal-control hardware 

for the Probe’s atmospheric entry. The fore and aft heatshields were made of carbon 

phenolic and phenolic nylon, respectively, which had been used often for Earth reentry 

vehicles but were subjected to conditions at Jupiter that the materials had never before 

experienced in flight. The deceleration module had to withstand the enormous stresses 

that occurred when the Probe hit the dense Jovian atmosphere at 171,000 kilometers per 

hour. The Probe slammed into and rapidly compressed the atmospheric gases ahead of it. 

It also experienced tremendous friction from gases streaming by it. The friction and atmospheric compression heated and ionized nearby atmospheric gases to temperatures twice 

as hot as the surface of the Sun, thereby generating an incandescent plasma envelope that 

surrounded the Probe as it fell. 

The Probe’s plunge through Jupiter’s upper atmosphere has been compared to 

a trip through a nuclear fireball. The job of the heatshields was to disperse the enormous 

thermal energy to which they were subjected before the sensitive electronics within the 

Probe got cooked. The shields did this by means of an ablative process in which heat 

energy was first absorbed as it vaporized the shields’ surface, then carried away by the 

gases produced. This process quickly eroded the shields (see figure 5.5). During the first 

2 minutes after atmospheric entry, this erosion reduced the forward heatshield mass from 

152 kilograms (335 pounds) to only 70 kilograms (154 pounds).20

Figure 5.5. Probe heatshield ablation. The forward heatshield, which was made of carbonphenolic, accounted for about one-half of the Galileo Probe’s total mass before entry (as 

shown in the left half of the figure). More than one-half of the heatshield’s mass was ablated 

during the entry period. The right half of the figure shows the final heatshield shape (in black) 

as determined from sensors within the heatshield. The ablated material is shown in grey. 

(NASA Ames photo number ACD96-0313-13)

Most of the Probe’s reduction in speed took place within those first 2 minutes. 

By the end of that time, the braking action of Jupiter’s atmospheric gases had cut the 

Probe’s speed to about 1,600 kilometers per hour (1,000 miles per hour), slow enough 

that parachutes could be deployed without getting ripped apart or melted. As will be 

explained in chapter 8, the parachutes helped to separate the Probe’s deceleration module 

components from its descent module. This descent module contained the payload—

delicate scientific instruments and electronics that would study the Jovian atmosphere. 

The descent module fell slowly through the Jovian atmosphere, its speed controlled by a 

parachute, while its instruments gathered data. These data were then transmitted back to 

the over-flying Orbiter, which stored them for later transmission to Earth.21

20	“Galileo Probe To Look for Secrets of Jupiter,” “Probe Mission Time Line,” and “The Probe Spacecraft,” all in

Galileo Probe Background; “The Galileo Probe Spacecraft,” http://spaceprojects.arc.nasa.gov/Space_Projects/

galileo_probe/htmls/probe_spacecraft.html (accessed 23 March 2004), available in folder 11626, NASA Historical

Reference Collection, Washington, DC; “Probe Mission Events,” http://spaceprojects.arc.nasa.gov/Space_Projects/

galileo_probe/htmls/Probe_Mission.html (accessed 17 October 2004), available in folder 11626, NASA Historical

Reference Collection, Washington, DC; “Galileo Probe Heat Shield Ablation,” http://spaceprojects.arc.nasa.gov/

Space_Projects/galileo_probe/htmls/Heat_Shield.html (accessed 18 October 2004), available in folder 11626, NASA

Historical Reference Collection, Washington, DC.

21	 “The Probe Spacecraft”; “The Galileo Probe Spacecraft”; “Probe Mission Events”; Ames Research Center, “Artwork

of Parachute Deployment,” http://spaceprojects.arc.nasa.gov/Space_Projects/galileo_probe/htmls/Parachute_

deployment.html (accessed 17 October 2004), available in folder 18522, NASA Historical Reference Collection,

Washington, DC.
118 MISSION TO JUPITER: A Histor y of the Galileo Project Chapter 5 I The Galileo spacecraft 119

Power Source for the Probe 

The Galileo Probe needed an electrical power source that would retain a high energy 

density during the years of interplanetary travel and be able to deliver its power quickly 

during the final phase of the Probe’s life as it approached, then plunged down through 

Jupiter’s ever-thickening atmosphere. To achieve such a power source, three parallel battery modules were developed, each containing 13 lithium–sulfur dioxide cells connected 

in series. NASA staff had chosen this design in 1976, 13 years before the Galileo launch, 

during the early development of the Probe. In making the selection, the discharge capabilities of sample cells and modules were evaluated while the batteries were subjected 

to rigorous high-g simulations of the launch and the plunge into Jupiter’s atmosphere. 

Life testing was an important part of the evaluation. The lithium–sulfur dioxide cells had 

to demonstrate that their voltage delay at turn-on, as well as their ampere-hour capacity, 

would remain within acceptable limits even after the multiyear journey through space.

Because the Galileo launch was repeatedly delayed, numerous battery-cell lots 

were fabricated. For maximum battery life, it was vital that the Probe carry the freshest 

cells possible. The 10th lot of cells, manufactured in 1988, became the lot used on the 

spacecraft. All of the lithium–sulfur dioxide cells were manufactured at Honeywell’s Power 

Sources Center in Pennsylvania. Each cell was designed to be the size of a D battery so 

that the company’s existing tooling and assembly experience could be exploited.22

Probe Entry Trajectory

The Probe’s trajectory was designed to intersect Jupiter’s atmosphere at an angle of only 8 

degrees below horizontal. The angle needed to be shallow so that the Probe would slow 

gradually and be able to withstand the forces of its deceleration, as well as the frictional 

heat it generated. But an angle of 8 degrees was also just steep enough to prevent the 

Probe from bouncing back out of the atmosphere like a stone skipping along the surface 

of a pond.23

Probe Scientific Experiments and Instrumentation

The Probe’s atmospheric exploration focused on advancing our understanding of Jupiter 

beyond what was known from Earth-based and previous spacecraft studies. These earlier 

investigations had to rely on remote observations of the Jovian atmosphere. Analysis of 

the planet’s complex, three-dimensional atmospheric dynamics had to be gleaned from 

two-dimensional images of cloud tops. But to fully understand the colorful swirls and 

rapid movements visible at the cloud tops, scientists needed to know about the dynamics and composition deep within the atmosphere and the nature of the powerful forces 

that drive Jovian weather patterns. The Probe had the advantage of being able to provide 

key in situ measurements of atmospheric characteristics as a function of depth. What 

the Probe found shed light not only on Jupiter’s atmosphere, but also on those of the 

other gas giant planets—Saturn, Uranus, and Neptune. However, the Probe did have the 

limitation that, at a given moment, it could take observations of only one place in the 

atmosphere. An important question that arose during the interpretation of Probe data was 

how typical it was of the rest of the Jovian atmosphere.

James Van Allen’s JOPSWG team, whose activities were discussed above, had 

furnished NASA in 1976 with a “rationale for the mission’s objectives and measurement 

requirements,” which helped NASA staff to develop its Announcement of Opportunity 

soliciting proposals for instrumentation to meet mission objectives. The announcement 

identified a series of specific objectives that an Atmospheric Entry Probe was to fulfill. 

These objectives were as follows: 

• Characterize structure (temperature, pressure, and density) of the Jovian 

atmosphere to a pressure depth of at least 10 bars (10 times the atmospheric 

pressure at sea level on Earth). 

• Determine the chemical composition of the Jovian atmosphere.

• Determine the location and structure of the Jovian clouds. 

• Measure the vertical energy flux to determine the local radiative energy 

balance. 

• Characterize the upper atmosphere.

• Determine the nature and extent of cloud particles. 

Furthermore, the Probe’s payload of science instruments was to weigh no more 

than 30 kilograms (66 pounds).24

Most of the scientists on Van Allen’s JOPSWG team who advocated an atmospheric entry probe either had served on the Pioneer Venus mission or had connections 

to it. Not surprisingly, their instrumentation formed the basis for the Probe’s straw-man 

payload design.25

Critical to the Probe’s success was a method of analyzing the Jovian atmosphere’s 

chemical composition. In selecting the right instrument for this, weight considerations 

could never be forgotten, since the total Probe payload weight could not exceed a very 

22	L. M. Hofland, E. J. Stofel, and R. K. Taenaka, “Galileo Probe Lithium–Sulfur Dioxide Cell Life Testing,” in IEEE 

Proceedings of the Eleventh Annual Battery Conference on Applications and Advances (held in Long Beach, CA, 9–12

January 1996), pp. 9–14.

23	“Galileo Probe To Look for Secrets of Jupiter.”

24	T. V. Johnson, C. M. Yeates, and R. Young, “Space Science Reviews Volume on Galileo Mission Overview,” Space

Science Reviews 60 (1992): 7–8; Galileo to Jupiter; “Galileo Project Information,” http://nssdc.gsfc.nasa.gov/planetary/

galileo.html (accessed 15 March 2000); Hofland, Stofel, and Taenaka, “Galileo Probe Lithium–Sulfur Dioxide Cell Life

Testing,” p. 9; “Galileo Probe To Look for Secrets of Jupiter.”

25	Johnson e-mail message.
120 MISSION TO JUPITER: A Histor y of the Galileo Project Chapter 5 I The Galileo spacecraft 121

modest 66 pounds. The neutral mass spectrometer (NMS)26 was a tried-and-true instrument 

for analyzing neutral-particle atmospheric chemical compositions that had been flown on 

the Pioneer Venus and other missions. It also had a modest weight. The version flown on 

Pioneer Venus’s large atmospheric probe weighed 9 kilograms, or about 20 pounds.27

Another common instrument used for chemical analysis is the gas chromatograph (GC).28 Although GCs and NMSs were used by the Pioneer Venus mission, a gas 

chromatograph was not considered an adequate replacement for a mass spectrometer 

aboard the Galileo Probe. According to Joel Sperans, former Probe Project Manager, 

“You get more information with a mass spec than with a gas chromatograph.”29 The issue 

before JOPSWG was more “how massive and costly [the mass spectrometer] could afford 

to be,” rather than whether or not to use a mass spectrometer.30 What the JOPSWG team 

did consider was using a combination gas chromatograph–mass spectrometer (GC/MS), 

a very powerful analytical tool. But JOPSWG was “mindful that the Viking [Mars] lander 

GC/MS at that time was the most expensive thing ever flown and had created huge programmatic problems for Viking.”31 JOPSWG concluded that “while a GC/MS would be 

nice for Jupiter, it was not required to meet our minimum objectives and would probably 

prove to be too costly and massive to be affordable.”32

NASA’s instrument-selection staff ended up agreeing with JOPSWG. Following 

the Galileo Announcement of Opportunity, NASA chose a proposal from Goddard Space 

Flight Center to build a basic mass spectrometer, “but with some clever additions of gas 

adsorption cells” that added GC/MS-like capabilities to the instrument.33

A critical question that the Probe addressed was, how much helium did the 

Jovian atmosphere contain? Precise measurements of helium quantities and of the ratio of 

hydrogen to helium would give important clues “as to the origins of the atmosphere and to 

the origins of the planet.” Although neutral mass spectrometer data would shed some light 

on helium abundance, scientists planning the mission thought they could “use a little more 

horsepower” in addressing the helium question. They decided to add another instrument, a 

helium abundance detector (HAD), to enhance the Probe’s helium analysis.34

Other key measurements that the Probe had to perform were of atmospheric 

temperature, pressure, density, and molecular weight as functions of depth. These parameters were measured by sensors in the atmospheric structure instrument (ASI). The 

parameter values also served as reference scales for other Probe experiments and as 

critical constraints in computer models of atmospheric composition and dynamics. 

The Probe’s nephelometer (NEP) and NMS studied Jovian cloud particle states 

(liquid versus solid) and the composition, structure, and location of cloud layers, while 

the lightning and radio emissions detector (LRD) searched for any lightning activity in 

the clouds. Sunlight penetrating into the atmosphere and infrared radiation upwelling 

from the planet were characterized using the Probe’s net flux radiometer (NFR). 

One reason for adding a lightning detector to the suite of Probe instruments was 

that Voyager imagery and plasma wave data had indicated the possible existence of lightning phenomena in the Jovian atmosphere, and in situ measurements were needed to verify 

the phenomenon. In addition, scientists were interested in the planet’s radio-frequency 

emissions, which had been received for many years by stations on Earth. Some scientists 

thought that Jovian lightning could provide an energy source for these emissions.35

An energetic particles instrument (EPI) was also added to the Probe to study 

ionized magnetospheric particles from an altitude of 5 Jupiter radii down to the atmosphere.36 A camera was not selected as a Probe instrument. The biggest problem with 

including a camera would have been its data generation rate. Digital pictures require a 

considerable amount of data, but the Probe was only capable of transmitting data at the 

rate of 128 bits per second. The total quantity of data that the Probe transmitted during 

its entire atmospheric descent was about one million bits, enough for only one or two 

images. According to Charlie Sobeck, who served as the Probe Deputy Project Manager 

and Systems Engineering Manager, “We just didn’t have the data [transmission] rate to 

support a camera.”37

Characteristics of the Probe’s instruments are summarized in table 5.1. Note 

the wide range of institutions involved in managing the Probe’s experiments. Photos 

of Probe instruments are included in figure 5.6. The findings obtained from the various 

Probe instruments’ measurements are discussed in detail in chapter 8, “Jupiter Approach 

and Arrival.” 

Neutral Mass Spectrometer. The NMS was a quadrupole mass spectrometer, 

developed by Goddard Space Flight Center in Maryland, that repeatedly measured 

chemical and isotope compositions of the gases in Jupiter’s atmosphere and recorded 

vertical variations in the compositions. The NMS weighed 27.8 pounds (12.6 kilograms) 

and drew only 29 watts of power (see table 5.1). At the time the instrument was built, it 

was considered to be very light and economical, although new generations of the NMS

are even smaller and draw less power.38

26	A mass spectrometer is an instrument that performs chemical analyses by converting molecules of a gas sample

into ions and then separating the ions according to their mass-to-charge ratio. The combination of different

masses from a substance, called its mass spectrum, provides a “fingerprint” of the substance. The Probe used its

mass spectrometer to identify the different chemical constituents of Jupiter’s atmosphere. “Mass Spectrometer,”

Microsoft® Encarta® Online Encyclopedia, 2001, http://encarta.msn.com, © 1997–2000 Microsoft Corporation, all

rights reserved.

27	 “Neutral Particle Mass Spectrometer,” National Space Science Data Center (NSSDC) Master Catalog: Experiment, 

http://nssdc.gsfc.nasa.gov/nmc/tmp/1978-078D-6.html (accessed 7 September 2000).

28	The gas chromatograph is a device that performs chemical analysis by separating the volatile constituents of a

substance. “Gas Chromatograph,” Encarta World English Dictionary (North American Edition), http://dictionary.msn.

com/find/entry.asp?refid=1861694680, © and (P) 2001 Microsoft Corporation, all rights reserved.

29	Joel Sperans, former Probe Project Manager, interview, tape-recorded telephone conversation, 26 October 2001.

30	Johnson e-mail message.

31	 Ibid.

32	 Ibid.

33	“The Pioneer Venus Multiprobe,” Pioneer Venus Project Information, http://nssdc.gsfc.nasa.gov/planetary/pioneer_

venus.html, 5 January 2001; Johnson e-mail message; “Probe Mass Spectrometer,” Galileo Messenger (August

1982): 2.

34	Joel Sperans interview.

35	“Galileo Project Information”; “Pioneer Venus Multiprobe”; “The Probe Science Instruments,” Galileo Messenger

(September 1995).

36	“Probe Instruments,” in Galileo Probe Background; “Probe Science Instruments.”

37	Charlie Sobeck, former Probe Deputy Project Manager and Systems Engineering Manager, interview, tape-recorded

telephone conversation, 26 October 2001.

38	Mass and power data were supplied by Charlie Sobeck, NASA Ames Research Center Probe Systems Engineering

Manager at the conclusion of the Probe project. Sobeck’s data were drawn from the “Galileo Probe Mass Properties

Report” of 2 April 1984 and Hughes space and communications group instrument fact sheets.
122 MISSION TO JUPITER: A Histor y of the Galileo Project Chapter 5 I The Galileo spacecraft 123

Instrument Function Mass in

kilograms

(pounds)

Power

Requirement

in watts

Principal Investigator(s) and

Institution(s)

Neutral mass

spectrometer

(NMS)

Analyzed gas composition. 13

(28)

29 Hasso Niemann,

NASA Goddard Space

Flight Center

Helium abundance

detector (HAD)

Determined atmospheric

hydrogen/helium ratios.

1.4

(3.1)

1.1 Ulf von Zahn, Bonn

Universitat & Institut fur

Atmosparenphysik an

der Universitat Rostock

Atmospheric

structure

instrument (ASI)

Recorded temperature, pressure,

density, and molecular weight.

4.1

(8.9)

6.3 Alvin Seiff, NASA

Ames Research Center

and San Jose State

University Foundation

Nephelometer

(NEP)

Located cloud layers and

analyzed characteristics of cloud

particles.

4.8

(11)

14 Boris Ragent, NASA

Ames Research Center

and San Jose State

University Foundation

Lightning and

radio emissions

detector (LRD) and

energetic particles

instrument (EPI)

LRD recorded radio bursts and

optical flashes. EPI measured

fluxes of protons, electrons, alpha

particles, and heavy ions orbiting

Jupiter’s magnetosphere.

2.7

(6.0)*

2.3** Louis Lanzerotti, Bell

Laboratories, University

of Florida, and Federal

Republic of Germany

Net flux

radiometer (NFR)

Determined differences between

light and heat being radiated

downward versus upward at

each altitude.

3.0

(6.7)

7.0 L. Sromovsky,

University of Wisconsin

Radio equipment Besides transmitting data up to

the Orbiter, the Probe’s radio

equipment served double duty

as an aid in measuring Jovian

wind speeds and atmospheric

absorption.

David Atkinson,

University of Idaho

*	 Combined mass of LRD and EPI.

**	 Combined power draw of LRD and EPI.

Data sources:

1.	 Mass and power data supplied by Charlie Sobeck, NASA Ames Research Center Probe Systems Engineering

Manager at the conclusion of the Probe project. Sobeck’s data were drawn from the “Galileo Probe Mass Properties

Report” of 2 April 1984 and Hughes space and communications group instrument fact sheets.

2. “The Atmosphere Structure Instrument,” Galileo Messenger (April 1981).

3. 	 “The Probe Science Instruments,” Galileo Messenger (September 1995).

4. “Probe Mass Spectrometer,” Galileo Messenger (August 1982).

5. “Net Flux Radiometer—Studying the Atmosphere,” Galileo Messenger (April 1987).

6. 	 “Probe Nephelometer,” Galileo Messenger (March 1983).

7. Ames Research Center, “Galileo Probe Background,” no date given. Copy available in folder 11626, NASA

Headquarters Historical Reference Collection.

Figure 5.6. Probe science instruments. (Adapted from JPL image number 230-900) 

A quadrupole mass spectrometer typically identifies gases by bombarding a 

neutral gas species with an electron beam to create ions. These ions are then passed 

through a four-pole “mass filter” that uses specific voltages between its poles to allow only 

ions of a certain mass-to-charge ratio to pass through the array and strike a detector. Other 

ions are deflected away before they reach the detector. The voltages between poles of the 

mass filter are varied rapidly, so that the mass-to-charge ratios allowed to pass through 

to the detector are also constantly varying. An ion’s mass-to-charge ratio can be surmised 

from the time that the ion strikes the detector.39

A mass spectrometer is calibrated and programmed to record the mass and 

intensity of ions (that is, the rate at which ions hit the detector) for a wide range of atomic 

masses. The Probe’s NMS measured all species with atomic masses from 1 through 52, 

as well as selected species of higher atomic masses, including krypton and xenon. In addition, the NMS also occasionally performed sweeps from 1 up to 150 atomic masses. The 

sampling range of the NMS was designed so that almost all of the Jovian atmospheric gases 

that entered the instrument could be analyzed. NMS measurements gave a cross section of 

Jovian atmospheric composition, and this aided greatly in understanding the dynamic processes that created the planet’s multihued, complex cloud formations.40

NEPHELOMETER

NET FLUX RADIOMETER (NFR)

HELIUM ABUNDANCE DETECTOR (HAD)

ATMOSPHERIC STRUCTURE INSTRUMENT

LIGHTING AND RADIO EMISSION

DETECTOR/ENERGETIC PARTICLE INSTRUMENT (LRD/EPI)

Shelf

Periphery

Atmospheric

Gas Inlet

Electronic

Unit

Gas Inlet

Assembly

Sensor Unit

NEUTRAL MASS SPECTROMETER (NMS)

Table 5.1. Atmospheric Probe instrumentation.

39	“Neutral Mass Spectrometer (NMS),” NSSDC Master Catalog: Experiment, NSSDC ID:1989-084E-3, http://nssdc.gsfc.

nasa.gov/database/MasterCatalog?sc=1989-084E&ex=3 (accessed 8 August 2003); Hugh Gregg, chemist and mass

spectrometer specialist, Lawrence Livermore National Laboratory, CA, interview, 2001; “Probe Mass Spectrometer,”

Galileo Messenger (August 1982), http://www.jpl.nasa.gov/galileo/messenger/oldmess/3Probe.html; Atmospheric

Experiment Branch in the Laboratory for Atmospheres at NASA Goddard Space Flight Center, “Introduction to the

Galileo Mission,” Introduction to the Branch Activities, http://webserver.gsfc.nasa.gov/ (accessed 8 August 2003).

40	“Neutral Mass Spectrometer”; “Probe Mass Spectrometer”; Gregg interview.
124 MISSION TO JUPITER: A Histor y of the Galileo Project Chapter 5 I The Galileo spacecraft 125

Gases entered the NMS through two inlet ports at the Probe’s apex. For the protection 

of the instrument, these ports remained sealed by metal-ceramic panels until after the Probe 

entered Jupiter’s atmosphere, at which time pyrotechnic devices blew the panels away.41

Jupiter’s atmosphere is starlike in composition, with hydrogen and helium composing approximately 90 percent and 10 percent of it, respectively. The atmosphere also 

contains minor amounts of methane, water vapor, ammonia, hydrogen sulfide, acetylene, 

and ethane, as well as the inert gases neon, argon, krypton, and xenon. Inert gases are so 

named because they do not generally combine with other elements to form compounds. 

They also do not settle out of a planetary atmosphere by liquefying or freezing, but 

remain suspended and in their gaseous state. Because of their stability, they are expected 

to occur in the same abundances as they did at the beginning of the solar system’s existence. Thus, their concentrations in the Jovian atmosphere give us information about our 

early planetary system. Cosmic abundances of the inert gases krypton and xenon are not 

well known, and so measurements taken by the Probe’s NMS instrument were especially 

important, for they offered the first opportunity to measure these elements in an undisturbed reservoir. Results obtained were being used to calibrate a large amount of data on 

noble gas abundances on Earth, the other inner planets, and meteorites.42

Voyager’s discovery of Jovian lightning raised the possibility that organic compounds exist on the planet. Organic compounds, which are the basis of life on Earth, can 

be formed when sparks arc across various mixtures of gases such as may be found on 

Jupiter. The Probe’s neutral mass spectrometer analyzed the Jovian atmosphere closely for 

various organic compounds such as hydrogen cyanide and acetonitrile.43

Helium Abundance Detector. The HAD, which was developed at Bonn University 

in Germany, determined the abundance ratio of helium to hydrogen in Jupiter’s atmosphere 

at pressures from 3 to 8 bars. The ratio was also measured by the NMS, but accurate measurements of this parameter were considered so important that the HAD, which was able 

to make the measurements with an uncertainty one-tenth that of NMS, was included in the 

Probe as well. When the instruments for the Probe were first chosen in 1977, the predominant scientific opinion was that Jupiter’s helium abundance was a relic of the distant past, 

in that it was the same as the helium abundance created during the Big Bang, and the same 

as that present in the solar nebula from which our Sun and planets were formed. Accurate 

measurements of Jupiter’s helium abundance would thus, it was believed, provide critical 

data on conditions at the instant of the universe’s creation.44

The prevailing theories were called into question when Voyager data from Saturn 

and Uranus, as well as more detailed information about Jupiter and the Sun, suggested 

that Jovian evolutionary processes could have altered the original helium abundance. 

HAD measurements are now seen as providing valuable clues about the origin and evolution of Jupiter itself.45

To make these measurements, the HAD used an optical sensor called a 

Jamin-Mascart two-arm interferometer, with an infrared light source. One arm of the 

interferometer was vented to Jupiter’s atmosphere, while the other arm was connected to 

a reference gas reservoir. The interferometer determined relative helium abundance by 

comparing the difference in refractive index between the atmosphere and the reference 

gas. (A medium’s index of refraction is calculated by dividing light waves’ speed through 

a vacuum by their speed through the medium.) When the light beam from the infrared 

source is split and directed through the gases in each of the interferometer’s arms, then 

merged back into one beam, an interference pattern of light and dark bands is formed. 

The particular pattern is dependent on the refractive index of the Jovian atmospheric gas, 

which is, in part, a function of the percentage of helium present. The HAD experimental 

apparatus was small compared to the NMS, weighing only 3.1 pounds (1.4 kilograms) and 

drawing only 1.1 watts of power (see table 5.1).46

Atmospheric Structure Instrument. The ASI, developed by San Jose State University 

Foundation in California, measured physical properties of the atmosphere—temperature, 

pressure, density, and molecular weight—over an altitude range from about 1,000 kilometers (600 miles) above the cloud deck down to the atmospheric depth at which the Probe 

ceased to operate, about 150 kilometers (90 miles). From the instrument’s measurements, 

Probe altitude and velocity of descent were calculated and then used in other experiments’ 

calculations as well as in the ASI experiment. Mission scientists were especially interested in 

the stability of the atmosphere and the levels at which various cloud layers occur. Stability 

observations were important for determining whether the Jovian atmosphere was turning over 

(like Earth’s lower atmosphere, the troposphere, does during a storm) or stagnantly layered 

(similar to Earth’s stratosphere, or to the troposphere in regions of temperature inversions).47

Nephelometer. The word nephele is Greek for cloud.48 The purpose of the 

Probe’s nephelometer was to determine the physical structure of Jupiter’s clouds, including cloud particle shapes, sizes, and concentrations, as well as locations of cloud layers. 

The shapes of the particles provided an indication of their state—solid (ice) or liquid.49

Understanding the characteristics of Jupiter’s clouds is essential to understanding 

the planet’s energy balance. Jupiter is very different from Earth. It is a giant planet, composed almost completely of hydrogen and helium, and it resembles a star perhaps more 

than it does a terrestrial-type planet. Like a star, Jupiter gives off more energy than it takes 

in. Jupiter radiates twice as much energy as it receives from the Sun, and it is this internal 

41	 “Neutral Mass Spectrometer”; “Probe Mass Spectrometer.”

42	“Neutral Mass Spectrometer”; “Probe Mass Spectrometer.”

43	“Neutral Mass Spectrometer”; “Probe Mass Spectrometer.”

44	“The Probe Science Instruments,” Galileo Messenger (September 1995).

45	 Ibid.

46	“The Probe Instruments,” in Galileo Probe Background; “Helium Abundance Detector (HAD),” NSSDC Master Catalog 

Display: Experiment, http://nssdc.gsfc.nasa.gov/cgi-bin/database/www-nmc?89-084E-01 (accessed 15 March

2000); Charles Sobeck, NASA Ames Research Center Probe Systems Engineering Manager, at the conclusion of the

Probe project, interview, tape-recorded telephone conversation, 26 October 2001.

47	 “Atmospheric Structure Instrument (ASI),” NSSDC Master Catalog Display: Experiment, http://nssdc.gsfc.nasa.gov/

cgi-bin/database/www-nmc?89-084E-02 (accessed 15 March 2000); “The Probe Instruments”; “The Atmosphere

Structure Experiment,” Galileo Messenger (April 1981); “Galileo Probe Mission Events.” Also, mass and power data

supplied by Charlie Sobeck, NASA Ames Research Center Probe Systems Engineering Manager, at the conclusion

of the Probe project. Sobeck’s data were drawn from the “Galileo Probe Mass Properties Report” of 2 April 1984 and

Hughes space and communications group instrument fact sheets.

48	Webster’s New World Dictionary of the American Language—College Edition (Cleveland and New York: World

Publishing Company, 1962), p. 984.

49	“Probe Nephelometer,” Galileo Messenger (March 1983).
126 MISSION TO JUPITER: A Histor y of the Galileo Project Chapter 5 I The Galileo spacecraft 127

heat that drives its complex weather patterns. Greater understanding of Jovian weather 

and of its cloud and atmospheric structure provides clues regarding Jupiter’s basic dynamics and internal processes.50

As the spinning Probe fell through the clouds, the nephelometer analyzed cloud 

particles by shining an infrared laser through them and measuring how the light beam 

scattered. Cloud particle sizes were determined by the intensity of scattered light intercepted by special sensing mirrors positioned at 5.8 degrees, 16 degrees, 40 degrees, 70 

degrees,. and 178 degrees off the direction of the laser beam.51

The nephelometer weighed 11 pounds (4.8 kilograms) and operated on 14 watts. 

The San Jose State University Foundation in California oversaw this experiment.52

Lightning and Radio Emissions Detector. Jupiter was named for the Roman god of 

the sky, who was said to have kept a stock of lightning bolts on hand. Before the Voyager 1 

flyby of Jupiter, scientists hypothesized that Jovian lightning served as an energy source for 

the planet’s nonthermal radio emissions, which had been readily detected on Earth. Photos 

that Voyager took of Jupiter’s night side appeared to show the existence of lightning, as 

did the craft’s plasma wave experiment, which detected “whistlers” (signals thought to be 

caused by electrical discharges propagating through the atmosphere). But to verify the 

existence of Jovian lightning and to delve deeper into its physical characteristics and understand how it is generated, how frequently it occurs, and how intense it is, in situ rather than 

brief flyby measurements needed to be made. Galileo’s LRD was well suited to this task. It 

was built to take into account large uncertainties regarding the nature of Jovian lightning. It 

also had the capacity to carry out the second part of the experiment, the measurement of 

radio-frequency emissions and magnetic field characteristics near Jupiter.53

The LRD was funded and built by the Federal Republic of Germany, one of 

several international partners on the Galileo project. The LRD consisted of two types 

of sensors: photodiodes to take optical observations and an antenna to make radio-frequency (RF) and magnetic measurements. The LRD shared its electrical system with the 

energetic particles instrument (EPI) described below.54

While Galileo sped through space on its way to Jupiter, the LRD experiment’s 

principal investigator, Louis Lanzerotti of Bell Laboratories and the University of Florida, 

and his team made numerous studies of lightning on Earth in order to calibrate a duplicate 

LRD. Doing so would help them better understand the data they would receive from 

Galileo, as well as aid in the ongoing interpretation of Earth’s electrical phenomena.55

Energetic Particles Instrument. The goal of the EPI experiment was to study fluxes 

of protons, electrons, alpha particles, and heavy ions orbiting in Jupiter’s magnetosphere 

at speeds of up to tens of thousands of miles per second. The EPI was the only Probe 

instrument to start recording data before the Probe entered the Jovian atmosphere. The 

EPI analyzed energies and angular distributions of charged particles at altitudes that ranged 

from 5 Jovian radii down to the top of the atmosphere. At 5 planetary radii, the study took 

place in the vicinity of a torus-shaped (donut-shaped) field of plasma that is thought to have 

originated from particles spewed out during the moon Io’s frequent volcanic eruptions. 

This torus of plasma and the magnetic field associated with it corotate with Jupiter, which 

means that they travel four times faster than Io and repeatedly overtake it.56

Between Io and Jupiter, four tiny moons orbit. At about 2 Jovian radii is a dust 

ring, and scientists believe that this ring and the four small moons influence the energetic 

particle populations in the region, sweeping up particles in their paths. The Probe’s EPI

experiment was the first to directly measure the region’s energetic particles, which travel 

at “relativistic” velocities (comparable to the speed of light). Prior to Galileo, the only 

means of “seeing” into Jupiter’s inner magnetosphere was to observe electromagnetic 

radiation that the fast-moving charged particles emitted.57

The EPI instrument employed two silicon-disc particle detectors with a brass particle 

absorber inserted between them. These parts were contained in a cylindrical tungsten shield 

that was open in the front, admitting particles as a telescope admits light. The EPI instrument 

was mounted under the aft heatshield of the Probe, which meant that only particles with 

enough energy to pass through the heatshield (at least 2.6 million electron volts, or “MeV”) 

would be counted. The energies of the particles were further determined by the parts of 

the instrument that they were able to penetrate. For instance, a particle that struck only the 

first silicon detector disc had to pass through the Probe’s heatshield but didn’t have enough 

energy to get through the brass absorber to the second detector. Such a particle had to have 

at least 2.6, but not more than 7, MeV of energy. Particles that could get through the brass 

absorber and hit the second silicon detector had energies of at least 7 MeV, and particles that 

could get through the tungsten shield had at least 20 MeV of energy.58

Net Flux Radiometer. The NFR, whose operation was overseen by the University of 

Wisconsin, measured heat and light energy, at different levels in the atmosphere, that was 

radiated both from below by Jupiter and from above by the Sun. The NFR’s optical system 

performed upward and downward observations in order to determine the difference in 

energy emitted between Jupiter’s internal heat source and that coming from the Sun (see 

figure 5.7). Because the Probe was spinning and the Sun was near the horizon, the NFR

was also able to locate and measure the opacities of substantial cloud layers around the 

Probe that scattered sunlight. Note from figure 5.7 the sudden increase in solar net flux at 

50	 Ibid.

51	 Ibid.

52	 “Nephelometer,” NSSDC Master Catalog Display: Experiment, http://nssdc.gsfc.nasa.gov/cgi-bin/database/wwwnmc?89-084E-05 (accessed 15 March 2000); “Probe Nephelometer.” Also, mass and power data supplied by Charlie

Sobeck, NASA Ames Research Center Probe Systems Engineering Manager, at the conclusion of the Probe project.

Sobeck’s data were drawn from the “Galileo Probe Mass Properties Report” of 2 April 1984 and the Hughes space

and communications group instrument fact sheets.

53	“The Probe Science Instruments,” Galileo Messenger (September 1995).

54	“Lightning and Radio Emission Detector (LRD),” NSSDC Master Catalog Display: Experiment, http://nssdc.gsfc.nasa.

gov/database/MasterCatalog?sc=1989-084E&ex=6 (accessed 15 March 2000); “Probe Instruments,” in Galileo Probe 

Background; “Probe Science Instruments.” Also, mass and power data supplied by Charlie Sobeck, NASA Ames

Research Center Probe Systems Engineering Manager, at the conclusion of the Probe project. Sobeck’s data were

drawn from the “Galileo Probe Mass Properties Report” of 2 April 1984 and the Hughes space and communications

group instrument fact sheets.

55	“Probe Science Instruments.”

56	“Probe Instruments,” in Galileo Probe Background; “Probe Science Instruments”; Ed Tischler, former experiments

manager for Probe instruments, interview, telephone conversation, 26 October 2001.

57	 “Probe Science Instruments.”

58	“Probe Instruments.”
128 MISSION TO JUPITER: A Histor y of the Galileo Project Chapter 5 I The Galileo spacecraft 129

0.5 bar pressure, immediately followed by a sharp decrease at 0.6 bar. This phenomenon 

is thought to be caused by scattering from an ammonia ice cloud layer.59

Radio-Science Experiment: Investigations Using Relay Radio Equipment. Dave 

Atkinson of the University of Idaho oversaw the use of radio equipment on the Probe 

and Orbiter to carry out two analyses: Doppler wind determination and atmospheric 

absorption. During the Probe’s descent through the Jovian atmosphere, its position and 

velocity were partly functions of the planet’s 28,000-mph rotational speed and its massive 

gravity field. These effects could be predicted. But atmospheric winds also had a significant 

effect. To study this effect, the Probe’s transmitter and antenna worked in conjunction 

with the Orbiter’s Probe relay radio hardware to conduct the Doppler wind experiment. 

The Probe’s transmitted frequencies were monitored during the entire descent by the 

Orbiter, and their Doppler shifts were noted.60 From the Doppler shifts, mission scientists 

were able to reconstruct the Probe’s trajectory during descent. After separating out the 

Doppler shifts expected to be caused by gravitational and rotational forces, they were 

able to surmise the shifts caused by Jovian winds and develop a profile of those winds at 

the time of descent. These data gave them clues as to the forces behind Jupiter’s global 

circulation patterns. They also provided confirmation, or “ground truth,” for the wind 

speeds determined by the Orbiter’s imaging team. The Doppler wind experiment will 

help future researchers understand the dynamics, not only of Jupiter, but also of Saturn, 

Uranus, and Neptune, the other gas-giant planets.61

The atmospheric absorption experiment was also carried out using the radio 

signal from Probe to Orbiter. This signal was affected by density variations in the atmosphere. In particular, electron densities in the planet’s ionosphere had a significant effect 

on the radio signal. Atmospheric absorption of the signal’s energy was determined by 

comparing the Probe’s transmitter output with the actual signal strength received by the 

Orbiter. Variation of atmospheric absorption with changes in the Probe’s position provided data on atmospheric structure as well as ionospheric electron densities.62

Orbiter Scientific Experiments

The Orbiter’s tasks, after relaying data from Probe to Earth, were to conduct its own 

investigations of Jupiter’s atmosphere and magnetosphere and to perform repeated flybys 

of Jovian satellites, collecting a wide range of data. The mission’s Announcement of 

Opportunity listed these objectives for the Orbiter’s scientific experiments:

• Define the topology and dynamics of the outer magnetosphere, magnetosheath, and bow shock.

• Describe the nature of magnetospheric particle emission.

• Determine the distribution and stability of trapped radiation.

• Study the magnetosphere-satellite interaction.

• Determine the surface composition of the satellites.

• Identify the physical state of the satellite surfaces and characterize their surface morphology.

• Measure the satellite magnetic, gravitational, and thermal properties and 

thereby obtain their geophysical characteristics.

• Study the satellite ionospheres and atmospheres, as well as the emission of gas.

• Conduct a synoptic study of the Jovian atmosphere.63

59	“Probe Instruments”; “Jupiter’s Net Fluxes,” http://spaceprojects.arc.nasa.gov/Space_Projects/galileo_probe/

htmls/NFR_results.html (accessed 19 October 2004), available in folder 18522, NASA Historical Reference Collection,

Washington, DC.

60	These are the shifts in frequency due to changes in velocity. They are similar to the changes in pitch one hears in an

ambulance siren as it approaches, rushes past, and speeds away.

61	 “Probe Instruments.”

62	 Ibid.

63	“Announcement of Opportunity for Outer-Planets Orbiter/Probe (Jupiter).” A synoptic climate analysis attempts to

characterize an entire weather situation that exists in a given area at a given time, and involves meteorological analysis

over a large region of the atmosphere, of a scale typically in excess of 2,000 kilometers. See Synoptic Climatology

Laboratory, Department of Geography, University of Delaware, March 2003, http://www.udel.edu/SynClim/; University

Corporation for Atmospheric Research, National Center for Atmospheric Research, “Definition of the Mesoscale,”

MetEd Meteorology and Training, 31 July 2003, http://meted.ucar.edu/mesoprim/mesodefn/print.htm.

Figure 5.7. Jupiter’s net radiation fluxes. The net flux radiometer experiment measured the 

difference between upward and downward energy flow (“net flux”) at the wavelengths of visible 

sunlight, as well as at the wavelengths of thermal infrared radiation, as a function of depth in 

Jupiter’s atmosphere. The sudden rise in solar net flux followed by an abrupt dropoff is probably 

caused by an ammonia ice cloud layer. (NASA Ames photo number ACD96-0313-6)
130 MISSION TO JUPITER: A Histor y of the Galileo Project Chapter 5 I The Galileo spacecraft 131

To meet these objectives, the Orbiter had to visit different parts of Jupiter’s magnetosphere, view the parent planet from many different angles, and fly by some of its 

satellites. This required that the spacecraft’s trajectory “be adjusted from time-to-time by 

powered maneuvers and by satellite flybys (involving gravity assists) in order to optimize 

observing conditions for diverse scientific purposes.”64 In their review of the Galileo mission plan, Johnson, Yeates, and Young compared the Probe’s and Orbiter’s tasks by saying 

that “while the Probe’s success is keyed to its ability to penetrate the Jovian atmosphere, 

the Orbiter’s success depends on its unique trajectory, which provides for unprecedented 

new measurements.”65

The instruments chosen to meet the Orbiter’s objectives had to be of several 

types. Some had to assess magnetic and electric fields, others needed to characterize particles of various energy levels, and those in a third group were to investigate 

Jovian satellites remotely. Figure 5.8 depicts the science instruments used for these ends. 

Instruments designed to take fields-and-particles data were mounted on the Orbiter’s 

spun section, where they swept through and measured the surrounding environment 

in all directions as the spacecraft flew through it. The Orbiter’s spun section carried 

not only these instruments, but also Galileo’s power supply, propulsion module, most 

of its computers and control electronics, its high-gain antenna, and one of its low-gain 

antennas. Instruments that remotely sensed satellite and planetary characteristics and that 

required stable, high-accuracy pointing were mounted on the Orbiter’s despun section.66

Figure 5.8. The Galileo Orbiter. Fields-and-particles instruments were mounted on the spun 

section (including the long science boom); remote sensing experiments were mounted on 

the Orbiter’s stable despun section (at the bottom of the spacecraft drawing). (Adapted from 

JPL image number P-31284) 

Instruments Designed To Measure Fields and Particles

The Orbiter required instruments that could measure the Jovian magnetosphere’s magnetic 

fields, as well as its electromagnetic and electrostatic wave characteristics over a range of 

frequencies. The instruments also needed to investigate the magnetic fields of Jupiter’s 

satellites. Furthermore, the Orbiter needed equipment to characterize the energies, angular 

distributions, and compositions of the particles that the magnetosphere contained. Mission 

planners had a good idea of what instruments would best accomplish these goals because 

similar experiments to those that Galileo performed had been conducted on the earlier 

Jupiter flybys of Pioneer and Voyager spacecraft, as well as on missions that had orbited 

Earth. These other missions had used magnetometers to characterize magnetic fields; particle detectors, plasma analyzers, and other instruments to study charged- and neutral-particle 

populations; and a plasma wave system to investigate wave phenomena. On-board communications equipment also had been utilized for radio-science experiments that helped 

determine satellite and atmospheric properties. According to Torrence Johnson, “The only 

major [Galileo] proposal issues were which universities and labs could produce the best 

instruments at the lowest cost and mass. Groups with instruments already on other missions 

usually have an advantage in this area, of course.”67 Table 5.2 lists the fields-and-particles 

instruments selected for the Orbiter and the organizations that supplied them. 

Table 5.2. Orbiter fields-and-particles instruments.

Instrument Function Mass in Kilograms

(Pounds)

Power Draw 

in Watts

Principal Investigator(s) 

and Institution(s)

Dust detection

system (DDS)

Measured mass and speed of

small Jovian dust particles and

studied the distribution of interplanetary dust.

4.1 (9.0) 1.8 Eberhard Grun,

Max Planck

Institut fur

Kernphysik,

Heidelberg,

Germany

Energetic particles detector

(EPD)

Determined angular distributions, temporal fluctuations,

intensities, and compositions

of energetic charged particles

in Jupiter’s magnetosphere

and investigated the processes

responsible for replenishing

particles that escape into interplanetary space.

10 (23) 6 D. J. Williams,

Johns Hopkins

University

Magnetometer

(MAG)

Measured magnetic fields of

Jupiter’s magnetosphere and

satellites. Characterized interplanetary magnetic fields and

those of asteroids.

7 (15) 4 Margaret

Kivelson, UCLA

PLASMA WAVE SEARCH COIL SENSOR

HIGH GAIN ANTENNA

(EARTHCOMMUNICATIONS

AND RADIO SCIENCE)

RADIOSOTOPE THERMOELECTRIC

GENERATOR

SPUN SECTION

ENERGETIC PARTICLES DETECTOR

PLASMA SCIENCE

SUN SHIELDS

DUST DETECTOR

SOLID STATE IMAGING

DESPUN SECTION

ULTRAVIOLET SPECTROMETER

PROBE RELAY

ANTENNA

RETROPROPULSION

MODULE

RADIOISOTOPE

THERMOELECTRIC

GENERATOR

PLASMA WAVE

E-FIELD SENSOR

PHOTOPOLARIMETER

RADIOMETER

NEAR INFRARED

MAPPING SPECTROMETER

MAGNETOMETER

SENSOR

64	“Announcement of Opportunity for Outer-Planets Orbiter/Probe (Jupiter).”

65	Johnson et al., pp. 19–20.

66	 Ibid., pp. 12–13.

67	Johnson et al., pp. 12, 19–20; Johnson e-mail; “Pioneer 10,” NSSDC Master Catalog: Spacecraft, last updated 28

February 2003, http://nssdc.gsfc.nasa.gov/nmc/tmp/1972-012A.html; “Pioneer 11,” NSSDC Master Catalog: Spacecraft,

last updated 7 September 2000, http://nssdc.gsfc.nasa.gov/nmc/tmp/1973-019A.html; “Voyager Project Information,”

NSSDC: Planetary Sciences, version 2.4, 20 May 2003, http://nssdc.gsfc.nasa.gov/planetary/voyager.html.
132 MISSION TO JUPITER: A Histor y of the Galileo Project Chapter 5 I The Galileo spacecraft 133

Instrument Function Mass in Kilograms

(Pounds)

Power Draw 

in Watts

Principal Investigator(s) 

and Institution(s)

Plasma subsystem (PLS)Measured densities, temperatures, bulk velocities, and composition of low-energy plasmas

(1.2 electron volts, or eV, to 50

keV), which make up the bulk of

Io’s plasma torus.

13 (29) 11 Lou Frank,

University of Iowa

Plasma wave

subsystem

(PWS)

Determined intensities of plasma

waves in Jovian magnetosphere,

as well as radio waves emitted

by Jupiter, Earth, and the Sun.

7 (16) 10 Donald Gurnett,

University of Iowa

Radio science Employed the Orbiter’s radio

telecommunication subsystem

and Earth-based instruments to

perform celestial mechanics and

relativity experiments, as well as

atmospheric studies.

John Anderson,

JPL, and H.

Taylor Howard,

Stanford

University

Sources:

1. “Galileo’s Science Instruments,” Galileo Home Page, http://www.jpl.nasa.gov/galileo/instruments/index.html

(accessed 19 February 2001).

2. “Galileo Magnetometer Project Overview,” Galileo Magnetometer Team’s Homepage, Institute of Geophysics and

Planetary Physics, UCLA, http://www.igpp.ucla.edu/galileo/framego.htm (accessed 2 January 2001).

3. “Dust Detection Experiment,” Galileo Messenger (December 1983).

4. “PLS—Plasma Subsystem,” Galileo’s Science Instruments, Galileo Home Page, http://www.jpl.nasa.gov/galileo/

instruments/pls.html>, accessed 15 March 2000.

5. “PWS—Plasma Wave Subsystem,” Galileo’s Science Instruments, Galileo Home Page, http://www.jpl.nasa.gov/

galileo/instruments/pws.html (accessed 15 March 2000); “Plasma Wave Investigation,” Galileo Messenger (July 1986).

6. “Energetic Particle Detector (EPD),” Galileo Messenger (February 1986).

Dust Detection System. The exquisite ring system surrounding Saturn has been 

seen by millions of people on Earth—many looking through fairly modest telescopes. 

Uranus’s rings have been viewed by far fewer people, although they too can be seen 

using more sophisticated Earth-based instruments. In contrast, Jupiter’s rings are too faint 

to spot with a telescope sitting on Earth. Their discovery, in fact, came from one fortuitous 

photograph taken by Voyager 1, in which the Sun happened to backlight the ring’s particles in just the right way for Voyager’s cameras to pick them up. Jupiter’s tenuous ring 

system is believed to be composed entirely of dust particles, making them quite different 

from Saturn’s rings, where constituents range from dust-size to the size of a house.68

Orbiter’s dust detection system (DDS) was designed to measure the motions of 

dust streams near Jupiter, as well as electrical charges on the larger particles. The DDS

could analyze as many as 100 dust impacts per second. These measurements cast light on 

the particles’ sources and transport mechanisms and on how dust streams interact with 

plasmas. Observations of Jupiter’s electrostatically charged dust were of particular interest. Scientists wanted to determine whether charges on dust particles led to particular ring 

structures, such as the radial spoke features that have been observed in Saturn’s rings. The 

theory was that fine, charged dust particles interact with a planet’s magnetic fields in such 

a way as to get forced out of the ring plane and leave voids in the rings.

Scientists also wanted to study “fluffy” aggregates of small particles. If such aggregates get strongly charged, mutually repulsive electrostatic forces can break them apart into 

small chunks, creating a swarm of micrometeoroids. Such swarms have been observed in 

Earth’s atmosphere by a detector on a Highly Eccentric Orbit Satellite (HEOS).69

The DDS was a modified micrometeoroid detector that had been used successfully 

on the HEOS-2 satellite. As particles flew through the instrument’s entrance grid, they induced 

currents in it from which their charges were surmised. The particles then struck a gold target 

and were detected by the plasma (ionized gas) generated from the impact. An electric field 

separated the ions and electrons of this plasma by their charges and directed each stream 

toward separate amplifiers, which produced current pulses whose rise times were functions of 

the particles’ speeds and whose amplitudes were functions of both the particles’ masses and 

speeds. From pulse rise time and amplitude, particle mass and speed were determined.70

Energetic Particles Detector (EPD). Jupiter’s magnetosphere is the region in which 

the planet’s intense magnetic field and charged particles are confined by the solar wind 

(see figures 5.9a and b). A planet’s magnetosphere can be thought of as a “bubble” in the 

solar wind. Charged particles in the Jovian magnetosphere have so much kinetic energy 

that radiation damage to the Orbiter was of great concern to project engineers—after 

all, the spacecraft would have to operate in the area for years. These energetic particles 

continuously escape into interplanetary space but are quickly replaced with new particles. 

One of the EPD’s main objectives was to study how this replacement occurred; the other 

was to analyze the dynamics of the Jovian magnetosphere.

Figure 5.9a. Jupiter’s magnetosphere, a region of intense magnetic fields and rapidly moving 

charged particles. The small black circle at the center shows the relative size of Jupiter. Also 

depicted are lines of Jupiter’s magnetic field. (Adapted from JPL image number PIA-03476)

68	“Dust Detection Experiment,” Galileo Messenger (December 1983).

69	Ibid.; “Heos 2,” NSSDC Master Catalog: Spacecraft, NSSDC ID:1972-005A, 17 May 2000, http://nssdc.gsfc.nasa.

gov/database/MasterCatalog?sc=1972-005A.

70	 “Dust Detection Experiment.”

Table 5.2. Orbiter fields-and-particles instruments. (continued)
134 MISSION TO JUPITER: A Histor y of the Galileo Project Chapter 5 I The Galileo spacecraft 135

Figure 5.9b. Parts of a planet’s magnetosphere. Deflected solar-wind particles flow around 

a magnetosphere. Note the magnetosphere’s typically elongated, rather than spherical, 

shape. The planet’s bow shock is the interface between the region of space influenced by 

the planet’s magnetic field and the region of the undisturbed, interplanetary solar wind. 

(Figure drawn by NASA History Office from the Web site http://www2.jpl.nasa.gov/galileo/

messenger/oldmess/Earth3.html)

 Voyager’s observations identified three possible sources for energetic-particle 

replenishment: the Sun, Jupiter’s ionosphere, and its moons. The Sun’s energetic-particle 

streams and its solar wind were deemed the most likely source for the helium, carbon, 

nitrogen, oxygen, neon, magnesium, silicon, and iron-bearing particles detected in 

Jupiter’s outer magnetosphere. The satellite Io and its plasma torus were suspected of 

furnishing the particles containing sulfur, sodium, and oxygen that had been seen closer 

Figure 5.10. The energetic particles detector (EPD) included two telescope systems: the lowenergy magnetospheric measurements system (LEMMS) and the composition measurement 

system. These determined particle charges, energies, and species. (JPL image no. 230-1027A) 

to the planet. Molecular-sized ions of the hydrogen isotope H3 were thought to have originated in Jupiter’s ionosphere. The relative contributions of these sources had been shown 

by Voyager data to vary considerably over time, and so years of observations by Galileo 

would be crucial to obtaining an accurate picture of Jovian magnetospheric processes.71

The EPD (figure 5.10) used magnetic deflection and various absorber materials 

to distinguish between incoming particle charges and types. The instrument consisted of 

two different telescope systems that together studied electrons with energies from 15,000 

electron volts (15 keV) to 11 million electron volts (11 MeV) and ions with energies from 

10 keV to 55 MeV. The EPD was able to identify ion species that ranged from hydrogen 

to iron. It determined particle energy, composition, and location in the vicinity of Jupiter 

and its satellites, and it also generated a three-dimensional map of particle distribution. 

The EPD principal investigator was from Johns Hopkins University, although the science 

team for the instrument included staff from Germany’s Max Planck Institut fur Aeronomie, 

the University of Alaska, the University of Kansas, and Bell Laboratories.72

Magnetometer. Within a magnetosphere, the magnetic field of the planet, rather 

than that of the solar wind, is dominant and controls the behavior of charged particles.73

The size of a magnetosphere is related to the strength of the planet’s field. Jupiter has a 

very strong magnetic field and an enormous magnetosphere. The Jovian magnetosphere 

is the largest single object in the solar system, with an average diameter of about 9 million 

miles (15 million kilometers). If our eyes could see its magnetic fields, it would appear 

from Earth to be larger than either our Moon or the Sun. It would occupy 1.5 degrees of 

sky, compared to the Sun’s 0.5 degrees of sky. And it would take up this space in our sky 

even though it is far more distant from Earth—about 710 million kilometers (440 million 

miles)—than is the Sun at 150 million kilometers (93 million miles).74

The mammoth scale of Jupiter’s magnetosphere arises from several factors. The 

solar wind which envelopes it has a low dynamic pressure at Jupiter’s distance from the 

Sun, down by a factor of more than 25 from its pressure at Earth. This is not surprising; the 

solar wind has hundreds of millions more miles to spread out and become increasingly 

diffuse as it streams outward through the solar system and eventually reaches Jupiter. 

Jupiter’s large size, with its radius 11 times that of Earth, also contributes to the scale of 

its magnetosphere, as does its hefty magnetic moment (a measure of the strength and orientation of a magnetic field), which is four orders of magnitude larger than that of Earth. 

Finally, the Jovian system fills its magnetosphere with a relatively dense plasma that is 

centrifugally accelerated by Jupiter’s rapid rotation and strong field. The result is that this 

plasma pushes outward and “inflates” the magnetosphere from within.75

A planet’s magnetosphere is not actually shaped like a sphere. Jupiter’s magnetosphere resembles a giant wind sock, with its rounded head facing toward the Sun (and 

71	 “Energetic Particle Detector (EPD),” Galileo Messenger (February 1986).

72	 Ibid.; “EPD—Energetic Particles Detector,” Galileo’s Science Instruments, Galileo Home Page, http://www.jpl.nasa.

gov/galileo/instruments/epd.html (accessed 15 March 2000); “Energetic Particles Detector (EPD),” NSSDC Master 

Catalog Display: Experiment, http://nssdc.gsfc.nasa.gov/cgi-bin/database/www-nmc?89-084B-06 (accessed 15

March 2000).

73	 “Magnetometers,” Galileo Messenger (May 1982).

74	 Ibid.; “Plasma Investigation,” Galileo Messenger (September 1985).

75	M. G. Kivelson, K. K. Khurana, J. D. Means, C. T. Russell, and R. C. Snare, “The Galileo Magnetic Field Investigation,”

Space Science Reviews 60 (1992): 357–383.

Planet

Magnetopause

Bowshock

Magnetosphere

Solarwind Magnetotail
136 MISSION TO JUPITER: A Histor y of the Galileo Project Chapter 5 I The Galileo spacecraft 137

the solar wind) and its long, flapping “magnetotail” stretched out away from the Sun. The 

tail is so long that it may extend as far as Saturn’s orbit, roughly 650 million kilometers 

(400 million miles) away from Jupiter. This is more than four times the distance from Earth 

to the Sun and nearly the distance from Jupiter to the Sun. Galileo was able to travel only 

about 11 million kilometers (7 million miles) down the tail. 

The Orbiter’s magnetometer investigated magnetic fields throughout Jupiter’s magnetosphere. Pioneer and Voyager missions also made observations of the Jovian magnetosphere’s size, 

shape, and structure as they flew by the planet, but they did not stay in the area long enough to 

record significant temporal changes. In its years of residence in the Jovian system, Orbiter’s magnetometer was able to study the dynamic nature of the magnetosphere more thoroughly.76

Jupiter’s four largest moons—Io, Europa, Ganymede, and Callisto, the “Galilean” 

satellites that were discovered by the Italian astronomer—all orbit within Jupiter’s magnetosphere. As Jupiter rotates, its magnetic field sets the charged particles in its magnetosphere in 

motion. These interact with the Galilean moons in ways that differ depending on how good 

an electrical conductor the moon is, how strong its magnetic field is, and whether it has an 

ionosphere. A focus of the Orbiter’s tour through the Jovian system was to study relationships 

among the planet, its magnetospheric particles, and its satellites. 

The Orbiter also characterized the electrical and magnetic properties of the satellites 

themselves. Evidence of a strong magnetic field suggested that a satellite might have a molten, 

electrically conducting metallic core such as is found inside Earth.77

During Galileo’s long voyage to Jupiter, its magnetometer studied the fields of interplanetary space and the solar wind. One aspect of this study was to correlate changes in solar activity 

with long-term changes in the solar wind’s field at great distances from the Sun. Also observed 

were the interactions of the solar wind with Venus, Earth, and the asteroids Gaspra and Ida.78

The magnetometer consisted of two clusters of three sensors each, mounted on 

the Orbiter’s 11-meter (36-foot) science boom. The sensors had to be placed a distance 

away from the rest of the spacecraft in order to minimize the effect of magnetic fields 

generated by electronic circuitry in the Orbiter and Probe, which could be confused with 

magnetic fields from outside the craft. The cluster that measured the faint fields originating in the distant Jovian magnetotail, as well as from the solar wind, was located at the 

end of the 11-meter boom. The cluster designed to measure the much stronger fields of 

Jupiter’s inner magnetosphere, and also to provide redundancy with the other cluster’s 

measurements, was mounted about 7 meters down the boom from the spacecraft. (Figure 

5.8 shows the two locations of the magnetometer sensor clusters.)79

Procedures were also carried out during data processing to reduce the effects of 

spacecraft-generated fields. Rotation of the spacecraft was used to distinguish between 

the natural magnetic fields that NASA wanted to measure and “engineering-induced” 

fields produced by and moving with the spacecraft. These latter fields were quantified and 

separated from the magnetospheric data before transmission to Earth.80

Galileo’s magnetometer system was designed by UCLA’s Earth and Space Sciences 

Department. The basic device that measured magnetic fields was called a ring-core sensor, 

manufactured by the Naval Surface Weapons Center in Silver Spring, Maryland. This 

sensor was chosen because of its proven stability and low noise.81 The magnetometer was 

calibrated in flight using a specially designed coil that generated a known magnetic field 

at a set frequency. This calibration enabled mission staff to eliminate the effects of yet 

another source of error in measurement—the bending and twisting of the long boom on 

which the magnetometer was mounted. These deformations altered the magnetometer’s 

orientation, and a calibration scheme was needed in order to determine its orientation 

accurately at any given time.82

Plasma Subsystem. A plasma is a collection of charged particles, usually made up 

of about equal numbers of ions and electrons. In some respects, plasmas act like gases. 

But unlike gases, they are good electrical conductors and are affected by magnetic fields. 

Plasmas are the most common form of matter, making up more than 99 percent of the 

visible universe.83

Galileo’s PLS measured the composition, energy, temperature, motion, and 

three-dimensional distribution of low-energy plasma ions passing by the spacecraft. Its 

objectives were to 1) determine the sources of magnetospheric plasma; 2) investigate the 

plasma’s interaction with Jupiter’s moons; 3) analyze the plasma’s role in generating energetic charged particles; 4) determine the nature of a sheet of current in Jupiter’s equatorial 

plane; and 5) evaluate the effects that rotational forces, electric currents aligned with magnetic fields, and other phenomena have on Jovian magnetospheric dynamics.84

Observations by the Pioneer and Voyager flybys had shown Jupiter’s magnetosphere to be an enormous reservoir of charged particles. Galileo added to the data 

collected by these missions but had the advantage of spending years, rather than weeks, 

in the vicinity of Jupiter. Galileo’s PLS was also far more advanced than the instruments 

of the other spacecraft. The Galileo PLS had the ability to identify chemical compositions 

of ion populations in Jupiter’s magnetosphere.

Galileo’s plasma instrument used two electrostatic analyzers to measure the 

energy per unit charge for electrons and positive ions (see figure 5.11). These analyzers 

also measured the particles’ directions of flow. In addition, three miniature mass 

spectrometers measured the mass per unit charge of the plasmas’ positive ions, which 

allowed identification of ions’ species.85

Plasma Wave Subsystem. Plasma waves are oscillations that involve both the 

charged particles of a plasma and the electric and magnetic fields associated with the 

plasma. There are two basic types of plasma waves: electrostatic waves, which resemble 

76	 “Magnetometers.”

77	 Ibid.

78	 Ibid.; “Magnetometer (MAG),” NSSDC Master Catalog Display: Experiment, http://nssdc.gsfc.nasa.gov/cgi-bin/

database/www-nmc?89-084B-03 (accessed 15 March 2000).

79	 “Magnetometers”; “Magnetometer (MAG).”

80	 “Magnetometers”; “MAG—Magnetometer,” Galileo’s Science Instruments, http://www.jpl.nasa.gov/galileo/instruments/

epd.html (accessed 24 January 2000).

81	 “Magnetometers”; “Galileo Magnetic Field Investigation”; “MAG—Magnetometer.”

82	“MAG—Magnetometer.”

83	“Perspectives on Plasmas—Basics,” Plasmas International, 1999, http://www.plasmas.org/basics.htm; “What Is

Plasma?” Coalition for Plasma Science, 2000, http://www.plasmacoalition.org/what.htm; “Plasma Investigation,”

Galileo Messenger (September 1985).

84	“PLS—Plasma Subsystem,” Galileo’s Science Instruments, http://www.jpl.nasa.gov/galileo/instruments/pls.html

(accessed 15 March 2000).

85	“Plasma Investigation.”
138 MISSION TO JUPITER: A Histor y of the Galileo Project Chapter 5 I The Galileo spacecraft 139

sound waves, and electromagnetic waves, which are more similar to light waves. The 

PWS was designed to analyze the properties of varying fields in these waves. The PWS

measured electric field oscillations in the plasma waves over frequencies ranging from 5 

hertz (cycles per second) to 5.6 megahertz, as well as magnetic field oscillations over the 

range of 5 hertz to 160 kilohertz. Plasma electric fields were measured with an electric 

dipole antenna, a simple design often used on Earth for radio reception. The magnetic 

fields were analyzed using two “search coil” magnetic antennas.86

When Voyager passed by Jupiter, it detected several kinds of plasma wave 

phenomena. The Galileo mission had a chance to study these phenomena in more detail 

and sought to understand how the waves were generated. The spacecraft made threedimensional measurements of the areas where the waves occurred, which included 

Jupiter’s equatorial plane and the plasma torus that encircles the planet and encloses the 

satellite Io. Jupiter and its nearest moon, Io, are strongly coupled in various ways that 

do not occur in our Earth-Moon system. By studying Jovian plasma wave phenomena, 

Galileo was able to improve our understanding of this relationship between Jupiter and 

Io. In particular, Galileo data shed light on how energy is transferred among Io, the plasma 

torus, and Jupiter’s ionosphere. The rate of energy transfer is sizable—1012 watts of power 

flows from Io to Jupiter. Galileo also collected data on the nature of certain Jovian polar 

auroras, which occur along magnetic field lines that pass through both the planet and Io.87

Figure 5.11. The plasma subsystem measured energy and mass per unit charge of plasma 

particles and was able to identify particular chemical species. 

As was mentioned above, all four of Jupiter’s Galilean satellites—Io, Europa, 

Ganymede, and Callisto—lie within the planet’s magnetosphere. This situation gives the 

plasma within this magnetosphere the opportunity to interact with each of the Galilean 

moons. Jupiter’s rapid rotation and strong magnetic field serve to accelerate plasmas in 

the magnetosphere to great speeds. Wakes and waves are formed as the plasma is swept 

past slower-moving satellites. Factors that influence plasma-moon interactions include 

a moon’s size, magnetic moment, and surface properties, as well as the nature of its 

atmosphere (or lack of an atmosphere). The flow velocity and magnetic field strength 

of the plasma are also important. The data collected by Galileo’s PWS were critical for 

understanding the complex interactions in the magnetospheric region.88

Radio Science. Three separate radio-science experiments were planned for 

Galileo that used the Orbiter’s radio equipment for functions other than communication 

with Earth. The experiments were to study celestial mechanics, search for gravity 

waves, and measure radio-wave propagation characteristics. Unlike the mission’s other 

investigations, approximately 80 percent of the measuring equipment for radio-science 

experiments was located on Earth, at our Deep Space Network (DSN) stations, rather 

than on the spacecraft. This equipment included arrays of ultrasensitive antennas and 

receiving systems.89

The celestial mechanics studies sought to detect tiny changes in Galileo’s 

trajectory caused by gravitational fields. These trajectory changes could be measured by 

the small increments they caused in the frequency of the radio signal transmitted by the 

spacecraft and received on Earth. The amount of frequency increment was dependent on 

the magnitude of the spacecraft’s velocity change. From the velocity changes caused by 

Galileo’s close passage to Jupiter or one of its satellites, scientists were able to determine 

details regarding the planet’s gravitational field and mass. Also, features regarding the 

internal distribution of that mass could be surmised. For instance, were there distinct 

layers of rock and ice within a satellite, or was it homogeneous throughout? Small 

increments in signal frequencies shed light on such issues. In addition, the data enabled 

Galileo scientists to determine more exactly the orbits of Jupiter and its satellites.90

Another radio-science experiment searched for gravitational waves. These are 

extremely faint perturbations in the interstellar medium caused by catastrophic events 

such as the collapse of a star. Normally, there is a constant level of gravitational-wave 

energy in space. This is occasionally overwhelmed by some momentous stellar or galactic 

event, which generates very long-wavelength, low-frequency oscillations with periods of 

about 20 minutes. 

Gravity waves perturb electromagnetic radiation that they encounter, and it is 

this property that was used by Galileo to search for such waves. If two black holes were 

to collide, the event would generate gravitational waves rippling through interstellar 

space. These waves would eventually encounter and interfere with Galileo’s radio signal 

to Earth, and receiving stations on Earth would pick up low-frequency “pops” indicative 

of this interference. The Galileo mission presented a rare opportunity because the radio 

86	“PWS—Plasma Wave Subsystem,” Galileo’s Science Instruments, http://www.jpl.nasa.gov/galileo/instruments/pws.

html (accessed 15 March 2000); “Plasma Wave Investigation,” Galileo Messenger (July 1986).

87	 “Plasma Wave Investigation.”

88	 Ibid.

89	“Radio Science,” Galileo Messenger (December 1986).

90	“Radio Science,” Galileo’s Science Instruments, http://www.jpl.nasa.gov/galileo/instruments/rs.html (accessed 15

March 2000); “Radio Science,” Galileo Messenger.

RAM

#2

2 K

RAM

#1

2 K

CPU ROM

INTERFACE

CONTROL

&

CLOCKS

SIGNAL

COND

CKTS

HARDWARE

MULTIPLIER

INTERNAL

CALIBRATE

CIRCUITS

MUX-ADC

ANALOG

PWR

CONTROL

FLP

CONTROL

INTERNAL BUS

MAGNETOMETER ELECTRONICS

MEMORY

KEEP ALIVE

PWR

REG

Functional block diagram of the experiment.

