Spacecraft Galileo: To Jupiter and Its Moons Spacecraft Galileo: To Jupiter and Its Moons By Elizabeth Howell, Space.com Contributor | November 26, 2012 04:55pm ET MORE An artist;s conception of the Galileo spacecraft, which was launched in 1989 for a mission to Jupiter. Credit: NASA Galileo was the first spacecraft to examine Jupiter and its moons for an extended period. It launched from the payload bay of space shuttle Atlantis in 1989, got some speed boosts by swinging past Earth twice and Venus once, then arrived at Jupiter at last in 1995. Circling the solar system's most giant planet for eight years, Galileo beamed back a string of discoveries to Earth despite encountering several mechanical problems. It discovered evidence of saltwater below the surfaces of three moons – Europa , Ganymede and Callisto – and also got close to the infamous "pizza moon" Io as its volcanoes belched into the atmosphere. When Galileo was almost out of fuel, NASA deliberately sent the spacecraft on a suicide plunge into Jupiter on Sept. 21, 2003.

The sacrifice, the agency said, was necessary to protect Europa – which likely has a subsurface ocean that could contain life . The case for Jupiter Jupiter was a target of interest for NASA for decades before Galileo's launch. Four spacecraft had flown by the giant planet before – Pioneer 10 and Pioneer 11 , and Voyager 1 and Voyager 2 . Pioneer 10, when it zoomed by the planet in 1973, discovered Jupiter's radiation was far less than scientists had anticipated. This made a long-term mission to Jupiter more feasible, since the spacecraft wouldn't need to be as heavily shielded (making it more expensive to launch.) NASA was intrigued by the thought of a Jupiter mission, but the idea went through years of budgetary discussions and management changes, as well as machinations in Congress. By 1977, NASA was far enough in its planning to propose a "Jupiter orbiting probe" in its budget, but the idea was thrown out by a congressional appropriations subcommittee with oversight of the agency. That decision was reversed after a massive lobbying effort by the scientific community, and Congress approved the project later that year. It would take another 12 years to get Galileo off the ground.

There were continual government funding threats to the mission and even the existence of NASA's Jet Propulsion Laboratory , where it was being built. Debates also arose about Galileo's planned launch vehicle. Then, just as the probe was readying for a space shuttle mission, Challenger exploded and killed seven astronauts in 1986, grounding the fleet for two years. Named for astronomer Galileo Galilei , the spacecraft launched at last on Oct. 18, 1989, from the payload bay of space shuttle Atlantis. To save on fuel, the probe whipped by Venus once and Earth twice to pick up speed, with the aim to reach Jupiter in 1995. Solving problems in space Operating missions for the long duration can be a marathon for both humans and spacecraft alike. As components on the spacecraft break down, it's up to the people on the ground to figure out how to resurrect them – or do without them. Galileo spacecraft observations: a three-color global scale view of Io obtained on 3 July 1999 (Orbit 21) with a resolution of 1.3 km per pixel is shown on the left.

The corresponding infrared image on the right was taken at 4.7 μm on October 16 2001 in daytime and has a spatial resolution of 30 km/pixel obtained . The near infrared picture shows the active volcanoes glowing thermal radiation. Credit: NASA/JPL/University of Arizona Galileo's most prominent failures were a high-gain antenna that failed to open – threatening the transmissions of data back to Earth – and a data tape recorder that temporarily jammed during the final approach to Jupiter. The sticky antenna, shaped somewhat like an umbrella, lay stowed on the spacecraft for nearly two years after launch. NASA determined it might be risky to set it free when Galileo was closer to the sun during its gravity assist by Venus. On April 11, 1991, controllers sent a command for Galileo to unfurl the antenna. The motors ran for eight minutes at higher power levels than expected, which indicated there might be a problem. The spacecraft then failed to send a signal to Earth saying the antenna had opened. A fault analysis determined that some of the antenna's "ribs" got stuck. Managers did everything from rapidly spinning the spacecraft to exposing the antenna to sunlight, with no luck.

To get around the problem, they came up with ways to compress data so that Galileo could send back more information to Earth, which helped save the crippled mission. NASA now relied on Galileo's on-board data recorder and low-gain transmission antennas to bring information back to Earth. That worked well until the recorder jammed for 15 hours while rewinding on Oct. 11, 1995 – just as the spacecraft was finally closing in on Jupiter. Fortunately, NASA found a work-around and was able to resume work a few weeks later. The tape recorder acted up in future years, though, necessitating more fixes. "Among the Galileo mission’s memorable achievements were the repeated successes of its staff in solving serious technical problems," wrote Michael Meltzer in the NASA publication Mission to Jupiter: A History of the Galileo Project. "In each instance, the team attacked potentially mission-ending problems and found ingenious ways to keep the spacecraft operational and productive." Entering Jupiter's system One of Galileo's first science targets was Comet Shoemaker-Levy 9 . Jupiter's gravity had pulled the comet towards the planet and broken it up into more than 20 pieces.

When the shards slammed into Jupiter in July 1994, the astronomical community eagerly watched. Galileo was en route to Jupiter at that time and snapped some shots of the hits. The spacecraft encountered "interplanetary dust storms" on its way to Jupiter, possibly from particles coming from within the Jovian system. At one point, Galileo was tracking 20,000 dust particles a day , compared to a typical one particle every three days. Galileo was still flying at Jupiter when in July 1995, it released a probe to smack the planet's atmosphere in December. When the probe finished its descent, NASA was surprised at its measurements of helium – half of what they expected – and the dryness of the region in which the probe flew. "These initial findings are encouraging scientists to rethink their theories of Jupiter's formation and the nature of planetary evolution processes," wrote NASA in a January 1996 press release . Planetary, moon and asteroid science Galileo itself reached Jupiter on Dec. 7, 1995, beginning its years of circling the planet and its moons.

These images show the trailing hemisphere of Jupiter's moon Europa taken by the Galileo spacecraft at a distance of about 677,000 km. The left image shows Europa in approximately true color and the right image shows Europa in enhanced color to bring out details. The bright feature towards the lower right of the disk is the 45 km diameter crater Pwyll. Credit: NSSDC Photo Gallery In the early days, Galileo took a close look at the faint rings of Jupiter to find out how they formed. Data from the spacecraft determined that meteoroids, crashing into small moons around Jupiter, were sending dust up around the planet . Over time, the dust coalesced into rings. While Galileo was often called a mission to Jupiter, the spacecraft also made extensive observations of the planet's largest moons. It found evidence of a liquid ocean underneath Europa's surface, sparking questions about what sort of life could lie underneath. From watching volcanoes at Io , Galileo's data showed the moon's volcanic activity could be as much as 100 times more than what is seen on Earth. And at Ganymede, Galileo found the first magnetic field around a moon. Galileo even made a few side discoveries.

