NASA's Galileo spacecraft continues to make dis-coveries about the giant planet Jupiter, its moons andits surrounding magnetic environment after more thanfive years in orbit around Jupiter. The mission wasnamed for the Italian Renaissance scientist GalileoGalilei, who dis-covered Jupiter'smajor moons in1610.

Mission OverviewGalileo's prima-

ry mission atJupiter began whenthe spacecraftentered into orbitaround Jupiter inDecember 1995,and its descentprobe, which hadbeen released fivemonths earlier,dove into the giantplanet's atmos-phere. The primarymission included a23-month, 11-orbittour of the Joviansystem, including10 close encoun-ters of Jupiter'smajor natural satellites, or moons.

Although the primary mission was completed inDecember 1997, the mission has been extended three

times to take advantage of the spacecraft's durabilitywith 24 more orbits. The extensions have enabledadditional encounters with all four of Jupiter's majormoons: Io, Europa, Ganymede and Callisto, as well asthe small moon Amalthea. Galileo will cross

Amalthea’s orbitalplane a second timebefore making amission-endingplunge into Jupiter'satmosphere inSeptember 2003.

Galileo was thefirst spacecraft everto measure Jupiter'satmosphere directlywith a descentprobe, and the firstto conduct long-term observations ofthe Jovian systemfrom orbit aroundJupiter. It found evi-dence for subsurfaceliquid layers of salt-water on Europa,Ganymede andCallisto, and it doc-umented extraordi-nary levels of vol-

canic activity on Io. During the interplanetary cruise,Galileo became the first spacecraft to fly by an aster-oid and the first to discover the moon of an asteroid.It was also the only direct observer as fragments from

Galileo Mission to Jupiter

NASA FactsNational Aeronautics andSpace AdministrationJet Propulsion LaboratoryCalifornia Institute of TechnologyPasadena, CA 91109

Ice rafts on Jupiter’s moon Europa, photographed by the Galileo spacecraft dur-ing a flyby February 20, 1997.

the Shoemaker-Levy 9 comet slammed into Jupiter inJuly 1994.

Launch. The Galileo spacecraft and its two-stageInertial Upper Stage (IUS) were carried into Earthorbit on October 18, 1989 by space shuttle Atlantis onmission STS-34. The solid-fuel upper stage thenaccelerated the spacecraft out of Earth orbit towardthe planet Venus for the first of three planetary flybys,or "gravity assists," designed to boost Galileo towardJupiter. In a gravity assist, the spacecraft flies closeenough to a planet to be propelled by its gravity, cre-ating a "slingshot" effect for the spacecraft. TheGalileo mission had originally been designed for adirect flight of about 3-1/2 years to Jupiter, using aplanetary three-stage IUS. When this vehicle wascanceled, plans were changed to use a liquid-fuelCentaur upper stage. Due to safety concerns after theChallenger accident, NASA cancelled use of theCentaur on the space shuttle, and Galileo was movedto the two-stage IUS; this, however, made it impossi-ble for the spacecraft to fly directly to Jupiter. Tosave the project, Galileo engineers designed a newand remarkable six-year interplanetary flight pathusing planetary gravity assists.

Venus and Earth flybys. After flying past Venusat an altitude of 16,000 kilometers (nearly 10,000miles) on February 10, 1990, the spacecraft swungpast Earth at an altitude of 960 kilometers (597 miles)on December 8, 1990. The spacecraft returned for asecond Earth swingby on December 8, 1992, at analtitude of 303 kilometers (188 miles). With this,Galileo left Earth for the third and final time andheaded toward Jupiter.

The flight path provided opportunities for scientif-ic observations. At Venus, scientists obtained the firstviews of mid-level clouds and gained new informa-tion about the atmosphere's dynamics. They alsomade many Earth observations, mapped the surface ofEarth's Moon, and observed its north polar regions.

