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)
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)


vi	 MISSION TO JUPITER: A History of the G

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, 

  NASA photo number S89-44175) 

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) 12

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

 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 
vi	 MISSION TO JUPITER: A History of the G

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, 

  NASA photo number S89-44175) 

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) 12

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

 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 
viii	 MISSION TO JUPITER: A History of the 

 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) 16

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) 16

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) 16

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) 16

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
viii	 MISSION TO JUPITER: A History of the 

 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) 16

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) 16

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) 16

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) 16

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
x	 MISSION TO JUPITER: A History of the 

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) 21

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 clos

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) 25

FIGURE pAGE 
x	 MISSION TO JUPITER: A History of the 

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) 21

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 clos

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) 25

FIGURE pAGE 






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
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


xxii	 MISSION TO JUPITER: A History 

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.