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| Artist's conception of the Galileo probe entry |
Galileo is a NASA spacecraft mission to Jupiter, launched October 18, 1989, and designed to study the planet's atmosphere, satellites and surrounding magnetosphere for 2 years starting in December 1995. It was named for the Italian Renaissance scientist who discovered Jupiter's major moons in 1610 with the first astronomical telescope.
This mission will be the first to make direct measurements from an instrumented probe within Jupiter's atmosphere, and the first to conduct long-term observations of the planet and its magnetosphere and satellites from orbit around Jupiter. It is already the first to encounter an asteroid, and the discoverer of another asteroid's moon.
The Jet Propulsion Laboratory designed and developed the Galileo Jupiter orbiter spacecraft and is operating the mission; NASA's Ames Research Center developed the atmospheric probe with Hughes Aircraft Company as the prime contractor. The German government is a partner in the mission through its provision of the spacecraft propulsion subsystem and two science experiments.
Like Voyager and some other previous interplanetary missions, Galileo used planetary gravitational fields as auxiliary propulsion stages. The spacecraft dipped into the gravitational fields of Venus and Earth to pick up enough velocity to get to Jupiter. This 38-month Venus-Earth-Earth Gravity Assist phase ended with the second Earth flyby on December 8, 1992. It provided, in addition to the velocity increment, opportunities for useful scientific observations and an exercise of the spacecraft's scientific capabilities.
Galileo's two planned visits to the asteroid belt provided the first and second opportunities for close observation of these bodies: in October 1991 the spacecraft flew by 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. In late July 1994 Galileo was the only observer in a position to obtain images of the far side of Jupiter when more than 20 fragments of Comet Shoemaker-Levy plunged into the night-side atmosphere over a 6-day period.
In December 1995 the Galileo atmospheric probe will conduct a direct examination of Jupiter's atmosphere, while the larger part of the craft, the orbiter, begins a 23-month, 11-orbit tour of the magnetosphere and the Galilean moons, including ten close satellite encounters.
Galileo's orbital science results will be transmitted to Earth over the low-gain antenna at significantly lower data rates than originally planned, because of the in-flight failure of the high-gain antenna to deploy as commanded in April 1991. The Project team has developed means to transmit the key scientific data and to accomplish most of the Project's Jupiter science objectives, using on-board data processing and compression, and various enhancements to the communications link performance, including new encoding systems and advanced technology in ground equipment.
The 2,223-kilogram (2-1/2-ton) Galileo orbiter spacecraft carries 10 scientific instruments; there are another six on the 339-kilogram (750-pound) probe. The spacecraft radio link to Earth and the probe-to-orbiter radio link serve as instruments for additional scientific investigations.
Galileo communicates with its controllers and scientists through the Deep Space Network, using tracking stations in California, Spain and Australia.
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| Artist's conception of the Galileo probe entry |
The Galileo mission had originally been designed for a direct flight of about 2 ½ years to Jupiter. Changes in the launch system after the Challenger accident, including replacement of the Centaur upper-stage rocket with the IUS, precluded this direct trajectory.
Galileo engineers designed a new interplanetary flight path using gravity assists, once with Venus and twice with Earth, which was called the Venus-Earth-Earth-Gravity-Assist or VEEGA trajectory.
Galileo's scientists seized the opportunities for scientific observation and measurement of Venus and the Earth-Moon system. At Venus, the scientists believe they confirmed the presence of lightning, and secured the first views of the mid-level clouds of Venus. During the Earth and Moon encounters, the spacecraft observed the Moon's north polar regions, compiled some compositional maps of the surface using spectral instruments, and made many Earth observations. The gravity-assist trajectory incidentally sent the spacecraft into the Asteroid Belt twice, letting the scientists closely observe two asteroids, Gaspra and Ida.
The VEEGA mission exposed the spacecraft to a hotter environment from Earth to Venus and back than the Earth-to-Jupiter transit for which Galileo was originally designed. The project's engineers devised a set of sunshades to protect the craft, and the top of the spacecraft was pointed close to the Sun, with the umbrella-like high-gain antenna furled (precluding high-rate communications except close to Earth) for protection from the Sun's rays, until well after the first Earth flyby in December 1990.
The spacecraft was scheduled to deploy its 4.8-meter (16-foot) high-gain antenna in April 1991 as Galileo moved away from the Sun and the risk of overheating ended, although both Earth encounters were designed to use the wide-angle, low-gain antenna. The antenna failed to deploy fully at that time.
