Reference Work Entry

Encyclopedia of Planetary Science

Part of the series Encyclopedia of Earth Science pp 258-263

Galileo mission

  • Karen Buxbaum
The Galileo spacecraft is in orbit about the planet Jupiter, to conduct an intensive and comprehensive investigation of the Jovian system using an atmospheric entry probe and a Jupiter orbiter (Figure G1). Galileo is instrumented to achieve much more than was possible with the Voyager (q.v.) flyby missions. The Galileo Probe has obtained the first in situ measurements of the Jupiter atmosphere, and the Galileo Orbiter is providing the first long-term close observations of the Jupiter system. The Orbiter encounters the Galilean satellites repeatedly and typically 100 times closer than Voyager did.
Figure G1

Galileo spacecraft configuration.

Jupiter, of all the planets, asteroids, comets and satellites, holds a prime key to understanding the evolution of the solar system. Essential to exploring the history of the solar system are measurements of elemental abundances — the chemical make-up of a variety of objects. Because it retains its primordial composition, Jupiter is a better cosmological ‘laboratory’ than any of the other planets.

The Jovian system is also a solar system in miniature, with massive gaseous Jupiter, circled by a retinue of satellites, all enveloped in an intense magnetosphere. Each of the four major satellites — Io, Europa, Ganymede and Callisto — shows a different level of geological activity, and each has followed a different evolutionary path.

The Galileo mission has three major and coequal general scientific objectives, namely to investigate (1) the chemical composition and physical state of Jupiter's atmosphere, (2) the chemical composition and physical state of the Jovian satellites, and (3) the structure and physical dynamics of the Jovian magnetosphere.

The Galileo Orbiter carried the probe to Jupiter, releasing it on its ballistic trajectory about 150 days before arrival at Jupiter. Probe release took place on 13 July 1995, over 80 million km from Jupiter. The Probe had no control system and was entirely passive during the 5 months from release to Jupiter arrival. The orbiter's configuration at the time of release resulted in the Probe being spin-stabilized at 10 rpm to achieve entry at zero angle of attack within a tolerance of ±6.0°.

After Probe release, the orbiter performed trajectory corrections in order to (1) overfly the probe during entry in order to record probe data for later relay to the Earth and (2) be in position for Jupiter orbit insertion (JOI). This Orbiter deflection maneuver used the orbiter's main engine for the first time in flight.

The Orbiter's arrival at Jupiter on 7 December 1995 satisfied the many competing mission objectives for arrival day. On 7 December the Orbiter was to be in position over the entry site to receive the uplink signal from the probe. The arrival geometry for both the Orbiter and the Probe is shown in Figure G2.
Figure G2

Jupiter arrival (lo flyby, probe relay and Jupiter orbit insertion): 7 December 1995.

The Probe entered Jupiter's atmosphere with a velocity of about 47 km s−1, at a location just above the Jovian equator, at latitude approximately 7° N at 22:04 UT on 7 December. The entry and descent events are depicted in altitude/time space on Figure G3 Following the Probe's descent mission of 61.4 min duration, the Orbiter began a 7-month orbit about Jupiter. For the subsequent 23 months, near each periapsis passage, the Orbiter executes a close encounter with one of Jupiter's Galilean satellites, using a gravity assist from the encounter to change its orbit in order to achieve the desired satellite encounter near the next periapsis. This satellite gravity-assist tour is the foundation of the Orbiter mission design.
Figure G3

Predicted probe entry/descent events.

The Probe investigations and instruments are identified in Table G1. A comprehensive description of these investigations is given in Yeates et al. (1985) and Russell (1992). About 40 min after entry, the probe penetrated to a depth corresponding to a pressure of greater than 10 bars, below the lowest cloud layers, in the well-mixed region of the Jupiter atmosphere where the bulk composition was measured. A 46-min engine burn subsequently placed the Orbiter in a 7-month orbit about Jupiter. During the first phase of this orbit the Orbiter returned the probe data back to Earth.

