Released from the orbiter on 13 July 1995 with an arrival of 7 December 1995, the probe has as its scientific objectives to: (1) determine the chemical composition of the Jovian atmosphere; (2) characterize the structure of the atmosphere to a depth of at least 10 bars; (3) investigate the nature of cloud particles and the location and structure of cloud layers; (4) examine the Jovian radiative heat balance; (5) study the nature of Jovian lightning activity; and, (6) measure the flux of energetic charged particles down to the top of the atmosphere. The probe is 1.25 m in diameter and 0.86 m in height. It is comprised of two two major segments: a deceleration module and a descent module.
The deceleration module consists of the fore and aft heat shields and their accompanying support structure and the thermal control hardware for the phases of the mission through entry into the atmosphere. The descent module, which contains the science instruments and the subsystems required to support them, is the package which descends through the atmosphere by parachute.
During entry into the Jovian atmosphere, as the probe is subjected to temperatures near 14000 K, the forward shield is expected to lose around 60% of its 145 Kg mass. A drogue parachute will be deployed, using a mortar, when the probe is at a velocity of about Mach 0.9 and a dynamic pressure of 6000 N/sq m. Once the drogue chute is released, explosive bolts will be fired to release the aft cover which in turn pulls out and strips off the bag containing the main parachute. This entire process is designed to take less and 2 s.
The Galileo descent module differs from the Pioneer Venus Large Probe design, which included a sealed pressure vessel, in that the mass has been minimized by venting the module and by protecting individual units as necessary with hermetically sealed housings. These housings are designed to survive to pressures of 20 bars and were tested to 16 bars.
Power to the probe is supplied by a LiSO2 battery with a capacity of about 21 A/hour. Mission requirements are for about 16.3 A/hour. The primary source of energy for pyrotechnic events is a set of four thermal batteries.
The duration of the probe's descent through the Jovian atmosphere is expected to last between 48-75 minutes, with the lower limit determined by the minimum required battery capacity and the upper limit by atmospheric pressure.