When taking pictures of the asteroid Ida, it discovered there was a smaller object orbiting around it . Later called Dactyl, this was the first confirmed satellite of an asteroid. In 2003, the aging spacecraft – battling radiation problems and other mechanical issues – was running low on fuel. NASA chose to send Galileo straight into Jupiter rather than leave it in orbit, just in case the spacecraft accidentally crashed into Europa and disrupted possible life there. Galileo broke up in Jupiter's atmosphere on Sept. 21, 2003. — Elizabeth Howell, SPACE.com Contributor NASA - Galileo to Jupiter (included NASA Ames partnership) Galileo to Jupiter (included NASA Ames partnership) 03.19.07 Launched in 1989 aboard space shuttle Atlantis, Galileo explored Jupiter and its moons. Upon arrival at Jupiter in December 1995, the Galileo spacecraft delivered a probe that descended into the giant planet's atmosphere. The orbiter completed many flybys of Jupiter's major moons, reaping a variety of science discoveries. The mission ended on Sept. 21, 2003, when the spacecraft plunged into Jupiter's atmosphere. Overview: Galileo was launched Oct.

18, 1989, from Kennedy Space Center, Florida, carried into Earth orbit in the cargo bay of space shuttle Atlantis. A two-stage solid-fuel motor called an inertial upper stage then propelled Galileo onto its interplanetary flight path. Although earlier plans called for Galileo to use a more powerful upper stage so that it could fly directly to Jupiter, the final flight took the spacecraft by other planets first so that it could gain energy from the gravity of each. Galileo flew past Venus on Feb. 10, 1990, and then twice past Earth -- once on Dec. 8, 1990, and again on Dec. 8, 1992. Also en route to Jupiter, Galileo flew close to two asteroids, the first such visits by any spacecraft. It encountered the asteroid Gaspra on Oct. 29, 1991, and the asteroid Ida on Aug. 28, 1993. During the latter part of its interplanetary cruise, Galileo was used to observe the collisions of fragments of Comet Shoemaker-Levy with Jupiter in July 1994. Galileo arrived at Jupiter on Dec.

7, 1995, entering orbit and dropping its instrumented probe into the giant planet's atmosphere. The Galileo atmospheric probe was managed by NASA Ames Research Center, Moffett Field, Calif. Measurements of temperature, pressure and vertical winds revealed several discoveries as the Galileo Probe plunged into Jupiter's atmosphere. The Atmosphere Structure Instrument (ASI) measured temperature, pressure and density. The probe also included a mass spectrometer that measured the chemical composition of Jupiter's atmosphere. "Measuring the composition of Jupiter's atmosphere was a primary scientific objective of the probe, because we knew it could change our understanding of Jupiter's formation and evolution," said Galileo probe project scientist Dr. Richard Young of NASA Ames Research Center, Moffett Field, Calif. "These latest probe results have done exactly that, and the measurements are the sort that could only have been obtained by in-situ measurements from an entry probe." The spectrometer detected in Jupiter's atmosphere higher than expected concentrations of argon, krypton and xenon, three chemical elements called noble gases because they are very independent and do not combine with other chemicals. Tiny traces of these gases are found in Earth's atmosphere, and argon is sometimes used like neon in advertising signs.

When it dropped 156 kilometers (97 miles) through Jupiter's atmosphere, the Galileo probe relayed data back to the main Galileo spacecraft more than 209,215 kilometers (130,000 miles) overhead for storage and transmission to Earth. The probe descended deeper into the atmosphere than expected, but was finally overcome by Jupiter's high temperatures and pressures. The Galileo spacecraft, meanwhile, orbited Jupiter and its moons for nearly four years, beaming back to Earth thousands of pictures and a wealth of scientific data. Its two-year, primary mission ended in December 1997, but the mission was continued with an initial two-year extension. Galileo discovered strong evidence that Jupiter's moon Europa has a melted saltwater ocean under an ice layer on its surface. The spacecraft also found indications that two other moons, Ganymede and Callisto, have layers of liquid saltwater as well. Other major science results from the mission include details of varied and extensive volcanic processes on the moon Io, measurements of conditions within Jupiter's atmosphere, and discovery of a magnetic field generated by Ganymede. The mission finally ended on Sept.

21, 2003, when the spacecraft plunged into Jupiter's atmosphere. This planned maneuver prevented the risk of Galileo drifting to an unwanted impact with the moon Europa, which may harbor a subsurface ocean. Key Mission People Galileo probe project scientist Dr. Richard Young of NASA Ames Research Center, Moffett Field, Calif. Related links: Galileo to Jupiter Galileo to Jupiter Courtesy of the Jet Propulsion Laboratory Table of Contents Galileo Probe The Galileo spacecraft was designed to study Jupiter's atmosphere, satellites, and surrounding magnetosphere for two years. The spacecraft was named in honor of Galileo Galilei , the Italian Renaissance scientist who discovered Jupiter's major moons in 1610. The Galileo spacecraft was carried into space by the shuttle Atlantis on October 18, 1989. Once released from the cargo bay, a two-stage Inertial Upper Stage (IUS) booster fired to accelerate the spacecraft out of Earth orbit toward the planet Venus . From there, Galileo will spend more than five years traveling to Jupiter, where it will become the first spacecraft to make direct measurements from an instrumented probe within Jupiter's atmosphere.

It will also become the first spacecraft to conduct long-term observations of Jupiter, its magnetosphere, and satellites from orbit around Jupiter. But Galileo has already earned a "first" -- the first spacecraft to encounter an asteroid and to photograph an asteroid's moon . The Galileo orbiter, weighing 2-1/2 tons, was designed and developed by the Jet Propulsion Laboratory, which manages the mission for NASA. The 340-kilogram (746-pound) atmospheric probe was developed by NASA's Ames Research Center. Hughes Aircraft Company acted as the prime contractor. The German government is a partner in the mission through its provisions of the spacecraft propulsion subsystem and two science experiments. The spacecraft carries 10 scientific instruments, six of which are located on the probe. The spacecraft's radio link to Earth and the probe-to-orbiter radio link are also used to conduct scientific investigations. Like Voyager and other interplanetary spacecraft, Galileo used gravity assists from Venus and Earth to gain enough momentum to travel to Jupiter. Galileo spent the first three years of its journey making flybys of Venus and Earth before it was ready to swing outward toward Jupiter.