SPACECRAFT

DATA

INTERFACE

OUTBOARD

SENSOR

INBOARD

SENSOR

SPACECRAFT

POWER
140 MISSION TO JUPITER: A Histor y of the Galileo Project Chapter 5 I The Galileo spacecraft 141

signal stretching from spacecraft to Earth constituted a gravitational-wave detector that was 

hundreds of millions of kilometers long. A detector of such enormous scale was critical 

for picking up gravity-wave phenomena. The detection of a gravity-wave signal could 

be confirmed by other spacecraft in the vicinity or by finding visual confirmation of the 

causal event using Earth-based telescopes. It would take a very large event, however, to 

generate gravitational waves capable of detection. John Anderson, JPL’s team leader for the 

celestial mechanics experiment, estimated that the collapse of a star with a mass 10 million 

times larger than the Sun might be necessary to produce a measurable phenomenon.91

Galileo’s radio-wave propagation studies measured the minute changes in 

frequency, power, time of flight to Earth, and polarization of Galileo’s radio signals that 

resulted from their passages through the atmospheres and ionospheres of Jupiter and 

its satellites. Atmospheres refract as well as scatter the radio signals that pass through 

them. The process of refraction bends and slows a radio signal, while scattering 

diffuses the electromagnetic waves of the signal. When the signal was received by a 

DSN station on Earth, these modifications to the signal showed up as amplitude and 

frequency changes. Analyses of the changes yielded Jovian system atmospheric 

information that included refractive indices and pressure and temperature profiles. 

Ionospheric electron densities were also estimated from these measurements.92

Galileo conducted a radio-wave experiment that studied the solar corona. When 

Galileo was on the opposite side of the Sun from Earth, a radio signal sent by the 

spacecraft had to pass through the solar corona in order to reach DSN antennas. Changes 

in the signal as it passed through the corona gave scientists data on its nature. 

Another radio-science experiment was conducted with the aid of the Probe. By 

monitoring its radio signal during its descent into the Jovian atmosphere, the Galileo team 

was able to collect information on wind-speed changes with respect to altitude and on the 

direction of heat flow—downward or upward—through the atmosphere.93

Partially in preparation for radio transmissions from Galileo, NASA built several 

sensitive new antennas and receiving systems at its DSN sites. These were able to detect 

the faint signal from Galileo continuously, which the old equipment would have had 

trouble doing. By the time it reached Earth, Galileo’s signal was so minuscule as to be 

over one billion times smaller than the sound of a transistor radio playing in New York—

as heard from Los Angeles. Moreover, the phenomena that the radio-science experiment 

observed were also minute. Detecting these phenomena was comparable to measuring 

the distance from New York to Los Angeles with an error no greater than the diameter of 

a human hair.94

Orbiter’s Remote Sensing Instruments

In order to satisfy the objectives stated in NASA’s Announcement of Opportunity for Outer 

Planets, the Orbiter’s remote sensing instruments (see table 5.3) needed to determine 

Jovian satellite compositions and surface morphologies, as well as making observations 

of Jupiter’s atmospheric characteristics. Central to these investigations was a means of 

capturing high-resolution optical images. As NASA had successfully done on many of its 

previous missions, it decided to keep control of the design and fabrication of the Orbiter’s 

imaging system rather than soliciting proposals for its development. The Agency instead 

solicited proposals from individual investigators who wanted to join the team that would 

use the imaging system. The selected investigator team did, however, get to refine the 

requirements for the imaging system’s color capability and some of its other features.95

The major instrument-selection issue with which NASA dealt for remote sensing 

experiments was what types of spectrometers were most appropriate to include on the 

mission. Spectrometers analyze reflected and emitted light, both in visible and nonvisible 

(infrared and ultraviolet) parts of the spectrum. Spectral data can give valuable information 

on the chemical compositions of planetary surface features and atmospheres. 

NASA considered developing an in-house spectrometer for the Orbiter, as it was 

doing for the imaging system. NASA had in mind a variant of the spectrometer flown 

on Voyager. This instrument, designed primarily to analyze atmospheric gases, had high 

resolution in the “far” infrared range (so named because its wavelengths were significantly 

longer than those of visible light). This part of the light spectrum was useful for studying 

thermal emissions, such as those from a planetary atmosphere.

The scientific community also made cases for ultraviolet and infrared reflection 

instruments. Those interested in satellite surface analysis thought that a reflection 

spectrometer operating in the near-infrared region (the section of the spectrum that 

begins with wavelengths just slightly longer than those of visible red light) was needed 

to achieve the Announcement of Opportunity’s objective to “determine the surface 

composition of the satellites.” Torrence Johnson remembers at least two proposals for 

a near-infrared instrument being submitted, one of which was from a team that he 

headed. His team’s concept for a near infrared mapping spectrometer (NIMS) won 

out over other proposals and convinced NASA to develop it rather than supplying a 

Voyager-type spectrometer for the mission.96, 97

The mission still needed a far-infrared instrument to study thermal emissions 

from the Jovian system. What NASA selected was a radiometer, an instrument that 

quantitatively measured electromagnetic radiation, and included it as one of the 

instruments in the photopolarimeter radiometer (PPR) experiment. The radiometer 

specialized in analyzing the intensity of radiant thermal energy, and that was very useful 

for “sounding” the atmosphere in several infrared bands of the spectrum, as well as 

for measuring temperatures of satellite surfaces. A radiometer did not have the strong 

chemical composition analysis capabilities that a spectrometer would have had, but 

NASA mission planners did not perceive such capabilities as necessary. NASA’s thinking 

was that the Probe’s in situ compositional experiments would provide in-depth analyses 

of Jupiter’s atmospheric composition and that the NIMS instrument would perform major 

91	 “Radio Science,” Galileo Messenger.

92	“Radio Science,” Galileo’s Science Instruments; “Radio Science,” Galileo Messenger.

93	“Radio Science,” Galileo Messenger.

94	 Ibid.

95	Announcement of Opportunity; Johnson e-mail.

96	Announcement of Opportunity; Johnson e-mail.

97	Spectroscopy, the study of spectra, is based on the idea that each chemical substance has its own characteristic

spectrum. See “Spectroscopy,” Microsoft Encarta Online Encyclopedia, 2001, http://encarta.msn.com. Infrared light

consists of electromagnetic waves whose wavelengths are too long for the human eye to see, whereas ultraviolet light

has wavelengths to short to be detected by our eyes. See “Infrared Radiation” and “Ultraviolet Radiation,” both in

Microsoft Encarta Online Encyclopedia, 2001, http://encarta.msn.com.
142 MISSION TO JUPITER: A Histor y of the Galileo Project Chapter 5 I The Galileo spacecraft 143

compositional studies of the satellites. (NIMS actually ended up supplying valuable 

atmospheric compositional information as well.)98

Mission planners also selected an ultraviolet spectrometer (UVS) because 

of its capability for making measurements of Jupiter’s high atmosphere, analyzing 

the ionized gas regimes surrounding the planet, and determining whether the satellites 

have atmospheres.99

Table 5.3. Orbiter remote sensing equipment.

Instrument Function Mass in Kilograms

(Pounds)

Power Draw 

in Watts

Principal Investigator(s) 

and Institution(s)

Near infrared

mapping

spectrometer

(NIMS)

Determined satellite

chemical compositions. Also

analyzed Jovian atmospheric

compositions.

18 (40) 12 Robert Carlson,

JPL

Photopolarimeter

Radiometer

(PPR)

Measured radiant thermal

energy intensities. Determined

distribution of Jupiter’s cloud

and haze particles. Analyzed

Jupiter’s energy budget.

5 (11) 11 J. E. Hansen,

Goddard Institute

for Space

Studies, NY

Solid state

imaging (SSI)

camera

Galileo’s main imaging

device. Especially useful for

studying satellite geology.

30 (65) 15 Michael Belton,

National Optical

Astronomy

Observatories

Ultraviolet

spectrometer

(UVS)

Located cloud layers and

analyzed characteristics of

cloud particles. Analyzed

Jupiter’s high atmosphere

and surrounding ionized gas

regions. Searched for satellite

atmospheres.

4.2 (9.2) 4.5 C. W. Hord and

Ian Stewart,

University of

Colorado

Sources:

1.	 “Galileo’s Science Instruments,” Galileo Home Page, http://www.jpl.nasa.gov/galileo/instruments/index.html 

(accessed 19 February 2001).

2. 	Institute of Geophysics and Planetary Physics, UCLA, “Galileo Magnetometer Project Overview,” Galileo 

Magnetometer Team’s Homepage, http://www.igpp.ucla.edu/galileo/framego.htm (accessed 2 January 2001).

3. “Ultraviolet Spectrometer,” Galileo Messenger (June 1983).

4. “The PPR: Finding More Than Meets the Eye,” Galileo Messenger (August 1989).

Near Infrared Mapping Spectrometer. NIMS analyzed Jovian cloud strata and 

temporal and spatial atmospheric composition variations. It determined temperature 

versus pressure in the region between 1 and 5 bars of pressure. (As noted earlier, a 

bar is equal to Earth’s atmospheric pressure at sea level.) NIMS also tried to answer the 

question, what are the Jovian moons made of? NIMS was able to recognize many common 

minerals, including silicates, nitrates, and carbonates. One product of the observations 

was a detailed geological map of each satellite. These data are expected to shed light on 

the processes governing solar system evolution.100

Galileo’s NIMS (figure 5.12) was a brand-new instrument that had never flown in 

space before. It was designed and built to be sensitive to the part of the electromagnetic 

Figure 5.12. NIMS was especially useful for studying gaseous chemical species and reflections 

from solid matter. The cone on the left was a radiative cooler. (JPL image 382-2165A)

spectrum that begins with the deepest red that we can see (whose wavelength is about 0.7 

micrometers) and then moves out of the visible to wavelengths as long as 5 micrometers. 

The near-infrared spectrum is quite useful for analyzing gaseous chemical species, as 

well as light reflected off solid matter. NIMS was thus a powerful diagnostic tool for both 

Jupiter’s gaseous atmosphere and the solid surfaces of its satellites.101

NIMS’s main components were a telescope and a spectrometer. The telescope 

focused light on the spectrometer, which acted like a prism and divided the near-infrared 

component of the light up into individual wavelengths, or “colors.” Most were not, however, 

colors that our eyes could see—their wavelengths were too long. An array of detectors 

measured the intensity of light at each individual wavelength. The relative intensities of 

different wavelengths of light reflected or emitted from a particular material constitute 

a unique “signature” of that substance. Large databases exist of characteristic spectral 98	Johnson e-mail; NASA Earth Observatory, “Radiometer,” Glossary, http://earthobservatory.nasa.gov:81/Library/

glossary.php3?xref=radiometer (accessed 9 August 2003); “Radiometer,” Microsoft Encarta Online Encyclopedia,

2001, http://encarta.msn.com.

99	Johnson e-mail.

100	“Near Infrared Mapping Spectrometer,” Galileo Messenger (July 1981).

101	 Ibid.
144 MISSION TO JUPITER: A Histor y of the Galileo Project Chapter 5 I The Galileo spacecraft 145

signatures from many thousands of different materials. These were used to interpret the 

NIMS data observed by Galileo and identify various constituents of the Jovian atmosphere 

and satellites. The overall amplitude of a signal received by NIMS was also of use, for it 

gave an indication of the quantity of the substance present.102

NIMS was built at JPL, with scientists on the team from the United States, England, 

and France. Because warm surfaces in the instrument would have emitted infrared 

(thermal) radiation that might have interfered with infrared signals from the objects being 

observed, NIMS’s detector array was lowered to a cryogenic temperature of 64 K (the 

equivalent of 64 Celsius degrees above absolute zero) using a radiative cooler. This action 

minimized background infrared radiation.103

Photopolarimeter Radiometer. The PPR was designed to measure intensity and 

polarization of light and was used during the Galileo mission to determine the following:

• Variations in intensity of reflected sunlight and emitted thermal radiation 

from both Jupiter and its satellites. 

• Vertical and horizontal distribution of Jupiter’s cloud and haze particles, as 

well as their shapes, sizes, and refractive indices.

• Jupiter’s energy budget.

• The thermal structure of the Jovian atmosphere and the vertical distribution 

of absorbed solar energy within it.

• Photometric, polarimetric, and radiometric properties of the Galilean moons 

(Io, Europa, Ganymede, and Callisto).104

The PPR (see figure 5.13) was an instrument system with several functions. Its 

photometer measured the intensity of incoming light and used various colored filters to 

pass only light in the desired spectral band. The signal intensity that the PPR observed 

could be correlated with the abundance of a particular material present. Of the seven 

bands of light that the photometer observed, one was in a region of the spectrum in 

which methane strongly absorbs light, and another was in a region where ammonia 

strongly absorbs light.105

The PPR’s polarimetric measurements determined the polarization characteristics 

of reflected sunlight. Polarization is the suppression of light-wave vibrations in certain 

directions, coupled with the allowance of light-wave vibrations in other directions (this is 

what polarized sunglasses do). Polarization is also related to the times at which vibrations 

in different directions reach their maxima and minima. For instance, in a particular beam 

of light, vertical vibrations may be “in phase” with those in the horizontal direction, meaning that they both reach their maxima at the same time. Alternatively, vertical vibrations 

may peak a fraction of a cycle before or after horizontal ones. A “phase angle” describes 

the amount by which vibrations in one direction are out of phase with those in another.

Figure 5.13. Photopolarimeter radiometer. (Image number 23-1158A) 

To measure polarization, the PPR passed incoming light through a prism, which 

separated it into beams of vertically and horizontally polarized components. These beams 

were directed onto a pair of photodiode detectors, which measured the intensities of the 

two polarized components by converting their light into electric current.106

In the Jovian atmosphere, as in that of Earth, cloud particles and atmospheric molecules play important roles in scattering and polarizing sunlight, as well as in determining the 

color of the atmosphere. On Earth, the blue of the sky is caused by the scattering of the shortwavelength blue components of white sunlight by particles suspended in the atmosphere. 

The scattering of these blue light waves is more pronounced than the scattering of waves 

with longer visible wavelengths, such as those of red light. This is why Earth’s sky does not 

appear red. The scattering of light by Jupiter’s atmospheric molecules also favors the blue 

wavelengths, as well as the violet wavelengths. This type of frequency-dependent scattering 

by small particles is called Rayleigh scattering; it occurs only when the atmospheric particles 

doing the scattering are very small compared to the wavelength of the light.107 

One of the PPR’s science goals was to measure the relative importance of Rayleigh 

scattering in the Jovian atmosphere above cloud-top level, from which an estimate of the amount 

of gas in that region could be made, as well as identification of cloud-top height variations.

The PPR’s radiometry function was to measure thermal infrared emissions. The 

PPR observed in seven radiometry bands, which corresponded to different regions of the 

infrared spectrum that are absorbed, to varying extents, by the atmosphere. Emissions in 102	 Ibid.

103	“Near Infrared Mapping Spectrometer”; “NIMS—Near Infrared Mapping Spectrometer,” Galileo’s Science Instruments,

http://www.jpl.nasa.gov/galileo/instruments/nims.html (accessed 3 January 2001).

104	“Photopolarimeter-Radiometer (PPR),” NSSDC Master Catalog Display, Experiment, http://nssdc.gsfc.nasa.gov/cgibin/database/www-nmc?89-084B-08 (accessed 15 March 2000).

105	“PPR—Photopolarimeter-Radiometer,” Galileo’s Science Instruments, http://www.jpl.nasa.gov/galileo/instruments/

ppr.html (accessed 15 March 2000); “The PPR: Finding More Than Meets the Eye,” Galileo Messenger (August 1989).

106	 “The PPR: Finding More Than Meets the Eye.”

107	 Georgia State University, “Rayleigh Scattering,” HyperPhysics Web site, 2000, http://hyperphysics.phy-astr.gsu.edu/

hbase/atmos/blusky.html; “Color,” Microsoft Encarta Online Encyclopedia, 2001, http://encarta.msn.com; “Rayleigh

Scattering,” Encarta World English Dictionary (North American Edition), 2001, http://dictionary.msn.com/find/print.

asp?refid=1861727369&search=&wwi=8274; “The PPR: Finding More Than Meets the Eye.”
146 MISSION TO JUPITER: A Histor y of the Galileo Project Chapter 5 I The Galileo spacecraft 147

those parts of the spectrum that are absorbed poorly could originate deeper down in the 

atmosphere and still be observed by the Orbiter. Thus, depending on which emissions it 

was observing, the PPR was able to detect signals from various atmospheric depths. This 

capability allowed the PPR to analyze a range of different levels of the Jovian atmosphere. 

From the characteristics of the thermal radiation observed, atmospheric temperatures at 

each depth could be estimated. The PPR could also make direct temperature measurements of the surfaces of Jupiter’s satellites. It was the only one of Galileo’s instruments 

with this capability.108

Solid State Imaging (SSI) Camera. The SSI camera (figures 5.14 and 5.15) was used 

for a wide range of science objectives, including the following:

• Analyzing Jupiter’s atmospheric dynamics and cloud structure.

• Measuring sizes, shapes, and features of Jupiter’s moons.

• Determining the geological processes that formed the satellites’ surfaces. To 

do this, the SSI mapped those surfaces at resolutions finer than 1 kilometer 

(0.6 mile) over a range of viewing angles and lighting conditions. 

• Identifying and mapping ice and mineral distributions on the satellites.

• Searching for auroral and other atmospheric emission phenomena on Jupiter’s 

night side, in the region around the planet, and on its satellite surfaces. 

The SSI camera needed to study both atmospheric motion and geological 

formations, which required a high-resolution, large-format design that could pick out 

and identify very small features. But the SSI was also used for composition analyses of 

satellite surfaces, which meant that it needed to employ a range of spectral filters to help 

in mineral identification. Analyses of low-light phenomena such as auroras, lightning, and 

the Jovian rings required a camera with an extremely sensitive detector and an optical 

system with very little scattering of light so as not to obscure faint images. 

To prevent damage to the SSI from Jupiter’s intense radiation, the instrument 

needed to be heavily shielded. It also had to be protected from propellant byproducts that 

might contaminate it during launch or in flight. Finally, the SSI had to be made as light as 

possible and require only limited electrical power to operate.109

To meet the above requirements, the Orbiter used a 1,500-millimeter (60-inch) 

focal-length Cassegrain reflecting telescope with a 176-millimeter (7-inch) aperture that 

had operated successfully on Voyager. Light from an image was collected by the telescope’s primary mirror and directed to a smaller, secondary mirror. This mirror then 

channeled the light through a hole in the center of the primary mirror and onto a solid-state 

silicon image-sensor array called a “charge-coupled device” (CCD). The CCD had 640,000 individual sensors, or “pixels,” to provide high-resolution images. Except where light entered, the 

CCD was surrounded by a 1-centimeter-thick (0.4-inch) tantalum shield to protect it from the 

intense radiation fields within Jupiter’s magnetosphere. Data collected by the CCD were digitized and sent to the Orbiter’s tape recorder for storage, eventually to be played back, edited, and 

compressed by the spacecraft’s computers, then sent to Earth. After the data were received by 

tracking stations on Earth, they were sent to JPL’s image-reconstruction equipment.110

The SSI used an eight-position filter wheel to obtain images of the same scenes in different spectral bands. These images were combined electronically on Earth to produce color images. 

The spectral response of the SSI ranged from light with wavelengths of about 0.4 micrometers to 

light of 1.1 micrometers (a micrometer is one-millionth of a meter). This range covered both the 

visible light spectrum (0.4 to 0.7 micrometers) and part of the near-infrared region.111

Texas Instruments provided the CCD for the SSI camera. The SSI used RCA microprocessors and contained 600 integrated circuits. The telescope, shutter, and filter designs were 

inherited from Voyager, but NASA improved its design to better reject scattered light, which could 

interfere with the imaging of faint objects. The electronics chassis was machined at JPL.

112

The SSI and three other optical remote sensing instruments (the near infrared 

mapping spectrometer, ultraviolet spectrometer, and photopolarimeter radiometer) were 

mounted on a movable “scan platform” connected to the despun section of the Orbiter. 

The scan platform could be oriented so as to point the instruments toward the object 

being studied. The optical axes of all four instruments were aligned so that they all looked 

toward the same areas at the same time. The simultaneous outputs from the four instruments could then be correlated so as to interpret various phenomena better.113

Ultraviolet Spectrometer. The UVS studied ultraviolet light from the Jovian system, 

which was especially helpful in understanding properties of Jupiter’s high atmosphere, 

the compositions of the planet’s satellites, and the nature of Io’s plasma torus (the donutshaped cloud of ionized gas originating from the moon and encircling Jupiter). Ultraviolet 

(UV) light includes wavelengths shorter than those of visible light but longer than those of 

x rays. The shorter the wavelength is, the higher the light wave’s frequency and energy are. 

As an illustration of this principle, UV light from the Sun is able to burn our skin. X rays, 

which have shorter wavelengths and are more energetic than UV light, can penetrate our 

bodies and do serious damage. Ultraviolet wavelengths begin at the wavelength of violet 

light, 4,000 angstroms (an angstrom is one ten-millionth of a millimeter), and range to as 

short as 150 angstroms, where the x-ray spectrum begins. The Orbiter’s UVS had detectors 

that could observe light in all but the shortest wavelengths of the ultraviolet spectrum.114

108	 “The PPR: Finding More Than Meets the Eye”; “PPR—Photopolarimeter-Radiometer.”

109	 “Solid-State Imaging,” Galileo Messenger (December 1984). 

110	 “Solid-State Imaging”; “SSI—Solid-State Imaging,” Galileo’s Science Instruments, http://www.jpl.nasa.gov/galileo/

instruments/ssi.html (accessed 9 January 2001).

111	 “Solid-State Imaging”; “SSI—Solid-State Imaging.”

112	 “Solid-State Imaging”; “SSI—Solid-State Imaging.”

113	 “Solid-State Imaging”.

114	 “UVS—Ultraviolet Spectrometer,” Galileo’s Science Instruments, http://www.jpl.nasa.gov/galileo/instruments/uvs.

html (accessed 15 March 2000); “Ultraviolet Spectrometer,” Galileo Messenger (June 1983); “Ultraviolet Radiation,”

Microsoft Encarta Online Encyclopedia, 2000, http://encarta.msn.com. 
148 MISSION TO JUPITER: A Histor y of the Galileo Project

Figure 5.14. The Orbiter’s SSI. (JPL image number 352-8284) 

Figure 5.15. Cutaway schematic of the SSI. Shows principal optical components, chargecoupled device detector, particle radiation shield, and front aperture. (JPL 230-537)

The UVS searched for evidence of certain complex hydrocarbon molecules in 

the Jovian atmosphere that, on Earth, were “building blocks” for life. In its investigation of 

the Galilean satellites Io, Europa, Ganymede, and Callisto, the UVS searched for evidence 

of atmospheres. Such evidence could indicate that volatiles were escaping from the satellites and that their compositions were still evolving. Reflected light from the satellites was 

picked up by the UVS and analyzed for signs of ammonia, ozone, and sulfur dioxide.

Particles from Io’s volcanic eruptions are believed to be the source of its ionized 

plasma torus. UVS measurements, as well as observations by Orbiter’s fields-and-particles 

instruments, of electrons and ions in the torus provided comprehensive data on Io’s evolution and its relationship with Jupiter’s magnetic field. 

The UVS was developed and built by the University of Colorado’s Laboratory for 

Atmospheric and Space Physics. The UVS team was led by principal investigator Charles 

Hord. Although the experiment’s main focus was to study the Jovian system, UVS measurements were also carried out during Galileo’s six-year voyage to Jupiter in order to 

obtain data at Venus, Earth and its Moon, and two asteroids.115

APERTURE

CORRECTOR

ELEMENTS

QUARTZ

CLEAR

APERTURE

DEPLOYABLE

APERTURE

COVER

FIELD

CORRECTOR

ELEMENTS

SECONDARY

MIRROR FILTER

SHUTTER

ELECTRONICS CHASSIS FOCAL POINT

CCD

PRIMARY

MIRROR

QUARTZ

RADIATION PLUG

RADIATOR

PLATE

TANTALUM

RADIATION

SHIELDING

115	 “Ultraviolet Spectrometer”; “Galileo,” Laboratory for Atmospheric and Space Physics, http://lasp.colorado.edu/

programs_missions/present/galileo/ (accessed 11 January 2001).
Whenthe Space Shuttle Atlantis lifted off from 

Kennedy Space Center at 12:53 p.m. EDT on 18 

October 1989, it began the Galileo Prime Mission, which 

explored Venus, two asteroids, a comet, and the Jovian system. The Shuttle carried in its 

cargo bay both the Galileo spacecraft and the Inertial Upper Stage (IUS) rocket to which 

the spacecraft was connected, bringing them up to a low-Earth orbit. At 7:15 p.m. EDT, 

the Shuttle crew deployed the 38,500-pound package that included the Galileo spacecraft 

and its IUS booster (see figure 6.1). As this spacecraft-rocket combination emerged from 

the Shuttle with its booms and antennas still folded, it “resembled a butterfly emerging 

from its cocoon, with all its appendages.”1

Once the spacecraft and IUS were deployed, the Shuttle flew away to a safe 

distance. An hour after deployment, the IUS fired, slowing Galileo’s orbital speed around 

the Sun by 10,000 mph. As a result, Galileo began falling toward the inner solar system 

and its first destination: Venus.2

The IUS booster rocket, which was supplied by the U.S. Air Force, consisted of 

two solid-fueled sections. The first, containing 21,400 pounds (about 9,700 kilograms) of 

Chapter 6

Galileo Deployment, the

Inner Solar System Tour, 

and the Asteroid Belt

1	 C. T. Russell, foreword to “The Galileo Mission,” Space Science Reviews 60, nos. 1–4 (1992); Everett Booth, “Galileo:

The Jupiter Orbiter/Space Probe Mission Report,” December 1999, p. 2, JPL internal document, Galileo—Meltzer

Sources, folder 18522, NASA Historical Reference Collection, Washington, DC; Kathy Sawyer, “Galileo’s 6-Year Flight

to Jupiter Begins,” Washington Post (19 October 1989): A4.

2	Russell, foreword to “The Galileo Mission”; Booth, “Galileo: The Jupiter Orbiter/Space Probe”; Sawyer, “Galileo’s 6-

Year Flight.”
150	 MISSION TO JUPITER: A History of the Galileo Project Chapter 6 I Galileo Deployment 151

propellant, lifted Galileo out of low-Earth orbit while the smaller second stage, containing 

6,000 pounds (about 2,700 kilograms) of propellant, “injected” Galileo into its interplanetary Venus-Earth-Earth gravity-assist (VEEGA) trajectory. After deployment from the 

Shuttle, the IUS received its commands from Onizuka Air Force Base, located south of San 

Francisco and very close to NASA Ames Research Center. In the event of an IUS malfunction, Onizuka could issue corrective commands.3

Checkout of Galileo Equipment and First Observations 

Shortly after launch, the Galileo flight team, whose members were located at Kennedy 

Space Center, Ames Research Center, and JPL, began remotely verifying the functionality 

of each of the spacecraft’s systems and subsystems. They feared that vibrations during 

launch and the IUS burn might have “flipped” some of the craft’s electrical relays, which 

would need to be reset. Such an occurrence would have been especially problematic for 

the Probe, so it was the first system examined. The Probe ran on nonrechargeable lithium 

batteries, and a flipped relay could have resulted in power being drained from the batteries. If the Probe’s batteries were depleted, it would not be able to function at Jupiter, and 

a major part of the mission would be ruined. Fortunately, none of the Probe’s relays had 

been flipped. Future tests were scheduled annually, with the next one to occur during the 

first Earth flyby in December 1990.4

A day after Galileo’s 18 October launch, it had already journeyed almost 300,000 

miles (480,000 kilometers) from its home planet and was speeding toward Venus at 

more than 9,000 miles per hour (14,500 kilometers per hour).5

 Several days after launch, 

Galileo’s mission operations team began sending commands to Galileo. In general, the 

spacecraft was operated by command sequences sent from Earth that were then stored 

for later execution. “Real-time” commands could also be sent as needed, however, for 

quicker response. By 27 October 1989, the mission operations team had sent over 250 

real-time commands.6

In the first few days of the mission, the spacecraft’s heavy ion counter was 

powered on and began detecting high-energy ions from solar flares. Emissions from solar 

flares were of immediate concern because they were energetic enough to disrupt sensitive 

equipment aboard the spacecraft, causing what was termed a single event upset (SEU). 

Fortunately, no SEUs were detected by Galileo. SEU incidents that had plagued earlier 

missions had been thoroughly studied, and special shielding and hardening of electronic 

components had been included in Galileo’s design.7

In late October, Galileo’s solid state imaging and near infrared mapping spectrometer instruments were successfully checked out. Data from these tests were stored on 

Galileo’s data memory subsystems tape recorder, to be played back later and transmitted 

to Earth as the spacecraft’s telemetry link capabilities allowed. The tape recorder would 

be used extensively during each planetary, satellite, and asteroid encounter in order to 

record and preserve valuable observation data until they could be forwarded to Earth. 

Galileo was being tracked by the Deep Space Network (DSN) system of antennas in California, Australia, and Spain.8

Testing to be performed on Galileo’s flight to Venus included the characterization 

of spacecraft equipment. One of the first pieces of equipment to be tested was the attitude-control subsystem, which would be needed during trajectory correction maneuvers 

(TCMs) in early November. The remaining scientific instruments would all be checked out 

and turned on in time for the Venus flyby.9

Figure 6.1. The deployed Galileo spacecraft and Inertial Upper Stage booster rocket. Note 

the two circular sun shields installed to protect the spacecraft’s sensitive equipment. 

(Image number S89-42940)

3	 NASA Office of Space Science and Applications, Mission Operation Report (Report No. E-829-34-89-89-01, 1989),

p. 1; Booth, “Galileo: The Jupiter Orbiter/Space Probe”; Sawyer, “Galileo’s 6-Year Flight.”

4	 “Checking Out the Probe,” Galileo Messenger (November 1989): 3.

5	 Kathy Sawyer, “Galileo Travels 292,500 Miles Toward Venus,” Washington Post (20 October 1989): A24.

6	 “Galileo: Up to Date,” Galileo Messenger (November 1989): 3.

7	 Ibid., p. 3.

8	 Ibid., p. 3; Douglas J. Mudgway, Uplink-Downlink: A History of the Deep Space Network, 1957–1997 (Washington, DC:

NASA SP-2001-4227, 2001).

9	 “Galileo: Up to Date,” p. 3.
152	 MISSION TO JUPITER: A History of the Galileo Project Chapter 6 I Galileo Deployment 153

Trajectory Correction Maneuvers

Included in the mission were opportunities for several TCMs that would optimize the 

spacecraft’s course and minimize propellant requirements. The first TCM after launch, 

called TCM-1, was initiated on 9 November 1989 and corrected for IUS trajectory-injection 

errors. The IUS burn had been extremely accurate so only a small correction to Galileo’s speed was required—less than a 2-meter-per-second change. TCM-1 also included a 

preventative maneuver. When traveling inside Earth’s orbit, Galileo had to orient its axis 

directly toward the Sun in order for the spacecraft’s Sun shields to shade the equipment 

and keep it from overheating. This orientation introduced a potential problem: orbital 

corrections that involved a thrust component directly away from the Sun would be inefficient to perform, requiring excessive fuel consumption. TCM-1 was designed to reduce to 

less than 1 percent the probability that such an “anti-Sun” maneuver would be needed.10

TCM-2, performed on 22 December 1989, removed the very small maneuverexecution errors of TCM-1 and aimed Galileo at the desired Venus flyby location. The 

third scheduled maneuver (TCM-3) on the Earth-Venus trajectory leg turned out not to be 

needed because the aimpoint at Venus achieved by TCM-2 had been “virtually perfect.”11

The Venus Encounter

Galileo reached and flew by Venus on 10 February 1990, less than four months after launch. 

The spacecraft instruments had been designed for a broad range of planetary investigations, 

and they took many useful measurements at Venus. But because Galileo was closer to the 

Sun than Earth orbit during this part of its trajectory, steps had to be taken to protect its 

equipment—in particular, its high-gain antenna (HGA)—from possible thermal damage. The 

HGA’s parabolic-shaped dish had to be kept folded up like an umbrella during this time. The 

HGA also had to stay pointed toward the Sun so that a sun shield mounted at its tip remained 

in the right position to keep the HGA shaded and cool. This configuration prevented the 

spacecraft from rotating to optimum orientations for data collection. In addition, keeping 

the HGA dish closed made it impossible to send data back to Earth at the normal rate for 

planetary missions. Two small low-gain antennas (LGAs) had to be used instead, transmitting 

data at rates so slow that most of the data collected at Venus had to be stored on the spacecraft’s tape recorder until Galileo flew close to Earth months later and could transmit more 

quickly. But the volume of data that could be stored on the tape recorder was limited, and 

this restricted the amount of data collected at Venus. In particular, it restricted the number of 

images collected because they required large amounts of digital storage space.12

Venus had already been visited by many U.S. and Soviet craft, which had 

performed flybys and sent atmospheric probes, balloons, and landers toward and onto 

the planet’s surface. Despite Galileo’s limitations and the previous data that had been 

collected, however, the spacecraft was able to perform an interesting set of observations. 

Some of Galileo’s instruments, such as its near-infrared mapping spectrometer (NIMS), had 

never been employed on a Venus-bound spacecraft. Others, such as Galileo’s solid state 

imaging system, constituted major improvements over instruments that had been used 

before. Galileo extended observations that had previously been made of particle, field, 

and plasma wave phenomena and of the planet’s limb region. As Galileo encountered 

Venus, the spacecraft’s instruments worked in concert with ground-based observations 

from Earth, yielding more accurate information than had been available about Venus’s 

plasma fields, cloud patterns, and lightning.13

Infrared Observations

Galileo’s NIMS revealed the structure not only of Venus’s clouds and dense atmosphere, 

but also of the surface beneath. NIMS was able to probe from a height of about 100 kilometers downward. For the first time, space scientists were able to map small-scale features 

in mid-latitude and equatorial regions. The NIMS experiment was particularly useful in 

studying the night side of Venus. Recent telescope observations had discovered that the 

Venusian atmosphere was transparent to limited parts of the infrared spectrum. Galileo’s 

observations at these wavelengths were able to collect thermal radiation from deep in the 

planet’s atmosphere, far below the visible clouds. As a result, Galileo was able to make 

night-side maps of Venus with resolutions three to six times better than those of Earthbased telescopes.14

Galileo was able to take night-side infrared images with high spatial resolution. 

The images indicated substantial lower cloud deck opacity variations that were centered 

at an altitude of 50 kilometers. The Galileo project team estimated that variations of 25 

percent in the cloud deck opacity were caused by a narrow cloud band between an altitude of 48 kilometers (the bottom of the Venusian cloud deck) and roughly 52 kilometers. 

The fact that this region had such a marked effect on the opacity of the entire cloud deck 

suggested dramatically different conditions inside this narrow band from those of the rest 

of the cloud deck.15

In order to support the Galileo mission’s investigation of Venus, Earth-based 

observations were carried out before, during, and after the spacecraft’s flyby. They included 

thermal maps taken of Venus by the NASA Infrared Telescope Facility on Hawaii’s Mauna 

Kea, as well as near-infrared images obtained by a global network of ground-based 

observatories. Astronomers on Earth saw a large dark cloud at low latitudes (within 40 

degrees of the equator) that extended halfway around Venus and persisted throughout the 

10	Louis A. D’Amario, Larry E. Bright, and Aron A. Wolf, “Galileo Trajectory Design,” Space Science Reviews 60, nos. 1–4

(1992): 26–28.

11	 Ibid., p. 28.

12	Torrence V. Johnson, Clayne M. Yeates, Richard Young, and James Dunne, “The Galileo Venus Encounter,” Science 

253 (27 September 1991): 1516; Michael J. S. Belton, Peter J. Gierasch, Michael D. Smith, Paul Helfenstein, Paul J.

Schinder, James B. Pollack, Kathy A. Rages, Andrew P. Ingersoll, Kenneth P. Klaasen, Joseph Veverka, Clifford D.

Anger, Michael H. Carr, Clark R. Chapman, Merton E. Davies, Fraser P. Fanale, Ronald Greeley, Richard Greenberg,

James W. Head III, David Morrison, Gerhard Neukum, and Carl B. Pilcher, “Images from Galileo of the Venus Cloud

Deck,” Science 253 (27 September 1991): 1531–1536.

13	Johnson et al., “The Galileo Venus Encounter,” pp. 1516–1517.

14	 “Galileo’s First Encounter: Venus is in View,” Galileo Messenger (February 1990): 1, 4; Johnson et al., “The Galileo

Venus Encounter,” p. 1517; R. W. Carlson, K. H. Baines, Th. Encrenaz, F. W. Taylor, P. Drossart, L. W. Camp, J. B.

Pollack, E. Lellouch, A. D. Collard, S. B. Calcutt, D. Grinspoon, P. R. Weissman, W. D. Smythe, A. C. Ocampo, G. E.

Danielson, F. P. Fanale, T. V. Johnson, H. H. Kieffer, D. L. Matson, T. B. McCord, and L. A. Soderblom, “Galileo Infrared

Imaging Spectroscopy Measurements at Venus,” Science 253 (27 September 1991): 1541.

15	Carlson et al., “Galileo Infrared Imaging,” pp. 1541–1544.
154	 MISSION TO JUPITER: A History of the Galileo Project Chapter 6 I Galileo Deployment 155

observing period. Mid-latitudes (40 to 60 degrees) were characterized by bright east-west 

cloud bands. The highest latitudes observable, between 60 and 70 degrees, remained dark 

and featureless, indicating greater cloud opacity. The dynamics of sulfuric acid droplets 

in the planetwide Venusian cloud layer are believed to affect opacity significantly. For 

instance, the greater transparency of the atmosphere in some areas may be caused by 

downwelling currents that transport sulfuric acid droplets to warmer regions below the 

cloud base, where they evaporate.16

Solid State Imaging System Observations

Another Galileo instrument well-suited to investigating Venus was the solid state imaging 

(SSI) system, which provided high-resolution tracking of small ultraviolet features and also 

conducted an optical search for lightning. These observations addressed fundamental questions about Venusian meteorology, chemical composition, and atmospheric evolution. 17

Solar Wind–Bow Shock Interactions

On its approach, Galileo repeatedly traversed Venus’s magnetospheric bow shock, 

mapping its structure and that of the solar wind upstream of the shock. A planet’s bow 

shock is the interface between the region of space influenced by the planet’s magnetic 

field and the undisturbed, interplanetary solar wind (a current of ions, or plasma, streaming outward from the Sun). A planet’s bow shock is very similar to the wave which 

appears in front of a boat as it passes through the water, or in front of a plane traveling 

at supersonic speeds. Planetary bow shocks slow the solar wind and help force it to go 

around the planet’s magnetosphere (see figure 6.2). 

The first hint that Galileo had crossed the Venusian bow shock was given by the 

craft’s magnetometer, which registered sudden changes in magnetic field strengths. The 

plasma detector reported heating and compression of plasma flows, with electron temperatures rising by up to 20 percent and densities increasing by as high as a factor of three.18

The solar wind interacts very differently with Venus from how it interacts with 

Earth. Our planet’s strong magnetic field deflects the solar wind while it is still far away 

from us, resulting in a bow shock that occurs at a distance of about 10 Earth radii (65,000 

kilometers or 40,000 miles) from the planet’s center. Venus’s weaker magnetic field cannot 

keep the solar wind so far away, and this results in a bow shock that occurs nearly on 

Venus’s surface, or in other words, one Venus radius from the planet’s center. (The radii of 

Venus and Earth are approximately the same.) The solar wind thus interacts directly with 

the Venusian ionosphere and dense atmosphere.19

The Search for Lightning

The spacecraft’s plasma wave instrument searched for radio signals characteristic of lightning and recorded nine events in frequency ranges that suggested lightning was the 

source. The occurrence of lightning in Venus’s atmosphere had been previously reported 

by others but was still a subject of controversy. The Soviet Venera spacecraft reported 

lightning at Venus, but there has always been a concern that the signals could actually 

have been electrostatic discharges from the spacecraft itself as it descended through the 

atmosphere. The U.S. Pioneer-Venus orbiter picked up very low-frequency “whistler” 

signals20 on the order of 100 hertz (cycles per second) that were interpreted as being 

generated by a lightning event deep in the Venusian atmosphere. But skeptics believed 

that the whistlers could also have been locally generated plasma waves that had nothing 

to do with a lightning event. 

16	D. Crisp, S. McMuldroch, S. K. Stephens, W. M. Sinton, B. Ragent, K.-W. Hodapp, R. G. Probst, L. R. Doyle, D.

A. Allen, and J. Elias, “Ground-Based Near-Infrared Imaging Observations of Venus During the Galileo Encounter,”

Science 253 (27 September 1991): 1538–1541; Glenn S. Orton, John Caldwell, A. James Friedson, and Terry Z. Martin,

“Middle Infrared Thermal Maps of Venus at the Time of the Galileo Encounter,” Science 253 (27 September 1991):

1536–1538.

17	 Johnson et al., “The Galileo Venus Encounter,” p. 1517.

18	Margaret Galland Kivelson, “Serendipitous Science from Flyby of Secondary Targets: Galileo at Venus, Earth, and

Asteroids; Ulysses at Jupiter,” Reviews of Geophysics, supplement (July 1995): 565–566; Mona Kessel, author and

curator, NASA Goddard Space Flight Center, “Collaborative Study of the Earth’s Bow Shock,” NASA SpaceLink, 

http://spdf.gsfc.nasa.gov/bowshock/ (accessed 11 November 2000) (available in Galileo—Meltzer Sources, folder

18522, NASA Historical Reference Collection, Washington, DC); “Bow Shock,” http://www.windows.ucar.edu/cgibin/tour_def/glossary/bow_shock.html, and “Magnetosphere,” http://www.windows.ucar.edu/tour/link=/glossary/

magnetosphere.html&edu=high, both from Windows to the Universe, copyright 1995–1999, The Regents of the

University of Michigan, 2000 (accessed 15 November 2004), (available in Galileo—Meltzer Sources, folder 18522,

NASA Historical Reference Collection, Washington, DC).

Figure 6.2. Solar wind-bow shock-magnetosphere interaction for a planet such as Earth. 

Venus’s bow shock occurs nearly on the planet’s surface.

19	Kivelson, “Flybys of Planets and Asteroids,” p. 565; L. A. Frank, W. R. Paterson, K. L. Ackerson, F. V. Coroniti, and V.

M. Vasyliunas, “Plasma Observations at Venus with Galileo, Science 253 (27 September 1991): 1528; D. J. Williams,

R. W. McEntire, S. M. Krimigis, E. C. Roelof, S. Jaskulek, B. Tossman, B. Wilken, W. Studemann, T. P. Armstrong, T. A.

Fritz, L. J. Lanzerotti, and J. G. Roederer, “Energetic Particles at Venus: Galileo Results,” Science 253 (27 September

1991): 1526.

20	Whistlers are packets of electromagnetic waves in which the higher frequencies travel faster than the lower

frequencies, reaching the receiving antenna first. When a whistler signal is played back at audible frequencies, it

sounds like a whistle descending in pitch from high to low. Earth lightning is known to generate whistlers (Kivelson,

“Flybys of Planets and Asteroids,” p. 566).

Planet

Magnetopause

Bowshock

Magnetosphere

Solarwind Magnetotail
156	 MISSION TO JUPITER: A History of the Galileo Project Chapter 6 I Galileo Deployment 157

A confirmation of lightning would be very important information because it 

would support the existence of convective storms in the Venusian atmosphere. It could 

also indicate active volcanism on the Venusian surface. On Earth, for instance, particles 

that rub together inside turbulent plumes of volcanic ash sometimes generate lightning; 

this might be happening on Venus as well. The Magellan spacecraft saw what appeared 

to be ash strewn downwind of volcanoes.21

Galileo’s plasma wave detector was able to measure electric fields up to 

5.6 megahertz (1 megahertz equals 106

 cycles per second). This was well above the 

frequency range of locally generated plasma waves that could give a false positive 

indication of lightning. The nine plasma wave impulses that Galileo detected were in 

a frequency band that was able to propagate upward through Venus’s atmosphere and 

ionosphere. Galileo’s plasma team concluded that “there was no reasonable hypothesis 

other than lightning-generated whistler noise that could explain the observations.”22

Due to the considerable interest in whether there is lightning activity in the 

Venusian atmosphere and what it would imply regarding atmospheric dynamics and 

possible volcanism, 10 pictures taken by Galileo’s SSI camera were devoted to this 

search. No optical lightning images were obtained, but this result was far from conclusive. If Venus lightning flashes had the power ranges and frequency of occurrence 

characteristics of terrestrial lightning, then it was only “marginally possible” that the 

phenomenon would be detected by the SSI. The SSI observations serendipitously captured a star, k-Geminorum, whose brightness was well known. This brightness was used 

to estimate that the minimum optical energy a lightning flash would need for the SSI

to detect it was about three orders of magnitude greater than the energy of a typical 

terrestrial flash.23

Venus Cloud Deck

Galileo’s SSI camera recorded useful images of the ever-changing states of Venus’s cloud 

tops. Very little was known about small-spatial-scale, short-timescale dynamics of the 

Venusian atmosphere. The SSI’s ability to image in the infrared allowed it to probe down 

to different cloud layers and follow the evolution of small dynamic phenomena in the 

atmosphere. Changes in cloud morphology were observed to occur far slower there than 

on Earth. Major changes in upper-level clouds observed by Galileo, as well as by the 

Pioneer spacecraft, evolved on timescales that ranged from days to years.24

Summary of Venus Tour

Important observations made by Galileo during its Venus flyby included the following:

•	 Probable detection of whistler phenomena that indicated the existence of 

lightning in the Venusian atmosphere.

•	 Interactions of Venus with the solar wind, which shed light on the nature of 

the planet’s bow shock and plasma environment.

•	 Mapping of small-scale midlatitude and equatorial features of the Venusian 

surface.

•	 Collection of high-resolution night-side Venus data that indicated existence 

of a high-opacity cloud deck centered at a 50-kilometer altitude, as well as 

substantial differences between northern- and southern-hemisphere clouds.

•	 Cloud morphology observations that implied that significant changes in upper 

levels of clouds occur on timescales of years, far slower than on Earth.

Return to Earth

Galileo’s post-Venus flyby of Earth in December 1990 marked the first time that a spacecraft 

had returned from interplanetary space, even if briefly. But to get to Earth from Venus, 

Galileo had to expend some of its fuel to perform another trajectory correction maneuver 

(TCM 4). When JPL’s Roger Diehl first discovered the VEEGA trajectory that would take 

Galileo to Jupiter, it appeared that the only time the craft could launch was in November 

1989. Otherwise, the planets would not be lined up correctly. But continued research by 

Louis D’Amario and Dennis Byrnes revealed that a course correction between Venus and 

Earth widened Galileo’s launch window to include most of October 1989 as well.25

TCM4 was the largest course correction that Galileo would have to perform to 

get to Jupiter, shifting the craft’s velocity by 35 meters per second. It was performed in 

two parts: first from 9 through 12 April, then from 11 to 12 May. This and smaller TCMs 

put Galileo on the precise path for an Earth flyby and gravity-assist. The craft flew by 

Earth on 8 December at an altitude of 595 miles, only 5 miles higher than had been predicted. This meant that the predicted aiming point for the flyby was 99 percent accurate. 

The time of closest approach was predicted to be 12:34:34:00 Pacific standard time (PST) 

and proved to be only half a second off.26

21	Johnson et al., “The Galileo Venus Encounter,” p. 1517; M. G. Kivelson, C. F. Kennel, R. L. McPherron, C. T. Russell,

D. J. Southwood, R. J. Walker, C. M. Hammond, K. K. Khurana, R. J. Strangeway, and P. J. Coleman, “Magnetic

Field Studies of the Solar Wind Interaction with Venus from the Galileo Flyby,” Science 253 (27 September 1991):

1518–1522; D. A. Gurnett, W. S. Kurth, A. Roux, R. Gendrin, C. F. Kennel, and S. J. Bolton, “Lightning and Plasma Wave

Observations from the Galileo Flyby of Venus,” Science 253 (27 September 1991): 1522; Richard A. Kerr, “Lightning

Found on Venus At Last?” Science 253 (27 September 1991): 1492.

22	Kivelson, “Serendipitous Science from Flybys of Secondary Targets,” pp. 566–567.

23	Belton et al., “Images from Galileo.”

24	 Ibid., pp. 1531–1536.

25	Peter N. Spotts, “NASA’s Galileo Mission Clears Hurdles for Jupiter Voyage,” Christian Science Monitor (3 December

1987): 5; Bill O’Neil, “From the Project Manager,” Galileo Messenger (July 1990): 1.

26	Bill O’Neil, “From the Project Manager,” Galileo Messenger (July 1990): 1; “A Closer Look at the Earth and Moon,”

Galileo Messenger (April 1991): 1
158	 MISSION TO JUPITER: A History of the Galileo Project Chapter 6 I Galileo Deployment 159

Earth’s Magnetic Field and the Solar Wind

Besides receiving its needed gravity-assist from Earth, Galileo also collected valuable data 

during its flyby of our planet and Moon. The first area studied was Earth’s interplanetary 

environment, which included our magnetosphere’s interaction with the solar wind. This 

plasma “wind” is extremely diffuse, with a density of only a few particles per cubic centimeter. While plasmas similar to the solar wind are difficult to maintain and analyze in 

Earth-based laboratories, the region of space near our planet can serve as a superb field 

lab for plasma study.27

The solar wind travels at speeds from 200 to 800 kilometers per second (450,000 

to almost 2,000,000 mph) and carries magnetic field lines from the Sun along with it. 

Earth’s magnetic field deflects the solar wind, and the interface between the two forms 

the boundary of Earth’s magnetosphere (see figure 6.2). The bow shock forms on Earth’s 

day side because it is this side that faces the Sun and the oncoming solar wind. The solar 

wind drags Earth’s magnetic field on its night side back into a tail (the magnetotail) whose 

length is over a thousand times greater than Earth’s radius.28

Galileo approached Earth from its night side, entering its magnetotail 560,000 

kilometers (350,000 miles) behind the planet. The magnetosphere happened to be in a 

very active state at the time, and Galileo observed several magnetic storms. Many whistlers, generated by lightning strokes, were also detected.29

Moon Observations

Although our Moon has been studied for many years from both ground-based observatories and spacecraft, Galileo was able to provide some unusual observations that gave 

us a new view of parts of the satellite. In its orbit around Earth, the Moon always shows 

us the same face. It wasn’t until the late 1960s and early 1970s that lunar orbiters showed 

us the Moon’s far side. Apollo 15 made the most extensive survey of the far side, and its 

data led scientists to postulate the existence of a large basin on the far side’s southern 

section. The new images that Galileo collected of the material covering this region led to 

the conclusion that the suspected impact basin did exist and was huge—1,940 kilometers 

(1,200 miles) across. If it was located on Earth, it would stretch from Mexico to Canada. 

Scientists now think that a 100-mile-diameter (160-kilometer-diameter) meteor formed the 

basin when it struck the Moon’s surface “like a small state coming at you from space.”30 It 

probably hit so hard that it made the whole Moon wobble and lurch while it blew a large 

chunk out of the satellite’s bottom half. 31

Seeing Earth in a New Light

Because of Galileo’s excellent instrumentation and unusual trajectory, it was able to carry 

out investigations that brought a new understanding of Earth’s ozone hole, auroras, and 

global weather patterns. The spacecraft’s near infrared mapping spectrometer (NIMS) 

was able to investigate very high clouds in Earth’s mesosphere, an extremely cold atmospheric region above the stratosphere. These mesospheric clouds, seen only since 1885, 

are believed to be caused by methane generated from industrial processes. The methane 

affects global air flows, causing increased warm air to blow into polar regions, incrementing the amount of ice melted each spring and releasing more water vapor into the 

atmosphere. This water vapor forms the mesospheric clouds and reacts with the ozone, 

breaking it down and causing an ozone hole.32

These mesospheric clouds are sometimes seen in September or October. Galileo 

saw them in December, which was a rare event that might indicate that Earth’s ozone 

population is indeed changing, as many scientists fear. The ozone layer helps shield our 

planet from harmful parts of the solar spectrum. A decrease in ozone could lead to a 

dangerous feedback loop in which increasing amounts of polar ice are melted, releasing 

more water vapor, which results in the destruction of ever-growing quantities of ozone. 

To develop reliable models of ozone depletion, atmospheric scientists have needed hardto-acquire data on mesospheric water quantities. Galileo’s investigations were important 

in filling this data gap.33

Can Life on Earth Be Surmised from Galileo’s Observations?

Carl Sagan and other scientists asked this question: was it possible, from Galileo’s observations, to deduce the existence of life on our planet? If Galileo were sending its data back 

to aliens on another world, what would they conclude about Earth? 

Aliens receiving Galileo data would know that Earth’s atmosphere had high 

oxygen and low carbon dioxide levels, in proportions that were not in equilibrium. There 

had to be a driving factor keeping these gases in disequilibrium, and the presence of life 

was one explanation. Galileo also detected narrow-band, pulsed, amplitude-modulated 

radio signals, as well as atmospheric methane in extreme thermodynamic disequilibrium. 

Methane is quickly oxidized to carbon dioxide and water. At thermodynamic equilibrium, not a single methane molecule should exist in our atmosphere. What mechanism, 

the aliens might wonder, could pump methane into our atmosphere so quickly that it 

outpaced oxidative processes? Decomposition of nonbiological organic matter would produce methane, but probably not at a sufficient rate. If the alien planet were similar to 

Earth, its scientists might know that the atmospheric methane levels could be caused by 

methane-generating bacteria, as well as activities such as agriculture, livestock-raising, 

and combustion processes.34

27	 “A Closer Look,” p. 2.

28	 Ibid., pp. 2–3.

29	 Ibid., pp. 3–4.

30	 Ibid., p. 4.

31	Kathy Sawyer, “Galileo Probe Reveals Giant Basin on Moon,” Washington Post (20 December 1990): A3.

32	“A Closer Look,” p. 4.

33	 Ibid., pp. 4–5.

34	Carl Sagan, W. Reid Thompson, Robert Carlson, Donald Gurnett, and Charles Hord, “A Search for Life on Earth from

the Galileo Spacecraft,” Nature 365 (21 October 1993): 715–721.
160	 MISSION TO JUPITER: A History of the Galileo Project Chapter 6 I Galileo Deployment 161

The existence of nitrous oxide (N2O) in our atmosphere at high disequilibrium 

levels is another indicator of possible biological processes. Although certain nonbiological mechanisms such as lightning are able to produce N2

O, other sources such as 

nitrogen-fixing bacteria and algae would provide more credible explanations for the gas’s 

abundance. Nitrogen-fixing bacteria and algae convert soil and oceanic nitrate (NO3

) to 

nitrogen gas and nitrous oxide.35

Spectral characteristics of Earth’s surface also provided interesting information on 

the question of life. Project engineers took data from the six spectral bands that Galileo’s 

SSI system observed and combined them in various ways to visualize spectral contrasts 

on the surface. Landmasses not obscured by clouds showed extreme albedo (reflectivity) 

contrasts. The colors of the lighter, more reflective areas were consistent with those of the 

rocks and soils that might be expected on a terrestrial-type planet with iron silicate surface composition. But there were widespread darker areas with a greenish tinge that, to 

an alien observer, could be puzzling. If photosynthetic processes occurred on the alien’s 

home planet, it might reason that Earth’s low albedo areas contained light-harvesting 

pigments. Widespread plant photosynthesis could explain the large atmospheric oxygen 

abundance on Earth.36

Galileo’s high-resolution visible and near-infrared imaging of Earth’s morphology, 

in which each pixel represented 1 to 2 square kilometers of surface, was not conclusive 

in establishing the existence of life. No geometric, artificial-appearing surface features or 

other compelling “artifacts of a technical civilization” were discerned. Previous studies of 

orbiting spacecraft imagery with resolutions finer than Galileo’s have indicated that, when 

surface features as small as 0.1 kilometer can be resolved, the chances of finding convincing artifacts of life in any random image is only about one in a hundred. Galileo’s poorer 

resolution would require that nearly all of Earth’s surface be imaged in order to detect a 

civilization such as ours. Galileo’s high-resolution imaging was mainly of Australia and 

Antarctica, representing a total of only 6 percent of Earth’s surface. In addition, these areas 

are among the most sparsely populated land regions on the planet.37

The atmospheric composition and spectral data received by Galileo were suggestive of some kind of life on Earth, but they were far from conclusive. The strongest 

indication that Galileo received of intelligent, technologically sophisticated life was from 

radio transmissions. These were detected only on Galileo’s night-side pass of Earth, when 

the signals were able to escape through the night-side ionosphere. Solar ultraviolet radiation provides a strong ionizing force when it shines on the day-side atmosphere, and it 

renders the ionosphere more opaque to radio waves.38

The radio transmissions that Galileo detected from ground-based antennas were 

not the only radio signals it received. Solar and auroral-related radio bursts were also 

recorded. Nevertheless, the ground-based signals had characteristics that suggested that 

the signals were being intentionally generated rather than naturally occurring. Their central frequencies remained constant over periods of hours. Naturally generated emissions 

almost always drift in frequency. The artificial signals also displayed amplitude modulation 

patterns, a characteristic not seen in naturally occurring emissions and one that might be 

interpreted as intentionally designed into the signals to transmit information.39

Satellites equipped differently from Galileo have clearly traced radio signals back 

to the surface transmitters that generated them. Galileo, however, was not able to do this. 

A technologically advanced alien civilization might be able to deduce the existence of 

surface radio transmitters from Galileo’s flyby data and make a strong case for an intelligent life-form on Earth, but such a conclusion would be far from certain.40

From the Galileo flyby, an alien observer previously unfamiliar with Earth could 

draw the following conclusions:

•	 Water is abundant on Earth in vaporous, liquid, and solid forms. If life does 

exist, it is likely water-based.

•	 The high abundance of oxygen, methane, and nitrous oxide gases in the 

atmosphere cannot readily be explained by nonbiological processes. 

•	 Spectral data from Earth’s land regions cannot be explained by geochemical 

processes but are suggestive of widespread photosynthetic processes. These 

could explain the atmospheric oxygen abundance. 

•	 No clear sign of a technologically advanced civilization was obtained from 

the spacecraft’s imaging activities.

•	 Amplitude-modulated, stable-frequency radio signals strongly suggest intelligent life.41

Out into the Asteroid Belt: The Gaspra Encounter

In planning the first spacecraft encounter with an asteroid, Galileo project staff set forth 

several objectives that included determining its size, shape, cratering characteristics, and 

composition, as well as surveying the surrounding environment. On 29 October 1991, a 

little over two years after Galileo had lifted off, it flew by the asteroid 951 Gaspra at a 

relative velocity of 8.0 kilometers per second (18,000 mph). The spacecraft was operating 

beautifully, with the exception of its high-gain antenna, which had failed to unfold into 

its intended dish shape. But there would be an opportunity to work on that problem, and 

the craft’s low-gain antennas were adequate, for the time being, to transmit the data back 

to Earth. The transmission was slow, however, and most of the data were not received 

until November 1992, when Galileo was once again nearing Earth.42

35	 Ibid., p. 718.

36	 Ibid., pp. 718–719.

37	 Ibid., pp. 718–719.

38	 Ibid., p. 720.

39	 Ibid., p. 720.

40	 Ibid., p. 720.

41	 Ibid., pp. 715–721.

42	“An Encounter with Gaspra,” and Bill O’Neil, “From the Project Manager,” both from Galileo Messenger (February

1992): 1, 8; “Gaspra’s Story Continues,” Galileo Messenger (February 1993).
162	 MISSION TO JUPITER: A History of the Galileo Project Chapter 6 I Galileo Deployment 163

Grigori Neujmin discovered Gaspra in 1916 at the Simeis Observatory in the 

Ukraine, naming the asteroid for a scientists’ resort on the Crimean Peninsula. Gaspra, 

believed to be made up of both rocky and metallic minerals that included iron, nickel, 

olivine, and pyroxene, is one of the 5,000 bodies composing the “main asteroid belt,” a 

donut-shaped area midway between Mars and Jupiter.43

The spacecraft’s images revealed Gaspra to have a cratered surface and an irregular shape. It is about 20 kilometers (12 miles) long, 12 kilometers (7.4 miles) wide, and 

11 kilometers (6.8 miles) thick, tiny compared to the largest asteroids, which range up to 

800 kilometers (500 miles) in diameter. Figure 6.3 depicts a series of progressively larger 

Gaspra images, taken as the spacecraft approached the asteroid. These images capture 

almost one full rotation of the asteroid. Gaspra’s irregular shape suggests that it was 

broken off a larger body through catastrophic collisions. Consistent with such a history is 

the prominence of groove-like linear features, believed to be related to fractures.

Figure 6.4, the highest resolution picture taken during the flyby, was formed 

from two images that Galileo took on 29 October 1991 from a distance of 5,300 kilometers (3,300 miles). This figure depicts an abundance of small craters—more than 600 of 

them with diameters ranging from 100 to 500 meters. Although many craters are visible 

on Gaspra, none approach the scale of the asteroid’s radius. Gaspra apparently lacks 

the large craters commonly seen on many planetary satellite surfaces. This characteristic 

supports the theory that Gaspra is of comparatively recent origin. Gaspra is probably 

relatively young and has not been around long enough to pick up the major scarring 

that many other bodies have. The abundance of small craters on Gaspra is not surprising, 

however, since there are far more small asteroids and rocks that could hit another asteroid 

than large ones.44

Figure 6.4. Highest resolution picture of Gaspra, constructed as a mosaic of two images 

taken by the Galileo spacecraft from 5,300 kilometers (3,300 miles) away, 10 minutes 

before closest approach on 29 October 1991. The large concavity on the lower right limb 

is about 6 kilometers (3.7 miles) across, while the prominent crater on the left is about 

1.5 kilometers (1 mile) in diameter. A striking feature of Gaspra’s surface is the presence 

of more than 600 small craters, 100 to 500 meters (330 to 1,650 feet) in diameter. (NASA 

image number PIA00118)

43	“An Encounter with Gaspra”; “Gaspra’s Story Continues.”

Figure 6.3. The Gaspra asteroid as seen by the approaching Galileo spacecraft. The images 

show Gaspra growing progressively larger in the field of view of Galileo’s SSI camera. 

The earliest view (upper left) was taken 164,000 kilometers (102,000 miles) from Gaspra, 

while the latest (lower right) was taken 16,000 kilometers (10,000 miles) away. (NASA image 

number PIA00079)

44	“An Encounter with Gaspra”; “Gaspra’s Story Continues”; “Gaspra Approach Sequence,” NASA’s Planetary 

Photojournal Catalog Page, http://photojournal.jpl.nasa.gov/cgi-bin/PIAGenCatalogPage.pl?PIA00079 (available

in Galileo—Meltzer Sources, folder 18522, NASA Historical Reference Collection, Washington, DC); “Gaspra—

Highest Resolution Mosaic,” NASA’s Planetary Photojournal Catalog Page, http://photojournal.jpl.nasa.gov/cgi-bin/

PIAGenCatalogPage.pl?PIA00118 (available in Galileo—Meltzer Sources, folder 18522, NASA Historical Reference

Collection, Washington, DC); “Galileo Mission to Jupiter: Educators’ Slide Set Vol. 1,” http://www2.jpl.nasa.gov/

galileo/slides/ (accessed 15 November 2004) (available in Galileo—Meltzer Sources, folder 18522, NASA Historical

Reference Collection, Washington, DC).
164	 MISSION TO JUPITER: A History of the Galileo Project Chapter 6 I Galileo Deployment 165

Galileo’s observations showed that the solar wind changed direction a few hundred kilometers from the asteroid. This behavior of the ionized, electrically conducting 

gas that composes the solar wind is typical of what happens when it encounters a magnetic region and is called a “field rotation.” Although this result suggested that Gaspra 

might have a magnetic field, it was not conclusive. Dr. Margaret Kivelson, a UCLA scientist 

who was the principal investigator for Galileo’s magnetometer studies, emphasized that 

the field rotation might not have been caused by an interaction with Gaspra. The solar 

wind frequently undergoes field rotations, and it might simply have been coincidental that 

the observed one took place near the asteroid.45

One of the big questions that interest space scientists is, what were the processes that formed the asteroids and gave them their properties? The scientific community 

believes that asteroids’ chemical compositions are related to those of meteorites, interplanetary fragments that have impacted Earth. If Gaspra indeed has a magnetic field, this 

finding could shed light on how it was formed and at what temperatures. Since asteroids 

are believed to have formed during the earliest stages of the solar system, the existence 

of a Gaspra magnetic field might also provide information on the magnetic environment 

of the ancient solar system. In addition, the strength of an asteroid’s magnetic field could 

give an indication of how rich it is in iron and iron-nickel alloys. One day, asteroids may 

be mined for their ores, and magnetic field strength may provide a clue as to the body’s 

economic value.46

Back to Earth

On 8 December 1992, two years after its last Earth encounter, Galileo once more flew by 

our planet, passing within 300 kilometers (190 miles) above the South Atlantic and using 

Earth’s gravity to swing it onto a direct trajectory to Jupiter. The accuracy of Galileo’s flyby 

maneuver was phenomenal. The spacecraft passed within 1 kilometer of its aimpoint, 

and as a result, a post-Earth trajectory correction maneuver (TCM-18) was canceled. This 

saved approximately 5 kilograms (11 pounds) of propellant that could be used later in 

the mission.47

Once again, the Galileo team was able to make valuable observations of our 

planet. The ultraviolet spectrometer (UVS) group, working with the near infrared mapping 

spectrometer (NIMS) group, discovered “a wealth of information” about the geocorona,48

the outermost part of Earth’s atmosphere (see figure 6.5). It is composed of diffuse hydrogen gas that has escaped from the lower reaches of the atmosphere. This hydrogen 

first absorbs, then reemits a certain part of the solar spectrum called Lyman-α radiation. 

Galileo was able to observe some of this reemitted radiation. The absorption and reemission process results in a force on the geocorona in the anti-Sun direction, creating a 

“geotail” feature of the geocorona (see figure 6.6).

Figure 6.5. The geocorona, shown here in a photograph taken with an ultraviolet camera, 

is a halo of low-density hydrogen around Earth that absorbs sunlight and emits “Lyman- ” 

radiation. (NASA image number AS16-123-19650)

45	“Gaspra’s Story Continues.”

46	“An Encounter with Gaspra”; “Gaspra’s Story Continues.”

47	Kathy Sawyer, “Galileo Swings by Earth, Speeds Toward Jupiter,” Washington Post (9 December 1992): A3; “A

Farewell to Earth” and Bill O’Neil, “From the Project Manager,” both from Galileo Messenger (February 1993): 1, 3.

48	“A Farewell to Earth,” Galileo Messenger (February 1993): 4.

SUNLIGHT MOON EARTH EARTH 

ESCAPING ATOMIC 

HYDROGEN 

RADIATION PRESSURE 

PUSHING HYDROGEN ATOMS 

Figure 6.6. The geocorona and its geotail. As the atomic hydrogen in the geocorona is exposed 

to sunlight, the hydrogen emits radiation. This process results in a force that sweeps much of 

the hydrogen in the direction opposite to that of the Sun, creating a geotail. 
166	 MISSION TO JUPITER: A History of the Galileo Project Chapter 6 I Galileo Deployment 167

The geocorona had previously been measured out to distances of 95,000 kilometers (59,000 miles) in the region of its geotail. During its second Earth flyby, Galileo 

detected an enormous geocorona bulge that extended 400,000 kilometers (250,000 miles) 

away from Earth in the geotail region—nearly to the Moon’s orbit and four times the 

thickness of the traditional geocorona model—and even observed very diffuse atomic 

hydrogen (about 1 atom per cubic centimeter) in the vicinity of the Moon. The Galileo 

team believed that all or most of this hydrogen was associated with the geotail, rather than 

being an aspect of the Moon’s tenuous atmosphere.