A special team was immediately formed to understand the failure and to propose corrective actions. After extensive tests and analyses, they determined that a few (probably three) of the antenna's 18 umbrella-like ribs were held by enhanced friction in the closed position. The flight team has undertaken various remote-controlled operations to relieve this condition, but as yet the ribs have not been freed. Accordingly, the team designed the two asteroid encounters to use the low-gain antenna and tape recorder for recovery of the scientific observations. The Jupiter encounter was subsequently redesigned to rely on the low-gain antenna.
Nine months into its two-year Earth-to Earth orbit, Galileo entered the asteroid belt, and on October 29, 1991, it had the world's first asteroid encounter, with Gaspra. It passed just 1,601 kilometers (1000 miles) from the stony asteroid's center at a relative speed of about 8 kilometers per second (18,000 miles per hour); scientists collected pictures of Gaspra and other data on its composition and physical properties. These revealed a cratered, complex, and irregular body about 36 by 22 by 20 kilometers (22 by 14 by 12 miles), with a thin covering of dirt-like "regolith" and a possible magnetic field.
On December 8, 1992, Galileo had its final rendezvous with Earth, a close 303-kilometer (188-mile) encounter. It left Earth for the third and last time with a solar-orbital speed of 39 kilometers per second (about 87,000 mph), headed at last for Jupiter.
The closest-approach distance was 2,391.2 kilometers (about 1500 miles), with a relative speed of nearly 12.4 kilometers per second or 28,000 miles per hour. The initial image and other data was transmitted over the low-gain antenna in September 1993. They revealed a surface covered with craters, suggesting a much greater age than previously thought, an older surface than Gaspra's. After a pause for the period of poor communications caused by solar conjunction and for needed engineering operations, receipt of Ida data resumed in February and March 1994.
Almost immediately, Galileo was found to have discovered the first moon of an asteroid. Ida's satellite was found in both a camera frame and an infrared scan. The 1.5-kilometer satellite, later named Dactyl, was estimated to be about 100 kilometers (60 miles) from the center of the asteroid, in a roughly circular orbit.
The discovery of Comet Shoemaker-Levy 9 in March 1993 provided an exciting new opportunity for Galileo's science teams as well as the rest of Earth's astronomers. In orbit around Jupiter and headed for atmospheric entry in July 1994, the fragmented comet was a unique observational subject as well as a series of natural probes of Jupiter's atmosphere.
The Galileo spacecraft, approaching Jupiter, was the only observation platform in line of sight to the impact area on Jupiter's far side. Although there was no additional funding available for this new "encounter," an observation program was planned for Galileo's remote sensing instruments. All observations had to be programmed in advance into the spacecraft computer, notwithstanding uncertainties in the predicted impact times. The data were to be stored on the spacecraft (one tape load plus some computer memory space) for playback at the 10 bits-per-second rate. Playback was scheduled to continue, with necessary interruptions for other activities, until late January 1995.
Galileo's imaging system used different methods in order to cover the time uncertainties (amounting to hours) of the impacts for different events. Repeated imaging, rather like a very slow motion picture, captured the beginning of the very last impact (fragment W); further playbacks, scheduled later, may extend the history of this event. Two kinds of smeared image, producing a streak representing the night-side impact fireball among smears representing Jupiter and some satellites, have provided a brightness history for one 46-second event so far, and additional playbacks should add to this. The photopolarimeter-radiometer detected at least three events, and the infrared spectrometer at least one; detailed data remain to be played back.
In July 1995, Galileo prepared for the complex Jupiter encounter by deploying the atmospheric probe, and then directing itself away from the atmospheric entry point and into the path that leads to the beginning of the orbital mission.
The spacecraft will first accurately adjust its trajectory to establish the atmospheric probe's 5-month free flight to Jupiter, and then turn to orient the probe so that it will enter the atmosphere in the correct attitude. Finally, it will spin up to 10 rpm and release the spin-stabilized probe.
Several days later, the Galileo orbiter will readjust its trajectory to aim for its own near-Jupiter passage, at a point about 200,000 kilometers (130,000 miles) above the planet, on December 7, 1995. This was the first use of Galileo's 400-newton rocket engine.
Galileo's Jupiter operations using the low-gain antenna call for a new flight software, in effect, a redesign of part of the spacecraft. In fact, the critical first part of the Jupiter science activity, arrival and probe relay, will use one new set of flight software, and the extended orbital operations another. These software will provide different ways of handling the science data (voluminous) and the telecommunications link (low rate), to optimize the scientific return, as well as other minor operational changes. The new software started operating in March 1995 for the first phase and March 1996, during the first Jupiter orbit, for the second phase.