Because of the modification in Galileo's trajecto-ry, the spacecraft was exposed to a hotter environ-ment than originally planned. To protect it from theSun, project engineers devised a set of sun shades andpointed the top of the spacecraft toward the Sun, withthe umbrella-like high-gain antenna stowed until wellafter the first Earth flyby in December 1990. Flightcontrollers stayed in touch with the spacecraft

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Low-gain antennaPlasma-wave antenna

Magnetometersensors

Sun shields

Extreme ultravioletspectrometer

Star scanner(at rear)

Thrusters

Radioisotopethermoelectricgenerators

Probe relayantenna

Dust counter (at rear)

Descentprobe

Scan platform:Ultraviolet spectrometerImaging cameraNear-infrared mapping spectrometerPhotopolarimeter

Spinning section of spacecraftNon-spinning section

Energetic-particle detector

through a pair of low-gain antennas, which send andreceive data at a much slower rate.

High-gain antenna problem. The spacecraft wasscheduled to deploy its 4.8-meter-diameter (16-foot)high-gain antenna in April 1991 as Galileo movedaway from the Sun and the risk of overheating ended.The antenna, however, failed to deploy fully.

A special team performed extensive tests anddetermined that a few (probably three) of the anten-na's 18 ribs were held by friction in the closed posi-tion. Despite exhaustive efforts to free the ribs, theantenna would not deploy. From 1993 to 1996,extensive new flight and ground software was devel-oped, and ground stations of NASA's Deep SpaceNetwork were enhanced in order to perform the mis-sion using the spacecraft's low-gain antennas.

Asteroid flybys. Galileo became the first space-craft ever to encounter an asteroid when it passedGaspra on October 29, 1991. It flew within just 1,601kilometers (1,000 miles) of the stony asteroid's centerat a relative speed of about 8 kilometers per second(18,000 miles per hour). Pictures and other datarevealed a cratered, complex, irregular body about 20by 12 by 11 kilometers (12.4 by 7.4 by 6.8 miles),with a thin covering of dirt-like "regolith" and a pos-sible magnetic field.

On August 28, 1993, Galileo flew by a secondasteroid, this time a larger, more distant asteroidnamed Ida. Ida is about 55 kilometers (34 miles) long,by 20 and 24 kilometers (12 by 15 miles). LikeGaspra, Ida may have a magnetic field. Scientistsmade a dramatic discovery when they found that Idaboasts its own moon, making it the first asteroidknown to have a natural satellite. The tiny moon,named Dactyl, has a diameter of only about 1.5 kilo-meters in diameter (less than a mile). Scientists stud-ied Dactyl's orbit in order to estimate Ida's density.

Comet event. The discovery of CometShoemaker-Levy 9 in March 1993 provided an excit-ing opportunity for Galileo's science teams and otherastronomers. The comet was breaking up as it orbitedJupiter and was headed to dive into the giant planet'satmosphere in July 1994. The Galileo spacecraft,approaching Jupiter, was the only observation plat-form with a direct view of the impact area on Jupiter'sfar side. Despite the uncertainty of the predictedimpact times, Galileo team members pre-programmed

the spacecraft's science instruments to collect dataand were able to obtain spectacular images of thecomet impacts.

Jupiter arrival. On July 13, 1995, Galileo'sdescent probe, which had been carried aboard the par-ent spacecraft, was released and began a five-monthfreefall toward Jupiter. The probe had no engine orthrusters, so its flight path was established by point-ing of the Galileo orbiter before the probe wasreleased. Two weeks later, Galileo used its 400-new-ton main rocket engine for the first time as it readjust-ed its flight path to arrive at the proper point atJupiter.

Arrival day on December 7, 1995, turned out tobe an extremely busy 24-hour period. When Galileofirst reached Jupiter and while the probe was stillapproaching the planet, the orbiter flew by two ofJupiter's major moons - Europa and Io. Galileo passedEuropa at an altitude of about 33,000 kilometers(20,000 miles), while the Io approach was at an alti-tude of about 900 kilometers (600 miles). About fourhours after leaving Io, the orbiter made its closestapproach to Jupiter, encountering 25 times more radi-ation than the level considered deadly for humans.