Table G1

Probe scientific payload


Mass (kg)



Atmospheric Structure Instrument (ASI)


Temperature: 0–540 K Pressure: 0–28 bar

Determine temperature, pressure, density and molecular weight as a function of altitude

Neutral Mass Spectrometer (NMS)


Covers 1–150 AMU

Determine chemical composition of atmosphere

Helium Abundance Detector (HAD)


Accuracy: 0.1%

Determine relative abundance of helium

Nephelometer (NEP)


0.2–20 μm particles, as few as 3 per cm3

Detect clouds and infer states of particles (liquid versus solid)

Net-Flux Radiometer (NFR)


6 infrared filters from 0.3 to > 100 μm

Determine ambient thermal and solar energy as a function of altitude

Lightning and Energetic Particles (LRD/EPI)


Fisheye lens sensors; 1 Hz-100 kHz

Verify the existence of lightning and measure energetic particles in inner magnetosphere

The Neutral Mass Spectrometer detected smaller than expected amounts of several constituents, including oxygen, water and neon. Some elements are more abundant than was expected. These include carbon, nitrogen and sulfur. Since the amounts of these elements are larger than found for the Sun, these results may contain clues concerning the process of planetary formation. The Probe entered an unusually hot and dry area of the Jupiter atmosphere, and as a result only a few misty clouds were detected. The Probe determined that the high winds present in the upper atmosphere extend to considerable depths. This finding eliminated the theory that the winds were driven by energy from the Sun; instead these > 500 km h−1 winds must be driven by heat from within Jupiter.

The Io encounter was originally introduced as a mission element to provide the gravity assist to reduce JOI propellant cost. Galileo's Io encounter at 1000 km altitude was 20 times closer than Voyager's and presented a unique scientific opportunity. However, the observations planned for this Io encounter were cancelled due to uncertainties in the performance of the onboard tape recorder.

On each successive orbit of Jupiter, the Orbiter is precisely navigated to a close encounter of one of the three outer Galilean satellites — Europa, Ganymede or Callisto. The prime mission tour design precludes a return to Io because of the radiation hazard (see Jupiter:​ magnetic field and magnetosphere). For each encounter the satellite flyby aiming point is selected to result in the satellite gravity assist that will change Galileo's orbit to the next desired one. While the Orbiter's remote sensing (or telescopic) instruments observe primarily at the satellite encounters, and image Jupiter when inside 50 R J, the fields and particles instruments gather data continuously during much of the Orbiter mission. The Orbiter scientific investigations and instruments are identified in Table G2. A comprehensive discussion of these is given in Russell (1992).
Table G2

Orbiter scientific payload


Mass (kg)



Solid-state imaging (SSI)


1500 mm, f/8.5 800 × 800 CCD, 8 filters 0.47° field of view

Map Galilean satellites at roughly 1 km resolution and monitor atmospheric circulation over 23 months while in orbit around planet

Near-Infrared mapping spectrometer (NIMS)


0.7–5.2 μm range, 0.03 μm resolution

Observe Jupiter and its satellites in the infrared to study satellite surface composition, Jovian atmospheric composition and temperature

Ultraviolet spectrometer (UVS)


1150–4300 Å

Measure gases and aerosols in Jovian atmosphere

Extreme ultraviolet spectrometer (EUV)


0.05–0.14 μm

Measure Io plasma torus temperature, scale height and composition; monitor aurora

Photopolarimeter—Radiometer (PPR)


Discrete visible and near-infrared bands, radiometry to > 42 μm

Determine distribution and character of atmospheric particles; compare flux of thermal radiation to incoming solar levels

Magnetometer (MAG)


32–16 384 gammas

Monitor magnetic field fore strength and changes

Heavy ion counter (HIC)


10 MeV–200 MeV

Monitor highly ionizing energetic particles

Energetic particle detector (EPD)


Ions: 0.020–55 MeV Electrons: 0.015–11 MeV

Measure high-energy electrons, protons and heavy ions in and around Jovian magnetosphere

Plasma detector (PLS)


1 eV–50 keV in 64 bands

Assess composition, energy and three-dimensional distribution of low-energy electrons and ions

Plasma wave (PWS)


6–31 Hz, 50 Hz-200 kHz, 0.1–5.65 MHz

Detect electromagnetic waves and analyze wave—particle interactions

Dust detector (DDS)


10−16−10−6 g, 2–50 km s−1

Measure particle mass, velocity and charge

Radio science (RS); Celestial mechanics


S-band signals

Determine mass of Jupiter and its satellites (uses radio system and low-gain antenna)

Radio science (RS): Propagation


S-band signals

Measure Jovian atmospheric structure and body radii (uses radio system and low-gain antenna)

The satellite tour shown in Figure G4 was chosen after a long tour design and selection process. It represents an intricately constructed compromise between the competing scientific objectives of the Orbiter instruments (O'Neil et al., 1992).
Figure G4

Galileo tour of Jovian satellites (R J = 71 398 km).