The orbiter will have been collecting and transmitting data from its magnetometer, dust detector, and extreme ultraviolet instrument on the space environment near Jupiter, plus a single color image of Jupiter made about two months before arrival. All other science data from the Jupiter-arrival phase will be stored in the tape recorder for playback during the first orbit, starting in the late spring of 1996. All the observations from this phase, including the relayed probe data, and certain spacecraft engineering telemetry, must fit in one tape load -- 900 megabits, the equivalent of some 150 full-resolution, uncompressed, single-color pictures.
The orbiter will fly close by Io, make its closest approach to Jupiter in position to receive and record the probe signals as the probe enters Jupiter's atmosphere, and finally, go into orbit around Jupiter, all in a period of about seven hours.
While the probe is still approaching Jupiter, the orbiter will have its first two satellite encounters. After passing within 33,000 kilometers (20,000 miles) of Europa, it will fly about 1,000 kilometers (600 miles) above Io's volcano-torn surface, about one-twentieth the closest flyby altitude of the Voyagers in 1979.
About four hours later, the probe will enter the upper atmosphere, 6.6 degrees north of Jupiter's equator, at more than 47 kilometers per second (100,000 miles per hour), and slow by aerodynamic braking for about 2 minutes before deploying its parachute and removing its heat shields.
Then it will float down about 200 kilometers or 125 miles through the clouds, passing from an atmospheric pressure of one-tenth that of the Earth's surface to about 25 Earth atmospheres in 75 minutes. It may not last much longer than that.
About 214,000 kilometers (133,000 miles) above, the orbiter will receive the probe's science-data transmissions and store them both in the spacecraft tape recorder and in computer memory for later transmission to Earth. The trajectory has been designed to put the orbiter within the conical beam of the probe's transmitter for the 75-minute probe mission duration.
The probe's observations from within the clouds of Jupiter, consisting of weather and atmospheric properties data, are uniquely valuable to the scientists, and every effort is being made to secure them.. This takes the form of tape-recording the complete probe data stream, as noted above, for delayed playback, and in addition storing a compressed and shortened version in onboard computer memory, to be read out to the scientists back on Earth a few days after the probe's atmospheric operations.
The Galileo spacecraft engineers had, for reliability reasons, doubled the spacecraft command and data subsystem's memory capacity during the spacecraft rework phase for the VEEGA redesigned mission, providing a memory reserve of 192 kilobytes. Stripping away associated engineering measurements and saving only the probe science, the spacecraft computer will be able to store almost 40 minutes of the planned 75-minute probe observing time, and capture the critical part of the atmospheric data.
About an hour after collecting the last of the probe data, the orbiter must brake with its main engine to go into orbit around Jupiter.
This, the first of eleven planned operational orbits, will have a period of about 7 months. A thrust maneuver at the farthest point of the orbit will raise the near-Jupiter point of this and all subsequent orbits, moving the spacecraft away from Jupiter's charged-particle belts, which can damage spacecraft sensors and computer chips.
During this orbit the complete tape-recorded probe data and the orbiter's pictures and other data of Europa, Io and Jupiter from December 7, 1995 will be played back, and then the new software for orbital science data processing will be installed in the spacecraft.
This new software effectively makes the Galileo orbiter into a different spacecraft from that which came from Earth to Jupiter and the one that carried out the Jupiter approach and probe relay mission. It changes the way the data from many of the instruments is packaged and transmitted to Earth. It will also permit drastic data compression, permitting the spacecraft to store and transmit perhaps 10 times the number of pictures and other measurements that would have been possible otherwise using the low-gain antenna, whose normal data rate is less than 1/10,000 the rate of the high-gain antenna in the old design. This process will use high-speed computer elements that actually belong to the attitude control subsystem, as well as the computer and reserve memory in the command and data subsystem.
