Galileo Project

Courtesy of the Jet Propulsion Laboratory

## Table of Contents

- Summary
- Launch Operations
- Earth to Jupiter
  - Venus
  - Earth (First Pass)
  - First Asteroid: Gaspra
  - Earth (Second Pass)
  - Second Asteroid: Ida
  - Approaching Jupiter
- At Jupiter
- Scientific Activities
- Spacecraft
  - Atmospheric Probe
  - Galileo Orbiter
- Ground Systems
- Jupiter's System
- Management
- Galileo Mission Events
- Spacecraft Characteristics
- Galileo Scientific Experiments
- Interdisciplinary Investigators

## Summary

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 to photograph an 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 prime contractor. The German government is a partner in the mission through its provision of the spacecraft propulsion subsystem and two science experiments. Scientists from six nations are participating in the mission.

Galileo used planetary gravitational fields as auxiliary propulsion stages, employing a well-tried technique used by Voyager and some other previous interplanetary missions. The spacecraft dipped into the gravitational fields of Venus (closest approach on February 10, 1990) 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. On October 29, 1991, the spacecraft flew by asteroid Gaspra, near the inner edge of the main asteroid belt, and obtained the world's first close-up asteroid images. On August 28, 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.

The 2,223-kilogram (2.5-ton) Galileo orbiter spacecraft carries 10 scientific instruments; there are another six on 339-kilogram (746-pound) probe. The spacecraft radio link to Earth and the probe-to-orbiter radio link serve as instruments for additional scientific investigations.

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 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.

Galileo communicates with its controllers and scientists through the Deep Space Network, using tracking stations in California, Spain and Australia.

## Launch Operations

The Galileo spacecraft was carried into Earth orbit on October 18, 1989, by space shuttle Atlantis, commanded by Donald E. Williams and piloted by Michael J. McCulley. Mission specialists Shannon W. Lucid, Ellen S. Baker and Franklin R. Chang-Diaz deployed Galileo and its IUS (Inertial Upper Stage) booster from the shuttle. The two-stage IUS solid rocket accelerated the spacecraft out of Earth orbit toward the planet Venus.

The Galileo mission was originally designed for a direct flight of about 2.5 years to Jupiter. Changes in the launch system after the Challenger accident, including replacement of the Centaur upper-stage rocket with the less powerful IUS, precluded this direct flight. Trajectory engineers designed a new interplanetary flight path using gravity assists, once with Venus and twice with Earth, to build up the speed to reach Jupiter, taking a total of just over six years. This is called the Venus- Earth-Earth-Gravity-Assist or VEEGA trajectory.

## Earth to Jupiter

Galileo's VEEGA trajectory includes three inner-planet encounters in the first part of its gravity-assisted flight to Jupiter. These provided opportunities for scientific observation and measurement of Venus and the Earth-Moon system. The mission also provides a chance to fly close to two asteroids, bodies of a type never before observed close up, and to obtain data on other phenomena in interplanetary space.

The instruments designed to observe Jupiter's atmosphere from afar also improved our knowledge of the atmosphere of Venus; sensors designed for the study of Jupiter's moons can add to information about our own planet and its satellite.

### Venus

The planet Venus is approximately the size and density of the Earth, and it has a solid surface beneath a cloudy atmosphere, but it does not otherwise resemble our planet. The atmosphere is deep and dense; cloud tops are some 65 kilometers (40 miles) above the surface, where the atmospheric pressure is more than 90 times that on Earth and the temperature near 750° Kelvin (480°C or 900°F).

The clouds of Venus are essentially opaque to visible light, and the surface can be observed only by radar from Earth or spacecraft or by a spacecraft hardy enough to land and survive on the surface. Many observations of these types have been made, but they have been of limited scope or resolution. NASA's Magellan mission, in orbit around Venus, has just completed a global, high-resolution radar survey.

Many features of the atmosphere were unknown, including details of the motion of the upper regions, the form of lower clouds, and the existence of lightning storms.

The Galileo spacecraft approached Venus on February 9, 1990, from the night side and passed across the sunlit hemisphere. Closest approach was about 10 p.m. PST, or about 1 a.m. EST February 10, at a distance of 16,000 kilometers (10,000 miles) above the cloudtops. For a day before and several days after closest approach, Galileo scientists collected measurements of charged particles, dust and magnetism, infrared and ultraviolet spectral observations, data for infrared lower-atmosphere maps, and 81 camera images.