These gravity-assist encounters provided Galileo with the opportunity to conduct brief scientific observations of Venus, at closest approach on February 10, 1990, and of Earth and the Moon , with closest approaches on December 8, 1990 and December 8, 1992. During this phase of the mission, the spacecraft made two planned visits to the asteroid belt beyond the orbit of Mars and provided the first opportunities for close observation of these bodies. In October 1991, Galileo flew by the asteroid Gaspra , obtaining the world's first close-up asteroid images. In August 1993, it flew by a second asteroid, Ida , and discovered the first confirmed asteroid moon, named Dactyl . In July 1994 Galileo was the only observer to obtain images of the far side of Jupiter when more than 20 fragments of Comet Shoemaker-Levy 9 plunged into Jupiter's nightside atmosphere over a six-day interval. Galileo will arrive at Jupiter, on December 7, 1995, and perform several important tasks to place itself in the correct orbit around Jupiter.

It will then relay 75 minutes of data from an atmospheric probe as it descends into Jupiter's atmosphere. The spacecraft will be making its closest flyby of Io at the same time, just 1,000 kilometers (620 miles) away, and perform scientific investigations of Io's volcanic moon. Once the probe's signal is lost and the probe is crushed from the tremendous pressure of Jupiter's atmosphere, the larger part of the spacecraft, the orbiter, will begin its two-year tour of the Jovian environment. It will make 11 orbits around the planet and many of its 16 moons, taking advantage of 10 close satellite encounters to conduct science investigations. Views of Galileo Galileo In this artist's rendering the Galileo probe, with its heat shield below and a parachute above, enters the turbulent upper atmosphere of Jupiter. Galileo released its probe in September 1995, about 150 days before it was scheduled to reach Jupiter. The orbiter's trajectory will be corrected to send it into Jupiter orbit. The probe was scheduled to enter the upper atmosphere on December 7, 1995, at about 48 kilometers (30 miles) per second.

Data from its six instruments were to be relayed to Earth via the orbiter during the probe's expected 75 minutes of operation before temperature and pressure destroy it. (Courtesy NASA/JPL) Galileo Deployment The Galileo spacecraft and its Inertial Upper Stage booster rocket were deployed from the space shuttle Atlantis October 18, 1989. Shortly thereafter, the booster rocket fired and separated, sending Galileo on its six-year journey to the planet Jupiter. Upon its arrival at Jupiter in December 1995, Galileo will release a probe into the atmosphere so that scientists can survey the composition of the planet's clouds. The orbiter will relay probe information and will survey and photograph Jupiter and some of its major satellites. (Courtesy NASA/JPL) The Journey Begins This artist's concept depicts the Inertial Upper Stage booster rocket firing shortly after its deployment from the shuttle Atlantis. Galileo's complex trajectory to Jupiter flung it by Venus once and then back to Earth for two fly-bys. Each pass added kinetic energy to the spacecraft, increasing its velocity so that it could reach Jupiter with a relatively small amount of fuel.

(Courtesy NASA/JPL) Hubble Views the Galileo Probe Entry Site This image, taken by the Hubble Space Telescope, shows the predicted site at which the Galileo Probe will enter Jupiter's atmosphere on December 7, 1995. At this latitude, the eastward winds have speeds of about 250 miles per hour (110 meters per second). The white oval to the north of the probe site drifts westward at 13 miles per hour (6 meters per second), rolling in the winds which increase sharply toward the equator. The four enlarged images of Jupiter's equatorial region show clouds sweeping across the predicted Galileo probe entry site, which is at the exact center of each frame (a small white dot has been inserted at the centered at the predicted entry site). (Credit: Reta Beebe, New Mexico State University, and NASA) Galileo Spacecraft Facts Galileo Spacecraft Facts Tweet Galileo was an unmanned NASA spacecraft which studied the planet Jupiter and its moons. It was launched on October 18, 1989, by the Space Shuttle Atlantis.

Galileo arrived at Jupiter on December 7, 1995, via gravitational assist flybys of Venus and Earth, becoming the first spacecraft to orbit Jupiter. Galileo Spacecraft consisted of an orbiter and probe. The probe was released into Planet Jupiter atmosphere on the 13th, July, 1995. The mission was named for the Italian Renaissance scientist Galileo Galilei, who discovered Jupiter's major moons in 1610. After traveling 2.4 billion miles in just over 6 years to reach Jupiter, Galileo missed its target at the Jovian moon Io by only 67 miles. That's like shooting an arrow from Los Angeles at a bull's-eye in New York and missing by only 6 inches! The Galileo spacecraft was made of materials that suggest it is much more like a fighter plane than a car. It used very lightweight materials such as beryllium to house the subsystems, aluminum for the structure, and carbon composites for the booms. On its first orbit around Jupiter, the Galileo spacecraft reached a maximum distance from Jupiter of about 20 million kilometers.

This is nearly half the distance between the orbits of Earth and Venus, Earth's closest planetary neighbor. Galileo returned 14,387 images with the Solid State Imaging Camera, 3,762 from Jupiter’s Orbit. Galileo made 34 orbits of Jupiter and traveled a total Distance of 4,631,778,000 km. Batteries only get you so far in outer space. The Galileo orbiter carried two radioisotope thermoelectric generators (RTGs),which where used to generate electrical power on board the spacecraft. There are 7.8 kilograms (17.2 pounds) of Plutonium-238 in each RTG. Galileo's roots date back to an early recommendation for an atmospheric probe that would explore Jupiter's atmosphere down to pressure levels 100 times that of Earth at sea level. This proposal eventually became JOP (for Jupiter Orbiter Probe), which then eventually became Galileo. When the Galileo Probe entered Jupiter's atmosphere, it was traveling at a speed of 106,000 miles per hour -- the fastest impact speed ever achieved by a man- made object.

At that speed, one could drive around the Earth at the equator in 14 minutes (assuming there were bridges across all the oceans) or to the Moon and back in only 5 hours! {googleAds} {/googleAds} On its journey from Earth to Jupiter, Galileo traveled 2.4 billion miles. Along the way, about 67 gallons of fuel from the propulsion system were used to control Galileo's flight path and to keep its antenna pointed at Earth. That's equivalent to getting 36 million miles per gallon! With that kind of mileage, one would use up only 4 tablespoons of gasoline to drive to the Moon and back! Since being launched from Earth on October 18, 1989, Galileo has traveled 2.4 billion miles in just over 6 years to reach Jupiter. That's an average speed of 44,000 miles per hour. At that speed, one could drive around the Earth at the equator (assuming there were bridges across all the oceans) in just over half an hour, or to the Moon and back in only 11 hours!