In addition, hardware changes on the ground and adjustments to the spacecraft-to-Earth communication system will provide a gain of as much as ten times in the average telemetry rate. As a consequence of the new software on the spacecraft and the hardware and other changes on Earth, Galileo will transmit almost one full tape-recorder load on average during each orbit around Jupiter. With this and the full probe mission, the scientists have concluded Galileo will achieve about 70% of its Jupiter science objectives. The first Ganymede close flyby in July 1996 will shorten and change the next orbit, and each time the orbiter returns to the inner zone of satellites it will make a close pass over one of them, using the satellite's gravity to change the spacecraft orbit around Jupiter. Galileo will fly by Ganymede four times, Callisto three, and Europa three. Io gets only the one close pass on arrival day, because Galileo will do a large maneuver in March 1996, lifting its orbit away from the inner Jovian system to minimize the spacecraft's exposure to the radiation-belt environment.
These satellite encounters will be at altitudes as close as 200 kilometers (125 miles) above the surfaces of the moons, and typically 100 to 1,000 times closer than the Voyagers' satellite flybys. Galileo's scanning instruments will scrutinize the surface and features of each. After a week or so of intensive observation, with its tape recorder full of data, the spacecraft will spend the next months in orbit playing out the information to Earth. Throughout the 23-month orbital phase, Galileo will continue observing the planet and the satellites and gathering data on the magnetospheric environment.
The Galileo science team have already conducted observations of Venus, Earth, the asteroids Gaspra and Ida, the Comet Shoemaker-Levy 9's impacts with Jupiter and the interplanetary medium during Galileo's long trip to Jupiter and have begun to publish some results from these studies. The experiments and principal scientists are listed at the end of this fact sheet.
Galileo's atmospheric probe masses 339 kilograms (750 pounds), and includes a deceleration module to slow and protect the descent module, which carries out the scientific mission.
The deceleration module consists of an aeroshell and an aft cover, designed to block the heat generated by friction during the sharp deceleration of atmospheric entry. Inside the shells are the descent module and its 2.5-meter (8-foot) parachute. The descent module carries a radio-relay transmitter and six scientific instruments. Each operating at 128 bits per second, the dual L-band transmitters send nearly identical streams of scientific data to the orbiter. Probe electronics are powered by batteries with an estimated capacity of about 18 amp-hours on arrival at Jupiter.
Probe instruments include an atmospheric structure group of sensors measuring temperature, pressure and deceleration; a neutral mass spectrometer and a helium-abundance detector supporting atmospheric composition studies; a nephelometer for cloud location and cloud-particle observations; a net-flux radiometer measuring the difference, upward versus downward, in radiant energy flux at each altitude; and a lightning/radio-emission instrument with an energetic-particle detector, measuring light and radio emissions associated with lightning and energetic particles in Jupiter's radiation belts (so that this instrument begins measuring some hours before the probe reaches atmosphere).
The Galileo orbiter spacecraft, in addition to supporting the probe activities, will support all the scientific investigations of Jupiter's satellites and magnetosphere, and remote observation of the giant planet itself.
At launch, the orbiter weighed about 2,223 kilograms (4,900 pounds), not counting the upper-stage-rocket adapter but including about 925 kilograms of usable rocket propellant.
This propellant is being expended in the small pulses that turn and orient the spacecraft and in the maneuvers which change the spacecraft's direction or speed of flight. The flight path changes include almost 30 relatively small maneuvers during the long gravity-assisted flight to Jupiter, three large thrust maneuvers in 1995 and 1996 including the one that puts the craft into its Jupiter orbit, and the 30 or so orbit-trim maneuvers planned for the satellite tour phase.
The propulsion module consists of twelve 10-newton thrusters, a single 400-newton engine, the monomethyl-hydrazine fuel, nitrogen-tetroxide oxidizer, and pressurizing-gas tanks, tubing, valves and control equipment. (A thrust of 10 newtons would support a weight of about one kilogram or 2.2 pounds at Earth's surface.) The propulsion system was developed and built by Messerschmitt-Bolkow-Blohm (MBB) and provided by the Federal Republic of Germany as a long-term partner in Project Galileo.
In addition to the scientific data acquired by its 10 instruments, the Galileo orbiter acquires and can transmit a total of 1,418 engineering measurements of internal operating conditions (temperatures, voltages, computer states and counts, and the like). The spacecraft transmitter operates at S-band frequency (2,295 megahertz).
Two low-gain antennas (one pointed upward or toward the Sun, and one on a deployable arm to point down, both mounted on the spinning section) supported communications during the Earth-Venus-Earth leg of the flight. The top-mounted antenna is currently carrying the communications load, including science data and playbacks, in place of the high-gain antenna, and is the basis of the redesigned Jupiter sequences. The other low-gain antenna has been re-stowed after supporting operations during the early VEEGA phase, and is not expected to be used again.