During the first Earth flyby in December 1990, Galileo investigated a series of 

high stratospheric clouds over Antarctica. Atmospheric scientists now know that these 

clouds are involved in the complex chemical reactions leading to the growing ozone hole 

over the South Pole. Due to the low temperatures of the Antarctic stratosphere, these 

clouds are composed of ice crystals, rather than water droplets, as is the case in warmer 

regions of the atmosphere. On the clouds’ ice-crystal surfaces, reactions occur that result 

in the release of ozone-destroying chlorine. At the time of Galileo’s first Earth flyby, scientists were surprised by the occurrence of these stratospheric ice clouds and hoped that 

they were an anomaly. Galileo’s NIMS instrument found the clouds during the second 

flyby as well, however. Dr. Robert Carlson, NIMS principal investigator, thought that they 

might actually be a frequent occurrence over the South Pole.49

Galileo was able to fly by the northern polar region of the Moon that had been 

photographed by Mariner 10 in 1973. This time, however, Galileo was able to image the 

region in infrared, as well as visible, and provide new data on geologic processes. Exposed 

lava flows observed by Galileo indicate that volcanism during the Moon’s early years was 

more frequent and widespread than had been thought. Galileo data also suggested that 

the Moon’s far side had a thicker crust. The Galileo imaging team was surprised to find 

some hidden maria (lunar “seas” made up of frozen lava flows) that had been overlain 

by other features. These “cryptomaria” were only visible in certain spectral bands that 

Galileo’s NIMS was fortunately able to detect.50

A Second Asteroid Encounter: Ida

Months after leaving Earth for the last time, Galileo once again entered the asteroid belt 

between the orbits of Mars and Jupiter. On 28 August 1993, Galileo flew to within 1,500 

miles (2,400 kilometers) of a new asteroid: 243 Ida. This asteroid draws its name from 

a Greek myth in which the god Zeus was raised by Ida the nymph. The Ida asteroid is 

considerably larger than Gaspra but is also highly irregular, measuring 56 by 24 by 21 

kilometers (35 by 15 by 13 miles). Like Gaspra, Ida is heavily cratered with small depressions (see figure 6.7). The cratering is more extensive on Ida, however, suggesting that it 

formed earlier than Gaspra.51

Galileo detected changes in the interplanetary magnetic field in the vicinity of 

Ida; as with Gaspra, these changes implied that Ida may have a magnetic field. Prior to 

Galileo’s journey, many space scientists did not believe that asteroids had magnetic fields. 

In planets such as Earth, magnetic fields are generated in dense metallic cores. But asteroids the size of Ida are too small and light to have gravitational fields strong enough to 

have pulled their dense metallic minerals into the center as the asteroids were forming. 

Nevertheless, other mechanisms could explain the formation of asteroidal magnetic fields. 

One theory is that the strong fields in the solar wind might have induced magnetism in the 

asteroids at some point in the past. Another theory is that asteroids may be pieces broken 

from larger parent bodies. These bodies could have had strong enough gravitational fields 

to form metal cores that generated magnetic fields. These fields then magnetized some of 

the minerals surrounding the cores. When chunks broke away from the parent bodies and 

formed asteroids, they carried magnetized elements with them.52

Figure 6.7. The asteroids Ida (left) and Gaspra (right). Gaspra was imaged on 29 October 

1991 at a range of 5,300 kilometers (3,300 miles). Ida was imaged on 28 August 1993 from 

a range of 3,000 to 3,800 kilometers (1,900 to 2,400 miles). The surfaces of Ida and Gaspra 

contain many small craters, evidence of numerous collisions. The fact that craters are more 

abundant on Ida suggests that it is older than Gaspra. (NASA image number PIA00332)

49	“ Farewell to Earth,” pp. 3–4.

50	“Observing the Moon in a Different Light,” Galileo Messenger (February 1993): 2–3.

51	 “Revisiting the Asteroids: This Time It’s Ida,” Galileo Messenger (February 1994): 4; “Galileo Mission to Jupiter.” 52	 “Revisiting the Asteroids,” p. 4.
168	 MISSION TO JUPITER: A History of the Galileo Project Chapter 6 I Galileo Deployment 169

Months after the Ida flyby, when Galileo’s images were played back, project scientists got a big surprise. On 17 February 1994, a search of imaging data revealed a small 

moon orbiting Ida! Its existence was confirmed eight days later in a search of NIMS data. 

This was the first conclusive evidence that natural satellites of asteroids exist. The piece of 

rock, heavily cratered as are Ida and Gaspra, is only approximately 1.6 kilometers (1 mile) 

in diameter, and it orbits about 100 kilometers (60 miles) from Ida’s center (see figure 6.8). 

The International Astronomical Union named it “Dactyl.” The name is derived from the 

Dactyli, Greek mythological beings who lived on Mount Ida.53

Galileo’s planetary gravity-assists and asteroid flybys were now completed, and the 

spacecraft continued on its way to Jupiter with a scheduled arrival date of 7 December 1995. 

Summary of Galileo’s Major Observations During Its Moon and Earth 

Flybys and Its Asteroid Encounters

Major accomplishments of Galileo’s first Moon and Earth flybys included the detection of 

an impact basin almost 2,000 kilometers in diameter on the Moon’s far side, along with a 

rare sighting of mesospheric clouds above Earth in December, possibly indicating changes 

in our planet’s ozone layer.

Galileo made these significant observations during its encounter with the 

asteroid Gaspra:

•	 Image data revealed Gaspra to have an irregular shape, linear features related 

to fractures, and abundant small craters, but a dearth of large ones. These 

data suggest that Gaspra may be relatively young and that it was formed 

from a chunk of some larger body. The asteroid is about 20 kilometers (12 

miles) long, 12 kilometers (7.4 miles) wide, and 11 kilometers (6.8 miles) 

thick, considerably smaller than the largest asteroids, which range up to 800 

kilometers (500 miles) in diameter. 

•	 Changes in direction of the solar wind a few hundred kilometers from the 

asteroid suggest that Gaspra may have its own magnetic field. If so, this field 

might provide data on the ancient solar system’s magnetic environment and 

could give an indication of how rich Gaspra is in iron and iron-nickel alloys. 

During its second Earth flyby, Galileo’s accomplishments included the following:

•	 Detection of a huge geocorona bulge that extended nearly to the Moon’s orbit.

•	 Another observation of mesospheric clouds, indicating that their existence 

during the first flyby was not an anomaly.

•	 Lunar lava-flow observations indicating more extensive volcanism during the 

Moon’s early years than previously thought.

•	 Data collected that suggested a thicker farside lunar crust than expected.

Galileo’s observations during its encounter with the asteroid Ida revealed the 

following information:

•	 The asteroid is 56 by 24 by 21 kilometers (35 by 15 by 13 miles), significantly 

larger than Gaspra. 

•	 It is more heavily cratered than Gaspra, a condition that suggests greater age. 

•	 The interplanetary magnetic field changes near Ida; this fact indicates that 

Ida may have its own magnetic field also. 

•	 Ida has its own moon: a 1.6-kilometer-diameter (1-mile-diameter) rock 100 

kilometers (60 miles) away, which has been named Dactyl.

53	“Ida Data Return,” Galileo Home Page, pp. 2–3, http://www2.jpl.nasa.gov/galileo/iaf/iaf_sect3.html (accessed 21

March 2000) (available in Galileo—Meltzer Sources, folder 18522, NASA Historical Reference Collection, Washington,

DC); Donald J. Williams, “Jupiter—At Last!” Johns Hopkins APL Technical Digest 17, no. 4 (1996): 352–353; “High

Resolution View of Dactyl,” NASA’s Planetary Photojournal Catalog Page, http://photojournal.jpl.nasa.gov/cgi-bin/

PIAGenCatalogPage.pl?PIA00297 (available in Galileo—Meltzer Sources, folder 18522, NASA Historical Reference

Collection, Washington, DC); “Galileo Mission to Jupiter.”

Figure 6.8. Dactyl, satellite of the asteroid Ida. The little moon is egg-shaped and only about 

1.6 kilometers (1 mile) across. The large crater is about 300 meters (1,000 feet) across. This 

image was taken on 28 August 1993 at a distance of 3,900 kilometers (2,400 miles) from the 

moon. (NASA image number PIA0029)

INAPRIL 1991, GALILEO WAS ON ITS WAY FROM ITS FIRST EARTH FLYBY to

its encounter with the asteroid Gaspra. Since the October 1989 launch, the 

4.8-meter-diameter (16-foot-diameter) parabolic dish of the spacecraft’s 

high-gain antenna (HGA) had been folded up like an umbrella and kept stowed behind 

a sun shield. This configuration had helped protect it from the extreme forces of Galileo’s 

launch and the intense solar radiation of the inner solar system tour. But Galileo was now 

far enough from the Sun that the HGA could be deployed, thus making Galileo capable 

of rapidly sending and receiving data. JPL’s flight team prepared to send commands to 

unfurl the HGA. (Figure 7.1 depicts the open antenna.)

The flight team was nervous. Conducting a major mechanical operation many 

millions of kilometers from the nearest set of hands is risky. If anything goes wrong, it 

isn’t possible to take out a toolbox and fix the problem. On 11 April 1991, the flight team 

initiated the deployment sequence. It radioed commands for Galileo to turn on two direct 

current (DC) electric motors in the mechanical drive system that was designed to open the 

HGA. The antenna deployment time, if both motors operated properly, was expected to 

be 165 seconds. Even if one of the motors or its gear train failed, the other motor should 

have been able to deploy the HGA in about 330 seconds.

The motors obeyed the commands and turned on, but they operated at higherthan-expected power levels for nearly 8 full minutes, as if they were laboring to open the 

Chapter 7

The High-Gain Antenna

Failure: A Disappointment

and a Challenge
172	 MISSION TO JUPITER: A History of the Galileo Project Chapter 7 I The High-Gain Antenna Failure: A Disappointment and a Challenge 173

Figure 7.1. The unfurled high-gain antenna. (Adapted from JPL image number 230-893B)

antenna. The motors were supposed to turn a worm gear,1 which in turn was supposed 

to push a ring up the axis of the antenna. This ring was connected to levers that were to 

open the antenna by spreading its ribs. The team expected a signal from Galileo confirming that the antenna had opened, but the signal never came.2

Analysis of the Problem

On the same day that the Galileo flight team tried to open the HGA and realized there was 

a problem, JPL formed an HGA deployment anomaly team from its staff of mechanical, 

electrical, thermal, materials design, reliability, and flight operations specialists. JPL also 

drew in personnel from Harris Corporation, which had built the HGA. 

By 30 April, the anomaly team had reviewed all videos taken from the Shuttle of 

Galileo’s deployment, trying to determine the status at that time of the HGA and its solar 

shade. The team studied all telemetry data from the spacecraft and evaluated the possible 

1

 Worm gears resemble coiled worms and are used when large gear reductions are needed. A worm gear is typically 

employed to reduce speed. In the Galileo spacecraft’s antenna assembly, each complete turn of the worm gear shaft 

was supposed to advance a ring a small amount up the shaft of the antenna. See Karim Nice, “How Gears Work,” 

HowStuffWorks, Inc., 2003, http://computer.howstuffworks.com/gear5.htm; “Worm Gear,” Flying Pig Co., United 

Kingdom, 2003, http://www.flying-pig.co.uk/mechanisms/pages/worm.html. 

2 “The High-Gain Antenna,” Galileo: The Tour Guide, http://www.jpl.nasa.gov/galileo/tour/2TOUR.pdf, p. 6 (accessed 

15 March 2000); “Unfurling the HGA’s Enigma,” Galileo Messenger (August 1991); Michael Dornheim, “Improper 

Antenna Deployment Threatens Galileo Jupiter Mission,” Aviation Week & Space Technology (22 April 1991): 25; Paula 

Cleggett-Haleim and Frank O’Donnell, “Galileo Antenna Deployment Studied by NASA,” NASA News, Washington, DC 

(19 April 1991); Michael R. Johnson, “The Galileo High Gain Antenna Deployment Anomaly,” NASA Technical Report 

No. N94-33319 in the proceedings of the 28th Aerospace Mechanisms Symposium (held at NASA Lewis Research 

Center, May 1994), pp. 360, 364.

effects of the different thermal expansion characteristics of the antenna’s various parts, as 

well as the effects of shock, vibration, and vacuum on the HGA.3

Mission staff received the following telemetry data from the spacecraft:

• Output from one of Galileo’s sun sensors was reduced at certain “clock 

angles.” The clock angle measured angular position on the spacecraft, with 

the origin at the craft’s rotational axis.

• The spacecraft’s spin-detector4

 output spiked 8 seconds after the HGA 

deployment attempt, indicating a sudden acceleration and deceleration of 

the spacecraft’s spin rate. 

• The spacecraft’s spin rate further decreased over the remainder of the deployment attempt.

• Electric current data from the two motors designed to open the HGA 

indicated that they had stalled 56 seconds after the start of the deployment 

sequence.

• The wobble of the spacecraft had increased.5

The surface of the antenna dish was constructed of a gold-plated molybdenum 

wire mesh stretched across 18 graphite-epoxy support ribs. The anomaly team thought 

that the reduction in output from the sun sensor might be due to shadowing by one or 

more of the ribs. After examining the sensor’s position relative to the HGA and the Sun, 

the team concluded that only one rib was capable of shadowing the sensor, and only 

at a partially deployed angle of 34 to 43 degrees. Analysis of the drop in output of the 

sun sensor narrowed this possible range of angles. The rib shadowing the sensor had 

deployed about 35 degrees from its fully stowed position.

Initially, the anomaly team thought that the sudden release of stuck ribs might 

have caused the spike in the spin-detector data, indicating a sudden change in spin rate. 

But the team noticed that the spin-detector spike occurred at the same time as an increased 

torque from the motors. Rib release was therefore discarded as an explanation for the 

spike because such a release should not have augmented the load on the motors. 

The spin rate of the spacecraft’s spun section was expected to decrease as the 

antenna dish opened, due to the increase in the antenna’s moment of inertia. This effect 

is similar to the slowing of a spinning ice skater when she extends her arms, increasing 

her moment of inertia. A decrease was measured in Galileo’s spin rate, but it was smaller 

than was expected from a fully deployed antenna. This was one more indication that the 

antenna had not completely opened. 

As mentioned above, the electric motors stalled 56 seconds into the deployment sequence. The anomaly team used its knowledge of the antenna’s mechanical drive 

3 “

Unfurling the HGA’s Enigma,” http://www.jpl.nasa.gov?galileo/messenger/oldmess/HGA.html (accessed 15 March 

2000). 

4 Galileo’s spin detector was a sensitive accelerometer mounted on the spun section of the spacecraft. This detector 

measured spin rate changes, which were affected by whether the HGA was opened or closed.

5 Johnson, “The Galileo High Gain Deployment Anomaly.”
174	 MISSION TO JUPITER: A History of the Galileo Project Chapter 7 I The High-Gain Antenna Failure: A Disappointment and a Challenge 175

system, the amount of current that the motors drew during deployment, and several tests 

on an antenna at JPL to determine that the most likely configuration of Galileo’s HGA dish 

was that three ribs on one side of the antenna had remained in their stowed position, stuck 

to the antenna’s central mast (see figure 7.2 and the photograph at the beginning of this 

chapter). The stuck ribs would have prevented the antenna dish from fully opening but 

would have allowed a rib opposite them to deploy just enough to shade the sun sensor 

by the amount indicated in the telemetry data. This scenario of an asymmetric, partially 

opened antenna dish could also explain the increase in the spacecraft’s wobble.6

Figure 7.2. HGA’s supporting ribs did not fully deploy. The HGA was not able to send usable 

data in its partially opened state.

Once the anomaly team determined the probable shape of the semideployed 

antenna, they analyzed the HGA’s design in great detail in order to identify what might 

have caused the problem. Four possible scenarios survived the team’s rigorous analysis:

• The sunshade, mounted on the tip of the central mast of the HGA to protect 

the antenna from overheating during the early months of the mission (when 

Galileo flew within Earth’s orbit), snagged in the wire mesh of the antenna dish. 

• The mechanical drive system got restrained in some way.

• The tips of several of the antenna’s ribs got stuck in their tuning-fork-like 

sockets.

• Restraint pins at the midpoints of several antenna ribs got stuck in their sockets.7

6 Johnson, “The Galileo High Gain Antenna Deployment Anomaly”; “Unfurling the HGA’s Enigma.”

7 Johnson, “The Galileo High Gain Antenna Deployment Anomaly.

STOWED

RIB

RIB #2

DEPLOYED

AT 35.3”

SUN GATE SENSOR

Exhaustive tests that the team performed on an antenna at JPL failed to show any 

way in which the sunshade could snag the antenna dish’s wire mesh such that ribs could 

be restrained in their stowed position. A restrained mechanical drive system was also eliminated as a likely cause of the antenna anomaly. The vicinity around the drive system had 

been closed off and inaccessible during successful prelaunch deployment tests of the flight 

antenna, as well as during installation of the antenna on the spacecraft, making damage to 

the drive system very unlikely. Based on the results of prelaunch testing, retention of the 

tips of the antenna dish’s ribs in their tuning-fork sockets was also deemed improbable. The 

sockets would have to have been damaged after the final deployment test or in flight. The 

team considered that the chances of this happening had been quite small. 

The anomaly team’s analysis left a failure of restraint pins to retract from their 

sockets as the most likely anomaly scenario. Each graphite/epoxy rib had two titanium 

restraint pins. The titanium pins were finished with a ceramic anodized coating, using a 

process called Tiodizing. This helped provide a good bond surface for the next coating, 

a molybdenum disulfide dry lubricant called Tiolube 460. This material, which derived its 

lubricity from its weak sulfur bonds’ sliding past each other, had been successfully used 

in the past on many different spacecraft.8

When the antenna was closed, the restraint pins remained seated in receptacles 

made of Inconel, an alloy composed of nickel, chromium, and iron. The function of the 

pins was to hold the antenna in a closed position while inside the Shuttle’s cargo bay 

(if open, the antenna would not fit within the bay). But the pins were supposed to slide 

smoothly out of their receptacles when attempts were made to deploy the antenna. The 

deployment anomaly team analyzed the possibilities that a “cold weld,” adhesion, or friction between pin and receptacle had prevented the antenna from unfurling.9

Cold welding can occur if metallic elements come into contact with each other 

under certain conditions, creating bonds across the interface. In order for any degree 

of cold welding to occur, surface oxide layers have to be removed. This could have 

happened on Galileo if the restraint pins and their receptacles rubbed against each other. 

Also necessary for cold welding is the plastic deformation10 of at least one of the two 

contacting metal surfaces. Such deformation involves the sliding and rotation of individual 

grains of one of the metals, leading to better bonding across the interface.11

Unexpected friction between restraint pin and receptacle, possibly great enough 

to prevent the antenna from opening, might have occurred if the first attempt to deploy 

the HGA had bent the pins slightly (see figure 7.3). Another possibility was that the 

metal surfaces of pins and receptacles had been galled and pitted from rubbing against 

each other. Opportunities for pins and receptacles to rub together, which could have led 

either to cold welds or to increased friction between the two parts, occurred during 

the spacecraft’s multiple truck rides across the country, under the stresses of launch, and 

during maneuvers in space such as Galileo’s deployment from the Shuttle.12

8 Ibid. 

9 Ibid.; Donald F. Lewis and Tim O’Donnell, Materials Analysis in Support of the Galileo High Gain Antenna (HGA) 

Deployment Anomaly (Pasadena, CA: JPL D-9814, November 1992), p. 10.

10 Plastic deformation refers to the ability of a metal to change shape without breaking or fracturing, with the cohesion 

between the molecules of the metal sufficient to hold them together. See “Ductility,” Electric Library’s Encyclopedia.

Com, http://www.encyclopedia.com/printable/14228.html (accessed 9 January 2002). 

11 Lewis and O’Donnell, p. 10; Johnson, “The Galileo High Gain Antenna Deployment Anomaly.”

12 Lewis and O’Donnell, p. 12; Johnson, “The Galileo High Gain Antenna Deployment Anomaly.”
176	 MISSION TO JUPITER: A History of the Galileo Project Chapter 7 I The High-Gain Antenna Failure: A Disappointment and a Challenge 177

Figure 7.3. A blowup view of the HGA in its closed, or “stowed,” configuration illustrates the 

suspected misalignment of its restraint pins. Rather than fitting into the centers of their 

housings, the pins might have been bent so as to hang up on the edges of the housings, thus 

preventing deployment of the antenna. Note: “LGA Assembly” at the top of the figure refers 

to the low-gain antenna assembly that was eventually used in place of the HGA.

Pin and Socket Analysis

The anomaly team used pin and socket pairs from an extra, non-flight-ready HGA at 

JPL to evaluate the various failure scenarios. Vibrational testing had previously been 

performed on the extra HGA, and so the pin and socket pairs already should have 

experienced considerable relative motion. 

Visual inspection of pins from the spare HGA revealed some plastic deformation. 

X-ray diffraction scans of a number of the pins indicated that both the Tiodize ceramic 

coating and the molybdenum disulfide lubricant coating had been destroyed in areas 

contacting the socket. Scans of nondeformed pins did not show the same damage to the 

coatings, suggesting that deformation of the contact areas was involved in the destruction 

of the Tiodize and “drylube” layers.13

In order to better understand the friction behavior of the HGA’s restraint pins 

as they rubbed against their sockets, JPL arranged with NASA Lewis Research Center to 

conduct a rigorous series of tests. JPL provided Lewis with Tiolubed and unlubricated 

titanium pins, as well as discs of the same Inconel material as the sockets. Lewis staff 

performed a series of friction tests, both under atmospheric conditions and in a vacuum, 

holding together the materials under load and displacing them relative to each other. The 

forces applied in these friction tests were adjusted so that the contact stresses between 

pins and discs were the same as those estimated for the HGA.14

13 Johnson, “The Galileo High Gain Antenna Deployment Anomaly.”

14 Lewis and O’Donnell, p. 82; Johnson, “The Galileo High Gain Antenna Deployment Anomaly.”

LGA ASSEMBLY

RIB

ASSEMBLY

MID-POINT

RESTRAINT

CRM

UPPER

SUPPORT

STRUCTURE VECTOR OF FORCE

Lewis Research Center reported that the coefficient of friction15 between the 

lubricated pins and Inconel in vacuum was 0.04, a very low value. In air, the sliding friction between lubricated pins and Inconel was nearly 10 times higher (a result that also 

had been obtained by testing at JPL). Lewis Research Center determined that the endurance life of the Tiolube coating on the pins was three orders of magnitude (roughly 1,000 

times) greater in a vacuum than in air. This finding indicated that rubbing a Tiolubed pin 

against its socket within Earth’s atmosphere wears away the Tiolube coating far faster than 

rubbing under vacuum conditions.16

Lewis Research Center also performed tests on bare titanium pins, simulating 

the behavior of HGA restraint pins whose Tiolube and Tiodized coatings had been worn 

away. As long as an atmosphere was present to react with the bare titanium and form 

oxide coatings, the coefficient of friction between pin and socket never exceeded 0.35, 

which was not high enough to trap the pins inside their sockets. By contrast, rubbing 

bare titanium pins against Inconel in a vacuum led to galling (a type of severe surface 

damage) and a coefficient of friction between the rubbing surfaces that was over three 

times greater than in an atmosphere.17

Why Galileo’s HGA Rib Restraint Pins Stuck in Their Sockets

From the mass of data it collected, the anomaly team developed a credible scenario 

for the failure of Galileo’s HGA to deploy. The first step in the failure mechanism was 

postulated to be deformation of the restraint pins. Whenever the antenna was folded 

into its stowed position, the restraint pins were pushed tightly into their sockets. Plastic 

deformation of the contact points on some of the pins may have resulted, damaging their 

Tiodized coatings, which were supposed to provide good bonding surfaces for the dry 

lubricant coating. 

During the HGA’s truck journeys across the country, the damage was 

compounded. The HGA made four long truck trips: first in 1982, when it was shipped 

from Harris Corporation’s Florida manufacturing facility to JPL; then from JPL to Kennedy 

Space Center (KSC) in late 1985 for the planned launch; back to JPL in 1986 after the 

Challenger tragedy; and a return to KSC in 1989 for the actual launch. The antenna was 

kept in its stowed position during the journeys and was subjected to “enough of a vibration environment to cause relative motion between the pins and sockets.”18 The particular 

cantilevered mounting configuration of the HGA in its shipping container amplified this 

relative motion, the anomaly team believed. The folded-up antenna lay horizontal in its 

15 The force of friction between two objects is equal to a constant number times the force holding the objects together. 

The constant number is called the coefficient of friction. The higher it is, the greater the friction. The coefficient of 

friction for rubber sliding on concrete is 0.8 (relatively high), while the coefficient for Teflon sliding on steel is 0.04 

(relatively low). Applying a layer of lubricant reduces the coefficient of friction by minimizing the contact between 

rough surfaces. A lubricant’s particles slide easily against each other and cause far less friction than would occur 

between bare surfaces. See “Friction,” Microsoft Encarta Online Encyclopedia, 2001, http://encarta.msn.com.

16	Lewis and O’Donnell, p. 82.

17 Johnson, “The Galileo High Gain Antenna Deployment Anomaly,” p. 375; Kazuhisa Miyoshi and Stephen V. Pepper, 

“Properties Data for Opening the Galileo’s Partially Unfurled Main Antenna,” NASA Technical Memorandum 105355, 

February 1992, p. 1. 

18 Johnson, “The Galileo High Gain Antenna Deployment Anomaly.”
178	 MISSION TO JUPITER: A History of the Galileo Project Chapter 7 I The High-Gain Antenna Failure: A Disappointment and a Challenge 179

container, and the restraint pins of ribs that were on the top and bottom of the prone 

antenna experienced the greatest relative motion with respect to their sockets. Because 

these truck journeys occurred in an atmosphere, the anomaly team believed that the 

Tiolube and Tiodized layers on the contact points of some of the pins were completely 

worn away by the time of Galileo’s launch. Vibration testing of the flight antenna at JPL 

may have compounded the problem. 

After the launch and deployment of Galileo from the Shuttle, the spacecraft was 

once again subjected to significant vibration when the Inertial Upper Stage fired, causing 

more relative motion of pins and sockets—but now the spacecraft was in a vacuum environment, with some titanium pins that had been rubbed bare of lubricant and Tiodizing. 

The anomaly team postulated that at this point, the rubbing of pin in socket galled the 

metal surfaces, resulting in a high coefficient of friction between the parts. 

Possibly due to their orientations in the shipping container, some pins and sockets were less damaged than others. The antenna’s mechanical drive system was able to 

eject most of the restraint pins from their sockets and deploy most of the HGA’s ribs, but 

the drive system’s motors stalled before the forces were large enough to eject the last 

three ribs. As a result, the antenna dish deployed only partially, assuming an asymmetric 

shape with the stuck ribs all on one side. This configuration created unexpected stresses 

on the antenna’s mechanical drive system. The anomaly team conducted tests on a mockup of the drive system in order to surmise “how it would respond to the odd loading 

condition created by the antenna.” The team found that the drive system did not respond 

well to its unbalanced load. Parts in the mocked-up drive system jammed against each 

other, resulting in the loss of two-thirds of the torque that the system had been expected 

to generate. If Galileo’s HGA drive system had experienced a comparable loss, it would 

have had very little available torque to free the ribs from their restraints. This reducedtorque condition and the higher-than-expected coefficient of friction between some pins 

and sockets were, the team believed, major factors in the HGA’s malfunction.19

In summary, four sequential conditions had to be generated in the HGA to cause 

the deployment failure:

1. High enough contact stress to plastically deform the titanium restraint pins 

and break the Tiodized coating used to bond the dry lubricant to the pins.

2. Relative motion—under atmospheric conditions—between pins and sockets. 

This removed the dry lubricant and damaged Tiodized coating from the contact areas, exposing rough, bare metal surfaces on mating parts.

3. Relative motion—in a vacuum—leading to galling of the contact surfaces 

and a very high coefficient of friction.

4. Asymmetric rib deployment resulting in large loads on the antenna’s mechanical drive system and loss of most of the system’s available torque.20

19 Ibid. 

20 Ibid.; Douglas Isbell, “Cooling Down Fails to Free Galileo Antenna,” Space News (26 August–8 September 1991).

Attempts To Free the Antenna

In addition to modeling the root causes of the HGA failure, the anomaly team also developed possible strategies for overcoming the problem. Although the stuck ribs could have 

been easily freed had the spacecraft still been on Earth, it was a bit more complicated 

to set them loose from many millions of miles away. Mission staff could accelerate the 

spacecraft, change its orientation, and exert some control over the antenna’s mechanical 

drive system, but they could do little else and had to build a rescue strategy based on 

these limited actions.

The first suggestion that the anomaly team made was to restow the antenna in 

its furled, “closed umbrella” configuration, then try the deployment sequence again. This 

strategy was quickly dropped because stowing the antenna required human intervention 

in order to carefully roll up the antenna dish’s wire mesh so that it would not snag either 

on itself or on other parts of the HGA. Also, ground testing of the spare antenna revealed 

that each time the HGA was stowed and redeployed, a rotating steel ballscrew within the 

drive mechanism galled its aluminum housing, resulting in a reduction of the torque that 

the drive mechanism was able to generate. 

The first actual attempt to break the stuck antenna ribs loose was to rotate the 

spacecraft toward and then away from the Sun. The anomaly team believed that the 

thermal expansion and contraction of much of the antenna’s structure due to heating and 

cooling would be considerably greater than the expansion and contraction of the antenna 

dish’s ribs, resulting in significant changes in the forces on the stuck restraint pins and 

sockets. Computer modeling indicated that after four to six thermal cycles of the antenna, 

the stuck pins might break loose from their sockets due to “infinitesimal sliding each time 

the forces changed.”21

JPL staff had high hopes that the HGA would eventually be deployed, either 

through the heating and cooling turns or through other means. Franklin O’Donnell, a JPL 

spokesman, stated in April 1991, shortly after the HGA first failed to deploy, “There’s a 

fair amount of optimism that [the problem] is correctable.” And Bill O’Neil, the Galileo 

Project Manager, affirmed, “We’re going to get on top of this. I’m pretty sure that we’ll get 

the antenna open, because we’ve seen other cases of antennas temporarily hung up that 

responded to subsequent exercising.”22

On 20 May 1991, the first of Galileo’s recommended thermal turns was performed; 

the spacecraft was rotated 38 degrees to expose the HGA assembly to solar heating. The 

spacecraft performed with precision, attaining the desired attitude within 4 milliradians 

(about 0.23 degrees of arc). Unfortunately, the HGA ribs remained stuck, as confirmed by 

spacecraft wobble data. Before the heating turn, the wobble had an angular magnitude 

of 3.47 milliradians. Afterwards, it was nearly identical—3.52 milliradians. If the antenna 

had deployed symmetrically, the spacecraft’s wobble should have shrunk to about 1 

milliradian (0.057 degrees of arc).23

Galileo performed a cooling turn on 10 July 1991, rotating so that the HGA dish 

pointed almost directly away from the Sun. This orientation put the whole HGA assembly 

21 Johnson, “The Galileo High Gain Antenna Deployment Anomaly.” 

22 John Noble Wilford, “Stuck Antenna Hinders Jupiter Mission,” New York Times (17 April 1991): A16; “NASA Considers 

Mission to Aid Faulty Galileo Probe,” Washington Post (30 April 1991): A4. 

23 “Unfurling the HGA’s Enigma.”
180	 MISSION TO JUPITER: A History of the Galileo Project Chapter 7 I The High-Gain Antenna Failure: A Disappointment and a Challenge 181

in the shade created by the rest of the spacecraft. There it sat for a 32-hour “cold soak.” 

Galileo then rotated again to warm the antenna back up, but still it remained stuck. The 

spacecraft performed another cooling turn on 13 August 1991. This time, JPL staff reduced 

Galileo’s power consumption in order to cool down the antenna even more than last time. 

The cold-soak duration was increased to 50 hours. Gary Coyle, the antenna task manager, 

determined that this more severe cold soak yielded a small fraction of an inch (0.004 inch) 

of greater contraction of the antenna tower, but it was not sufficient to free the trapped 

ribs. A check on 19 August 1991 of the spacecraft’s wobble and of the partially obscured 

sun sensor revealed no change from the previous configuration.24

Flight controllers were still able to send commands and monitor spacecraft 

operations, utilizing Galileo’s small low-gain antenna (LGA). But this antenna was only 

capable of transmitting and receiving data at a very slow rate. Project staff believed that 

the large HGA was essential for transmitting the voluminous data and numerous digitized 

photographs that Galileo would collect once it reached Jupiter.25

As the spacecraft approached the asteroid Gaspra, mission managers turned their 

attention to the upcoming flyby that was slated to occur in late October 1991. However, 

they made plans to try to free the stuck HGA with more thermal maneuvers in December. 

By that time, Galileo would have passed the asteroid and be starting its loop back toward 

Earth for its second flyby. The craft’s December 1991 position would be the farthest it 

would get from the Sun in several years, and it would thus provide an opportunity for 

cooling the craft to temperatures even lower than during the last cooling turn in August. 

NASA managers believed that the December opportunity would provide the best chance for 

several years to free the crucial antenna.26

On 13 December 1991, the Galileo team commanded its spacecraft to turn so 

that the HGA faced 165 degrees away from the Sun and was in the shade of the craft’s 

sun shield. The team kept Galileo in that position for 50 hours, hoping that lowering 

the antenna’s temperature to -274o

F would shrink its 78-inch central tower and free the 

hung-up pins. They calculated that it would be necessary for the tower to contract by 

only 0.073 inches.27

By 16 December 1991, preliminary data sent from Galileo indicated that the latest 

repair attempt had not worked. The information from the spacecraft “seems to have the same 

signature as before,” said Bill O’Neil. He was referring to the piece of data from one of Galileo’s 

sun sensors that said that the antenna was still casting the same shadow as before. This diagnosis 

was confirmed after further analysis of sun-sensor data and the spacecraft’s wobble.28

NASA continued trying to deploy the HGA. Repeated thermal cycling attempts 

over the next several months were not successful, however, in freeing the HGA’s stuck 

pins.29 The anomaly team suggested another recovery technique. Galileo had two LGAs. 

LGA-2 was mounted on top of a mast that was about 2 meters long. This mast was able 

24 Ibid. 

25	 “Stuck Antenna Hinders Jupiter Mission.”

26	 Isbell, “Cooling Down Fails to Free Galileo Antenna.” 

27 JPL Public Information Office, “Galileo Mission Status: December 11, 1991,” 11 December 1991.

28 Kathy Sawyer, “Galileo Antenna Apparently Still Stuck,” Washington Post (17 December 1991): A14; Kathy Sawyer, 

“$1.4 Billion Galileo Mission Appears Crippled,” Washington Post (18 December 1991): A3. 

29 “NASA Fails to Free Galileo’s Antenna,” Washington Times (11 February 1992): A2; “Spacecraft Antenna Stuck; NASA 

Will Keep Trying,” Philadelphia Inquirer (17 April 1992); Douglas Isbell, “NASA Plans to Get Most Out of Disabled 

Galileo,” Space News (20–26 April 1992): 9. 

to swing 145 degrees, at the end of which it hit a hard stop and imparted a significant 

impulse to the spacecraft structure. The Galileo team tried swinging LGA-2 six times, but 

the maneuver was not able to free the stuck HGA ribs.30

Leonard Fisk, NASA’s Associate Administrator for Space Science, said that more 

radical solutions, such as using Galileo’s thrusters to shake the spacecraft, might free 

the antenna. Some NASA staff were reluctant to try such drastic measures, worrying that 

Galileo’s long appendages such as its 11-meter (36-foot) boom might be damaged. Such 

an occurrence could conceivably end the mission.31

In spite of some staff members’ misgivings, the anomaly team developed an 

HGA recovery strategy that involved pulsing the mechanical drive system’s dual electric 

motors at 1.25 and 1.875 hertz (cycles per second). During testing of the HGA mechanical 

drive system, the team had found that the system had a mode of oscillation that resulted 

from a coupling of the electric motor armature inertia with the gearbox. As a result, the 

drive system was able to produce pulsing torques that were 40 percent greater than 

its normal maximum torques. When the spare HGA had been subjected to the pulsing 

torque, the team had observed that it oscillated at the same frequencies. The team hoped 

that if this pulsing was tried on Galileo’s HGA, the augmented force on the stuck restraint 

pins might be enough to free them.32

The anomaly team made aggressive plans to free the antenna. They set their 

sights on what might have been one of the best opportunities to deploy the antenna—

Galileo’s flyby of Earth in December 1992. This would be the closest it would get to the 

Sun for the remainder of the mission, and it would be a good time to try a combination of 

thermal turns and pulsed torques. As 1992 drew to a close, project staff sent commands to 

Galileo to rotate so that the HGA faced 45 degrees from the Sun (turning directly toward 

the Sun might overheat and damage the antenna further). In addition, staff initiated a 

“hammering” process, rapidly pulsing the drive motors over 13,000 times during a 

three-week period between December 1992 and January 1993. Disappointingly, even 

these radical efforts did not deploy the antenna. The staff also tried other approaches, 

such as spinning the spacecraft up to its maximum rotation rate of 10 rpm and then 

pulsing the antenna drive motors once again, but that strategy did not work either.33 

After numerous unsuccessful efforts to deploy Galileo’s HGA, it appeared likely 

that scientists on Earth would not be able to free the remote antenna. The massive size 

of Galileo—nearly 2.5 tons—made such a task especially difficult. There did not seem to 

be any way to shake the spacecraft hard enough to slide the stuck pins out of their niche. 

Project Manager Bill O’Neil described the situation as “trying to dislodge something caught 

in the smokestack of the Queen Mary by using a rowboat pushing from the stern.34

30 Johnson, “The Galileo High Gain Antenna Deployment Anomaly.”

31 Isbell, “Cooling Down Fails to Free Galileo Antenna”; Michael A. Dornheim, “Galileo Engineers Plan Aggressive Actions 

to Free Stuck Antenna,” Aviation Week & Space Technology (29 June 1992): 48. 

32 Johnson, “The Galileo High Gain Antenna Deployment Anomaly.”

33 Sean McNamara, “Space Mission: Mend Galileo Glitch,” USA Today (29 December 1992): 3A; “Galileo Spacecraft 

Antenna Fails to Open,” Washington Post (21 January 1993): A10; “New Telecommunications Strategy,” Galileo Home 

Page, July 1995, http://www.jpl.nasa.gov/galileo/hga_fact.html. 

34 Kathy Sawyer, “$1.4 Billion Galileo Mission Appears Crippled,” Washington Post (18 December 1991): A3. 
182	 MISSION TO JUPITER: A History of the Galileo Project Chapter 7 I The High-Gain Antenna Failure: A Disappointment and a Challenge 183

Should the HGA Anomaly Have Been Prevented?

An analysis of the HGA anomaly by John F. Kross in the periodical Ad Astra concluded 

that human error led to the antenna’s failure to open. “The problem should have been 

anticipated,” Kross wrote. “After all, the [molybdenum disulfide] lubricant was applied just 

once, nearly a decade before launch. Travel is notoriously fraught with risk . . . . When 

asked if this excess travel was ever addressed as a potential problem, project manager 

William O’Neil claims, ‘I believe that (the travel induced lubricant problem) was not raised 

as a concern; it was not recognized that . . . these cross country trips of the antenna in the 

stowed position . . . would be deleterious to the lubrication.’”35

The Ad Astra article pointed fingers for the HGA anomaly not only at NASA 

technical and managerial staff, but also at Washington, DC, legislators. “Funding uncertainties and political machinations by multiple congresses and administrations left their 

scars,” said the article. “A perceived lack of funds sent Galileo down its lubricant-wearing 

journey, symbolic perhaps of the rocky fiscal road the project always faced . . . shrinking 

slices of the budgetary pie meant reliance on success-oriented missions with little room 

for error.”36

One of the results of limited budgets was that no “proof-test model” Galileo 

spacecraft could be constructed. Proof-test spacecraft served as valuable sources of spare 

parts for the flight spacecraft. Although the immediate causes of the HGA’s difficulties 

were damaged restraint pin coatings, a Space News analysis suggested that the root of 

the problem may well have been NASA’s failure to provide a backup flight-ready HGA. 

According to Robert Murray, Galileo program manager in 1991, the flight-ready antenna 

“was a one-of-a-kind unit and we had no spare.” JPL did have an extra HGA that it used 

for testing, but it had no unit that could be substituted for the HGA aboard Galileo. As a 

result, the HGA that was installed on Galileo “was protected more than” usual from rigorous testing, Murray said. The flight-ready HGA never received the full thermal evaluation 

that, according to the Space News analysis, might have revealed the deployment problem. In 

1983 and 1984, the HGA developed prelaunch problems in the form of snags in the antenna 

dish’s fragile metal mesh. To guard the flight-approved HGA from further damage, project 

managers decided to perform the majority of additional prelaunch testing on a damaged 

backup unit. According to antenna task manager Gary Coyle, the actual flight-approved 

HGA was only unfurled approximately a half-dozen times before launch.37

Former Galileo Project Manager John Casani had a different view of the reasons 

leading to the HGA anomaly. He had wanted more flight-ready spare parts to be available 

for Galileo. In fact, he had wanted a complete proof-test spacecraft to be fabricated, as 

had been done on previous missions. Such a backup craft would have served as a “spares 

bed” for the HGA, as well as for other Galileo systems. A proof-test craft would have 

given the Galileo mission “some robustness and assurance that you wouldn’t lose time in 

the schedule if you have a system or subsystem that has to be refurbished or fails. If you 

have spares, you can keep going with the test program.” Nevertheless, Casani did not 

believe that having a proof-test model would have prevented the HGA failure. Nor did he 

think that, in the case of the HGA, funding restrictions were to blame. “The problem [with 

35 John F. Kross, “Silent in the Heavens,” Ad Astra (December 1991): 14–15.

36 Ibid. 

37 Douglas Isbell, “Galileo Distress Apparent in ’80s Tests by NASA,” Space News (27 May–2 June 1991): 4. 

the HGA] was that we didn’t recognize the physics of what was going on at the time,” 

he said. “It was a problem that wouldn’t show up in tests. Because it was a combination 

of wearing that lubricant off and then launching (the spacecraft) and it being in a hard 

vacuum for a period of time.” Casani noted that the Galileo team was never able to get 

the HGA failure to recur in tests on Earth. “The combination of losing the lubricant during 

transportation, the vibration under vacuum during the launch phase, and the period of 

time under vacuum in space . . . [all] contributed to it. So, I don’t think we would have 

found the problem. I guess you can say we should have discovered it through analysis 

or inspection, but I don’t think there was anything that happened in the way of budget 

restrictions or anything else that would have revealed that.”38 

Using air instead of truck transport for the spacecraft’s trips between JPL and 

Florida might have prevented the HGA anomaly. Antennas similar to Galileo’s HGA had 

been successfully deployed on Earth-orbiting Tracking and Data Relay Satellites (TDRSs). 

But these spacecraft were typically flown to their launch site, rather than driven. Tom 

Williams, NASA’s Deputy Project Manager in 1991 for Advanced TDRSs, commented that 

“air shipping generally places less loads, less vibrations on a satellite.” Air transport was 

expensive, however. Williams estimated that continental flights of a TDRS cost about 

$65,000 per plane ride. Such a flight was considered for Galileo, but it appeared more 

cost-effective to use ground transportation.39

There were also other reasons for choosing truck transport. JPL had a tradition of 

shipping by land that went back a long time. Not only is air transportation very expensive, 

but the plane also must be scheduled far in advance, eliminating much-needed flexibility. 

The Galileo team might have found it quite difficult either to delay or to move up the 

shipment of the spacecraft. In addition, there is no place to land a plane at JPL, so mission 

staff would have had to load the spacecraft onto a truck anyway in order to transport it to 

the airport. There, the spacecraft would have been taken off the truck and put onto the 

airplane. The plane would have had to fly to Patrick Air Force Base because there was 

no suitable landing strip at Kennedy Space Center. The antenna would then have been 

taken off the airplane and put onto another truck. John Casani commented, “We always 

felt that not only was . . . [air transport] more expensive, but it was actually more hazardous, because once you get . . . [the spacecraft] onto a truck, you can move it down to 

Florida safely in a matter of three or four days . . . you were exposing the spacecraft to 

less damage potential from the handling than by putting it on an airplane, which required 

a lot more handling.”40

The antenna deployment failure was a huge setback for the mission. But thanks 

to the ingenuity and creativity of the Galileo team, the event proved not to be a missionending failure. In fact, one of the most notable achievements of the mission was the 

innovation of systems to work around the antenna failure.

38 John Casani interview, tape-recorded telephone conversation, 29 May 2001. 

39 Kross, p. 14. The $65,000 for cross-country plane transportation was a 1991 estimate. Air costs for Galileo’s 1985 and 

1989 cross-country trips might have been a little less.

40 Casani interview. 
184	 MISSION TO JUPITER: A History of the Galileo Project Chapter 7 I The High-Gain Antenna Failure: A Disappointment and a Challenge 185

41 “NASA Is Optimistic on Salvaging Jupiter Missions,” NY Times (12 June 1992): D17; Kathy Sawyer, “Galileo’s Mission 

to Jupiter Can Be Salvaged, Scientists Say,” Washington Post (12 June 1992): A2; “Effort Begins to Free Space 

Probe’s Balky Antenna,” from UPI, 29 December 1992; Michael A. Dornheim, “Prospects Dim for Stuck Antenna,” 

Aviation Week & Space Technology (11 January 1993); “New Telecommunications Strategy,” Galileo Home Page, July 

1995, http://www.jpl.nasa.gov/galileo/hga_fact.html.

42 “Galileo’s Mission to Jupiter Can Be Salvaged, Scientists Say,” p. A2.

43 Ibid., p. A2; Robert C. Cowen, “Galileo on Course, But One Antenna Still Won’t Unfurl,” Christian Science Monitor

(13 January 1993): 6. 

Even with a Stuck Antenna, the Mission Could Go On

As attempts to free the stuck HGA failed, Galileo planners attacked the difficult question 

of what to do if the antenna could not ultimately be opened. An operational HGA would 

have been able to transmit data from Jupiter at 134,000 bits per second, versus the LGAs’ 

more limited capability of only 10 to 40 bits per second. The difference between the 

two types of antennas was comparable to the difference between a powerful searchlight 

and a dim incandescent bulb. The HGA’s reflecting dish was supposed to unfurl into a 

parabolic shape, the same shape as the reflector of a searchlight or an automobile headlight. This parabolic reflector would have focused the HGA’s energy into a tight beam 

directed toward Earth. The LGAs, on the other hand, emitted unfocused broadcasts in the 

same way that a bare lightbulb gives off light in all directions. Only a tiny fraction of the 

LGA’s broadcast would be directed toward Earth. Because the received signal would be 

extremely faint, data would have to be sent at a much lower rate in order to ensure that 

the contents were decipherable. The minuscule data transmission rate would especially 

impact the number of digitized photos that could be sent. Project managers had hoped 

to receive 50,000 images of the Jovian system—enough to make movies of the planet’s 

swirling weather patterns and document their rapidly changing dynamics.41

In spite of the LGA’s slow transmission rate, project staff devised ways to 

complete 70 percent of the mission’s scientific goals. “The exciting news we have today,” 

said Bill O’Neil, “is that we have found a way to return a majority of the science.”42 Many 

of the scientific studies were able to do just fine using the LGA to transmit their data. 

Some experiments required only a few images. The magnetic fields investigation required 

none at all. 

Project scientists worked hard on developing approaches for compressing the 

data that Galileo would collect so that maximum use of the LGA’s limited data-transmission capacity could be realized. Through squeezing more data into fewer computer bits, 

the scientists hoped to increase the transmission capacity of the LGAs at least a hundred 

times. Lead scientist Torrence Johnson estimated that the LGAs would then be able to transmit 2,000 to 4,000 images during the two years that were originally planned for Galileo to 

explore the Jovian system. These would be the highest priority pictures, with fine resolution of detail. What would mostly be lost were lower resolution images, motion pictures of 

Jupiter’s atmosphere, and details of the magnetic field and associated particles.43

Project engineers developed two sets of new flight software that would help 

compensate for the HGA difficulties. The Phase 1 software was radioed to Galileo first, 

and it began operating in March 1995 as the craft was making its approach to Jupiter. 

NASA designed Phase 1 to ensure that the Atmospheric Probe’s most important data made 

it safely to Earth. Because the data could not be sent as quickly to Earth with the LGAs 

as with the HGA, dependable data storage aboard Galileo took on new importance. The 

Phase 1 software provided a partial backup for Probe data.44

After the Probe’s valuable data were received by Earth, NASA radioed Galileo the 

Phase 2 software in March 1996. Its job was data compression—shrinking the enormous 

files of information that Galileo was collecting while retaining the most scientifically important material. Only then would the modified data be transmitted to Earth. Phase 2’s first task 

was to process data collected during the Orbiter’s final approach to the Jovian system in late 

1995, including data from its close encounters with the satellites Europa and Io.45

Phase 2 used two different approaches to data compression. One was termed 

“lossless compression,” in which the data package could be expanded to its original state 

once it reached Earth and all information in the original file could be recovered. This 

approach is one that personal computers commonly employ (in “zipping” and “unzipping” 

files) in order to augment their data-transmission capabilities. The other approach used in 

the Galileo mission was called “lossy” compression, in which mathematical approximations were employed to abbreviate the total amount of data sent to Earth. One way the 

software did this was to minimize or eliminate the least valuable part of data sets, such 

as the dark background of space in many of the images. The approximations had to be 

carefully chosen so that the information lost was not critical.46

Gerry Snyder was a systems engineer who worked on the Galileo project in the 

late 1970s but then left. He was brought back to the project in 1992, when the trouble with 

the HGA occurred and data flow through the LGA had to be maximized. The spacecraft 

had been handling data using an inefficient system called “time division multiplex,” or 

TDM. If, for instance, one of the spacecraft’s instruments was generating the same data 

time after time, TDM would send the repetitive information again and again to Earth, 

wasting valuable transmitting time. This method wouldn’t have been a problem if the 

HGA had been operating, for it would have sent data at a high enough rate (134,000 bits 

per second) that Earth would have received all the data it wanted, even if some of it was 

repetitive. But the LGA could only send a trickle of information (about 10 to 40 bits per 

second), and it all had to count. What Snyder did when he rejoined the Galileo project 

was to implement a “packetized data” system in which the spacecraft computer kept a 

watch on the data flow and only sent a packet to Earth when there was something new. 

The packetized system selected just the data that mission personnel on Earth needed to 

reconstruct what the spacecraft was doing and observing.47

When the spacecraft computer used the packetized-data approach, an image 

taken by Galileo’s SSI camera could be reconstructed on Earth, employing as little as 

one-eightieth of the data that TDM would have sent, with very good visual quality. 

The Galileo spacecraft only had two or so months after an encounter to send what it 

had collected to Earth before the tape recorder had to store information from the next 

encounter. Because the spacecraft had to transmit data at a very slow rate, it could not 

send all the information it collected. Galileo scientists had to make tough decisions 

44 “New Telecommunications Strategy.”

45 Ibid.

46 Ibid.

47 Galileo mission support crew interview, JPL, 5 August 2001. 
186	 MISSION TO JUPITER: A History of the Galileo Project 

about what to transmit and what to erase. Packetized data were important because they 

allowed significantly more of the observational data to be sent to Earth.48

The Galileo team also augmented data flow by improving the ground antenna 

systems of Earth’s Deep Space Network. This provided “a larger, more sensitive ear”49 that 

could extract additional information out of the faint signals that Galileo’s LGAs transmitted. Of the DSN’s three stations around the world, the one in Canberra, Australia, received 

particular attention during the upgrades. Australia is in the Southern Hemisphere, and 

Jupiter was in the southern sky during much of the mission. Enhancements to the DSN 

included the linking of Canberra’s 70-meter and two 34-meter antennas so that all could 

receive Galileo’s signals concurrently, thereby allowing the electronic signals generated by each antenna to be combined. Another antenna in Australia, the Parkes Radio 

Telescope, was also arrayed with the Canberra antennas, as was the DSN’s 70-meter 

antenna in Goldstone, California, when its view of Galileo overlapped with Australia’s. 

The intercontinental array began operations just in time for Galileo’s flyby of the Jovian 

moon Callisto on 4 November 1996. The event occurred when Galileo was at one of 

its most distant locations from Earth, which added to the difficulty of picking up the 

spacecraft’s whisper of a signal. With the array, more of Galileo’s signal was captured, 

allowing a greater rate of data transmission.50

These creative maneuvers for doing more with less were geared toward 

achieving 80 percent of the originally planned atmospheric research, 60 percent of the 

magnetic field studies, and 70 percent of the work on observing Jupiter’s satellites. 

Overall, the improvements enabled Galileo to attain 70 percent of its original science 

goals. Bill O’Neil summed up the Galileo recovery effort by saying, “All in all, it’s still 

an excellent mission.”51 In addition, Galileo’s hardware, software, and antenna-arraying 

modifications significantly enhanced deep space communications for other missions. This 

benefit was foreseen by Paul Westmoreland, Director of Telecommunications and Mission 

Operations for JPL at the time, who stressed that Galileo’s “data compression and encoding techniques will be especially useful for the new era of our faster, better, cheaper 

interplanetary missions . . . . This opens the way for mission developers to reduce future 

spacecraft and operations costs by using smaller spacecraft antennas and transmitters.”52

48 Gerry Snyder, Galileo mission support systems engineer, interview, JPL, 5 August 2001.

49 “Galileo’s Mission to Jupiter Can Be Salvaged,” p. A2.

50 “New Telecommunications Strategy”; Mary Beth Murrill, “NASA Links Antennas To Maximize Galileo Data 

Return,” JPL Public Information Office news release, 7 November 1996, accessed at http://www.jpl.nasa.gov/

galileo/status961107.html. 

51 “Galileo Mission to Jupiter Can Be Salvaged, Scientists Say”; Cowen, “Galileo on Course, but One Antenna Still Won’t 

Unfurl”; Murrill, “NASA Links Antennas To Maximize Galileo Data Return.” 

52 Murrill, “NASA Links Antennas To Maximize Galileo Data Return.”
galileo 

FLEW BY EARTH FOR THE LASt TIME

on 8 December 1992. Finished with its 

Venus and Earth gravity-assists, Galileo 

headed toward the outer solar system on a leg of its voyage that lasted three years and 

ultimately brought it to Jupiter (see figure 8.1). Two experiences of this long journey were 

described in previous chapters: Galileo’s encounter with the asteroid Ida and its little 

moon (chapter 6) and the continued efforts to deploy the spacecraft’s high-gain antenna 

(chapter 7). Other major mission events that occurred during the long space voyage and 

immediately after arrival at Jupiter included the following:

• A serendipitous encounter with a comet.

• A trip through the most intense interplanetary dust storm that a spacecraft 

had ever confronted.

• A serious malfunction of Galileo’s tape recorder, the spacecraft’s main datastorage device, which threatened the success of the entire mission.

• The descent of Galileo’s Probe into the atmosphere of Jupiter.

• The insertion of the Galileo Orbiter into a stable trajectory around Jupiter.

Chapter 8

Jupiter Approach and

Arrival
188	 MISSION TO JUPITER: A History of the Galileo Project Chapter 8 I jupiter approach and arrival 189

1	 Sonoma State University Department of Physics and Astronomy, “The Bruce Medalists—Fred Lawrence Whipple,” 10

September 2003, http://www.phys-astro.sonoma.edu/BruceMedalists/ (available in Galileo—Meltzer Sources, folder

18522, NASA Historical Reference Collection, Washington, DC); “At Age 92 Astronomer Fred Whipple Is Joining NASA

Space Team to Explore Three Comets in 2002,” Cornell News (26 July 1999), http://www.news.cornell.edu/releases/

July99/Whipple.bpf.html (available in Galileo—Meltzer Sources, folder 18522, NASA Historical Reference Collection,

Washington, DC).

2	 “Comet Shoemaker-Levy Background,” Comet Shoemaker-Levy Collision with Jupiter, http://www2.jpl.nasa.

gov/sl9/background.html (accessed 13 March 2000) (available in Galileo—Meltzer Sources, folder 18522, NASA

Historical Reference Collection, Washington, DC); David M. Harland, Jupiter Odyssey: The Story of NASA’s Galileo 

Mission (United Kingdom: Springer-Praxis Books, 2000); “Comet,” Microsoft Encarta Online Encyclopedia, http://

encarta.msn.com, 2001 (available in Galileo—Meltzer Sources, folder 18522, NASA Historical Reference Collection,

Washington, DC).

3	 JPL, “Comet Shoemaker-Levy Background”; Harland, Jupiter Odyssey; “Comet Freight Train to Collide with Jupiter,”

Comet Shoemaker-Levy Collision with Jupiter, http://www2.jpl.nasa.gov/sl9/back1.html (accessed 13 March 2001)

(available in Galileo—Meltzer Sources, folder 18522, NASA Historical Reference Collection, Washington, DC).

4	 JPL, “Comet Shoemaker-Levy Background”; Harland, Jupiter Odyssey.

Figure 8.1. The Galileo trajectory from launch to Jupiter arrival. This drawing charts Galileo’s 

flightpath to Jupiter through its three planetary gravity-assists and two asteroid encounters. 

Gravity-assists included those from Venus (February 1990) and from Earth (December 1990 

and December 1992). Encounters with the asteroids Gaspra and Ida are also depicted. Galileo 

arrived at Jupiter on 7 December 1995. (NASA photo number S89-44173)

A Midmission Opportunity: The Comet Spectacle

In July 1992, five months before Galileo made its last pass by Earth, a comet was being 

torn to pieces as it skimmed by Jupiter, passing less than one-third of a Jovian radius 

(about 21,000 kilometers or 13,000 miles) above the planet’s surface. The comet was 

passing closer to the planet than its Roche limit. Within the Roche limit, the unequal gravitational attractions of a planet on a body’s near and far sides will rip that body apart.

Astronomer Fred L. Whipple has compared comets to “dirty snowballs”1

 in that 

their nuclei, which contain almost all of their mass, are typically conglomerates of ice and 

dust, held together by mutual gravitational attraction. In particular, Whipple suggested that 

within comet nuclei, icy cores lie inside thin, insulating layers of dust. It was this ice-anddust nucleus that Jupiter’s gravitational force was fragmenting. Afterwards, the shattered 

comet continued on its path away from Jupiter and toward interplanetary space. It could 

go only so far, however. Although its pieces traveled many millions of kilometers, they 

could not escape Jupiter’s massive gravity and would eventually be pulled back.2

On 24 March 1993, eight months after the comet’s fragmentation, world-class 

comet-seekers Eugene and Carolyn Shoemaker and David Levy were taking a photo of 

the night sky with the Palomar Observatory’s 0.4-meter Schmidt telescope. Two days 

after the observation, Carolyn Shoemaker examined the photographic plates and saw a 

strange linear object with a fan of light on one side. Subsequent analyses by observatories 

in Hawaii, Arizona, and Texas confirmed that the object was a comet orbiting Jupiter that 

had made a very close approach on 7 July 1992. Imaging conducted by the Spacewatch 

Telescope on Arizona’s Kitt Peak and the Mauna Kea Observatory in Hawaii exposed the 

comet’s unusual form—a line of numerous pieces resembling a “string of pearls.”3

In the past, the Shoemakers and David Levy had jointly discovered eight other 

comets, so they named this one Shoemaker-Levy 9 (S/L-9). Further analyses showed that 

the comet’s fragments would return to Jupiter approximately two years after their encounter with the planet, during the period from 16 through 22 July 1994. This time, the comet 

would not fly by the planet; it would smash into it. When this happened, the Galileo 

spacecraft, though still far away, would have an excellent view of the collision sites. This 

was fortuitous, because the comet’s fragments would hit Jupiter’s night side, away from the 

Sun. Earth-based telescopes would be able to see evidence of the impact, such as ejecta 

blown into space, but would not be able to observe the impact sites themselves.4

More energy was released during the Shoemaker-Levy spectacle than from the 

largest nuclear warhead explosions ever conducted on Earth. The event represented the 

first time in human history when people had discovered a body in the sky and been able 

to predict its collision with a planet more than seconds in advance. The Shoemaker-Levy 

crash into Jupiter also gave credence to astronomers’ predictions that similar events may 

occur on our home planet. This was one of the reasons why thousands of astronomers 

on Earth, both professional and amateur, were so interested in studying the details of 

Shoemaker-Levy’s demise. The comet’s crash on Jupiter demonstrated one of the solar 

system’s basic physical processes. Better knowledge of the phenomenon could help predict how serious the damage to Earth might be if various-sized bodies struck it. 

A similar event to Shoemaker-Levy might in fact have occurred on Earth 360 

million years ago, leaving evidence that can still be studied today. Radar images of Earth 

have revealed a chain of craters in the central African country of Chad that may have been 

created by a fragmented comet or asteroid comparable in size to Shoemaker-Levy. Such an 

impact may also have contributed to the mass biological extinctions that were occurring 
190	 MISSION TO JUPITER: A History of the Galileo Project Chapter 8 I jupiter approach and arrival 191

at the time. In addition, there is growing evidence that the energy from a large comet or 

asteroid crash caused the extinction of dinosaurs 65 million years ago.5

The Collision

In the days prior to impact, fragments of the broken-up comet stretched in a line across 5 

million kilometers (3 million miles) of space, more than 12 times the distance from Earth 

to our Moon. They sped like a runaway freight train toward their rendezvous with Jupiter. 

This “train” expanded in length as it flew through space, due to the different velocities 

between fragments. Some astronomers, believing it was unlikely that the comet crash 

would be seen through small amateur telescopes, discouraged the public from expecting too much of the impact. An article in Nature entitled “The Big Fizzle Is Coming”6

hypothesized that each comet fragment would ablate and burn up like a meteoroid in 

Jupiter’s upper atmosphere. The article said that because the fragments did not have sufficient momentum or structural integrity, they would probably not penetrate deeply into 

the atmosphere, where they might have exploded with thousands of megatons of energy. 

The result was thus predicted to be a spectacular meteor shower, but not the giant fireball 

explosions that other researchers thought would occur.7

How wrong this prediction turned out to be. From 16 through 22 July 1994, 21 

discernible pieces of Comet Shoemaker-Levy 9 collided with Jupiter in an event that was 

called “simply spectacular and beyond expectations.”8

 When the first comet fragment hit 

Jupiter at over 320,000 kilometers per hour (200,000 mph), it generated an intensely bright 

fireball at least 3,000 kilometers (1,900 miles) high. Although the impact site was hidden 

from Earth because it was on Jupiter’s night side, the fireball was so large and bright that 

even 2-inch amateur refractor telescopes on Earth could see it. The Shoemaker-Levy event 

was the first collision of two solar system bodies ever observed. Impacts of the cometary 

fragments, which ranged in size up to 2 kilometers, resulted in the generation of plumes 

spewing material many thousands of kilometers up from Jupiter; hot bubbles of gas in the 

atmosphere; and large, dark, scarlike spots on the atmosphere that persisted for weeks. 

Some of these spots were two to three times the size of Earth.9

5	 “Comet Freight Train to Collide with Jupiter”; Douglas Isbell and Mary Hardin, “Chain of Impact Craters Suggested

by Spaceborne Radar Images,” http://www2.jpl.nasa.gov/sl9/news80.html (accessed 17 March 2001) (available in

Galileo—Meltzer Sources, folder 18522, NASA Historical Reference Collection, Washington, DC); Ray L. Newburn,

Jr., “Background Material for Science Teachers: July 1994—Periodic Comet Shoemaker-Levy 9 Collides with Jupiter,”

http://www.seds.org/sl9/Educator/intro.html (accessed 27 December 2001) (available in Galileo—Meltzer Sources,

folder 18522, NASA Historical Reference Collection, Washington, DC).

6	 Paul Weissman, “The Big Fizzle is Coming,” Nature (14 July 1994), http://www.jpl.nasa.gov/sl9/news9.html (available

in Galileo—Meltzer Sources, folder 18522, NASA Historical Reference Collection, Washington, DC).

7	 Amara Graps, Comet Shoemaker Levy 9 Impact: Analysis and Interpretation (6 Months After Event) (presentation at

Macworld Exposition, San Francisco, CA, 4 January 1995).

8	 JPL, Comet Shoemaker-Levy Collision with Jupiter, http://www2.jpl.nasa.gov/sl9/sl9.html (accessed December

2004) (available in Galileo—Meltzer Sources, folder 18522, NASA Historical Reference Collection, Washington, DC).

9	 Graps, presentation at Macworld; JPL, Comet Shoemaker-Levy Collision with Jupiter, http://www2.jpl.nasa.gov/sl9/

sl9.html (accessed 17 March 2001) (available in Galileo—Meltzer Sources, folder 18522, NASA Historical Reference

Collection, Washington, DC); “Comet Shoemaker-Levy 9 Collision with Jupiter,” National Space Science Data Center 

(NSSDC) Web site, http://nssdc.gsfc.nasa.gov/planetary/comet.html (accessed 13 March 2001) (available in Galileo—

Meltzer Sources, folder 18522, NASA Historical Reference Collection, Washington, DC); “Results of SL9’s Impacts

Thrill Scientists,” JPL Universe, 29 July 1994, http://www2.jpl.nasa.gov/sl9/uni1.html (available in Galileo—Meltzer

Sources, folder 18522, NASA Historical Reference Collection, Washington, DC).

Effects of the comet-fragment collisions were discerned by most Earth-based 

observatories, as well as by spacecraft that included Galileo, the Hubble Space Telescope, 

Ulysses, and Voyager 2. Each fragment impact began with a flash that gradually declined 

in brightness (see figure 8.2). When Galileo observed a fragment impact event in ultraviolet light, the total lifetime of the event appeared to last about 10 seconds, while in infrared 

light, events appeared to continue for 90 seconds or more.

Figure 8.2. Comet Shoemaker-Levy 9 fragment W impact with Jupiter. These four images 

of Jupiter and the luminous night-side impact of fragment W of comet Shoemaker-Levy 9 

were taken by the Galileo spacecraft on 22 July 1994, when it was 238 million kilometers 

(148 million miles) from Jupiter. The images were taken at intervals of 21 3 seconds using a 

green filter (visible light). The first image, taken at an equivalent time to 8:06:10 Greenwich 

mean time (1:06 a.m. Pacific daylight time, or PDT), shows no impact. In the next three 

images, a point of light appears, brightens so much as to saturate its picture element, and 

then fades again, 7 seconds after the first picture. (NASA image number PIA00139)

The fragments released enormous energy when they crashed into Jupiter. 

Ultraviolet data taken by Galileo showed a 20-percent increase in the total brightness 

of the entire planet during the impact of one large comet fragment (the fragment labeled 

“G”). During the collisions of other fragments, the brightness of Jupiter typically increased 

10 percent. Fragment G’s impact was followed by the eruption of spectacular plumes 

of hot gases. Fragment G created a fireball 7 kilometers (5 miles) in diameter that 

burned with a temperature of at least 8,000 K (14,000o

F), hotter than the Sun’s surface. 

Galileo’s near infrared mapping spectrometer (NIMS) observed the fireball’s evolution. 