Descent probe. Eight minutes later, the orbiterstarted receiving data from the descent probe, whichslammed into the top of the Jovian atmosphere at acomet-like speed of 170,000 kilometers per hour(106,000 miles per hour). In the process the probewithstood temperatures twice as hot as a layer rough-ly corresponding to the Sun's surface. The probeslowed by aerodynamic braking for about two min-utes, then deployed its parachute and dropped its heatshield.

Like the other gas giant outer planets, Jupiter hasno solid surface. The wok-shaped probe kept trans-mitting data for nearly an hour as it parachuted downthrough Jupiter's atmosphere. Temperatures exceeding150 degrees Celsius (302 degrees Fahrenheit) eventu-ally caused electronics to fail at a depth correspond-ing to about 22 times sea-level air pressure on Earth,a pressure more than double what had been the mis-sion objective. That descent took the probe about 130kilometers (81 miles) below the level correspondingto Earth sea-level air pressure. As it descended, theprobe relayed information to the orbiter about sun-light and heat flux, pressure, temperature, winds,

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lightning and the composition of the atmosphere.

An hour after receiving the last transmission fromthe probe, at a point about 200,000 kilometers(130,000 miles) above the planet, the Galileo space-craft fired its main engine to brake into orbit aroundJupiter.

This first loop around Jupiter lasted about sevenmonths. Galileo fired its thrusters at its farthest pointin the orbit to keep it from coming so close to thegiant planet on later orbits. This adjustment prevent-ed possible damage to spacecraft sensors and comput-er chips from Jupiter's intense radiation environment.

During this first orbit, new software was installedto give the spacecraft extensive new onboard data-processing capabilities. It enabled data compression,permitting the spacecraft to transmit up to 10 timesthe number of pictures and other measurements thatwould have been possible otherwise.

In addition, hardware changes on the ground andadjustments to the spacecraft-to-Earth communicationsystem increased the average telemetry rate tenfold.Although the problem with the high-gain antenna pre-vented some of the mission's original objectives frombeing met, the great majority of them were. So manyobjectives were achieved that scientists feel Galileohas produced considerably more science than everenvisioned at the project's start 20 years ago. Datacompression features enabled the science of the mis-sion to be possible and was a significant achievement.

Orbital tour. During its primary mission orbitaltour, Galileo's itinerary included four flybys ofJupiter's moon Ganymede, three of Callisto and threeof Europa. These encounters were about 100 to 1,000times closer than those performed by NASA'sVoyager 1 and 2 spacecraft during their Jupiter flybysin 1979. During each flyby, Galileo's instrumentsscanned and scrutinized the surface and features ofeach moon. After about a week of intensive observa-tion, with its tape recorder full of data, the spacecraftspent the next one to two months in orbital "cruise,"sending to Earth data stored on the onboard taperecorder.

Mission extensions. Galileo's prime missionended in December 1997. Since then, three missionextensions have taken advantage of the spacecraft'sability to continue returning valuable scientific dis-

coveries.

The two-year Galileo Europa Mission used eightclose encounters with Europa to examine that moonintensively, then made four passes near Callisto andtwo near Io. This first extended mission added signifi-cantly to the evidence that a liquid ocean has existedand probably still exists beneath Europa's icy surface.The spacecraft came so close to Europa that it couldsee features the size of a schoolbus. Galileo alsoreturned information about thunderstorms in Jupiter'satmosphere during this extended mission. The space-craft is exposed to harsh radiation from Jupiter's radi-ation belts whenever it comes near the planet, so

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Close encounters by the Galileo orbiter

Orbit Target Date Altitude(Universal Time) (kilometers and miles)

0 Io Dec. 7, 1995 897 km (558 mi)1 Ganymede June 27, 1996 835 km (519 mi)2 Ganymede Sept. 6, 1996 261 km (162 mi)3 Callisto Nov. 4, 1996 1136 km (706 mi)4 Europa Dec. 19, 1996 692 km (430 mi)5 none6 Europa Feb. 20, 1997 586 km (364 mi)7 Ganymede April 5, 1997 3102 km (1928 mi)8 Ganymede May 7, 1997 1603 km (996 mi)9 Callisto June 25, 1997 418 km (260 mi)