Virtually all these data were tape-recorded on the spacecraft and played back in November 1990, because Galileo's communications were constrained during this early phase of flight.

The spacecraft was originally designed to operate between Earth and Jupiter; at Jupiter, sunlight is 25 times weaker than at Earth and temperatures are much lower. The VEEGA mission exposed the spacecraft to a hotter environment from Earth to Venus and back than that for which Galileo was designed. The spacecraft 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 main antenna furled (precluding high- rate communications) for protection from the Sun's rays, until well after the first Earth flyby in December 1990.

The 81 pictures of the upper clouds, the infrared images of lower-level cloud patterns (never observed before), various data on solar wind interactions and other observations make up Galileo's Venus results.

### Earth (First Pass)

Approaching Earth for the first time about 14 months after launch, the Galileo spacecraft took the opportunity to measure the magnetic tail extending far beyond the dark side of Earth. The spacecraft also observed parts of both the near and far sides of the Moon, compiling maps of mineral composition and showing gigantic impact basins in the south of the far side and on the limb between near and far.

After passing Earth, Galileo observed the planet's sunlit side, gathering enough images to constitute a brief motion picture showing the planet making a full rotation.

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. The antenna failed to deploy fully at this time. A special team was immediately formed to fully understand the failure and 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 operations to relieve this condition, but as yet the ribs have not been freed. Accordingly, the team designed the Gaspra asteroid encounter to use the low-gain antenna and tape recorder, in a manner similar to the Venus encounter. Although there is no significant prospect of deploying the antenna, controllers will make one last attempt in March 1996.

### First Asteroid: Gaspra

Nine months after the Earth passage, Galileo entered the asteroid belt, and two months after that, on October 29, 1991, it performed the world's first asteroid encounter. From Earth-based observations, Gaspra was believed to be a fairly representative main-belt asteroid, probably similar in composition to stony meteorites.

The spacecraft passed about 1,600 kilometers (1,000 miles) from Gaspra at a relative speed of about 8 kilometers per second (18,000 miles per hour). Scientists collected several pictures of Gaspra, and measurements to indicate composition and physical properties. The last (and best) two images were played back to Earth in November 1991 and June 1992, revealing a cratered and very irregular body about 19 by 12 by 11 kilometers (12 by 7.5 by 7 miles). The rest of the data, including low resolution images of more of the surface, were transmitted in late November 1992.

### Earth (Second Pass)

Thirteen months after the Gaspra encounter, the spacecraft completed its two-year elliptical orbit around the Sun and arrived back at Earth. Galileo requires a much larger elliptical orbit (with a six-year period) to reach as far as Jupiter, and the second flyby of Earth pumped the orbit up to that size.

Passing about 300 kilometers (185 miles) above Earth's surface, or 40 kilometers (25 miles) above the altitude at which it was deployed from the space shuttle more than three years before, Galileo used Earth's gravitation to change its flight direction and pick up about 3.7 kilometers per second (8,000 miles per hour).

Each gravity-assist flyby requires several rocket-thrusting sessions, using Galileo's onboard propulsion module, to refine the flight path. (Asteroid encounters require similar maneuvers to obtain the best observing conditions.)

Passing the Earth for the last time, the spacecraft's scientific equipment made observations of the planet, both for comparison with Venus and Jupiter and to aid in Earth studies. It observed the north polar regions of our Moon, for comparison with Jupiter's satellites and obtained new data on lunar regions never explored before.

### Second Asteroid

Nine months later, on August 28, 1993, Galileo flew within 2,400 kilometers (1,500 miles) of asteroid Ida. This asteroid is more than twice as large as Gaspra and more distant. Ida is about 56 x 24 x 21 kilometers (35 x 15 x 13 miles) in size. Like Gaspra, it is believed to represent a main-belt asteroid in composition which formed from the collisional breakup of a larger body. They are both classified as S-type asteroids and are likely composed of silicates or metal-rich sliicates. Relative velocity for this flyby was nearly 12.6 kilometers per second or 28,000 miles per hour.

Galileo also discovered that Ida had a small moon, Dactyl. This marked the first discovery of a moon around an asteroid. The satellite is approximately egg-shaped, measuring about 1.2 by 1.4 by 1.6 kilometers (0.75 by 0.87 by 1 miles). More than a dozen craters larger than 80 meters (250 feet) in diameter are clearly evident, indicating that the moon has suffered numerous collisions from smaller solar system debris during its history.