In only 1.5 minutes, it expanded to a size hundreds of miles across and cooled 

swiftly to 400 K (260o

F).10 

10	Comet Shoemaker-Levy 9; C. W. Hord, W. R. Pryor, A. I. F. Stewart, K. E. Simmons, J. J. Gebben, C. A. Barth,

W. E. McClintock, L. W. Esposito, W. K. Tobiska, R. A. West, S. J. Edberg, J. M. Ajello, and K. L. Naviaux, “Direct

Observations of the Comet Shoemaker-Levy Fragment G Impact by Galileo UVS,” Geophysical Research Letters

22, no. 12 (15 June 1995): 1565; James Wilson, “Galileo Comet SL9 Observations,” JPL Public Information Office

news release, 31 October 1994, http://www2.jpl.nasa.gov/sl9/gll22.html (available in Galileo—Meltzer Sources, folder

18522, NASA Historical Reference Collection, Washington, DC).
192	 MISSION TO JUPITER: A History of the Galileo Project Chapter 8 I jupiter approach and arrival 193

13	Graps; Donald Savage, Jim Elliott, and Ray Villard, “Hubble Observations Shed New Light on Jupiter Collision,”

NASA news release number 94-161, 29 September 1994, Comet Shoemaker-Levy Home Page (JPL), http://www.jpl.

nasa.gov/sl9/hst15.html (available in Galileo—Meltzer Sources, folder 18522, NASA Historical Reference Collection,

Washington, DC); R. M. West, “Comet Shoemaker-Levy 9 Collides with Jupiter: The Continuation of a Unique

Experience,” ESO Messenger (European Southern Observatory, Germany, September 1994); “Comet,” Microsoft 

Encarta Online Encyclopedia, 2001, http://encarta.msn.com (available in Galileo—Meltzer Sources, folder 18522,

NASA Historical Reference Collection, Washington, DC). 

14	H. A. Weaver, M. F. A’Hearn, C. Arpigny, D. C. Boice, P. D. Feldman, S. M. Larson, P. Lamy, D. H. Levy, B. G. Marsden,

K. J. Meech, K. S. Noll, J. V. Scotti, Z. Sekanina, C. S. Shoemaker, E. M. Shoemaker, T. E. Smith, S. A. Stern, A. D.

Storrs, J. T. Trauger, D. K. Yeomans, and B. Zellner, “The Hubble Space Telescope (HST) Observing Campaign on

Comet Shoemaker-Levy 9,” Science 267 (3 March 1995): 1287; “Astronomy: A.2 Comets and Asteroids,” Microsoft 

Encarta Online Encyclopedia, 2001, http://encarta.msn.com/find/print.asp?&pg=8&ti=1741502444&sc=23&pt=1&pn

=5 (available in Galileo—Meltzer Sources, folder 18522, NASA Historical Reference Collection, Washington, DC).

15	 “Aurora (phenomenon),” Microsoft Encarta Online Encyclopedia, 2001, http://encarta.msn.com (available in

Galileo—Meltzer Sources, folder 18522, NASA Historical Reference Collection, Washington, DC); R. M. West, “Comet

Shoemaker-Levy 9 Collides with Jupiter.”

16	H. B. Hammel, R. F. Beebe, A. P. Ingersoll, G. S. Orton, J. R. Mills, A. A. Simon, P. Chodas, J. T. Clarke, E. De Jong,

T. E. Dowling, J. Harrington, L. F. Huber, E. Karkoschka, C. M. Santori, A. Toigo, D. Yeomans, and R. A. West, “HST

Imaging of Atmospheric Phenomena Created by the Impact of Comet Shoemaker-Levy 9,” Science 267 (3 March

1995): 1293.

Scientists divided Shoemaker-Levy’s collision events into three phases:

• Meteor phase: The incoming comet fragments hit the Jovian atmosphere and 

heated. 

• Fireball phase: The fragments exploded into a fireball of extremely hot gas 

and debris that expanded out of the atmosphere, then cooled to form plumes 

that many observing stations recorded. 

• Splash phase: The plume material fell back into the Jovian atmosphere and 

heated it, which led to intense thermal emissions.11 

Preliminary analysis of Galileo NIMS data indicated that the splash-phase material might have contained the hot remnants of water molecules that originated either from 

the comet’s vaporized ice or from Jupiter’s presumed water clouds. The splash material 

also contained carbon and hydrogen, derived either from Jovian atmospheric methane 

or from the comet’s hydrocarbons. Either Jupiter’s atmosphere or the comet may have 

contributed micrometer-sized carbon particles (a micrometer is 1 millionth of a meter, or 

about 40 millionths on an inch) that formed the immense black patches that persisted in 

Jupiter’s high atmosphere for weeks after the comet impact. The patches may also have 

been caused by sulfur that erupted during the comet impact from a lower Jovian cloud 

layer of condensed ammonium hydrosulfide.12

Interpretation of the Data

In the months that followed Shoemaker-Levy’s crash into Jupiter, data collected by Galileo, 

other spacecraft, the Hubble Space Telescope, and Earth observatories were painstakingly 

analyzed. Scientists believe that the comet was a body that had orbited the Sun for 4.5 billion 

years and had only recently been captured by Jupiter, possibly just a few decades earlier. It 

was the first comet ever observed orbiting Jupiter (comets generally orbit the Sun). 

The composition of Shoemaker-Levy’s impact plumes was investigated in many 

different wavelength bands using spectroscopic techniques. Surprisingly, the data indicated that the plumes were devoid of water. Since 1949, when the astronomer Fred L. 

Whipple proposed that comets were “dirty snowball” conglomerates of dust and ices of 

various compounds, the space science community has believed that comets typically 

contain large percentages of water ice. Halley’s Comet, for instance, was found in 1986 

to contain about 80 percent water ice. Did Shoemaker-Levy’s hydrogen and oxygen molecules combine with other elements to form new molecules? Or, alternatively, could the 

“comet” have actually been an asteroid? Comets and asteroids have much in common. 

They are primordial in origin, in that they were both formed 4.6 billion years ago, along 

with the solar system’s planets and their satellites. Also, both comets and asteroids are 

found near Jupiter. But the main difference between the two types of bodies is that 

comets are composed largely of ices, while asteroids are virtually devoid of ices because 

they formed too close to the Sun.13

In spite of the lack of water detected, some scientists believe that ShoemakerLevy very likely had the composition of a comet. Sensitivity limitations of receiving stations 

may have been responsible for the lack of typical comet emissions being observed spectroscopically. The strongest evidence that Shoemaker-Levy was indeed a comet was the 

existence of a persistent, circularly symmetric coma (an envelope of gas and dust) around 

each of its fragments. A comet’s coma is formed when the Sun sublimates some of its ices 

into gases, which also results in a release of a portion of the comet’s dust.14

Ultraviolet image data suggested that the comet initiated enhanced auroral phenomena in Jupiter’s atmosphere. Auroras are luminous atmospheric activities caused by 

charged particles entering an atmosphere and colliding with and exciting its gas molecules, which then emit electromagnetic radiation in the visible as well as other parts of 

the spectrum. Jovian auroral displays were first observed near the planet’s northern pole. 

Scientists believe that the comet fragments’ collisions with Jupiter formed many charged 

particles, which then moved rapidly along magnetic field lines, plunged into the Jovian 

atmosphere near the poles, and created auroral displays.15 

Images taken in the hours after the larger fragments of Shoemaker-Levy hit 

Jupiter show transient dark rings, reminiscent of the circular ripples that propagate outward when a stone is thrown into a pond, surrounding the collision sites. The dark rings 

are likely some sort of wave phenomenon. It is not clear, however, why we can see them. 

The rings occur high in the Jovian atmosphere; the gases there are normally transparent. 

The wave phenomenon may cause partial condensation of the gases into visible droplets 

or particles. Either an upward displacement or a rarefaction could initiate condensation 

by lowering the temperature. Alternatively, the comet fragment impacts might render 

normally transparent gases visible due to the chemical changes and condensation occurring during the explosive collision.16 

11	A Comet’s Fiery Dance at Jupiter,” Galileo Messenger (May 1995), http://www2.jpl.nasa.gov/sl9/gll29.html (available

in Galileo—Meltzer Sources, folder 18522, NASA Historical Reference Collection, Washington, DC).

12	 Ibid.
194	 MISSION TO JUPITER: A History of the Galileo Project Chapter 8 I jupiter approach and arrival 195

20	 “Orbiter Deflection Maneuver Status July 27,” JPL Galileo Home Page, http://www2.jpl.nasa.gov/galileo/odm.html (accessed

22 March 2001) (available in Galileo—Meltzer Sources, folder 18522, NASA Historical Reference Collection, Washington, DC).

21	JPL, “Critical Engine Firing Successful for Jupiter-Bound Galileo,” JPL Public Information Office news release, 27

July 1995, http://www2.jpl.nasa.gov/galileo/oldpress/odm.html (available in Galileo—Meltzer Sources, folder 18522,

NASA Historical Reference Collection, Washington, DC); “Orbiter Deflection Maneuver Status July 27.

22	Sawyer, “Galileo Sends Probe Toward Jupiter Study.”

23	JPL, “Galileo Flying Through Intense Dust Storm on Way to Jupiter,” JPL Public Information Office news release, 29

August 1995, http://www2.jpl.nasa.gov/galileo/duststorm.html (available in Galileo—Meltzer Sources, folder 18522,

NASA Historical Reference Collection, Washington, DC); A. L. Graps, E. Grun, H. Svedhem, H. Kruger, M. Horanyi,

A. Heck, and S. Lammers, “Io as a Source of the Jovian Dust Streams,” Nature 405 (4 May 2000): 48–50; M. Horanyi,

G. Morfill, and E. Grun, “Mechanism for the Acceleration and Ejection of Dust Grains from Jupiter’s Magnetosphere,”

Nature 363 (13 May 1993): 144–146. 

17	 JPL, “Probe Release Status Updates,” JPL news release, 13 July 1995, http://www2.jpl.nasa.gov/galileo/release.html

(available in Galileo—Meltzer Sources, folder 18522, NASA Historical Reference Collection, Washington, DC).

18	Kathy Sawyer, “Galileo Sends Probe Toward Jupiter Study,” Washington Post (14 July 1995): A3.

19	 JPL, “Probe Release Status Updates.”

The Orbiter and Probe Separate

On 12 July 1995 (PDT), Galileo released its 750-pound Jupiter Atmospheric Probe (see 

figure 8.3). The spacecraft was 83 million kilometers (52 million miles) and five months 

away from Jupiter at the time.17 In December 1995, the Probe would become the first 

“Earth emissary” to plunge into the atmosphere of one of the solar system’s gas-giant planets. Alvin Seiff, principal investigator for the Probe’s atmospheric-structure experiments, 

had proposed back in 1969 that a Jupiter entry vehicle be designed and constructed. He 

commented that the temperatures to which the Probe would be subjected, which would 

exceed those at the surface of the Sun, would be “unprecedented, enormous, inconceivable,” and he went on to say that conducting a successful Probe mission into Jupiter’s 

atmosphere would involve “the most challenging entry problem in the solar system, 

except for [entering] the Sun itself.”18 

Figure 8.3. An artist’s concept of the Galileo Probe inside its aeroshell being released from the 

mother ship, the Galileo Orbiter, prior to arrival at Jupiter. (NASA photo number ACD95-0229)

News of the Probe’s release took 37 minutes to travel at the speed of light 

across the 664 million kilometers (412 million miles) of space from Galileo to its 

control crew in California. At 11:07 p.m. PDT on Wednesday, 12 July 1995, points on 

a Mission Support Area monitor displaying Doppler-shift data took a sudden dip. This 

tiny frequency shift in the signal resulted from a small change in the Orbiter’s velocity 

along the Earth-line of some tens of millimeters per second, and it indicated that the 

Probe had been sent on its freefall path toward Jupiter. The news was greeted with 

jubilation. William J. O’Neil, Galileo Project Manager, commented at the time, “We’re 

just elated. It’s been a long haul for us.”19

Critical Course Corrections

On 27 July 1995, two weeks after the Probe’s release, the Galileo Orbiter fired its 400-

newton main engine for 5 minutes and 8 seconds, performing an important maneuver that 

would deflect it away from a collision course with Jupiter. NASA maintained a real-time 

telemetry link with the spacecraft throughout the burn, tracking changes in the Orbiter’s 

velocity. These changes could be calculated from the Orbiter’s accelerometer data, as 

well as from Doppler shifts in the craft’s radio signals. From this information, the Orbiter’s 

engine performance was deduced.20

This was the first major burn by Galileo’s German-manufactured engine, and 

it demonstrated the engine’s operability. Firing the engine had to wait until the release 

of the Atmospheric Probe because the Probe had been mounted in front of the engine 

nozzle. The burn altered the Orbiter’s speed by 61 meters per second (about 140 mph), 

within a meter per second of the planned change. The trajectory modification resulting 

from this maneuver was enough for the Orbiter to miss Jupiter by 230,000 kilometers 

(140,000 miles) rather than colliding with the planet. The course change targeted the 

spacecraft to fly by Jupiter’s volcanic moon Io on 7 December 1995, the same day that the 

Probe would plunge into Jupiter’s atmosphere.21

The Orbiter had a critical task to perform in conjunction with the Probe. The 

Orbiter had to fly high overhead and receive data from the Probe as it fell through the 

Jovian atmosphere, then relay the information back to Earth. This information would 

have to be transmitted at a far slower rate than originally intended because of the failure 

to deploy the Orbiter’s high-gain antenna (see chapter 7 on the high-gain antenna failure 

for more details). However, the Probe’s Project Manager, Marcie Smith of Ames Research 

Center, was optimistic that the Orbiter’s low-gain antennas would be sufficient to eventually transmit all of the data collected by the spacecraft.22

Interplanetary Dust Storm

In August 1995, when the Galileo Orbiter was 63 million kilometers (39 million miles) from 

Jupiter, it entered the most intense interplanetary dust storm ever observed. The storm was 

so large that it took several months for the spacecraft to journey through it. During normal 

interplanetary travel, a spacecraft comes across about one dust particle every three days. In 

plowing through the most severe of the several storms that Galileo encountered, the spacecraft counted up to 20,000 particles per day. The particles’ impact trajectories on Galileo’s 

dust detector clearly indicated that they originated from somewhere in the Jovian system.23 
196	 MISSION TO JUPITER: A History of the Galileo Project Chapter 8 I jupiter approach and arrival 197

28	Graps et al., “Io as a Source.”

29	 Ibid.

30	 Ibid.

31	 “Galileo Flying Through Intense Dust Storm.” 

24	Horanyi, Morfill, and Grun, “Mechanism for the Acceleration and Ejection of Dust Grains.”

25	E. Grun, M. Baguhl, D. P. Hamilton, R. Riemann, H. A. Zook, S. Dermott, H. Fechtig, B. A. Gustafson, M. S. Hanner, M.

Horanyi, K. K. Khurana, J. Kissel, M. Kivelson, B. A. Lindblad, D. Linkert, G. Linkert, I. Mann, J. A. M. McDonnell, G.

E. Morfill, C. Polanskey, G. Schwehm, and R. Srama, “Constraints from Galileo Observations on the Origin of Jovian

Dust Streams,” Nature 381 (30 May 1996).

26	 Ibid.

27	 “Galileo Flying Through Intense Dust Storm”; Horanyi, Morfill, and Grun, “Mechanism for the Acceleration and Ejection

of Dust Grains”; Graps et al., “Io as a Source.” 

These interplanetary streams of dust were first discovered by the Ulysses spacecraft in 1992 and were greeted with much surprise by the space science community. 

Interplanetary space had been known to contain dust, but the presence of discrete streams, 

at the densities observed, had been totally unexpected. How could large populations of 

dust escape Jupiter’s immense gravitational field? The answer appears to be that the dust 

first encountered extensive plasmas, electrically charged ionized gases, in Jupiter’s magnetosphere, and these induced charges on the dust particles. The dust’s motion could 

then be controlled not only by Jupiter’s gravitational field, but also by its electromagnetic fields, which exert strong forces on charged particles. Models of these field-particle 

interactions developed at the University of Colorado and Germany’s Max Planck Institute 

indicated that only particles in a certain size range could gain sufficient energy from electromagnetic forces to escape Jupiter’s gravity. This constraint proved useful in identifying 

the sources of the particles.24

Scientists originally suspected that the Shoemaker-Levy comet could be the 

source of the dust, emitting particulates either when it flew close by Jupiter in 1992 and 

fragmented or when it smashed into the planet in 1994. But no correlation could be found 

between the dust storms and comet events. Some of the dust streams occurred before 

known comet events, while others occurred long after these events.25

The scientific community also thought that Jupiter’s gossamer ring, one of the 

faint particle rings surrounding the planet, could be the dust source. The gossamer ring is 

made up mainly of micrometer-sized particles that are located in a disc between 1.8 and 

3.0 Jupiter radii. These particles orbit Jupiter like a multitude of tiny moons. Collisions 

commonly occur between ring particles, and the sub-micrometer-sized particles that result 

from these collisions are accelerated away from Jupiter by the planet’s powerful electromagnetic fields. The gossamer ring generates a fairly constant flux of escaping particles, 

but the dust streams observed by Ulysses and Galileo varied dramatically with time. 

Magnetic field effects could modulate the emissions of dust, but Galileo measurements of 

the interplanetary field showed little variation between when a dust storm was observed 

and when the spacecraft saw no dust streaming. This suggested that Jupiter’s gossamer 

ring was not the source of the dust storms.26

Particulates from the satellite Io were another potential source of the dust. To 

investigate a possible Io origin of the particles, Amara Graps of the Max Planck Institute 

led a team that analyzed several years of Galileo dust-detector data. The team found 

that during a dust storm, oscillations in the rate of particle impacts on Galileo’s detector 

were related to the rotational periods of Jupiter and Io. The fact that Io’s rotation period 

appeared to influence the dust particles’ behaviors pointed to the moon as a likely source 

of the dust. Also, calculations showed that Io had usually been located somewhat near 

(within 50o

 longitude) the origin of dust particles’ trajectories.27 

Scientists on Graps’s team considered several mechanisms that might have generated the particulates. They could have been created as a result of impacts on Io’s surface 

that pulverized and ejected fragments of minerals out into space. An impact-generated 

dust cloud existed around another Jovian moon, Ganymede, and scientists studied it in 

order to understand better whether a similar mechanism on Io was a likely source of 

the dust storm’s particles. The study concluded that there was little chance that impactgenerated particulates from Io were the dominant source of the intense dust streams that 

Galileo encountered.28 

The Max Planck Institute team also revisited the possibility that the dust originated from Jupiter’s ring system. If dust particles then passed close by Io, could the 

satellite have influenced their trajectories and in some way ejected the particles from the 

Jovian system? The team concluded that this scenario, too, was very unlikely.29 

Io’s volcanoes appeared to be a much more likely source of the dust. The submicrometer sizes of the particles observed by Galileo in the interplanetary dust storms 

corresponded well to particle sizes in Io’s volcanic plumes. Io’s volcanic dust production 

rate, as measured by Voyager, was large enough that the escape of only a small fraction 

of it would be sufficient to generate the dust streams observed by Galileo. Furthermore, 

plausible models existed for the ejection of Io’s sub-micrometer-sized volcanic dust into 

interplanetary space by Jovian electromagnetic forces.

The characteristics of Jovian-created interplanetary dust storms may provide 

glimpses into Jupiter’s environment that cannot be easily obtained in other ways. In the 

future, dust-stream measurements might be used to monitor Io’s volcanic plume activity 

and composition; they might also serve as a source of data on the dynamics of Jupiter’s 

magnetosphere.30

Galileo’s dust detector was roughly the size of a large kitchen colander. In addition to counting particle impacts, it measured their energy and direction of flight, from 

which sizes and speeds were calculated. Dr. Eberhard Grun of Germany’s Max Planck 

Institute of Nuclear Physics was the principal investigator on Galileo’s dust-detector experiment. He also placed dust detectors aboard the Ulysses spacecraft that flew by Jupiter in 

1992 on its way to the Sun and the Cassini spacecraft that passed by the Jovian system in 

2000 and 2001 en route to Saturn.31

The Tape-Recorder Crisis

Galileo’s reel-to-reel tape recorder was originally intended to serve only as a backup to 

the high-gain antenna (HGA), which was supposed to send images to Earth in near-real 

time. But the failure of the HGA to unfurl left the Galileo team dependent on data storage and slow data transmission through the spacecraft’s low-gain antennas (LGAs). The 

tape recorder became a vital link in the strategy developed to work around the HGA’s 

problems. Critical data remained stored on the recorder’s 560 meters (1,850 feet) of 6-

millimeter-wide (or a quarter of an inch) Mylar tape until they got painstakingly sent to 

Earth at an average rate of two to three images per day. After the images were received 

on Earth, their section of tape could be overwritten with new data. It was only with a 
198	 MISSION TO JUPITER: A History of the Galileo Project Chapter 8 I jupiter approach and arrival 199

37	Franklin O’Donnell, “Galileo Spacecraft Tape Recorder To Be Tested,” JPL Public Information Office news release, 20

October 1995, http://www2.jpl.nasa.gov/galileo/dmsoct20.html (available in Galileo—Meltzer Sources, folder 18522,

NASA Historical Reference Collection, Washington, DC).

38	“Rewind Almost Unhinges Galileo”; O’Donnell, “Galileo Spacecraft Tape Recorder to be Tested”; Donald J. Williams,

“Jupiter—At Last!” Johns Hopkins APL Technical Digest 17, no. 4 (1996): 346.

39	“Rewind Almost Unhinges Galileo.”

40	 Ibid.

41	 Ibid.

42	 “The Journey to Jupiter.”

32	“Rewind Almost Unhinges Galileo Mission,” Washington Post (29 October 1995): A6. 

33	“The Journey to Jupiter,” section 2 in Galileo Tour Guide, JPL Galileo Home Page, p. 10, http://www2.jpl.nasa.gov/

galileo/tour/tourtoc.html (accessed 22 March 2001) (available in Galileo—Meltzer Sources, folder 18522, NASA

Historical Reference Collection, Washington, DC).

34	“Rewind Almost Unhinges Galileo.”

35	 Ibid.

36	 Ibid. 

working tape recorder that the mission could still attain approximately 70 percent of its 

original science objectives.32 

As Galileo began its final approach to Jupiter, the Orbiter took a global image of 

the planet on 11 October 1995 that included the Probe atmospheric-entry site. As usual, 

the Orbiter digitally stored this image on its tape recorder. Shortly after NASA sent the 

command for the tape recorder to rewind so that the image could be transmitted to Earth, 

project staff got an unwelcome surprise: Signals from the Orbiter indicated that the reelto-reel recorder had rewound for playback as intended but then failed to stop spinning. 

What did that indicate? Had the tape broken so that a reel was turning endlessly? Or were 

the capstans, the recorder’s speed-regulating spindles that were supposed to pull the tape 

along, slipping instead of pulling? Perhaps the recorder actually had stopped and the signals from the Orbiter were incorrect.33 

Within hours of the tape recorder failure, in what Project Manager William J. 

O’Neil called “an amazing coincidence,” an Earth-based duplicate of Galileo’s recorder 

also malfunctioned during a routine simulation of operations to be performed at Jupiter. 

Due to faulty circuitry, the recorder failed to sense that it had reached the end of its Mylar 

tape. According to O’Neil, “The thing just tore the tape off the reel and left it flapping.”34 

The Galileo team feared that the same thing had happened aboard the spacecraft, which 

was now 870 million kilometers (540 million miles) from Earth.35 

For 15 hours, Galileo’s tape recorder remained stuck in its rewind setting. NASA

staff, including Project Manager William J. O’Neil, “sank into despair.” After all that 

Galileo had been through and survived, it now appeared that a broken tape recorder 

would irreparably damage the mission. The recorder was the spacecraft’s main datastorage system; if it could not be fixed, much of the imaging that was so vital to the 

mission might not be transmittable to Earth.36 

Commands were sent for the recorder to stop, and project engineers began an 

intricate analysis of the problem. Meanwhile, other staff began developing a strategy to 

keep the mission going even if the tape recorder proved to be permanently broken. Once 

again, Galileo’s redundant systems and the project staff’s ingenuity provided a way out of 

the crisis. The staff found that memory storage sites in Galileo’s main on-board computer 

could partially replace the tape recorder’s function. O’Neil said of this effort, “Some of us 

did calculations and became convinced that, remarkably, there was a way. We were really 

buoyed up by the realization that we had a really good mission even without the tape 

recorder. That’s when we came out of our depression . . . .” Using the computer’s memory 

sites instead of the tape recorder, the Galileo team estimated that it would still achieve 

over half of the mission’s original research goals. All of the Atmospheric Probe’s objectives 

could still be achieved, as well as the Orbiter’s survey of the Jovian magnetic and chargedparticle environment. The principal loss of data would be “the number of images and 

other high-speed spectral data that could be returned by the spacecraft,”37 according to 

project scientist Torrence Johnson. The spacecraft would be able to send to Earth perhaps 

only hundreds instead of thousands of images of Jupiter and its satellites.38 

On 20 October, NASA performed the first test of Galileo’s tape recorder since 

its breakdown by sending a play command. The recorder played back a few seconds of 

data, and this sparked tremendous elation among the staff. The tape had not broken and 

the recorder could still operate! 

O’Neil speculated that an “uphill/downhill problem” had caused the tape 

recorder malfunction. On 10 October, just before the problem was noticed, engineers had 

rewound the tape back to its very beginning. When the tape was near either of its ends, 

it took more force from the recorder’s capstans to move the tape along. O’Neil compared 

the situation to “a car trying to make it up the hill when the road is iced over.” He was 

fairly confident that the recorder’s three capstans had just started spinning like a car spinning its wheels and had not moved the tape along at all.39 

Why had the capstans lost traction? Galileo’s tape recorder was the same type 

as those installed in other spacecraft, where they had good records of reliability. But on 

other missions, the recorder had been constantly in use. On Galileo’s six-year-long cruise 

to Jupiter, it was rarely used. O’Neil believed that the grease in the recorder’s bearings 

might have gotten stiff, thus making the reels harder to turn and leading to the capstan 

slippage. He thought that a period of running the recorder might help redistribute the 

grease and make the recorder less prone to another malfunction.40 

NASA staff worried that the section of tape involved in the recorder failure 

might have been weakened by “being polished by the capstan for 15 hours”41 and be 

ready to break. To guard against this, they sent commands for the recorder to wind tape 

onto one reel, wrapping 25 times around the potentially damaged section. This secured 

that area of tape against more stresses that could tear it. Throughout the rest of 

the mission, that section of tape remained “off-limits” for further recording. One 

consequence of this decision was that some of the approach images of Jupiter became 

permanently unavailable.42 

Project engineers still had questions about what the recorder’s safe modes of 

operation were and did not want to unknowingly damage the recorder further. They 

did not know how fast the tape could safely move or whether it could reliably move in 

both directions. Until these questions were answered, Galileo’s management decided to 

use the recorder as little as possible. In the immediate future, it would be devoted to the 

mission’s highest priorities, which were to help the Orbiter attain a delicate insertion into 

a Jupiter orbit and to store data sent from the Probe as it plunged down through Jupiter’s 

atmosphere. In order to have the maximum probability of attaining these objectives, 

Galileo’s management sacrificed certain of the tape recorder’s less essential operations. 
200	 MISSION TO JUPITER: A History of the Galileo Project Chapter 8 I jupiter approach and arrival 201

43	Bill O’Neil interview, telephone conversation, 15 May 2001.

44	“Arrival Day—December, 1995,” Frequently Asked Questions—Jupiter, JPL Galileo Home Page, http://galileo.jpl.nasa.

gov/jupiter/arrival_day.html (accessed 25 March 2001) (available in Galileo—Meltzer Sources, folder 18522, NASA

Historical Reference Collection, Washington, DC); “Rewind Almost Unhinges Galileo.”

45	Glenn Orton, “Online from Jupiter—Field Journal from Glenn Orton,” NASA Quest, 30 November 1995, http://quest.

arc.nasa.gov/galileo/bios/fjournals/orton-ofj13.html (available in Galileo—Meltzer Sources, folder 18522, NASA

Historical Reference Collection, Washington, DC). 

46	Kathy Sawyer, “Jupiter Probe Set for Big Plunge,” Washington Post (4 December 1995): A3. 

These included the storage of the first-ever closeup images of the satellites Europa and Io 

when the Orbiter flew by them during its 7 December 1995 Jupiter arrival. 

Bill O’Neil, Galileo Project Manager from 1992 to 1997, commented that the 

most unpleasant task he ever had to perform on the mission was to tell his staff that there 

was no way the imaging of Europa and Io, which had been planned and anticipated for 

years, could be carried out on Jupiter Arrival Day, given the uncertain condition of the 

spacecraft’s tape recorder. The 7 December 1995 arrival day had, in fact, been picked 

because Galileo would have the opportunity to fly far closer than Voyager had to two of 

Jupiter’s most interesting moons—Europa and Io—on the same day. Canceling the imaging operations was a hurtful decision that O’Neil regretted to make, but it was clear to him 

what had to be done. He could not risk losing the Atmospheric Probe data and possibly 

crippling the spacecraft for the remainder of the mission.43 

At the time that O’Neil made this decision, it was a huge disappointment to many 

scientists on his staff because the Orbiter was not scheduled to return to Io during Galileo’s 

primary mission. Mission scientists feared that their only chance to obtain closeup pictures 

of the moons might have been lost. NASA was considering an extended Galileo mission that 

could include another encounter with Io, but this mission could only be carried out if the 

spacecraft continued to operate. With all that had gone wrong with it already, it was highly 

questionable that the spacecraft would survive long enough to return to Io.44 

Galileo’s Final Approach

In the weeks before Arrival Day, telescopes around the world prepared to make as many 

observations as possible of Jupiter in order to best determine what the Atmospheric 

Probe was heading into. The international effort included the Swedish Telescope 

at La Palma in the Canary Islands, Mt. Wilson in California, the French observatory 

Pic-du-Midi in the Pyrenees, Mauna Kea in Hawaii, Yerkes Observatory near Chicago, 

the McMath Solar Telescope at Kitt Peak in Arizona, and others.45

According to Galileo Project Manager William J. O’Neil, the moment of truth 

for the project would occur at about 2 p.m. PST on Friday, 7 December 1995, when the 

spacecraft would begin “the most hazardous arrival at a planet ever attempted.”46 The 

750-pound Probe would plunge into the Jovian atmosphere at a speed about 50 times 

greater than that of a rifle-shot bullet, then survive an ordeal that scientists compared 

to a trip through a nuclear fireball. Meanwhile, NASA would await the crucial signal, 

relayed by the Orbiter, that the Probe had survived and was still operating. This would 

be the first spacecraft signal ever received from inside the atmosphere of one of the 

giant outer planets.47

The Probe’s science instruments would be focused on analyzing Jupiter’s complex 

chemical makeup, measuring the temperatures and pressures of its atmosphere, determining 

the amounts of water contained in its cloud banks, observing its lightning, and experiencing 

its intense winds, which were believed to race around the planet at speeds of up to 480 

kilometers per hour (300 mph). Within hours of its arrival, after its descent of more than 

650 kilometers (400 miles) below Jupiter’s cloud tops, the Probe, which would have long 

since discarded its heatshields, would then be destroyed by temperatures high enough to 

vaporize metal. After the Orbiter, which would be flying 214,000 kilometers (133,000 miles) 

directly overhead, ceased to receive signals from the descending Probe, it would then fire 

its main engine and move on to its next task—inserting itself into Jupiter orbit. From there, 

it would begin its years-long study of the planet and its satellites.48

Because of the distance from Earth to Jupiter, over 800 million kilometers (half 

a billion miles), mission control could not quickly repair any problems that arose. Radio 

transmissions, which travel at the speed of light, take almost an hour each way at that 

distance. In most cases, when signs of trouble arise, a robotic spacecraft is designed to 

go into a “safing mode” and await further instructions from Earth. But during the arrival 

sequence, things would be happening too quickly for such a strategy. William O’Neil said 

that if problems arose during arrival, “we can’t have . . . [the spacecraft] stop. We can’t be 

in that loop. It would take an hour to see that something’s wrong, and another hour to 

transmit a fix back to the spacecraft. For anything that’s likely to happen, the spacecraft 

will sense it and keep going.”49 NASA’s task had been to make the arrival sequence failsafe, designing the spacecraft to accomplish its objectives as independently as possible 

from mission staff back on Earth.50

On 16 November 1995, the Orbiter’s magnetometers indicated that the spacecraft 

was crossing Jupiter’s bow shock, the shock wave preceding its magnetosphere, which is 

the region where Jupiter’s magnetic field, rather than the interplanetary field, predominates. 

The bow shock in front of a magnetosphere is very similar to the wave generated in front of 

a boat as it travels through the water. A planet’s bow shock helps to guide the charged particles of the solar wind around the planet’s magnetosphere and, by so doing, aids in forming 

a boundary between the interplanetary magnetic field environment and that of the planet.51

The bow shock detected by the Orbiter was not stationary; it moved back and 

forth in response to gusts of the solar wind. According to Dr. Margaret Kivelson of UCLA, 

principal investigator of Galileo’s magnetometer experiment, “As the solar wind velocity 

increased, the bow shock moved inside the position of the spacecraft, leaving Galileo 

again in the solar wind.” This crossing and recrossing of the bow shock occurred several 

47	 Ibid.; Ames Research Center, “Galileo Probe Mission Events,” http://ccf.arc.nasa.gov/galileo_probe/htmls/probe_

events.html (accessed 11 March 2000) (available in Galileo—Meltzer Sources, folder 18522, NASA Historical

Reference Collection, Washington, DC); “Galileo Less Than Three Weeks From Jupiter,” JPL Universe, 17 November

1995, http://www2.jpl.nasa.gov/galileo/status951117.html (available in Galileo—Meltzer Sources, folder 18522, NASA

Historical Reference Collection, Washington, DC).

48	Sawyer, “Jupiter Probe Set for Big Plunge”; “Galileo Less Than Three Weeks From Jupiter.” 

49	Sawyer, “Jupiter Probe Set for Big Plunge.”

50	 Ibid.

51	 “Bow Shock,” Windows to the Universe, University Corporation for Atmospheric Research (UCAR), Regents of the

University of Michigan, 2001, http://www.windows.ucar.edu/cgi-bin/tour_def/glossary/bow_shock.html (available in

Galileo—Meltzer Sources, folder 18522, NASA Historical Reference Collection, Washington, DC).
202	 MISSION TO JUPITER: A History of the Galileo Project Chapter 8 I jupiter approach and arrival 203

56	Sawyer, “Galileo Starts Tour of Giant Planet.”

57	 Ibid.

58	Bob Holmes, “Tears of Joy as Galileo Meets Jupiter,” New Scientist (16 December 1995): 5.

59	 Ibid.

52	JPL, “Galileo Crosses into Jupiter’s Magnetosphere,” JPL Public Information Office news release, 1 December 1995,

http://www2.jpl.nasa.gov/galileo/status951201.html (available in Galileo—Meltzer Sources, folder 18522, NASA

Historical Reference Collection, Washington, DC).

53	JPL, “Arrival at Jupiter,” section 6 inGalileo: The Tour Guide, June 1996, p. 47, http://www2.jpl.nasa.gov/galileo/tour/6TOUR.

pdf (available in Galileo—Meltzer Sources, folder 18522, NASA Historical Reference Collection, Washington, DC).

54	Kathy Sawyer, “Galileo Starts Tour of Giant Planet,” Washington Post (8 December 1995): A1; “Arrival at Jupiter.”

55	Sawyer, “Galileo Starts Tour of Giant Planet”; JPL, “Probe Science Results,” section 7 in Galileo: The Tour Guide, June

1996, p. 57, http://www2.jpl.nasa.gov/galileo/tour/7TOUR.pdf (available in Galileo—Meltzer Sources, folder 18522,

NASA Historical Reference Collection, Washington, DC).

times between the first encounter on 16 November 1995, when Orbiter was about 15 

million kilometers (9 million miles) from Jupiter, and 26 November, when the spacecraft 

had approached to a distance of 9 million kilometers (6 million miles). At that point, the 

Orbiter passed for the time being into “Jupiter space,” close enough to the planet that 

its bow shock could no longer be blown back far enough by the solar wind to cross the 

spacecraft’s position again. William J. O’Neil said of the event, “With the spacecraft now in 

the magnetosphere, we begin our first direct measurements of the Jupiter system.”52

Arrival Day: 7 December 1995

The date 7 December was a day of anticipation and tension as the men and women who 

ran the Galileo Project watched monitors and readouts and waited for news that the 

mission was going as planned. The 24 hours of Arrival Day were the most eventful, and 

possibly the busiest, of the whole mission. The Orbiter flew close by two Galilean moons; 

the Probe discovered a new radiation belt and dived into Jupiter’s thick atmosphere at a 

velocity far faster than that of a speeding bullet; and then the Orbiter performed a long, 

critical burn to enter a Jovian orbit.53

Two of the first events of Arrival Day were the Orbiter’s flybys of the moons 

Europa (at 3:09 a.m. PST) and Io (at 7:46 a.m. PST). The Orbiter passed 32,500 kilometers 

(20,200 miles) from Europa, but only 890 kilometers (550 miles) from Io, closer than originally planned (see figure 8.4). The Orbiter’s trajectory had been altered to make maximum 

use of Io’s gravitational field for decelerating the spacecraft, thereby conserving propellant 

for later in the mission. Approximately 4 hours after its Io encounter, at 11:54 a.m. PST, the 

Orbiter made the closest pass it would perform of Jupiter’s cloud tops, traveling through a 

region of intense radiation. The Orbiter was heavily shielded to protect its electronics, but 

nevertheless, the radiation levels during this time shot up more abruptly than expected 

and almost reached the craft’s design limit. One of the Orbiter’s navigation systems temporarily lost its bearings, but a backup navigation system took over. Once again, the value of 

NASA’s policy of building redundant systems into its spacecrafts was demonstrated.54 

At about 8 a.m. PST, 6 hours before atmospheric entry, an internal alarm “woke 

up” the Probe and turned on its instruments to begin warming them up. When the Probe 

passed between Jupiter’s ring and the upper atmosphere at about 11 a.m. PST (3 hours 

before entry), its energetic particle instrument (EPI) detected unexpected high-energy 

helium atoms and a belt of radiation 10 times stronger than Earth’s Van Allen belts. This 

new phenomenon provided data related to Jupiter’s high-frequency radio emissions, as well 

as to emissions from other celestial bodies with magnetospheres and trapped radiation.55

Figure 8.4. Arrival Day events. Notice the two different trajectories of the Probe and Orbiter. 

(Adapted from image number P-45516A) 

Descent of the Probe

Minutes after 2:00 p.m. PST, the Probe sped by Jupiter’s cloud tops on its way down 

into the planet’s atmosphere. If it survived its incandescent deceleration, it was expected 

to transmit a message within a few minutes. The first signal from the Probe was to be 

a single data bit, relayed from the Orbiter to Earth. The trip across interplanetary space 

would take 52 minutes, and the signal should have arrived by just after 3 p.m. PST—but 

it did not. Faces in JPL’s control room “drained of color as the minutes stretched on and 

nothing appeared on their screens.”56 At 3:13 p.m. PST, when a voice finally announced, 

“We have confirmation of telemetry lock,”57 it meant that the Probe had survived its hellish atmospheric entry and was still operating. Galileo staff “broke into whoops of joy and 

wild applause. There were even a few tears.”58

“It’s wonderful,” said David Morrison, chief of the space sciences division at Ames 

Research Center in Mountain View, California. “We have a spacecraft inside the atmosphere 

of a giant planet for the first time. We have been waiting twenty years for this moment.”59

The one-bit signal had actually arrived at 3:04 p.m. PST, right on schedule, 

but the receiving equipment on Earth had taken 10 minutes longer than expected to 

process the signal and send it to Galileo’s staff. Due to Galileo’s slow transmission rate 

EARTH

OCCULTATION

ZONE

SUN

OCCULTATION

ZONE

ONE WAY LIGHT TIME = 52 min

BEGIN PROBE PRE-ENTRY SCIENCE

EARTH EUROPA FLYBY

(32,500 KM)

Io FLYBY

(1,000 km)

SUN

RELAY

JUPITER

JOI

C/A

9.3

h

h

4.4

h 4.6

PROBE

ORBITER

75m

65 m

49m

9.4

EVENT TIMES (STC @ S/C):

EUROPA C/A

IO C/A

JUPITER C/A

PROBE ENTRY

RELAY STRAT

RLY CHECK #1*

RLY CHECK #2*

RELAY END

JOI START (12/8)

JOI END (12/8) 

13:09

17:46

21:54

22:04

22:07

22:12

22:24

22:22

00:27

01:16 

* CHECKS IF RVCR IN LOCK
204	 MISSION TO JUPITER: A History of the Galileo Project Chapter 8 I jupiter approach and arrival 205

through its low-gain antennas, it would take days to get even a preliminary look at the 

Probe’s findings and months before all the data were received. Project staff did know 

that the Orbiter had received the Probe’s long-awaited transmissions for over an hour, 

so extensive data on the Jovian atmospheric environment were expected to be transmitted eventually to Earth receiving stations.60

Figure 8.5 and table 8.1 portray the times, atmospheric pressures, altitudes, 

and temperatures at which key events occurred during the Probe’s approach, entry, 

and parachute descent into Jupiter’s atmosphere. Since Jupiter does not have a solid 

surface, altitudes were measured relative to the level at which Jovian atmospheric pressure equals Earth sea-level atmospheric pressure (1.0 bar pressure). The listed times are 

relative to the moment when the Probe crossed the 450-kilometer altitude level (“atmospheric entry time”), which occurred at 22:04:44 universal time (UT). Note the shallow 

angle (only 8.4 degrees below horizontal) at which the Probe entered the Jovian atmosphere. The radio signal from the Probe ended 61.4 minutes into the entry, when the 

high atmospheric temperatures caused the radio transmitter to fail. 

Figure 8.5. Probe entry and descent into the Jovian atmosphere. (NASA photo number ACD96-0313-4) 

Deployment of the Probe parachutes (see figure 8.6) occurred only after the 

Probe had slowed sufficiently (to about 1,600 kilometers per hour or 1,000 mph) to 

prevent air friction from immediately ripping the parachutes apart. First, the pilot or 

drogue chute was deployed from inside the aft heatshield (or aft cover) with the aid 

of a mortar. The aft heatshield was released from the Probe, and the drogue parachute 

pulled off the aft cover and extracted the main chute. These events all occurred within 

60	Sawyer, “Galileo Starts Tour of Giant Planet”; Ames Research Center, “Galileo Probe Mission Events,” http://ccf.arc.

nasa.gov/galileo_probe/htmls/probe_events.html (accessed 11 March 2000) (available in Galileo—Meltzer Sources,

folder 18522, NASA Historical Reference Collection, Washington, DC).

a period of 1.26 seconds. The forward heatshield was dropped 9 seconds later from the 

rapidly slowing descent module. When the forward shield had fallen to a distance of 

30 meters below the descent module, the module’s science experiments were begun. 

The main parachute eventually decreased descent module velocity to 430 kilometers 

per hour (265 mph).61 

Figure 8.6. Atmospheric Probe parachute deployment sequence and separation of deceleration 

module components from descent module. After the high-speed entry phase during which the 

atmosphere slowed down the Probe’s speed, a mortar deployed a small drogue parachute, 

the drogue chute opened, the aft cover of the deceleration module was released, the drogue 

parachute pulled the aft cover off and deployed the main parachute, and the forward heatshield 

was dropped. (NASA photo number AC89-0146-2)

61	 Ames Research Center, “Probe Mission Events,” http://spaceprojects.arc.nasa.gov/Space_Projects/galileo_probe/

htmls/Probe_Mission.html, and “Artwork of Parachute Deployment,” http://spaceprojects.arc.nasa.gov/Space_

Projects/galileo_probe/htmls/Parachute_deployment.html (accessed 17 October 2004) (hard copies available in

Galileo—Meltzer Sources, folder 18522, NASA Historical Reference Collection, Washington, DC).
206	 MISSION TO JUPITER: A History of the Galileo Project Chapter 8 I jupiter approach and arrival 207

Table 8.1 includes a timeline of the life of the Probe, from launch through 

cessation of the Probe signal.

Table 8.1. Timeline of Atmospheric Probe events.62

Time before or

after atmospheric

entry (e)

date action

Distance from

Jupiter in

Kilometers (Miles)

Speed Relative

to Jupiter in

Kilometers

(Miles) Per Hour

E - 6 years,

50 days

18 October 1989 Galileo launch.

E - 147 days 12 July 1995

(PDT)

Probe separation: Orbiter/Probe

spacecraft spins at up to 10

rpm for stability. Spin axis is

lined up with Probe trajectory. Explosive nuts accomplish

separation of Probe from Orbiter.

Three springs push Probe away.

Orbiter rocket engine fires to

alter craft trajectory so that it will

not impact Jupiter but will orbit

the planet in position to pick up

the Probe’s signals.

81,520,000

(50,660,000)

20,448

(12,706)

7 December 1995 Probe Atmospheric Entry Day

E - 6 hours Probe timer initiates craft’s operation. Batteries are activated,

and EPI is biased for operation.

600,000

(373,000)

76,700

(47,600)

E - 3 hours Measurement of Jupiter’s inner

radiation belts begins.

360,000

(224,000)

97,200

(60,400)

62	Julio Magalhães (ARC), “Structure of Jupiter’s Atmosphere,” 19 September 1997, http://spaceprojects.arc.nasa.gov/

Space_Projects/galileo_probe/htmls/ASI_results.html (accessed 13 December 2004) (available in Galileo—Meltzer

Sources, folder 18522, NASA Historical Reference Collection, Washington, DC); Julio Magalhães, “Probe Mission

Events,” 19 September 1997, http://spaceprojects.arc.nasa.gov/Space_Projects/galileo_probe/htmls/Probe_Mission.

html (accessed 13 December 2004) (available in Galileo—Meltzer Sources, folder 18522, NASA Historical Reference

Collection, Washington, DC); Julio Magalhães, “Galileo Probe Mission Events Timeline,” Expresní Astronomické 

Informace, 30 November 1995, http://astro.sci.muni.cz/pub/galileo/probe_mission_events.html (accessed 13 December

2004) (available in Galileo—Meltzer Sources, folder 18522, NASA Historical Reference Collection, Washington, DC);

JPL, “Probe Mission Time Line,” Galileo Probe Background, NASA Ames Research Center Public Affairs news release,

http://ccf.arc.nasa.gov/dx/basket/storiesetc/GalBkgd.html (accessed 11 March 2000) (available in Galileo—Meltzer

Sources, folder 18522, NASA Historical Reference Collection, Washington, DC); “Galileo Probe Mission Science

Summary,” 30 December 1996, http://spaceprojects.arc.nasa.gov/Space_Projects/galileo_probe/htmls/Science_

summary.html (accessed 13 December 2004) (available in Galileo—Meltzer Sources, folder 18522, NASA Historical

Reference Collection, Washington, DC).

Time before or

after atmospheric

entry (e)

date action

Altit ude* in

Kilometers

(Miles)

Speed in

Kilometers

(miles) Per

Hour

Pressure in

Bars

Temperat ure in

Degrees Celsius

(Fahrenheit)

E + 0 Probe atmospheric

entry. Rapid deceleration and intense heating of atmospheric

gases near the Probe

begin. Atmospheric

structure instrument

begins recording;

atmospheric pressure, density, and

temperature inferred

from data.

450 (280) 171,000

(106,000)

0.0000001 352 (666)

E + 35	

seconds

Period of maximum

heat generation and

ablation begins.

Heatshield burnoff

sensors begin

measurements of

heatshield mass loss.

220

(1400

0.0001 Near Probe:

14,000

(25,000);

away from

Probe: -100

(-150)

E + 55	

seconds

Peak aerodynamic

stresses and maximum forces of up to

250 g’s experienced.

0.006 Away from

Probe: - 120

(-184)

E + 80	

seconds

Main heat pulse and

heatshield ablation

end.

E + 172	

seconds

Pilot parachute	

deployed

23 (14) 0.4 -145 (-229)

E + 173	

seconds	

Aft heatshield separates and pulls out

main parachute

23 (14) 0.4 -145 (-229)

E + 3.0	

minutes

Main parachute slows

descent module.

Three explosive separation bolts fire, and

forward heatshield

falls away.

21 (13) 650 (400) 0.45 -145 (-229)

E + 3.8	

minutes

Orbiter locks on

Probe radio signal.

16 (10) 0.56 -135 (-211)

E + 6.4	

minutes

Zero altitude attained	

(defined as the point

where pressure = 1

bar, or Earth surface

pressure).

0 (0) 1 -107 (-161)

Table 8.1. Timeline of Atmospheric Probe events. (continued)
208	 MISSION TO JUPITER: A History of the Galileo Project Chapter 8 I jupiter approach and arrival 209

*Altitude is measured relative to a point in the atmosphere with pressure of 1 bar, the 

pressure on Earth’s surface.

 As the Probe continued to fall into the planet after completing its mission, it 

was slowly destroyed by the increasing heat. This was not the short-lived heat of atmospheric friction and compression against which the Probe’s now-discarded heatshields 

protected it during the first minutes of entry. This was a steady-state thermal barrage 

that would claim one part of the craft after another. Soon after the Probe stopped transmitting, mission scientists hypothesized that its parachute melted, then its aluminum 

fittings. A couple of hours later, the aluminum vaporized. Four hours after that, even the 

ultratough titanium shell of the Probe had melted, and, after an additional 2 hours, the 

titanium evaporated as the temperature climbed to 1,700oC (3,100oF) and the pressure 

rose to 5,000 times that of Earth’s surface. The Probe had been reduced to its atomic 

components and become “one with the atmosphere of Jupiter.”63 

Jupiter Orbit Insertion (JOI)

After the Orbiter finished receiving data from the Atmospheric Probe, it had to adjust 

its trajectory quickly to avoid heading off into deep space, away from the planet and 

satellites it was supposed to study. The Orbiter had to fire its 400-newton engine in such 

a way as to slow itself down relative to Jupiter and allow the giant planet’s gravitational 

field to capture it. NASA scientists scheduled this burn when the Orbiter was close to 

Jupiter, only four Jupiter radii (RJ) distant, for the further the JOI maneuver was performed 

from the planet, the less the gravitational force and the greater the spacecraft’s propellant 

requirements would have been. The Orbiter’s burn sequence began at 5:20 p.m. PST and 

reduced the craft’s velocity by over 600 meters per second until the Orbiter was finally 

captured. JOI, the actual capture of the Orbiter by Jupiter, occurred approximately 45 

minutes into the 49-minute burn.64

Once the Orbiter was captured, it became the first artificial satellite to join 

Jupiter’s family of moons and particles. Figure 8.7 depicts the elongated elliptical shape 

of the Orbiter’s 7-month-long initial orbit, as well as the shapes of the spacecraft’s next 

10 orbits. During most of the initial orbit, the spacecraft slowly transmitted data to Earth 

that had been received from the Probe during its brief plunge into the Jovian atmosphere 

and stored on the craft’s tape recorder. Mission engineers tested the tape recorder to 

determine its present capabilities and loaded and tested new software sent from Earth 

that would make data transmission more efficient. Data taken during the first Io flyby 

(on Arrival Day, 7 December 1995) were also sent to Earth. Because of uncertainty with 

the tape recorder’s capabilities, images of Io, which would have required large storage 

volumes, had not been stored on the tape recorder, but other Io-related data that required 

less storage volume had been collected. Scientific findings from Io and other Jovian 

moons are discussed in detail in chapter 9. 

When the spacecraft approached the apojove (furthest point from Jupiter) of its 

orbit on 14 March 1996, it fired its main engine once again. This maneuver changed the 

craft’s trajectory so that its perijove (closest point to Jupiter of its orbit) was raised from 4 

RJ to about 10 RJ, above the altitude of Jupiter’s intense radiation belts. Nearly 50 percent 

of the radiation dosage allowed for Galileo’s primary mission had been received during its 

initial low-altitude Jupiter flyby. To ensure that the spacecraft’s instruments lasted as long 

as possible, it was important to limit future radiation exposure.65

Figure 8.7. Orbiter’s first 11 orbits around Jupiter. Notice that the apojove of the first orbit is 

over 250 Jupiter radii (RJ) away from the planet. (Adapted from JPL image number P45516B)

Scientific Results from the Probe

Data collected during the Probe’s descent into Jupiter’s atmosphere made it necessary 

for scientists to revisit many of their beliefs about the formation and evolution of our 

solar system’s giant gaseous planets. Measurements of atmospheric composition, wind 63	“Arrival at Jupiter,” p. 51; “The Probe Story: Secrets and Surprises from Jupiter,” Galileo Messenger (April 1996).

64	“Jupiter Orbit Insertion Burn Status,” Galileo Jupiter Orbit Insertion December 7, 1995, http://www2.jpl.nasa.gov/

galileo/countdown/plot.html (accessed 29 March 2000) (available in Galileo—Meltzer Sources, folder 18522, NASA

Historical Reference Collection, Washington, DC); JPL, “Galileo Mission Status,” JPL Public Information Office

news release, 7 December 1995, http://www2.jpl.nasa.gov/galileo/status9512071.html (available in Galileo—Meltzer

Sources, folder 18522, NASA Historical Reference Collection, Washington, DC).

65	Donald J. Williams, “Jupiter—At Last!” Johns Hopkins APL Technical Digest 17, no. 4 (1996): 347; William J. O’Neil,

“Project Galileo,” paper number AIAA–90-3854 in AIAA Space Programs and Technologies Conference Proceedings

(held in Huntsville, AL, 25–28 September 1990).

Table 8.1. Timeline of Atmospheric Probe events. (continued)

Time before or

after atmospheric

entry (e)

date action

Altit ude* in

Kilometers

(Miles)

Speed in

Kilometers

(miles) Per

Hour

Pressure in

Bars

Temperat ure in

Degrees Celsius

(Fahrenheit)

-20 (-12) 2 -66 (-87)

-96 (-60) 12 84 (183)

E + 61.4	

minutes

Probe signal no

longer detected.

Pressure is approximately equal to that

at an Earth ocean

depth of 700 feet.

-146

(-91)

22 153 (307)
210	 MISSION TO JUPITER: A History of the Galileo Project Chapter 8 I jupiter approach and arrival 211

velocities, temperatures, cloud characteristics, electrical storms, and elemental and molecular abundances painted a very different picture of Jupiter from what was expected.66

Where Is the Water?

One of the most puzzling mysteries that the Probe’s findings raised was, where was the 

water that everyone expected to see? Data from the Voyager spacecraft that had flown 

by Jupiter in 1979 suggested a water abundance such that the amount of oxygen present 

in the atmosphere (most of which would be in the water molecules) would amount to 

twice the oxygen abundance of the Sun. Analysis of the atmospheric waves propagating 

across Jupiter’s cloud tops as a result of the Shoemaker-Levy comet’s impact indicated 

that Jupiter might have a water abundance such that the oxygen level would be 10 times 

the solar level. Preliminary data released six weeks after the Probe’s 7 December 1995 

descent, however, revealed a planetary atmosphere far dryer than expected, with a water 

content corresponding to an oxygen abundance no greater than that of the Sun, and possibly far less.67

A virtual absence of water and, in particular, water clouds was eventually 

confirmed by several of the Probe’s science instruments. Its nephelometer, which was 

designed to analyze cloud formations near the Probe, saw none of the dense water 

clouds that were expected based on flyby spacecraft and Earth-based telescope observations. The net flux radiometer, which could make measurements of distant clouds, did 

not observe a thick water cloud deck either. The atmospheric structure instrument (ASI), 

which determined the temperature, pressure, and density structure of Jupiter’s atmosphere 

from its uppermost regions down to a region where the pressure was about 22 times that 

on Earth (or 22 bars), found evidence that the deep Jovian atmosphere (100 to 150 kilometers, or 60 to 90 miles, below visible, nonwater clouds) was dryer than expected, at 

least in the region that the Probe visited. The ASI measured a vertical temperature gradient 

indicative of a dry atmosphere without condensation. The lightning and radio emission 

detector (LRD), which searched for optical lightning flashes in the Probe’s vicinity and 

found none, also supported an absence of extensive water clouds. The neutral mass 

spectrometer (NMS), whose job was to determine the chemical composition of the 

atmosphere, measured far less oxygen than in the Sun’s atmosphere. Since most of Jupiter’s 

oxygen would be found in water molecules in the form of water vapor, the NMS data 

implied a surprisingly dry atmosphere.68

The water situation launched a spirited debate on the details of Jupiter’s formation and evolution. Some theories of planet formation postulated that collisions with 

small, water-bearing bodies such as comets and asteroids had played an important role 

not only in Jupiter’s formation, but also in that of other planets. But if Jupiter had indeed 

captured significant amounts of water from these collisions, then where was it? Jupiter’s 

powerful gravitational force should have prevented the water from escaping. If the lack 

of water observed by the Probe was typical of Jupiter as a whole, not just of the area 

the Probe examined, then collisions with water-bearing bodies may not have contributed 

much at all to the planet’s composition. If such collisions were insignificant for Jupiter, 

they also may have been unimportant for the solar system’s other planets. Comets, for 

instance, might not have played as large a part in the delivery of water for Earth’s oceans 

as some studies had suggested.69 

Theories of how the planets formed were also reevaluated. According to nucleosynthesis theories, our solar system’s Sun and planets were created from the same primitive 

interstellar cloud of matter. This cloud consisted of a primordial component (one that 

had existed since the beginning of time)70 generated in the Big Bang that began the universe, as well as younger material manufactured in stars and then injected into interstellar 

space through supernova explosions and stellar winds (streams of particles similar to the 

solar wind). But the Probe had measured only a fraction of the Sun’s oxygen content in 

Jupiter’s atmosphere. Where was the water (and its inherent oxygen) that should have 

remained from Jupiter’s creation in the same nebula from which the Sun and the other 

planets emerged?71

One theory suggested that Jupiter actually did contain water levels at or above 

those of the Sun, with the bulk of the water trapped deep in the planet’s interior. According 

to this theory, Jupiter began as a solid protoplanet of ice and rock that grew to 8 to 10 

times the mass of Earth through accretion of ice grains and dust from the original solar 

nebula. As the solid proto-Jupiter grew, its increasing gravity attracted surrounding lighter 

gases such as helium and hydrogen from the nebula, producing a dry atmosphere similar 

to that detected by the Probe. Such a process could have permanently locked water ice in 

the solid core of Jupiter while keeping atmospheric regions nearly devoid of water. 

This theory had to account for the enhanced atmospheric carbon, sulfur, and 

nitrogen levels measured by the Probe, which were notably enriched compared to solar 

abundances. In order to explain such abundances, scientists postulated that methane, 

ammonia, hydrogen sulfide, and other volatiles that were originally locked in the core 

escaped as it heated up. The theory had serious problems, however. Dr. Tobias Owen of 

the University of Hawaii questioned how water ice remained trapped in a planetary core 

believed to be very hot when many other compounds were supposed to have volatized, 

escaped, and entered the Jovian atmosphere.72

Another theory, one that has gained considerable acceptance in the space science community, proposed that Jupiter’s atmosphere was not globally dry and that the 

Probe had just stumbled into an atypical entry site comparable to Earth’s desert regions. 

This theory was strongly supported by Earth-based observations of Jupiter, including 

66	Williams, “Jupiter—At Last!” p. 353. 

67	Kathy Sawyer, “Jupiter Retains Atmosphere of Mystery,” Washington Post (23 January 1996): A3; Douglas Isbell and

David Morse, “Galileo Probe Data Spurs New Concepts for Jupiter’s Circulation and Formation,” NASA news release

96-103, 21 May 1996, http://www2.jpl.nasa.gov/galileo/status960521.html (available in Galileo—Meltzer Sources,

folder 18522, NASA Historical Reference Collection, Washington, DC); M. Roos-Serote, P. Drossart, Th. Encrenaz, R.

W. Carlson, and L. Kamp, “New NIM 5 Micron Observations: Where Is the Water on Jupiter?” (paper presented at the

31st Annual Meeting of the Division for Planetary Sciences (DPS), session 51: “Outer Planet Atmospheres Posters,”

14 October 1999).

68	“Galileo Probe Mission Science Summary”; Isbell and Morse, “Galileo Probe Data Spurs New Concepts for Jupiter’s

Circulation and Formation.”

69	Sawyer, “Jupiter Retains Atmosphere of Mystery”; Isbell and Morse, “Galileo Probe Data Spurs New Concepts for

Jupiter’s Circulation and Formation”; Christopher F. Chyba, “Impact Delivery and Erosion of Planetary Oceans in the

Early Solar System,” Nature 343 (11 January 1990): 129–133.

70	Encarta Online Encyclopedia, http://encarta.msn.com (available in Galileo—Meltzer Sources, folder 18522, NASA

Historical Reference Collection, Washington, DC).

71	B. J. Conrath, D. Gautier, R. A. Hanel, and J. S. Hornstein, “The Helium Abundance of Saturn from Voyager

Measurements,” Astrophysical Journal 282 (15 July 1984): 807–815.

72	 “Isbell and Morse, “Galileo Probe Data Spurs New Concepts for Jupiter’s Circulation and Formation”; “Astronomy,”

Microsoft Encarta Online Encyclopedia, 2001, http://encarta.msn.com (available in Galileo—Meltzer Sources, folder

18522, NASA Historical Reference Collection, Washington, DC).
212	 MISSION TO JUPITER: A History of the Galileo Project Chapter 8 I jupiter approach and arrival 213

infrared telescope measurements taken by JPL’s Glenn Orton, which showed the Probe’s 

entry site to be situated on the edge of a prominent “hotspot,” a patch of atmosphere that 

was clearer and drier than that of most of the rest of the planet, with a marked absence 

of clouds. Hotspot areas cover about 1 percent of the planet and about 15 percent of the 

surface in equatorial areas.73 

Clouds

Most space science researchers predicted that the Probe would encounter three substantial 

Jovian cloud layers: an upper ammonia ice-particle cloud layer at an altitude at which atmospheric pressure was 0.5 to 0.6 bar, a middle ammonium hydrosulfide (NH4

SH) ice cloud 

with its base at 1.5 to 2 bars, and a lower water cloud deck, expected to be the densest of 

all, with its base at 4 to 5 bars (see figure 8.8). Scientists envisioned this scenario based on 

the known thermodynamic properties of these compounds, Jovian atmospheric models, 

and past Earth-based and spacecraft-based observations of large regions of Jupiter. Once 

again, the Probe sent back data that surprised the scientific community. Cloud decks were 

far more tenuous than expected or completely absent.74

Figure 8.8. Jupiter’s expected cloud layers, along with the clouds actually observed at the 

Probe entry site. (NASA image number ACD96-0313-7)

Two experiments on the Probe used different approaches to detect possible 

clouds. The net flux radiometer (NFR), designed to measure energy balance between 

solar flux from above and thermal energy from below, recorded variations in sky 

brightness in the 0.4- to 0.6-bar level of the atmosphere. This phenomenon corresponded 

to altitudes from 23 down to 14 kilometers (14 to 9 miles) above the level at which pressure 

equaled 1.0 bar,75 the pressure found at sea level on Earth. The sky brightness variations and 

altitude suggested scattered clouds that were likely composed of ammonia ice particles.76

The Probe’s nephelometer (NEP) detected a tenuous layer between 0.46 and 

0.55 bar—nearly the same altitude range as above—that could have been the predicted 

ammonia cloud. NEP data, however, had an uncertainty associated with them. The NEP 

operated by sending out a laser beam and observing scattered laser light from cloud 

particles passing between the body of the Probe and a deployed 0.1-meter arm. The NEP 

measured scattering only from matter in the near vicinity of the Probe, so if the Probe 

happened to be passing through even a small inhomogeneous region, the measurements 

could lead to a fallacious picture of the surrounding atmosphere. 

The NEP recorded denser particle concentrations at pressures ranging from 0.76 

to 1.34 bars, or altitudes from roughly 9 to -10 kilometers (i.e., from 9 kilometers above 

to 10 kilometers below the 1-bar level in the atmosphere). Though far more tenuous than 

predicted, these particle populations were possibly the expected ammonium hydrosulfide 

clouds. The NFR detected a similar structure at 1.35 bars. The NEP also found one welldefined tenuous cloud structure with a base at the 1.6-bar pressure level.

Scientists had predicted the occurrence of the thickest clouds of all, composed 

of water, in the 1.9- to 4.5-bar range (roughly in the region of -20 to -50 kilometers). The 

NEP, however, detected only a weak structure in that region which might have been a thin 

water cloud, although the data were inconclusive. The NFR did not detect this structure 

at all. In summary, the Probe’s search for Jovian clouds did not identify any thick, dense 

cloud layers, nor did it conclusively identify any water clouds.77

Helium Abundance and the Evolution of the Solar System

One of the Probe’s primary objectives was to analyze the composition of the Jovian 

atmosphere. Jupiter’s strong gravitational field (due to its large mass), its low atmospheric 

temperatures, and the low strength of the solar wind at the planet’s distance from the Sun 

all have contributed to the retention of even the lightest elements, such as hydrogen and 

helium. The mean escape time for light elements in Jupiter’s atmosphere is far longer than 

the age of the solar system. Thus, Jupiter should contain close to the original abundance 

of elements that it had when it was formed from the primitive solar nebula, the cloud of 

73	 “The Probe Story: Secrets and Surprises from Jupiter,” Galileo Messenger (April 1996); “Probe Mystery Solved:

Jupiter as Wet (and Dry) as Earth,” Galileo Messenger (July 1997); Richard E. Young, “The Galileo Probe Mission

to Jupiter: Science Overview,” Journal of Geophysical Research 103, no. E10 (25 September 1998): 22,775–

22,776.

74	Sawyer, “Jupiter Atmosphere of Mystery”; Young, “Galileo Probe Mission to Jupiter: Science Overview,”

p. 22,780. Caption information from NASA image number ACD96-0313-7.

75	Altitudes are arbitrarily measured from the level at which the pressure is 1 bar. Negative altitudes denote levels

beneath the 1-bar level.

76	 “The Probe Story: Secrets and Surprises from Jupiter.”

77	Boris Ragent, David S. Colburn, Kathy A. Rages, Tony C. D. Knight, Philip Avrin, Glenn S. Orton, Padmavati A.

Yanamandra-Fisher, and Gerald W. Grams, “The Clouds of Jupiter: Results of the Galileo Jupiter Mission Probe

Nephelometer Experiment,” Journal of Geophysical Research 103, no. E10 (25 September 1998): 22,891–22,909;

Alvin Seiff, Donn B. Kirk, Tony C. D. Knight, Richard E. Young, John D. Mihalov, Leslie A. Young, Frank S. Milos,

Gerald Schubert, Robert C. Blanchard, and David Atkinson, “Thermal Structure of Jupiter’s Atmosphere Near the

Edge of a 5-μm Hot Spot in the North Equatorial Belt,” Journal of Geophysical Research 103, no. E10 (25 September

1998): 22,857–22,889; L. A. Sromovsky, A. D. Collard, P. M. Fry, G. S. Orton, M. T. Lemmon, M. G. Tomasko, and

R. S. Freedman, “Galileo Probe Measurements of Thermal and Solar Radiation Fluxes in the Jovian Atmosphere,”

Journal of Geophysical Research 103, no. E10 (25 September 1998): 22,929–22,977; “Lightning,” Microsoft Encarta 

Online Encyclopedia, http://encarta.msn.com (available in Galileo—Meltzer Sources, folder 18522, NASA Historical

Reference Collection, Washington, DC); Young, “Galileo Probe Mission to Jupiter: Science Overview,” p. 22,781.
214	 MISSION TO JUPITER: A History of the Galileo Project Chapter 8 I jupiter approach and arrival 215

The helium mass abundance of the primitive solar nebula had been estimated, 

based on stellar evolution models, at 27.5 percent, with the remaining percentage mainly 

hydrogen. The best pre-Galileo estimates of Jovian atmospheric helium abundance were 

derived from Voyager data and calculated to be about 18 percent. This estimate suggested 

that significant gravitational settling of helium out of the atmosphere may have occurred, 

although not as much as on Saturn.80

Two independent Galileo Probe measurements taken by the helium abundance 

detector and the neutral mass spectrometer indicated a helium abundance in Jupiter’s 

atmosphere of 24 percent, higher than the 18 percent expected from Voyager data and 

close to the primitive solar nebula value of 27.5 percent. Jupiter is so large that a 24-

percent helium abundance implies that there are on the order of 10 Earth masses more 

helium in Jupiter’s atmosphere than had been predicted from Voyager data. 