10 Callisto Sept. 17, 1997 535 km (333 mi)11 Europa Nov. 6, 1997 2043 km (1270 mi)12 Europa Dec. 16, 1997 201 km (125 mi)13 none14 Europa March 29, 1998 1644 km (1022 mi)15 Europa May 31, 1998 2515 km (1562 mi)16 Europa July 21, 1998 1834 km (1140 mi)17 Europa Sept. 26, 1998 3582 km (2226 mi)18 Europa Nov. 22, 1998 2271 km (1411 mi)19 Europa Feb. 1, 1999 1439 km (894 mi)20 Callisto May 5, 1999 1321 km (821 mi)21 Callisto June 30, 1999 1048 km (651 mi)22 Callisto Aug. 14, 1999 2299 km (1429 mi)23 Callisto Sept. 16, 1999 1052 km (654 mi)24 Io Oct. 11, 1999 611 km (380 mi)25 Io Nov. 26, 1999 301 km (187 mi)26 Europa Jan. 3, 2000 351 km (218 mi)27 Io Feb. 22, 2000 198 km (123 mi)28 Ganymede May 20, 2000 809 km (502 mi)29 Ganymede Dec. 28, 2000 2338 km (1452 mi)30 Callisto May 25, 2001 138 km (86 mi)31 Io Aug. 6, 2001 194 km (120 mi)32 Io Oct. 16, 2001 184 km (114 mi)33 Io Jan. 17, 2002 102 km (63 mi)34 Amalthea Nov. 5, 2002 160 km (99 mi)

Planned Encounter35 Jupiter Sept. 21, 2003 impact

examination of Io, closest in of Jupiter's four majormoons, was saved until after the Europa flybys. AsGalileo approached Io, engineers worked through thenight to counteract radiation effects to the onboardcomputer. Galileo provided new insight into Io'sintense volcanic activity and captured images of anactively erupting fire fountain.

A second extended mission, the GalileoMillennium Mission, was initially approved through2000, then extended further to gain more discoveriesand send the spacecraft to a controlled impact intoJupiter's atmosphere in 2003. During the GalileoMillennium Mission, Galileo has made additionalclose flybys of all four of Jupiter's major moons. Amajor benefit to science was obtained by havingGalileo still functioning well when NASA's Saturn-bound Cassini spacecraft passed Jupiter in December2000. Observations and measurements by Galileo andCassini were coordinated to examine Jupiter's hugemagnetosphere and other parts of the Jupiter systemin ways that neither spacecraft could have done alone.During this time, Galileo became the first interplane-tary spacecraft to make an interstellar discovery,namely that the star cluster Delta Velorium was bina-ry, with one star periodically eclipsing the other. Thediscovery was made because the star scanner failed torecognize this standard star. Later, Galileo flybysover Io's north and south poles in 2001 provided mea-surements useful for determining whether that moongenerates its own magnetic field.

In November 2002, Galileo swung closer toJupiter than ever before, flying by the moonAmalthea, which is less than one-tenth the size of Ioand less than half as far from Jupiter. Scientists usedmeasurements of Galileo's radio signal to estimate themass and density of Amalthea. They are also studyingdata collected as Galileo flew through Jupiter's gos-samer ring to gain new understanding of the magneticforces and the energetic charged particles close to theplanet. Galileo sampled for the first time the inner-most regions of the magnetosphere and radiationbelts, a region equivalent to Earth’s Van Allen belts.

Galileo's final orbit is an elongated loop awayfrom Jupiter. In September 2003, the spacecraft willhead back for a direct impact and burn up as it plowsinto Jupiter's dense atmosphere.

