The Galileo Space Craft SCIENCE Kit Fact Sheet

Highlights of Galileo's Mission to Jupiter

The Galileo mission to Jupiter is named for the Italian Renaissance scientist who discovered Jupiter's four largest moons. The Mariner, Pioneer and Voyager spacecraft completed an initial reconnaissance of the solar system, and Galileo is the flagship of a fleet of new spacecraft designed to enter into orbit and return detailed information from the planets for extended periods of time: Magellan's target is Venus; Mars Global Surveyor is scheduled to launch in 1996; Cassini is being planned to orbit Saturn in future years.

The Journey to Jupiter

Originally, Galileo was to launch from the Space Shuttle on a direct flight to Jupiter using a three-stage IUS (Interim Upper Stage, later renamed Inertial Upper Stage upon cancellation of the Space Tug). When delayed due to Shuttle development problems in 1979, Galileo had to be redesigned for separate launches of its orbiter and atmospheric probe in 1984, since Jupiter's changing position demanded too much launch energy for a combined craft. In 1981, NASA dropped development of the three-stage IUS, and adopted the powerful, cryogenic-fueled Centaur for Galileo's upper stage. The project had a re-combined orbiter and probe, with a two-year flight directly to Jupiter. Then Centaur was deleted from the Federal budget, and Galileo had to adopt a two-stage Air Force IUS, develop a third stage for it, and prepare to fly a trajectory looping past Earth to gain the required energy. This May, 1985 launch scenario was called Delta-V EGA (Velocity change via the new third stage, plus Earth Gravity Assist). In 1982, Congress directed NASA to restart development of the Centaur. Launch was postponed until May, 1986, and the trajectory was redesigned once again for a direct flight. After Challenger's tragic explosion in January 1986, Centaur was cancelled for reasons of crew safety. A two-stage IUS was the only alternative, but it lacked the energy required for a direct flight, or even the Delta-V EGA flight. Enter the VEEGA trajectory: one Venus flyby, and two Earth flybys for Gravity Assist would do it. Designed for the cold of deep space, Galileo would need sunshades and strict attitude control to survive the inner solar system. Appropriate changes were made to the spacecraft during the years while NASA was recovering from Challenger's failure.

Launch Day

In early October, 1989, as Space Shuttle Atlantis was preparing to launch Galileo, groups of concerned people protested the launch because of the spacecraft's complement of plutonium, used for electric power and heat. Chances of accidental dispersion of the radioactive material were recognized as very small, and on October 7 the federal court overruled them. October 12's launch was delayed because of a Shuttle computer problem, and five days later, it was delayed again because of rain showers near the emergency landing strip. Atlantis succeeded in placing the combined Galileo orbiter and probe, with a two-stage IUS, in earth orbit October 18, 1989. The IUS performed flawlessly, Galileo began its six-year VEEGA journey to Jupiter, and Atlantis returned to Earth five days later after completing separate scientific and engineering tasks.

En route

Along the way to Jupiter, Galileo made observations of Venus and Earth, and has taken the first images of two minor planets in the asteroid belt, Gaspra and Ida, discovering a moon orbiting the latter. Once out of the inner solar system, an attempt was made to unfurl the High-Gain Antenna and begin normal communications. This and other attempts proved unsuccessful. Galileo will have to rely on its low-gain antennas to communicate with Earth from the distance of Jupiter. Upgrades in Galileo's on-board software, and improvements in ground systems will make it possible to recover 70% of the originally planned science data, using the low-gain antennas on board. Additional attempts to unfurl the High-gain Antenna, made again nearing Jupiter in late 1995, were not sucessful.

Finally There!

Upon arrival at Jupiter in December, 1995, Galileo used its closest encounter with Jupiter's satellite Io for a gravity assist to help reduce speed. Next, it tracked the atmospheric probe, released 150 days before, as it slammed into Jupiter's atmosphere at about 47 kilometers per second. During aerobraking, temperatures on the probe's heat shield briefly became twice as hot as the Sun's photosphere, and it experienced deceleration forces of up to 350 times earth gravity. Once slowed, parachutes opened to ease it down through the clouds, while it radios data for about an hour back to the orbiter passing overhead. Following this activity, Galileo fired the engine on its German-built Retro Propulsion Module, and entered into orbit around Jupiter.

Galileo's Future

During its two-year tour of the Jovian system, the Galileo Orbiter will monitor the behavior of Jupiter's atmosphere, magnetosphere, and radiation belts. It will continue to loop in orbit for at least two years, examining Jupiter's diverse family of satellites from close encounter distances of as small as 140km.

Communications

The two-way data link between spacecraft and Earth is maintained using complexes of large radio telescopes in Spain, Australia, and the U.S., which comprise NASA's Deep Space Network. Additional telecommunications support is provided by the Federal Republic of Germany. Project Galileo and the Deep Space Network are managed for NASA by Caltech's Jet Propulsion Laboratory. The atmospheric probe is managed by NASA's Ames Research Center. Galileo's scientific investigations are undertaken by more than 100 scientists from six nations.

Click Here to view a large illustrated graphic of Galileo (66 kbytes). Image courtesy JPL/NASA.

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