Jupiter has some truly high velocity winds-- they blow at speed as high as 260 miles per hour at Jupiter's cloud tops! Magnetic fields can be powerful entities. Jupiter's magnetosphere strips away 1 ton of material from Io a second. Io's orbital motion through Jupiter's magnetosphere generates electricity--an electric current of 3 million amps! Using Galileo's on board instruments to observe the asteroid Gaspra was a challenge somewhat akin to attempting to spot the Goodyear Blimp through a soda straw from five miles away, while sitting in a car going 90 mph. Jupiter's moon Io has some of the most dramatic-appearing volcanoes around. Geysers on Io spew out at speeds as high as 1 km/sec (2,300 mph). On Earth, Mt. Etna's ejecta erupt out at a "mere" 112 mph; terrestrial volcanoes rarely exceed 200 mph. Io's gravity is low (1/6 that of Earth), so the plumes are huge--reaching as much as 162 miles high.

If you could move Old Faithful to Io, it would shoot up a plume of water and ice over 21 miles high (note, though, that we have no detection of water on Io and the most likely fluid/gas for the plume geysers is sulfur dioxide). Io is arguably the most volcanically active body yet known. We've seen at least 200 volcanic caldera bigger than 12 miles in diameter on its surface. Jupiter's tug on the Galilean moon Io causes tides, just like our own moon raises tides on the Earth's surface. A typical ocean tide on Earth is about one meter (3 feet). Io's surface tides (no water!), though, are truly something to behold: as great as 330 feet high! How many people have worked on Galileo? Nobody knows for sure, but it's been estimated that roughly 10,000 people have worked directly on Galileo since the Project's start in 1977. That's excluding people associated with the Space Shuttle and the Inertial Upper Stage booster.

Because the asteroid Gaspra is so small (about 19 x 12 x 11 kilometers, or 12 x 7.5 x 7 miles), its surface gravitational force is two thousand times smaller than that of the Earth's, yielding an escape speed of only 10 meters per second (22 miles per hour); an Olympic-caliber sprinter could run himself into orbit! A 200 pound person would weigh 0.1 pounds! The Gaspra asteroid flyby was yet another example of outstanding navigation: at closest approach, Galileo was just 1.5 seconds and 1 kilometer (0.6 miles) from the aim point. Even so, taking the picture was a dramatic achievement. One of Galileo's scientists said "It was like taking a picture of a large house in San Francisco from Los Angeles." When the Galileo Photopolarimeter Radiometer detected the flashes of light caused by Comet Shoemaker-Levy 9 crashing into Jupiter in July 1994, it was using a 4 inch telescope, and it was as far away from Jupiter as Mars is from the Sun.

Galileo's second Earth flyby brought the spacecraft within 303 kilometers (182 miles) of the Earth's surface. The gravity assist added 3.7 kilometers per second (8,300 miles per hour) to the spacecraft's speed in its solar orbit. As always, Galileo's navigation was impeccable: the spacecraft was within a kilometer of its intended path, and was just 0.1 second early. Jupiter's volume is about 1,400 times that of the Earth. In fact, its volume is half again bigger than all of the Solar System's other planets, moons, asteroids, and comets combined. Since Galileo is going into orbit around Jupiter (unlike the two Voyager spacecraft, which flew by the planet), it can fly by Jupiter and its moons at far closer distances than did Voyager, which means that Galileo's pictures will be a dramatic improvement over those from Voyager. Comparing Voyager images with those to be sent back from Galileo is like viewing a book at the base of the Empire State Building from the top story, as opposed to holding that book in your hands.

Galileo's power sources, radioisotope thermo-electric generators, are putting out only about 520 watts on 7 December. That's not even enough to run a kitchen toaster! Sources: Galileo meets Jupiter and the Moons Galileo meets Jupiter and the Moons Spacecraft Galileo finally arrived at Jupiter in December 1995. Galileo's companion, the Probe, arrived too, although both were travelling separately and had not communicated with each other since they separated five months before, in July. Galileo's first task at Jupiter was to get close to the big planet so it could listen for the Probe, about to dive into Jupiter's cloud tops at thousands of miles an hour! Galileo listened, and listened. The time for the Probe to signal was late! Was it OK? The Probe signaled a minute later than expected, but it was okay. It had broken through the atmosphere and was detecting all sorts of things about Jupiter's wonderful clouds. The Probe sent all the information back to Galileo, listening overhead. The Probe heroically transmitted for nearly an hour until it got swallowed up by Jupiter and became one with the giant planet.

Galileo now had to get into orbit, and fire its engine *just right* or it would fly past Jupiter, into empty space! Now Galileo was in orbit around Jupiter, something no Earth spacecraft had ever done! Galileo would not only get to study Jupiter lots of times, but each time around, it would swing close by a moon of Jupiter and look at that too. First stop, Ganymede! Spacecraft Galileo flew by Ganymede and discovered a strong magnetic area around it, called a magnetosphere. On the third pass around, Galileo flew over Callisto. Callisto has a huge crater called Valhalla which radiates over one whole side of this dark moon. Finally Galileo flew really close past Europa. Europa appears to have hardly any craters. It seems all covered with ice cracks. Does it have an ocean under the ice? We're not sure, but it looks like it could. Around and around Galileo flies, looking at this moon, that moon, at Jupiter, or at its thin ring. Even long after its mission is over, Galileo will be orbiting Jupiter, maybe for as long as thousands of years!

We can listen and get information from Spacecraft Galileo for about another two years, until Jupiter's strong radiation affects it so much that we can't communicate anymore. But until that time, Galileo will hopefully tell us more and more about mysterious Europa... ...and then Galileo will try to get close to Io and check out the spectacular volcanos there. Wait! We're getting a message from Spacecraft Galileo - "I hope you enjoyed my adventures at Jupiter. I sure had fun with Spacecraft Cassini when it swung by NASA - Mission to Jupiter Mission to Jupiter 05.29.07 Editor's Note: This is the 26th in a series of essays on exploration by NASA's Chief Historian, Steven J. Dick. The recent publication in the NASA History series of Michael Melzer's Mission to Jupiter: A History of the Galileo Project details another extraordinary voyage of exploration, and highlights the trials, tribulations and triumphs in the long process required for success in planetary science. The Galileo spacecraft was launched Oct. 18, 1989, arrived at Jupiter on Dec. 7, 1995, and undertook its tour of Jupiter over the next two years.

This Prime Mission would have been quite enough to declare success, but it was followed by the Galileo Europa Mission, extending for two more years until December 1999. During this time the spacecraft studied not only studied Europa but also other Jovian satellites, especially (and appropriately) the other "Galilean satellites" discovered by Galileo almost 400 years before. Aside from satellites, the Galileo spacecraft would also study the magnetic fields, particles and atmosphere of Jupiter itself, notably with its releasable Atmospheric Probe. Even then, Galileo's explorations were not over; its lifetime was further extended by the Galileo Millennium Mission, which ran from January, 2000 through the end of mission operations in January 2003. Galileo's death occurred by design when it was sent on a collision course with Jupiter on Sept. 21, 2003 and burned up in Jupiter's dense atmosphere. Galileo represented a new phase in the study of the outer planets. The Pioneer probes and Voyagers 1 and 2 together completed the preliminary reconnaissance of those gas giants.