SpacecraftOrbiter. The Galileo orbiter weighed 2,223 kilo-

grams (2-1/2 tons) at launch and measured 5.3 meters(17 feet) from the top of the low-gain antenna to thebottom of the descent probe. The orbiter features aninnovative "dual-spin" design. Most spacecraft arestabilized in flight either by spinning around a majoraxis, or by maintaining a fixed orientation in space,referenced to the Sun and another star. As the firstdual-spin planetary spacecraft, Galileo combinesthese techniques. A spinning section rotates at about 3rpm, while a non-spinning section provides a fixedorientation for cameras and other remote sensors. Astar scanner on the spinning side determines orienta-tion and spin rate; gyroscopes on the non-spinningside provide the basis for measuring turns and point-ing instruments.

The power supply, propulsion module and most ofthe computers and control electronics are mounted onthe spinning section. Two low-gain antennas mountedon the spinning section supported communicationsduring the Earth-Venus-Earth leg of the flight. Onewas pointed upward toward the Sun, with the otherwas mounted on a deployable arm to point down. Theupward-pointing antenna is currently carrying thecommunications load, while the lower antenna hasbeen re-stowed and there are no plans to use it again.

The Galileo orbiter spacecraft carries 11 scientificinstruments. Another seven were on the descentprobe.

The orbiter's spinning section carries instrumentsto study magnetic fields and charged particles. Theinstruments include magnetometer sensors mountedon an 11-meter-long (36-foot) boom to minimizeinterference from the spacecraft's electronics, a plas-ma instrument to detect low- and medium-energycharged particles, and a plasma-wave detector tostudy electromagnetic waves generated by the parti-cles. There are also a high-energy particle detector, adetector of cosmic and Jovian dust, an extreme ultra-violet detector associated with the ultraviolet spec-trometer, and a heavy ion counter to assess potentiallyhazardous charged-particle environments the space-craft flies through.

Galileo's non-spinning section carries instrumentssuch as cameras that need to be held steady. Theseinstruments include the camera system; the near-

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infrared mapping spectrometer to make multispectralimages for atmosphere and surface chemical analysis;the ultraviolet spectrometer to study gases; and thephotopolarimeter-radiometer to measure radiant andreflected energy. The camera system obtains imagesof Jupiter's satellites at resolutions from 20 to 1,000times better than the best possible from NASA'sVoyager spacecraft; its charge-coupled-device sensoris much more sensitive than previous spacecraft cam-eras and is able to detect a broader color band.Galileo's non-spinning section also carries a dishantenna that picked up the descent probe's signalsduring its fall into Jupiter's atmosphere.

The spacecraft's propulsion module consists oftwelve 10-newton thrusters and a single 400-newtonengine, which use monomethyl-hydrazine as fuel andnitrogen tetroxide as the oxidizer. The propulsion sys-tem was developed and built by Messerschmitt-Bolkow-Blohm and provided by the Federal Republicof Germany as NASA's major international partner onGalileo.

Because radio signals take on the order of 45 min-utes (plus or minus a few minutes depending onwhere the spacecraft is relative to Earth) to travelfrom Earth to Jupiter and back, the Galileo spacecraftwas designed to operate from computer instructionssent to it in advance and stored in spacecraft memory.A single master sequence of commands can cover aperiod ranging from weeks to months of quiet opera-tions between flybys of Jupiter's moons. During busyencounter operations, one sequence of commandscovers only about a week. Galileo’s internal proces-sor was an old rebuilt version of the RCA 1802 chip,similar to those used to run the old PacMan computergames of the 1980’s. The flight team designed a spe-cial form of the software language called ‘machinelanguage’ in order to change its functionality overtime. After subtracting the power needed to runessential spacecraft systems, the power left for theexperiments was on the order of 60 watts (enough topower a light bulb).

The flight software includes fault-protectionsequences designed to automatically put Galileo in asafe state in case of computer glitches or otherunforeseen circumstance. Electrical power is provid-ed by two radioisotope thermoelectric generators.Heat produced by the natural radioactive decay of

plutonium is converted to electricity (570 watts atlaunch, 432 in Galileo's final days in 2003) to operatethe orbiter's equipment. This is the same type ofpower source used on other NASA missions includingViking to Mars, Voyager and Pioneer to the outerplanets, and Cassini to Saturn.

