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Jupiter is one of the most compelling destinations in the sky. Not only is it the largest planet in the Sun’s family, it also presides over a miniature solar system of 53 known moons, a system of rings and an immense and powerful magnetic field. These moons run the gamut from icy Ganymede, larger than the planet Mercury, to scorching Io, constantly erupting with the solar system’s most active volcanoes. Europa, with its underground ocean of liquid water, has risen to one of the top slots on the list of places to search for potential extraterrestrial life.
Four spacecraft (Pioneer 10 & 11, then Voyager 1 & 2) had previously flown by the Jupiter system, but the Galileo mission would be the first to enter orbit around the planet. Like the famed astronomer for which it was named, Galileo would study the King of Planets over an extended period, in finer detail than was ever possible before.
To accomplish this, the Galileo orbiter carried 10 science instruments, along with a descent probe that it released directly into Jupiter’s atmosphere.
Galileo changed the way we look at our solar system. When the spacecraft plunged into Jupiter's crushing atmosphere on Sept. 21, 2003, it was being deliberately destroyed to protect one of its own discoveries—a possible ocean beneath the icy crust of the moon Europa.
The spacecraft was the first to fly past an asteroid, Gaspra, and the first to discover a moon of an asteroid, tiny Dactyl orbiting Ida. It provided the only direct observations of a comet colliding with a planet, when it witnessed Shoemaker-Levy 9 impact Jupiter.
The Galileo spacecraft logged quite a few other firsts during its 14-year mission to Jupiter. Among its discoveries: an intense radiation belt above Jupiter's cloud tops, helium in about the same concentration as the Sun, extensive and rapid resurfacing of the moon Io because of volcanism and a magnetic field at Ganymede.
The orbiter carried a small probe that became the first to sample the atmosphere of a gas planet. The probe measured temperature, pressure, chemical composition, cloud characteristics, sunlight and energy internal to the planet, and lightning. During its 58-minute life, the probe penetrated 124 miles (200 kilometers) into Jupiter's violent atmosphere before it was crushed, melted and/or vaporized by the intense pressure and temperature.
Galileo data allowed the creation of the first detailed maps of Jupiter’s major moons. It also gave scientists the most detailed look yet at the structure of the planet’s magnetic field and radiation belts.
The Galileo mission is also an example of innovative problem solving. When the spacecraft’s main antenna failed to deploy as planned, a special team performed extensive tests and determined that a few (probably three) of the antenna's 18 ribs were held by friction in the closed position. Despite exhaustive efforts to free the ribs, the antenna would not deploy.
From 1993 to 1996, extensive new flight and ground software was developed, and ground stations of NASA's Deep Space Network were enhanced in order to perform the mission using the spacecraft's low-gain antennas. Despite the fact that the spacecraft was millions of miles away in deep space, the fixes worked, and most of the mission’s planned observations were carried out.
Launch: Oct. 18, 1989 from Kennedy Space Center, Fla., aboard space shuttle Atlantis on mission STS-34
Venus flyby: Feb. 10, 1990, at altitude of 10,000 miles (about 16,000 kilometers)
Earth flybys: Dec. 8, 1990, at altitude of 597 miles (960 kilometers); Dec. 8, 1992 at altitude of 188 miles (303 kilometers)
Asteroid Gaspra flyby: Oct. 29, 1991, at a distance of 1,000 miles (1,600 kilometers)
Asteroid Ida flyby: Aug. 28, 1993, at 1,400 miles (2,400 kilometers)
Comet Shoemaker-Levy 9 observations: Impacts of comet fragments into Jupiter seen while en route July 16-22, 1994
Atmospheric probe release: July 12, 1995
Jupiter arrival and orbit insertion: Dec. 7, 1995
Probe atmospheric entry and relay: Dec. 7, 1995
Primary mission: October 1989 to December 1997
Extended missions: Three, from 1997 to 2003
Mission end: Sept. 21, 2003
Jupiter's Storms and Rings
Using data from the Galileo Probe's plunge into the top cloud layers of Jupiter, Galileo discovered that Jupiter has thunderstorms many times larger than Earth's. These storms result from the vertical circulation of water in the top layers, leaving large areas where air descends and becomes dry like the Sahara desert, and other areas where water rises to form the thunderstorms. Galileo has also found that Jupiter's rings are made of small dust grains blasted off the surface of Jupiter's four innermost satellites by the impacts of meteoroids.