### Approaching Jupiter

Some 2 1/2 years after leaving Earth for the third time and five months before reaching Jupiter, Galileo's probe separated from the orbiter which has been carrying it since before launch. The separation occurred on July 12, 1995.

The spacecraft precisely adjusted its trajectory to establish the atmospheric probe's 5-month free flight to Jupiter, and then turned to orient the probe so that it will enter the atmosphere in the correct attitude. Finally, it spun up to 10 rpm and released the spin-stabilized probe. Several days later, the Galileo orbiter readjusted its trajectory to aim for its own Jupiter encounter.

## At Jupiter

Early in December 1995 the Galileo orbiter and probe will approach Jupiter separately. They will have travelled about 4 billion kilometers (2.5 billion miles) in a complex multiple looping path for more than six years. For the last 60 days of the approach, the orbiter carries out a comprehensive program of observations of Jupiter and measurements of its environment in space.

The probe will enter the atmosphere to make direct measurements. The orbiter will fly close by Io, receive the probe signals for relay to Earth, and 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 1/20 the closest flyby altitude of Voyager in 1979.

A few hours later, the probe will enter the upper atmosphere, about 6 degrees north of Jupiter's equator, at about 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 (125 miles) through the clouds, passing from a pressure of 1/10 that on Earth's surface to about 25 Earth atmospheres in 75 minutes. The probe batteries are not expected to last beyond this point, and the radio-communications link will be terminated.

About 214,000 kilometers (133,000 miles) above, the orbiter will receive, store and transmit the probe's science data. Next, the orbiter must thrust with its main engine to go into orbit around Jupiter.

This, the first of ten planned operational orbits, will have a period of about 8 months. Additional maneuvers and the first Ganymede close flyby in July 1996 will shorten the orbit, and each time the orbiter returns to the inner zone of satellites it will make a gravity-assist close pass over one of them to change its orbit while making close observations. These satellite encounters will be at altitudes as close as 200 kilometers (125 miles) above the surfaces of the moons, typically about 100 times closer than the Voyagers' satellite flybys. Throughout the 23- month orbital phase, Galileo will continue observing the planet and the satellites and gathering data on the magnetospheric environment.

## Scientific Activities

Galileo's scientific experiments are being carried out by more than 100 scientists from six nations. These are supported by dedicated instruments and the radio subsystems on the Galileo orbiter and probe. NASA has appointed thirteen interdisciplinary scientists whose studies reach across more than one Galileo instrument data set. The experiments and principal scientists are listed at the end of this fact sheet.

## Spacecraft

The Galileo mission and systems were designed to investigate three broad aspects of the Jupiter system: the planet's atmosphere, the satellites and the magnetosphere. The spacecraft was constructed in three segments, which help focus on these areas: the atmospheric probe; a non-spinning section of the orbiter carrying cameras and other remote sensors; and the spinning main section of the orbiter spacecraft which includes the fields and particles instruments, designed to sense and measure the environment directly as the spacecraft flies through it. The spinning section also carries the main communications antenna, the propulsion module, flight computers and most support systems.

### Atmospheric Probe

The probe weighs about 340 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 slowing from the probe's arrival speed of about 47 kilometers per second to subsonic speed in less than 2 minutes.

After the aft cover is released, the descent module deploys its 2.5-meter (8-foot) parachute and drops the aeroshell; its radio-relay transmitter and all six of its instruments go to work (two instruments started storing data on the way in). 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 measuring temperature, pressure and deceleration;
- a neutral mass spectrometer and a helium-abundance interferometer 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.

### Galileo Orbiter

The orbiter, 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, including those carried out on the way to Jupiter.

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 is used in almost 30 relatively small maneuvers during the long gravity-assisted flight to Jupiter, three large thrust maneuvers including the one that puts the craft into its Jupiter orbit, and the 30 or so trim maneuvers planned for the satellite tour phase.

The propulsion module consists of twelve 10-newton thrusters, a single 400-newton engine, and the fuel, 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 partner in Project Galileo.

