Galileo is a NASA spacecraft mission to Jupiter, launched in 1989 and currently in the middle of its two-year study of the planet's atmosphere, satellites and surrounding magnetosphere. It was named for the Italian Renaissance scientist who discovered Jupiter's major moons in 1610 with the first astronomical telescope.

This mission became the first to make direct measurements from an instrumented probe within Jupiter's atmosphere and is the first to conduct long-term observations of the planet and its magnetosphere and satellites from orbit around Jupiter. It was also the first to encounter an asteroid and to photograph an asteroid's moon.

The Jet Propulsion Laboratory designed and developed the Galileo Jupiter orbiter spacecraft and is operating the mission; NASA's Ames Research Center developed the atmospheric probe with Hughes Aircraft Company as prime contractor. The German government is a partner in the mission through its provision of the spacecraft propulsion subsystem and two science experiments. Scientists from six nations are participating in the mission.

To pick up enough velocity to get to Jupiter, Galileo took advantage of a billiard-like technique called gravity assist by dipping into the gravitational fields of Venus and Earth. The 38-month Venus-Earth-Earth Gravity Assist phase ended with the second Earth flyby on December 8, 1992. It provided, in addition to the velocity increment, opportunities for useful scientific observations and an exercise of the spacecraft's scientific capabilities.

Galileo's two planned visits to the asteroid belt provided the first and second opportunities for close observation of these bodies: in October 1991 the spacecraft flew by asteroid Gaspra, obtaining the world's first close-up asteroid images; in August 1993 it flew by a second asteroid, Ida, and discovered the first confirmed asteroid moon.

In late July 1994 Galileo was the only observer in a position to obtain images of the far side of Jupiter when more than 20 fragments of Comet Shoemaker-Levy plunged into the night-side atmosphere over a 6-day period.

In December 1995 the Galileo atmospheric probe conducted a direct examination of Jupiter's atmosphere, while the larger part of the craft, the orbiter, began a 23-month, 11-orbit tour of the magnetosphere and the Galilean moons (Callisto, Europa, Ganymede, and Io). The tour will include ten close satellite encounters, three of which have already been successfully performed.

Galileo's orbital science results are being transmitted to Earth over the low-gain antenna at significantly lower data rates than originally planned, because of the in-flight failure of the high-gain antenna to deploy as commanded in April 1991. The Project team developed means to transmit the key scientific data and to accomplish the Project's Jupiter science objectives, using on-board data processing and compression, and various enhancements to the communications link performance, including new encoding systems and advanced technology in ground equipment.

The 2,223-kilogram (2-1/2-ton) Galileo orbiter spacecraft carries 10 scientific instruments; there were another six on the 339-kilogram (746-pound) probe. The spacecraft radio link to Earth and the probe-to-orbiter radio link have served as instruments for additional scientific investigations.

Galileo communicates with its controllers and scientists through the Deep Space Network, using tracking stations in California, Spain and Australia.