Image courtesty NASA/JPL.
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Galileo
The Galileo spacecraft was launched on October 19, 1989. Galileo had two parts: an orbiter and a descent probe that parachuted into Jupiter's atmosphere. Galileo's main mission was to explore Jupiter and its moons and rings . The Galileo mission ended on September 21, 2003, when mission controllers crashed Galileo into Jupiter's atmosphere.
The Galileo mission was a huge success! Originally, the Galileo orbiter was expected to last about two years in orbit around Jupiter. Even though Galileo was damaged by radiation in the magnetosphere of Jupiter, it kept working for eight years. It sent back hundreds of pictures of the four large Galilean moons of Jupiter (Io, Europa, Ganymede, and Callisto).
Galileo arrived at Jupiter in December 1995. On December 7, 1995, Galileo's probe made a dangerous dive into Jupiter's atmosphere, where it was hit by 400 mph winds. The probe was heated to temperatures twice as high as those at our Sun's surface as it screamed into Jupiter's atmosphere. Although it did not carry a camera, the probe's instruments measured atmospheric pressure, density, and composition, and explored the planet's radiation belts. It found very little water in the atmosphere, which has caused scientists to change their ideas about how Jupiter formed.
The orbiter made many discoveries during its eight years looping around Jupiter. It found evidence for layers of salt water below the surface on Europa, Ganymede, and Callisto. It measured high levels of volcanic activity on Io. When Shoemaker-Levy slammed into Jupiter in 1994, Galileo had the only direct view of the comet striking Jupiter's atmosphere. Galileo determined that Jupiter's rings are formed from dust hurled up by meteor impacts on planet's the inner moons. Measurements by the orbiter's magnetometer revealed that Io, Europa, and Ganymede have metallic cores, while Callisto does not. Also, Galileo discovered that Ganymede makes its own magnetic field; it is the first moon know to do so. The orbiter also found that Europa, Ganymede, and Callisto all have thin atmospheres.
During it's trip from Earth to Jupiter, Galileo passed by and studied two asteroids. Galileo was the first spacecraft to fly by an asteroid when it passed Gaspra in 1991. In 1993, Galileo flew past the asteroid Ida and made the first discovery of a moon orbiting an asteroid - the tiny rock Dactyl.