The orbiter's maximum communications rate is 134 kilobits per second (the equivalent of about one black-and-white image per minute) with the high-gain antenna; there are other data rates, down to 10 bits per second, for transmitting engineering data when the Earth-spacecraft geometry makes communication difficult. The Galileo spacecraft acquires and can transmit a total of 1,418 engineering measurements (temperatures, voltages, computer states and counts, and the like). The spacecraft transmitters operate at S-band and X-band (2,295 and 8,415 megahertz) frequencies.

The high-gain antenna is a 4.8-meter (16-foot) umbrella-like wire-mesh reflector, designed to be unfurled after the first Earth flyby. Two low-gain antennas (one pointed forward and one aft, both mounted on the spinning section) support communications during the Earth-Venus-Earth leg of the flight and whenever the main antenna is not deployed and pointed at Earth.

Because the time delay in radio signals from Earth to Jupiter and back is more than one hour, 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 a few days or less.

These sequences operate through flight software installed in spacecraft computers in various subsystems and scientific instruments.

In the command and data subsystem 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 software has about 37,000 lines of code, including 5,500 lines devoted to fault protection.

Electrical power is provided to Galileo's equipment by two radioisotope thermoelectric generators. Heat produced by natural radioactive decay of plutonium is converted to approximately 500 watts of 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 measure turns and instrument pointing.

Instruments which 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 escape interference from the spacecraft;
- a plasma instrument detecting low-energy charged particles;
- a plasma-wave detector to study waves generated by the particles;
- a high-energy particle detector; and,
- a detector of cosmic and Jovian dust.

The spinning section also carries the Heavy Ion Counter, an engineering experiment added to assess the potentially hazardous charged-particle environments the spacecraft flies through, and an added 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 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 was designed to obtain images of Jupiter's satellites at resolutions from 20 to 1,000 times better than Voyager's best.

This section also carries a dish antenna to track the probe in Jupiter's atmosphere and pick up its signals for relay to Earth.

## Ground Systems

Galileo communicates with Earth via NASA's Deep Space Network. DSN has a complex of large antennas with receivers and transmitters located in the California desert, in Australia and in Spain, linked to a network control center at JPL in Pasadena, California. The spacecraft receives commands, sends science and engineering data, and is tracked by doppler and ranging measurements through this network. The German Space Operations Center and tracking station at Weilheim will also support Galileo cruise science activities. At JPL, mission controllers including about 275 scientists, engineers and technicians supported the mission at launch; nearly 400 will support Jupiter operations. Their responsibilities include commanding the spacecraft, interpreting the engineering and scientific data it sends in order to understand how it is performing and responding, and analyzing navigation data obtained by the Deep Space Network. The controllers use a set of complex computer programs to help them control the spacecraft and interpret the data.

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 make certain 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 three-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 regularly reports its status and health through an extensive set of engineering measurements. Interpreting these data into trends and averting or working around equipment failures is a major task for the Galileo flight team. Conclusions from this activity become an important input, along with scientific plans, to the sequence design process. This too 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, the position and velocity of the spacecraft 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 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.

## Jupiter's System

Jupiter is the largest and most rapidly rotating planet in the solar system. Its radius is more than 11 times Earth's, and its mass is 318 times that of our planet. It is made mostly of light elements, principally hydrogen and helium. Its atmosphere and clouds are deep and dense, and a significant amount of energy is emitted from its interior. 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. Atmospheric forms and dynamics were observed in increasing detail with the Pioneer and Voyager flyby spacecraft, and Earth-based infrared astronomers have recently studied the nature and vertical dynamics of deeper clouds.

Sixteen satellites are known. The four largest, discovered by the Italian scientist Galileo in 1610, are about the size of small planets. 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. 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-and-half ice and rock, with mostly ice or frost surfaces.

Of the other moons, eight orbit irregularly 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.

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 magnetic field 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.

## Management

The Galileo Project is managed for NASA's Office of Space Science and Applications by the Jet Propulsion Laboratory, a division of the California Institute of Technology. This responsibility includes designing, building, testing, operating and tracking Galileo. William J. O'Neil is project manager, Torrence V. Johnson is project scientist, Neal E. Ausman Jr. is mission director, and Matthew R. Landano is deputy mission director. The Federal Republic of Germany has furnished the orbiter's retro-propulsion module and some of the instruments and is participating in the scientific investigations. The radioisotope thermoelectric generators were designed and built by the General Electric Company for the U.S. Department of Energy.

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 Benny Chin and the probe scientist is Richard E. Young.