The difference between Jupiter’s 24-percent atmospheric helium abundance and 

Saturn’s 6-percent abundance is dramatic, but perhaps not surprising. Jupiter, with its 

greater mass and dimensions, has retained more primordial thermal energy in its interior 

than Saturn has. Theoretical calculations of the solubility curve of helium in metallic 

hydrogen suggest that Jupiter is still too warm for helium droplets to form within its metallic hydrogen region. If this is indeed the case, differentiation of the planet’s helium from 

its hydrogen through gravitational settling of the helium droplets will not occur.81

Other Substances in the Jovian Atmosphere

In the Jovian atmosphere, the heavier elements carbon, nitrogen, oxygen, and sulfur 

occur principally in the compounds methane, ammonia, water, and hydrogen sulfide, 

respectively. These may have been deposited in part from encounters with meteorites 

and comets. Some hydrocarbons with two or three carbon atoms appear to be present. 

Only upper limits could be estimated for heavier hydrocarbons, and these were extremely 

small. The dearth of complex hydrocarbons makes it very unlikely that life similar to that 

of Earth will be found in the Jovian atmosphere. 

Also detected in the atmosphere were noble gases, including argon, krypton, 

and xenon, in amounts up to three times the levels found in the Sun. This was surprising because noble gases are difficult for a planet to trap. Detection of relatively large 

concentrations raised questions in the scientific community regarding what in the planet’s 

formation process allowed Jupiter to acquire such amounts of the gases. One explanation suggested that if Jupiter trapped the gases through cooling processes that caused 

them to condense, this would have required extremely cold temperatures of about 

 -240°C (-400°F), colder than the surface of Pluto. For such low temperatures to exist, 

Jupiter might have been formed out in the area around the Kuiper Belt and later dragged 

inward to its present location closer to the Sun. 

Another possibility is that the solar nebula, the huge cloud of gas and dust that 

gave birth to our solar system, was much colder than scientists currently believe. A third 

proposal was that the solid, icy materials that transported noble gases to Jupiter began 

forming in the original interstellar cloud of gas and dust long before it collapsed to form 

matter out of which the solar system was created. If Jupiter has indeed retained most of its 

original elemental abundances, then the study of the planet’s composition should provide 

not only a view of its current conditions, but also a glimpse back in time to the early solar 

system environment (see figure 8.9).78

Figure 8.9. The Jovian atmospheric composition provides a “tracer” for planetary history. 

(NASA photo number ACD96-0313-9)

Space scientists believe that conditions in the primitive solar nebula were such 

that helium and hydrogen did not separate from each other during the process of planetary formation, but remained homogeneously mixed. Jupiter’s atmosphere still consists 

mainly of hydrogen and helium. What the space science community did not know, however, was whether Jovian atmospheric helium abundance had remained the same as in 

the primitive solar nebula. A significantly lower helium percentage in the present Jovian 

atmosphere would indicate that segregation had occurred, perhaps through gravitational 

settling of the helium, which is heavier than hydrogen. 

Theories of planetary evolution predict a layer of hydrogen, forming at high 

pressures in Jupiter’s and Saturn’s interiors, that has the characteristics of a metal. As 

Jupiter grows older, it continuously cools. If its interior cools sufficiently, helium would 

become immiscible in the metallic hydrogen layer. Droplets of helium would form and 

fall toward the center of the planet, separating from hydrogen and eventually leading to a 

depletion of helium in the planet’s atmosphere. Such a process may be quite appreciable 

on Saturn, where Voyager data have indicated that the atmospheric helium abundance is 

less than one-quarter that of the primitive solar nebula environment.79

78	Conrath et al., “Helium Abundance of Saturn from Voyager Measurements,” pp. 807–815; Richard E. Young, “The

Galileo Probe Mission to Jupiter: Science Overview,” pp. 22,776–22,778.

79	Conrath et al., “Helium Abundance of Saturn from Voyager Measurements,” pp. 807–815; Young, “Galileo Probe

Mission to Jupiter: Science Overview.”

80	“Galileo Probe Mission to Jupiter: Science Overview,” pp. 22,776–22,777.

81	Young, “Galileo Probe Mission to Jupiter: Science Overview,” p. 22,777; Williams, “Jupiter—At Last!” p. 353; Conrath

et al., “Helium Abundance of Saturn from Voyager Measurements,” p. 807.
216	 MISSION TO JUPITER: A History of the Galileo Project Chapter 8 I jupiter approach and arrival 217

the solar nebula. That would make those icy materials far older and more primitive than 

had been thought. The last two hypotheses suggest that giant planets such as Jupiter 

might form closer to their stars than current theories predict. This would help explain 

the new observations of planetary systems around other stars, in which giant planets are 

commonly located close to their stars.82 

Galileo’s neutral mass spectrometer determined that the carbon-to-hydrogen 

ratio in Jupiter’s atmosphere was almost three times the solar value. In other words, 

the Jovian atmosphere has been enriched in carbon relative to the Sun. Two possible 

explanations for this are an internal-to-Jupiter mechanism and an external influence on 

the planet. The internal theory proposes that as solar nebula material accreted over many 

years to form Jupiter, the pressure in the planet’s core increased and caused carbon 

in the form of methane to outgas from the core. The external theory postulates that 

Jupiter’s overabundance of carbon relative to the Sun came from an influx of late-accreting 

planetesimals impacting the planet.83

One study of Galileo mission atmospheric science predicted that ammonia density would vary considerably with altitude in the Jovian atmosphere. Solar energy–initiated 

photochemical reactions in the upper atmosphere, condensation of ammonia gas into 

cloud droplets when temperatures dropped below about -130o

C, dissolution of ammonia 

in water clouds, and chemical reactions with hydrogen sulfide to form ammonium hydrosulfide clouds were all expected to affect ammonia concentrations at different altitudes.84

Observations of radio signals broadcast at 1,387 megahertz from Probe to Orbiter 

showed attenuation, or lessening of signal strength, that was presumed to result from constituents of the Jovian atmosphere. Clouds had been expected to attenuate radio signals, 

but the Probe descended through a relatively cloud-free region, so clouds could have had 

little effect. High concentrations of atmospheric water and hydrogen sulfide might have 

had a marked attenuation effect, but at the concentrations of these chemicals that were 

measured by the Probe’s neutral mass spectrometer, their effect would be negligible. JPL

scientists inferred that the observed radio attenuation must be mainly due to atmospheric 

ammonia concentrations. From the data, they were able to derive an ammonia concentration profile as a function of depth in the atmosphere. This profile showed that for 

atmospheric pressures greater than 6 bars, corresponding to depths below -60 kilometers, 

ammonia abundance rose to four times the predicted value, based on the solar abundance 

of nitrogen, one of the components of ammonia. 

The Probe’s net flux radiometer, although not able to quantify ammonia abundance explicitly, collected data consistent with an ammonia abundance significantly 

higher than solar. The neutral mass spectrometer was not able to determine ammonia 

abundance, possibly due to a contamination problem. 

The high ammonia concentration inferred from radio attenuation data does not 

agree with data from Voyager measurements or Earth-based microwave spectroscopy, 

which predicted abundances closer to the solar value. Voyager studies, however, probably 

measured average values in the Jovian atmosphere rather than values in an anomalous 

region such as the one into which the Probe descended.85

Dynamics of Jupiter’s “Hotspots” and Their Relation to Atmospheric Composition Profiles

Concentrations of hydrogen sulfide and water, as well as ammonia, rose noticeably with 

depth in the atmosphere. One of the most puzzling features of the Probe’s atmospheric 

data was the lack of correlation between cloud decks and abundances of the water, 

hydrogen sulfide, and ammonia thought to form or be involved in the formation of clouds. 

Abundance increases of hydrogen sulfide, for instance, did not occur as expected near 

the altitude of the ammonium hydrosulfide cloud deck, but in a region that scientists had 

thought to be well mixed, and thus where more constant abundances were expected.86 

Scientists postulated that downdrafts at the Probe entry site would suppress 

abundances of water, ammonia, and hydrogen sulfide in the upper atmosphere while 

concentrating them at lower depths. The occurrence of downdrafts in the relatively dry 

hotspot into which the Probe plunged would be analogous to phenomena in Earth’s desert 

regions, where dry air descends after drying out during ascent. A downdraft phenomenon 

at the Probe entry site is difficult to explain for several reasons, however. Dry air generally has a lower density than moist air, which means that if masses of moist and dry air 

are next to each other, the heavier, moist air will tend to sink to the lower altitude and 

displace the dry air upward. Downflowing air in the dry hotspot that the Probe entered 

would be more plausible if the air surrounding the hotspot were as dry or nearly as dry. 

Researchers have asked, however, what would make the surrounding regions dry. 

Another proposed mechanism for the downdrafting was that the less dense, 

dry air of the hotspot was entrained in large-scale convection currents (flows initiated by 

differences in density due to temperature variations) and carried down into the atmosphere. This process would rely on descending air currents’ merging and trapping less 

dense, dry air between them. This model was derived from fluid simulations of intense 

convection. A similar entrainment process may occur in Earth’s atmosphere, with convection currents responsible for dragging stable stratospheric air down into the troposphere. 

Convection currents, which typically form separate regions, or cells, of upflows and 

downflows, can possibly explain the regular spacing of hotspots observed on the Jovian 

surface—if the hotspots are associated with downdrafting cells.87 

Later modeling efforts by groups at NASA Ames Research Center and the 

University of Louisville suggested that the downwellings in hotspots were part of

large-scale, long-lived Jovian circulation patterns, which helped to explain why hotspots 

such as the one at the Probe entry site persisted for many years. Ames’s Adam Showman 

and the University of Louisville’s Timothy Dowling, director of the school’s Comparative 

Planetology Laboratory, generated models indicating that if large-scale pressure differences 82	Young, “Galileo Probe Mission to Jupiter: Science Overview,” p. 22,777; Ames Research Center, “Jupiter’s

Atmospheric Composition,” 19 September 1997, http://spaceprojects.arc.nasa.gov/Space_Projects/galileo_probe/

htmls/Composition.html (accessed 31 October 2004) (available in Galileo—Meltzer Sources, folder 18522, NASA

Historical Reference Collection, Washington, DC); Robert Roy Britt, “Jupiter’s Composition Throws Planet-formation

Theories into Disarray,” Space.com, 17 November 1999, http://www.space.com/scienceastronomy/solarsystem/

jupiter_elements_991117.html (available in Galileo—Meltzer Sources, folder 18522, NASA Historical Reference

Collection, Washington, DC); Jane Platt and Kathleen Burton, “Galileo Probe Results Suggest Jupiter Had an Ancient,

Chilly Past,” JPL news release, 17 November 1999.

83	Young, “Galileo Probe Mission to Jupiter: Science Overview,” p. 22,777.

84	 Ibid.

85	W. M. Folkner, R. Woo, and S. Nandi, “Ammonia Abundance in Jupiter’s Atmosphere Derived from the Attenuation of

the Galileo Probe’s Radio Signal,” Journal of Geophysical Research 103, no. E10 (25 September 1998): 22,847, 22,854;

Young, “Galileo Probe Mission to Jupiter: Science Overview,” p. 22,777.

86	Young, “Galileo Probe Mission to Jupiter: Science Overview,” pp. 22,778–22,779.

87 Young, “Galileo Probe Mission to Jupiter: Science Overview,” p. 22,779; Seiff et al., “Thermal Structure of Jupiter’s

Atmosphere,” p. 22,885. 
218	 MISSION TO JUPITER: A History of the Galileo Project Chapter 8 I jupiter approach and arrival 219

are present, the air that regularly flows west to east just north of Jupiter’s equator also 

moves “dramatically up and down every few days.”88 As the air moves up and drops 

in temperature, the water, ammonia, and other vapors it contains condense into liquid 

droplets that form clouds, and they are left behind. The air that then flows downward is 

very dry, devoid of material that can form more clouds, and thus produces a clear area 

such as the one the Probe entered (see figure 8.10). According to JPL’s Dr. Robert Carlson, 

principal investigator for the NIMS experiment, the air in these dry, clear areas contains 

only 1 percent of the water that the air around the dry spots has. 

Jupiter’s hotspots actually are not particularly hot; the temperature at their visible depth is typically about 0o

C (32oF), though that is far warmer than the -130oC (-200oF) 

temperatures found at surrounding cloud tops. The hotspots might more accurately be 

called “clear spots” or “bright spots” because their transparency allows increased infrared 

radiation emitted from the planet below to pass through the atmosphere and be received 

by both spacecraft and Earth-based instruments. 

These spots are typically the size of North America. Researchers’ computer simulations recreated some observed features of the spots and of Jupiter’s equatorial cloud 

plumes. The simulations showed that winds on the rim of a hotspot can get stronger and 

stronger with depth, a characteristic that the Probe noticed as well.89

Jupiter’s equatorial region is depicted in figure 8.10. The dark region near the 

center of the mosaic is an equatorial hotspot similar to the Galileo Probe’s entry site. 

Jupiter’s Winds and the Energy Sources That Drive Them

Pre-Galileo models of Jovian meteorology predicted that wind speeds at the cloud tops 

would be 360 to 540 kilometers per hour (220 to 330 mph). If these winds were generated as winds on Earth are—by solar energy and the release of latent heat as water vapor 

condenses—the wind speeds would be expected to drop with atmospheric depth and 

reach zero at some point. This is not what was found at Jupiter. The Probe’s Doppler wind 

experiment, which correlated Doppler shifts in the frequency of the Probe’s radio signal 

with wind speed, produced totally unexpected data. The winds below Jupiter’s cloud tops 

were fierce, blowing at speeds of up to 720 kilometers per hour (450 mph), and these 

speeds remained fairly constant to depths far below the clouds. Scientists sought to determine an energy source that would plausibly drive this wind pattern and be compatible 

with other observations, such as the Probe’s atmospheric entry point’s being on the edge 

of a desertlike Jovian hotspot. Energy sources considered included the following:

• Conversion of ortho/para states of hydrogen to attain equilibrium.

• Latent heat release by condensation of water and other compounds.

• Direct heating of the atmosphere through solar-energy absorption.

• Heat fluxes from inside the planet.90

Figure 8.10. A mosaic of Jupiter’s equatorial region, photographed in the near-infrared 

and showing Jupiter’s main visible cloud deck. The smallest resolved features are tens of 

kilometers in size. The dark hotspot near the center is a hole in the equatorial cloud layer 

and resembles the Probe’s entry site. Circulation patterns and composition measurements 

suggest that dry air is converging and sinking over these holes, maintaining their cloudfree appearance. The bright oval in the upper right of the mosaic and the other, smaller, 

bright features are examples of moist air upwellings resulting in condensation. (NASA 

photo number PIA01198)

Ortho/Para Conversion. Quantum mechanical constraints produce a coupling 

between the rotational states of molecular hydrogen and the spin states of its protons. This coupling results in two distinct spin states of molecular hydrogen. Molecules 

with antiparallel nuclear spins are termed para-hydrogen, while molecules with parallel 

nuclear spins are called ortho-hydrogen. In Jupiter’s atmosphere, ortho and para hydrogen are not in equilibrium with each other, and there are significant energy differences 

between the two states. Converting from one hydrogen state to another can release 

significant energy. One study of this energy source has indicated, however, that the 

ortho/para conversion rate likely to occur in Jupiter’s atmosphere is too slow to be a 

major factor in driving the observed Jovian winds.91 

88	JPL, “Computer Simulation Reveals Ups and Downs of Jupiter’s Winds,” JPL Media Relations Office news release,

8 September 2000, http://www.jpl.nasa.gov/releases/2000/glhotspot.html (available in Galileo—Meltzer Sources,

folder 18522, NASA Historical Reference Collection, Washington, DC).

89	JPL, “Computer Simulation Reveals Ups and Downs of Jupiter’s Winds”; Douglas Isbell and Jane Platt, “Galileo Finds

Wet Spots, Dry Spots and New View of Jupiter’s Light Show,” NASA news release 97-123, 5 June 1997, http://nssdc.

gsfc.nasa.gov/planetary/text/gal_wet_dry.txt (available in Galileo—Meltzer Sources, folder 18522, NASA Historical

Reference Collection, Washington, DC).

90	Williams, “Jupiter—At Last!” p. 353; “The Probe Story: Secrets and Surprises from Jupiter.” 

91	Michael D. Smith and Peter J. Gierasch, “Convection in the Outer Planet Atmospheres Including Ortho-Para Hydrogen

Conversion,” Icarus 116 (July 1995): 159; Young, “Galileo Probe Mission to Jupiter: Science Overview,” p. 22,783.
220	 MISSION TO JUPITER: A History of the Galileo Project Chapter 8 I jupiter approach and arrival 221

Latent Heat Release from Water Vapor Condensation. Latent heat release through 

condensation of water vapor was investigated as a possible driving force for large-scale 

convection of the Jovian atmosphere. Various researchers developed a range of models 

approximating such a system, but their results did not exhibit good agreement with observations of the Jovian atmosphere. A review of these models found that they tended to 

oversimplify the vertical structure, boundary conditions, and dynamics of Jupiter’s atmosphere so much that the models couldn’t reproduce vital properties of the atmosphere.92

Direct Solar Heating of the Atmosphere. The Probe observed high wind speeds that 

extended significantly below the region of clouds as well as the depth to which solar energy 

penetrates. The downward solar flux measured by the Probe’s net flux radiometer was 

effectively zero by the time the Probe had descended to the 10-bar pressure level, while the 

winds that the Probe observed at cloud level extended deep into the atmosphere to at least 

the 22-bar pressure level. No dynamic model has convincingly explained how solar energy 

absorbed in the upper reaches of the atmosphere would be likely to transport momentum to 

the atmosphere’s deep interior in order to maintain the wind flows there. Thus, direct solar 

heating does not appear to be a likely driving mechanism for the observed wind patterns. 

Heat Fluxes from Inside the Planet. An internal heat source could provide the 

thermal energy to drive convective motions in the atmosphere from below, leading to the 

observed zonal wind structure. Such a mechanism would explain the atmospheric flows 

that the Probe observed, in which the winds extend deep into the Jovian atmosphere. 

A heat source internal to the planet may well prove to be the dominant energy source 

for observed winds.93

Lightning and Radio-Frequency Signals

The Probe’s lightning and radio emissions detector (LRD) sought evidence of both nearby 

and distant electrical discharge events from cloud to cloud or between cloud and planet. 

The LRD was the only one of the Probe’s instruments able to perform in situ measurements of Jovian atmospheric characteristics, as well as remote sensing observations. The 

LRD searched for optical flashes from nearby electrical discharges and radio waves emitted from more distant events. 

The thick Jovian clouds that scientists thought the Probe would encounter suggested that the LRD would see many cloud-to-cloud lightning flashes, but it did not. 

Considering that the ammonia and ammonium hydrosulfide cloud decks were far more 

tenuous than predicted and that the anticipated dense water clouds were largely absent, it 

was not surprising that the LRD did not record any optical flashes. The LRD did, however, 

receive radio signals that may have been signatures of as many as 50,000 distant lightning 

strikes that took place up to an Earth diameter away. The signals indicated the existence 

of lightning strikes 100 times stronger than a typical lightning discharge on Earth. But 

based on the data that the LRD was able to collect, the occurrence rate of Jovian lightning 

events (measured in number of flashes per square kilometer of atmosphere per year) was 

only about one-hundredth the frequency of discharges on Earth. This dearth of lightning 

events may have been due to the atypical location in the Jovian atmosphere through 

which the Probe descended.94 

Thermal Structure of the Jovian Atmosphere at the Probe Entry Site

The Galileo Probe atmospheric structure instrument (ASI) conducted the first in situ study 

of the Jovian atmosphere and measured its temperatures, pressures, and densities from 

1,029 kilometers (638 miles) above the 1-bar level to 133 kilometers (82 miles) below it. 

ASI data for these thermal characteristics versus height in the Jovian atmosphere served 

as an altitude scale for other experiments. For instance, if a certain atmospheric pressure 

was observed in another experiment, this could be correlated with an altitude by using 

the ASI data. 

The primary goal of the ASI experiment was to measure thermal characteristics 

of the atmosphere below cloud level, a region that was inaccessible to remote sensing 

by flybys such as Voyager or by Earth-based instruments. A second goal of the ASI was 

to define upper-atmosphere thermal structure starting in the exosphere, a tenuous outer 

layer of the atmosphere, and continuing down to cloud-top level. The ASI found temperatures in the upper Jovian atmosphere hotter than could be explained by a heating 

source of sunlight alone. Scientists postulated the precipitation of energetic particles and 

dissipation of waves traveling through the atmosphere as two possible mechanisms for 

additional upper-atmosphere heating. Temperature oscillations observed from the tropopause (the altitude where the troposphere ends and the stratosphere begins) up to the 

highest altitude measured by the ASI were consistent with wave phenomena. The viscosity of the atmosphere would, over time, damp the amplitudes of the waves, converting 

the energy they carried into thermal energy. Scientists’ calculations show that the energy 

dissipated by the postulated waves would be comparable to the amount needed to heat 

the Jovian atmosphere to its observed levels.95

Upper-atmosphere pressure measurements showed a region that was more dense 

than expected. A surprisingly thick layer of a constant temperature of roughly -113o

C 

(171o

F) was encountered in Jupiter’s stratosphere, from an altitude of 290 kilometers (180 

miles) down to 90 kilometers (56 miles). A second isothermal (constant-temperature) 

region was found as well, just above the tropopause. This region was 25 kilometers (16 

miles) thick, with a temperature of about -161o

C (-258oF). In the lower regions of the 

atmosphere measured by the Probe, temperature increased with pressure, although at a 

slightly slower rate than expected.96

92	Peter J. Gierasch, “Jovian Meteorology: Large-Scale Moist Convection,” Icarus 29 (December 1976): 445–454.

93	Young, “Galileo Probe Mission to Jupiter: Science Overview,” pp. 22,783–22,785.

94	K. Rinnert, L. J. Lanzerotti, M. A. Uman, G. Dehmel, F. O. Gliem, E. P. Krider, and J. Bach, “Measurements of Radio

Frequency Signals from Lightning in Jupiter’s Atmosphere,” Journal of Geophysical Research 103, no. E10 (25

September 1998): 22,979–22,992; Klaus Rinnert and L. J. Lanzerotti, “Radio Wave Propagation Below the Jovian

Ionosphere,” Journal of Geophysical Research 103, no. E10 (25 September 1998): 22,993; “The Probe Story: Secrets

and Surprises from Jupiter.”

95	Seiff et al., “Thermal Structure of Jupiter’s Atmosphere,” p. 22,857; JPL, “Probe Science Results,” section 7 in Galileo: 

The Tour Guide, June 1996, p. 54, http://www.jpl.nasa.gov/galileo/tour/6TOUR.pdf (available in Galileo—Meltzer

Sources, folder 18522, NASA Historical Reference Collection, Washington, DC). 

96	Seiff et al., “Thermal Structure of Jupiter’s Atmosphere”; JPL, “Probe Science Results.”

The Jupiter Orbiter tour was more than simply a 

series of flybys of the planet’s moons, where photographs 

were taken and fields measured. The tour was the culmination 

of an exploration plan developed over 20 years earlier to study the Jovian system thoroughly. The multifunction Galileo spacecraft was intended to explore the various sectors 

of the Jupiter system in an integrated manner, examining interactions among all sectors 

so as to better understand the whole. The first part of the exploration plan, involving in 

situ measurements of the parent planet, had been accomplished by Galileo’s Atmospheric 

Probe. The second part of the plan depended on the Orbiter to analyze comprehensively 

the planet’s satellites, fields and particles, and magnetosphere1

. The Orbiter concentrated 

its satellite observations on Jupiter’s four major moons, the ones that Galileo Galilei discovered nearly four centuries ago, although some of the Orbiter’s time was also spent 

observing several of Jupiter’s smaller moons.

Discovery of the Galilean Satellites

Exploration of the Jovian system began long before the 20th century. During the summer 

of 1609, a mathematics lecturer at the University of Padua named Galileo Galilei heard of 

a new optical instrument, a telescope, that had been displayed in Holland. He constructed 

one for himself and verified its ability to discern “sails and shipping that were so far off 

1	 JPL, “The Tour,” section 8 in Galileo: The Tour Guide, June 1996, p. 59, http://www.jpl.nasa.gov/galileo/tour/8TOUR.

pdf; Torrence V. Johnson interview, tape-recorded telephone conversation, 31 July 2001.

Chapter 9

The Orbiter Tour

We’re able to observe lava flowing on a surface that is similar to what we 

had here on Earth two billion years ago. We can almost provide a window 

into our own past. 

—Dr. Eilene Theilig, Galileo Project Manager, 8 May 2001
224	 MISSION TO JUPITER: A History of the Galileo Project Chapter 9 I The orbiter tour 225

with the parent planet could be repeatedly evaluated at close range. The results of these 

evaluations have sparked the imaginations of scientists around the world and have radically altered our previous picture of the Jovian system.

Attaining Scientific Goals During the Tour

Each Galileo orbit of Jupiter usually included a close encounter with one of Jupiter’s 

satellites8

 and a cruise period that typically lasted a couple of months.9 During each 

encounter period, the Orbiter collected a range of data at high rates. The Orbiter returned 

some real-time data to Earth on fields-and-particles characteristics, but it had to store 

most of the other data on its tape recorder due to limitations in the transmission rate of 

the low-gain antenna. The stored data included satellite images, ultraviolet and infrared spectra, and additional fields-and-particles measurements, especially those taken at 

the closest approach to the satellite. Subsequent cruise periods gave the spacecraft the 

chance to play back and send the recorded data to Earth, as well to take additional fieldsand-particles measurements. 

Project management established three working groups for collecting three types 

of scientific data: magnetospheric, atmospheric, and satellite. In situ measurements of 

Jupiter’s magnetosphere were acquired nearly continuously during the Orbiter tour, but 

with higher resolution during satellite encounters as well as during the spacecraft’s journey down Jupiter’s magnetotail (which occurred between the close flybys in orbits C9 

and C10). The spacecraft performed remote sensing of Jupiter’s atmosphere and of the 

planet’s satellites primarily within a few days of each satellite flyby.10

Each of the scientific working groups had its own aims and needs for acquiring 

and storing data. Unfortunately, these needs could not always be met simultaneously by 

the spacecraft. This shortcoming was due in part to the limited data-storage capacity of 

the tape recorder and the slow transmission rate of the low-gain antenna. These factors 

restricted the amount of data that could be gathered during each satellite encounter and 

returned to Earth before the next encounter. Another restriction was the limited amount 

of memory available to retain computer code sequences. Such sequences directed the 

spacecraft to perform the necessary operations for attaining scientific objectives (such 

as orienting the scan platform in a specific direction). Due to the Orbiter’s operational 

limitations, negotiations and bargaining among the scientific working groups for access 

to the spacecraft became an important part of how mission business was conducted.11

Mission Support Activities During Satellite Encounters

Key to the success of the Galileo project were the vital activities performed by the engineers, scientists, and technicians of the mission support staff, whose job it was to ensure 

that it was two hours before they were seen with the naked eye.” Galileo was not satisfied with the capabilities of his first model telescope, however, and he quickly developed 

improved versions that could observe new features on the Moon and find many more 

stars in the night sky than had been seen before. It was on one of the evenings when he 

was searching the sky that he saw something that startled him:2

On the seventh day of January in this present year 1610, at the 

first hour of night, when I was viewing the heavenly bodies with a 

telescope, Jupiter presented itself to me; and because I had prepared a 

very excellent instrument for myself, I perceived (as I had not before, 

on account of the weakness of my previous instrument) that beside 

the planet there were three starlets, small indeed, but very bright.3

Galileo continued to watch these starlets over the next several days. At first, he 

could not understand how, overnight, Jupiter could move from a position west of two 

of them to a new position east of all three of them. The movements of Jupiter that had 

been recorded by other astronomers predicted that the planet should have moved in the 

opposite direction. Galileo’s initial thoughts were that the other astronomers had somehow miscalculated Jupiter’s movement.4

On 11 January 1610, a fourth starlet appeared. After a week of observation, 

Galileo discovered that these four bright bodies never left the vicinity of Jupiter; instead, 

they appeared to be carried along with the planet as it journeyed through the sky. The 

starlets did change their positions with respect to each other and to Jupiter, though. Finally, 

Galileo deduced that he was not observing stars, which do not change their positions relative to each other, but planetary bodies that were in orbit around Jupiter. By showing that 

not all bodies revolved around Earth, Galileo’s revelation helped to support the Copernican 

view of the solar system, which places the Sun rather than Earth at the center.5

The planetary bodies that Galileo discovered turned out to be Jupiter’s four 

largest moons, now known as the Galilean satellites. They were called Io, Europa, 

Ganymede, and Callisto, which are the Greek names of four lovers of the god Zeus in 

Greek mythology (or Jupiter in Roman mythology).6,7

Although Galileo Galilei discovered Jupiter’s four major moons, his telescope 

was capable of seeing them only as points of light. Subsequent telescopes and space 

vehicles garnered much valuable data regarding these moons, but it was not until the 

Galileo Orbiter tour that the different characteristics of these bodies and their interactions 

2

 J. R. Casani, “A Name for Project Galileo,” JPL interoffice memo GLL-JRC-78-53, 6 February 1978, John Casani 

Collection, Galileo Correspondence 12/77–2/78, folder 43, box 5 of 6, JPL 14, JPL Archives. Biographical sketch 

of Galileo Galilei included in this memo was written by Mary Jo Smith, Galileo program engineer for NASA 

Headquarters.

3

 Galileo Galilei, The Starry Messenger (Sidereus Nuncius), 1610. One of many places this book can be accessed is at 

The History Guide, http://www.historyguide.org/earlymod/starry.html, 2002. 

4

 Galileo to Jupiter: Probing the Planet and Mapping Its Moons, JPL Document No. 400-15 7/79 (Washington, DC, GPO 

No. 1979-691-547), p. 1. 

5 The Discovery of the Galilean Satellites,” JPL Galileo Project Home Page, p. 1, http://www.jpl.nasa.gov/galileo/

ganymede/discovery.html (accessed 12 March 2000). 

6 Casani, “A Name for Project Galileo.”

7 NASA, “Welcome to the Planets—Glossary,” Planetary Data System, last updated 15 November 2004, http://pds.jpl.

nasa.gov/planets/special/glossary.htm (accessed 12 June 2005).

8 During part of Galileo’s fifth orbit after Jupiter Orbit Insertion (JOI), the spacecraft had to be out of communication 

with Earth due to a solar conjunction; no satellite encounter was planned for that orbit. 

9 An exception to this occurred on the first orbit after JOI, in which Galileo’s orbit was very elongated and its cruise 

period was nearly half a year in duration. 

10 JPL, “The Tour,” pp. 60–61.

11 Ibid., p. 61.
226	 MISSION TO JUPITER: A History of the Galileo Project Chapter 9 I The orbiter tour 227

At 6:00 on Sunday evening, 5 August 2001, the spacecraft seemed to be performing as hoped as it neared its closest approach to Io. But the moon exists within an intense 

radiation environment whose high-energy particles had degraded instrument and spacecraft performance repeatedly during past encounters. Mission support was concerned 

about additional radiation incidents. Each near-Jupiter orbit increased the odds of more 

serious damage from exposure to the planet’s radiation belts. Galileo’s solid state imaging 

(SSI) camera was one component that had not been operating at full performance. The 

camera’s passage through radiation belts had resulted in completely saturated, overexposed images. Project engineers had developed procedures for minimizing the loss of 

images that had plagued the camera since summer 2000. Cycling the power—turning it 

off, then on again—sometimes helped, at least temporarily. So did reloading the camera’s 

software. Mission support personnel implemented these procedures eight times during 

encounter I31, just before key observations.15

Another Galileo component affected by radiation was the star scanner, whose 

measurements determined spacecraft orientation. According to Gerry Snyder, a systems 

engineer and member of the mission support team, Galileo’s attitude-control system 

normally used light collected by the star scanner from three or four stars to accurately 

determine the direction in which the spacecraft was pointing. But in the intense radiation environment close to Jupiter, high-energy electrons flooded Galileo’s star scanner 

detector and masked the light from the fainter stars. This noise could cause serious problems. Spacecraft and sequence team chief Duane Bindschadler explained that when the 

background counts from radiation went up, the star scanner saw intermittent flashes. The 

attitude-control system software could mistake the flashes for dim stars and try to act on 

incorrect knowledge. Because the attitude-control system would not know which way 

the spacecraft was pointing, the system would crash, restart, and crash again. Meanwhile, 

the scan platform would freeze up. The spacecraft computer would hold it stationary to 

prevent any instruments from getting damaged. During these times, the remote sensing 

instruments mounted on the scan platform would not be able to perform observations 

that Galileo’s scientists were counting on, and much of the encounter would be wasted. 

To avoid such problems during passes through high-radiation regions, mission 

personnel commanded the star scanner to temporarily lock in on only one bright star—a 

star with a signal well above the noise produced by the radiation. Then the attitude-control 

software would not be confused and could safely maintain Galileo’s orientation. The star used 

during I31 was Alpha Eridani, the brightest star in the constellation of Eridanus, the River.16

Jupiter Occultation and Deep Space Network Limitations

On I31, the relative positions of spacecraft, Jupiter, and Earth allowed less time than other 

encounters for mission support staff to deal with problems. Gerry Snyder explained that 

the “health and safety of the vehicle.”12 Their activities intensified just before and during 

a close encounter. Mission support activities surrounding an Io encounter in 2001 serve 

as examples of typical actions taken during a flyby. They give a picture of the many data 

streams that teams had to watch and the various actions they needed to take. 

Tape-Recorder Preparation

In the week before the 5 August 2001 encounter with Io (the abbreviation for this 

encounter was “I31”), mission support guided the spacecraft through several preparatory 

housekeeping tasks. Routine maintenance of the on-board tape recorder was a critical 

task. Playback and transmission of the recorder’s data from the previous flyby had to be 

stopped and the tape wound halfway down its length and back again. This action helped 

prevent the tape from sticking to the recorder’s head, as happened in October 1995 when 

Galileo was first approaching Jupiter. A tape jam would have eliminated most of the 

spacecraft’s data-storage potential.13

Targeting Maneuvers

Mission support closely oversaw the spacecraft’s final targeting maneuvers on its approach 

to a satellite. This oversight involved precisely controlled firings of Galileo’s 12 thrusters to ensure that the spacecraft reached its scheduled satellite rendezvous at the correct 

time and place. Galileo’s thrusters delivered only a modest amount of propulsion (about 

2.2 pounds of thrust each) and had to fire for several hours to nudge the 1,300-kilogram 

(2,900-pound) spacecraft onto a slightly different trajectory. With some thrusters pointing 

forward, some backward, and some to the sides, the choice of which to fire and when 

determined in what direction the spacecraft moved. Mission support personnel also used 

the thrusters to rotate the spacecraft, with no change to its orbital path, in order to point 

its antenna in new directions.

Typically, the final targeting maneuvers changed Galileo’s speed by only a few 

tenths of a meter per second. This was minuscule compared to the craft’s speed of 7.1 

kilometers per second (16,000 mph) as it flew by Io.14

Coping with Radiation Effects

Galileo was a robotic spacecraft that needed little attention when working correctly. Its 

control sequences had all been programmed into its computer, and if it followed them without 

incident, the crew mainly sat back and tracked the craft’s progress. But when Galileo malfunctioned, the crew got busy and sometimes had to work marathon hours through the night. 

15	Guy Webster, “Spacecraft to Fly Over Source of Recent Polar Eruption on Io,” JPL news release 2001-161, July 2001;

“Today on Galileo—Saturday, August 4, 2001: The Encounter Begins,” SpaceRef.com Web site, 4 August 2001,	

http://www.spaceref.com/news/viewsr.html?pid=3380 (accessed 13 June 2005) (available in Galileo—Meltzer Sources,

folder 18522, NASA Historical Reference Collection, Washington, DC).

16 Gerry Snyder (mission support systems engineer) and Duane Bindschadler (spacecraft and sequence team chief)

interviews, JPL, 5 August 2001; “This Week on Galileo: The Io 31 Encounter Begins,” JPL DOY 2001/216, 4 August 2001,

http://www2.jpl.nasa.gov/galileo/news/thiswk/today010804.html (available in folder 18522, NASA Historical Reference

Collection, Washington, DC).

12 Nagin Cox interview, tape-recorded conversation, 15 May 2001.

13 “This Week on Galileo: Galileo Continues Preparations for Next Week’s Io Flyby,” JPL DOY 2001/211-215, 30 July 

2001, http://www2.jpl.nasa.gov/galileo/news/thiswk/today010730.html; “This Week on Galileo July 30–August 3, 

2001,” SpaceRef.Ca, 2 August 2001, http://www.spaceref.com/news/viewsr.html?pid=3369 (available in folder 

18522, NASA Historical Reference Collection, Washington, DC); “Today on Galileo—Sunday, August 5, 2001—Day 2 

of the Io 31 Encounter,” SpaceRef.Ca, 5 August 2001, http://www.spaceref.ca/news/viewsr.html?pid=3385 (available 

in folder 18522, NASA Historical Reference Collection, Washington, DC). 

14 “This Week on Galileo—July 30–August 3, 2001.” 
228	 MISSION TO JUPITER: A History of the Galileo Project Chapter 9 I The orbiter tour 229

Galileo had been occulted behind Jupiter for 3 hours (a fairly long occultation) and out 

of radio contact with Earth until about 6 p.m. PDT on 5 August—just hours before the 

craft’s closest approach to Io. During this occultation, mission support personnel instructed 

the spacecraft to stay in “fill data” mode, which meant that the craft ceased sending valuable 

information to Earth until after occultation was over and data could actually be received.17

At about 8 p.m. PDT, 2 hours before the closest approach, the craft again traveled out of radio contact with Earth, but for a different reason from before. It flew into 

a region of space that was not in sight of two of NASA’s three Deep Space Network 

(DSN) stations (the ones in California and Australia). The large antennas of NASA’s DSN

typically provide the communication link with the Agency’s interplanetary spacecraft, 

which included Voyagers 1 and 2, Ulysses, the Mars Global Surveyor, Galileo, and others. 

Normally, at least one of the DSN’s three stations stayed in contact with Galileo. During 

the close encounter, only the station in Spain had a line-of-sight view of Galileo and could 

potentially communicate with it, but the Spanish station’s large antenna that typically 

tracked Galileo had been put temporarily out of service. It was being upgraded to handle 

the increased number of missions anticipated for 2003 and 2004. Data concerning Galileo’s 

status during the I31 flyby had to be received several hours afterward by the DSN’s other 

stations, and any rescue attempts on the part of mission support had to wait until then. 

The Deep Space Operations Center

During flybys, mission support crews carefully monitored many types of data to determine the health of the spacecraft. Information received by the three DSN stations was 

sent to JPL’s Deep Space Operations Center. This facility is called the “darkroom” because 

of its subdued lighting, which allows better viewing of its banks of monitors. One bank 

of monitors reports on the state of communications between JPL and the DSN. Another 

bank enables operators to check that the appropriate data get routed to support personnel 

for the right project—the darkroom monitors track many interplanetary missions, and it 

would not do for Martian data to get sent to Galileo personnel or vice versa.18

Galileo data arrived as packets of information, signified on the screens by various 

number and letter codes. The darkroom’s computers are programmed to display certain 

signs when data change beyond prearranged limits. The data codes might suddenly turn 

bold, or the surrounding screen might change to red or some other color. If the out-ofspec data happened to be, for instance, the temperature of the spacecraft, mission support 

needed to determine whether the craft was heating up or cooling down, then take appropriate actions before sensitive instruments were damaged or a flyby was ruined.19

Galileo sent information down to Earth in compressed packets of data that 

allowed more information than originally planned to be transmitted each second. Gerry 

Snyder, who had started with the Galileo project in the late 1970s but then left, was 

brought back on the project in 1992 when the trouble with the high-gain antenna occurred 

and data flow through the low-gain antenna had to be maximized. The spacecraft had 

been handling data using an inefficient system called “time division multiplex” (TDM). If, 

for instance, one of the spacecraft’s instruments was generating the same data time after 

time, TDM would send the repetitive information again and again to Earth, wasting valuable transmitting time. This procedure would not have been a problem if the high-gain 

antenna had been operating, for it would have sent data at a high enough rate (134,000 

bits per second) for Earth to receive all the data it wanted, even if some of the information 

was repetitive. But the low-gain antenna could send only a trickle of information (about 

10 to 40 bits per second), and it all had to count. What Snyder did when he rejoined 

the Galileo project was to implement a “packetized data” system in which the spacecraft 

computer kept a watch on the data flow and sent a packet to Earth only when there was 

something new. The packetized system selected just the data that mission personnel on 

Earth needed to reconstruct what the spacecraft was doing and observing. 

Using the packetized-data approach, an image taken by Galileo’s SSI camera could 

be reconstructed on Earth with very good visual quality, employing as little as one-eightieth 

of the data that TDM would have sent. The Galileo spacecraft had only two or so months 

after an encounter to send what it had collected to Earth before the tape recorder had to 

store information from the next encounter. Because the spacecraft had to transmit data at 

a very slow rate, not all the information collected could actually be sent. Tough decisions 

had to be made as to what to transmit and what to erase. Packetizing data was important 

because it allowed significantly more of the information collected to be sent on to Earth.20

Galileo’s Data Transmissions

The data Galileo transmitted to Earth were of two basic types: 1) engineering data that 

related to the spacecraft’s health and operating performance and 2) science data, which 

pertained to the various experiments that Galileo was conducting. Mission support personnel studied the former carefully and passed on the latter to the various science teams 

around the country and in Europe. Mission support sent on science data packets without 

ever actually looking at the data they contained. Such data were very sensitive and carefully protected by the science team. That team might have been waiting for the data for 

many years and had likely spent a large sum of money building the instrumentation for its 

experiment. The team wanted to be sure that it controlled access to its own data.21

Navigating and Orienting Galileo

For mission engineers to control the Orbiter’s trajectory accurately, they needed to analyze both radio tracking data and optical data (positions of stars and satellites used as 

reference points). This information was then used to make the needed modifications in 

spacecraft flight direction and orientation.22

One of the critical “housekeeping” chores of the mission was to keep the spacecraft’s low-gain antenna (LGA) pointed within 4 degrees of Earth in order to maintain 

radio contact. For optimal signal reception on Earth, it was better to maintain the LGA

20	Gerry Snyder interview.

21 Ibid.

22 JPL, “The Tour,” p. 62.

17	Gerry Snyder interview.

18 Galileo mission support crew interview, JPL, 5 August 2001; JPL, “Galileo Millennium Mission Status,” JPL Media Relations

Office news release, 6 August 2001.

19 Galileo mission support crew interview.
230	 MISSION TO JUPITER: A History of the Galileo Project Chapter 9 I The orbiter tour 231

direction within 1 degree of Earth. Two of the data streams that mission support personnel 

watched related to spacecraft attitude control and indicated how far off the spacecraft’s 

antenna was from where it had been commanded to point.23

Spacecraft attitude data were also used in turning the Orbiter to allow its instruments to view different parts of the sky that might otherwise be blocked by the science 

boom or the main body of the spacecraft. Such reorientations of the Orbiter were called 

“spacecraft inertial turns” (SITURNS) or, more commonly, “science turns.” The number 

of science turns that could be made was severely limited by the amount of propellant 

aboard. Only 20 kilograms of propellant were budgeted for science turns during the mission; a 90-degree turn and return used about 3 kilograms.24

Preventing Spacecraft Problems

Mission support engineers such as Gerry Snyder had to keep tabs on many different data 

streams and quickly spot potential spacecraft trouble or conditions that would interfere 

with science experiments. One data stream relayed spin error, which was the difference 

between the spacecraft’s actual and intended spin rates. The “star code” record gave an 

indication of what Galileo’s star scanner navigation sensor was doing, while the “running 

count”—the number of background pulses per unit of time that the star scanner was 

seeing—related to radiation levels around the spacecraft. In low-radiation regions, normal 

background counts ran about 170 to 270 per second, allowing the star scanner to track both 

faint and bright stars. But when the counts exceeded 1,000 per second, many faint stars 

were masked by the background and the scanner could track only bright stars reliably.25

The “slew count” was a measure of the times that the spacecraft’s scan platform, 

on which the remote sensing instruments were mounted, had been reoriented to allow 

an instrument to make a particular observation. Other data indicated whether it would be 

necessary to perform a “repulsive maneuver,” in which one or more of the science instruments 

had to be turned off or put into safe modes because, for instance, fumes from engine firing 

might present risks to them. The spacecraft’s central data system had fixed responses for such 

occurrences, like turning off an instrument’s high voltage in order to prevent arcing.26

Another data stream provided the “Doppler residuals,” important to the navigation team in determining how close Galileo was to its aim point for the flyby. There 

was also a sequence of events (SOE) log that listed all spacecraft commands and ground 

events (for instance, actions that the DSN stations took).27

Using Gravity-Assists To Minimize Propellant Requirements

Project engineers had the task of “steering” the spacecraft along its satellite tour so that 

it could fly by one moon after another with a minimum expenditure of fuel. The better 

the engineers were at their job, the longer the craft could operate (assuming that its 

other systems held together). To minimize propellant use, each satellite encounter was 

designed to make optimal use of the moon’s gravitational force to alter the course of the 

spacecraft toward its next target. Only a small amount of propellant was needed to finetune the Orbiter’s trajectory. The entire Prime Mission satellite tour required the thrusters 

to supply a total change in speed of only about 100 meters per second, about 60 times 

less than if gravity-assists had not been used. Galileo staff viewed this trajectory design as 

a “10-cushion shot in a celestial billiard ball game,” with the caveat that in this case, small 

corrections in billiard ball direction could be made along the way.28

The Prime Mission

The trip from Earth to Jupiter, the Probe’s exploration of the Jovian atmosphere, and an 

Orbiter tour consisting of 11 orbits of Jupiter constituted Galileo’s Prime Mission. Table 

9.1 lists major milestones of the Prime Mission.

Table 9.1. Overview of Galileo’s Prime Mission.29

Launch 18 October 1989

Probe Release 12 July 1995 (PDT)

Jupiter Arrival Day 7 December 1995

Probe atmospheric penetration 7 December 1995

Prime Mission Orbiter tour (11 orbits of Jupiter) 7 December 1995 to 7 December 1997

On Jupiter Arrival Day (7 December 1995), the Galileo spacecraft was given 

a gravity-assist from Io and then subjected to the Jupiter orbit insertion (JOI) maneuver, which slowed the spacecraft down and let it be “caught” by the planet. These two 

actions placed the Orbiter on its proper trajectory to tour the Jovian moons. The Jupiter 

orbit insertion maneuver involved an orbit around the planet, which is referred to as the 

spacecraft’s “zeroth” orbit. The spacecraft’s “first,” and by far longest, orbit around Jupiter 

followed the JOI and lasted nearly seven months. On 27 June 1996, this initial orbit culminated in a close encounter with Ganymede, the largest of the four Galilean satellites.

Figure 9.1 depicts the “flower petal” pattern of the 11 Jovian orbits on Galileo’s 

Prime Mission. This tour included four close encounters with Ganymede, three with 

Europa, and three with Callisto. No Io encounters were planned for the Prime Mission 

(besides the flyby on Arrival Day) because mission scientists feared that the high radiation 

levels so close to Jupiter could damage the spacecraft and possibly end the project. 

28	JPL, “The Tour,” p. 62.

29 Project Galileo Outreach Office, “The Galileo Mission at Jupiter—Fact Sheet,” Bringing Jupiter to Earth, revision	

3, November 1998, http://www.jpl.nasa.gove/galileo.

23	Gerry Snyder interview.

24 JPL, “The Tour,” p. 65.

25 Galileo mission support crew interview.

26 Gerry Snyder interview.

27 Brad Compton, Galileo mission control team chief, interview, JPL, 5 August 2001.
232	 MISSION TO JUPITER: A History of the Galileo Project Chapter 9 I The orbiter tour 233

After the first Jupiter orbit of seven months, subsequent orbits were much 

shorter, ranging from one to two and a half months. As figure 9.1 shows, orbits 2 through 

11 followed much less eccentric (elongated) elliptical paths than the first orbit. The Prime 

Mission ended in December 1997, two years after Jupiter arrival.30

Figure 9.1. “Flower petal” plot of the Prime Mission Orbiter tour, depicting close encounters 

with satellites. Note the eccentricity of the first orbit as compared to the others. Due to a solar 

conjunction, the spacecraft did not have a close encounter with any satellites on its fifth orbit. 

(Adapted from JPL image number P45516B)

Naming the Orbits of Jupiter

The Galileo mission used a two-character code to specify each orbit. The first character 

was the first letter of the name of the moon that would receive a flyby on the orbit, while 

the second character indicated the number of the orbit. Table 9.2 lists the code name for 

each Jupiter orbit on the Prime Mission satellite tour, the satellite encountered during the 

orbit, and the date of closest approach to the satellite. Note that on the fifth orbit (J5), 

no close satellite encounter occurred.31 Galileo was out of communication during part of 

J5 due to a solar conjunction, when the Sun was between Jupiter and Earth, so mission 

engineering staff did not plan a close flyby for this orbit (figure 9.2 illustrates a solar conjunction situation).32 

SUN 

Sun-Earth-Craft 

(SEC) Angle 

Earth

Jupiter

33	JPL, “The Tour,” p. 60.

30	JPL, “The Tour,” p. 59.

31 Ibid., p. 60.

32 “Galileo Highlights of Jupiter Orbital Tour 92-14a,” Project Galileo Homepage, http://www.jpl.nasa.gov/galileo/

tourhilites.html (accessed 19 May 2001).

Table 9.2. Prime Mission satellite tour. Includes the naming convention for Jupiter orbits and 

dates of closest approach to the designated satellite on each orbit.33

Orbit Satellite Encounter Closest Approach

G1 Ganymede 27 May 1996

G2 Ganymede 6 September 1996

C3 Callisto 4 November 1996

E4 Europa 19 December 1996

J5 solar conjunction no close flyby

E6 Europa 20 February 1997

G7 Ganymede 5 April 1997

G8 Ganymede 7 May 1997

C9 Callisto 25 June 1997

C10 Callisto 17 September 1997

E11 Europa 6 November 1997

Figure 9.2. Solar conjunction occurs during the Galileo mission when the Sun lies directly 

between Jupiter and Earth. The Sun is a strong source of electromagnetic activity, and it wreaks 

havoc with the spacecraft’s radio signal, essentially reducing the spacecraft’s data-transmission 

rate to Earth to almost zero for the two and a half weeks spanning the conjunction. Mission 

planners and telemetry engineers define this problem time as occurring when the Sun-EarthCraft (SEC) angle is less than 7 degrees (see figure), although a relatively “quiet” Sun can allow 

data to be successfully transmitted to Earth at SEC angles as small as 3 to 5 degrees. During 

Galileo’s primary mission, solar conjunction periods occurred from 11 to 28 December 1995 and 

11 to 28 January 1997. During the Galileo Europa Mission (GEM), solar conjunctions occurred 

from 14 February 1998 to 4 March 1998 and from 22 March 1999 to 10 April 1999. A Galileo 

Millennium Mission (GMM) solar conjunction period ran from 28 April 2000 to 17 May 2000.
234	 MISSION TO JUPITER: A History of the Galileo Project Chapter 9 I The orbiter tour 235

The Galileo Europa Mission

The Galileo project would have been considered a success even if the spacecraft had stayed 

operational only through the end of the Prime Mission on 7 December 1997, two years after 

Jupiter Arrival Day. The Orbiter was an extremely robust machine, however, with many 

backup systems. It showed no sign of quitting at the end of the Prime Mission, so it was 

given a highly focused set of new exploration objectives, defined in part by the findings of 

the Prime Mission. These new objectives centered on investigating Europa in great detail, 

and the new mission was, appropriately, called the “Galileo Europa Mission” (GEM). Mission 

objectives were not limited to Europa, however; they included analyses of other satellites, as 

well as of Jovian fields and particles and atmospheric characteristics. During GEM, some of 

the most important and spectacular observations of the volcanic moon Io were taken.34

GEM ran for slightly over two years, from 8 December 1997 to 31 December 

1999. It was a low-cost mission with a budget of only $30 million.35 At the end of the 

Prime Mission, most of the 200 Galileo staff members, including Project Manager Bill O’Neil, 

left for other assignments. The remaining bare-bones crew, about one-fifth the size of the 

Prime Mission, was left to run GEM and achieve the objectives of four separate studies:

• Europa campaign.

• Io campaign.

• Io plasma torus study.

• Jupiter water study.

Europa Campaign

The spacecraft analyzed Europa’s crust and atmosphere and searched for further evidence 

of a past or present ocean beneath the moon’s ice crust. The Galileo team compared 

GEM image data with previous images and identified surface changes and signs of 

spewing ice volcanoes that might indicate the presence of a subsurface ocean. Because 

a flowing, salty ocean can generate a magnetic field, the Orbiter needed to determine 

whether the magnetic signals near Europa were generated from within the moon. By 

measuring variations in the pull of the moon’s gravity, the thickness of the ice crust and 

depth of a possible ocean were estimated. 

The Europa campaign involved a one-year study of the moon that included 

eight consecutive close flybys. During these encounters, the Orbiter conducted extensive 

remote sensing and fields-and-particles observations. A timeline for the Europa campaign 

and other parts of GEM is included in table 9.3.36

Table 9.3. Galileo Europa Mission key events. 

Event Dates Orbits Closest Approach in 

Kilometers (Miles)

Europa campaign 16 December 1997 to 4 

May 1999 

8 orbits: E12–E19 201 km (125 mi), 16 

December 1997 (E12)

Jupiter water/Io torus 

studies

5 May to 10 October 1999 4 orbits: C20–C23 467,000 km (290,000 

mi), 14 September 

1999 (C23)

Io campaign 11 October to 31 

December 1999

2 orbits: I24–I25 300 km (186 mi), 26 

November 1999 (I25)

The mission’s engineering staff expected to collect Europa images with resolutions better than 50 meters. The spacecraft flew close enough to the moon that it was able 

to take in situ sampling of its very tenuous atmosphere.

Jupiter Water Study

GEM’s agenda included making detailed observations of storm and wind patterns in 

Jupiter’s atmosphere and mapping the water distribution. Besides contributing to our 

knowledge of Jovian meteorology, this study also helped scientists better understand 

Earth’s weather system.37 The Orbiter flew the closest to Jupiter that it had since Arrival 

Day. In particular, it sought to examine the billowing thunderstorms that grow to heights 

several times those on Earth. The craft studied the water circulation patterns in the Jovian 

atmosphere’s top layers that cause large areas of the surface to be drier than our Sahara 

Desert while other parts receive rainfall similar to that in Earth’s tropical regions. 

Io Plasma Torus Study

The Orbiter explored and mapped the density of the donut-shaped cloud of charged particles that surrounds Io’s orbit. During passes through this cloud, the spacecraft mapped 

the density of the sulfur streaming from Io’s erupting volcanoes. The spacecraft also measured the sodium and potassium that particles accelerated by Jupiter’s rotating magnetic 

field “sand-blast” off the moon’s surface.38

34	“Galileo Europa Mission (GEM) Fact Sheet,” JPL Galileo Home Page, http://www.jpl.nasa.gov/galileo/gem/fact.html 

(accessed 31 March 2000).

35 “Ice, Water, and Fire: The Galileo Europa Mission,” extracted from The Planetary Report (January/February 1998, courtesy

of the Planetary Society), Galileo Europa Mission (GEM) Home Page, http://www.jpl.nasa.gov/galileo/gem/gem1.html

(accessed 31 May 2001).

36	“GEM Fact Sheet”; “Ice, Water, and Fire: The Galileo Europa Mission”; JPL, “The Europa Mission—Exploring Through

1999,” JPL Galileo Home Page, http://www.google.com/search?q=cache:KSWkENJMpCQJ:www2.jpl.nasa.gov/galileo/

Page_2_GEM_Fact_Sheet.pdf+The+Galileo+Europa+Mission+%E2%80%93+Exploring+Through+1999&hl=en&ie=UTF

-8 (accessed 20 November 2004) (available in folder 18522, NASA Historical Reference Collection, Washington, DC).

37 “GEM Fact Sheet”; “The Galileo Europa Mission—Exploring Through 1999.”

38 “GEM Fact Sheet”; “Ice, Water, and Fire: The Galileo Europa Mission.”
236	 MISSION TO JUPITER: A History of the Galileo Project Chapter 9 I The orbiter tour 237

Io Campaign

Now that the Prime Mission was successfully completed and the Orbiter still operated 

well, mission engineers decided to attempt close flybys and a detailed examination of Io. 

During the first Jupiter orbit of the Prime Mission, the Orbiter’s perijove (its point of closest approach to Jupiter) had been raised to keep the craft out of the most intense parts 

of Jupiter’s radiation belts (through which Io orbits). The radiation intensity in those belts 

exceeds human fatality levels, and there was a high risk that traveling through the belts 

would damage the Orbiter’s equipment and possibly end the mission. For the spacecraft 

to attempt close flybys of Io at this point, its perijove had to be reduced. This was done 

through a series of maneuvers that involved four Callisto encounters. Thus, the Galileo 

Europa Mission provided opportunities to study at close range not only Europa and Io, but 

Callisto as well. For six months in 1999, Galileo used gravity-assists from Callisto, as well as 

fine tuning from the craft’s thrusters, to gradually halve its closest approach to Jupiter.39

The first Io flyby occurred in October 1999. The spacecraft was still operating after 

passing through the high-radiation region near the moon, and so it flew by Io again six 

weeks later. The Io campaign conducted a comprehensive examination of the satellite’s volcanism, atmosphere, and magnetosphere. Project planners obtained high-resolution images 

and a compositional map of the moon. The Orbiter was able to sample and analyze volcanic particulates and, as at Europa, particles from Io’s tenuous atmosphere. The spacecraft 

also made measurements to determine whether Io generated its own magnetic field.40

GEM Power, Propellant, and Data-Recording Issues

Project engineers predicted that throughout GEM, the Orbiter’s radioisotope thermoelectric generators would be able to supply ample power for spacecraft functions and that 

sufficient propellant would remain to power the craft’s thrusters. However, they thought that 

by GEM, the Orbiter’s tape recorder would have surpassed its design limit for starts and stops. 

GEM’s scope would be significantly reduced if the tape recorder failed. In such an event, project 

engineers would load the Orbiter’s on-board computer with a program that would allow the 

craft’s instruments to receive and transmit a severely limited amount of data in real time.41

Organizational and Operational Changes During GEM

To meet its scientific goals with a bare-bones budget of $15 million per year, GEM

employed only 20 percent of the original personnel to operate the Orbiter and analyze 

its reduced data stream. Engineering and science teams streamlined and automated their 

operations and software. On each flyby, the spacecraft took only two days of data versus 

the seven days it had taken during the Prime Mission. Minimal Jovian magnetic field data 

were collected. Orbiter turning maneuvers were kept to a minimum. The GEM team did 

not include the expertise to deal with unexpected problems, as the Prime Mission had. 

When issues arose, specialists who had gone on to other missions were temporarily 

brought back and placed on “tiger teams” to work through the problems quickly.42

The Galileo Millennium Mission

Because the Orbiter spacecraft continued to operate so well, a further extension to the 

original project, the Galileo Millennium Mission (GMM), was added to pursue answers 

to key questions raised during GEM. The original GMM schedule ran from January 2000 

through March 2001, but it was then extended to the end of mission operations in January 

2003.43 The spacecraft met its demise in September 2003, when its trajectory took it on a 

collision course toward Jupiter and it burned up in the planet’s atmosphere.44

GMM conducted additional investigations of Europa, including a magnetic field 

measurement key to detecting the presence of liquid water. GMM also added to our 

knowledge of Io, studied the dynamics of Ganymede’s unique magnetosphere, determined particle sizes in Jupiter’s rings, and performed a joint investigation with the Cassini 

spacecraft, whose closest approach to Jupiter was on 30 December 2000. Primary science 

goals of the joint Galileo-Cassini study were to accomplish the following:

• Analyze how interactions with the solar wind affect Jupiter’s magnetosphere 

and, in particular, its auroral regions.

• Improve our understanding of Jupiter’s atmosphere, particularly its active 

storm regions.

• Analyze dynamics of Jovian dust streams.

• Observe Io in eclipse in order to understand airglow phenomena and

monitor hotspots.45

Earth-based telescopes joined Galileo and Cassini in studying Jupiter during 

GMM. Table 9.4 gives dates for GMM’s close encounters and Jovian magnetosphere–solar 

wind study.46 Some of Galileo’s instruments were not operating at full performance during 

GMM because exposure to Jupiter’s intense radiation belts had damaged them. This was not 

surprising; the total radiation that the spacecraft had received was three times the amount 

that its systems had been built to withstand. But even with its impaired systems, Galileo 

continued to make valuable observations and generate important scientific data. 

42	 Ibid.

43 Eilene Theilig e-mail message, “Re: Definition of GMM,” 20 September 2001.

44 Guy Webster, “Galileo Gets One Last Frequent-Flyer Upgrade,” JPL Media Relations Office news release, 15 March

2001. 

45 J. K. Erickson, Z. N. Cox, B. G. Paczkowski, R. W. Sible, and E. E. Theilig, “Project Galileo Completing Europa, Preparing

for Io” (paper number IAF-99-Q.2.02 of the International Astronautical Federation, Paris, France, from the 50th International

Astronautical Congress, Amsterdam, The Netherlands, 4–8 October 1999), p. 15; J. K. Erickson, D. L. Bindschadler, E.

E. Theilig, and N. Vandermey, “Project Galileo: Surviving Io, Meeting Cassini” (paper IAF-00-Q.2.01 of the International

Astronautical Federation, Paris, France, from the 51st International Astronautical Congress, Rio de Janeiro, Brazil, 2–6

October 2000), pp. 14–15.

46 Erickson et al., “Project Galileo Completing Europa, Preparing for Io.”

39	“GEM Fact Sheet”; “Ice, Water, and Fire: The Galileo Europa Mission.”

40 “The Galileo Europa Mission—Exploring Through 1999”; “GEM Fact Sheet.”

41 “Ice, Water, and Fire: The Galileo Europa Mission.”
238	 MISSION TO JUPITER: A History of the Galileo Project Chapter 9 I The orbiter tour 239

Table 9.4. GMM activities. During GMM, the spacecraft conducted a study of the Jovian 

magnetosphere–solar wind interaction in conjunction with the Cassini spacecraft (which was 

passing by Jupiter on its way to Saturn) and with Earth-based telescopes.47

Orbit Date of Closest Encounter

E26 3 January 2000

I27 22 February 2000

G28 20 May 2000

Magnetosphere–solar wind interaction 15 June 2000 to 15 November 2000

G29 28 December 2000

C30 25 May 2001

I31 5 August 2001

I32 16 October 2001

I33 17 January 2002

Amalthea 34 5 November 2002

Jupiter 35 (impact) 21 September 2003

Fundamental Questions Addressed by the Orbiter Tour

Certain achievements of the Orbiter tour emerge as particularly memorable because they 

addressed highly compelling questions that cut across the boundaries of different disciplines. Among these questions were the following:

• What did our own planet look like during its early years, billions of years 

before humans appeared? The Galileo mission’s study of the moon Io helped 

answer this question.

• Are we alone in the universe? In other words, does life exist on any other 

planet than Earth? And if so, how rare is its occurrence? Although Galileo 

could not answer this question, data sent back from encounters with Europa 

raised tantalizing possibilities that will influence planetary exploration policy 

for many decades to come. 

• What is the impact of Jupiter’s intense, complex magnetic field and its massive 

gravitational field on its moons and rings? This question probes at the basic 

nature of the outer planet satellite environment and was addressed by many 

of the Orbiter’s moon encounters, especially those with Io.

• How does the Jovian system interact with surrounding space? In particular, what effect does the solar wind have on the shape and characteristics of 

Jupiter’s magnetosphere and particle fluxes, and what were the results of collisions between Jupiter’s satellites and interplanetary meteoroids and comets?

• What is the nature of Jupiter’s weather, especially its patterns of water distribution and its dynamic, long-lasting storms (so much more persistent than 

Earth’s weather systems)? 

Although many Orbiter tour activities addressed the above questions, other 

actions taken by the spacecraft could not be so readily categorized. However, what can 

be said about all of the Orbiter tour’s observations is that they strove to answer the question, what is out there? If Galileo’s scientific activity had been limited to the pursuit of 

well-defined, preconceived objectives, unexpected phenomena may have been missed. 

The Orbiter tour resulted in discoveries that addressed the questions above, but it also 

repeatedly brought to light the unexpected. 

The following sections attempt to capture the excitement and importance of the 

Orbiter tour’s discoveries, including those that came as complete surprises. They also 

spotlight what an Earth-based flight team was able to do to keep an aging, problemridden spacecraft operational from a distance of nearly half a billion miles. Again and 

again, Galileo’s multitalented team of engineers and scientists found ways to save the 

mission when the Orbiter’s electronic or mechanical systems shut down. The flight team’s 

innovations were among the most impressive mission achievements. The team’s efforts 

enabled Galileo to become more than just a robot spacecraft: Galileo became a virtual 

extension of human senses, allowing those on Earth to ride along to Jupiter and observe 

what was there. 

Io: Volcanoes, Mountains, and Magnetism

Io’s Volcanism

Io’s fiery surface has provided scientists with a window on dynamic volcanism and colossal lava eruptions such as those that raged on Earth many eons ago. Galileo data revealed 

that Io was the most geologically active body in the solar system, with more than 100 

erupting volcanoes. Dr. Alfred McEwen, a member of Galileo’s imaging team from the 

University of Arizona, spoke of Io’s “gigantic lava flows and lava lakes, and towering, 

collapsing mountains. Io makes Dante’s Inferno seem like another day in paradise.”48

The last comparable lava eruptions on Earth occurred 15 million years ago. It has 

been over 2 billion years since lava as hot as the 1,500o

C (2,700oF) molten material found 

on Io has flowed on our planet. “No people were around to observe and document these 

past events,” said Dr. Torrence Johnson, Galileo project scientist. “Io is the next best thing 

47	 Ibid.; Eilene Theilig e-mail message, 24 September 2001.

48	Jane Platt, “Jupiter’s Moon Io: A Flashback to Earth’s Volcanic Past,” JPL news release no. 99-097, 19 November 1999,

http://www.jpl.nasa.gov/releases/99/ioishot.html. 
240	 MISSION TO JUPITER: A History of the Galileo Project Chapter 9 I The orbiter tour 241

Table 9.5. Io encounters. (continued)

ENCOUNTER Closet Approach in

KILOMETERS (Miles)

DATE IN PACIFIC TIME MISSION COMMENTS

I31 200 (120) 5 August 

2001

Galileo 

Millennium 

Mission

Magnetic measurements 

indicated a weak or absent 

internally generated field. 

Spacecraft directly sampled 

fresh sulfur dioxide “snowflakes” from a volcanic vent.

I32 181 (112) 16 October 

2001

Galileo 

Millennium 

Mission

Galileo observed the Loki 

volcano (largest in the solar 

system) and a new eruption 

in the southern region of 

the moon.

I33 102 (63) 17 January 

2002

Galileo 

Millennium 

Mission

This was the closest of all 

the flybys. The moon 

provided a gravity-assist 

necessary for Galileo’s 

ultimate collision course 

with Jupiter. A safing 

event prevented most of 

the planned data from 

being collected.

Io has an even more active volcano than Pele: Loki, the most powerful volcano 

in the solar system. Galileo’s photopolarimeter radiometer and near infrared mapping 

spectrometer have constructed detailed temperature maps of Loki; these maps show that 

the volcano generates more heat than all of Earth’s active volcanoes combined. 

