Galileo orbited Jupiter for almost eight years, and made close passes by all its major moons. Its camera and nine other instruments sent back reports that allowed scientists to determine, among other things, that Jupiter’s icy moon Europa probably has a subsurface ocean with more water than the total amount found on Earth. They discovered that the volcanoes of the moon Io repeatedly and rapidly resurface the little world. They found that the giant moon Ganymede possesses its own magnetic field. Galileo even carried a small probe that it deployed and sent deep into the atmosphere of Jupiter, taking readings for almost an hour before the probe was crushed by overwhelming pressure.

10 Key Science Discoveries

10 Key Science Discoveries


A global ocean of liquid water exists under the icy surface of Jupiter’s moon Europa.


Galileo magnetic data provide evidence that the moons Ganymede and Callisto also likely have a liquid saltwater layer.


Galileo discovered the first moon around an asteroid—tiny Dactyl orbits the asteroid Ida.

Ida and Dactyl in Enhanced Color


Ganymede is the first moon known to possess a magnetic field.


Galileo’s atmospheric probe discovered that Jupiter has thunderstorms many times larger than Earth's.


The probe measured atmospheric elements, and found that their relative abundances were somewhat different than on the Sun, indicating Jupiter's evolution since the planet formed.


Io's extensive volcanic activity may be 100 times greater than that found on Earth. The heat and frequency of eruption are reminiscent of early Earth.


Io's complex plasma interactions in Io's atmosphere include support for currents and coupling to Jupiter's atmosphere.


Europa, Ganymede, and Callisto all provide evidence of a thin atmospheric layer known as a 'surface-bound exosphere.’


Jupiter's ring system is formed by dust kicked up as interplanetary meteoroids smash into the planet's four small inner moons. The outermost ring is actually two rings, one embedded within the other.

Io Erupts



  • Galileo was the first spacecraft to orbit an outer planet.
  • It was the first spacecraft to deploy an entry probe into an outer planet's atmosphere.
  • It completed the first flyby and imaging of an asteroid (Gaspra, and later, Ida).
  • It made the first, and so far only, direct observation of a comet colliding with a planet’s atmosphere (Shoemaker-Levy 9).
  • It was the first spacecraft to operate in a giant planet magnetosphere long enough to identify its global structure and to investigate its dynamics.

Quick Facts

Quick Facts

Oct. 18, 1989 from Kennedy Space Center, Fla., aboard space shuttle Atlantis

Asteroid Gaspra Flyby:
Oct. 29, 1991, at about 1,000 miles (1,601 kilometers)

Comet Shoemaker-Levy 9:
impacts of comet fragments into Jupiter observed while en route in July 1994

Jupiter arrival and orbit insertion:
Dec. 7, 1995

Probe atmospheric entry and relay:
Dec. 7, 1995

Number of Jupiter orbits during entire mission:

Number of flybys of Jupiter moons:
Io 7, Callisto 8, Ganymede 8, Europa 11, Amalthea 1

Primary Mission:
October 1989 to December 1997

Extended Missions:
three, from 1997 to 2003

Mission end:
spacecraft entered Jupiter’s atmosphere on Sept. 21, 2003

Speed of atmospheric entry at mission end:
106,000 miles per hour (47 kilometers per second) -- equivalent of traveling from Los Angeles to New York City in 82 seconds

Total distance traveled, launch to impact:
about 2.8 billion miles (4.6 billion kilometers)

Spacecraft stats:

  • Size: 17 feet (5.3 meters) high; magnetometer boom extended 36 feet (11 meters) to one side
  • Weight: 4,902 pounds (2,223 kilograms), including 260 pounds (118 kilograms) of science instruments and 2,040 pounds (925 kilograms) of propellant
  • Power: 570 watts (at launch) from radioisotope thermoelectric generators

Science Instruments:
Solid-state imaging camera, near-infrared mapping spectrometer, ultraviolet spectrometer, photopolarimeter radiometer, magnetometer, energetic particles detector, plasma investigation, plasma wave subsystem, dust detector, heavy ion counter

Atmospheric probe stats:
50 inches (127 centimeters) in diameter, 36 inches (91 centimeters) high, weighed 750 pounds (339 kilograms)

Approximate number of people who worked on some portion of the Galileo mission:

Total from start of planning through end of mission was $1.39 billion. International contribution estimated at an additional $110 million

More than 100 scientists from United States, Great Britain, Germany, France, Canada and Sweden carried out Galileo's experiments.

Notable Explorers

What's Next

What’s Next

The Galileo mission ended on Sept. 21, 2003, when the spacecraft was intentionally commanded to plunge into Jupiter’s atmosphere, where it was destroyed. However, to this day scientists continue to study the data it collected.

The Juno mission is currently orbiting Jupiter, following up on many of Galileo’s observations and studying the planet’s internal structure.

Galileo Spacecraft Model

Galileo Spacecraft Model

Galileo was the first spacecraft to orbit Jupiter.

More Resources

More Resources

Galileo fact sheet

End-of-mission press kit

Eyes on the Solar System simulation:

More images taken by the Galileo mission

National Space Science Data Center Master Catalog: Galileo Orbiter

National Space Science Data Center Master Catalog: Galileo Probe

Galileo News