## Galileo Mission Events

Mission Event | Date
--- | ---
Launch: STS-34 Atlantis and IUS | October 18, 1989
First trajectory-change maneuver | November 9-11, 1989
Venus flyby (about 16,000 km altitude) | February 10, 1990
Venus data playback | November 19-21, 1990
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 | August 28, 1993
Probe release | July 13, 1995
Jupiter arrival (includes Io flyby at about 1000 km, Probe entry and relay, Jupiter orbit insertion) | December 7, 1995
Orbital tour of Galilean satellites Europa, Ganymede, Callisto | December '95-October '97
First Ganymede encounter | July 1996

## Spacecraft Characteristics

Characteristic | Orbiter | Probe
--- | --- | ---
Mass, kilograms (pounds) | 2,223 (4890) | 339 (746)
Usable propellant mass | 925 (2,035) | --
Height (in-flight) | 6.15 m (20.5 ft) | 86 cm (34 in.)
Instrument payload | 11 experiments | 6 experiments
Payload mass, kg (lb) | 118 (260) | 30 (66)
Electric power | RTGs, 570-470 watts | Lithium-sulfur battery, 730 wh

## Galileo Scientific Experiments

### Probe

Experiment/Instrument | Principal Investigator | Objectives
--- | --- | ---
Atmospheric Structure | Alvin Seiff, NASA Ames Research Center | Temperature, pressure, density, molecular weight profiles
Neutral Mass Spectrometer | Hasso Niemann, NASA Goddard Spaceflight Center | Chemical composition
Helium Abundance | Ulf von Zahn, Bonn University, FRG | Helium/hydrogen ratio
Nephelometer | Boris Ragent, NASA Ames Research Center | Clouds, solid/liquid particles
Net Flux Radiometer | Larry Sromovsky, University of Wisconsin | Thermal/solar energy
Lightning/Energetic | Louis Lanzerotti, Bell Particles Laboratory | Detect lightning, measure energetic particles

### Orbiter (Despun)

Experiment/Instrument | Principal Investigator | Objectives
--- | --- | ---
Solid-State Imaging Camera | Michael Belton, NOAO (Team Leader) | Galilean satellites at Camera 1-km resolution or better, other bodies correspondingly
Near-Infrared Mapping Spectrometer | Robert Carlson, Jet Propulsion Laboratory | Surface/atmospheric composition, thermal mapping
Ultraviolet Spectrometer | Charles Hord, University of Colorado | Atmospheric gases (includes aerosols, etc.), extreme UV sensor on spun section
Photopolarimeter Radiometer | James Hansen, Goddard Institute for Space Studies | Atmospheric particles, thermal/reflected radiation

### Orbiter (Spinning)

Experiment/Instrument | Principal Investigator | Objectives
--- | --- | ---
Magnetometer | Margaret Kivelson, UCLA | Strength and fluctuations of magnetic fields
Energetic Particles | Donald Williams, Johns Hopkins APL | Electrons, protons, heavy ions in atmosphere
Plasma | Lou Frank, University of Iowa | Composition, energy, distribution of ions
Plasma Wave | Donald Gurnett. University of Iowa | Electromagnetic waves and wave-particle interactions
Dust | Eberhard Grun, Max Planck Institute for Chemistry | Mass, velocity, charge of submicron particles
Radio Science: Celestial Mechanics | John Anderson, JPL (Team Leader) | Masses and motions of bodies from spacecraft tracking
Radio Science: Propagation | H. Taylor Howard, Stanford University | Satellite radii, atmospheric structure, from radio propagation

### Engineering Experiment

Experiment/Instrument | Principal Investigator | Objectives
--- | --- | ---
Heavy Ion Counter | Edward Stone, Caltech | Spacecraft charged-particle environment

## Interdisciplinary Investigators

- Fraser P. Fanale, University of Hawaii
- Peter Gierasch, Cornell University
- Donald M. Hunten, University of Arizona
- Andrew P. Ingersoll, California Institute of Technology
- David Morrison, NASA Ames Research Center
- Michael McElroy, Harvard University
- Glenn S. Orton, Jet Propulsion Laboratory
- Toby Owen, State University of New York
- James B. Pollack, NASA Ames Research Center
- Christopher T. Russell, University of California at Los Angeles
- Carl Sagan, Cornell University
- Gerald Schubert, University of California at Los Angeles
- James Van Allen, University of Iowa

Galileo to Jupiter

##### Calvin J. Hamilton