Launched in 1989, the Galileo spacecraft (named after the Italian Renaissance scientist who
discovered Jupiter's major moons in 1610 with the first astronomical telescope) arrived at
Jupiter on December 7, 1995, when it fired its main engine for a successful orbit capture
around Jupiter. On that day, Galileo's atmospheric probe plunged into Jupiter's atmosphere
and relayed information on the structure and composition of the solar system's largest planet.
The spacecraft's orbiter will spend the next two years orbiting the giant planet, studying
Jupiter and its moons (encountering one moon during each orbit), and returning a steady
stream of images and scientific data. The first two encounters were successfully performed
with Jupiter's largest moon, Ganymede, on June 27, 1996 and September 6, 1996.
This mission will be the first to make direct measurements from an instrumented probe
within Jupiter's atmosphere, and the first to conduct long-term observations of the planet and
its magnetosphere and satellites from orbit around Jupiter. It is already 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
Scientists from six nations are participating in the mission.
Like Voyager and some other previous interplanetary missions, Galileo used planetary
gravitational fields as auxiliary propulsion stages. The spacecraft dipped into the gravitational
fields of Venus and Earth to pick up enough velocity to get to Jupiter. This 38-month
Venus-Earth-Earth Gravity Assist phase (see figure) 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 for approximately one hour, (see figure) while the larger part of the craft, the
orbiter, began a 23-month, 11-orbit tour of the magnetosphere and the Galilean moons,
including ten close satellite encounters (see figure).
Galileo's orbital science results were 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 has 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 are another six on the 339-kilogram (746-pound) probe. The spacecraft radio link to
Earth and the probe-to-orbiter radio link serve as instruments for additional scientific
Galileo communicates with its controllers and scientists through the Deep Space Network,
using tracking stations in California, Spain and Australia.