Descent probe. Galileo's descent probe had amass of 339 kilograms (750 pounds). It was slowedand protected by a deceleration module consisting ofan aeroshell and an aft cover designed to block heatgenerated by friction during atmospheric entry. Insidethe aeroshells were a descent module and its 2.5-meter (8-foot) parachute. The descent module carrieda radio transmitter and seven scientific instruments.These were devices to measure temperature, pressureand deceleration, atmospheric composition, clouds,particles, and light and radio emissions from lightningand energetic particles in Jupiter's radiation belts.

Science ResultsAmong the Galileo mission's key science findings

since reaching Jupiter are the following:

� The descent probe measured atmospheric ele-ments and found that their relative abundances weredifferent than on the Sun, indicating Jupiter’s evolu-tion after the planet formed out of the solar nebula.

� Jupiter's atmosphere has numerous, large thun-derstorms concentrated in specific zones above andbelow the equator, where winds are highly turbulent.Although individual lightning strokes appear less fre-quently than on Earth, they are up to 1,000 timesmore powerful than terrestrial lightning. Clouds ofammonia, the first ever detected in a planet's atmos-phere, form only from material brought up from thelower regions of the atmosphere, that is in 'fresh'clouds. It is still a mystery why only fresh clouds,covering about 1 percent of Jupiter, show the ammo-nia signature while the remaining clouds, which prob-ably also contain ammonia, lose their sharp spectralsignature.

� Evidence supports a theory that liquid oceansexist under Europa's icy surface. There are placeswhere recognizable features that once were wholehave been separated from each other by new, smoothice. These areas indicate that when the older featureswere separated, they floated for a time in liquid water,much as icebergs float in Earth's polar regions.

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Scientists believe the water later froze solid, creating"rafts" of ice, similar to some seen on a smaller scaleat Earth's polar regions. There are also indications ofvolcanic ice flows, with liquid water flowing acrossthe surface of Europa. These discoveries are particu-larly intriguing, since liquid water is a key ingredientin the process that may lead to the formation of life.Europa is crisscrossed by faults and ridges, withregions where large pieces of crust have separatedand shifted. Faults are breaks in the moon's outercrust where the crust on either side has shifted.Ridges indicate that sections of the crust have alsomoved.

� Galileo magnetic data provide evidence notonly that Europa still has a liquid-saltwater layerunder its ice, but that such layers also exist, fartherbelow the surface, on Ganymede and Callisto.Magnetometer readings show that magnetic fieldsaround those moons vary in ways indicating the pres-ence of electrically conducting layers -- possiblyoceans -- under the surface.

� Europa, Io and Ganymede all have metalliccores. Processes on these three inner moons havepermitted denser elements to separate out and sink tothe moons' respective centers. On the other hand, thecomposition of the more distant moon Callisto is fair-ly uniform throughout, indicating it did not follow thesame evolutionary path as the other three moons.

� Europa, Ganymede and Callisto each provideevidence of having a surface-bound exosphere, theterm for a thin atmospheric layer composed of ions--electrically charged gases, and neutral gases sur-rounding the moon, and loosely bound to the surface.The presence of these gases proves the efficacy of aprocess previously well-known only in the laboratorycalled 'sputtering'. It refers to collisions of chargedparticles from the magnetosphere with the icy surfacethat release water vapor and other molecules.

� Ganymede generates a magnetic field, just asEarth does. In fact, Ganymede is the first moon ofany planet known to possess an intrinsic magneticfield, though other of Jupiter's moons have secondarymagnetic fields induced by the strong magnetic fieldof Jupiter itself. Ganymede's magnetosphere - thesmall magnetic "bubble" created by its field withinthe surrounding, more powerful, magnetic field ofJupiter - is actually somewhat larger than Mercury's, a

planet of similar size. There even appears to betrapped radiation in a miniature radiation belt similarto Earth's Van Allen radiation belts on magnetic fieldlines close to Ganymede. The discovery of a magnet-ic field within Ganymede challenges theoretical mod-els of how planetary magnetic fields arose on Earthand other celestial bodies.