Hot Active Volcanoes on Io
Now considered the solar system's most active body, Io's volcanoes were first discovered by Voyager 1 in 1979. They result from 328-foot (100 meter) tides in its solid surface. By taking Io's temperature with Galileo's instruments, scientists now know that some of Io's volcanoes are hotter than Earth's. From this, scientists surmise that lava made of silicate material rich in magnesium erupts from below Io's surface.
An Ocean on Europa
Possessing more water than the total amount found on earth, Europa appears to have had a salty ocean beneath its icy cracked and frozen surface. Galileo images show ice "rafts" the size of cities that appear to have broken off and drifted apart, a frozen "puddle" smooths over older cracks, warmer material bubbles up from below to blister the surface, evaporative-type salts are exposed. A remarkable lack of craters shows the surface to be relatively young. Europa has a very thin oxygen atmosphere and an ionosphere.
Ganymede's Magnetic Field
Internal tidal friction causes surprising effects on the solar system's largest moon. Galileo revealed that Ganymede has its own magnetic field. Perhaps from a slightly different orbit in its past, enough heat from tidal friction caused the separation of material inside Ganymede and this stirring of a molten core or iron sulfide is believed to generate Ganymede's magnetic field.
Does an Ocean Hide Beneath Callisto's Surface?
There is evidence to support the existence of a subsurface ocean on Callisto. The ocean would have to be deep enough inside the moon that it does not affect the heavily cratered surface on top. Instead the ocean could be showing itself indirectly, through the magnetic field it generates. This could come from electric flow in a salty ocean generated by Jupiter's strong magnetic field passing through it.
During one of Galileo’s extended missions, NASA’s Cassini spacecraft swung by Jupiter in late 2000 on its way to Saturn. For a few weeks, both Cassini and Galileo were able to simultaneously observe the giant planet in a coordinated campaign.
Scientists and engineers also used Galileo’s extensive time near Jupiter to observe the effects of a powerful radiation field on the hardware and operations of a spacecraft.
Assembly, Testing and Launch
The Jet Propulsion Laboratory managed the Galileo mission for NASA. It built and tested the spacecraft in Pasadena, Calif. NASA's Ames Research Center in Mountain View, Calif. managed the atmospheric descent probe, which was built by Hughes Aircraft Company. The German government supplied the propulsion module. The spacecraft’s electrical power came from radioisotope thermoelectric generators, which were designed and built by General Electric Co. for the U.S. Department of Energy.
Unlike most planetary missions, which ride to space aboard an expendable rocket, Galileo set sail for Jupiter from the cargo bay of Space Shuttle Atlantis. Shuttle mission STS-34 lifted off from Kennedy Space Center’s Pad 39-B on Oct. 18, 1989. The crew included Donald E. Williams, Commander; Michael J. McCulley, Pilot; Franklin R. Chang-Diaz, Mission Specialist 1; Shannon W. Lucid, Mission Specialist 2; and Ellen S. Baker, Mission Specialist 3.
The shuttle crew released Galileo into space above Earth the same day, with Astronaut Shannon Lucid performing the delicate maneuvers. Then the two-stage Inertial Upper Stage rocket ignited to propel the spacecraft on its journey.
Galileo didn't have enough fuel to fly directly to Jupiter, but the spacecraft borrowed energy from Venus and Earth to make the long trip. Mission planners designed a flight path nicknamed "VEEGA"—Venus-Earth-Earth Gravity Assist. Galileo would slingshot once by Venus, and twice by Earth, gathering enough momentum to reach distant Jupiter.
First stop: Venus. The Galileo team tried out the spacecraft's instruments and study of the thick, toxic clouds that cloak our sister planet. Flying by our home planet twice, Galileo saw the Earth and Moon together—as someone from another world might view us.
After the first Earth flyby, Galileo's umbrella-shaped high-gain antenna did not open as planned. But the Galileo team worked to reprogram the spacecraft to send back data through a smaller antenna. Engineers at NASA's Deep Space Network upgraded their antennas as well. The result allowed scientists to capture almost all the information originally planned.
