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By Marty McDowell/NASA

On October 18, 1989, the United States launched Galileo. The Galileo spacecraft was designed to study Jupiter's atmosphere, satellites, and surrounding magnetosphere. The spacecraft was named in honor of Galileo Galilei, the Italian Renaissance scientist who discovered Jupiter's major moons in 1610. The spacecraft is currently orbiting Jupiter and performing an extended study of the planet's moons.

The Galileo mission consists of two spacecraft: an orbiter and an atmospheric probe. Launched during the STS 34 flight of the Atlantis orbiter, the two spacecraft were kicked out of Earth orbit by an inertial upper stage (IUS) rocket, sending them careening through the inner solar system. The trajectory which the spacecraft followed was called a VEEGA (Venus-Earth-Earth Gravity Assist), traveling first in toward the Sun for a gravity assist from Venus before encountering the Earth two times (spaced two years apart). These encounters with Venus and the Earth allowed Galileo to gain enough velocity to get it out to Jupiter.


False color image of the Minos Linea region on icy and fractured surface of Europa.

Image courtesy of NASA.


During the flybys of Venus and the Earth, Galileo scientists took the opportunity to study these two planets as well as the Moon, making some unprecedented observations as a result. In addition, following each Earth flyby, Galileo made excursions as far out in the solar system as the asteroid belt, enabling scientists to make the first close-up studies of two asteroids, Gaspra and Ida. As is this were not sufficient, Galileo scientists were fortunate to be the only ones with a direct view of the Comet Shoemaker-Levy 9 fragment impacts on Jupiter. All of this was prior to the primary missions of sending an atmospheric probe into Jupiter's atmosphere and studying Jupiter, its satellites, and its magnetosphere for two years with the orbiter.

Interplanetary studies were also made sporadically by some of the other Galileo instruments, including the dust detector, magnetometer, and various plasma and particles detectors, during its six year journey to Jupiter.

The probe was released from the orbiter 147 days prior to its entry into the Jovian atmosphere on 7 December 1995.

Science Objectives

The science goals of the Galileo Probe were to:

  1. determine the chemical composition of the Jovian atmosphere;
  2. characterize the structure of the atmosphere to a depth of at least 10 bars;
  3. investigate the nature of cloud particles and the location and structure of cloud layers;
  4. examine the Jovian radiative heat balance;
  5. study the nature of Jovian lightning activity; and,
  6. measure the flux of energetic charged particles down to the top of the atmosphere.

The objectives of the Galileo Orbiter are to:

  1. investigate the circulation and dynamics of the Jovian atmosphere;
  2. investigate the upper Jovian atmosphere and ionosphere;
  3. characterize the morphology, geology, and physical state of the Galilean satellites;
  4. investigate the composition and distribution of surface minerals on the Galilean satellites;
  5. determine the gravitational and magnetic fields and dynamic properties of the Galilean satellites;
  6. study the atmospheres, ionospheres, and extended gas clouds of the Galilean satellites;
  7. study the interaction of the Jovian magnetosphere with the Galilean satellites; and,
  8. characterize the vector magnetic field and the energy spectra, composition, and angular distribution of energetic particles and plasma to a distance of 150 Rj.

Scientific firsts of the Galileo mission

Although Galileo was not the first mission to explore Jupiter (actually, it is the sixth), it has established a number of "firsts" during its journey.

  • First mission to make a close flyby of an asteroid (Gaspra).
  • First mission to discover a satellite of an asteroid (Ida's satellite Dactyl).
  • First multispectral study of the Moon.
  • First atmospheric probe to enter Jupiter's atmosphere.
  • First spacecraft to go into orbit around Jupiter.
  • First direct observations of a comet impacting a planet (Shoemaker-Levy 9).

Scientific results of the Galileo mission (so far)

A comprehensive list of the science results of Galileo would be longer than space permits. Here, then, is a short list of some important discoveries (in no particular order).

  • The discovery of a satellite (Dactyl) of an asteroid (Ida).
  • Confirmation of the existence of a huge ancient impact basin in the southern part of the Moon's far side (inferred from Apollo data but never before mapped).
  • Evidence of more extensive lunar volcanism than previously thought.
  • Discovery of an intense interplanetary dust storm (the most intense ever observed).
  • Discovery of an intense new radiation belt approximately 50,000 km (31,000 miles) above Jupiter's cloud tops.
  • Jovian wind speeds in excess of 600 kilometers per hour (> 400 mph) were detected.
  • Far less water was detected in Jupiter's atmosphere than estimated from earlier Voyager observations and from models of the Comet Shoemaker-Levy 9 impact.
  • Far less lightning activity (about 10% of that found in an equal area on Earth) than anticipated. The individual lightning events, however, are about ten times stronger on Jupiter than the Earth.
  • Helium abundance in Jupiter is very nearly the same as its abundance in the Sun (24% compared to 25%).
  • Extensive resurfacing of Io's surface due to continuing volcanic activity since the Voyagers flew by in 1979.
  • Preliminary data support the tentative identification of intrinsic magnetic fields for both Io and Ganymede.
  • Evidence for liquid water ocean under Europa's surface.

Source: NASA.


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Space References (Books):
Dickinson, Terence. Nightwatch: A Practical Guide to Viewing the Universe. Firefly Books, 1998.
Greene, Brian. Elegant Universe: Superstrings, Hidden Dimensions, and the Quest for the Ultimate Theory. Vintage, 2000.
Hawking, Stephen. Illustrated Brief History of Time, Updated and Expanded Edition. Bantam, 1996.
Hawking, Stephen. Theory of Everything: The Origin and Fate of the Universe. New Millenium, 2002.
Hawking, Stephen. The Universe in a Nutshell. Bantam, 2001.
Kaku, Michio. Hyperspace: A Scientific Odyssey Through Parallel Universes, Time Warps and the Tenth Dimension.
Kranz, Gene. Failure Is Not an Option: Mission Control from Mercury to Apollo 13 and Beyond. Berkley Pub Group, 2001.
Sagan, Carl; Druyan, Ann. Comet, Revised Edition. Ballantine, 1997
Sagan, Carl. Cosmos, Reissue Edition. Ballantine, 1993
Sagan, Carl. Pale Blue Dot: A Vision of the Human Future in Space. Ballantine, 1997

Space References (Videos):
Cosmos. PBS, 2000.
Stephen Hawking's Universe. PBS, 1997.
Hyperspace. BBC, 2002.
Life Beyond Earth PBS, 1999.
The Planets
. BBC, 1999.
Understanding The Universe. A&E, 1996.



Galileo being deployed during the Shuttle STS 34 flight.

Courtesy of NASA

Launched: October 18, 1989

Destination: Jupiter

Arrival: December 7, 1995

Return: N/A

Nation: U.S.

Mission: Orbit and atmosphere studies

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