Destination: Jupiter. Launched: Oct. 18, 1989. Achieved Orbit: Dec. 7, 1995. Mission ended: Sept. 2003. Mission: To study the chemical composition and physical state of the largest planet in the solar system, its atmosphere, and four of its moons. During its 2-year prime mission, Galileo made 11 orbits around Jupiter and visited and photographed Jupiter's large moons Io, Callisto, Ganymede, and Europa. Originally slated to end its explorations in Dec. 1997, Galileo has been favored with a series of two-year extensions from NASA and Congress. The first extension, dubbed the Galileo Europa Mission (GEM), included eight flybys of Europa, four flybys of Callisto, and two flybys of Io by the end of 1999. The spacecraft also observed the smaller moons Amalthea, Thebe, and Metis on Jan. 3, 2000. After so many successful flybys, Galileo was granted another tour, the Galileo Millennium Mission, which collected data on Io and Europa. On Feb. 22, 2000, Galileo made the closest pass ever of Io at 124 mi (200 km). Two flybys of Ganymede were conducted May 20 and Dec. 28, 2000. Also in Dec. 2000, Galileo embarked on a joint scientific expedition with the Saturn-bound Cassini spacecraft to make simultaneous observations of the Jupiter system from two vantage points.
In May of 2002, Galileo finished its observations of Io; the results revealed Io—now known as the most active body in the solar system—to be more heavily populated by active volcanoes than had been expected. Galileo's last flyby was a close brush with Amalthea, an inner satellite of Jupiter, on Nov. 5, 2002. Intense radiation caused a computer failure that forced a switch to backup circuits and a temporary shutdown, but Galileo was able to record significant scientific data. In Sept. 2003, Galileo self-destructed in a final, fatal plunge into Jupiter's atmosphere. Galileo was named for the Italian astronomer Galileo Galilei, who discovered the four great moons of Jupiter that were the major targets of this mission.