Historical Development


Galileo was launched on 18 October 1989 aboard the shuttle Atlantis (STS-34, OV-104). The inertial upper stage (IUS-19) placed Galileo on its Earth-to-Venus trajectory. Following the IUS burns Galileo configured itself for solo flight and separated from the IUS on 19 October. All aspects of the launch were essentially perfect.

The Galileo Venus—Earth—Earth gravity assist (VEEGA) trajectory is illustrated in Figure G5 and is discussed in detail in D'Amario t al. (1989). The Venus gravity assist in February of 1990 was the first of three gravity assists designed to put Galileo into its transfer orbit to Jupiter. The second two gravity assists were with the Earth, both on 8 December in 1990 and 1992.
Figure G5

The Galileo VEEGA trajectory to Jupiter.

Venus gravity assist

The 10 February 1990 Venus gravity assist provided Galileo with its first target of opportunity. All of the orbiter's science instruments gathered data during the flyby. With its magnetospheric instruments, energetic particles were detected, bowshock crossings were indicated, and the plasma wave instrument saw evidence of lightning discharges. With the remote sensing instruments, Venus was observed over many days, providing new scientific insight as well as early indications of the health and performance of Galileo's scan platform instruments. Because the high-gain antenna had to remain furled under its sunshade until Galileo traveled beyond Earth's orbit, all the Venus science had to be recorded. The vast majority of the Venus data were not received on Earth until mid-November 1990, when the Galileo-to-Earth communication range became short enough that science data rates could be achieved over one of Galileo's low-gain antennas. Results from the Galileo Venus encounter were presented in a special issue of the journal Science (1991, 253, 1515–50).

Earth gravity assist 1

The first Earth gravity assist required that Galileo fly virtually up the Earth's magnetotail (see Earth:​ magnetic field and magnetosphere). This provided unique measurements of the magnetotail. The fields and particles instruments provided nearly continuous measurements from 30 days before Earth encounter to 8 days after. From the day of closest approach and continuing for 7 days, periodic Earth and Moon observations were made. Beginning 2 1/2 days after the flyby, Galileo photographed the Earth every minute for 25 h. These images were processed at the Jet Propulsion Laboratory to produce a color movie of the Earth making one full rotation on its axis. Excellent images were also obtained revealing part of the lunar farside not visible from the Earth. Multispectral global maps of lunar compositional units were also obtained. Nearly 3000 imaging frames were taken during the Earth—Moon encounter.

High-gain antenna deployment anomaly

Because of the VEEGA trajectory, which took Galileo into the inner part of the solar system, it was necessary to keep Galileo's high-gain antenna (HGA) furled under its tip shade for thermal protection. HGA unfurling (deployment) was scheduled for the earliest thermally acceptable time, 11 April 1991.

As seen in Figure G5, the antenna is analogous to an inverted umbrella. In a normal deployment the ribs unfurl symmetrically, in about 3 min. Analysis indicates that during Galileo's HGA deployment attempt, three adjacent HGA ribs stuck to the central tower, causing the deployment mechanism to stall and leaving the HGA in a partially and asymmetrically deployed configuration. This conclusion is based on extensive analysis of the flight telemetry, computer modeling and testing of the flight spare HGA at the Jet Propulsion Laboratory. The configuration of the antenna subsequent to the initial deployment attempt as estimated by the anomaly investigation team is illustrated in Figure G6 (O'Neil, 1991; O'Neil, 1993).
Figure G6

Sun gate obscuration by rib no. 2.

From mid-1991 until early 1993 thermal cycling as well as dynamic excitation of the antenna system were carried out in an attempt to free the antenna. None of the activities freed any of the stuck ribs. In 1993 work began to modify ground and flight software and to adapt operating plans to fly the Jupiter mission with the low-gain antenna 1 for telemetry, tracking and commands.