On Galileo's first trip through the asteroid belt, the spacecraft took detailed images of an asteroid named Gaspra—the first close approach to an asteroid. On a second pass through the asteroid belt, Galileo discovered a miniature moon orbiting asteroid Ida. This tiny body was named Dactyl.
In 1994, Galileo was perfectly positioned to watch the fragments of comet Shoemaker-Levy 9 crash into Jupiter. The spacecraft made the only direct observations of the impact. Earth-based telescopes had to wait to see the impact sites as they rotated into view.
The Galileo spacecraft and probe traveled as one for almost six years. In July 1995, the probe was released to begin a solo flight into Jupiter.
Five months later, the probe sliced into Jupiter's atmosphere at 106,000 mph (47 kilometers per second). It slowed, released its parachute, and dropped its heat shield. As the probe descended through 95 miles (153 kilometers) of the top layers of the atmosphere, it collected 58 minutes of data on the local weather. The data were sent to the spacecraft overhead, then transmitted back to Earth.
It appeared that Jupiter's atmosphere is drier than had been previously thought. Measurements from the probe showed few clouds, and lightning only in the distance. It was only later that we discovered that the probe had entered an area called a "hot spot."
Towards the end of the 58-minute descent, the probe measured winds of 450 mph (724 kilometers per hour)—stronger than anything on Earth. The probe was finally melted and vaporized by the intense heat of the atmosphere.
To get into orbit around Jupiter, the spacecraft had to use its main engine. An error could send Galileo sailing past the planet. There was just one chance to get it right. After hours of anxious waiting, mission controllers confirmed that the spacecraft was safely in orbit. Galileo was alive and well and had begun its primary mission. The maneuver was precisely carried out, and Galileo entered orbit around Jupiter.
On December 7, 1995 Galileo began its prime mission: a two-year study of the Jovian system.
Galileo traveled around Jupiter in elongated ovals—each orbit lasted about two months. By traveling at different distances from Jupiter, Galileo could sample different parts of the planet's extensive magnetosphere. The orbits were designed for close-up flybys of Jupiter's largest moons.
To keep track of Galileo's journey, each orbit was numbered, and named for the moon that the spacecraft encountered at closest range. During orbit "C-3" for example—the third orbit around Jupiter—Galileo flew near the moon Callisto.
The data collected on Jupiter and the moons were stored on the on-board tape recorder. During the rest of the orbit, the data were sent to Earth using the low-gain antenna. At the same time, measurements are made of Jupiter's magnetosphere and transmitted back to Earth.
The intriguing data gathered during the eleven orbits of the prime mission left many questions to be answered. Since Galileo obviously was capable of much more, its mission was extended.
Galileo's prime mission ended on December 7, 1997. With more to learn, and the spacecraft in good health, NASA approved a two-year study called "GEM"—the Galileo Europa Mission. For fourteen more orbits, the spacecraft focused on ice, water, and fire: the icy moon Europa, which might have an ocean; Jupiter's majestic thunderstorms; and the fiery volcanoes of Io.
The spacecraft came so close to Europa that if there were something there the size of a school bus, Galileo would have detected it. The additional observations of Europa supported the theory that an ocean of water currently exists below the surface. NASA began considering plans for future missions to orbit Europa, and perhaps to send a lander.
Approaching Io—Jupiter's innermost moon—meant surviving Jupiter's intense radiation, so these encounters were saved until last. When radiation upset the spacecraft's computer, engineers worked all night to get them back online. But Galileo came through again, and even observed a lava fountain erupting on Io.
These successful flybys led to another exciting mission—the Galileo Millennium Mission, extending into 2001. Data was collected on Io and Europa, and studies made of the effects of radiation on a spacecraft close to Jupiter.
Spacecraft and Instruments
Some scientific instruments on the Galileo spacecraft observed from a distance while others measured fields and particles directly. The Galileo atmospheric descent probe was dropped directly into Jupiter's cloudtops to collect data about the atmosphere.
At launch, the orbiter weighed 2-1/2 tons (2,223 kilograms) and measured 17 feet (5.3 meters) from the top of the low-gain antenna to the bottom of the probe.
