Project Galileo: Final Mission Status

Abstract

After a 14-year odyssey, the historic Galileo mission to Jupiter ended on September 21, 2003 when the spacecraft entered the atmosphere of the giant planet it had studied for almost seven and a half years. The planned destruction of the orbiter was necessary to satisfy planetary protection concerns about Europa, a prime target in the search for extraterrestrial life. Almost 11 months earlier, on November 5 , 2002, the spacecraft flew to within 71,500 km of Jupiter's cloud-tops, sampling the inner magnetosphere and the Gossamer ring. The trajectory allowed Galileo to obtain the first density estimate of Amalthea, a small inner moon. This encounter presented challenges both in preparing for this risky flyby and in recovering from this traverse deep within the radiation belts. By limiting the observations to two primary experiments, radio science and fields and particles, the flight team was able to simplify sequence design and facilitate a robust strategy to continue data acquisition in the event of an anomaly. Based on previous experience, changes were made to onboard fault protection routines to either facilitate recovery or keep Galileo from entering safe mode (and subsequently canceling the science command sequence). Not unexpectedly, two new types of hardware problems were manifested during this perijove passage. About 16 minutes after Amalthea closest approach, the extreme radiation levels caused erratic behavior in the Command and Data Subsystem phase lock loops. This resulted in multiple swaps of the timing chains and entry into spacecraft safe mode. An autonomous science recovery sequence designed to continue recording fields and particles data was initiated but did not run to completion because of the specific type of hardware anomaly. This problem was resolved as Galileo moved outside the region of highest radiation levels. The second problem occurred when high-energy protons were encountered with sufficient flux to case significant displacement damage in optical electronic circuits responsible for control of the tape recorder drive mechanism. The resolution of this anomaly is discussed in Section 3.2. Designed to withstand 150 krad inside a 2.2 g/cm2 shell, the spacecraft is remarkably healthy after sustaining over 650 krad but is showing the effects of both age and radiation. Radiation effects include damage to electronic parts in the attitude control subsystem, the computer memory, the tape recorder and some science instruments. Software patches and modified operating strategies were implemented to work around most of the radiation effects. A summary of spacecraft performance in the harsh jovian environment and a report of final subsystem and instrument status are provided.