Galileo probe

The Galileo mission (e.g. O'Neill, 2002; Bienstock, 2004; Hunten et al., 1986) was conceived early in the 1970s. In 1975 initial work started at NASA Ames for a Jupiter orbiter and probe for launch in 1982 on the Space Shuttle, with Jupiter arrival in 1985 after a Mars flyby en route. The project was transferred to JPL, and was approved by Congress in 1977. Development difficulties with the Space Shuttle led to a slip, and over the following years political pressures from various NASA centres led to several redesigns and different upper stages. Eventually, Galileo was set for a May 1986 launch on the Shuttle with a powerful Centaur upper stage. The Challenger disaster, however, interrupted the Shuttle launch schedule, and a re-examination of safety considerations ruled out the Centaur upper stage with its volatile cryogenic propellants. The revised mission, with a two-stage inertial upper stage (IUS) solid propellant upper stage would launch (after yet more delays) on October 18, 1989.

The low energy of the launcher then required Galileo to make one Venus and two Earth flybys to reach Jupiter. Although this trajectory afforded two asteroid flybys, the thermal design reworking needed to protect the spacecraft in the inner solar system led inadvertently to the failure of the high-gain antenna deployment mechanism, which drastically reduced the downlink performance during the scientific mission.

The release date of the probe was driven by trade-off of the higher fuel penalty (for the orbiter to retarget from the probe entry trajectory to the orbit insertion trajectory) at a later release, against the poorer accuracy of an earlier release. In fact, the coast after release was some 148 days, close to the battery limit of 150 days, with entry on 7 December 1995.

The entry point was driven largely by the need to minimize the entry speed, which is extremely high given Jupiter's deep gravity well. Jupiter's rapid rotation provides an opportunity to mitigate the entry speed, in that the receding (evening) limb is moving at some 11 km s"1 (the problem is analogous to landing an aircraft on an aircraft carrier - it is easier if the carrier is moving away from the aircraft, such that the velocities are subtracted).

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