There were 10 scientific instruments aboard the spacecraft. Some of the instruments were mounted on long arms so they didn’t pick up unwanted signals from the spacecraft. The probe used six instruments plus its radio to investigate Jupiter's atmosphere. In addition, scientists used Galileo's radio system for experiments.
The orbiter featured an innovative "dual-spin" design. Most spacecraft are stabilized in flight either by spinning around a major axis, or by maintaining a fixed orientation in space, referenced to the Sun and another star. As the first dual-spin planetary spacecraft, Galileo combined these techniques. A spinning section rotated at about 3 rpm, and a "despun" section was counter-rotated to provide a fixed orientation for cameras and other remote sensors. A star scanner on the spinning side determined orientation and spin rate; gyroscopes on the despun side provided the basis for measuring turns and pointing instruments.
The power supply, propulsion module and most of the computers and control electronics were mounted on the spinning section. The spinning section also carried instruments to study magnetic fields and charged particles. These instruments included magnetometer sensors mounted on a 36-foot (11-meter) boom to minimize interference from the spacecraft's electronics; a plasma instrument to detect low-energy charged particles; and a plasma-wave detector to study electromagnetic waves generated by the particles. There was also a high-energy particle detector and a detector of cosmic and Jovian dust, an extreme ultraviolet detector associated with the ultraviolet spectrometer, and a heavy ion counter to assess potentially hazardous charged-particle environments the spacecraft flew through.
Galileo's de-spun section carried instruments that need to be held steady. These instruments included the camera system; the near-infrared mapping spectrometer to make multispectral images for atmosphere and surface chemical analysis; the ultraviolet spectrometer to study gases; and the photopolarimeter-radiometer to measure radiant and reflected energy. The camera system obtained images of Jupiter's satellites at resolutions from 20 to 1,000 times better than the best possible from NASA's Voyager spacecraft; its charge-coupled-device (CCD) sensor was much more sensitive than previous spacecraft cameras and is able to detect a broader color band. Galileo's de-spun section also carries a dish antenna that picked up the descent probe's signals during its fall into Jupiter's atmosphere.
The spacecraft's propulsion module consisted of 12 2.25-pound-force (10-newton) thrusters and a single 90-pound-force (400-newton) engine which used monomethylhydrazine fuel and nitrogen-tetroxide oxidizer. The propulsion system was developed and built by Messerschmitt-Bolkow-Blohm (MBB) and provided by the Federal Republic of Germany as NASA's major international partner on Galileo.
Because radio signals take more than one hour to travel from Earth to Jupiter and back, the Galileo spacecraft was designed to operate from computer instructions sent to it in advance and stored in spacecraft memory. A single master sequence of commands could cover a period ranging from weeks to months of quiet operations between flybys of Jupiter's moons. During busy encounter operations, one sequence of commands covered only about a week.
These sequences operated through flight software installed in the spacecraft computers, with built-in automatic fault protection software designed to put Galileo in a safe state in case of computer glitches or other unforeseen circumstance.
Electrical power was provided by two radioisotope thermoelectric generators. Heat produced by natural radioactive decay of plutonium was converted to electricity (570 watts at launch, 485 at the end of the mission) to operate the orbiter spacecraft's equipment. This is the same type of power source used on other NASA missions including Viking to Mars, Voyager and Pioneer to the outer planets, Ulysses to study the Sun, and Cassini to Saturn.
Galileo's descent probe had a mass of 339 kilograms (750 pounds), and included a deceleration module to slow and protect the descent module. The probe measured about 50 inches (127 centimeters) in diameter. Inside the heat shield, the scientific instruments were protected from ferocious heat during entry. The probe had to withstand extreme heat and pressure on its high-speed journey at 106,000 miles per hour.
The deceleration module consisted of an aeroshell and an aft cover designed to block heat generated by friction during atmospheric entry. Inside the aeroshells were the descent module and its 8-foot (2.5 meter) parachute. The descent module carried a radio transmitter and seven scientific instruments. These were devices to measure temperature, pressure and deceleration, atmospheric composition, clouds, particles, and light and radio emissions from lightning and energetic particles in Jupiter's radiation belts.