Because radio signals take more than one hour to travel from Earth to Jupiter and back, the Galileo spacecraft was designed to operate from programs sent to it in advance and stored in spacecraft memory. A single master sequence program can cover from weeks to months of quiet operations between planetary and satellite encounters. During busy encounter operations, one program covers only about a week.
These sequences operate through flight software installed in the spacecraft computers in the command and data subsystem (CDS), attitude control subsystem, and many of the scientific instruments. In the CDS software, there are about 35,000 lines of code, including 7,000 lines of automatic fault protection software, which operates to put the spacecraft in a safe state if an untoward event such as an onboard computer glitch were to occur. The articulation and attitude control (AACS) software has about 37,000 lines of code, including 5,500 lines devoted to fault protection.
As noted above, two new generations of flight software have been developed to allow the spacecraft to carry out its scientific activities during the Jupiter approach and first encounter (March 1995 t0 March 1996) and during subsequent orbital operations. The "Phase 1" new flight software involves changes to about 10 percent of the CDS software, with a net increase of a few percent. The "Phase 2" orbital operations software, entailing extensive onboard data processing and compression, nearly doubles the size of the CDS software and increases the AACS software by 10 percent. This was only made possible because the prior flight software was stored four times in the CDS memory; this will be reduced to "double redundancy" at most in the orbital design.
Electrical power is provided to Galileo's equipment by two radioisotope thermoelectric generators. Heat produced by natural radioactive decay of plutonium is converted to electricity (570 watts at launch, 485 at the end of the mission) to operate the orbiter equipment for its eight-year baseline mission. This is the same type of power source used by the Voyager and Pioneer Jupiter spacecraft in their outer-planet missions.
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. Galileo represents a combination of these techniques, and is the first dual-spin planetary spacecraft. A spinning section rotates at 3 rpm, and a "despun" section is counter-rotated to provide a fixed orientation for cameras and other remote sensors. A star scanner on the spinning side is used to determine orientation and spin rate. Gyros are located on the despun side to provide the basis for measuring turns and pointing instruments.
Scientific instruments to measure fields and particles, together with the main antenna, the power supply, the propulsion module, most of the computers and control electronics, are mounted on the spinning section. The instruments include magnetometer sensors, mounted on an 11-meter (36-foot) boom to minimize interference from the spacecraft; a plasma instrument detecting low-energy charged particles and a plasma-wave detector to study electromagnetic waves generated by the particles; a high-energy particle detector; and a detector of cosmic and Jovian dust. It also carries a heavy ion counter, an engineering experiment added to assess the potentially hazardous charged-particle environments the spacecraft flies through, and an extreme ultraviolet detector associated with the UV spectrometer on the scan platform.
The despun section carries instruments and other equipment whose operation depends on a steady pointing capability. The instruments include 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 will obtain images of Jupiter's satellites at resolutions from 20 to 1,000 times better than Voyager's best. The camera's CCD sensor is more sensitive and has a broader color detection band than the vidicons of Voyager.
This section also carries a dish antenna to track the probe in Jupiter's atmosphere and pick up its signals for recording and later relay to Earth.
As indicated above, the Galileo spacecraft carries out its complex operations, including maneuvers, scientific observations and communications, in response to stored sequences which are sent up to the orbiter periodically through the Deep Space Network in the form of command loads.
Designing these sequences is a complex process balancing the desire to carry out scientific observations with the need to safeguard the spacecraft and mission. The sequence design process itself is supported by software programs which, for example, display to the scientist maps of the instrument coverage on the surface of a satellite for a given spacecraft orientation and trajectory. Notwithstanding these aids, a typical seven-day satellite encounter will take efforts spread over many months to design, check and recheck. The controllers also use software designed to check the command sequence against flight rules and constraints.
The spacecraft status and health are monitored through data from 1418 onboard measurements. The Galileo flight team interprets these data into trends to avert or work around equipment failure. Their conclusions become an important input, along with scientific plans, to the sequence design process. The telemetry monitoring is supported by computer programs written and used in the mission support area.
Navigation is the process of estimating, from radio range and doppler measurements and spacecraft camera astronomy, the position and velocity of the spacecraft, so as to predict its flight path and to design course-correcting maneuvers. These calculations must be done with computer support. The Galileo mission, with its complex gravity-assist flight to Jupiter and 10 gravity-assist satellite encounters in the Jovian system, is extremely dependent on consistently accurate navigation.