Subsurface volcanic mechanisms on Loki are somewhat different from those on 

Pele. According to Dr. Rosaly Lopes-Gautier, a member of the spectrometer team, “Unlike 

the active lava lake at Pele, Loki has an enormous caldera [a crater at the top of a volcano] 

that is repeatedly flooded by lava, over an area larger than the state of Maryland.”52

On Thanksgiving night, 25 November 1999 (during the I25 encounter), the 

Orbiter’s solid state imaging camera and its near infrared mapping spectrometer captured 

images of a dazzling volcanic display. The craft observed a fountain of sparkling lava that 

extended more than a mile above the satellite’s surface. As a means of comparison, lava 

fountains on Earth rarely exceed a few hundred yards in height. The Orbiter’s images 

depicted a curtain of fiery lava erupting from a massive crater. Dr. Torrence Johnson commented that the “lava was so hot and bright, it over-exposed part of the camera picture 

and left a bright blur in the middle.”53 The lava fountains were hot enough and tall enough 

that they were seen by NASA’s infrared telescope atop Mauna Kea in Hawaii.

Because lava fountains appear infrequently and briefly, it is hard to capture 

images of them. According to Galileo scientist Dr. Alfred McEwen, “Catching these 

fountains was a one-in-500 chance observation.”54

49	 Ibid.

50 Ibid.; Ron Cowen, “Close Encounter: Galileo Eyes Io,” Science News (11 December 1999): 382.

51 Erickson et al., “Project Galileo: Completing Europa, Preparing for Io,” p. 15; Cowen, “Close Encounter: Galileo Eyes Io”;

JPL, “Galileo Sees Dazzling Lava Fountain on Io,” JPL Media Relations Office news release, 17 December 1999; Theilig

e-mail message, 24 September 2001.

52	Platt, “Jupiter’s Moon Io”; Encarta World English Dictionary (North American Edition), 2001. 

53 JPL, “Galileo Sees Dazzling Lava Fountain on Io,” JPL Media Relations Office news release, 17 December 1999.

54 Ibid.

to traveling back in time to Earth’s earlier years. It gives us an opportunity to watch, in 

action, phenomena long dead in the rest of the solar system.”49

Table 9.5 lists the Io encounters that Galileo made during its Prime, Europa, and 

Millennium Missions. On 11 October 1999, during GEM’s I24 encounter, Galileo’s camera 

captured a closeup at Io’s Pele volcano of hot lava, at most a few minutes old, breaking 

through the surface crust. The lava formed a thin, curving line 10 kilometers (6 miles) 

long and up to 50 meters (150 feet) wide. Galileo scientists hypothesized that, under the 

surface crust at Pele, there might be an extremely active lava lake that constantly exposes 

new molten material. The curved line may mark the edge of the molten lake where it 

presses against a rock wall and breaks through to the surface. The Kilauea volcano in 

Hawaii has such a structure and behavior, although Pele’s lava lake appears to be 100 

times larger than those found in Hawaii.50

Table 9.5. Io encounters.51

ENCOUNTER Closet Approach in

KILOMETERS (Miles)

DATE IN PACIFIC TIME MISSION COMMENTS

Jupiter 

Arrival Day

1,000 (600) 7 December 

1995

Prime 

Mission

Spacecraft did no imaging 

due to tape-recorder 

problems.

I24 611 (379) 11 October 

1999

Galileo 

Europa 

Mission

Spacecraft safed at 19 

hours before encounter due 

to radiation memory hit. 

Obtained valuable imaging 

of Io volcanism. Observed a 

10-kilometer-long eruption of 

Pele volcano.

I25 300 (186) 25 November 

1999

Galileo 

Europa 

Mission

Spacecraft safed at 4 hours 

before encounter due to 

software problem. Collected 

dramatic pictures of Io 

volcanic activity. Observed 

mile-high lava fountain.

I27 198 (123) 22 February 

2000

Galileo 

Millennium 

Mission

Discovered volcanoes that 

change from hot to cool in 

several weeks. Spacecraft 

safed due to transient bus 

reset. Some Io 27 data 

played back during 

Ganymede 28.
242	 MISSION TO JUPITER: A History of the Galileo Project Chapter 9 I The orbiter tour 243

Io’s Mountains

Although Io is far smaller than Earth, the satellite has mountains soaring up to 16 

kilometers (52,000 feet) high—significantly taller than those on our planet. Scientists are 

not sure how these huge mountains formed. They do not appear to be volcanic in origin. 

Galileo images have captured evidence of how they die: concentric ridges covering the 

mountains and surrounding plateaus suggest that immense landslides are triggered as the 

mountains collapse under the force of their own weight. These ridges strongly resemble 

the terrain surrounding Olympus Mons, the largest peak on Mars.55

The Cause of Io’s Geologic Activism

Io’s geological phenomena may provide important analogs of early Earth events, but the 

mechanism driving the moon’s tectonics appears to be far different from anything our 

planet has experienced. Galileo scientists believe that Io’s geologic activism is related 

to intense frictional heating generated by the constant deformation of the moon by 

Jupiter’s powerful gravitational forces. “Jupiter’s massive gravity field distorts the shape 

of Io in the same way that tides are raised in Earth’s oceans by the gravitational tugs 

of the Sun and Moon,” said Dr. Torrence Johnson. The face of Io closest to Jupiter is 

pulled harder than its other, outer face; this difference in force results in a stretching 

effect on the moon. Irregularities in the moon’s orbit around Jupiter cause “body tides” 

to rise and fall. The orbital perturbations are, in turn, caused by gravitational nudges 

from the moons Europa and Ganymede. As a result of the varying tides, Io is stretched 

and released like a rubber ball. Friction created by this process heats and melts rock 

and produces the volcanoes and lava flows seen all over its surface, as well as the huge 

geysers that spew sulfur dioxide onto Io’s landscape. Though it is less than a third of 

Earth’s size, Io generates twice as much heat.56

Summary of Io Statistics

Table 9.6 summarizes some of the facts known about Io. Note that although its distance 

from Jupiter is roughly the distance of our Moon from Earth, Io moves far quicker, orbiting 

Jupiter in less than two days, while our Moon takes a month to complete its orbit. Also, 

note that Io’s orbital period and its rotational period (the length of its day) are the same. 

This means that, like our Moon, Io always presents the same face to Jupiter. Just as we 

on Earth never see the far side of our Moon, an observer on Jupiter will never see the far 

side of Io. In fact, all four of the Galilean satellites have this property. The mechanism 

that causes it is related to the tides induced in satellites by the gravitational pull of the 

parent planet. Although we normally think of tides as occurring in an ocean, they also 

occur in the solid material of a planet or moon, making the body slightly elongated. It is 

the gravitational pull of the parent body on these elongations that can “tidally lock” the 

satellite so that it always shows the same side to the planet.57

Table 9.6. Io statistics.58

Discovery 7 January 1610, by Galileo Galilei

Diameter 3,630 kilometers (2,256 miles)

Mass 8.94 x 1022 kilograms (2 x 1023 pounds)

Mass relative to that of Earth 0.014960 times the mass of Earth

Surface gravity (Earth = 1) 18.3 percent of Earth’s gravity

Mean distance from Jupiter 422,000 kilometers (261,000 miles)

Mean distance from Jupiter in radii 5.905 RJ

Mean distance from the Sun 5.20 AU (1 AU = distance from Sun to Earth)

Orbital period 1.77 days

Rotational period 1.77 days

Density 3.57 grams per cubic centimeter

Typical subsolar temperature ≈ -135o

C (-211oF)

Surface composition Sulfur and frozen sulfur dioxide 

Discovery of a Possible Iron Core and Magnetic Field

On 3 May 1996, JPL and Science magazine reported that Galileo might have found a giant 

iron core taking up half the moon’s diameter. The spacecraft also observed a large “hole” 

in Jupiter’s magnetic field near Io, where the strength of the magnetic field took a sudden 

drop of about 30 percent. This suggested that Io might be generating its own field. At 

the time of this discovery, no other planetary moons in our solar system were known to 

generate their own magnetic fields (although many have fields induced by the magnetism 

of the parent planet). Only Earth, Mercury, and the outer gas-giant planets were known 

to generate their own fields. 

“It’s an astonishing result and completely unexpected,” said Dr. Margaret 

Kivelson of UCLA and the leader of Galileo’s magnetic fields investigation team. “The data 

suggest that something around Io—possibly a magnetic field generated by Io itself—is 

creating a bubble or hole in Jupiter’s own powerful magnetic field. But it’s not clear to us 

just how Io can dig such a deep and wide magnetic hole.”59

The existence of a large, dense core within Io was deduced from Galileo data 

taken during the spacecraft’s flyby within 900 kilometers (560 miles) of the moon on 

55	Don Savage and Guy Webster, “Jupiter Millennium Mission: The Galileo and Cassini Encounter at the Fifth Planet,” NASA

press kit; Platt, “Jupiter’s Moon Io.” 

56 Douglas Isbell (NASA Headquarters) and Mary Beth Murrill (JPL), “Galileo Finds Giant Iron Core in Jupiter’s Moon Io,”

NASA press release 96-89, 3 May 1996, http://www.solarviews.com/eng/galpr4.htm.

57	Ron Baalke, Today on Galileo—May 23, 2001, JPL e-mail newsletter, 23 May 2001. 

58 JPL, “A Summary of Facts About Io,” Io, A Continuing Story of Discovery, 22 March 1999, http://www.jpl.nasa.gov/

galileo/io/io_summary.html.

59 Isbell and Murrill, “Galileo Finds Giant Iron Core.” 
244	 MISSION TO JUPITER: A History of the Galileo Project Chapter 9 I The orbiter tour 245

Jupiter Arrival Day, 7 December 1995. In particular, Doppler data generated with the 

spacecraft’s radio carrier wave were analyzed to measure Io’s external gravitational 

field. A metallic core composed of iron and iron sulfide with a radius of about 900

kilometers (560 miles), or 52 percent of the moon’s mean radius, would be consistent 

with the spacecraft’s observations. Scientists think that the core was formed as a result of 

interior heating of the moon into a molten state, causing the moon’s heavier components, 

such as iron, to sink toward its center. The internal heat source of the moon could have 

been either its original heat of formation or frictional heating resulting from perpetual 

distortions of the moon due to Jupiter’s strong gravitational field.60

The Rapid Timescale of Io’s Geological Changes

On Earth, we think of geological changes as happening ever so slowly, over periods 

of thousands or millions of years. Large alterations to Earth’s surface rarely occur 

quickly, unless there is a cataclysmic event such as a massive volcanic eruption. But 

Io is a moon where cataclysmic events are the norm. For instance, a Galileo observation of a section of Io’s surface taken only five months after the previous observation 

revealed that a new dark spot the size of Arizona had appeared, indicating dramatic 

volcanic activity.61

The visible change, almost 250 miles in diameter, occurred during the five 

months between Galileo’s 7th (G7) and 10th (C10) orbits of Jupiter. The observations 

were not taken during an Io close encounter, but from much further away. The alteration in Io’s surface occurred around a volcanic center named Pillan Patera (after the 

South American god of thunder, fire, and volcanoes). Pictures revealing the changes 

were captured by the Orbiter’s SSI camera. The pictures showed marked differences 

between the Pillan Patera region on 4 April 1997 and 19 September 1997. In June of 

1997, both Galileo and the Hubble Space Telescope observed an active plume over 

Pillan 120 kilometers (75 miles) in height, and both Galileo and ground-based astronomers observed an intense hotspot.62

The composition of the rapidly created dark spot appeared to be different 

from that of typical deposits. “Most of the volcanic plume deposits on Io show up as 

white, yellow, or red due to sulfur compounds. However, this new deposit is gray, 

which tells us it has a different composition, possibly richer in silicates than the other 

regions,” said Galileo imaging team member Dr. Alfred McEwen, a research scientist 

at the University of Arizona in Tucson. “While scientists knew that silicate volcanism 

existed on Io from high temperatures, this may provide clues as to the composition of 

the silicates, which in turn tells us about Io’s evolution. Io is probably primarily composed of silicates, which is the type of volcanic rock found on Earth,” McEwen said. 

“But the extreme volcanism of Io may have led to the creation of silicate compositions 

that are unusual on Earth.”63

A dark region southwest of the Pele volcano, which was first seen during the 

Voyager flybys in 1979, is similar in appearance to the Pillan deposits. The Io images 

showing the changes in Pillan Patera also revealed alterations in a plume deposit at 

the Pele volcano, which may indicate that both plumes were active at the same time and 

interacted with one another.64

Io’s Ionosphere

In analyzing Galileo data, scientists discovered that in December 1995, during the spacecraft’s first encounter with Io, the craft may have flown through a dense, high-altitude 

ionosphere of the moon. An ionosphere is a region of electrically charged gas. Such a layer 

occurs at the top of some planetary atmospheres, including that of Earth. The scientists 

thought that the ionosphere consisted of volcanic gases that rose to very high altitudes.65

According to Dr. Louis A. Frank of the University of Iowa, principal investigator on Galileo’s plasma science experiment, the spacecraft’s sensors registered a dense 

region of ionized oxygen, sulfur, and sulfur dioxide 900 kilometers (560 miles) above Io. 

No one expected an ionosphere at such an altitude because 1970s data from the Pioneer 

and Voyager spacecraft only saw particulate layers as high as a few hundred kilometers. 

Scientists also were surprised that the ionized gases found by Galileo stayed with Io rather 

than being swept away by forces from Jupiter’s rotating magnetosphere.

The difference between what the Pioneer 10 spacecraft’s 1973 radio occultation experiments revealed—an ionosphere only 50 to 100 kilometers (30 to 60 miles) 

high—and what Galileo saw indicates that Io’s ionospheric activity is highly variable. The 

moon’s ionosphere may grow and shrink with volcanic activity. The Galileo results gave 

credence to the “stealth plume” theory of project scientist Torrence Johnson. He proposed 

that because of the moon’s weak gravity, invisible gases from Io’s volcanoes rose to great 

heights far beyond those achieved by dust and other visible volcanic ejecta.66 

Arrival Day Io Flyby: The Great Disappointment

Galileo flybys of the satellite Io were filled with surprises, drama, and sometimes disappointment. Since the beginning of the Galileo program in the late 1970s, its scientists 

had been looking forward to the spacecraft’s Io flyby on Jupiter Arrival Day (7 December 

1995). Because of the high radiation levels around Io and their effect on sensitive

instruments, the scientists knew that Arrival Day might be the only time they would get 

a detailed look at Io—the only time they could obtain high-resolution, closeup images. 

But concerns regarding Galileo’s malfunctioning tape recorder made it necessary for the 

Project Manager, Bill O’Neil, to cancel all remote sensing operations during the flyby. 

Only fields-and-particles instruments were allowed to take and record data. Those 

60	J. D. Anderson, W. L. Sjogren, and G. Schubert, “Galileo Gravity Results and the Internal Structure of Io,” Science 272,

no. 5262 (3 May 1996): 709; Isbell and Murrill, “Galileo Finds Giant Iron Core.” 

61 Douglas Isbell (NASA Headquarters) and Jane Platt (JPL), “Galileo Finds Arizona-Sized Volcanic Deposit on Io,” NASA

news release 97-257, 5 November 1997, http://www.jpl.nasa.gov/galileo/status971105.html.

62 Ibid.

63 Ibid.

64	 Ibid.

65 	Douglas Isbell (NASA Headquarters) and Mary Beth Murrill (JPL), “High Altitude Ionosphere Found at Io by Galileo

Spacecraft,” NASA news release 96-216, 23 October 1996.

66 Ibid.
246	 MISSION TO JUPITER: A History of the Galileo Project Chapter 9 I The orbiter tour 247

instruments needed the tape recorder to run continuously, but at low speeds, and 

project staff thought that the recorder could handle this. Remote sensing instruments 

such as the SSI camera required the recorder to run discontinuously, with abrupt starts 

and stops, and at high speeds. That kind of operation might well have permanently 

crippled the recorder and, with it, the entire mission. It is hard to imagine the frustration 

felt by scientists who had waited much of their careers for the Io flyby when they learned 

that they would not receive the data for which they had hoped.67

I24 and I25: The Great Flybys That Almost Weren’t

Much to the delight of scientists around the country, Galileo survived its Prime Mission 

and continued to perform beautifully. Two more Io missions were planned during the Galileo 

Europa Mission, after the spacecraft had had a chance to study Europa in detail. The missions were much anticipated, but the observations that were planned almost did not happen. 

During the I24 and I25 flybys, the first Io encounters since Jupiter Arrival Day, problems 

arose that once again threatened to ruin the chances of close-flyby Io observations. 

I24. After the extremely limited flyby on Jupiter Arrival Day, the next Io encounter, I24, occurred during GEM. Its closest approach took place on 11 October 1999. The 

entire encounter sequence lasted four days, beginning on 10 October and running to 14 

October. Mission scientists were tremendously interested in the first images of Io taken 

from a low-altitude flyby skimming only 611 kilometers (379 miles) above the surface. 

The SSI camera and NIMS were going to observe numerous volcanic features, including 

the terrain around Pele and Pillan Patera. NIMS would also produce a regional map and 

a plume image, in addition to searching for undiscovered hotspots. Other remote sensing instruments would take key atmospheric and radiometric measurements. But all this 

almost did not happen.68

Io is the Galilean moon that is closest to Jupiter. It lies in a region of intense 

radiation from the parent planet that “can wreak havoc with spacecraft instruments.”69 

During the flyby, this radiation interfered with the Orbiter computer’s memory. The 

Orbiter’s on-board flight software was programmed to detect anomalous situations in 

the spacecraft’s many subsystems. Any time the spacecraft computer executed a portion 

of memory that had an error in it, the craft’s flight software was designed to detect it and 

put the Orbiter into a “safe mode.” This meant that all nonessential functions were turned 

off to make the spacecraft as safe as possible from a power, thermal, and communications 

perspective until mission staff back on Earth could figure out what had happened and 

how to get the craft functioning again without damaging it. A safing incident occurred at 

3:09 a.m. Pacific time on Sunday, 11 December 1999. The closest approach to Io was only 

19 hours away.70, 71

According to Dr. Duane Bindschadler, Team Chief of the spacecraft and sequence 

team, what occurred on I24 was that high-energy electrons penetrated the Galileo spacecraft, hit a memory chip, and physically altered a single bit of memory. This caused the 

spacecraft to “safe.”72

Bringing Galileo out of safe mode was orders of magnitude more complex than 

restoring a personal computer that has crashed. For one thing, the travel time for radio 

transmissions from Earth to Jupiter was 35 to 55 minutes one way (depending on the relative positions of the two planets). Querying the on-board computer, analyzing what had 

gone wrong, and responding to the situation constituted a very slow, multistep process. 

The early “safings” aboard Galileo typically took a week to 10 days to bring the spacecraft 

gradually back to full operation, although as more was learned about the craft’s limitations and operating characteristics, the recovery time grew shorter. But 19 hours was 

a very short time in which to bring the craft out of safe mode. Once again, it seemed, 

Galileo might pass close by Io and not be able to send the imaging data that scientists so 

desperately wanted.73 Scientists had been waiting for 20 years for close-encounter images 

from Io. It was critical not just for JPL, but also for NASA and the U.S. interplanetary space 

program, that Galileo do well. 

The Galileo engineering staff had to develop and implement a recovery plan 

on the fly, altering the spacecraft’s code sequences so that it would obtain the necessary 

data. Late Sunday afternoon, the staff sent out its code modifications, and they worked 

as hoped, getting the spacecraft fully operational by 8 p.m. Pacific time, a mere 2 hours 

before the closest approach. Nagin Cox, the Deputy Team Chief for the spacecraft and 

sequence team, said, “I think I will be proud of the Galileo team and those few days for 

the rest of my life . . . . Without them, there [would have been] no recovery.”74

“It was a heroic effort to pull this off,” said Jim Erickson, Galileo Project Manager. 

“The team diagnosed and corrected a problem we’d never come across before, and they 

put things back on track.” Because of this effort, his staff could look forward to seeing the 

closest pictures of Io ever taken, as well as observations of the moon’s surface chemistry, 

thermal characteristics, gravitational field, and magnetic properties.75

The Orbiter was not able to carry out and record all of its planned observations. 

Some photopolarimeter radiometer (PPR) observations of Jupiter and Io’s night side 

were lost, as well as some fields-and-particles data of Io’s plasma. But the spacecraft 

did obtain numerous high-resolution images of Io’s surface, including the Pillan Patera, 

Zamama, Prometheus, and Pele volcanic eruption centers. The Orbiter also obtained 

global-scale color images and an observation of Io while in eclipse, as well as atmospheric 

airglow and auroral emissions. In addition, excellent infrared measurements were 

67	J. K. Erickson, D. L. Bindschadler, E. E. Theilig, and N. Vandermey, “Project Galileo: Surviving Io, Meeting Cassini” (paper

IAF-00-Q.2.01 of the International Astronautical Federation, Paris, France, from the 51st International Astronautical

Congress, Rio de Janeiro, Brazil, 2–6 October 2000), p. 7; Bill O’Neil, former Galileo Project Manager, interview,	

tape-recorded conversation, 15 May 2001. For more detailed information on the tape-recorder anomaly, see chapter 8. 

68 J. K. Erickson, Z. N. Cox, B. G. Paczkowski, R. W. Sible, and E. E. Theilig, “Project Galileo: Completing Europa, Preparing

for Io” (paper IAF-99-Q.2.02 of the International Astronautical Federation, Paris, France, from the 50th International

Astronautical Congress of the International Astronautical Federation, Amsterdam, The Netherlands, 4–8 October 1999),	

pp. 13–14. 

69 JPL, “Galileo Succeeds in Historic Flyby of Jupiter’s Volcanic Moon,” JPL Media Relations Office news release,	

11 October 1999.

70	Nagin Cox and Bill O’Neil interviews, tape-recorded telephone conversations, both conducted on 15 May 2001. Nagin

Cox was the Deputy Team Chief for the spacecraft sequence team at the times of Io 24 and 25. Bill O’Neil is a former

Galileo Project Manager. 

71 JPL, “Galileo Succeeds in Historic Flyby.” 

72 Duane Bindschadler, Team Chief of the Galileo spacecraft and sequence team, interview, JPL, 5 August 2001.

73 Bill O’Neil interview, 15 May 2001.

74 Nagin Cox interview, 15 May 2001.

75 JPL, “Galileo Succeeds in Historic Flyby.”
248	 MISSION TO JUPITER: A History of the Galileo Project Chapter 9 I The orbiter tour 249

taken of various volcanic centers and mountains, and two infrared regional maps 

were constructed.76

I25. Galileo encounters, according to mission staff, seemed to occur again and 

again over holidays, when normal people are home enjoying their families and taking 

it easy. I25, the second and last Io encounter during GEM, occurred on Thursday, 25 

November 1999, at 8:40 p.m. Pacific time—Thanksgiving evening. Project staff ate their 

Thanksgiving dinners in Galileo’s Mission Control Center while waiting for the closest 

approach, which was to be a mere 300 kilometers (186 miles) above the moon’s surface. 

Remembering the problems that had happened on I24, engineering staff had been counting down the hours. At 12 hours from closest encounter, everything was fine. Things 

remained so as the time grew shorter: 10 hours . . . 8 . . . 7 . . . . Then, at 4 hours out, the 

spacecraft safed again. A software problem that had been present since launch interacted 

with the software patch that had been loaded into the Orbiter’s computer to help bring it 

out of safe mode during the I24 encounter. The spacecraft safed once more but did not 

shut down to as deep a level as on the last Io encounter. Nevertheless, trying to pull the 

Orbiter out of safe mode at 4 hours from the closest approach was very ambitious, if not 

outright futile. “With so little time to spare, it would have been easy to think, no way can 

we do this,” said Galileo Project Manager Jim Erickson. “But our team members jumped 

to the challenge, in some cases leaving behind half-eaten Thanksgiving dinners.”77

“I have very little memory of those four hours,” said Nagin Cox, “because [we 

were] so incredibly focused and intense. With that amount of time to recover, everybody 

simply had to do their jobs as they knew them in their heads. There was not enough time 

to rely upon the automated tools that normally help us make sure we are doing the right 

thing.” The team formulated command sequences, got them to Jupiter, and recovered the 

spacecraft, 500 million miles away, only 3 minutes before the closest encounter with Io. 

“There was clapping and cheering and smiles all around. There was a lot of adrenalin 

flowing. We were all thrilled and a bit surprised at having recovered so quickly.” But they 

could not celebrate for long because the flyby was upon them. When the team had gotten 

I24 back online, they had had 2 hours to catch their breath before the flyby. On I25, they 

had all of 3 minutes.78

Dr. Eilene Theilig, who worked on Galileo for 11 years and became its Project 

Manager in January 2001, said that the recovery of I25 was her most exciting moment on 

the mission. “Just working through that with the people involved and the teamwork, and the 

fact that everybody knew just what to do and was right on top of it was an exciting evening. 

And then to see an image that we got back, which was our first image of an active volcano. 

I mean, we caught the curtain of fire. All of that tied together had to be the highlight.”79

The literally last-minute recovery of I25 allowed stunning observations to be 

taken and sent to Earth. The encounter’s high-resolution imaging data and spectral 

maps met many of the flyby’s ambitious remote sensing goals. Galileo’s SSI and 

NIMS systems caught one of the most impressive events ever recorded anywhere in the 

solar system—an actively erupting volcanic fountain that generated a curtain of lava 20 

kilometers (12 miles) long. Capturing such a massive eruption was “extremely fortuitous,” 

based on similar events on Earth, which typically last only a few hours to one to two days. 

Even considering the more frequent volcanic activity found on Io, Galileo was very lucky 

to have seen such a massive eruption.80

The I25 encounter was a smashing success, due largely to the frenetic efforts 

of the project staff who recovered it. But some important observations were never made 

because of the 4 hours the spacecraft spent in safe mode. Among the most significant 

scientific data that were lost were the fields-and-particles observations that were to 

address the question of whether Io has an internally generated magnetic field, which 

would have bearings on the internal structure of the moon. The observations were to 

have helped determine the extent to which electrical conductivity within Io and its 

atmosphere could have generated the moon’s observed magnetic signature. In fact, the 

polar path of the flyby had been chosen largely in order to help answer this question.81

Galileo Millennium Mission Observations of Io

I27. The first Io flyby of the Galileo Millennium Mission (GMM) occurred on 

22 February 2000, when the Orbiter skimmed above the satellite surface at an altitude 

of only 198 kilometers (123 miles). The plan for this encounter was influenced to a 

large extent by the data returned from I24 and I25 but had to be very limited in scope 

due to a solar conjunction that occurred during the latter part of the I27 cruise period. 

The solar conjunction interfered with data transmission by reducing the rate at which 

the data could be sent to Earth. The spacecraft was also near its maximum distance 

from Earth at this time, which further reduced the data transmission rate. Both of these 

factors significantly reduced the volume of information that could be sent before 

the next satellite encounter (G28, a flyby of Ganymede) required the space on the 

recording tape. With these constraints, Galileo staff focused on the time of closest 

approach to Io as yielding the most valuable data. Approximately 75 percent of the 

tape-recorded observations were taken during a 3-hour period beginning 40 minutes 

before closest approach.82

The spacecraft safings in I24 and I25 caused data gaps in the survey of the 

Io plasma torus. The observations during I27 filled in some of those gaps. The Orbiter 

also measured the global distribution of daytime temperatures on Io and took extensive 

visible and infrared images of the moon. Results from previous Io encounters indicated 

that images taken at low-Sun-angle conditions (when the Sun was close to the satellite’s 

horizon) were especially helpful in establishing the shapes and geology of the satellite’s 

features. Radio science measurements of the moon’s gravitational field were made to 

improve characterizations of the field and to refine Io interior structure models.83

The high-resolution images taken by Galileo’s NIMS during I27 revealed 14 

volcanoes in a region that was previously thought to contain only 4. The region covers 76

Erickson, Bindschadler, Theilig, and Vandermey, “Project Galileo: Surviving Io, Meeting Cassini,” p. 9. 

77 Nagin Cox interview, 15 May 2001; JPL, “Galileo Mission Status: November 25, 1999,” JPL Media Relations Office news

release, 25 November 1999; Erickson, Bindschadler, Theilig, and Vandermey, “Project Galileo: Surviving Io, Meeting

Cassini,” p. 4. 

78 Nagin Cox interview.

79 		Eilene Theilig, former Galileo Project Manager, interview, tape-recorded conversation, 8 May 2001.

80	Erickson, Bindschadler, Theilig, and Vandermey, “Project Galileo: Surviving Io, Meeting Cassini,” p. 11. 

81 Ibid., p. 11. 

82 Ibid., pp. 12-13.

83 Ibid., pp. 13–14.
250	 MISSION TO JUPITER: A History of the Galileo Project Chapter 9 I The orbiter tour 251

about 5 percent of Io’s surface and is approximately three times larger than Texas. 

Integrating data from I24, I25, and I27 revealed that some of the moon’s smaller, fainter 

volcanoes turned rapidly on and off, altering from hot and glowing objects to cool and 

dim ones in the space of a few weeks. On the other hand, the moon’s larger, brighter 

volcanoes tended to remain active for years and possibly decades, based on Galileo data 

and the images taken by Voyager when it flew by the Jovian system in 1979.84

During I27, Galileo noted dramatic changes in Loki, the most powerful known 

volcano in the solar system. According to Dr. John Spencer of Lowell Observatory in 

Arizona and a co-investigator for Galileo’s radiometer experiment, most of Loki’s caldera, 

a region half the size of Massachusetts (about 10,000 square kilometers or 4,000 square 

miles), appeared to have been covered by fresh lava in the four and a half months 

between I24 (11 October 1999) and I27 (22 February 2000). 

Images of the Chaac Patera volcanic region revealed a caldera wall 2.8

kilometers (1.7 miles) in height, with a slope of 70 degrees—twice as high and far steeper 

than Earth’s Grand Canyon. This finding indicates that the rocks around Chaac Patera are 

very strong to be able to support such an extreme topography. The spacecraft also imaged 

a caldera filled with bright white deposits containing sulfur dioxide of a purity higher 

than at any other place on Io. Mission scientists believed that it might be a layer of sulfur 

dioxide ice. “There are processes on Io for which we have no terrestrial experience,” said 

Dr. Alfred McEwen of the University of Arizona and a member of Galileo’s imaging team. 

“Strange new observations like these will provide fodder to current and future scientists 

for understanding the processes that have shaped this fascinating world.”85

A series of extreme ultraviolet (EUV) scans of the Io plasma torus and Jupiter, 

planned as a final observation for I27, had to be curtailed because of yet another spacecraft safing event. Project staff believe that this safing was also induced by radiation 

exposure, but not of the spacecraft’s memory, as was the case in I24. This radiation 

exposure caused a “transient bus reset,” which was triggered by inadvertent short circuits 

across the spin bearing assembly that connected the Orbiter’s spun and despun sections. 

Transient bus resets continued to occur up to several times during each encounter. A software patch loaded into the Orbiter’s computer after a Europa encounter (E19) allowed the 

spacecraft to recognize a bus reset and refrain from going into safe mode. This patch was 

typically enabled when the Orbiter flew within 15 RJ

 of Jupiter, where the most intense 

radiation occurs. On I27, however, a bus reset unexpectedly occurred at 29 RJ

, when the 

patch had been disabled. As a result, the spacecraft safed.86

Besides interrupting the extreme ultraviolet scan, the safing event also impacted 

the return of other Io data to Earth. As mentioned above, the amount of data returnable 

to Earth was already severely limited because of a solar conjunction and because of 

the extreme distance of the spacecraft from Earth. Loss of playback time due to the 

safing event further impacted data return. To help remedy the situation, Galileo managers 

decided to carry some I27 data into the next orbit (G28, a Ganymede flyby) and play it 

back at that time. This would reduce the amount of tape that could be used to record 

G28 data, but Galileo management perceived Io data as having a high enough science 

value to justify this.87

I31. GMM activities included a Callisto encounter on 25 May 2001 (C30), on 

which the moon’s gravitational field was used to set up two encounters with Io (I31 

and I32). During I31, which occurred on 5 August 2001 at about 10 p.m. PDT, Galileo 

skimmed as close as 200 kilometers (120 miles) above Io’s surface. A north polar path was 

picked for the I31 flyby because magnetic readings above the pole might help to reveal 

whether Io generated its own magnetic field. According to Torrence Johnson, determining 

that was important for understanding the processes taking place in Io’s hot interior. “All 

of our previous magnetic measurements at Io have been on equatorial passes, and from 

those we can’t tell whether the field at Io is induced by Jupiter’s strong magnetic field or 

produced by Io itself,” said Johnson. Initial interpretations of I31 data suggested that either 

the moon did not have an internally generated field or it was extremely weak, according 

to the principal investigator for the magnetometer experiment, Margaret Kivelson.88

As a side benefit of the flyby, Galileo sped through a region that had been occupied by a giant gas plume from the volcano Tvashtar seven months earlier. This was the 

first time that the spacecraft had an opportunity to sample a plume’s constituents directly. 

Io’s polar plumes, however, appeared to be short-lived, according to Project Manager 

Eilene Theilig. It was not clear whether remnants of the Tvashtar plume would still be 

there when Galileo passed through.89

Galileo’s investigations have often turned up the unexpected, and the I31 flyby 

was no exception. The Tvashtar volcano was quiet, but the spacecraft spotted a new eruption from a previously unknown volcano 600 kilometers away. The plume this new volcano 

spewed out was the tallest yet observed, soaring 500 kilometers (300 miles) above the 

moon’s surface. Fortuitously, Galileo’s flightpath took it through the outskirts of this plume, 

and the craft’s plasma science instrument detected particles that had erupted from Io’s interior only minutes earlier. Louis Frank, leader of the plasma experiment, commented that 

although Galileo had taken wonderful images of Io’s volcanoes, it had never before “caught 

the breath” of one. The particles Galileo detected were not hot volcanic embers, but rather 

tiny sulfur dioxide snowflakes of 15 to 20 molecules each. The snowflakes formed as hot 

gases from the vent rose through Io’s frigid, thin atmosphere and condensed.90

I32. On 18 September 2001, Galileo performed an “orbit trim” maneuver in 

which the spacecraft’s thrusters were pulsed briefly three times to “gently nudge” the 

vehicle toward its 16 October encounter with Io. Up to this point, this was the smallest 

maneuver of the mission, using only 15 grams of propellant. But it was necessary because 

the amount of fuel remaining in Galileo’s tanks was “getting so low that spending 15 

grams today may save us from spending 20 grams next week to reach the same place.” 

Project staff were painstakingly trying to get the maximum science results out of the 

84 Susan Mitgand and Mary Beth Murrill, “Dynamic Terrain and Volcanoes Galore on Io,” JPL Media Relations Office news

release, 31 May 2000. 

85 Ibid.

86 Erickson, Bindschadler, Theilig, and Vandermey, “Project Galileo: Surviving Io, Meeting Cassini,” p. 14; Nagin Cox interview.

87 Erickson, Bindschadler, Theilig, and Vandermey, “Project Galileo: Surviving Io, Meeting Cassini,” p.14. 

88 Guy Webster, “Spacecraft to Fly Over Source of Recent Polar Eruption on Io,” JPL news release 2001-161, 2 August 2001;

Guy Webster, “Galileo Millennium Mission Status,” JPL news release 2001-166, 6 August 2001; Margaret G. Kivelson,

Krishan K. Khurana, Christopher T. Russell, and Raymond J. Walker, “Magnetic Signature of a Polar Pass Over Io,”

Abstract P11A-01, Eos Transactions, American Geophysical Union 82, no. 47, Fall Meeting Suppl., 2001.

89 Webster, “Spacecraft to Fly Over Source.”

90 “Dashing Through the Snows of Io,” NASA Science News (16 October 2001).
252	 MISSION TO JUPITER: A History of the Galileo Project Chapter 9 I The orbiter tour 253

space vehicle’s activities and still attain the end-of-mission goal of impact with Jupiter in 

September of 2003.91

During the 15 October 2001 flyby, Galileo passed closer to Io than ever before—

only 181 kilometers (112 miles) above the moon’s south pole. The spacecraft made 

observations of several volcanic areas on the moon’s surface, including Loki, the most 

powerful volcano in the solar system, as well as a new hotspot and plume eruption in the 

moon’s southern region that had been discovered on the I31 flyby in August.92

Magnetic field measurements had a high priority during I32, as they did on I31, 

because they could help determine whether Io generated an intrinsic magnetic field of its 

own within the greater magnetic field of Jupiter. Analysis of the measurements indicated 

that “an internal magnetic field is negligibly small and probably absent.”93 This finding 

implied that Io’s molten iron core did not have the same type of convective overturning 

by which Earth’s molten core generates a magnetic field. This result fits a model in which 

Io’s core is heated from the outside, by tidal flexing of the layers around it, rather than 

being heated from its center.94

I33. This was the closest, and last, flyby that Galileo performed past any of 

Jupiter’s four major moons. The spacecraft cruised to within 102 kilometers (63 miles) 

of Io’s surface on 17 January 2002. The close encounter gave Galileo the gravityassist necessary for it to end its multiyear mission. “The reason we’re going so close,” 

according to Eilene Theilig, “is to put Galileo on a ballistic trajectory for impact into 

Jupiter in September 2003.”95 The propellant needed to steer the spacecraft and keep 

its antenna pointed toward Earth was nearly exhausted. Mission scientists wanted to 

avoid the small chance that Galileo might crash into Europa after its mission ended 

and possibly contaminate it with Earth microorganisms. If that happened, it would 

not be clear whether any life-forms that might ultimately be discovered on Europa 

originated there or on Earth. 

On I33, scientists hoped to observe how several of Io’s regions had changed over 

the years. The spacecraft was also supposed to make direct measurements of charged 

particles and magnetic fields around the moon, as well as using its camera and other 

instruments for infrared and thermal imaging. Plans for science observations did not go 

as hoped, however. While approaching the moon, 28 minutes before closest approach, 

the spacecraft placed itself in safe mode. In this mode, on-board fault protection 

software instructed the craft’s cameras and science instruments to stop taking data and 

await further instructions from Earth. Mission scientists thought that the safing incident 

probably had resulted from the radiation environment near Jupiter. The flight team 

worked through the day and evening to restore spacecraft operations. Unfortunately, 

three of the tracks of data that were to be put on Galileo’s tape recorder were lost.96

Eilene Theilig said of this last Io flyby, “As expected, visiting Io has proved to be a challenging and risky endeavor. It’s disappointing not to get the observations of Io that were planned 

for this encounter, but I am very proud of the flight team that has kept Galileo functioning in 

orbit more than three times longer than originally planned and revived it once more.”97

Galileo’s observations revealed the incredibly dramatic nature of Io geology, but 

there is still much more about the moon to study. Dr. Rosaly Lopes-Gautier, a research scientist 

at JPL and member of the Galileo spectrometer team, estimated the total quantity of volcanoes 

on Io. She based her estimate on the volcanoes seen so far and on the fact that volcanism 

appears to be uniformly distributed across the surface. According to Lopes-Gautier, “we can 

expect Io to have some 300 active volcanoes, most of which have not been discovered.”98

Europa: A Possible Ocean Under the Ice Crust

Europa looks like broken glass that is repaired by an icy glue oozing up from below.99

Frozen water appears to cover Europa’s entire surface, resembling “a broken pane of 

glass, its shards repaired by an icy glue from below.”100 The “shards” also look like the 

fragmented chunks of ice seen in Earth’s polar regions during springtime thaws (see 

figure 9.3) and are separated by a network of low ridges. The surface of Europa also contains gigantic ice rafts, cold-water volcanoes, and probably an underwater ocean.101 

Table 9.7. Europa data.102

Discovery 7 January 1610, by Galileo Galilei

Diameter 3,138 kilometers (1,946 miles)

Mass 4.8 x 1022 kilograms (1.1 x 1023 pounds)

Mass relative to that of Earth 0.0083021

Surface gravity (Earth = 1) 13.5 percent of Earth’s gravity

Mean distance from Jupiter 670,900 kilometers (416,000 miles)

91	Ron Baalke, “This Week On Galileo—September 17–23, 2001,” 18 September 2001.

92 JPL, “Galileo Millennium Status,” news release, 16 October 2001.

93 Margaret G. Kivelson, Fran Bagenal, William S. Kurth, Fritz M. Neubauer, Chris Paranicas, and Joachim Saur, “Magnetic

Interactions With Satellites,” in Jupiter, ed. Fran Bagenal, Tim Dowling, and Bill McKinnon (Port Melbourne, Australia:

Cambridge University Press, 2004).

94 JPL, “Jupiter’s Io Generates Power and Noise, But No Magnetic Field,” JPL news release, 10 December 2001,	

http://galileo.jpl.nasa.gov/news/release/press011210.html.

95 Donald Savage and Guy Webster, “Farewell, Io: Galileo Paying Last Visit to a Restless Moon,” NASA news release	

02-10, 15 January 2002.

96 JPL, “Galileo Millennium Mission Status,” news release, 17 January 2002; JPL, “Farewell to Io!” Today on Galileo, news

release, 18–20 January 2002; Donald Savage and Guy Webster, “Farewell, Io: Galileo Paying Last Visit to a Restless

Moon,” NASA news release 02-10, 15 January 2002.

97 JPL, “Galileo Millennium Mission Status,” news release, 18 January 2002, http://www.jpl.nasa.gov/galileo/news/

release/press020118.html.

98 	 Mitgand and Murrill, “Dynamic Terrain and Volcanoes Galore on Io.”

99 JPL, “Europa: Another Water World?” Moons and Rings of Jupiter—Galilean Moons: Europa, 1 October 2001, http://

www2.jpl.nasa.gov/galileo/moons/europa.html (available in folder 18522, NASA Historical Reference Collection,

Washington, DC).

100 JPL,“Why Europa?” Galileo Solid State Imaging (SSI) Team Education and Public Outreach Web site, 17 September

1999, http://www2.jpl.nasa.gov/galileo/sepo/education/europa/whyeuropa.html (accessed 22 November 2004)

(available in folder 18522, NASA Historical Reference Collection, Washington, DC).

101 “Why Europa?” and “Diving Water on Europa,” Science@NASA, 9 September 1999, http://science.nasa.gov/newhome/

headlines/ast09sep99_1.htm.

102 “A Summary of Facts about Europa,” JPL Galileo Home Page, http://www.jpl.nasa.gov/galileo/europa/e-summary.html.
254	 MISSION TO JUPITER: A History of the Galileo Project Chapter 9 I The orbiter tour 255

Table 9.7. Europa data. (continued)

Mean distance from Jupiter in radii 9.5 RJ

Mean distance from the Sun 5.203 AU

Orbital period 3.551181 days

Rotational period 3.551181 days

Density 3.01 grams per cubic centimeter

Orbit speed 13.74 kilometers per second (30,740 mph)

Visual albedo 0.64

Surface composition Water ice

Figure 9.3. Europa’s surface is covered with water ice. Straight and curved low ridges form 

the boundaries of ice fragments that resemble those seen in Earth’s polar regions during 

springtime thaws. The white and blue colors outline areas that have been blanketed by 

a fine dust of ice particles. The unblanketed surface has a reddish-brown color that has 

been painted by mineral contaminants carried and spread by water vapor released from 

below the crust when it was disrupted. The original color of the icy surface probably was a 

deep blue seen in large areas elsewhere on the moon. The colors in this picture have been 

enhanced for visibility. (NASA image number PIA01127)

Voyager’s View of Europa

In 1979, Voyager’s twin spacecraft provided Earth with its first closeup view of Europa. The pictures 

showed pale yellow, icy plains with mottled red and brown areas. Long cracks extended over the 

surface for thousands of kilometers. None of these features rose more than a few kilometers 

above the surface, which placed Europa among the smoothest bodies in our solar system.103

Galileo Prime Mission Europa Encounters

Galileo passed above Europa’s south pole at a distance of 32,000 kilometers (20,000 miles) 

when it first reached the Jovian system. Due to tape-recorder limitations, observations had to 

be extremely restricted to ensure the satisfactory storage of data from the Probe’s plunge into 

Jupiter’s atmosphere. After the Probe data were received, stored, and sent to Earth, the tape 

recorder could store more satellite data. The Orbiter’s remote sensing observations (including extensive imaging activities) were initiated shortly before the Ganymede encounter (G1) 

in June 1996. On each orbit, the spacecraft made observations not only of the satellite being 

encountered, but also of other satellites, as well as Jupiter and its fields and particles.104

Europan observations taken during the G1 and G2 encounters revealed areas 

that appeared to be ice floes that had once moved and rotated, indicating water or soft ice 

below the surface. The images also showed many long lines on the moon, which appeared 

in groups of three—two dark lines with a lighter line between them. Scientists thought that 

these were cracks in Europa’s icy surface due to upwellings of water or soft ice from below. 

Debris carried in the upwellings might have caused the dark lines. Deposits observed along 

the lines may have been caused by explosive geyser-like activity.105

The spacecraft’s C3 orbit, in which it carried out a close Callisto encounter on 4 

November 1996, also included a “nontargeted” pass by Europa on 6 November at an altitude of 34,800 kilometers (21,600 miles). Nontargeted encounters are secondary flybys on 

a given orbit, up to a distance of 100,000 kilometers (60,000 miles). Nontargeted encounters such as this Europa flyby are not accidental, but are designed into the orbit. They are 

untargeted in the sense that no effort is made to control the encounter conditions. With 

the limited amount of propellant available, it would be very difficult, if not impossible, to 

control the close flybys past two satellites on the same orbit and still achieve the necessary 

gravity effects to reach the next encounter on the tour. One of the primary objectives of 

nontargeted encounters is to take global observations of the satellite, which are possible 

because of the greater flyby distances.106

E4. Galileo’s first close encounter with Europa (E4) occurred on 19 December 

1996 Greenwich mean time (GMT), about a year after Jupiter Arrival Day. Galileo’s 

primary science objectives during E4 were to conduct remote sensing observations of 

Europa’s surface, collect data on the moon’s interactions with Jupiter’s magnetosphere, 

and analyze Jovian atmospheric features. During E4, the Orbiter flew as close as 692

kilometers (429 miles) above the moon’s surface. The encounter sequence ran from 15 to 

22 December 1996 GMT. It included occultations of the Sun and Earth by both Jupiter and 

Europa, which provided an opportunity to search for indications of an ionosphere and 

atmosphere on the moon. The return of data from E4 was limited by a solar conjunction 

104	 W. J. O’Neil, N. E. Ausman, J. A. Gleason, M. R. Landano, J. C. Marr, R. T. Mitchell, R. J. Reichert, and M. A. 

Smith, “Project Galileo at Jupiter” (paper number IAF-96-Q.2.01 of the International Astronautical Federation, Paris, 

France), pp. 27–31. Taken from a compilation of papers, The Flight of Project Galileo as Reported Annually to the IAF/

AIAA, with the number 1625-592, and published by NASA in May 1997. All papers in the compilation were presented 

at the 47th International Astronautical Congress, Beijing, China, 7–11 October 1996.

105 O’Neil et al., “Project Galileo at Jupiter,” pp. 27–31.

106 W. J. O’Neil, J. K. Erickson, J. A. Gleason, G. R. Kunstmann, J. M. Ludwinski, R. T. Mitchell, and R. J. Reichert, “Project 

Galileo Completing Its Primary Mission” (paper number IAF-97-Q.2.02 of the International Astronautical Federation, 

103	 “Europa: Another Water World?” Paris, France, from the 48th International Astronautical Congress, Turin, Italy, 6–10 October 1997), pp. 3, 5–6.
256	 MISSION TO JUPITER: A History of the Galileo Project Chapter 9 I The orbiter tour 257

on 19 January 1997, occurring approximately midway between the E4 and E6 encounters. 

During this time, the Sun’s proximity to the signal path from Galileo to Earth completely 

blocked the return of data for about 10 days and kept the data-transmission rate low for 

many days before and after this period.107

J5. No close encounter to a Jovian moon was designed into the spacecraft’s 

fifth orbit around Jupiter because Earth and Jupiter were in solar conjunction about the 

time that the closest approach would have occurred, and there would have been minimal communication capability between the spacecraft and Earth. There also would have 

been insufficient capability during the cruise part of the orbit, after the close encounter, 

to return collected data to Earth.108

E6. The Orbiter’s next close encounter with Europa took place on 20 February 

1997 at an altitude of 586 kilometers (363 miles). The main scientific objective was to conduct high-resolution coverage of Europa. This was a similar objective to E4, but with some 

new Europa surface terrain observed. Monitoring of Io was also conducted. During E6, 

the spacecraft would fly as close as about 400,000 kilometers (250,000 miles) to Io. Jupiter 

atmospheric observations during E4 involved a coordinated effort by all of the Orbiter’s 

remote sensing instruments to analyze white oval features in the infrared and ultraviolet 

regions of the spectrum. Four occultations of Earth occurred during E6—two by Europa, 

one by Io, and one by Jupiter. The radio science occultation measurements made during 

these events provided data on atmospheric profiles of the moons and Jupiter, and also on 

Europa’s gravitational field.109

E11. The Orbiter’s last close encounter of its Prime Mission took place on 6 November 

1997 at an altitude of 2,042 kilometers (1,266 miles). The encounter sequence lasted seven 

days, from 2 to 9 November 1997 GMT. The encounter included the longest recording to date, 

lasting almost 3 hours, of Jovian magnetospheric data close to Europa. The data were helpful 

not only in the study of Europa, but also for analyzing Io’s plasma torus, whose charged particles are strongly influenced by the magnetic fields they encounter. Primary science objectives 

of E11 included more remote sensing of the moon’s surface and more Jovian atmospheric 

observations. Another objective was to obtain the highest resolution images yet taken of four 

small, inner Jovian satellites: Thebe, Metis, Amalthea, and Adrastea.110

Galileo Europa Mission (GEM) Encounters

The first eight close encounters on GEM, orbits E12 through E19, were with the satellite 

Europa. Due to reduced resources at the start of GEM (a smaller project budget and only 

a fraction of the project staff), encounter sequences were shortened from typically seven 

days during the Prime Mission to about two days. During the Prime Mission, Galileo’s 

Europa observations had to compete with many other science objectives, including observations of Jupiter’s atmosphere and other satellites. During GEM, Europa encounters got 

the lion’s share of Orbiter resources. Details of these Europa encounters are included in 

table 9.8. Note the range of regions and surface features that Galileo observed.111

Table 9.8. GEM Europa encounters.112

Orbit Closest Approach in Kilometers

(Miles)Kilometers () Date Observations

E12 196 (122) 16 December 1997 The instruments observed the Conamara 

ice raft region and took stereo images of 

the Pwyll crater region. Stereo imaging 

discerned the topography of a region.

E13 3,562 (2,212) 10 February 1998 No remote sensing or magnetospheric 

data were collected because of the solar 

conjunction, which reduced the capacity 

to transmit science data to Earth. Radio 

science data for studying Europan 

gravitational field and internal structure 

were taken.

E14 1,645 (1,022) 28 March 1998 Stereo imaging of Mannann’an crater and 

Tyre Macula dark spot was accomplished. 

The spacecraft observed banded terrain, 

bright plains, and ice rafts.

E15 2,515 (1,562) 31 May 1998 The spacecraft carried out stereo and 

color imaging of Cilix massif, previously 

believed to be the largest mountain on 

Europa (but E15 data revealed that it was 

an impact crater). Created near-terminator 

maps of unexplored mottled terrain. (The 

terminator is the boundary between the 

part of the moon that is illuminated and 

that which is dark.) Because of the low 

Sun angles near the terminator, shadows 

cast by uneven terrain are more measurable, and the heights of mounds, ridges, 

and ice rafts can be determined.

E16 1,830 (1,136) 21 July 1998 A spacecraft safing event prevented 

Europan science observations. The cause 

of the event was believed to be debris 

generated in the slip rings between the 

spun and despun sections of the Orbiter. 

The spacecraft passed over the Europan 

south pole.

107 Ibid.

108 Ibid., p. 7.

109 Ibid., p. 7.

110 R.T. Mitchell, Z. N. Cox, J. K. Erickson, B. G. Paczkowski, and E. E. Theilig, “Project Galileo: The Europa Mission”

(paper number IAF-98-Q.2.01 of the International Astronautical Federation, Paris, France, from the 49th International

Astronautical Congress, Melbourne, Australia, 28 September to 2 October 1998), p. 8.

111		Ibid.

112 Ibid.; J. K. Erickson, Z. N. Cox, B. G. Paczkowski, K. W. Sible, and E. E. Theilig, “Project Galileo Completing Europa,

Preparing for Io,” paper number IAF-99-Q.2.02 in Proceedings of 50th International Astronautical Congress of the 

International Astronautical Federation (held in Amsterdam, The Netherlands, 4–8 October 1999); UCLA Institute of 

Geophysics and Planetary Physics, “Mission Data,” Galileo Magnetometer Team’s Home Page, 27 October 2000, 

http://www.igpp.ucla.edu/galileo/qlook/mission.htm (accessed 24 November 2004).
258	 MISSION TO JUPITER: A History of the Galileo Project Chapter 9 I The orbiter tour 259

Table 9.8. GEM Europa encounters. (continued)

Orbit Closest Approach in Kilometers

(Miles)Kilometers () Date Observations

E17 3,582 (2,224) 26 September 

1998

A south polar pass (like that of E16) allowed observations of many of the targets missed during E16. The spacecraft 

searched for evidence of large-scale 

shifting of surface features, which would 

indicate a possible liquid sublayer. The 

spacecraft’s instruments took images 

of the Agenor Linea-Thrace Macula region, Libya Linea, a strike-slip fault zone, 

Rhiannon Crater, Thynia Linea, and south 

polar terrain (for comparison with E4 and 

E6 equatorial terrain images). Thermal 

maps of Europa were generated. Radioscience analyses of the Europan gravity 

field were made over a 20-hour period. 

The instruments also made ultraviolet 

observations of Europa outgassing and 

atmospheric emissions.

E18 2,273 (1,412) 22 November 1998 A safing event terminated science observations 6 hours before the Europan closest approach. The primary collection was 

of radio-science Doppler data.

E19 1,439 (894) 1 February 1999 The instruments carried out global- and 

regional-scale mapping, along with imaging of Tegid Crater, Rhadmanthys Linea 

volcanic features, mottled terrain, and a 

dark spot. The ultraviolet instruments also 

made observations of atmospheric emissions and possible outgassing. A safing 

event terminated science observations 4 

hours after the Europan closest approach. 

Outbound distant observations of Europa 

(as well as Jupiter and Io) were lost.

Galileo Millennium Mission Europa Encounter

The Europa satellite encounter on 3 January 2000 (E26) marked the beginning of the 

Galileo Millennium Mission (GMM). During this encounter, the spacecraft flew as close 

to Europa as 351 kilometers (218 miles) and passed slightly south of the prominent Pwyll 

impact crater. Only limited observations were made during E26 due to factors such as the 

decreasing periods of Galileo orbits (allowing less time to develop orbital sequences), a 

smaller workforce and budget than during GEM, and reduced downlink resources. 

The E26 encounter with Europa began with real-time data collection113 by Galileo’s 

six fields-and-particles instruments, which continued the survey of Jupiter’s magnetosphere 

that had been carried out during previous encounters. The fields-and-particles instruments 

259

began their observations on 1 January 2000 at a distance from Jupiter of 29 RJ, and they 

continued it until 5 January, when the spacecraft was only about 15 RJ from the planet.114

Recorded observations during E26 included high-resolution pictures near the 

Europan terminator,115 images of three of the four Jovian inner moons (Thebe, Amalthea, 

and Metis), and observations of the Loki volcanic region on Io. Near the terminator line, 

the Sun angle is low, and shadows are cast that can help determine the heights of surface features. The spacecraft took low-Sun-angle images of Europan surface features that 

included possible volcanic flows, ejecta from the Callanish impact basin, and the point of 

intersection of two ridges. These features were chosen because they related to issues of 

subsurface water inside the moon.116

The E26 flyby was also designed to better characterize Europa’s magnetic field 

signature and to help determine whether the moon either generated its own magnetic 

field through some internal process or had only a magnetic field induced by Jupiter’s 

magnetism. Scientists hoped that E26 data would show whether the Europan magnetic 

field was constant, indicating an intrinsic, internally generated field, or time-varying, 

suggesting an induced field whose characteristics were affected by Europa’s location 

within Jupiter’s magnetosphere.117

Europa Scientific Findings

Lack of Craters. Europa has relatively few craters, as compared to Ganymede 

and Callisto, or, for that matter, to Earth’s Moon. Models of current cratering rates due to 

impacts with comets and meteoroids indicate that our Moon’s surface has been geologically inactive for more than a billion years. Geological activity during this period would 

have obliterated many of the craters. Earth has been impacted at least as many times as 

the Moon during the last billion years, but geological processes such as plate tectonics 

and volcanic flows, as well as continuous weathering, have smoothed over most of our 

cratered areas.118

Like our Moon, the Jovian satellites Ganymede and Callisto are heavily cratered, 

suggesting that they too have very old and inactive surfaces. It is unlikely that Europa has 

somehow avoided meteoroid and comet impacts when Ganymede and Callisto, whose 

orbits are not far outside Europa’s, show evidence of such collisions. Since Jupiter was hit 

only a few years ago with numerous fragments of the Shoemaker-Levy comet, it is likely 

that the planet, as well as its moons, sustained similar impacts in the recent past. If Europa 

has indeed been subjected to many collisions in the past, it suggests that the moon has an 

active geology capable of rapidly obliterating the impact craters.119

114		 Erickson, Bindschadler, Theilig, and Vandermey, “Project Galileo: Surviving Io, Meeting Cassini,” pp. 11–12.

115 The terminator is the boundary between the part of a moon or a planet that is illuminated by sunlight and the part that 

is dark. Encarta World English Dictionary (North American Edition), 2001.

116 Erickson, Bindschadler, Theilig, and Vandermey, “Project Galileo: Surviving Io, Meeting Cassini,” pp. 11–12.

117 Ibid., p. 12.

118 “Europa; Another Water World?”; Clark R. Chapman, “What’s Under Europa’s Icy Crust?” Southwest Research 

Institute’s Technology Today (Fall 1998), http://www.swri.edu/3pubs/ttoday/fall98/europa.htm (available in folder 

18522, NASA Historical Reference Collection, Washington, DC).

119 “Europa; Another Water World?”

113		 During real-time data collection, data were sent to Earth as they were gathered, rather than being stored on the

spacecraft’s tape recorder and sent to Earth at a later time.
260	 MISSION TO JUPITER: A History of the Galileo Project Chapter 9 I the orbiter tour 261

Dr. Clark Chapman, a planetary scientist at the Southwest Research Institute in 

Boulder, Colorado, estimated the age of Europa’s surface based on current data on the 

frequency at which solar system bodies are impacted and scarred by comet and meteoroid 

hits. Some of these data were developed by Gene and Carol Shoemaker, codiscoverers of the 

Shoemaker-Levy comet that hit Jupiter in July 1994. The Shoemakers led a telescopic survey 

of comets in near-Jupiter space, defining an impact rate. According to current data, a crater 20 

kilometers in diameter or larger is created on Europa approximately every million years. Fewer 

than 10 such craters have been observed on Europa, a finding that indicates a surface averaging 

less than 10 million years old. Chapman believes that the sections of the surface located in nearly 

crater-free regions are far younger than this. “We’re probably seeing areas a few million years old 

or less, which is about as young as we can measure on any planetary surface besides Earth.”120

Images from Galileo support the theory of active Europan tectonics that have 

wiped out old craters. The images depict blocks of crust resembling icebergs floating on 

an invisible ocean (see figure 9.4). Some of these blocks are tilted or rotated out of place, 

as if local stresses broke them free and turned them. Dark bands of ice and rock spread 

outward from central ridges. A mechanism that might account for these features is “tidal 

flexing,” which occurs as Europa is pulled in different directions by the gravitational fields 

of Jupiter and the other Galilean moons. Europa gets stretched and compressed up to 

several tens of meters each day, fracturing its brittle ice crust and possibly leading to eruptions of ice volcanoes or geysers that shower the surface with material from below.121

Not all scientists agreed that Europa’s surface was young. Dr. Michael Carr, a 

geologist with the U.S. Geological Survey and another Galileo imaging team member, put 

Europa’s surface age at closer to one billion years. He compared the number of smaller 

craters on Ganymede with those on Earth’s Moon and found indications that the cratering 

rate in the Jovian system was significantly less than in the Earth-Moon system. From this, 

he concluded that the relatively smooth surface of Europa was simply due to a lower rate 

of impacts with comets and meteoroids. As more observations of Europa are taken, this 

debate on the age of its surface may be resolved. Of special interest is evidence of current 

geological activity such as erupting geysers. If observations can establish that the satellite’s 

surface has changed since the Voyager flyby in 1979, or during the years of Galileo flybys, 

then this evidence will support the hypothesis of a dynamic geology that can rapidly alter 

surface features.122

Evidence for a Subsurface Ocean. The results of many different research 

efforts, including surface observational data, laboratory experiments, magnetometer data, and gravity measurements, suggest that a liquid ocean may exist beneath 

Europa’s icy crust. 

The very young age of the Europan surface (see the discussion in the previous 

section) indicates that the surface is being rapidly renewed, possibly by the movements 

of a low-viscosity region below it.123 The moon’s surface topography also supports this 

possibility, especially the fractured chunks of ice that have been observed. “In some areas, 

the ice is broken up into large pieces that have shifted away from one another, but obviously fit together like a jigsaw puzzle,” says Dr. Ronald Greeley, a geologist at Arizona 

State University and scientist on the Galileo imaging team. “This shows that the ice crust 

has been or still is lubricated from below by warm ice or maybe even liquid water.”124

NIMS observations of Europa’s surface added to the case for a subsurface 

ocean. In 1998, NASA reported that Europa’s surface appeared to contain magnesium and sodium salts (sulfates and carbonates). The salt deposits were associated 

with surface regions that had experienced the most recent disruption. These were 

important findings because a likely source of such salt deposits is a brine ocean 

below the ice crust. If the water in this ocean is carbonated, it would explain how 

enough pressure could be generated to produce the sprays of debris that have been 

observed on the surface.125

Another piece of data supporting the existence of a brine ocean was provided 

by laboratory experiments involving salt deposits. Magnesium sulfate salts were formed 

from brines in experiments simulating the conditions that might be expected on a moon 

121	“Europa: Another Water World?”

122 Savage and Platt, “New Images Hint.” 

123 Krishan K. Khurana and Margaret G. Kivelson, “Potential for a Subsurface Ocean on Europa and Its Suitability for 

Life,” abstract P21C-06 in Eos Transactions, AGU 82, no. 47, Fall Meeting Suppl., 2001; Chapman, “What’s Under 

Europa’s Icy Crust?” 

124 Douglas Isbell (NASA Headquarters) and Mary Beth Murrill (JPL), “Jupiter’s Europa Harbors Possible ‘Warm Ice’ or 

Liquid Water,” NASA news release 96-164, 13 August 1996.

125 Mitchell et al., “Project Galileo: The Europa Mission,” p. 13; Ronald Greeley, Christopher F. Chyba, James W. Head III, 

Thomas B. McCord, William B. McKinnon, Robert T. Pappalardo, and Patricio Figueredo, “Geology of Europa,” in Jupiter, 

ed. Fran Bagenal, Tim Dowling, and Bill McKinnon (Port Melbourne, Australia: Cambridge University Press, 2004).

120 Chapman,“What’s Under Europa’s Icy Crust?”; Donald Savage (NASA Headquarters) and Jane Platt (JPL), “New Images

Hint at Wet and Wild History for Europa,” NASA news release 97-66, 9 April 1997.

Figure 9.4. A comparison of the size of Europa’s iceberglike surface structures with features 

of Earth’s San Francisco Bay area. Both images show areas of equal size, 34 by 42 kilometers 

(21 by 26 miles), and resolution, 54 meters (59 yards). North is to the top of the picture. 

Europa’s crustal plates, ranging up to 13 kilometers (8 miles) across, have been broken apart 

and “rafted” into new positions, superficially resembling the disruption of pack ice on polar 

seas during spring thaws on Earth. The size and geometry of Europa’s features suggest that 

motion was enabled by water underneath the ice rafts. (JPL image number PIA00597)
262	 MISSION TO JUPITER: A History of the Galileo Project Chapter 9 I the orbiter tour 263

with a subsurface ocean that finds channels to the surface. The brines in the experiment 

were rapidly cooled on cold surfaces under a vacuum, and the resulting salt deposits 

exhibited reflectance spectra more similar to those for Europa than salts produced at 

room temperature.126

Galileo’s magnetometer provided perhaps the strongest evidence for a liquid 

briny ocean on Europa. The magnetometer data showed that the direction of Europa’s 

magnetic field changes in a manner suggesting that it is induced by Jupiter’s field rather 

than generated independently of Jupiter through processes within the moon itself. The 

importance of an induced Europan magnetic field is that it can be explained by the existence of a spherical shell of electrically conducting material (such as salt water) just under 

Europa’s surface. The Jovian field would induce electric currents in this conducting layer, 

and these currents would generate the observed Europan field.127

Data supporting an induced field were gathered by the team of Dr. Margaret 

Kivelson, a UCLA professor and principal investigator for Galileo’s magnetometer experiment. The team had originally been looking for evidence of an internally generated 

Europan magnetic field. The team found, however, that the axis of the observed Europan 

magnetic field differed from the moon’s rotation axis by approximately 90 degrees—difficult to explain for an internally generated field. Earth’s magnetic field, for instance, is 

much more closely aligned with our planet’s axis of rotation. The direction of the Europan 

field axis could be explained, however, if the field was induced by Jupiter’s field.128

To settle the question of the type of field that Europa has, the Kivelson team 

asked JPL to design a Galileo flyby when the moon was passing through a region of the 

Jovian magnetic field in which the field was pointing opposite to the direction in which 

it had been pointing during previous flybys. If the Europan field was indeed induced by 

Jupiter’s field, the opposite polarity of the Jovian field should cause the moon’s field to 

flip its polarity. An internally generated Europan field, however, would not flip. A Europa 

flyby (E26) was planned for January 2000, when the spacecraft would fly through a 

reversed Jovian field region. The spacecraft magnetometer observations taken during the 

flyby conclusively demonstrated that Europa’s field polarity reversed when Jupiter’s field 

was pointing in the reverse direction, supporting the existence of an induced Europan 

field. Furthermore, a detailed analysis of data collected during several Europan flybys 

demonstrated that a global, salty Europan ocean approximately 6 to 100 kilometers (4 to 

60 miles) in depth, with a salinity similar to that of Earth’s ocean, could explain the field 

observed by the magnetometer.129

Gravity measurements revealed that Europa’s surface layer or layers have a 

low density, similar to that of either liquid or frozen water. These results were consistent with the existence of an induced magnetic field and subsurface ocean, but only 

if one of the surface layers was liquid. Europa’s induced magnetic field is believed to 

originate from currents in an electrically conducting layer near the moon’s surface. 

Since ice is not a good conductor of electricity, it is a poor candidate for the composition of the layer responsible for the induced field. A layer of liquid water containing 

dissolved salt ions has a much higher conductivity and can better explain Europa’s 

induced magnetic field.130

If a liquid-water ocean does exist, it is not the Sun that keeps it in a liquid 

state. At Europa’s great distance from the Sun, the intensity of solar radiation is less 

than 4 percent what it is on Earth’s surface.131 A mechanism that might generate enough 

heat to maintain a liquid Europan ocean is the moon’s tidal flexing. Thermal energy produced by the satellite’s expansion and contraction could be sufficient to melt part of its ice 

crust, thereby creating invisible seas below the surface. Heating from radioactive sources 

in the moon may add to the melting.132

A liquid-water sea cannot exist uncovered on the surface of Europa. Due to the 

moon’s nearly nonexistent atmospheric pressure, water near the top of such a sea would 

quickly evaporate and be released into space or settle as snow or ice crystals on the moon’s 

surface. In addition, the low temperature (-162o

C or -260oF) would rapidly freeze a layer 

of water. These processes would quickly seal in a liquid-water sea, preventing further loss. 

Therefore, if liquid water does exist on Europa, it is under the moon’s ice crust.133

Could a Europan Ocean Support Life?

Once the existence of an ocean beneath the ice crust of Europa was suspected, a 

most tantalizing question arose both within and outside the space science community: 

Might life exist on Europa? The three main factors that exobiologists seek when searching 

for extraterrestrial life are water, organic compounds, and adequate heat. The presence of 

a liquid-water ocean would satisfy the first criterion, and organic compounds are known 

to be prevalent in our solar system. As to the third criterion, imaging data of Europa 

demonstrate the existence, at least at one time and very likely at present, of enough heat 

to create convection currents that drove or drive ice flows on the surface of the moon. 

