Galileo (spacecraft) - Mission Overview

Mission Overview

Workon the spacecraft began at JPL in 1977, while the Voyager 1 and 2 missions were still being prepared for launch. Early plans called for a launch on Space Shuttle Columbia on what was then codenamed STS-23 in January 1982, but delays in the development of the Space Shuttle allowed more time for development of the probe. Under the original launch schedule, Galileo would have been launched on an Inertial Upper Stage booster. As the shuttle program got underway, Galileo was scheduled for launch in 1984, but this later slipped to 1985 and then to 1986. The mission was initially called the Jupiter Orbiter Probe; it was christened Galileo in 1978.

Once the spacecraft was complete, its launch was scheduled for STS-61-G on-board Atlantis in 1986. It was to use the Centaur-G liquid hydrogen-fueled booster stage for a direct trajectory to Jupiter. However, the mission was further delayed by the hiatus in launches that occurred after the Space Shuttle Challenger disaster. New safety protocols introduced as a result of the disaster prohibited the use of the Centaur-G stage on the Shuttle, forcing Galileo to use a lower-powered Inertial Upper Stage solid-fuel booster. The mission was re-profiled in 1987 to use several gravitational slingshots, referred to as the "VEEGA" or Venus Earth Earth Gravity Assist maneuvers, to provide the additional velocity required to reach its destination. Venus was flown by at 05:58:48 UT on February 10, 1990 at a range of 16,106 km. Having gained 8,030 km per hour in speed, the spacecraft flew by Earth twice, the first time at a range of 960 km at 20:34:34 UT on 8 December 1990 before approaching the minor planet 951 Gaspra to a distance of 1,604 km at 22:37 UT on 29 October 1991. Galileo then performed a second flyby of Earth at 303.1 km at 15:09:25 UT on 8 December 1992, adding 3.7 km per second to its cumulative speed. Galileo performed close observation of a second asteroid, 243 Ida, at 16:51:59 UT on 28 August 1993 at a range of 2,410 km. The spacecraft discovered Ida has a moon Dactyl, the first discovery of a natural satellite orbiting an asteroid. In 1994, Galileo was perfectly positioned to watch the fragments of the comet Shoemaker-Levy 9 crash into Jupiter, whereas terrestrial telescopes had to wait to see the impact sites as they rotated into view. After releasing its atmospheric probe on 13 July 1995, the Galileo orbiter became the first man-made satellite of Jupiter at 00:27 UT on 8 December 1995 when it fired its main engine to enter a 198-day parking orbit.

Galileo's prime mission was a two-year study of the Jovian system. The spacecraft traveled around Jupiter in elongated ellipses, each orbit lasting about two months. The differing distances from Jupiter afforded by these orbits allowed Galileo to sample different parts of the planet's extensive magnetosphere. The orbits were designed for close-up flybys of Jupiter's largest moons. Once Galileo's prime mission was concluded, an extended mission followed, starting on December 7, 1997; the spacecraft made a number of daring close flybys of Jupiter's moons Europa and Io. The closest approach was 180 km (110 mi) on October 15, 2001. The radiation environment near Io in particular was very unhealthy for Galileo's systems, and so these flybys were saved for the extended mission when loss of the spacecraft would be more acceptable.

Galileo's cameras were deactivated on January 17, 2002, after they had sustained irreparable radiation damage. NASA engineers were able to recover the damaged tape recorder electronics, and Galileo continued to return scientific data until it was deorbited in 2003, performing one last scientific experiment —a measurement of the moon Amalthea's mass as the spacecraft swung by it.