� Galileo made observations of satellite surfacesthat revealed, among many phenomena, the follow-ing: patterns of long, looping cracks that extend forhundreds of miles over the surface of Europa, causedby successive warming and fissuring of the surface; astrikeslip fault on Europa as long as the Californiasegment of the San Andreas fault; a surface onGanymede formed by high tectonic activity, withfaulting and fracturing; evidence of extensive thoughstill mysterious erosion of Callisto's primordial sur-face that creates a smoother appearance for some ofthe craters found there.

� Io's extensive volcanic activity may be 100times greater than that found on Earth's. It is continu-ally modifying its surface. Many changes have beenrecorded since Voyager's visit in 1979, and othermajor changes have been seen during the course ofthe Galileo mission. During one four-month interval,an area the size of the state of Arizona was blanketedby volcanic debris thrown out of the volcano Pillan.Imaging and spectral analyses have shown that mostof the eruptions on Io must be composed of liquid sil-icate rock, which contain silicon-oxygen compounds.The temperatures of these lavas are too high for othermaterials, such as sulfur, and in fact are even signifi-cantly hotter than most eruptions on Earth today. Thecomposition of these hot lavas may be more similar toa type of volcanism that occurred on the Earth morethan 3 billion years ago.

� Galileo characterized the complex plasma envi-ronment of Io that is derived from the volcanic activi-ty. The spacecraft showed: that plasma (a fluid-likegas composed solely of charged particles) flowing inthe donut-shaped torus of Io, slows in until it stalls inthe polar regions of Io, and that the plasma performsin a similar fashion in the wake created as plasmafrom Jupiter flows around Io. The mission measuredthe intense currents generated by the interaction of theplasma with Io and its neutral particle cloud, discov-ered that energetic electrons bounce along the field

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lines that connect Io with the charged particle regionof Jupiter's atmosphere. Galileo also identified theprocess through which material lost from Io's sur-roundings is transported outward from Jupiter througha mechanism previously only theorized, namely theinterchange of structures known as ' magnetic fluxtubes' that carry low density plasma in from large dis-tances.

� Jupiter's ring system is formed by dust kickedup as interplanetary meteoroids smash into the plan-et's four small inner moons. The outermost ring isactually two rings, one embedded within the other.

� Galileo was the first spacecraft to dwell in agiant planet magnetosphere long enough to identifyits global structure and to investigate its dynamics.Despite some superficial similarities of behavior,Jupiter exhibits dynamics that differ from those famil-iar from Earth's magnetosphere. Unlike Earth, wherethe aurora are generated as Earth and the Sun interact,it has become clear that Jupiter's main auroral ovallinks to those regions of the magnetosphere (the mid-dle-magnetosphere) dominated by the powerfuleffects of Jupiter's rotation. Jupiter exhibits strongdawn-to-dusk asymmetries in the flow of plasma thatare not found in Earth's magnetosphere. The sub-storm process, a terrestrial phenomena that is a resultof the interaction of Earth and the Sun, may occur inJupiter's magnetosphere, with consequences that dif-fer greatly from those familiar at Earth.

Orbiter Scientific ExperimentsThe following is a list of the science instruments

on each part of the spacecraft, with the name of theprincipal investigator responsible for the instrumentand notes on the experiment's main object of study.

Remote sensing instruments on non-spinningsection:

� Camera - Dr. Michael Belton, NationalOptical Astronomy Observatories. Galilean satellites,high resolution, atmospheric small-scale dynamics.

� Near-infrared mapping spectrometer - Dr.Robert Carlson, Jet Propulsion Laboratory. Surface,atmospheric composition thermal mapping.

� Photopolarimeter-radiometer - Dr. JamesHansen, Goddard Institute for Space Studies.Atmospheric particles, thermal/reflected radiation.

� Ultraviolet spectrometer/extreme ultravioletexplorer - Dr. Ian Stewart, University of Colorado.Atmospheric gases, aerosols.

Instruments studying magnetic fields andcharged particles, located on spinning section:

� Magnetometer - Dr. Margaret Kivelson,University of California, Los Angeles. Strength andfluctuations of magnetic fields.