But Galileo undertook a much more systematic, in-depth, and holistic analysis of the entire Jupiter system, featuring close flybys of its satellites, and intense study of its particles and fields. And even before the spacecraft reached Jupiter it visited Venus, two asteroids, and twice returned to Earth, along the way observing a spectacular celestial crash. The Galileo spacecraft and its Inertial Upper Stage booster rocket were deployed from the space shuttle Atlantis October 18, 1989. Shortly thereafter, the booster rocket fired and separated, sending Galileo on its six-year journey to the planet Jupiter. Upon its arrival at Jupiter in December 1995, Galileo released a probe into the atmosphere to survey the composition of the planet's clouds. The orbiter has relayed probe information, surveyed its surroundings, and photographed Jupiter and the Galilean satellites, among others. Image credit: NASA. Like many spacecraft, Galileo had an extended and painful birth. The total mission from launch to destruction lasted 14 years, but the "Jupiter Orbiter Probe" project from which it emerged had begun another 14 years before launch.

Funding was won from Congress in 1977 and the project was renamed Galileo the following year. The spacecraft was originally scheduled to be deployed in 1984 aboard the Space Shuttle, which would not fly its first mission until April 1981. Once deployed in Earth orbit, the plan was for it to be launched by a Centaur upper stage toward Jupiter. Meanwhile, in the early 1980s the ever-watchful Office of Management and Budget proposed not only that Galileo should be cancelled, but that planetary exploration should be halted and JPL shut down. All three made it through that crisis, and in December 1985 the spacecraft was transported to the Kennedy Space Center for launch on the Shuttle Atlantis. The following month, in one of NASA's tragic and memorable moments, the Space Shuttle Challenger was lost with all its crew at launch. Return to flight for the Shuttle program would take almost three years. During that time it was decided the liquid-hydrogen-fueled Centaur was too dangerous to take aboard the Shuttle, and that a solid-fueled Inertial Upper Stage would be used instead.

Because of the lower thrust of the IUS, and in order to minimize fuel consumption, Venus and two Earth gravity assists (affectionately termed VEEGA) replaced the direct trajectory, greatly lengthening the travel time to Jupiter. Like its birth, Galileo's journey was full of drama -- as full as any of the ocean voyages in the Age of Discovery (minus the natives). Four months after launch it reached Venus for the first gravity assist. By December 1990 the spacecraft had returned to Earth for another gravity assist. Several months later, the mission encountered a major problem. Now far enough from the Sun, attempts were made to deploy its high gain antenna, but to no avail. That meant data return would be severely limited. Engineers were forced to use the low gain antenna, analogous to using a dim light bulb compared to a powerful searchlight. Nevertheless, in due course and with a great deal of patience and ingenuity, 70 percent of the mission's scientific goals were completed. In the midst of this problem the spacecraft proceeded to the asteroid belt, where Galileo encountered Gaspra on October 29, 1991, capturing spectacular images.

A year later it was back to Earth for its second Earth gravity assist. On Aug. 28, 1993 Galileo was in the asteroid belt again, encountering Ida at a distance of only 1,500 miles. Among other surprises, images examined months later revealed a moon of Ida, later named Dactyl. Finally Jupiter was in Galileo's sights, but meanwhile unprecedented events were unfolding at Galileo's final destination. Comet Shoemaker-Levy 9 had broken up, and from July 16-29, 1994 21 pieces of the comet collided with Jupiter, giving it a spectacular series of "black eyes." Still 140 million miles from Jupiter, Galileo captured images nonetheless, even as most telescopes on (and orbiting) Earth were trained on the event from their much more distant vantage point. At the U. S. Naval Observatory where I was working at the time, I recall our astonishment that we could see the dark splotches on Jupiter, even with small telescopes and from the middle of Washington, D.C.

A year later, at 39 million miles from Jupiter, Galileo survived the most intense interplanetary dust storm ever recorded. At about this time, with 147 days to go to Jupiter, the Jupiter Atmospheric Probe separated from Galileo, in freefall toward its destination. This "family portrait," a composite of the Jovian system, includes the edge of Jupiter with its Great Red Spot, and Jupiter's four largest moons, known as the Galilean satellites. From top to bottom, the moons shown are Io, Europa, Ganymede and Callisto. Europa, the smallest of the four moons, is about the size of Earth's moon, while Ganymede is the largest moon in the solar system. North is at the top of this composite picture in which the massive planet and its largest satellites have all been scaled to a common factor of 15 kilometers (9 miles) per picture element. The Solid State Imaging (CCD) system aboard NASA's Galileo spacecraft obtained the Jupiter, Io and Ganymede images in June 1996, while the Europa images were obtained in September 1996.

Because Galileo focused on high resolution imaging of regional areas on Callisto rather than global coverage, the portrait of Callisto is from the 1979 flyby of NASA's Voyager spacecraft. Image credit: NASA. By the time Galileo finally arrived at the Jupiter system in December 1995, the comet impact sites had dissipated. But the spacecraft had plenty to observe. On the day of arrival the Atmospheric Probe plunged into Jupiter, slowed by drogue parachutes, while Galileo flew by Europa and Io. The spacecraft then settled down to its Prime Mission Orbiter tour, which consisted of 11 orbits of Jupiter. They were not normal orbits, but orbits that unfolded in a flower petal pattern while maximizing scientific visits to the Jovian moons. Thus, the first orbit lasted 7 months, culminating in a close encounter with Ganymede. Ganymede, Callisto and Europa were visited multiple times over the next two years as part of the Prime Mission. During the extended Galileo Europa Mission, Io was also visited, mapping its cloud of charged particles and the density of sulfur streaming from Io's erupting volcanoes.

Further observations of Io and Jovian system dynamics were made during the Millennium Mission extension, including the eruptions of Io's Loki volcano, the largest and most powerful in the solar system. Europa proved no less fascinating, with its water-ice surface and large ocean lurking beneath. And the Atmospheric Probe results proved revolutionary. As Melzer puts it, Probe data "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 velocities, temperatures, cloud characteristics, electrical storms and elemental and molecular abundances painted a very different picture of Jupiter from what was expected." Galileo found a turbulent Jovian atmosphere, complete with lightning and thunderstorms a thousand times the size of those on Earth. Distinct from the Pioneer and Voyager planetary flyby missions, Galileo was the first to orbit the giant planet, and completed a total of 34 orbits by the end of its mission in 2003. Melzer also makes clear how complex spacecraft development has become, both in terms of institutions, technology and highly trained individuals interacting at many levels.

