To Galileo Mission Summary
During the last six months (June 6 to November 25, 1994), the spacecraft finished data return from the Ida encounter, recorded and returned data from the Comet Shoemaker-Levy 9 collision with Jupiter, continued a solar wind scintillation experiment, attempted to free the stuck ribs on the High Gain Antenna, and performed operational activities associated with health, attitude maintenance, and telecommunications link characterization.
Ida science data return was completed on June 26, 1994. Overall, 98.7 percent of the Ida science data transmitted by the spacecraft were successfully received and processed through the ground data system during the 1994 Ida data playback campaign.
Please see the feature article, The Brilliant Death of Comet SL9, for a complete summary of SL9 activities.
On June 25, the Galileo Dust Detector Subsystem (DDS) registered an increase in the rate of dust impacts reminiscent of the 11 dust streams that Ulysses detected coming from Jupiter. The particles had a mean impact speed of 17 km/s. Observations will continue and further analysis is planned.
Data collection on the solar wind experiment is scheduled to continue through December 28, 1994. This experiment is designed to measure the charged-particle environment very near the Sun by measuring the effect of those particles on the radio signal beamed from Galileo to Earth.
Several routine Retropropulsion Module 10-N maintenance flushing activities were completed. Regular science data acquisition from the Extreme Ultraviolet Spectrometer, DDS, and Magnetometer has continued successfully in parallel with SL9 data return, using the Command and Data Subsystem (CDS) Memory Readout (MRO) technique. Several Data Management Subsystem conditioning activities were performed.
Seven Ultrastable Oscillator tests were performed to verify the instrument's health and to collect gravitational red-shift experimental data. Long-term trend analysis is continuing.
The Acquisition Sensor was routinely calibrated.
Two Energetic Particle Detector motor maintenance exercises were performed, which stepped the motor through its eight operating positions and then returned it to the normal position (Sector 4). The motor maintenance was successfully verified by an MRO.
Conjunction command tests were performed at Sun-Earth-spacecraft (SEC) angles ranging from 10.0 to 6.0 degrees. The command modulation was turned on at the station (DSS-63) and the spacecraft-received signal was monitored to see if the Command Detector Unit (CDU) locked up and maintained lock. Preliminary results show that the CDU stayed in lock throughout the tests, though further analysis is planned.
Five Block V Receiver tests were performed over DSS-14. The Block V Receiver successfully acquired and tracked the fully suppressed carrier signal, and flowed telemetry and Doppler data to JPL. Full Spectrum Recorder (FSR) tests were also performed over DSS-14 and -43. A detailed test result briefing is scheduled with the Project on December 20, 1994.
On September 13, 1994, the Galileo spacecraft experienced its first computer memory cell failure in five years of interplanetary flight. This corresponds to one failed word out of about a half-million words in Galileo's aggregate computer memories.
During development of Galileo's advanced computer memory, it was recognized that perhaps dozens of cells would randomly fail over the eight-year mission lifetime. Engineers have been very surprised that no failures occurred earlier! To protect against such failures, special fault protection was included on board the spacecraft. Immediately upon detecting the failure on September 13, the fault-protection software executed as planned, stopping the ongoing sequence (which was playing back SL9 science data) and reconfiguring the spacecraft elements to a safe state.
Galileo engineers have isolated the exact location of the memory failure. In the near term, the Project will stop using 80 bytes of memory surrounding the one failed byte--a very small loss when compared to the almost 400,000-byte capacity of the spacecraft central computer. The sequence team has rewritten the background sequences, adjusting for lost time and working around the bad memory cell. Following verification and transmission of the "new" sequence, the spacecraft was back in full operation on September 25. Other than a small loss of some SL9 data, the memory failure should have no long-term effects on the Jupiter mission.
A special minisequence was transmitted August 26 to configure the spacecraft for the High-Gain Antenna (HGA) motor "hammer" activity on August 29. On that day, the HGA dual-drive motors were pulsed 1080 times in an attempt to free the stuck ribs. Prior to this activity, the motors were last pulsed in early 1993. Telemetry measurements indicate that the ribs are still stuck and there was no evidence of change in the antenna configuration. A routine Star Scanner checkout followed the hammering activity. There was no expectation that the HGA ribs would release, but this hammering was done as a final pre-Jupiter arrival check.
The AC/DC bus imbalance measurements have not exhibited significant change throughout this period.
The EJ-8 Final Sequence and Command Generation package was approved on October 10, 1994. The sequence covers spacecraft activities from October 17, 1994, to January 30, 1995, and includes the SL9 science data return, solar wind experiment, and Block V receiver and conjunction tests, as well as FSR and command conjunction tests.
The Project has approved the AACS 13.0 In-Flight Load (IFL) package for command generation. This IFL uses the time period from February 14 to 23, 1995.
Also approved was the Jupiter Orbit Encounter (JOE) Final Sequence and Command Generation package. This sequence covers spacecraft activities from December 3, 1995, to January 3, 1996, and includes windows for the Europa and Io encounters, Probe Relay, JOI, Probe symbol return, and Orbit Trim Maneuvers (OTMs) 1 and 2.
The Project also approved the JOCB Orbit Profile package. This orbit profile covers spacecraft activities from May 14 to June 29, 1996. The primary activity in JOCB is the playback of the arrival day science data (which includes the Io, Europa, and Torus observations). It also includes windows for OTMs 4 and 5, as well as real-time and playback collection of science data.
The Ganymede 1 (G1) Orbit Activity Plan was approved by the Project. This plan covers spacecraft activities from June 29 to September 1, 1996, and includes windows for OTMs 6, 7, 8, and 9, as well as a targeted encounter with G1 and completion of Jupiter Approach (JA)/JOE science playback data.
As a result of a meeting held to review the status of parallel operations, the Mission Telemetry System (MTS) support of Galileo was fully decommitted effective August 19, six weeks before the target date established in 1993. MTS is the original system that supported Voyager, Ulysses, Magellan, and earlier projects. All future telemetry operations will use the more powerful, compact, and cost-effective Advanced Multimission Operations System/Mission Ground Data System.
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