� Energetic particle detector - Dr. DonaldWilliams, Johns Hopkins Applied Physics Laboratory.Distribution of electrons, protons and ions of highenergy.

� Plasma investigation - Dr. Lou Frank,University of Iowa. Composition, energy, distributionof ions and electrons of median and low energy.

� Plasma wave subsystem - Dr. Donald Gurnett,University of Iowa. Electromagnetic waves andwave-particle interactions.

� Dust-detection subsystem - Dr. HaraldKrueger, Max Planck Institut für Kernphysik. Mass,velocity, charge of particles smaller than a micrometerin size.

Engineering Experiment:

� Heavy ion counter - Dr. Edward Stone,California Institute of Technology. Spacecraft'scharged-particle environment. Energy distribution ofheavy ions.

Radio Science:

� Celestial mechanics - Dr. John Anderson, JetPropulsion Laboratory. Masses and internal struc-tures of bodies from spacecraft tracking.

� Propagation - Dr. H. Taylor Howard, StanfordUniversity. Size and atmospheric structure ofJupiter's moons from radio propagation.

Scientific Experiments on Descent Probe� Atmospheric structure - Dr. Alvin Seiff, San

Jose State University Foundation. Temperature, pres-sure, density, molecular weight profiles.

� Neutral mass spectrometer - Dr. HassoNiemann, NASA Goddard Space Flight Center.Chemical composition.

� Helium abundance - Dr. Ulf von Zahn,

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Institut für Atmospharenphysik, Universitat Rostock.Helium/hydrogen ratio.

� Nephelometer - Dr. Boris Ragent, San JoseState University Foundation. Clouds and particles.

� Net flux radiometer - Dr. Larry Sromovsky,University of Wisconsin. Thermal/solar energy pro-files.

� Lightning and radio emissions/energeticparticles - Dr. Louis Lanzerotti, Bell Laboratories,and Dr. Klaus Rinnert, Max Planck-Institut fürAeronomie; Harald Fischer, Institut für Reine undAngewandte Kernphysik, Universitat Kiel. Lightningdetection, energetic particles.

� Doppler wind experiment - Dr. DavidAtkinson, University of Idaho. Measure winds, learntheir energy source.

Ground SystemGalileo communicates with Earth via NASA's

Deep Space Network, a worldwide system of largeantenna complexes with receivers and transmitterslocated in Australia, Spain and California's MojaveDesert, linked to a network control center at the JetPropulsion Laboratory in Pasadena, Calif. Throughthis network, the spacecraft receives commands,sends science and engineering data, and is tracked byDoppler and ranging measurements.

Doppler measurements detect changes in the fre-quency of the spacecraft's radio signal that reveal howfast the spacecraft is moving toward or away fromEarth. In ranging measurements, radio signals trans-mitted from Earth are marked with a code which isreturned by the spacecraft, enabling ground con-trollers to keep track of the distance to the spacecraft.

ManagementThe Galileo project is managed for NASA's

Office of Space Science, Washington, D.C., by the JetPropulsion Laboratory, Pasadena, a division of theCalifornia Institute of Technology. JPL designed andbuilt the Galileo orbiter, and operates the mission.

At NASA Headquarters, Dr. Barry Geldzahler isGalileo program manager and Dr. Denis Bogan isprogram scientist.

At JPL, the position of project manager has beenheld successively by John Casani, Richard Spehalski,

Bill O'Neil, Bob Mitchell, Jim Erickson, Dr. EileneTheilig and, currently, Dr. Claudia J. Alexander. Dr.Torrence V. Johnson is project scientist.

NASA's Ames Research Center, Moffett Field,Calif., managed the descent probe, which was built byHughes Aircraft Co, El Segundo, Calif. The positionof probe manager was held successively by JoelSperans, Benny Chinn and Marcie Smith. The probescientist is Dr. Richard E. Young.

Galileo's experiments are being carried out bymore than 100 scientists from the United States, GreatBritain, Germany, France, Canada and Sweden.

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