This circumstance does not prove that life actually exists—only that it might.134

Scientists consider Europa to be one of the handful of solar system locations that “possess an environment where primitive forms of life could possibly exist.”135 Mars and the Saturnian 

moon Titan are other such places. It cannot be stressed enough, however, that “could exist” does 

not by any means imply that the existence of life is even probable in those places. The search for 

extraterrestrial life-forms has to proceed slowly and meticulously, and all conclusions need to be 

based on good science. In the words of Daniel S. Goldin, former NASA Administrator:

126		Greeley et al., “Geology of Europa.”

127 Mitchell et al., “Project Galileo: The Europa Mission,” p. 9; Khurana and Kivelson, “Potential for a Subsurface Ocean”;

Webster, “Galileo Evidence Points to Possible Water World.”

128 Krishan K. Khurana interview, telephone conversation, 8 October 2003.

129 Khurana and Kivelson, “Potential for a Subsurface Ocean”; Khurana interview, 8 October 2003; Webster, “Galileo

Evidence Points to Possible Water World.”

130	Khurana and Kivelson, “Potential for a Subsurface Ocean”; Mitchell et al., “Project Galileo: The Europa Mission,” p. 9;

Webster, “Galileo Evidence Points to Possible Water World.”

131 		JPL, “Jupiter, Gas Giant,” Galileo: Journey to Jupiter, 17 June 2002, http://www.jpl.nasa.gov/galileo/jupiter/

jupiter.html.

132 “Europa:Another Water World?”; “Galileo Finds Evidence for an Ocean on Europa”; Kathy Sawyer, “Galileo Mission’s

Perseverance Extends Studies of Jupiter’s Moons,” Washington Post (26 December 1997): A3.

133 LarryPlatonic, “An Ocean Discovered: Europa Surrenders Her Secrets,” Galileo Messenger (May 1997); “Divining Water

on Europa.”

134 Douglas Isbell (NASA Headquarters) and Franklin O’Donnell (JPL), “Ice Volcanoes Reshape Europa’s Chaotic 

Surface,” NASA news release 97-12, 17 January 1997; “Europa: Another Water World?”

135 Douglas Isbell (NASA Headquarters) and Mary Beth Murrill (JPL), “Jupiter’s Europa Harbors Possible ‘Warm Ice’ or 

Liquid Water,” NASA news release 96-164, 13 August 1996.
264	 MISSION TO JUPITER: A History of the Galileo Project Chapter 9 I the orbiter tour 265

A few days ago, I greeted the possibility of ancient microbial life 

on Mars with skeptical optimism, and invited further scientific examination and debate . . . . The potential for liquid water on Europa is 

an intriguing possibility, and another step in our quest to explore the 

solar system, the stars, and the answer to the great mystery of whether 

life exists anywhere else in the cosmos . . . . Once again, NASA will 

ask the scientific process to work.136

What Europan Energy Source Could Support Life? Europa has possibly had abundant water, rich in organics and minerals, from the time it was created. On Earth, life 

appeared within 700 million years of the planet’s formation. Europa is as old as Earth, and 

the moon’s oceans may have had adequate time for life to evolve in them.137

But what type of energy might support life on Europa? Most life on Earth 

depends, either directly or indirectly, on photosynthesis. According to Christopher Chyba 

of the Search for Extra-Terrestrial Intelligence (SETI) Institute, the first link in Earth’s 

food chain “is chlorophyll’s conversion of sunlight into chemically stored energy. But 

imagine an ocean on Europa, a huge, bottled-up body of water capped with miles of ice. 

Photosynthesis isn’t going to work there.” Nonetheless, according to Chyba, there are 

other ways to “make a metabolic living in those dark seas.”138 

According to Chyba and his colleague, Kevin Hand of Stanford University, the 

energy necessary to support life on Earth is usually obtained through oxidation-reduction 

reactions in which substances such as carbon and oxygen bond to share an electron and 

release energy during the reaction. In Earth’s oceans, molecular oxygen, a product of 

photosynthesis, acts as the oxidizing agent in such reactions. But would molecular oxygen 

or other oxidizing agents be present in the depths of a Europan ocean? Chyba and Hand 

think perhaps yes. High-speed particles accelerated in Jupiter’s magnetosphere routinely 

bombard Europa’s ice crust, where they form oxidants such as hydrogen peroxide and 

molecular oxygen. The critical question is whether these molecules eventually get transported through Europa’s ice crust—for instance, through geological upheavals where 

sections of the crust are overturned—and into an ocean below. “We can’t be certain at this 

point whether the oxidants would actually make it into the water, even over geological 

timescales,”139 said Chyba.

Even if such molecules do not get transported through the ice, other mechanisms 

exist that might supply molecular oxygen to the oceans. One of these is the radioactive 

decay of potassium isotope 40K, which should be present in both the Europan ice crust 

and the liquid water below. This decay would split water molecules, generating oxygen 

that could conceivably support biological processes.140

Some scientists believe that geochemical energy could also help to support 

Europan life, although at significantly lower abundances than on Earth. This view is 

based in part on discoveries of Earth-based ecological communities on the ocean floor 

near volcanic vents and in hot springs, where life-forms employ chemical energy found 

in near-boiling water. Using models of geochemical activity from rock weathering combined with volcanic activity, Bruce Jakosky of the University of Colorado and Everett 

Shock of Washington University in St. Louis calculated the potential amount of life 

that geochemical energy could support on various planets. They found that sufficient 

chemical energy could be found on Mars for roughly 20 grams of organisms per square 

centimeter of surface to develop over 4 billion years. They estimated that a somewhat 

smaller amount of life could develop on Europa. For purposes of comparison, sufficient 

geochemical energy exists on Earth for 20 grams of organisms per square centimeter of 

surface to grow in only 1,000 years—4 million times faster than on Mars. Jakosky and 

Shock suggest that the most likely place to look for life on Europa is not in an ocean, 

but in the rocks under the ocean. Internal heat sources could possibly transfer life-sustaining energy to organisms living in the rocks.141

A Europan-Like Region on Earth. Can life exist in the frigid conditions of an icy 

solar system body such as Europa? The best way to find out is to send a spacecraft to 

Europa to conduct in situ analyses. But until such a mission is launched, studying similar 

regions that are a bit closer can help to determine how extreme an environment can be 

and still support life. One such extreme environment that resembles Europa in several 

ways is Lake Vostok in Antarctica.142

In 1974, an airborne research team passed over the Soviets’ Vostok station in 

Antarctica. The team’s sounding instruments detected a body of water that was located 

beneath 3,700 meters (12,000 feet) of ice and was about 50 by 220 kilometers (30 by 

140 miles) in size—roughly as large as Lake Ontario. Scientists thought that the lake was 

kept liquid by geothermal energy, combined with the insulation provided by its nearly 

4-kilometer-thick ice blanket. 

In 1998, Richard Hoover of NASA Marshall Space Flight Center and Dr. S. S. 

Abyzov of the Russian Academy of Scientists used an electron microscope to examine 

ice cores from above Lake Vostok for evidence of microbial life. They found cyanobacteria, bacteria, fungi, spores, pollen grains, diatoms (a type of algae), and, according to 

Hoover, “some really bizarre things . . . not recognizable as anything we’ve ever seen 

before.” For instance, some of the microbes were covered with a strange, fibrous structure made out of “nanofilaments” only 30 to 40 nanometers in diameter. An unusual 

feature of many of the cyanobacteria taken from ice at a depth of 1,200 meters (about 

a third of the way down to the water) was that they contained abnormal amounts of 

antimony, a toxic heavy metal.143

Richard Hoover explained that when the microorganisms froze, they went 

into an “anabiotic” state, which means that they shut down their functions and became 

136	Laurie Boeder (NASA Headquarters contact), “Statement by Administrator Daniel S. Goldin on the Release of New 

Galileo Spacecraft Images of Europa,” NASA news release 96-166, 13 August 1996. 

137 Larry Palkovic, “An Ocean Discovered: Europa Surrenders Her Secrets.”

138 SETI Institute, “Hidden Oceans Could Still Support Life,” news release, Mountain View, CA, 14 June 2001, 

http://www.seti-inst.edu/general/press_release/hiddenoceans.html.

139 Ibid.

140 Ibid.

141 “Limited Energy for Mars and Europa Life, Scientists Report,” SpaceViews—The Online Publication of Space Exploration

(26 August 1998), http://www.spaceviews.com/1998/08/26a.html.

142 “The Frosty Plains of Europa,” Science@NASA, 3 December 1998, http://science.nasa.gov/newhome/headlines/

ast03dec98_1.htm.

143 Ibid.; Marshall Space Flight Center, “Clues to Possible Life May Lie Buried in Antarctic Ice,” 5 March 1988, http://

science.nasa.gov/newhome/headlines/ast05,ar98_1.html; “Exotic-Looking Microbes Turn Up in Ancient Antarctic

Ice,” Science@NASA, 13 March 1998, http://science.nasa.gov/newhome/headlines/ast12mar98_1.htm; “Diatom,”

Microsoft Encarta Online Encyclopedia, 2001.
266	 MISSION TO JUPITER: A History of the Galileo Project Chapter 9 I the orbiter tour 267

inactive, as if in suspended animation. Frozen microbes can often be revived, however. 

Russian scientists have revived and cultured bacteria, yeast, fungi, and other microbiota 

from ice cores. 

The ice that harbored the Vostok finds varied in age up to 400,000 years. 

The ice was extracted using technology specially developed by scientists at the St. 

Petersburg Mining Institute in Russia for drilling ice cores without contaminating the 

samples. The Lake Vostok studies showed that life can exist under surprisingly harsh 

conditions. A subsurface Europan ocean may have a temperature profile, chemical composition, and ice cap resembling Lake Vostok’s. Although such a correlation 

cannot prove the existence of life on Europa, it does make it easier to accept the possibility of life on the Jovian satellite.144

Europa’s Atmosphere. In 1977, researchers suspected that an oxygen atmosphere might exist on icy bodies such as Ganymede and Europa. If so, water vapor 

would be present in at least small quantities. Solar radiation would slowly dissociate the water molecules into hydrogen and oxygen. The hydrogen, being lighter than 

oxygen, would escape the moons far more rapidly. Particles impacting the moons’ surfaces would dislodge additional molecules of surface materials and send them into the 

moons’ atmospheres.145

Scientists predicting a Europan atmosphere containing gaseous oxygen had 

to wait until the sensitive Hubble Space Telescope (HST) could confirm their theory. 

In February 1995, almost a year before Galileo reached Jupiter, a Johns Hopkins team 

using the HST reported that they had identified an “extremely tenuous atmosphere of 

molecular oxygen” around Europa. This was not an atmosphere that was likely to support life. Its surface pressure was estimated at one hundred-billionth that of Earth. Doyle 

Hall of Johns Hopkins, principal investigator of the team that detected that atmosphere, 

said that if all the oxygen estimated to be in the Europan atmosphere were compressed 

to the surface pressure of Earth’s atmosphere, it would fill “only about a dozen Houston 

Astrodomes.” The temperature of this thin atmosphere was -145o

C (-230oF).146

During Galileo’s December 1996 and February 1997 Europa encounters (E4 and 

E6), the spacecraft performed six occultation experiments in which Europa was positioned between the spacecraft and Earth. This configuration affected the radio signal 

sent from Galileo to Earth. Measurements of the signal by the Deep Space Network stations in Goldstone, California, and Canberra, Australia, showed that the radio beam from 

Galileo had been refracted by a layer of charged particles around Europa. According to

Dr. Arvydas Kliore of JPL, the source of these ions was most likely water molecules from 

Europa’s icy surface, dislodged by high-energy particles from Jupiter’s magnetosphere 

to form a thin atmosphere. Water molecules from this atmosphere were then ionized, 

which could have been done by ultraviolet radiation from the Sun or by collisions with 

additional energetic particles from the Jovian magnetosphere.147

Ganymede: Jupiter’s Largest Moon Has a Ravaged Surface and a Magnetic Field

Ganymede, with a diameter of 5,270 kilometers (3,270 miles), is larger than Earth’s 

Moon, larger than Pluto, and larger even than Mercury. It has been extensively bombarded by comets and meteorites and distorted by the same tectonic forces that make 

mountains and move continents on Earth. Ganymede has its own magnetosphere, a 

bubble-shaped region of charged particles surrounding the moon. This discovery was 

surprising to Galileo scientists because although many planets have magnetospheres, 

satellites generally do not.148

General Ganymede data are included in table 9.9.

Table 9.9. Ganymede statistics.149

Discovery 11 January 1610, by Galileo Galilei

Diameter 5,270 kilometers (3,270 miles)

Mass 1.48 x 1023 kilograms

Mass relative to that of Earth 0.0247 times the mass of Earth

Surface gravity (Earth = 1) 14.5 percent of Earth’s gravity

Mean distance from Jupiter 1,070,000 kilometers (664,000 miles)

Mean distance from Jupiter in radii 15.1 RJ

Mean distance from the Sun 5.203 AU

Orbital period 7.154553 days

Rotational period 7.154553 days

Density 1.94 grams per cubic centimeter

Orbit speed 10.9 kilometers per second (24,400 mph)

Surface composition Dirty ice

Ganymede Gravity-Assists Helped Make Critical Orbit Changes

Because of Galileo’s trajectory as it approached the Jovian system, the spacecraft went 

into orbit around Jupiter inclined to the equatorial plane of the planet, as well as to 

the orbital planes of the Galilean satellites. This situation had to be changed. Orbital 

mechanics calculations showed that if Galileo, with its limited propellant, were to have 144 “Exotic-Looking Microbes Turn Up in Ancient Antarctic Ice”; “Frosty Plains of Europa.”

145 NASA Langley Research Center, State of Knowledge of Europa, taken from the Report of the Europa Orbiter Science 

Definition Team, June 1999, http://centauri.larc.nasa.gov/outerplanets/Europa_SDT.pdf. Available from the Outer 

Planets Program Library, an online library on the Langley Research Center Aerospace Systems, Concepts, and 

Analysis Web site. The library is accessible at http://centauri.larc.nasa.gov/outerplanets/. The Europa document is 

also available in Galileo—Meltzer Sources, folder 18522, NASA Historical Reference Collection, Washington, DC.

146 Donald Savage (NASA Headquarters), Tammy Jones (Goddard Space Flight Center), and Ray Villard (Space 

Telescope Science Institute), “Hubble Finds Oxygen Atmosphere on Jupiter’s Moon Europa,” NASA news release 

95-17, 23 February 1995.

147 Jane Platt (JPL), “Galileo Spacecraft Finds Europa Has an Atmosphere,” JPL Public Information Office news release, 

18 June 1997

148Galilean Satellite Sheet,” National Space Sciences Data Center (NSSDC) Planetary Sciences Home Page, http://

nssdc.gsfc.nasa.gov/planetary/factsheet/galileanfact_table.txt; Mary Beth Murrill, “Galileo Spacecraft Makes New 

Discoveries at Ganymede,” JPL Public Information Office news.

149 Ganymede Fact Sheet,” JPL Galileo Home Page, http://www.jpl.nasa.gov/galileo/ganymede/fact.html (accessed 

6 June 2001). 
268	 MISSION TO JUPITER: A History of the Galileo Project Chapter 9 I the orbiter tour 269

close encounters with more than one of the Galilean satellites, the spacecraft’s orbit 

would have to be altered so that it lay within the orbital plane of the satellites. Until a 

zero-inclination condition could be achieved, all of Galileo’s close encounters had to be 

with the same moon. Ganymede was chosen as this moon because its large mass and 

orbital location were favorable for changing the spacecraft’s orbital inclination, as well 

as its orbital period (the time the spacecraft took to complete an orbit around Jupiter), 

which was longer than desired. Galileo’s initial orbital period was 210 days, or about 

seven months. A gravity-assist during G1 reduced the spacecraft’s orbital period to 72 

days, which allowed more orbits and thus more close encounters to be carried out 

each year. The G1 gravity-assist also slightly reduced the craft’s orbital inclination. 

The second encounter, G2, was planned largely to put Galileo into an orbit that was 

coplanar with the satellites’ orbits, thus enabling subsequent encounters with Europa, 

Io, and Callisto.150

Findings from Ganymede Flybys

Galileo’s first close encounter of its Prime Mission satellite tour, G1, was with Ganymede 

on 27 June 1996 at an altitude of 835 kilometers (518 miles). The second Ganymede 

encounter, G2, took place on 6 September 1996 at an altitude of 260 kilometers (161 

miles). G1 involved an equatorial-region flyby at closest approach, while G2 passed 

near a pole. Although Galileo’s trajectory during these encounters was governed by 

the orbital mechanics factors described above, by good fortune it also provided an 

excellent opportunity for radio-science observations of different parts of the moon. 

These observations used the fact that any change in Galileo’s velocity caused the frequency of the radio signal received on Earth to change. When the spacecraft passed 

close to Jupiter or to a moon, that body pulled on the craft, altering its velocity. The 

amount of velocity change depended on the mass of the body and on how that mass 

was internally distributed. Thus, by measuring the change in frequency of the radio 

signal received on Earth, the mass and internal structure of Jupiter or one of its satellites was estimated.151

The radio-science and other data received on Earth clearly indicated that 

Ganymede’s interior was differentiated, probably into a core and a mantle, which 

were enclosed in an ice shell.152 Besides revealing the moon’s differentiated interior, 

the Ganymede encounters also provided important data supporting the existence of 

a self-generated magnetic field, a possible subsurface ocean, and past volcanism that 

might explain extensive surface features. A summary of Galileo’s Ganymede flybys 

and observations is included in table 9.10.

150 W. J. O’Neil, N. E. Ausman, J. A. Gleason, M. R. Landano, J. C. Marr, R. T. Mitchell, and R. J. Reichert, “Project Galileo 

at Jupiter” (paper number IAF-96-Q.2.01 of the International Astronautical Federation, Paris, France, presented at the 

47th International Astronautical Congress, Beijing, China, 7–11 October 1996), p. 29.

151 O’Neil et al., “Project Galileo at Jupiter”; “Radio Science,” Galileo’s Science Instruments, JPL Galileo Home Page, 

http://www.jpl.nasa.gov/galileo/instruments/rs.html (accessed 10 July 2001).

152 Douglas Isbell (NASA Headquarters) and Mary Beth Murrill (JPL), “Big Icy Moon of Jupiter Found to Have a ‘Voice’ 

After All”; “Europa Flyby Next for Galileo,” NASA news release 96-255, 12 December 1996, http://www.jpl.nasa.gov/

galileo/status961212.html.

153 O’Neil et al., “Project Galileo: Completing Its Primary Mission”; O’Neil et al., “Project Galileo at Jupiter”; Erickson et al., 

“Project Galileo Completing Europa, Preparing for Io”; Erickson et al., “Project Galileo: Surviving Io, Meeting Cassini.”

Table 9.10. Ganymede encounters.153

Orbit Closest Approach in

Kilo­meters (Miles)

Date Mission Comments

G1 835 (519) 27 June 1996 Prime A gravity-assist during G1 

reduced Galileo’s orbital period from 

210 to 72 days, which allowed more 

orbits and close encounters each 

year. The perijove of orbit (point of 

closest approach to Jupiter) was increased to keep the spacecraft out 

of the most intense radiation regions. 

A radio-science experiment analyzed 

Ganymede’s gravitational field and 

internal structure. The instruments 

detected evidence of a self-generated 

magnetosphere around the moon.

G2 260 (161) 6 September 

1996

Prime A Ganymede gravity-assist put Galileo 

into coplanar orbit with other Galilean 

satellites, permitting subsequent encounters with them. A radio-science 

experiment analyzed Ganymede’s 

gravitational field and internal structure. G1 and G2 radio-science and 

other data revealed that Ganymede 

had an interior that was probably differentiated into a core and a mantle. 

The plasma wave experiment and 

magnetometer data gave evidence of 

an internally generated magnetic field.

G7 3,102 (1,926) 5 April 1997 Prime The spacecraft flew over the high 

latitudes of Ganymede and took 

high-resolution observations of 

high-energy impact regions, as well 

as Jupiter magnetosphere and 

aurora observations.

G8 1,603 (995) 7 May 1997 Prime The spacecraft passed over the midlatitudes of Ganymede, with closest 

approach longitudes 180o apart from 

those of the G7 encounter, allowing 

new terrain to be imaged.

G28 1,000 (600) 20 May 2000 GMM Galileo’s closest approach coincided 

with Cassini’s. Joint Galileo-Cassini 

observations revealed solar wind effects and magnetospheric dynamics. 

High-resolution Ganymede images 

also were taken. Magnetometer data 

suggest that a salty water layer exists 

beneath the icy crust.
270	 MISSION TO JUPITER: A History of the Galileo Project Chapter 9 I the orbiter tour 271

Table 9.10. Ganymede encounters. (continued)

Orbit Closest Approach in

Kilo­meters (Miles)

Date Mission Comments

G29 2,321 (1,441) 28 December 

2000

GMM Real-time data were transmitted as 

Galileo flew from the inner magnetosphere through the magnetopause 

and bow shock and into the solar 

wind. Remote sensing instruments 

targeted Jupiter, its rings, and the 

Galilean satellites.

Evidence of a Magnetosphere

Evidence from the G1 and G2 encounters indicates that Ganymede possesses a planetlike, self-generated magnetosphere, shielding the moon from the magnetic influence 

of its parent body, Jupiter. The Orbiter’s plasma wave subsystem (PWS), which used a 

dipole antenna as a receiver, picked up electromagnetic wave activity characteristic of a 

magnetosphere and closely matching the electromagnetic signatures of Earth’s, Saturn’s, 

and Jupiter’s magnetospheres. “The data we get back is in the form of a spectrogram, and 

reading it is kind of like looking at a musical score,” said Dr. Donald Gurnett, a physicist 

at the University of Iowa and principal investigator of Galileo’s PWS. “The instant I saw 

the spectrogram, I could tell we had passed through a magnetosphere at Ganymede.” The 

approach to the moon was relatively quiet, and then “all of a sudden, there’s a big burst 

of noise that signals entry into Ganymede’s magnetosphere. Then, for about 50 minutes, 

we detected the kinds of noises that are typical of a passage through a magnetosphere. 

As we exited the magnetosphere, there was another big burst of noise.”154

Abrupt changes in field intensity and direction also gave indications of a magnetosphere. As the spacecraft approached the moon, the magnetometer measured, as expected, the 

field of Jupiter, which was spatially fairly uniform and pointed south. Then the field suddenly 

increased nearly five times in strength and swung around to point directly at Ganymede. 

Ganymede provided the first example ever observed of a moon’s magnetosphere 

contained within the magnetosphere of the parent planet. “We knew Ganymede was an 

interesting place,” said Torrence Johnson, Galileo project scientist. “What we have just 

found makes it even more exciting.”155

Source of Ganymede’s Magnetism. UCLA’s Dr. Margaret Kivelson, principal investigator for Galileo’s magnetometer experiment, confirmed the detection of a large magnetic field 

increase near Ganymede. Combined with the discovery of a self-generated magnetic field, 

the radio-science data indicated that Ganymede had a metallic core beginning at a depth of 

400 to 1,300 kilometers (250 to 800 miles) below the surface. Depending on whether the core 

was pure iron or an iron/iron sulfide alloy, mission scientists estimated that it could account 

for as little as 1.4 percent or as much as one-third of Ganymede’s total mass. Galileo scientists 

suspected that the moon’s magnetic field is generated in a similar way to that of Earth’s field, 

through a dynamo mechanism originating in the electrically conducting core.156

Surface Terrain

Ganymede’s surface is a mixture of two types of terrain—40 percent highly cratered 

dark regions and 60 percent light, grooved terrain called “sulcus,” meaning a groove or 

burrow. The dark regions on Ganymede are old and rough in texture. The dark, cratered 

terrain is believed to be the original crust of the satellite. Lighter regions are believed to 

be smoother and younger, having relatively few craters. The light-colored, grooved sulcus 

terrain may be formed by the release of water from beneath the surface. Galileo observed 

grooved ridges as high as 700 meters (2,000 feet) that ran for thousands of kilometers 

across Ganymede’s surface.157

Ganymede’s larger craters are nearly flat and lack the central depressions 

common to craters often seen on Earth’s Moon. But our Moon has a rocky surface, whereas 

Ganymede has an icy one. Scientists think that the gradual flattening of Ganymede’s large 

craters over millions of years is probably due to the softness of its icy surface. These 

large, flat “phantom craters” are called palimpsests, which is a term that originally referred 

to reused, ancient writing materials on which older writing was still visible underneath 

newer writing. Ganymede’s palimpsest craters range from 50 to 400 kilometers (30 to 250 

miles) in diameter, and they are surrounded by rays of ejecta.158

Does Ganymede Have a Hidden Ocean?

Ganymede has a strong, self-generated magnetic field and magnetosphere. In addition, 

it has a secondary field induced by Jupiter’s magnetism. This makes Ganymede’s magnetic data more complex to interpret than, for instance, data from Europa, which has 

only the secondary induced field. UCLA’s Dr. Margaret Kivelson, principal investigator 

on Galileo’s magnetometer experiment, believes that Ganymede’s induced field has 

to originate in a material “more electrically conductive than solid ice.” A liquid layer 

several kilometers thick, with a salt content approximately that of Earth’s oceans and 

located within 200 kilometers (120 miles) of Ganymede’s surface, could be responsible 

for the induced field.159

Infrared spectrometer analysis has identified sections of Ganymede’s surface 

containing salt minerals. The salt-bearing materials may be mostly frozen magnesium 

sulfate brines that originally made their way to the surface through cracks, according to 

Dr. Thomas McCord, a University of Hawaii geophysicist. Image data show linear fractures on Ganymede that resemble such features on Europa. In addition, the salt minerals 

on Ganymede’s surface are similar to minerals seen on Europa. 

Galileo imaging data taken on the G28 encounter (May 2000) suggested further 

similarities between Ganymede and Europa. The Ganymede images show details of 

Arbela Sulcus, a bright, striated band of materials spread across an older, more heavily cratered landscape. According to Dr. Robert Pappalardo, a planetary scientist at 

154 Isbell and Murrill, “Big Icy Moon of Jupiter Found to Have a ‘Voice’ After All.”

155 Murrill, “Galileo Spacecraft Makes New Discoveries at Ganymede.”

156 Isbell and Murrill, “Big Icy Moon of Jupiter Found to Have a ‘Voice’ After All.”

157 Ganymede Fact Sheet,” JPL Galileo Home Page, http://www.jpl.nasa.ov/galileo/ganymede/fact.html (accessed 

6 June 2001). 

158 Ibid.

159 Guy Webster, “Solar System’s Largest Moon Likely has a Hidden Ocean,” JPL Media Relations Office news release, 

16 December 2000.
272	 MISSION TO JUPITER: A History of the Galileo Project Chapter 9 I the orbiter tour 273

Brown University, “It is possible that Arbela Sulcus has formed by complete separation of 

Ganymede’s icy crust, like bands on Europa.”160

A subsurface ocean needs sufficient heat to maintain it in a liquid state. The tidal 

flexing that is thought to maintain Europa’s ocean probably is not the mechanism responsible for one on Ganymede, whose nearly circular orbit would minimize such flexing. In 

order to explain the possible existence of a subsurface liquid-water layer on Ganymede, 

Galileo scientists made estimates of the thermal energy available from radioactivity in 

the satellite’s rocky interior. According to Dr. Dave Stevenson, a planetary scientist at the 

California Institute of Technology in Pasadena, Ganymede has sufficient radioactive heat 

sources to maintain a stable layer of water at a depth of roughly 150 to 200 kilometers (90 

to 120 miles) beneath the surface.161

A Mechanism To Explain Ganymede’s Bright Bands of Frozen Water

Although tidal flexing may not be the energy source maintaining a Ganymede ocean, it 

likely contributed to the melting of the ice crust in the past and possibly to volcanism. 

A billion years ago, when Ganymede was moving into its present position relative to 

the other Galilean moons, it did not have nearly as circular an orbit as it does now. At 

different points in its old, elliptical orbit, the direction and magnitude of tidal forces on 

the moon would vary, slightly altering the shape of the moon. The expansion and contraction of different parts of the moon’s interior would have resulted in heating, perhaps 

enough to have thinned its icy surface and enlarged a buried ocean, making it closer to 

the surface than now.162

During this time of tidal distortions, cracking of the crust and eruptions of slushy 

ice or water onto Ganymede’s surface may have been more common than after the moon 

attained a nearly circular orbit. Such eruptions could explain Ganymede’s extensive 

bright swaths of frozen water that overlay its darker, heavily cratered ice. Stereo images 

taken by Galileo show that the smoothest, brightest ice swaths lie in troughs 0.5 to 1 

kilometer below the dark, heavily cratered areas and that volcanic, calderalike features 

occur at the edges of the swaths. The bright bands’ low elevations and flatness could 

have resulted from water or slushy ice erupting and then flooding low-lying areas before 

refreezing. The eruptions might not have had enough energy to break through the additional ice of higher elevation surface areas.163

Louise M. Procter of the Johns Hopkins Applied Physics Laboratory said that 

the bright swaths’ low elevation “is the most persuasive evidence that they could 

be volcanic” in origin. She raises the question, however, that if icy eruptions were 

common on Ganymede a billion or so years ago, then why is there not more visible 

evidence of them? According to Paul M. Schenk of Houston’s Lunar and Planetary 

Institute, whose team created paired Voyager-Galileo images of areas on Ganymede, 

surface buckling and stretching since the eruptions could have obscured such visible 

evidence of icy eruptions in the past. Further analysis of Galileo and other spacecraft data 

may resolve the issue of how much volcanism occurred on the moon’s surface in the past.164

Callisto: Why Is This Galilean Moon So Different from the Others?

The Scarred Face of Callisto

Callisto, the outermost of the four Galilean satellites, has the most pockmarked surface of 

any solar system body yet observed. It is the presence of so many craters that supports the 

theory of a very old, unchanging Callisto surface. Although scientists do not know the exact 

rate of impact-crater formation on Callisto, they estimate that it must have taken several billion 

years for the moon to accumulate its depressions. Active geological processes on the moon 

probably would have obliterated many of these craters. Since so many craters exist, scientists 

assume that Callisto has been geologically inactive for billions of years.165

Callisto’s heavily cratered face is useful to scientists in providing a record of the 

bombardment history of the Jovian system. According to Dr. Torrence Johnson, Galileo 

project scientist, “The craters on Callisto are the visible record of what sizes of comets and 

other objects have pelted Jupiter and its moons and with what frequency . . . .”166

One puzzling feature regarding Callisto is that it appears to have notably few 

small craters. Galileo took high-resolution images in order to study more extensively 

the densities of small craters and how some of their features appear to be degraded or 

eroded. A dearth of small craters could reflect an active resurfacing mechanism that operates efficiently only on small scales. It could also indicate that relatively few small comets 

and meteoroids fly through the Jovian system and impact bodies within it.167

Callisto Encounters and General Data

Galileo made its first close pass of Callisto (C3) on 4 November 1996. The spacecraft 

flew to within 1,106 kilometers (686 miles) of the satellite’s surface. Earth received 

the signals confirming the encounter 46 minutes later.168 A summary of this and other 

Callisto encounters is included in table 9.11.

160 Ibid.; “Hydrated Salt Minerals on Ganymede’s Surface: Evidence of an Ocean Below,” JPL Galileo Home Page, http://

www.jpl.nasa.gov/galileo/ (accessed 12 July 2001). 

161 Webster, “Solar System’s Largest Moon Likely Has a Hidden Ocean.” 

162 R. Cowen, “Images Suggest Icy Eruptions on Ganymede,” Science News (3 March 2001): 133. 

163 Ibid.

164 Ibid.

165 Douglas Isbell (NASA Headquarters) and Franklin O’Donnell (JPL), “Galileo Makes Close Pass By Callisto,” NASA 

news release 96-226, 4 November 1996. 

166 Donald Savage (NASA Headquarters) and Guy Webster (JPL), “Galileo Gets One Last Close Encounter With Jupiter’s 

Callisto,” NASA news release 01-97, 22 May 2001. 

167 Southwest Research Institute, Boulder, CO, “October 1998 Update: Callisto’s Perplexing Craters,” Galileo Research (Imaging 

of the Galilean Satellites of Jupiter), 10 November 1998 (available in Galileo—Meltzer Sources, folder 18522, NASA Historical 

Reference Collection, Washington, DC); Savage and Webster, “Galileo Gets One Last Close Encounter With Jupiter’s Callisto.”

168 Isbell and O’Donnell, “Galileo Makes Close Pass By Callisto.”
274	 MISSION TO JUPITER: A History of the Galileo Project Chapter 9 I the orbiter tour 275

Table 9.11. Callisto encounters.169

Orbit Closest Approach in 

Kilo­meters (Miles)

Date Mission Observations

C3 1,136 (705) 4 November 

1996

Prime Observations supported the theory that 

Callisto has a homogeneous internal 

structure, 60 percent rock and 40 percent 

ice.

C9 418 (260) 25 June 

1997

Prime The spacecraft passed through and studied the magnetotail region of the Jovian 

magnetosphere during the period between the C9 and C10 flybys. Analysis of 

the C3, C9, and C10 data suggests that 

Callisto may have a subsurface, salty 

ocean that is responsible for a variable 

magnetic field induced by Jupiter’s field.

C10 539 (335) 17 

September 

1997

Prime C10 data suggest that the internal 

structure of the moon is not homogeneous, but partially differentiated, with a 

higher percentage of rock than ice having 

settled toward the center of the satellite. 

Callisto is probably less differentiated 

than the other Galilean moons.

C20 1,315 (817) 5 May 1999 GEM The perijove reduction campaign began; 

it involved incremental changes in the 

closest approach to Jupiter carried out 

over four Callisto encounters (C20–C23). 

The campaign was designed to set up 

flybys of Io, the Galilean moon closest to 

Jupiter.

C21 1,047 (650) 30 June 

1999

GEM NIMS studied the trailing edge of 

Callisto. The SSI camera observed 

dark surface material. The PPR studied equatorial region.

C22 2,296 (1,426) 14 August 

1999

GEM The spacecraft observed Callisto’s ionosphere and measured the distribution of 

free electrons.

C23 1,057 (656) 16 

September 

1999

GEM The spacecraft observed Callisto’s 

ionosphere, measured the distribution of 

free electrons, and completed the 

perijove reduction campaign.

C30 138 (86) 25 May 2001 Post-GMM The spacecraft observed the Asgard, 

Valhalla, and Bran craters in the 

closest flyby to date (in order to set up 

an Io encounter in August 2001). Camera 

problems were possibly due to 

continued radiation exposure that 

affected distant images taken of Io. 

Problems were corrected before the 

closest approach to Callisto.

General data regarding Callisto are included in table 9.12.

Table 9.12. Callisto data.170

Discovery 7 January 1610, by Galileo Galilei

Diameter 4,806 kilometers (2,980 miles)

Mass 1.077 x 1023 kilograms (about 2.4 x 1023 pounds) 

Mass relative to that 

of Earth

0.01807

Surface gravity 

(Earth = 1)

12.7 percent of Earth’s gravity

Mean distance from 

Jupiter

1,883,000 kilometers (1,167,000 miles)

Mean distance from 

Jupiter in radii

26.6 RJ

Mean distance 

from Sun

5.203 AU

Orbital period 16.68902 days

Rotational period 16.68902 days

Density 1.86 grams per cubic centimeter

Orbit speed 8.21 kilometers per second (18,400 mph)

Surface composition Ice

Callisto’s Internal Structure

Galileo’s observations of Callisto reveal that it is a mixture of metallic rock and ice and 

that, unlike Ganymede, it has no identifiable central core. Data taken on Galileo’s first 

Callisto encounter (C3) in November 1996 seemed to support the idea that Callisto had a 

homogeneous structure made up of 60 percent rock containing iron and iron sulfide and 

40 percent compressed ice. Scientists believed that since Callisto was located further from 

Jupiter than the other Galilean moons, it was not subjected to the same gravitational pull 

and tidal forces and thus did not experience the heating that melted parts of the other 

moons. This heating allowed heavier materials on the three other Galilean moons to sink 

and lighter ones to rise, leading to the formation of differentiated layers. “Callisto had a 

much more sedate, predictable and peaceful history than the other Galilean moons,” said 

Dr. John Anderson, a planetary scientist at JPL.

171

Later data from Galileo’s third Callisto encounter in September 1997 (C10) led scientists 

to modify their view of Callisto from a moon with a totally undifferentiated interior to one that 

is not completely uniform. According to Anderson, researchers have found signs that “interior 

materials, most likely compressed ice and rock, have partially settled, with the percentage 

of rock increasing toward the center of Callisto.” The satellite is still unlike Io, Europa, and 

169 Guy Webster (JPL), “Galileo Succeeds in Its Closest Flyby of a Jovian Moon,” JPL Media Relations Office news 

release, 25 May 2001; JPL, “Galileo Millennium Mission Status,” JPL Media Relations Office news release, 24 May 

2001; Erickson et al., “Project Galileo: Surviving Io, Meeting Cassini,” p. 19.

170 “Callisto, A Continuing Story of Discovery,” JPL Project Galileo Home Page, http://www.jpl.nasa.gov/galileo/callisto/

#overview (accessed 8 July 2001).

171 Douglas Isbell (NASA Headquarters) and Jane Platt (JPL), “Galileo Returns New Insights Into Callisto and Europa,” 

NASA news release 97-110, 23 May 1997. 
276	 MISSION TO JUPITER: A History of the Galileo Project Chapter 9 I the orbiter tour 277

Ganymede, all of which have differentiated interiors with distinct layers. The gravitational tidal 

forces believed to have heated these other moons and led to their differentiation appears to 

have only “half-baked” Callisto and somewhat separated its constituents.172

According to Dr. Gerald Schubert, a UCLA planetary physics professor and Galileo 

gravity investigator, “Learning about the structure of these celestial bodies enhances our 

knowledge of how all planets and moons form and evolve, including our own Earth and 

Moon.” Interior structure models of Jovian satellites were derived from radio Doppler 

data gathered by Galileo flybys. The moons’ gravitational signatures, which are dictated 

by the densities and distributions of the materials inside them, affected the spacecraft’s 

velocity and slightly altered the frequency of its radio signals. By analyzing those alterations, 

scientists were able to characterize the compositions and structures of the satellites.173

Magnetic Activity and a Possible Subsurface Sea

The initial analysis of plasma wave observations from Galileo’s Callisto C3 encounter 

did not find evidence of a magnetic field or magnetosphere. Magnetometer studies of 

the satellite did not find such evidence either. The lack of a Callisto magnetosphere is 

consistent with its apparent lack of a metallic core. This is different from the situation 

at Ganymede, where a metallic core is likely responsible for that moon’s self-generated 

magnetic field and magnetosphere.174

After evidence began to pour in supporting a subsurface ocean on Europa, 

data from Galileo’s November 1996 (C3), June 1997 (C9), and September 1997 (C10) 

encounters were reexamined. Variations in Europa’s Jupiter-induced magnetic field had 

been attributed to a possible brine ocean beneath its surface; researchers wondered if 

such a feature might also be found at Callisto. Dr. Margaret Kivelson, planetary science 

professor at UCLA and Galileo’s magnetometer experiment principal investigator, and 

her colleagues found signs that Callisto did indeed have a variable magnetic field 

that could be explained by the presence of Jupiter-induced electrical currents flowing 

near the moon’s surface. Kivelson’s team set out to identify the source of the currents. 

Callisto was known to have an atmosphere, but it was extremely tenuous and lacking 

in charged particles that could carry an electric current. Scientists did not believe that it 

could generate the satellite’s observed magnetic field. Callisto’s ice crust could not be 

responsible for the field, either, for ice is too poor an electrical conductor. Because of the 

lack of other explanations for the observed magnetic field, a subsurface briny ocean on 

Callisto, such as likely exists on Europa, was suspected. Lending further credence to this 

theory, Galileo’s observations showed that the electric currents induced in Callisto flowed 

in opposite directions at different times, a characteristic consistent with the existence of 

a brine ocean. “Until now, we thought Callisto was a dead and boring moon, just a hunk 

of rock and ice,” said Kivelson. “The new data certainly suggests that something is hidden 

below Callisto’s surface, and that something may well be a salty ocean.”175

Although the presence of a Europan ocean raises the possibility of life’s past 

or present existence, scientists consider it far less likely that life might exist on Callisto. 

According to Dr. Torrence Johnson, Galileo project scientist, “The basic ingredients of 

life—what we call ‘pre-biotic chemistry’—are abundant in many solar system objects such 

as comets, asteroids and icy moons. Biologists believe liquid water and energy are then 

needed to actually support life, so it’s exciting to find another place where we might have 

liquid water. But, energy is another matter, and currently, Callisto’s ocean is only being 

heated by radioactive elements, whereas Europa has tidal energy as well.”176

Observations of Jupiter’s Fields, Particles, and Atmosphere Made During 

the Satellite Tour

Some of the most interesting results reported by Galileo science teams during the satellite 

tour are related to the formation of the Jovian ring system. Jupiter has a main bright ring, 

a pair of faint “gossamer rings,” and a cloudlike ring halo. The gossamer rings are located 

just outside the main ring; at its inner edge, the main ring merges gradually into the halo 

(see figure 9.5). Scientists believe that when interplanetary meteoroids and comet fragments 

collided with Jupiter’s four small inner moons, Amalthea, Adrastea, Metis, and Thebe, they 

generated dust that formed the Jovian ring system. Galileo data also indicate that Amalthea’s 

and Thebe’s orbital characteristics are related to main- and gossamer-ring thicknesses.177

Figure 9.5. Jupiter’s inner satellites and ring components. (NASA image number PIA01627)

172 Jane Platt (JPL), “Galileo Mission Finds Strange Interior of Jovian Moon,” JPL Media Relations Office news release, 

4 June 1998. 

173 Ibid.

174 Isbell and Platt, “Galileo Returns New Insight Into Callisto and Europa.”

175 Douglas Isbell (NASA Headquarters) and Jane Platt (JPL), “Jupiter’s Moon Callisto May Hide Salty Ocean,” NASA 

news release 98-192, 21 October 1998.

176 Ibid.

177 Erickson et al., “Project Galileo: Completing Europa, Preparing for Io,” p. 16; “Jupiter’s Ring System,” Rings Node 

Home Page, http://pds-rings.seti.org/jupiter/jupiter.html (accessed 7 September 2005) (available in folder 18522, 

NASA Historical Reference Collection, Washington, DC).
278	 MISSION TO JUPITER: A History of the Galileo Project Chapter 9 I the orbiter tour 279

During GEM, Galileo was able to observe a unique Jupiter atmospheric event—

the merging of two “white oval” storms. For half a century, three storms that appear as 

white ovals when seen from above have been observed in a band in Jupiter’s equatorial 

region. At some time prior to the E17 orbit, two of the three ovals merged into one. 

Scientists could not determine the exact time that this occurred because Jupiter was in 

superior conjunction (i.e., behind the Sun from Earth’s point of view). No viewing of 

the event was possible from Earth, and Galileo did not witness the event. Observations 

of the merged ovals were carried out during E17 by Galileo’s near infrared mapping 

spectrometer, solid state imaging camera, and ultraviolet spectrometer. Prior to the merger, 

each oval had roughly two-thirds the diameter of Earth. The combined oval was as large 

as Earth and had characteristics that its parents did not. It was not visible at particular 

infrared frequencies, which indicated that it was not as deep as its parent storms. Those 

ovals had upwelling winds in their centers, with downwelling surrounding currents.178 

Galileo also acquired the highest resolution pictures ever taken of Jupiter’s 

auroral phenomena. This was accomplished during a campaign coordinated with the 

Hubble Space Telescope and instruments on Earth. The auroras seen in Galileo images 

were lower in the Jovian atmosphere than expected. Atmospheric scientists are generating 

new models to explain the auroral data supplied by Galileo and the other sources.179

Observations of Jupiter’s Inner Moons

Four tiny moons orbit closer to Jupiter than the Galilean moons. Metis, the innermost, 

and Adrastea, the next closest, were discovered through analyses of Voyager data taken in 

1979.180 Amalthea, the third moon, was discovered by Edward Emerson Barnard in 1892. 

Thebe, like the two closest moons, was also discovered in 1979 through analysis of Voyager 

data. Table 9.13 lists characteristics of Jupiter’s inner moons. The Jovian moons were named 

after women, boys, and nymphs who were associated with the Roman god Jupiter.181

Table 9.13. Jupiter’s inner moons.182

Satellite Discovery Distance from Jupiter in Kilometers

(Miles)

Diameter or Dimensions in Kilometers (Miles)

Metis 1979 128,000 (70,500) 40 (25)

Adrastea 1979 129,000 (80,000) 20 (12)

Amalthea 1892 181,300 (112,700) 270 x 166 x 150 (168 x 103 x 93)

Thebe 1979 222,000 (138,000) 100 x 90 (60 x 56)

There are dramatic differences in size between the Galilean moons (whose diameters 

range from 3,138 kilometers to 5,270 kilometers) and Jupiter’s inner moons (the largest of which, 

irregularly shaped Amalthea, is only 270 kilometers long and 166 kilometers across).183

During the Galileo Europa Mission (GEM), project engineers adjusted the spacecraft’s perijove so that Galileo would fly close by Io. This action also opened up the 

opportunity to study Jupiter’s innermost moons in greater detail. Before the summer 

of 1999, each elliptical orbit of the spacecraft took it no closer to Jupiter’s center than 

Europa’s orbit, 1,100,000 kilometers (700,000 miles) distant. This path kept Galileo well 

away from the highest intensity magnetic fields and charged particles in the planet’s 

radiation belts but also kept it quite far from the inner moons. The GEM perijove adjustment brought Galileo as close as about 400,000 kilometers (250,000 miles) from Jupiter’s 

center. During August and November 1999, and again in January 2000, Galileo’s solid state 

imaging camera obtained high-resolution pictures that revealed many features of three 

of Jupiter’s innermost moons: Thebe, Amalthea, and Metis (see figure 9.6). These images 

are important because before Galileo, spacecraft and Earth-based telescopes were able to 

observe small moons such as Thebe and Metis only as specks of light.184

Figure 9.6. Thebe, Amalthea, and Metis (left to right), taken in January 2000 by Galileo’s 

solid state imaging camera. The images resolve surface features as small as 2 kilometers 

(1.2 miles). The prominent impact crater on Thebe is about 40 kilometers across. The large 

white region near the south pole of Amalthea is the brightest patch of surface material seen 

anywhere on these three moons. (JPL image number PIA02531)

Previous spacecraft images had shown a round, bright spot on Amalthea. Galileo 

images taken during August and November 1999 revealed this “spot” to be a long, bright streak 

that was possibly formed by ejecta from a nearby meteoroid crater. It could also be the crest of a 

local ridge (see figure 9.7, where this streak appears in the upper left of each image).185

178 Ibid., pp. 8, 16.

179Mitchell et al., “Project Galileo: The Europa Mission,” p. 13.

180 Donald Savage (NASA Headquarters) and Guy Webster (JPL), “The Jupiter Millennium Mission,” NASA press kit, 

October 2000, p. 14, http://www.jpl.nasa.gov/jupiterflyby/documents/jupiterflyby.pdf; Jane Platt, “New Views of 

Jupiter’s Tiny Moons Are Online,” JPL Media Relations Office news release, 24 April 2000.

181 Savage and Webster, “Jupiter Millennium Mission,” p. 14. 

182 “Inner Satellites and Rings,” JPL NASA Home Page, http://galileo.jpl.nasa.gov/moons/rings.html (accessed 8 July 2001); 

“Jupiter’s Inner Moons,” from the University of Arizona’s The Nine Planets, Students for the Exploration and Development 

of Space (SEDS) Web site, http://seds.lpl.arizona.edu/nineplanets/nineplanets/amalthea.html (accessed 8 July 2001).

183 Savage and Webster, “Jupiter Millennium Mission,” p. 13. 

184 David Brand, “Galileo Takes Risky Trip to Dribble Back Data Revealing Best Images Yet of Jupiter’s Cratered Inner 

Moons,” Cornell University News Service, 24 April 2000, http://www.news.cornell.edu/releases/April00/Simonelli.

moons.deb.html. 

185 Ibid.
Figure 9.7. Bright streak on Amalthea. Galileo’s solid state imaging camera obtained the left 

image in August 1999 and the right in November 1999. The images show features as small as 

3.8 kilometers (2.4 miles) across. The bright linear streak in the top left of the images is about 

50 kilometers (30 miles) long. The large impact crater near the right-hand edge of the images 

is about 40 kilometers (25 miles) across. Two ridges, tall enough to cast shadows, extend from 

the top of the crater in a V shape resembling two rabbit ears. (JPL image number PIA02532)

On 4 November 2002, on a trajectory called “Amalthea 34,” Galileo flew closer than it 

ever had to Jupiter and Amalthea. Galileo’s closest distance to the surface of Amalthea was 160 

kilometers (99 miles). Scientists used data from the encounter in order to better determine the 

mass and density profile of Amalthea. Amalthea 34 observations, combined with previously 

determined shape and volume information, generated a bulk density (weight per volume) 

estimate near 1 gram per cubic centimeter, considerably lower than had been envisioned from 

the moon’s dark albedo and its expected rocky composition. Scientists suggested a highly 

porous “rubble pile” of low-density rock or rock/ice mixtures, consistent with Amalthea’s 

intense collisional past, as the most likely explanation for the low density.186

Just after the Amalthea encounter, Galileo’s tape recorder failed to play back 

the data collected. This failure caused considerable grief for the project team because 

the Orbiter had not only collected potentially important Amalthea data, but it also had 

entered an extremely interesting region of Jupiter’s magnetosphere. In this inner magnetosphere region, Jupiter resembles a star called a pulsar. The planet’s magnetic field is so 

powerful in the inner magnetosphere that it imparts high energies to electrons trapped 

there and ejects them. Astronomers have detected pulsed radio signals indicative of this 

phenomenon. The Galileo spacecraft had just collected vital in situ data that could help 

our understanding of inner magnetospheric processes, but if the tape recorder could not 

be made to run, this information would never get to Earth.

After a detailed diagnostic investigation, the flight team concluded that the problem was not stuck tape, as had happened when Galileo was first approaching Jupiter in 

1995, but was instead the result of radiation damage of one or more of the instrument’s 

infrared light-emitting diodes (LEDs). The damage was believed to consist of the displacement of atoms in the LEDs’ crystal lattices, which degraded the LED optical output to only 

20 percent of its full power. Laboratory experiments suggested that for the tape recorder 

to run properly, the LED output had to be at least 50 percent. 

The Galileo flight team conducted an exhaustive analysis of possible ways to 

work around the problem and developed a strategy that might partially repair the damaged lattices. JPL would send commands to the spacecraft to initiate electric currents 

passing through the LEDs. One member of the team in particular, Greg Levanas, had studied the tape-recorder design in considerable depth and believed that these currents would 

eventually cause some of the LEDs’ atoms that had been physically dislodged by radiation 

to return to their original locations in their crystal lattices. Levanas’s strategy did not immediately fix the LEDs, but after multiple applications of electric current, LED optical output 

increased to 60 percent, allowing the tape recorder to begin running again and download 

its stored data. Claudia Alexander, who later became the Galileo Millennium Mission Project 

Manager, called the recovery of the tape recorder after its being damaged by Jupiter’s severe 

radiation environment “one of the finest technical accomplishments”187 of the mission.188

The Demise of the Spacecraft

After fourteen years, Galileo is out of gas and it’s going out with a bang! 

—Gay Hill, “End of Mission” Webcast announcer189

Galileo’s final orbit took it on an elongated loop away from Jupiter, from which it returned 

on 21 September 2003 to plow into the parent planet’s 60,000-kilometer-thick atmosphere.190

This demise was planned in order to avoid any chance that the spacecraft might strike and 

contaminate the moon Europa, where scientists believe that simple life-forms may exist. If 

such life-forms are discovered in future missions, scientists must be sure that they are not Earth 

organisms that were accidentally carried to Europa aboard Galileo.191

On 21 September 2003, hundreds of current and former Galileo project members 

and their families converged on JPL “for a celebration to bid the spacecraft goodbye.”192

Through speeches and toasts by project staff, the event attempted to put closure on the 

mission and to articulate its specialness. One of the memorable comments was by T. V. Johnson, 

project scientist, who said that “we haven’t lost a spacecraft; we’ve gained a new stepping stone 

to the future.” This echoed the sentiments of others who considered the mission’s value to be 

187 Claudia Alexander interview, telephone conversation, 31 October 2003.

188 Gregory C. Levanas, systems engineer, interview, Pasadena, CA, 21 September 2003; G. Levanas to Claudia 

Alexander and Tiffany Chiu, “Galileo OP133 Infrared Radiation Presentation,” JPL Interoffice Memorandum 3483-

014-2003, 25 August 2003 (provided to author by Claudia Alexander, Galileo Millennium Mission Project Manager); 

Gary M. Swift, Gregory C. Levanas, J. Martin Ratliff, and Allan H. Johnston, “In-Flight Annealing of Displacement 

Damage in GaAs LEDs,” JPL internal presentation, 2003 (exact date not known) (provided to author by Claudia 

Alexander); Claudia Alexander e-mail, 11 September 2003; Alexander interview, 31 October 2003. 

189 Gay Hill, announcer on JPL’s “End of Mission Webcast,” originally Webcast on 21 September 2003, accessible from 

http://www.jpl.nasa.gov/webcast/galileo/.

190 Webster, “Galileo Gets One Last Frequent-Flyer Upgrade”; Donald Savage (NASA Headquarters) and Guy Webster 

(JPL), “The Jupiter Millennium Mission,” NASA press kit, October 2000, p. 13, http://www.jpl.nasa.gov/jupiterflyby/

documents/jupiterflyby.pdf; Porter Anderson, “Eilene Theilig: Faith, by Jupiter,” CNN.Com/CAREER Web site, 16 

July 2001, http://www.cnn.com/2001/CAREER/jobenvy/07/16/eilene.theilig/index.html.

191 “Galileo Spacecraft May Be Sent Crashing—But Not Soon,” San Francisco Chronicle “SF Gate” online news service, 

Breaking News section, 2 March 2000, http://www.sfgate.com/.

192 JPL, “Galileo End of Mission Status, September 21, 2003,” news release, http://www.jpl.nasa.gov/releases/2003/129.cfm.

280	 MISSION TO JUPITER: A History of the Galileo Project Chapter 9 I the orbiter tour 281

186 T.V. Johnson and J. D. Anderson, “Galileo’s Encounter with Amalthea” (abstract no. 7902 from the EGS-AGU-EUG 

(European Geophysical Society, American Geophysical Union, and European Union of Geosciences) Joint Assembly 

meeting held in Nice, France, 6–11 April 2003); Joseph A. Burns, Damon P. Simonelli, Mark R. Showalter, Douglas 

P. Hamilton, Carolyn C. Porco, Henry Throop, and Larry W. Esposito, “Jupiter’s Ring-Moon System,” in Jupiter, ed. 

Fran Bagenal, Tim Dowling, and Bill McKinnon (Port Melbourne, Australia: Cambridge University Press, 2004); JPL, 

“Galileo’s Amalthea Flyby a ‘Partial Success,’” Galileo Millennium Mission Status series, JPL Media Relations Office 

news release, 6 November 2002. 
not only in its discoveries and engineering advances, but also in the path that Galileo blazed 

toward future exploration. For instance, NASA Administrator Sean O’Keefe commented in a 

phone call to those assembled at JPL that Galileo was a “marvelous chapter of NASA’s exploration history” and that “we can utilize this great set of achievements to an even larger end.” In 

particular, the probable discovery of a warm saltwater ocean under Europa’s ice was such a 

compelling find that additional visits to the moon are already being planned.193

The spacecraft disintegrated in Jupiter’s dense atmosphere at 11:57 a.m. PDT on 

21 September. However, the Deep Space Network tracking station in Goldstone, California, 

did not receive the craft’s last signal until 12:43 p.m. PDT, due to the time delay for the 

signal to travel to Earth. During the last seconds of Galileo’s existence, the audience at 

JPL’s Von Karman Auditorium counted down as if this were a launch, not the demise, of a 

spacecraft. Later in the day, past and present Galileo staff toasted the spacecraft (with soft 

drinks and water). One of the most memorable toasts was given by Claudia Alexander, 

Galileo’s last Project Manager: “Following Balboa—who found an ocean—Galileo found 

another ocean. Here’s to you and all who follow in your path.”194

An aesthetic view of the spacecraft was given in a toast by Theodore Iskendarian, 

an engineer on the project. He said that “Galileo was a work of art—especially the dual 

spin, the antenna, the feathery umbrella antenna. Not a brick-like spacecraft.”195 Former 

Galileo Project Manager William O’Neil also expressed his view of Galileo’s appearance 

in his Webcast comment, “It’s without equal . . . and it’s beautiful besides.”196

While the spacecraft was most in the spotlight during the celebration (including in 

a musical revue), the Galileo team got many kudos as well. In the words of Greg Levanas, 

Galileo systems engineer, “We as a team would not take no for an answer. And the spacecraft just kept going.”197 Former Project Manager Eilene Theilig spoke in the same vein when 

she said, “I think this mission was special because of all the obstacles that arose. People 

drew together to surmount them.”198 And in a phone message to all those assembled in JPL’s Von 

Karman Auditorium, NASA Administrator Sean O’Keefe called the mission “an extraordinary job 

well done . . . a testimonial to the persistence NASA demonstrates.”199

 Two former Galileo project managers summed up the mission very concisely. In his 

toast, Jim Erickson said, “Everything we did was on the backs of the guys and women who went 

before. Not only did we have a great mission, but we went out in style.” And in Claudia Alexander’s 

words, “We learned mind-boggling things. This mission was worth its weight in gold.”200

193 T. V. Johnson and Sean O’Keefe (NASA Administrator), speaking at the Galileo end-of-mission event at JPL, 

Pasadena, CA, as reported by author, 21 September 2003. 

194 Claudia Alexander, speaking at the Galileo end-of-mission event at JPL, Pasadena, CA, as reported by author, 21 

September 2003. 

195 Theodore Iskendarian, cognizant engineer for Galileo’s linear boom actuator, interview, JPL, 21 September 2003.

196 William O’Neil, former Galileo Project Manager, speaking on the “End of Mission Webcast.” 

197 Greg Levanas, systems engineer, speaking on the “End of Mission Webcast.” 

198 Eilene Theilig, former Galileo Project Manager, interview, 21 September 2003.

199 Sean O’Keefe, speaking at the Galileo end-of-mission event at JPL, Pasadena, CA, 21 September 2003.

200 Jim Erickson and Claudia Alexander, speaking at the Galileo end-of-mission event at JPL, Pasadena, CA, 21 

September 2003.

282	 MISSION TO JUPITER: A History of the Galileo Project 
The Galileo project’s many achievements would not have

been possible without an exceptional team that included a succession of notable project managers, scientists, engineers, and mission 

support personnel. The Galileo team was drawn from different NASA sites and a range 

of universities, research laboratories, and institutes. Earlier chapters, especially chapters 3 

and 5, describe the significant contributions from this wide spectrum of organizations.

At the heart of the Galileo team were tremendously dedicated people who were 

key to the mission’s success. It is beyond the scope of this book to document the individual contributions of all, or even a large fraction of the people responsible for Galileo’s 

achievements. This chapter does, however, examine the experiences of several key members of the team who have articulated the mission experience and the importance of the 

work done by the Galileo team.

John R. Casani: Galileo Project Manager, 1977–88

I love the interaction with these people. Brilliant, creative people, with lots of 

challenges. Still trying to do stuff that’s never been done before.

—John Casani in an interview with the author on 29 May 2001

When John Casani was a 20-year-old student at the University of Pennsylvania in 1952, 

he read a series of articles in Colliers’ magazine that had a pivotal effect on his life. These 

were articles by Wernher von Braun, the German rocket engineer who came to the United 

States after World War II. Von Braun’s articles promoted space exploration. He touted the 

Chapter 10

Profiles of Selected People

Important to the Mission
284 MISSION TO JUPITER: A Histor y of the Galileo Project Chapter 10 I profiles of selected people important to the mission 285

most frequently recommended the name “Galileo,” which in Casani’s view was especially 

appropriate because it was the astronomer Galileo who had discovered Jupiter’s four 

largest moons and because the Galileo mission’s primary objectives included “the study of 

Jupiter and the Galilean satellites.”4

Table 10.1. Galileo Project managers.5

John Casani October 1977–February 1988

Dick Spehalski February 1988–March 1990

Bill O’Neil March 1990–December 1997

Bob Mitchell December 1997–June 1998

Jim Erickson June 1998–January 2001

Eilene Theilig January 2001–August 2003

Claudia Alexander August 2003–September 2003

Other managers of the Galileo project (see table 10.1) considered Casani to be 

a strong leader who could command the respect of both the congressional and scientific 

communities. These skills were absolutely essential to Galileo, for the project was nearly 

killed several times by budget-conscious members of Congress and presidential staffs 

(see chapters 2 and 3). John Casani made many trips to Washington, DC, to present his 

views and garner support for the mission. He lauded the valuable science findings that 

Galileo would produce and stressed the importance of keeping planetary exploration 

alive. He made government officials in Washington aware that JPL had no other major 

in-house missions now that Voyager was on its way. If Congress did not approve the 

Galileo mission, the loss of the project would put JPL in danger of losing 400 very highly 

qualified engineers and scientists who were counting on participating in the project. 

Casani stressed that JPL was a critical national resource that would be severely damaged 

if Congress killed Galileo.6

Casani also played on the space program’s appeal to the United States public. 

He reminded members of Congress of the excitement that Voyager’s images had generated. When dealing with presidential staff, Casani adjusted his approach to the particular 

aims of the administration at the time. President Reagan, for instance, strongly supported 

Strategic Defense Initiative (“Star Wars”) military defense technology. Casani capitalized 

on this by pointing out that Galileo was going to travel to Jupiter, over 400 million miles 

away, with an expected navigational accuracy that was absolutely remarkable. Casani 

4 Gia Scafidi, “John Casani Awarded Honorary Doctorate Degree,” JPL Media Relations Office news release, 31 May 

2000; Casani interview, 29 May 2001; “A Name for Project Galileo,” JPL interoffice memo GLL-JRC-78-53, 6 February 

1978, John Casani Collection, Galileo Correspondence 12/77–2/78, folder 43, box 5 of 6, JPL 14, JPL Archives. 

Quotation is from “A Name for Project Galileo.” 

5 Guy Webster, JPL Press Office, telephone conversation, 29 August 2001; J. R. Casani to multiple addresses, “Revised 

Guidelines for Galileo Implementation,” 30 January 1978, JPL Interoffice Memo GAL-JRC-78, folder 18522, NASA 

Historical Reference Collection, Washington, DC; Claudia Alexander, e-mail message, 11 September 2003. 

6 Casani interview, 29 May 2001.

construction of space stations and flying to the Moon, and his visions of what was possible made a deep impression on Casani. In 1956, a year after Casani graduated from the 

University of Pennsylvania with a bachelor’s degree in electrical engineering, he hunted 

down a job at the Jet Propulsion Laboratory. Although he initially worked on inertial guidance systems for guided missiles, his position at JPL would give him the chance to help 

create some of the projects that von Braun had only imagined.1

At the time that Casani started work at JPL, NASA had not yet been formed. JPL

was then a contractor to the Army’s Ballistic Missile Agency in Huntsville, Alabama. After 

Congress created NASA, the new Agency gave various missions to its different Centers. 

Right from the beginning, JPL’s mission was to develop interplanetary space travel, and 

Casani was deeply involved in this effort. Early Casani assignments at JPL included work 

on the Explorer, Pioneer, and Ranger missions. Explorer 1 was the first U.S. Earth satellite, 

which launched in January 1958 and discovered a radiation belt around Earth. Pioneer 3 

discerned the two bands of radiation that encircled Earth. Pioneer 4 passed within 37,000 

miles of the Moon before achieving a permanent solar orbit. 

Even though his background was in electrical engineering, Casani soon transitioned into systems engineering, serving as the lead design engineer on the Ranger-class 

spacecraft and as the team lead on the Ranger 1 and 2 missions. Later, Casani was also the 

lead design engineer on the Mariner-class spacecraft and the team lead on the Mariner 3 

and 4 missions. These were the first uses of the Mariner-class spacecraft. (The first Mariner 

missions, Mariner 1 and Mariner 2, actually used the older Ranger-class spacecraft design.)

Casani soon moved up the management chain into positions of increasing 

responsibility. On the 1969 Mariner Mars project, Casani was the Deputy Spacecraft System 

Manager; later, on the 1973 Mariner Venus-Mercury project, Casani served as Spacecraft 

System Manager. In April 1976, he assumed the role of Project Manager for the highly successful Voyager mission to the outer planets; the mission’s two spacecraft launched from 

Cape Kennedy in August and September 1977.2

Casani’s Management of the Galileo Mission

As Casani and his boss, Bob Parks, JPL’s Assistant Laboratory Director for Flight Projects, 

were driving to the airport to return to Pasadena after the Voyager spacecraft launches, 

Parks offered Casani a new assignment. Would he manage what was then called the 

Jupiter Orbiter Probe, or JOP 77? Casani said yes. But one of the first things he did 

after assuming the Project Manager post was to solicit suggestions from his staff for a 

new mission title. Planetary missions were traditionally given names such as Mariner 

Venus 62 and Mariner Mars 64, and Casani thought that “all those letters and numbers 

lose their significance to anybody except people who work”3

 on the project. His staff 

1 John Casani interview, tape-recorded telephone conversation, 29 May 2001; “Biography of John C. [sic] Casani,” John

Casani Collection, Galileo Correspondence February 1978-March 1978, folder 42, box 5, JPL 14, JPL Archives.

2 “Biography of John C. [sic] Casani”; “Early NASA Aeronautics and Astronautics Chronology (Pre-Apollo),” sections

1958 and 1959, and “Aeronautics and Astronautics Chronology,” section 1961, all on the NASA History Division

Web site, http://www.hq.nasa.gov/office/pao/History/timeline.html, excerpted from Eugene M. Emme, Aeronautics

and Astronautics: An American Chronology of Science and Technology in the Exploration of Space, 1915–1960,

(Washington, DC: NASA, 1961).