In addition to the programs which directly operate the spacecraft and are periodically transmitted to it, the mission operations team uses software amounting to 650,000 lines of programming code in the sequence design process; 1,615,000 lines in the telemetry interpretation; and 550,000 lines of code in navigation. These all had to be written, checked, tested, used in mission simulations and, in many instrument cases, revised before the mission could begin.
Science investigators are located variously at JPL or at their home laboratories, linked by computer communications. From either location, they are involved in developing the sequences affecting their experiments and, in some cases, helping to change preplanned sequences to follow up on unexpected discoveries with second looks.
The earliest Earth-based telescopic observations showed bands and spots in Jupiter's atmosphere; one storm system, the Red Spot, has been seen to persist over three centuries. The light and dark bands and some of the spots have disappeared and reappeared over periods of many years, and as the quality of Jupiter's observation has improved so has the amount of variability seen in the clouds.
Atmospheric features were seen in greatly improved detail with the Pioneer and Voyager missions of the 1970's, which provided four episodes for the study of changes as well. In addition, the Voyager encounters in the spring and summer of 1979 allowed the observation of short-term variations in real time as Jupiter turned beneath the spacecraft's cameras. Earth-based infrared astronomers have recently studied the nature and vertical dynamics of deeper clouds, and the new Earth- and orbit-based telescopes observe large-scale atmospheric developments and climate changes at our greatest planet, most notably during the Shoemaker-Levy impact events.
Sixteen satellites are known. The four largest, discovered by the Italian scientist Galileo in 1610, are about the size of small planets, and were seen by Voyager's experimenters to have varied terrain. The innermost of these, Io, has active sulfurous volcanoes, discovered by Voyager 1 and further observed by Voyager 2 and Earth-based infrared astronomy. Io and Europa are about the size and density of Earth's moon (3-4 times the density of water) and probably mostly rocky inside. Europa may also exhibit surface activity. Ganymede and Callisto, further out from Jupiter, are the size of Mercury but less than twice as dense as water; their interiors are probably about half ice and half rock, with mostly ice or frost surfaces which show distinct and interesting features.
Of the others, eight are in inclined, highly eccentric orbits far from the planet, and four (three discovered by the Voyager mission in 1979) are close to the planet. Voyager also discovered a thin ring system at Jupiter in 1979. In March 1993 a fragmented comet named Shoemaker-Levy 9 was discovered in orbit around Jupiter; however, the orbit lasted only until July 1994, ending with widely observed impacts in the atmosphere.
Jupiter has the strongest planetary magnetic field known; the resulting magnetosphere is a huge teardrop-shaped, plasma-filled cavity in the solar wind pointing away from the Sun. The inner part of the magnetically-constrained charged-particle belt is doughnut-shaped, but farther out it flattens into a disk. The magnetic poles are offset and tilted relative to Jupiter's axis of rotation, so the field appears to wobble around with Jupiter's rotation (about every 10 hours), sweeping up and down across the inner satellites and making waves throughout the magnetosphere.
NASA's Ames Research Center, Moffett Field, California, is responsible for the atmosphere probe, which was built by Hughes Aircraft Company, El Segundo, California. At Ames, the probe manager is Marcie Smith and the probe scientist is Richard E. Young.
Launch: STS-34 Atlantis and IUS October 18, 1989
First trajectory-change maneuver November 9-11,89
Venus flyby (about 16,000 km altitude) February 10, 1990
Venus data playback November 19-21,90
Earth 1 flyby (about 1000 km) December 8, 1990
Asteroid Gaspra flyby (about 1600 km) October 29, 1991
Earth 2 flyby (about 300 km) December 8, 1992
Asteroid Ida flyby (about 2400 km) August 28, 1993
Probe release July 13, 1995
Jupiter arrival (includes December 7, 1995
Io flyby at about 1000 km,
Probe entry and relay,
Jupiter orbit insertion)
Orbital tour of Galilean satellites Dec '95-Nov '97
First Ganymede encounter July 4, 1996
Spacecraft Characteristics
Orbiter Probe
Mass 2,223 kg (4890 lb) 339 kg (746 lb)
Usable propellant mass 925 kg (2,035 lb) --
Height (in-flight) 6.15 m (20.5 ft) 86 cm (34 in.)
Instrument payload 10 instruments* 6 instruments
Payload mass 118 kg (260 lb) 30 kg (66 lb)
Electric power RTGs, 570-470 w Lithium-sulfur
battery, 730 w-h
Two radio science experiments use telecommunications link rather than dedicated flight instruments.