The lead NASA field center for the project was the Jet Propulsion Laboratory in Pasadena, California, with oversight from the Solar System Exploration Division at NASA Headquarters in Washington. NASA's Ames Research Center developed the Jupiter probe that entered the planet's atmosphere. NASA's Lewis Research Center (now Glenn) in Cleveland helped design the spacecraft propulsion system and integrate the payload with the shuttle. The shuttle itself, as well as the astronauts, were under the jurisdiction of Johnson Space Center in Texas, while Kennedy Space Center provided the launch facilities. The Department of Energy provided the controversial plutonium-powered electrical generators for the spacecraft. Finally, the Galileo project involved international cooperation with the Federal Republic of Germany, which provided instrumentation, propulsion for the Orbiter and additional telemetry from the German Space Operations Center. Galileo the man would have marveled at Galileo the machine. And we should marvel too at what we can do, and see, in the new Age of Discovery. Further Reading Melzer, Michael. Mission to Jupiter: A History of the Galileo Project (NASA SP-2007-4231, NASA History Division, Washington, 2003).

Available from the NASA Center for Aerospace Information, 301-621-0390, or online. This book is also available online at . Steven J. Dick New findings from Galileo's Jupiter visit (photos) | ZDNet Published: May 13, 2011 -- 08:36 GMT (01:36 PDT) Caption by: Andy Smith Here's a look at the volcanically active moon Io (right) and its hot spots (left) shown in an infrared color-coded image taken in 2001. The hottest areas are indicated in white and the hottest area on the map is where the arrow points. Red is the next hottest color, then yellow. Published: May 13, 2011 -- 08:36 GMT (01:36 PDT) Caption by: Andy Smith Here's the first of NASA's top ten scientific images showing the moon, Europe, and its ice shell moving slowly over a briny ocean that's about 60 miles deep. Europa is now seen as a potential habitat for extraterrestrial life.

Published: May 13, 2011 -- 08:36 GMT (01:36 PDT) Caption by: Andy Smith This closer look at the ice surface of Europa shows a white area that is covered by a fine dust of ice particles while the brown area "has been painted by mineral contaminants carried and spread by water vapor released from below the crust when it was disrupted," according to NASA. Published: May 13, 2011 -- 08:36 GMT (01:36 PDT) Caption by: Andy Smith En route to Jupiter, Galileo snapped this photo of the asteroid 243 Ida from about 6,500 miles. The bright spot to the right is the first known moon to orbit an asteroid. The color has been enhanced, it's actually gray. Published: May 13, 2011 -- 08:36 GMT (01:36 PDT) Caption by: Andy Smith This view of an area called Galileo Regio on Jupiter's moon Ganymede show an area that's been pepper by impact craters for several billion years.

Published: May 13, 2011 -- 08:36 GMT (01:36 PDT) Caption by: Andy Smith This volcano in the Tvashtar region of Io was found to be hotter than any active volcano on Earth. Published: May 13, 2011 -- 08:36 GMT (01:36 PDT) Caption by: Andy Smith Active volcanos on Io are highlighted during an eclipse. Red indicates the most heat.. Published: May 13, 2011 -- 08:36 GMT (01:36 PDT) Caption by: Andy Smith This image of Jupiter's moon Callisto shows a region that contains no small impact craters - meaning they must have been eroded away. Published: May 13, 2011 -- 08:36 GMT (01:36 PDT) Caption by: Andy Smith The light blue region around the Great Red Spot shows an ammonia ice cloud. It has been formed by updrafts of ammonia from deep within Jupiter's atmosphere. The Great Red Spot is a storm that has been raging for at least 300 years.

It's about twice as big as the diameter of Earth. Published: May 13, 2011 -- 08:36 GMT (01:36 PDT) Caption by: Andy Smith These images show lightning storms on the night side of Jupiter. Published: May 13, 2011 -- 08:36 GMT (01:36 PDT) Caption by: Andy Smith In the final image included in Galileo's best, Jupiter's Gossamer rings "are a short lived dynamic phenomena that is the result of small dust particles being ejected from the moons as a result of impacts due to energetic particles in the magnetosphere," according to NASA. Published: May 13, 2011 -- 08:36 GMT (01:36 PDT) Caption by: Andy Smith One of Galileo's top discoveries is a magnetic field on the solar system's largest moon, Ganymede. Published: May 13, 2011 -- 08:36 GMT (01:36 PDT) Caption by: Andy Smith Evidence indicates there most likely is a subsurface ocean on the moon Callisto.

Shown here in true color on the left and enhanced right, "reveals a gradual variation across the moon's hemisphere, perhaps due to implantation of materials onto the surface from space," according to NASA. Published: May 13, 2011 -- 08:36 GMT (01:36 PDT) Caption by: Andy Smith In 1994 during its approach to Jupiter, Galileo provided the only direct observation of the comet Shoemaker-Levy 9's collision with the planet. Fragment W is shown here. Published: May 13, 2011 -- 08:36 GMT (01:36 PDT) Caption by: Andy Smith Here are true and false colors of an equatorial hotspot. When Galileo arrived at Jupiter, it sent a probe into the planet's atmosphere which indicated thunderstorms many times larger than Earth's. Published: May 13, 2011 -- 08:36 GMT (01:36 PDT) Caption by: Andy Smith Some of Jupiter's more interesting smaller moons as revealed by Galileo.

Published: May 13, 2011 -- 08:36 GMT (01:36 PDT) Caption by: Andy Smith Donate Jupiter Jupiter is the largest planet in the solar system. Its diameter is 88,846 miles (142,984 kilometers), more than 11 times that of Earth, and about one-tenth that of the sun. It would take more than 1,000 Earths to fill up the volume of the giant planet. When viewed from Earth, Jupiter appears brighter than most stars. It is usually the second brightest planet -- after Venus. Jupiter is the fifth planet from the sun. Its mean (average) distance from the sun is about 483,780,000 miles (778,570,000 kilometers), more than five times Earth's distance. Ancient astronomers named Jupiter after the king of the Roman gods. Astronomers have studied Jupiter with telescopes based on Earth and aboard artificial satellites in orbit around Earth. In addition, the United States has sent six space probes (crewless exploratory craft) to Jupiter.