The full Galileo Atmospheric Entry Probe mission and the Orbiter's insertion into Jupiter orbit were accomplished without the HGA. Tracking, telemetry and command will continue over the low-gain antenna (LGA-1), albeit at low telemetry rates. The orbiter playback of the probe data took place in the first few months of the 7-month initial orbit at Jupiter. The orbiter's computer systems were loaded with new flight software which enables the satellite tour to be conducted using the LGA-1 and the on-board tape recorder for collection of high-priority science data (O'Neil et al., 1992).

Gaspra encounter

On 29 October 1991 (at 22:37:01 UTC) Galileo became the first spacecraft to visit an asteroid. Galileo flew by Gaspra (q.v.) at a distance of 1600 km. Gaspra is a main-belt, S-type (silicate: olivinerich) asteroid. The greatest challenge of the Gaspra encounter arose from the limited accuracy of its ground-based ephemeris. Its position was uncertain by hundreds of kilometers, while Galileo's camera field of view for the highest resolution images is only tens of kilometers. Originally, the HGA was to be used to transmit the optical navigation pictures to Earth in real time. Due to the HGA anomaly, only four frames were taken, recorded on the on-board tape recorder, and then played back to Earth over the LGA-1. As planned, all of the orbiter science instruments, except the Heavy Ion Counter, collected data. In November 1991 Galileo returned the first-ever picture of an asteroid. All the Gaspra data were returned to Earth over the low-gain antenna on approach to the second Earth gravity assist (8 December 1992; see Gaspra).

Earth gravity assist 2

The second Earth flyby was particularly interesting because it provided. Galileo with unprecedented observations of the north polar region of the Moon. Infrared spectroscopy with the Galileo Near Infrared Mapping Spectrometer (NIMS) was very successful. A magnetospheric survey was also made at the second flyby, providing a view of the Earth's magnetosphere complementary to that which other spacecraft have been able to provide. In response to the possibility that the HGA might never be successfully deployed, significant observing time was also spent exploiting the high data rates available over the LGA-1 to obtain comprehensive calibrations of the science instruments before returning to the lower data rates characteristic of the LGA-1 when Galileo is not near the Earth.

Earth—Jupiter cruise

On the direct Earth-to-Jupiter leg, Galileo encountered a second asteroid. Galileo flew by Ida on 28 August 1993. The observation strategy was based on the results of the Gaspra encounter. Unlike the Venus and Gaspra encounters, there was no opportunity to achieve high telemetry rates over LGA-1 any time after the Ida encounter in 1993. Ida data were returned during 1993 and 1994 when telecommunication capability enabled playback of the onboard tape recorder. Since the enhanced flight software was not available until after Jupiter arrival, then-current spacecraft capabilities were used. Therefore, only the very highest priority data from the Ida encounter were returned to Earth. Galileo successfully imaged Ida in 1993 and made the first discovery of a Moon orbiting an asteroid (see Ida).

During Earth—Jupiter cruise, Galileo continued to return periodic data from the Magnetometer, the Dust Detector and the Extreme Ultraviolet Spectrometer. Radio science investigations also continued, including a multi-spacecraft search for gravity waves.

In July 1994, one and a half years before Jupiter arrival, the Galileo instruments were trained on Jupiter to capture data during the impacts of the’ string of pearls’ comet Shoemaker-Levy 9 (see Comet:​ impacts on Jupiter). Unlike Earth-based telescopes, Galileo could see the impact sites directly. Galileo observations helped determine the precise times of the impacts. Galileo observations also helped determine the sizes and temperatures of the impact fireballs, providing uniquely valuable information on these spectacular events.

In summer of 1995, Galileo flew through a series of interplanetary dust storms on its way to Jupiter. The dust particles, finer than particles in a cloud of smoke, apparently originate within the Jovian system. They may be a product of volcanic eruptions on the moon Io, or they may originate within the planet's rings. They must first be electrically charged, and then they may be accelerated by Jupiter's powerful magnetic field. The velocities of the particles may be as high as 200 km s−1.

The Galileo mission is remarkable for the resilience and ingenuity of the mission team. The failure of the HGA and problems with other mechanical subsystems of the spacecraft have been mitigated, if not totally overcome, by remarkable efforts by the mission team. Galileo is the only spacecraft in history to receive a completely new version of the flight software used by its central computing system while in flight to the primary mission objective.

Galileo's rendezvous with Jupiter is providing detailed new information about the Jovian system and the evolution of our solar system.

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© Chapman & Hall 1997
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