3 Casani interview, 29 May 2001.
286 MISSION TO JUPITER: A Histor y of the Galileo Project Chapter 10 I profiles of selected people important to the mission 287

To my mind, the debate is one of why do we have a NASA? The S in NASA

doesn’t stand for science. You can argue that everything should only be 

evaluated in terms of science. My own personal view is that NASA’s a part 

of a president’s armory for executing national and foreign policy and it can 

be used any way the president sees fit to use it. And many presidents have 

felt, and I agree, that it’s a powerful mechanism for motivating and inspiring people. Young people and older people alike . . . . Kids and people like 

heroes. What if instead of Admiral Byrd going to the South Pole, it was just an 

automated sled? It might have been able to do more science than he could, 

but it wouldn’t have inspired the kind of interest and public reaction that 

[Byrd] did. Having heroes is an important part of any enterprise like this.12

Our Space Program’s Impact 

One thing that gave Casani satisfaction over the years was the importance and interest that so many people in the world assigned to the U.S. space program. Once, on a 

street in Argentina, a man called out to him by name. The man, an Argentinian lawyer, 

followed our space program in great detail, and was very pleased to actually meet 

Casani and welcome him to Argentina. On another occasion, one of Casani’s colleagues 

was on a wildlife safari in Africa, during which his guide commented on the similarities between their all-terrain vehicle and NASA’s Mars Rover vehicles. These were not 

isolated incidents. Casani and his colleagues regularly met men and women in many 

different countries who were not involved in the U.S. space program but were well 

aware of what the program was doing. Sometimes, these people even knew intricate 

details about the particular equipment used on different space missions because those 

missions were of high interest to them.13

Casani believes that young people are inspired by our space program, just as he 

was inspired as a 20-year-old by Wernher von Braun’s articles. “There are a lot of people 

in science and technology now because of the Apollo program,” Casani says. “I’m a person 

who happens to believe that our whole world is technology-oriented. Anything we can do 

to increase people’s understanding, interest, or thirst for technology is good.”14

Future Plans

As to the future, Casani plans on working for JPL and the space program as long as he can 

“continue to contribute a little bit.” He retired in 1999, but shortly afterwards, JPL management asked him to return to the Laboratory to lead the failure investigations of NASA’s two 

ill-fated Mars missions. When that was done, NASA and JPL management asked him to advise 

on a range of other issues. Daniel Goldin, NASA Administrator at the time, invited him to sit 

on the Agency’s prestigious Advisory Council. Casani says that if he can persist in remaining 

“relevant,” he will keep on trying to help the space program in whatever way he can.15

12 Casani interview, 29 May 2001.

13 Ibid.

14 Ibid.

15 Ibid.

impressed upon Reagan’s administration that such a feat would, in a very nonthreatening 

way, convey to people around the world the level of technological capability that the 

United States had. Other countries would think that if the United States could achieve 

such an amazing feat with a civilian enterprise, imagine what it could do in a military situation. Galileo provided a way of demonstrating, without building military equipment, the 

technological strength of our country.7

The Challenger accident in 1986 resulted in millions of people around the 

nation and the world sharing a sense of sorrow and personal loss. As JPL employees, Casani and his staff felt this loss very intensely. But for them, there was also the 

possible loss of all they had been working toward for many years. “We had so much 

invested in the mission from 1977 through 1986,”8

 said Casani. After Challenger’s 

explosion, NASA management delayed Galileo’s launch from May 1986 until June 

1987 and then, due to safety concerns, canceled the use of the Centaur liquid-fueled 

upper stage in the Space Shuttle. This was a low point for Galileo’s staff, who feared 

that the spacecraft might never get off the ground. Casani’s task was to keep his staff’s 

morale up during this tremendously difficult time and focus its efforts on redesigning the mission and keeping it on track. The Centaur had to be replaced with a less 

powerful solid-fueled upper stage, but project engineers had not yet figured out how 

such an upper stage would be able to get Galileo to Jupiter. 

The eventual discovery of the six-year VEEGA trajectory, in which Galileo 

first flew toward the Sun and then looped around Venus for a gravity-assist, buoyed 

Casani’s spirits and those of his staff. “The most exciting time in my life,” he said, 

began with “the realization that there was another way of doing [the mission] using 

the gravity assist of Venus.”9

Casani and his staff successfully put the Galileo project back on the path 

toward a launch. By 1988, Casani decided that “in terms of the [Galileo] mission, everything 

was very, very secure”10 and accepted an excellent opportunity to enter the upper 

management structure of JPL, eventually becoming its Assistant Laboratory Director for 

Flight Projects.11

The Robotic Versus Human Spaceflight Debate

An endless debate in the United States space science community focuses on why NASA

puts the majority of its budget into human spaceflight, which is extremely expensive, 

when considerably more science per dollar can be achieved using robotic spacecraft. 

Although Casani participated in many robotic missions and managed several of them, 

he saw the need for both types of missions. He understood the less tangible benefits 

offered by a human mission, over and above the science findings. Casani’s understanding of the value of human expeditions was illustrated in the following statement:

7 Casani interview, 29 May 2001.

8 Ibid. 

9 “From the Project Manager,” Galileo Messenger (February 1986); “From the Project Manager,” Galileo Messenger

(July 1986); Casani interview, 29 May 2001.

10 Casani interview, 29 May 2001.

11 Dick Spehalski, e-mail message, March 2002.
288 MISSION TO JUPITER: A Histor y of the Galileo Project Chapter 10 I profiles of selected people important to the mission 289

Dick Spehalski: Galileo Project Manager, 1988–90

If we had launched in 1986, if the Challenger had worked and we had been 

able to launch on schedule, we would have found out that [Galileo’s] thrusters 

would fail. We would probably have lost the mission.

—Dick Spehalski, former Project Manager of Galileo

The most exciting moment of the Galileo mission for Dick Spehalski was the launch, or, 

more specifically, the countdown before the launch. “Spe” had been on the project since 

its official start date in 1977 and, in his managerial functions under John Casani, had helped 

work through one potentially mission-ending barrier after another. Now that the Galileo 

spacecraft was finally on the launchpad, tucked carefully inside the Space Shuttle, the 

countdown to takeoff seemed incredibly long. Spehalski’s adrenaline got him through those 

final hours, he said. He had been waiting for 12 years for this day. 

A year earlier, in February 1988, when Casani left the project, Spehalski had 

been chosen as his successor because JPL management viewed him as the best person 

to get Galileo off the ground and into space. Spehalski wanted to get the launch behind 

him. When the Shuttle Atlantis finally fired its rockets and carried Galileo aloft, then successfully deployed the spacecraft, Spehalski felt totally exhilarated but almost exhausted. 

“At the end of it,” he said, “you feel like gee, I was just put through this wringer and I

don’t have any energy left.” His work was not over, however. “Now we’ve got to fly this 

thing. It doesn’t ease up.”20

During John Casani’s tenure, Dick Spehalski had been the Flight System 

Integration Manager. Spehalski had been responsible for mechanical engineering tasks 

in the design and construction of the Galileo Orbiter. It also had been his job to develop 

smooth interfaces between the Orbiter and its Atmospheric Probe, the Centaur upper 

stage launch vehicle (which was scrapped after the Challenger disaster), the spacecraft’s propulsion system (used to maneuver the craft in interplanetary space and 

in the Jovian system), and the radioisotope thermal generator (RTG) electric power 

system. Spehalski had to work closely with industry contractors building Galileo’s 

components; with the U.S. Department of Energy, which designed and fabricated 

the RTGs; and with the Federal Republic of Germany, which designed and built the 

spacecraft’s propulsion system.21

Working with another country that was building a critical part of the spacecraft 

but was not under JPL’s direct control was one of the most difficult challenges that 

Spehalski faced during the entire mission. He does not hold back in expressing strong 

opinions, either verbally or in his publications.22 Spehalski criticized the decision to 

include a major international partner on Galileo in so vital a role. Although he recognized 

the additional capabilities that Germany brought to the mission, the cultural differences, 

along with varying ways of doing business, a lack of frequent face-to-face meetings, and 

the dissimilar motivations of the two governments led to serious problems. 

20Dick Spehalski, interview, tape-recorded telephone conversation, 4 May 2001; Dick Spehalski, e-mail message, 

March 2002. Quotations are from the interview.

21 “Meet the Team,” Galileo Messenger (December 1982). 

22 R. J. Spehalski, “Galileo Spacecraft Integration: International Cooperation on a Planetary Mission in the Shuttle Era,” 

Earth-Orient. Applic. Space Technol. 4, no. 3 (1984): 139–150.

Torrence V. Johnson: Galileo Project Scientist Since the Beginning of the Mission

Galileo gave us a chance to explore the entire Jupiter system . . . looking at the 

interactions of all parts of the system and not just . . . one piece of the elephant.

—Torrence Johnson, Galileo project scientist, in an interview with the author on 31 July 2001

As Galileo’s project scientist, Torrence “T. V.” Johnson served as the mission’s primary 

science advisor and the principal interface between project staff and investigators on 

science experiments. The mission’s project scientist was also an ex officio member of all 

of Galileo’s science teams. The criticality of Johnson’s position was exemplified by the 

responsibilities that fell to him after Galileo’s high-gain antenna failed to open. The Galileo 

team had to face the fact that the spacecraft was no longer capable of sending back all the 

data that scientists on Earth had been hungrily awaiting, some for as many as 20 years. As 

project scientist, Johnson needed to decide who got what. “The hardest thing I had to do 

as a project scientist was to figure out which objectives had the highest priority and how 

we were going to recover from not being able to send back all the data we wanted. I had 

to basically give the different science areas priority ratings, and give them allocations. Say, 

you’re only going to get 30% of the data. See what you can do with it.”16

Johnson’s involvement with Galileo dated back to the mid-1970s, when he was a 

member of NASA’s Mariner Jupiter Uranus planning group and the Jupiter Orbiter Probe (JOP) 

science working group. The JOP group developed the scientific rationale for the project that 

would later be called Galileo and was key in convincing Congress to fund the project.17

T. V. Johnson’s excitement about space science, and especially about NASA

interplanetary missions, is infectious. He has refused speaking engagements where he 

was given only 1 hour to talk about Galileo’s most important accomplishments. He sums 

up how the space program makes our lives better by first saying that it is not because 

such inventions as Tang and Teflon came out of it. The value in exploring space is much 

grander than that. By studying Io’s volcanoes or Jupiter’s Great Red Spot or a black hole in 

a distant galaxy, “you are trying to use the universe as a laboratory to test out your theories which were developed here on Earth under very different conditions. That improves 

those theories . . . . It’s not going to put more chicken in the kitchen but it is connective 

with the general human enterprise of trying to understand how the universe works.”18

Space science, in Johnson’s view, is simply an extension of our basic humanity. 

That is reason enough to conduct it. He is aware, however, of the tangible benefits that 

frequently surface from our quest to understand “how planets tick.” For instance, a friend 

of his who pursued space science because of a fascination with Jupiter’s clouds is now 

using the knowledge of atmospheric chemistry that he gained to do some of the world’s 

leading research into ozone-depleting chemicals.19

16 “Meet the Team,” Galileo Messenger (December 1984): 4; Torrence V. Johnson interview, tape-recorded telephone 

conversation, 31 July 2001.

17 “Meet the Team.”

18 Johnson interview, 31 July 2001.

19 Ibid.
290 MISSION TO JUPITER: A Histor y of the Galileo Project Chapter 10 I profiles of selected people important to the mission 291

going full bore trying to solve the problem with the thrusters. Ultimately the Germans 

came through by finding a way to use the thrusters without overheating them. We flew 

the mission without incident.”27

Bill O’Neil: Galileo Project Manager, 1990–97

Some of the senior engineers in the Probe area, when that signal came to say 

that everything worked, they actually were crying.

—Bill O’Neil, former Project Manager of Galileo

For Bill O’Neil, who was Galileo’s Project Manager from 1990 until 1997, nothing on the 

mission was more exciting than Jupiter Arrival Day, especially the Probe’s plunge into 

the Jovian atmosphere. “All the chips were on the table that day . . . . For five months 

we’d had no idea of the condition of the Probe.” Due to Jupiter’s enormous mass and 

gravity, it was extremely difficult to plan a successful entry into its thick atmosphere 

without the Probe falling so fast that it would burn up before taking all of its measurements. O’Neil believed that the entry maneuver was more difficult than it would have 

been at any other planet in the solar system. “The Probe had to survive this tremendous 

heating, and the entry loads that were estimated could go up as high as 350 g’s. The 

parachute would have to work properly, and the separation of the heatshield would 

have to be proper.” It was not until the Probe was on parachute and transmitting that 

scientists on Earth could confirm that the atmospheric entry had been successful. O’Neil 

remembers the pandemonium at JPL when this information was received. “Some of the 

senior engineers in the Probe area, when that signal came to say that everything worked, 

they actually were crying.”28

O’Neil, who studied aeronautical engineering in college, was hired by JPL in 

1963. He worked on the Surveyor Moon lander, the Mariner Mars 1971 orbiter, and the 

Viking Mars landers and orbiters prior to transferring to the Galileo project in 1980.29

O’Neil brought key experience from his other missions that helped him on Galileo. 

On the Lunar soft-landing Surveyor mission, he learned intricate details of launch systems and trajectory design. His association with Hughes Space and Communications 

Company during the Lunar Surveyor project was useful because Hughes also built 

Galileo’s Atmospheric Probe under contract to Ames Research Center, and O’Neil once 

again had to work closely with the company. O’Neil’s tenure on the Viking mission 

was advantageous because the project involved a challenging atmospheric entry and an 

orbital insertion similar to those on the Galileo mission.30

When O’Neil joined the Galileo team in 1980, he assumed the position of Science 

and Mission Design Manager. In that capacity, he became a key figure in planning out 

details of the project. He was also responsible for interfacing with Galileo scientists and 

developing the science operations that the spacecraft would perform. 

27 Ibid.; Spehalski e-mail message, March 2002. Quotation is from the interview. 

28 Bill O’Neil interview, tape-recorded telephone conversation, 15 May 2001.

29 “Meet the Team,” Galileo Messenger (May 1982): 1. 

30 O’Neil interview, 15 May 2001.

The German-built propulsion system was an absolutely critical part of NASA’s 

spacecraft, but its design and construction were not under NASA’s direct control. NASA

had made a cooperative agreement with Germany’s Bundesministerium fuer Forschung 

und Technoligie (BMFT), which used a local industrial contractor, MesserschmittBoelkow-Blohm, to fabricate Galileo’s propulsion module. The German organizations 

resisted NASA’s and JPL’s desires to set requirements for the propulsion system’s design, 

guide its construction, and implement inspection schedules for verifying that the system 

was reliable for flight. In Spehalski’s words, “the challenge is to get them to do what 

you want done with their money and resources . . . . This challenge leads to a lot of 

international harangue and argument . . . .”23 A great deal of unplanned time was spent by 

both parties negotiating requirements and reviewing the design and test results, thereby 

causing significant cost and schedule impacts.24

Dick Spehalski’s most difficult period on the Galileo mission began when he 

found out, one year before launch, that the thrusters Germany had built for the spacecraft 

had a serious problem. Discovering this was a strange benefit of the Challenger disaster 

and the delay in Galileo’s launch resulting from it. If Galileo had been sent into space in 

1986 as planned, NASA probably would have lost the mission. 

Galileo’s thrusters had a critical design flaw—the hot gases generated for propulsion burned holes through the walls of their combustion chambers. Germany had agreed 

to build Galileo’s thrusters partly to develop a product that it could market for other 

applications. One of these applications was in communications satellites. It was after such 

a satellite was launched that the thruster problem was discovered. The satellite’s thrusters overheated and failed, catastrophically, with the resultant loss of that mission. After 

that event, NASA’s German partners tested Galileo’s thrusters and found that they were 

susceptible to the same problem.25

Spehalski, his staff, and their German partners went through an extremely 

hectic period after the thruster problem was identified. The thrusters had to be made 

reliable in order for Galileo to be sent into space. Germany’s BMFT, its industrial 

partner Messerschmitt-Boelkow-Blohm, and JPL staff had to find the root cause of the 

thruster failure and come up with a fix for the problem that satisfied JPL’s requirements 

sufficiently to give the mission a green light to launch. And they had to do this in 

only one year, because management did not want to delay the launch yet again. But 

Spehalski added another task that increased the pressure on his staff even more. The 

team had to develop a backup design for a thruster system, in case Germany could not 

make their existing thrusters flight-ready in time for the launch. Spehalski explained 

that he “didn’t want the mission held hostage to whether or not the Germans were 

going to be successful.”26 So he contracted with a supplier in southern California who 

made thrusters for the Air Force, and he negotiated to get a high priority within its 

production facilities. His staff quickly completed the backup thruster design during 

“a very stressful period. At JPL, designers were going full bore on the backup design 

while in Germany, the German team with JPL propulsion specialists’ support were 

23 Spehalski interview, 4 May 2001; Spehalski e-mail message, March 2002. 

24 Spehalski, “Galileo Spacecraft Integration.”

25 Spehalski interview, 4 May 2001; Spehalski e-mail message, March 2002; Jim Erickson interview, tape-recorded

telephone conversation, 8 May 2001.

26 Spehalski interview, 4 May 2001.
292 MISSION TO JUPITER: A Histor y of the Galileo Project Chapter 10 I profiles of selected people important to the mission 293

Bob Mitchell: Galileo Project Manager, 1997–98

When the Challenger explosion accident occurred, we found ourselves in a 

situation . . . where we didn’t have any way to get to Jupiter . . . . Now we just 

took this on as kind of our own personal challenge.

—Bob Mitchell, former Galileo Project Manager

After the Challenger tragedy took place in January 1986 and after NASA management 

decided, a few months later, that it was too dangerous to fly the Centaur upper stage in 

the Shuttle, Galileo mission staff found themselves in trouble. The solid-fueled alternative upper stages to the Centaur did not have enough thrust to propel Galileo on a direct 

trajectory to the Jovian system.36

Bob Mitchell was Galileo’s Mission Design Manager and Navigation Team Chief 

at the time. His job was to figure out a way for the spacecraft to reach Jupiter. Given the 

constraints imposed, Mitchell’s task appeared to be extremely difficult, if not impossible. 

In his view, “most people had pretty much accepted that the Galileo project, the spacecraft, was going to make a one-way trip to the Smithsonian.”37 In other words, the mission 

was on the verge of getting scrapped.38

Galileo management desperately needed its staff to find a spacecraft trajectory 

that would allow the mission to be completed using a solid-fueled upper stage. To meet 

this goal, management brought together a group of four people with some of the best 

trajectory mechanics knowledge at JPL: Dennis Byrnes, Louis D’Amario, Roger Diehl, 

and their leader, Bob Mitchell. They made it their personal mission to design a trajectory 

with sufficiently low energy requirements that a solid-fueled booster could drive Galileo 

to Jupiter. The team conducted numerous brainstorming sessions in which they developed “really far-out, unconstrained ideas.”39 They envisioned various gravity-assists that 

might give the spacecraft the extra energy it needed. But the team had to abide by one 

constraint on the process that proved to be very frustrating. They had been told by their 

managers that Galileo could not fly closer to the Sun than just inside Earth’s orbit. Galileo 

designers worried that if the spacecraft flew any closer to the Sun, it would receive more 

thermal input than its instruments could withstand.40

After a month of “pretty serious brainstorming and head-scratching,” Mitchell’s 

team concluded that there was no trajectory that was going to work, given the constraints 

imposed. So one day, he said to the group, “Forget about this inside-of-the-Earth constraint. If you need to go inside Earth’s orbit, let’s do that.” They all went off and did some 

more calculations, and one morning, Roger Diehl came into Mitchell’s office with a new 

idea. The team had been considering gravity-assists by Earth, Venus, and other bodies, 

but what Diehl envisioned was a Venus gravity-assist followed by two Earth assists—in 

other words, a “VEEGA” trajectory. The thing that was new about Diehl’s scheme, and the 

36 Bob Mitchell interview, tape-recorded telephone conversation, 10 May 2001. 

37 Ibid.

38 Dick Spehalski commented that statements about the spacecraft ending up in the Smithsonian Institution were more 

“figurative than literal” in that if Galileo had not been launched, he did not think it would actually have been displayed in 

the Smithsonian. In an e-mail message to the author (March 2002), Spehalski wrote that although “people made such 

comments,” if Galileo had not been launched, “the hardware most likely would have been used on other missions.” 

39 Mitchell interview, 10 May 2001.

40 “Meet the Team,” Galileo Messenger (August 1982): 4; Mitchell interview, 10 May 2001.

In O’Neil’s opinion, Galileo’s premier accomplishment was the discovery of 

strong evidence for liquid water under Europa’s ice crust. About this discovery, O’Neil 

said, “I find it absolutely ironic that the most significant thing that Galileo achieved has 

also given Galileo the death sentence. That it must be crashed into a body to make sure 

that it will never strike Europa.” Galileo’s observations of Europa raised the possibility that 

the moon harbored life. To make sure that Galileo would never contaminate the moon’s 

ocean with microorganisms brought from Earth, in 2003 the spacecraft was sent at high 

speed into Jupiter’s thick atmosphere, where it met a fiery demise.31

The most unpleasant task that O’Neil ever had to perform on Galileo occurred 

soon after its tape recorder jammed, a couple of months before arrival at Jupiter. Mission 

engineers got the recorder working again, although they did not know for how long. It 

was clear to O’Neil what he had to do to protect the tape recorder, but taking the appropriate actions was difficult because he knew that they would have hurtful consequences. 

He had to ensure that the tape recorder remained functional for its most important Arrival 

Day function—capturing data from the Atmospheric Probe’s descent. That meant not risking its use on other extremely important operations. 

“It was the worst decision that I ever had to make. I had to say there was no 

way we could do the approach imaging”32 of Io and Europa that had been planned since 

the inception of the project. In fact, mission planners had picked 7 December 1995 as 

Arrival Day because of the excellent opportunity to view both moons before Jupiter Orbit 

Insertion took place, a maneuver that carried some risk of failure. These two satellite 

encounters had been precisely calculated and much anticipated, all the more so because 

no one knew whether Galileo would have a chance to view the moons closely again. 

The intense radiation around Io might preclude any more flybys in the future. “It fell to 

me to tell the scientists, without any debate—there was no point in debate as far as I was 

concerned”—that the images and other data for which they had been waiting could not 

be stored on the tape recorder and thus would be lost.33

A detailed analysis of the tape-recorder problem after Jupiter Arrival Day indicated that O’Neil’s decision had been the right one. According to O’Neil, “After the fact, 

when we fully diagnosed [the tape recorder problem], we know that [the recorder] absolutely would have failed, had we tried to do that imaging.” Such a failure would have 

prevented most of the Probe’s descent data from ever being sent to Earth.34

One of the tasks that O’Neil was thankful he did not have to perform was 

declaring the mission a failure. “Can you imagine the circumstance that we had the whole 

world, at least a lot of the nations and Europe, watching our day of arrival, and we had 

the auditorium at JPL all full of VIPs and we had the senior, the top management of NASA. 

Imagine if I had to go to the podium at the end of the day . . . and tell them that the mission had failed! That would clearly have been the most difficult task.”35

31 Ibid. 

32 Ibid.

33 Ibid.

34 O’Neil interview, 15 May 2001.

35 O’Neil interview, 15 May 2001.
294 MISSION TO JUPITER: A Histor y of the Galileo Project Chapter 10 I profiles of selected people important to the mission 295

Immediately following the antenna failure, nobody on Erickson’s team had a 

clue how to solve the problem. Erickson said that the most difficult thing he ever had to 

do on the mission was “facing the 60 people who worked for me, trying to have them not 

despair of Galileo working.”45 He had to maintain their morale and convince them that 

they would discover a means of making the mission successful. Ultimately, they did.46

Erickson was attracted to the Galileo mission soon after its inception in 1977 

because it “seemed to be the most challenging mission on the horizon. The hardest 

job.” Galileo management liked his work and kept giving him new jobs to do, including 

serving as deputy manager of the Galileo Engineering Office, manager of the Galileo 

Science and Sequence Office, Deputy Project Manager, and finally, in 1998, Galileo 

Project Manager.47

Eilene Theilig: Galileo Project Manager, 2001–03

I really like watching people presented with a problem, and using their creativity 

to overcome it. And we’ve had lots of examples of that . . . . The people who work 

here are really dedicated. It’s not just a job.48

—Eilene Theilig, former Galileo Project Manager

What Eilene Theilig enjoyed about working on the mission was that it changed 

character every few years. The spacecraft always presented her and the rest of the 

team with challenges, as did the evolving complexion of the mission—the six-year 

interplanetary journey, the two-year Primary Mission observations of Jupiter and its 

moons, and the extended missions activities that had to be accomplished with greatly 

reduced staffs and budgets.49

Theilig was tremendously impressed with the design and construction of a 

spacecraft “that is so robust and is still going . . . . It’s done far more than people 

thought it would. With the troubles that we’ve overcome, we still made numerous highlevel, high-priority scientific observations at Jupiter.” Observations that the spacecraft 

made have implications that reach far beyond the space science community. For 

instance, the possible identification of water on Europa is driving many Earth studies 

of “extremophiles”—life-forms that exist in areas we did not think possible, like in 

Earth’s very deep oceans or under kilometers of Antarctic ice.50

Galileo’s most important accomplishment, in Theilig’s mind, was to observe lava 

flowing on the surface of Io, in an environment similar to what existed on Earth two billion 

45 Jim Erickson interview, telephone conversation, 8 May 2001. 

46 Ibid.

47 Ibid.; Jane Platt and Mary Beth Murrill, “New Managers Appointed for Cassini and Galileo Missions,” JPL Media

Relations Office news release, 4 June 1998. 

48 Eilene Theilig interview, tape-recorded telephone conversation, 8 May 2001.

49 Porter Anderson, “Eilene Theilig: Faith, by Jupiter,” CNN.com/CAREER Web site, 16 July 2001, http://www.cnn.

com/2001/CAREER/jobenvy/07/16/eilene.theilig/index.html (available in folder 18522, NASA Historical Reference

Collection, Washington, DC).

50 Theilig interview, 8 May 2001.

reason Mitchell thinks that nobody had thought of it before, was that the second Earth 

encounter would not give Galileo any more energy. But it turned out that the spacecraft 

did not need additional energy; what it needed was to have its trajectory bent in a different direction. The second Earth encounter gave Galileo just the bending required to send 

it in the right direction.41

Mitchell called Bill O’Neil, who was then the science and Mission Design 

Manager, and relayed Diehl’s idea to him. O’Neil and Casani, Galileo’s Project Manager, 

agreed that a VEEGA trajectory could work. At that point, team members Louis D’Amario 

and Dennis Byrnes took Diehl’s initial trajectory concept and refined it to a much greater 

level of accuracy. D’Amario’s and Byrnes’s contributions were critical because Diehl’s 

work was based on design tools that did not have the accuracy to demonstrate that the 

VEEGA trajectory would actually be successful.42

Working out the VEEGA trajectory was the high point of the Galileo mission for 

Mitchell. He says about his experience during the effort: “Even though Roger was the one 

that made the discovery, and Lou and Dennis were the ones that designed it to where it 

was really workable and flyable, I was the leader of the team, and the involvement, the 

effort, the result of that was very exciting and very rewarding. We took a situation where 

we almost surely had the spacecraft going to the Smithsonian and turned it around.”43

Mitchell believes that the most significant result of the Galileo mission was the 

detailed understanding that we attained of the Jovian system. The importance of the 

spacecraft’s discoveries was underlined in a meeting, which Mitchell attended, of the 

mission’s entire science group. One of the scientists asserted that as a result of the data 

collected by Galileo at Jupiter, they were going to “have to throw away all the textbooks 

and start to rewrite ’em on Jupiter.” And all the other scientists in the room agreed.44

Jim Erickson: Galileo Project Manager, 1998–2001

Immediately after the high-gain antenna had failed . . . [I had to go] into a 

meeting to explain what we had just seen in the telemetry, and what it meant 

to the team, and what it meant to the project . . . [and] to keep everybody from 

getting into the mode of, “Gee, the mission has failed.” . . . We were a bunch 

of smart people, and we would find a way to solve this problem.

—Jim Erickson, former Galileo Project Manager

Jim Erickson was Galileo’s sequence team chief at the time that the high-gain antenna 

failed to open on Galileo’s interplanetary trajectory to Jupiter. Erickson’s job as sequence 

team chief was to run the team of 60 people who took all of the project staff’s “science experiment desires” and “engineering desires” and created sequences of commands 

necessary for the spacecraft to achieve those desires. Transmission of these sequences to 

Galileo depended on the spacecraft’s having an operable antenna. The high-gain antenna’s 

failure jeopardized the transmissions and threatened to shut down the mission.

41 Mitchell interview, 10 May 2001. 

42 Ibid.

43 Ibid. 

44 Ibid.
296 MISSION TO JUPITER: A Histor y of the Galileo Project Chapter 10 I profiles of selected people important to the mission 297

pressure. The Galileo spacecraft could not be stopped while support crew analyzed a 

malfunction. If a problem was not fixed “on the fly,” before an encounter was completed, 

the opportunity might be lost to collect key data that a science team had been waiting 

years to receive.

One of many critical mission support personnel was Nagin Cox. Her experiences 

on various Galileo teams mirror those of many of her colleagues and illustrate the vital 

responsibilities that these people had. During most of Galileo’s Prime Mission, Nagin Cox 

served on the Orbiter engineering team, focusing on “system fault protection.” Galileo 

was so far away from Earth, and it took such a long time to communicate with the craft, 

that it needed to “take care of itself for a while” when problems arose. Cox helped to 

maintain and apply the on-board algorithms that protected Galileo and put it into a safe 

mode or told it how to respond if something went wrong. These fault protection algorithms were complex enough that a separate team was formed to work on them and 

ensure that Galileo was always configured properly to take care of itself.56

Cox was appointed deputy team chief for the spacecraft and sequence team on the 

Galileo Europa Mission, which ran from 1997 through 1999. Her Galileo experiences were 

most intense on GEM, after project staff had been greatly reduced from Prime Mission levels 

of about 200 people to only 60. The spacecraft was beginning to exhibit the cumulative 

effects of radiation exposure, and new problems in its operation were surfacing. Project 

staff became a very tight-knit squad as they dealt with these issues that often surfaced only 

hours before an encounter. A breakneck pace of work ensued for two years and included 

many satellite encounters and long days and nights keeping the spacecraft operational. 

Just a week following GEM’s kickoff on 8 December 1997, or, in Cox’s words, “seven days 

after we were handed the keys,” radiation-related malfunctions in the ship’s gyros arose. 

The gyros were used to point Galileo’s scan platform, on which sat the SSI camera and 

other remote sensing instruments, and also to help ascertain spacecraft orientation. The 

team determined that a damaged field-effect transistor switch was responsible for the gyro 

problem. The switch was supposed to respond to the spacecraft’s degree of rotation but 

had begun to over-react, putting out more pulses than it should have for every degree of 

movement. For example, the spacecraft might only have rotated 10 degrees, but the signal 

that the gyros sent would say that it had rotated perhaps 50 degrees. Cox’s team had to write 

an algorithm to account for the gyros’ altered behavior.57

When Cox was hired by JPL, it was the fulfillment of a desire that she’d had 

since she was 14. She said that “working at JPL is all I’ve ever wanted to do.” She was 

more attracted to JPL’s robotic space exploration program than to the human missions 

conducted by other NASA sites because robotic spacecraft could travel the furthest into 

the unknown, uncharted regions of space. When the chance came to join the Galileo 

mission, it was particularly enticing. In her words, “What could be more cool than the 

first spacecraft to go into orbit around [an outer] planet? The first probe to take in situ 

measurements of a gas giant? For some engineers, . . . once it’s launched, they’re fine 

with that, and they tend to go on and build something else. Or once it’s in orbit around 

Jupiter, they’re fine with that, and they’d like to go do the hard engineering work getting 

something else into orbit. For me, I was totally thrilled and entranced to actually be doing 

56 Cox interview, 15 May 2001. 

57 Ibid.

years ago. As a planetary geologist specializing in lava flows, she was thrilled by the Io pictures, in part because they provided us with windows into our own planet’s distant past.51

Theilig’s love of her work came across very strongly. Her enthusiasm extended 

to both the people on the project and the science achieved. But she tried hard to balance 

her never-ending Galileo tasks with nonscientific pursuits. She recognized that hers was “a 

career type that could eat you up.” Although she spent many hours after the workday and on 

weekends at JPL, she was also deeply committed to her faith community, in which she was 

an elder. “Our society is so mobile that very few of us live in an area with the large extended 

family that we used to have,” Theilig said. “I think faith communities can help fill that gap.”52

Theilig found the Galileo project very welcoming to women. She noted the high 

number of women working on the mission, particularly in the science and design areas. 

Theilig’s assignments on the mission were largely engineering-oriented, and she found it 

taxing to be a scientist trying to fit into engineering niches. “The terminology and the way 

engineers think was more of a challenge for me [than being a woman on the project].”53

Of the many technical challenges she has faced, the most difficult was redesigning the mission after the high-gain antenna failed to open and figuring out how the team 

would operate the spacecraft at Jupiter, given the new constraints. She and the rest of 

the flight team had to create a very different mission quickly, one that did not depend 

on real-time transmission of most of its data. Galileo staff had to modify the spacecraft’s 

on-board programming to consolidate, compress, and store as much of the science data 

as possible in order to send it to Earth slowly through the spacecraft’s low-gain antenna. 

That whole reprogramming had to be done while the team was also very busy flying the 

spacecraft and collecting data, such as from the Shoemaker-Levy comet. Meeting all of 

these objectives at the same time was extremely difficult. But it was also exhilarating. In 

Theilig’s words, “I really like watching people presented with a problem and using their 

creativity to overcome it. And we’ve had lots of examples of that.”54

 Mission Support Personnel

If you cut us, we bleed Galileo.55

—Nagin Cox, quoting Shadan Ardalan, Galileo engineer

Galileo’s managers fought the political and budget battles and made the high-level technical decisions that gave the mission life and kept it alive during crises. But it was the 

mission support staff who provided hands-on expertise, quickly interpreting data streaming onto their monitors from the spacecraft, determining whether the craft was doing what 

it was supposed to, and fixing it from 450 million miles away when it ran into problems. 

Successful Jovian moon flybys would not have been achieved without the vital activities performed by mission support engineers and technicians, sometimes under extreme 

51 Ibid. 

52 Porter Anderson, “Eilene Theilig: Faith, by Jupiter.”

53 Theilig interview, 8 May 2001. 

54 Ibid.

55 Nagin Cox interview, tape-recorded telephone conversation, 15 May 2001. Cox was quoting a statement made by

Shadan Ardalan, Galileo Attitude and Articulation Control Subsystem Engineer.
298 MISSION TO JUPITER: A Histor y of the Galileo Project

the day-to-day operations. I’d walk outside at night and look up at Jupiter, and think, our 

ship’s up there.”58

As Galileo approached Io prior to its I24 encounter, the responsibility of Cox’s 

position as deputy team chief weighed heavily on her. She wondered whether she had 

just spent the last guilt-free night of her life. If anything went wrong on the encounter, if 

her team didn’t get the data from Io that the science teams were counting on, she would 

feel partly responsible. 

One of Cox’s new duties would be to lead the mission’s anomaly response team. 

For two years, she would serve as the project’s “first call,” which meant that whenever a 

problem occurred with the spacecraft, she would be contacted first by the mission controller on duty. She would stay on call 24 hours a day, 7 days a week, unless she made 

prior arrangements. It turned out to be a challenging two years, but Cox gained a deep 

satisfaction from being “so connected to the ship,” as well as to her team.59

Although mission support activities were not as widely publicized as those of 

Galileo’s managers, mission support personnel’s actions were no less vital to the completion of the Galileo mission. It was mission support’s charge to keep the spacecraft running 

and provide the key link between the dreams and hopes of Galileo’s scientific teams and 

the achievement of those desires. 

This chapter has highlighted only some of the personnel who made Galileo a 

success. The Galileo team members brought many different skill sets to the project, but 

they all shared a high degree of dedication to the mission and a determination to do whatever it took to solve the many problems that arose, some of which could have ended the 

mission. For all of these people, Galileo was more than just a job. 

58 Ibid. 

59 Ibid.
Thisbook has attempted to capture the importance of the 

Galileo mission to our country and to the world. The Galileo 

spacecraft explored new, exciting territory, using instruments that 

were, in essence, the eyes, ears, and fingertips of humankind. As the Galileo Probe buried 

itself in Jupiter’s atmosphere, as the Orbiter skimmed over Io’s fire fountains and Europa’s 

ice and possible buried ocean, it was we who were exploring uncharted frontiers.

These were not merely geographic frontiers being studied, but technological 

ones as well. To even get the Galileo spacecraft to Jupiter, NASA and the rest of the U.S. 

space science community had to push the envelope of what was possible. First, they had 

to create a vision for the mission that was so compelling that it would inspire a skeptical 

Congress and several recalcitrant White House administrations to fund the project. Then, 

they had to create a space vehicle robust enough to stay alive through all kinds of unexpected occurrences. And the challenge did not end with the spacecraft’s launch. When 

systems aboard Galileo malfunctioned and threatened to shut down our eyes and ears, 

mission staff had to invent new ways of operating and then reprogram the spacecraft 

through a thin lifeline of impaired radio communication from a distance of hundreds of 

millions of miles.

The mission logged so many notable achievements. One of the most difficult 

was simply getting the spacecraft off the ground. Years of battles with OMB and Congress, 

accompanied by one redesign of the spacecraft after another (including a major redesign 

after the Challenger disaster), made the eventual launch of Galileo a triumph of perseverance and willpower. Overcoming postlaunch mechanical difficulties with the spacecraft 

was also a premier achievement. For instance, the ingenuity that was required to keep the 

mission going after its main antenna refused to open can only be described as awesome. 

Chapter 11

conclusion
300	 MISSION TO JUPITER: A History of the Galileo Project 

No less impressive were the years of discoveries within the Jovian system. In situ 

profiling of Jupiter’s upper atmosphere, observations of Io’s extreme volcanism, and the 

discovery that an internal Europan ocean might harbor life were products of the Galileo 

effort that will long be remembered. So will the studies of the Shoemaker-Levy comet 

and the massive interplanetary dust storm. What may be less remembered, but was no 

less critical, were the decades of united effort by thousands of engineers, scientists, technicians, and politicians to create and carry out one of the greatest and most successful 

voyages of discovery ever attempted. 

Figure 11.1. The demise of Galileo. 
40K	 potassium-40, an isotope of potassium

ABMA	 Army Ballistic Mission Agency

AGU	 American Geophysical Union

AIAA	 American Institute of Aeronautics and Astronautics

ASC	 Astro Sciences Center; part of IITRI

ASI	 atmospheric structure instrument

AU	 Astronomical Unit (distance from Earth to the Sun)

BMFT	 Bundesministerium fuer Forschung und Technoligie

Caltech	 California Institute of Technology

CB	 Citizens Band

CCD	 charge-coupled device

CFC	 chlorofluorocarbon

CH4 methane

COMPLEX	 Committee on Planetary and Lunar Exploration

DC	 direct current

DDS	 dust detection system

DOD	 Department of Defense

DOE	 Department of Energy

DPS	 Division for Planetary Sciences

DSN	 Deep Space Network

EDT	 eastern daylight time

EGA	 Earth gravity-assist

EGS	 European Geophysical Society

EPD	 energetic particles detector

EPI	 energetic particles instrument

ESRO	 European Space Research Organization

ESTEC	 European Space Research and Technology Centre

EUG	 European Union of Geosciences

EUV	 extreme ultraviolet

FEIS	 final environmental impact statement

FSAR	 final safety analysis report

FY	 Fiscal Year

GAO	 General Accounting Office

GC	 gas chromatograph

GC/MS	 gas chromatograph–mass spectrometer

GEM	 Galileo Europa Mission

GMM	 Galileo Millennium Mission

GMT	 Greenwich mean time

Acronyms and Abbreviations
302 MISSION TO JUPITER: A Histor y of the Galileo Project Acronyms and Abbreviations 303

GPO	 Government Printing Office

H2 molecular hydrogen

H2O	 water

H3	 hydrogen isotope

HAD	 helium abundance detector

HEOS	 Highly Eccentric Orbit Satellite

HGA	 high-gain antenna

HST	 Hubble Space Telescope

HUD	 Housing and Urban Development

IITRI	 Illinois Institute of Research Technology Research Institute

INSRP	 Interagency Nuclear Safety Review Panel

IRAS	 Infrared Astronomy Satellite

IRD	 Interface Requirements Document

IUS	 Interim Upper Stage; later, Inertial Upper Stage

JOI	 Jupiter Orbit Insertion

JOP	 Jupiter Orbiter Probe

JOPSWG	 JOP science working group

JPL	 Jet Propulsion Laboratory

JSC	 Johnson Space Center

keV	 thousand (kilo) electron volts

KSC	 Kennedy Space Center

LED	 light-emitting diodes

LEMMS	 low-energy magnetospheric measurements system

LGA	 low-gain antenna

LRD	 lightning and radio emissions detector

MeV	 million electron volts

mph	 miles per hour

N2

O	 nitrous oxide

NASA	 National Aeronautics and Space Administration

NEP	 nephelometer

NEPA	 National Environmental Policy Act

NFR	 net flux radiometer

NH3 ammonia

NH4SH	 ammonium hydrosulfide

NIMS	 near infrared mapping spectrometer

NMS	 neutral mass spectrometer

NO3 oceanic nitrate

NOAA	 National Oceanic and Atmospheric Administration

NSSDC	 National Space Science Data Center

OMB	 Office of Management and Budget

PDT	 Pacific daylight time

PLS	 plasma subsystem

PPR	 photopolarimeter radiometer

psi	 pounds per square inch

PST	 Pacific standard time

PWS	 plasma wave subsystem

RF	 radio frequency

RFP	 Request for Proposal

RJ Jupiter radii

RPM	 retropropulsion module (part of the Jupiter Orbiter)

rpm	 revolutions per minute

RTG	 radioisotope thermal generator

S/L-9	 Comet Shoemaker-Levy 9

SAF	 Spacecraft Assembly Facility (JPL)

SAG	 Science Advisory Group

SAR	 safety analysis report

SBA	 spin bearing assembly

SEC	 Sun-Earth-Craft

SEDS	 Students for the Exploration and Development of Space

SETI	 Search for Extraterrestrial Intelligence

SITURNS	 spacecraft inertial turns, also called science turns

SOE	 sequence of events

SOPE	 Strategy for Outer Space Exploration

SRM	 solid rocket motor

SSI	 solid state imaging

Star Wars	 nickname for the Strategic Defense Initiative

TCM	 trajectory correction maneuver

TDM	 time division multiplex

TDRS	 Tracking and Data Relay Satellite

TOPS	 Thermoelectric Outer Planet Spacecraft

U.S.	 United States

UCAR	 University Corporation for Atmospheric Research

UCLA	 University of California, Los Angeles

UT	 universal time

UV	 ultraviolet

UVS	 ultraviolet spectrometer

VEEGA	 Venus-Earth-Earth gravity-assist

VLF	 Very Low Frequency

VOIR	 Venus Orbiter Imaging Radar
A

Adrastea 256, 277–278

Aftergood, Steven 98

Air Force 12, 33, 47, 50, 54–58, 65, 105,

	 149–150,183, 290

Aldrich, Arnold 85

Alexander, Claudia xix, 281–282, 285

Allnut, Robert 57, 59

Amalthea xi, 238, 256, 259, 277–280

Ames Research Center (Ames, ARC) vi, xix, 5,	

11–12, 20–22, 26–27, 30–33, 38–39, 61, 111,

115–117, 121–122, 124–126, 128, 150, 195,

200–201, 203–206, 216–217, 291

Amphitrite 45, 66–68, 76, 83, 104

Antarctica (antarctic) 160, 166, 265–266, 295

Apollo xviii, 4, 11, 16, 19–22, 24, 26, 85,	

	 98–99,114, 158, 284, 287

Army Ballistic Missile Agency (ABMA) 10,	

	 13–14, 284

Atlantis v, vii, 45, 70, 78, 100, 103–105, 149, 289

Atlas 14–15, 65

atmospheric structure instrument (ASI) 39,	

	 121–123, 125, 206–207, 210, 221

Atomic Energy Commission 98

Ausonia 94, 104

autonomous spacecraft 50–51

B

Baker, James 54

Beggs, James 45, 52, 57–58, 67–68

Boeing 33, 43–44, 47, 54–55, 58, 65

Boland, Eddie 35, 43, 45–47, 80

bow shock vi–vii, 20, 129, 134, 154–55,	

	 157–158, 201–202, 270

Bureau of the Budget 24–26

Bush, George 49, 75, 100, 102

Byrnes, Dennis 157, 293–294

C

Callisto xiii, 136, 139, 144, 148, 186, 209, 224,	

	 231–233, 236, 251, 255, 259, 268, 273–277

Carter, Jimmy 49

Casani, John xv, xix, xxi, 5–7, 32, 37–38,	

44–45, 49, 50–51, 53–56, 58–60, 62, 66–70,

81–83, 93–94, 105, 108, 111, 115, 182–183,

224, 283–287, 289, 294

Centaur 12, 15, 31–33, 42–48, 51–60, 63, 65–66,	

70, 76–77, 79–83, 93, 103–104, 266, 286, 289,

293

Challenger v, xiii, 3, 45, 70–97, 103–104, 177,

	 286, 289–290, 293, 299

Christic Institute 97, 102

Chyba, Christopher 211, 261, 264

cleanroom 62

comet ix, 4, 7, 48, 51, 53, 66, 149, 187–193, 196,

210–211, 215, 239, 259–261, 267, 273, 277,

296, 300

Committee to Bridge the Gap 97–98, 100

COMPLEX 12, 31–32

Congress (U.S.) v, xv, xix, 9, 11–12, 16, 19–21,

24–27, 28–29, 34–38, 43–48, 52, 56–59, 69,

80, 86, 95, 182, 284–285, 288, 299–300

Convair 44–45, 58

Cox, Nagin xix, xxi, 226, 237, 246–248, 250,

	 256–257, 296–298

D

Dactyl viii, 3, 168–169

D’Amario, Louis 82, 152, 157, 293–294

deceleration module ix, 61, 115–117, 205

Deep Space Network (DSN) 3, 108, 139–140,

	 151, 186, 227–228, 230, 266, 282

Department of Defense (DOD) 44, 50, 53–55,

	 78, 87, 96–97

Department of Energy (DOE) xv, 5, 78, 96–100

descent module vi, ix, 61, 115–117, 205, 207

Diehl, Roger 157, 293–294

dual-spin design 34, 112–114

dust x, 4–5, 8, 21, 29, 41, 51, 67, 109, 127,

130–133, 187–188, 192–193, 195–197, 211,

215, 237, 245, 254, 277, 300 

dust detector system (DDS) 41, 67, 109, 	 	

	 130–133, 195–197

Index

Note: Terms such as “NASA,” “Jet Propulsion Laboratory (JPL),” “Orbiter,” and “Probe” are not included 

because of their high frequency.
306 MISSION TO JUPITER: A Histor y of the Galileo Project INDEX 307

I

ice (extraterrestrial) vi, x, xi, 4, 125, 128, 139, 146,	

188, 192, 211–213, 215–216, 234–236, 250, 253–

277, 280, 282, 292, 295, 299

ice (terrestrial, including ice on Challenger) xi, 73,	

	 159, 166, 265–266, 295

Ida viii, ix, 3, 94, 104, 136, 166–169, 187–188

Inertial Upper Stage (IUS) v, vii, 33, 37, 41–48,	

54–59, 65, 80–81, 93, 103–105, 111, 149–152,

178

infrared vi, x, 17, 39, 67, 121, 125–126, 128, 141–	

145, 147, 151–153, 156, 159–160, 164, 166, 191,

212, 218–219, 225, 241, 247–249, 252, 256, 271,

278, 280–281

injection module 44, 54, 57, 60

International Court of Justice 101

interplanetary dust 4, 131, 187, 195–197, 300

Io xiii, 2, 4, 29, 127, 134–139, 144, 147–148, 185,	

195–197, 200–203, 209, 224, 226–228, 231–253,

256–259, 268–269, 274–275, 277, 279, 288, 292,

295–296, 298–300

J

Japan 7, 50–51

Johnson Space Center (JSC) v, vi, 5, 74, 76, 116

Johnson, Lyndon 16, 19–20, 24–25

Johnson, Torrence (T.V.) xvii–xix, 1–3, 31, 36, 50,	

88, 109–110, 119–120, 130–131, 141–142, 152–

154, 156, 184, 199, 223, 239, 241–242, 245, 251,

270, 273, 277, 280–282, 288

Joint Environment Simulator v, 88–89

Jupiter Orbiter Insertion (JOI) 111, 203, 208–209,	

	 225, 231, 292

Jupiter Orbiter Probe (JOP) 12–13, 30–38, 43,	

	 109–110, 284, 288

Jupiter Orbiter Probe Science Working Group	

	 (JOPSWG) 33, 109–110, 119–120

Justice Department 101

K

Kennedy Space Center (KSC) 5, 42, 45, 51, 60,	

69–70, 72–75, 77, 81, 93–94, 100–104, 149–150,

177–178, 183, 284

Kennedy, John 16, 19, 25

Keyworth, George 44, 52–53

Kivelson, Margaret 40, 131, 135, 154–156, 164,	

	 196, 201, 243, 251–252, 261–263, 270–271, 276

L

Ladish Company 86

Lake Vostok 265–266

lander 10, 17, 114–115, 120, 153, 291

Langley Research Center 115, 266

Lanzerotti, Louis 39, 122, 126, 155, 221

Levanas, Greg xix, 281–282

Levy, Eugene 53

Lewis Research Center 5, 58–59, 62, 172, 176–177

life (extraterrestrial) 4, 124, 148, 215, 238, 252,	

	 261, 263–266, 277, 281, 283, 292, 295, 300

life (terrestrial) 124, 159–161

lightning (Jovian) 4, 39, 121–122, 124, 126, 146,	

	 201, 210, 213, 220–221

lightning (Venusian) 153–157

lightning and radio emissions detector (LRD) 121–	

	 123, 126–127, 210, 220

Lockheed 10, 15

Lopes-Gautier, Rosaly 241, 253

low-gain antenna (LGA) viii, 93, 103, 130, 152,	

161, 176, 180–181, 184–186, 195, 197, 204, 225,

228–229, 296

Lowery, Bill 58

Lund, Robert 72

M

Magellan 52, 80–81, 156

magnetic field vi, vii, xviii, 1–4, 10, 17, 21, 29,	

38–40, 108–109, 111–113, 126–127, 130–139,

148, 154–158, 164, 167, 169, 184, 186, 193, 196,

201, 234–239, 243–244, 249, 251–252, 256, 259,

262–263, 267–271, 274, 276–280

magnetometer (MAG) 40, 95, 108, 112, 130–132,	

	 135–137, 201, 262, 269, 276

magnetosphere vi, vii, 40, 107, 109, 121, 129,	

131–139, 142, 154, 158, 164, 201, 223, 245, 251,

256–258, 261–262, 266, 269–271, 274, 276

Malow, Dick xix, 34–35, 46

Mariner 12, 16, 20, 27–32, 67, 166, 284, 288, 291

Mark, Hans 51

Markey, Edward 78

Marshall Space Flight Center 10, 16, 22, 72, 79,	

	 84–85, 87, 90, 265

Maryland Safe Energy Coalition 100

mass spectrometer 39, 120–124, 137, 210, 215–216

McDonald, Al xix, 72–75, 86–87

Meese, Edwin 52

E

electromagnetic x, 40, 63, 68, 127, 131,

	 138–141, 143, 155, 193, 196–197, 233, 270

energetic particle detector (EPD) vii, 130–136

energetic particle instrument (EPI) 121–123, 126–	

	 127, 202, 206

Environmental Protection Agency (EPA) 96

Erickson, Jim xix, 237, 240, 247–251, 255–257,

	 259, 269, 274, 277, 282, 285, 290, 294–295

Europa x, xi, xiii, xvii, 4, 136, 139, 144, 148, 185,

200–203, 209, 224, 231–238, 240, 242, 246, 250,

252–269, 271–272, 275–279, 281–282, 292, 295,

297, 299–300

Europe xv, xviii, 6–7, 12, 18, 29–31, 40, 50–51, 59,	

	 193, 229, 280, 292

Evans, David 50

F

Flandro, Gary 18–19, 22, 29

Fletcher, James 8, 11, 25, 27–28, 35, 69, 79

Florida Coalition for Peace and Justice 96, 100–102

flyby 3–5, 9, 10–12, 16–18, 20–24, 27–30, 44–45,

66–67, 83, 93, 99, 108, 126, 129–131, 137, 151–

158, 161, 163–164, 166, 168–169, 171, 180–181,

186, 202–203, 209–210, 221, 223–281, 292, 296

Ford, Gerald 31, 34

Frank, Louis 40, 132, 155, 245, 251

Frank, Steve 101

Frosch, Robert 43, 45–48

G

Gagnon, Bruce 96–97, 100

Galilean moons (Galilean satellites) 1, 38, 136,

139, 144, 148, 202, 223–224, 231, 242, 253, 260,

267–270, 272–275, 278–279, 285

Galileo Europa Mission (GEM) x, xiii, 233–237,

	 240, 246, 248, 256–258, 274, 278–279, 297

Galileo Galilei xxi, 1, 2, 38, 223–225, 243, 253,

	 267, 275

Galileo Millennium Mission (GMM) x, xiii, 228,

233, 237–241, 249–253, 255, 258, 269–270, 274,

280–281

Ganymede xiii, 4, 136, 139, 144, 148, 197, 209,

224, 231–233, 242, 249–250, 255, 259, 261, 266–

267–273, 275–276

gas chromatograph (GC) 120

gas chromatograph–mass spectrometer (GCMS) 120

Gasch, Oliver 101–102

Gaspra vii, viii, ix, 3, 94, 104, 136, 161–164, 166–169,

	 171, 180, 188

General Dynamics 44–45, 58, 65

geocorona viii, 164–166, 169

Germany xv, xviii, 5–6, 31, 38, 41, 49, 101, 111,

	 122, 124, 126, 131, 135, 193, 195–197, 289–291

Glenn Research Center (also see Lewis Research	

	 Center) 5, 59

Goddard Space Flight Center (GSFC) 4, 10, 16,

21–22, 29, 31, 39–40, 119–123, 125–126, 131,

133, 135–136, 142, 144, 154, 190, 218, 266–267

Goldin, Daniel 263–264, 287

Grand Tour 11–12, 17–19, 21–29, 34

gravity waves 139–140

gravity-assist v, viii, ix, 18, 26, 29, 30, 41–44, 47–48,	

54–56, 59, 82, 84, 103, 150, 157–158, 168, 187–

188, 230–231, 236, 241, 267–269, 286, 293

Greens 101

Guastaferro, Angelo 43

H

Halley’s Comet 7, 48, 51, 192

Hardy, George 72

Hearth, Donald 18–19

heatshield vi, ix, 12, 29–30, 61, 103, 116–117, 127,	

	 201, 204–205, 207

helium 2, 4, 39, 61, 120, 122–124, 202, 211, 213–215

helium abundance detector (HAD) 120–125

Herman, Dan 33–34

Hibbs, Albert 10

high-gain antenna (HGA) viii, 3, 93, 103, 109,	

130, 152, 161, 171–187, 195, 197, 228–229, 288,

294–296

House of Representatives 7, 11–13, 27, 34–35, 38,	

	 45–46, 58, 78, 80, 85–86, 108

Hubble Space Telescope 12–13, 34–35, 79, 87,	

	 191–193, 244, 266, 278

Hughes Aircraft Company, Hughes Space and	

	 Communications Company 5, 33–34, 61–63, 81,	

	 121–122, 125–126, 291

hydrogen v, viii, 2, 15, 29, 33, 39, 42–43, 47–48,	

58, 65–66, 75–76, 79–80, 89, 103, 120, 122,	

124–125, 135, 164–166, 192, 211, 214–219, 264, 266
308 MISSION TO JUPITER: A Histor y of the Galileo Project INDEX 309

solid rocket motor (SRM) 72, 79, 86–87, 90–93

solid state imaging camera (SSI) vii, xi, 142, 146–	

148, 151, 153–154, 156, 160, 162, 185, 227, 229,

241, 244, 246, 248, 253, 274, 278–280, 297

Soviet Union (Soviet) 7, 16, 25, 50–51, 153, 155,	

	 265–266

Space Science Board (SSB) 11, 16, 25–27, 31, 34, 53

Space Science Steering Committee 38

Space Shuttle (Shuttle) v, xiii, 3, 5, 12, 27, 29–31,	

33, 35, 37, 41–48, 51, 53–58, 62–66, 69–74, 76–

81, 83–93, 96, 99–105, 115, 149, 172, 175, 178,

286, 289, 293

spectrometer 15, 39–43, 109–110, 120–124, 130,	

137, 141–144, 147–148, 151, 153, 159, 164, 191,

210, 215–216, 241, 253, 271, 278

Spehalski, R.J. xix, 5, 105, 285–286, 289–291, 293

Sperans, Joel xix, 61, 120

spin-stabilized 11, 20–21, 26, 34

Stewart, Homer Joe 18

Stockman, David 44, 49–50, 53–54

Straly, Warren 13–14

Stuhlinger, Ernst 14

sun shield vii, 93, 103, 130, 150, 152, 171, 180

T

tape recorder 3, 93, 147, 151–152, 185, 187,	

197–200, 209, 225–226, 229, 236, 240, 245–246,

250–253, 255–259, 280–282, 285, 292

Thebe xi, 256, 259, 277–279

Theilig, Eilene xix, 223, 237–238, 240, 246, 248–

	 249, 251–253, 282, 285, 295–296

thermal blanket 64, 103, 111, 114

Thermoelectric Outer Planet Spacecraft (TOPS)	

	 12, 26–28 

Thompson, James 85, 90

three-axis-stabilized 11, 20, 26, 34

Thurmond, Strom 44, 50–51 

Titan (launch vehicle) 15, 31–32, 42, 47, 81, 88

Titan (satellite of Saturn) 29, 81, 88, 263

trajectory correction maneuver (TCM) 63, 151–	

	 152, 157, 164

Transit navigational satellite 98

Truly, Richard 85

U

UCLA xix, 18, 40, 131–132, 137, 142, 164, 201, 243,	

	 257, 262, 270–271, 276

ultraviolet spectrometer (UVS) 40, 109, 130, 142,	

	 147–148, 164, 191, 278

V

Van Allen belts 53, 202

Van Allen, James 22, 33, 53, 69, 109, 119

VEEGA trajectory v, 83–84, 93–94, 103, 150, 157,	

	 187–188, 286, 293–294

Venus v, vii, ix, xvii, 1, 7, 16, 26, 32, 48, 51–52,	

80, 83–84, 93–95, 103–104, 119–121, 136, 148–

157, 187–188, 284, 286, 293–294

Viking 25, 31–32, 36, 81, 114, 120, 291

volcano (volcanism, volcanic) v, x, xvii, 2, 4, 127,

148, 156, 166, 169, 195, 197, 234–236, 239–253,

258–259, 261, 263, 265, 268, 272–273, 288, 300

von Braun, Wernher 84, 283–284, 287

Voss, Robert G. 13–14

Voyager xvii, xviii, 3–4, 7, 12, 16–19, 21–29, 31–	

32, 37, 51–52, 69, 81–83, 108–109, 111, 114,

121, 124, 126, 131–132, 134–138, 141, 146–147,

191, 197, 200, 210–211, 214–217, 221, 228, 245,

250, 254, 261, 272, 278, 284–285

W

Waff, Craig xix, 10, 13–36, 41, 48–51, 53–55, 57–

	 58, 110

Wald, Patricia 103

water x, xi, 4, 73, 92, 98, 124, 154, 159, 161, 166,	

192–193, 201, 210–220, 234–239, 253–267, 269,

271–272, 277, 282, 292, 295

Webb, James 20

Weinberger, Caspar 54

Welch, Jasper 57

whistler 126, 155–158

White House 19–20, 44, 48, 51–55, 57, 75, 86, 100, 299

White Sands Missile Range 61

Wolfe, Al 50, 60

Metis xi, 256, 259, 277–279

Minovitch, Michael 18

Mitchell, Bob xix, 18, 52, 255–256, 261–263, 268,	

	 278, 285, 293–294

Moon (of Earth) x, xvii, 2, 4, 10, 19–20, 85, 93, 98,	

114, 135, 138, 148, 157–158, 166, 168–169, 190,

224, 239, 241–242, 259, 261, 271, 276, 284, 291

Moore, Jesse 42

Morton Thiokol 71–73, 79, 85–91

Murray, Bruce 19, 28, 32–33, 36, 46–47, 50, 57

Murray, Robert 182

N

National Academy of Sciences 10–12, 16–17, 25–	

	 27, 31–32, 49, 52–53

National Environmental Policy Act (NEPA) 101–103

National Oceanic and Atmospheric Administration	

	 (NOAA) 40, 96

National Research Council 87–90

National Space Council 101

near infrared mapping spectrometer (NIMS) vii,	

39, 109, 130, 141–144, 147, 151, 153, 159, 164,

166, 168, 191–192, 218, 241, 246, 248–249, 261,

274, 278

nephelometer (NEP) 39, 121–123, 125–126, 213

net flux radiometer (NFR) 121–123, 127–128, 210,	

	 212–213, 216

neutral mass spectrometer (NMS) 120–126, 210,	

	 213, 215–216

Newburn, Ray 9, 10, 13, 190

Nixon, Richard 11–12, 24–27, 31

Nuclear Regulatory Commission 96

O

Office of Management and Budget (OMB) 11–12,	

	 25–26, 34, 44, 48–49, 52, 54, 57, 299

O’Keefe, Sean 282

O’Neil, William (Bill) 180–182, 184, 186, 194, 198–	

202, 245, 247, 255, 268–269, 282, 285, 291–292,

294

Orbital Science Corporation 81

O-ring 71–74, 90–92, 94–95

Orr, Verne 58

Outer Planets Working Group 11, 22–24

oxygen v, 15, 65–66, 75–76, 80, 89, 134, 159–161,	

	 192, 210–211, 215, 245, 264, 266

ozone 148, 159, 166, 168, 288

P

Paine, Thomas 25–26

parachute ix, 44, 50, 61–62, 117, 204–205, 207–	

	 208, 291

photopolarimeter radiometer (PPR) vii, 40, 109,	

	 130, 141–142, 144–147, 241, 247, 274

Pioneer (spacecraft) 11–13, 20–23, 26–32, 37, 84,	

95, 108, 114, 119–121, 131, 136–137, 155–156,

245, 284

plasma subsystem vii, 40, 109, 127, 130–132, 137–	

	 138, 154

plasma wave subsystem (PWS) 40, 109, 121, 130–	

	 132, 137–139, 153–156, 270

plutonium 5, 64, 69, 77–78, 83, 97–101, 104, 111

Presidential Commission (also see Rogers	

	 Commission) 71, 75, 79, 87, 91

Prime Mission x, xiii, 149, 231–234, 236, 240, 246,	

	 255–257, 268, 295, 297

Q

Quayle, Dan 101

R

radioisotope thermal generator (RTG) 15, 22, 64,	

77–78, 81, 83, 97–100, 109, 111, 114–116, 130, 289

Reagan, Ronald 44, 48–49, 51–52, 54, 58, 76, 79,	

	 285–286

Rogers, William and the Rogers Commission	

	 71–80, 84, 86–87, 91

S

Sagan, Carl 159–161

Schmitt, Harrison 58

Schubert, Gerald 213, 244, 276

Science Advisory Group (SAG) 11–12, 27, 29–30

Search for Extra-Terrestrial Intelligence (SETI)	

	 Institute 263–264, 277

Senate xxi, 7, 11, 13, 27, 34–35, 41, 44, 46, 50,	

	 58, 99

Shoemaker-Levy comet ix, 4, 187–193, 196, 210,	

	 259–260, 296, 300

Sinkin, Larry 97

Sobeck, Charlie xix, 121–122, 125–126

solar conjunction x, 225, 232–233, 249–250, 255–257

solar wind vi, vii, xiii, 21, 29, 40,133–136, 154–	

158, 164, 167, 169, 201–202, 211, 213, 237–239,

269–270
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No monograph 28.

Chambers, Joseph R. Concept to Reality: Contributions of the NASA Langley Research 

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Renstrom, Arthur G. Wilbur and Orville Wright: A Chronology Commemorating the OneHundredth Anniversary of the First Powered Flight on December 17, 1903. NASA SP2003-4532, 2002.

Bowles, Mark D. and Robert S. Arrighi. NASA’s Nuclear Frontier: The Plum Brook 

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Matranga, Gene J., Wayne C. Ottinger, Calvin R. Jarvis, and Christian D. Gelzer.

Unconventional, Contrary, and Ugly: The Lunar Landing Research Vehicle. NASA SP2006-4535, 2006.

Logsdon, John M., editor, with Roger D. Launius, David H. Onkst, and Stephen J.

Garber. Exploring the Unknown: Selected Documents in the History of the U.S. Civil Space 

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Logsdon, John M., general editor, with Ray A. Williamson, Roger D. Launius, Russell J.

Acker, Stephen J. Garber, and Jonathan L. Friedman. Exploring the Unknown: Selected 

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NASA SP-4407, 1999.

Logsdon, John M., general editor, with Amy Paige Snyder, Roger D. Launius, Stephen J.

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4407, 2001.

Siddiqi, Asif A. Challenge to Apollo: The Soviet Union and the Space Race, 1945–1974.

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Launius, Roger D. and Aaron K. Gillette, compilers, Toward a History of the Space 

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Launius, Roger D., and J. D. Hunley, compilers, An Annotated Bibliography of the Apollo 

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Launius, Roger D. Apollo: A Retrospective Analysis. Monograph in Aerospace History, No.

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Hansen, James R. Enchanted Rendezvous: John C. Houbolt and the Genesis of the LunarOrbit Rendezvous Concept. Monograph in Aerospace History, No. 4, 1995.

Gorn, Michael H. Hugh L. Dryden’s Career in Aviation and Space. Monograph in

Aerospace History, No. 5, 1996.

Powers, Sheryll Goecke. Women in Flight Research at NASA Dryden Flight Research 

Center, from 1946 to 1995. Monograph in Aerospace History, No. 6, 1997.

Portree, David S. F. and Robert C. Trevino. Walking to Olympus: An EVA Chronology. 

Monograph in Aerospace History, No. 7, 1997.

Logsdon, John M., moderator. Legislative Origins of the National Aeronautics and Space 

Act of 1958: Proceedings of an Oral History Workshop. Monograph in Aerospace History,

No. 8, 1998.

Rumerman, Judy A., compiler, U.S. Human Spaceflight, A Record of Achievement 1961–

1998. Monograph in Aerospace History, No. 9, 1998.

Portree, David S. F. NASA’s Origins and the Dawn of the Space Age. Monograph in

Aerospace History, No. 10, 1998.

Logsdon, John M. Together in Orbit: The Origins of International Cooperation in the 

Space Station. Monograph in Aerospace History, No. 11, 1998.

Phillips, W. Hewitt. Journey in Aeronautical Research: A Career at NASA Langley 

Research Center. Monograph in Aerospace History, No. 12, 1998.

Braslow, Albert L. A History of Suction-Type Laminar-Flow Control with Emphasis on 

Flight Research. Monograph in Aerospace History, No. 13, 1999.
318 MISSION TO JUPITER: A Histor y of the Galileo Project

McCurdy, Howard E. Low Cost Innovation in Spaceflight: The History of the Near Earth 

Asteroid Rendezvous (NEAR) Mission. NASA SP-2005-4536, 2005.

Seamans, Robert C. Jr. Project Apollo: The Tough Decisions. NASA SP-2005-4537, 2005.

Lambright, W. Henry. NASA and the Environment: The Case of Ozone Depletion. NASA

SP-2005-4538, 2005.

Chambers, Joseph R. Innovation in Flight: Research of the NASA Langley Research Center 

on Revolutionary Advanced Concepts for Aeronautics. NASA SP-2005-4539, 2005.

Phillips, W. Hewitt. Journey Into Space Research: Continuation of a Career at NASA 

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Electronic Media, SP-4600 Series

Remembering Apollo 11: The 30th Anniversary Data Archive CD-ROM. NASA SP-4601, 1999.

The Mission Transcript Collection: U.S. Human Spaceflight Missions from Mercury 

Redstone 3 to Apollo 17. NASA SP-2000-4602, 2001, CD-ROM.

Shuttle–Mir: The United States and Russia Share History’s Highest Stage. NASA SP-2001-

4603, 2002, CD-ROM.

U.S. Centennial of Flight Commission presents Born of Dreams-Inspired by Freedom.

NASA SP-2004-4604, 2004, DVD data disk.

Of Ashes and Atoms: A Documentary on the NASA Plum Brook Reactor Facility. NASA

SP-2005-4605, DVD video.

Taming Liquid Hydrogen: The Centaur Upper Stage Rocket Interactive CD-ROM.	

NASA SP-2004-4606, 2004.

Fueling Space Exploration: The History of NASA’s Rocket Engine Test Facility DVD. NASA

SP-2005-4607, DVD video.

Conference Proceedings, SP-4700 Series

Dick, Steven J. and Keith L. Cowing, ed. Risk and Exploration: Earth, Sea and the Stars.

NASA SP-2005-4701.

Dick, Steven J. and Roger D. Launius. Critical Issues in the History of Spaceflight. NASA

20064702.