Astronomers witnessed a spectacular event in July 1994, when 21 fragments of a comet named Shoemaker-Levy 9 crashed into Jupiter's atmosphere. The impacts caused tremendous explosions, some scattering debris over areas larger than the diameter of Earth. Physical features of Jupiter Jupiter is a giant ball of gas and liquid with little, if any, solid surface. Instead, the planet's surface is composed of dense red, brown, yellow, and white clouds. The clouds are arranged in light-colored areas called zones and darker regions called belts that circle the planet parallel to the equator. Orbit and rotation Jupiter travels around the sun in a slightly elliptical (oval-shaped) orbit. The planet completes one orbit in 4,333 Earth days, or almost 12 Earth years. As Jupiter orbits the sun, the planet rotates on its axis, an imaginary line through its center. The axis is tilted about 3¡. Scientists measure tilt relative to a line at a right angle to the orbital plane, an imaginary surface touching all points of the orbit. Jupiter rotates faster than any other planet. It takes 9 hours 56 minutes to spin around once on its axis, compared with 24 hours for Earth.

Scientists cannot measure the rotation of the interior of the giant planet directly, so they have calculated the speed from indirect measurements. They first calculated the speed using an average of the speeds of the visible clouds that move with interior currents, except for a more rapid zone near the equator. Jupiter sends out radio waves strong enough to be picked up by radio telescopes on Earth. Scientists now measure these waves to calculate Jupiter's rotational speed. The strength of the waves varies under the influence of Jupiter's magnetic field in a pattern that repeats every 9 hours 56 minutes. Because the magnetic field originates in Jupiter's core, this variation shows how fast the plant's interior spins. Jupiter's rapid rotation makes it bulge at the equator and flatten at the poles. The planet's diameter is about 7 percent larger at the equator than at the poles. Mass and density Jupiter is heavier than any other planet. Its mass (quantity of matter) is 318 times larger than that of Earth. Although Jupiter has a large mass, it has a relatively low density. Its density averages 1.33 grams per cubic centimeter, slightly more than the density of water. The density of Jupiter is about 1/4 that of Earth.

Because of Jupiter's low density, astronomers believe that the planet consists primarily of hydrogen and helium, the lightest elements. Earth, on the other hand, is made up chiefly of metals and rock. Jupiter's mix of chemical elements resembles that of the sun, rather than that of Earth. Jupiter may have a core made up of heavy elements. The core may be of about the same chemical composition as Earth, but 20 or 30 times more massive. The force of gravity at the surface of Jupiter is up to 2.4 times stronger than on Earth. Thus, an object that weighs 100 pounds on Earth would weigh as much as 240 pounds on Jupiter. The atmosphere of Jupiter is composed of about 86 percent hydrogen, 14 percent helium, and tiny amounts of methane, ammonia, phosphine, water, acetylene, ethane, germanium, and carbon monoxide. The percentage of hydrogen is based on the number of hydrogen molecules in the atmosphere, rather than on their total mass. Scientists have calculated these amounts from measurements taken with telescopes and other instruments on Earth and aboard spacecraft. These chemicals have formed colorful layers of clouds at different heights.

The highest white clouds in the zones are made of crystals of frozen ammonia. Darker, lower clouds of other chemicals occur in the belts. At the lowest levels that can be seen, there are blue clouds. Astronomers had expected to detect water clouds about 44 miles (70 kilometers) below the ammonia clouds. However, none have been discovered at any level. The planet Jupiter's Great Red Spot is a huge mass of swirling gas. At its widest, it is about three times the diameter of the Earth. Image credit: NASA Jupiter's most outstanding surface feature is the Great Red Spot, a swirling mass of gas resembling a hurricane. The widest diameter of the spot is about three times that of Earth. The color of the spot usually varies from brick-red to slightly brown. Rarely, the spot fades entirely. Its color may be due to small amounts of sulfur and phosphorus in the ammonia crystals. The edge of the Great Red Spot circulates at a speed of about 225 miles (360 kilometers) per hour. The spot remains at the same distance from the equator but drifts slowly east and west. The zones, belts, and the Great Red Spot are much more stable than similar circulation systems on Earth.

Since astronomers began to use telescopes to observe these features in the late 1600's, the features have changed size and brightness but have kept the same patterns. Temperature The temperature at the top of Jupiter's clouds is about -230 degrees F (-145 degrees C). Measurements made by ground instruments and spacecraft show that Jupiter's temperature increases with depth below the clouds. The temperature reaches 70 degrees F (21 degrees C) -- "room temperature" -- at a level where the atmospheric pressure is about 10 times as great as it is on Earth. Scientists speculate that if Jupiter has any form of life, the life form would reside at this level. Such life would need to be airborne, because there is no solid surface at this location on Jupiter. Scientists have discovered no evidence for life on Jupiter. Near the planet's center, the temperature is much higher. The core temperature may be about 43,000 degrees F (24,000 degrees C) -- hotter than the surface of the sun. Jupiter is still losing the heat produced when it became a planet.

Most astronomers believe that the sun, the planets, and all the other bodies in the solar system formed from a spinning cloud of gas and dust. The gravitation of the gas and dust particles packed them together into dense clouds and solid chunks of material. By about 4.6 billion years ago, the material had squeezed together to form the various bodies in the solar system. The compression of material produced heat. So much heat was produced when Jupiter formed that the planet still radiates about twice as much heat into space as it receives from sunlight. Magnetic field Like Earth and many other planets, Jupiter acts like a giant magnet. The force of its magnetism extends far into space in a region surrounding the planet called its magnetic field. Jupiter's magnetic field is about 14 times as strong as Earth's, according to measurements made by spacecraft. Jupiter's magnetic field is the strongest in the solar system, except for fields associated with sunspots and other small regions on the sun's surface. Scientists do not fully understand how planets produce magnetic fields. They suspect, however, that the movement of electrically charged particles in the interior of planets generates the fields. Jupiter's field would be so much stronger than Earth's because of Jupiter's greater size and faster rotation.

Jupiter's magnetic field traps electrons, protons, and other electrically charged particles in radiation belts around the planet. The particles are so powerful that they can damage instruments aboard spacecraft operating near the planet. Within a region of space called the magnetosphere, Jupiter's magnetic field acts as a shield. The field protects the planet from the solar wind, a continuous flow of charged particles from the sun. Most of these particles are electrons and protons traveling at a speed of about 310 miles (500 kilometers) per second. The field traps the charged particles in the radiation belts. The trapped particles enter the magnetosphere near the poles of the magnetic field. On the side of the planet away from the sun, the magnetosphere stretches out into an enormous magnetic tail, often called a magnetotail, that is at least 435 million miles (700 million kilometers) long. Radio waves given off by Jupiter reach radio telescopes on Earth in two forms -- bursts of radio energy and continuous radiation. Strong bursts occur when Io, the closest of Jupiter's four large moons, passes through certain regions in the planet's magnetic field. Continuous radiation comes from Jupiter's surface as well as from high-energy particles in the radiation belts.

Callisto, a moon of Jupiter, is covered with craters produced when asteroids and comets struck its icy surface. Beneath the surface may be an ocean of salty liquid water. Image credit: NASA Satellites Jupiter has 16 satellites that measure at least 6 miles (10 kilometers) in diameter. It also has many smaller satellites. Jupiter's four largest satellites, in order of their distance from Jupiter, are Io, Europa, Ganymede, and Callisto. These four moons are called the Galilean satellites. The Italian astronomer Galileo discovered them in 1610 with one of the earliest telescopes. Io has many active volcanoes, which produce gases containing sulfur. The yellow-orange surface of Io probably consists largely of solid sulfur that was deposited by the eruptions. Europa ranks as the smallest of the Galilean satellites, with a diameter of 1,945 miles (3,130 kilometers). Europa has a smooth, cracked, icy surface. The largest Galilean satellite is Ganymede, with a diameter of 3,273 miles (5,268 kilometers). Ganymede is larger than the planet Mercury.

Callisto, with a diameter of 2,986 miles (4,806 kilometers), is slightly smaller than Mercury. Ganymede and Callisto appear to consist of ice and some rocky material. The two satellites have many craters. Ganymede, a moon of Jupiter, has craters and cracks on its surface. Asteroids and comets that hit Ganymede made the craters. The cracks are due to expansion and contraction of the surface. Image credit: NASA Jupiter's remaining satellites are much smaller than the Galilean moons. Amalthea and Himalia are the next largest. Potato-shaped Amalthea is about 163 miles (262 kilometers) in its long dimension. Himalia is 106 miles (170 kilometers) in diameter. Most of the remaining satellites were discovered by astronomers using large telescopes on Earth. Scientists discovered Metis and Adrastea in 1979 by studying pictures that had been taken by the Voyager spacecraft. Rings Jupiter has three thin rings around its equator. They are much fainter than the rings of Saturn. Jupiter's rings appear to consist mostly of fine dust particles.

The main ring is about 20 miles (30 kilometers) thick and more than 4,000 miles (6,400 kilometers) wide. It circles the planet inside the orbit of Amalthea. The impact of Comet Shoemaker-Levy 9 In March 1993, astronomers Eugene Shoemaker, Carolyn Shoemaker, and David H. Levy discovered a comet near Jupiter. The comet, later named Shoemaker-Levy 9, probably once orbited the sun independently, but had been pulled by Jupiter's gravity into an orbit around the planet. When the comet was discovered, it had broken into 21 pieces. The comet probably had broken apart when it passed close to Jupiter. Calculations based on the comet's location and velocity showed that the fragments would crash into Jupiter's atmosphere in July 1994. Scientists hoped to learn much about the effects of a collision between a planet and a comet. Scars from the crash of Comet Shoemaker-Levy 9 appear on Jupiter's surface as a series of maroon blotches in this photo. The comet broke into 21 pieces before it hit Jupiter in 1994.

Image credit: Hubble Space Telescope Comet Team and NASA Astronomers at all the major telescopes on Earth turned their instruments toward Jupiter at the predicted collision times. Scientists also observed Jupiter with the powerful Hubble Space Telescope, which is in orbit around Earth; and the remotely controlled space probe Galileo, which was on its way to Jupiter. The fragments fell on the back side of Jupiter as viewed from Earth and the Hubble Space Telescope. But the rotation of Jupiter carried the impact sites around to the visible side after less than half an hour. Scientists estimate that the largest fragments were about 0.3 to 2.5 miles (0.5 to 4 kilometers) in diameter. The impacts were directly observable from Galileo, which was within about 150 million miles (240 million kilometers) from Jupiter. However, damage to certain of the probe's instruments limited its ability to record and send data. The impacts caused large explosions, probably due to the compression, heating, and rapid expansion of atmospheric gases. The explosions scattered comet debris over large areas, some with diameters larger than that of Earth. The debris gradually spread into a dark haze of fine material that remained suspended for several months in Jupiter's upper atmosphere.

If a similar comet ever collided with Earth, it might produce a haze that would cool the atmosphere and darken the planet by absorbing sunlight. If the haze lasted long enough, much of Earth's plant life could die, along with the people and animals that depend on plants. Flights to Jupiter The United States has sent six space probes to Jupiter: (1) Pioneer 10, (2) Pioneer-Saturn, (3) Voyager 1, (4) Voyager 2, (5) Ulysses, and (6) Galileo. Pioneer 10 was launched in 1972 and flew within 81,000 miles (130,000 kilometers) of Jupiter on Dec. 3, 1973. The probe revealed the severe effects of Jupiter's radiation belt on spacecraft. Pioneer 10 also reported the amount of hydrogen and helium in the planet's atmosphere. In addition, the probe discovered that Jupiter has an enormous magnetosphere. Pioneer-Saturn flew within 27,000 miles (43,000 kilometers) of Jupiter in December 1974.

The craft provided close-up photographs of Jupiter's polar regions and data on the Great Red Spot, the magnetic field, and atmospheric temperatures. Voyager 1 and Voyager 2 flew past Jupiter in March and July 1979, respectively. These craft carried more sensitive instruments than did the Pioneers, and transmitted much more information. Astronomers used photographs taken by the Voyagers to make the first detailed maps of the Galilean satellites. The Voyagers also revealed sulfur volcanoes on Io, discovered lightning in Jupiter's clouds, and mapped flow patterns in the cloud bands. Ulysses was launched in October 1990 and passed by Jupiter in February 1992. The European Space Agency, an organization of Western European nations, had built the probe mainly to study the sun's polar regions. Scientists used the tremendous gravitational force of Jupiter to put Ulysses into an orbit that would take it over the sun's polar regions. As Ulysses passed by Jupiter, it gathered data indicating that the solar wind has a much greater effect on Jupiter's magnetosphere than earlier measurements had suggested. Galileo began its journey to Jupiter in October 1989.

The craft released an atmospheric probe in July 1995. In December 1995, the probe plunged into Jupiter's atmosphere. The probe penetrated deep into the cloud layers and measured the amount of water and other chemicals in the atmosphere. Also in December 1995, Galileo went into orbit around Jupiter. Over the next several years, the craft monitored Jupiter's atmosphere and observed the planet's major satellites. Galileo's mission was extended in 1997 and again in 1999. Eventually, however, the craft ran low on fuel. In September 2003, mission managers intentionally crashed Galileo into Jupiter's atmosphere to avoid any risk of the craft crashing into and contaminating Jupiter's moon Europa. Galileo's observations of Europa had shown that it might have an ocean below its surface capable of supporting life.

Question: In 1995, which probe sent back the first major pictures of Jupiter?
Answer: GALILEO