To On My Own--At Last! The Galileo Atmospheric Probe Release
Using the newly loaded flight software (see story, Reloading the Operating System on Galileo's PC) for the first time, the team verified that Probe data--telemetry in symbol form--could be stored in the spacecraft central computer's spare memory and recovered with a memory readout (MRO). The data compression feature of the new flight software and the direct use of Command and Data Subsystem (CDS) memory (bypassing the tape recording process) provided good data quickly--within a day. Processing of the same data by the usual method, a Low-Rate Probe Record (LPR) format (also a new flight software capability) recording on tape that is then downlinked to Earth, verified use of the new method.
The Probe will be powered by a three-module lithium battery during its solo flight and descent to Jupiter. Tests of the open circuit voltage (OCV) indicate it is holding at 39 V, even at its advanced age of more than 6 years. But how do you test battery capacity and avoid using precious energy intended for the Probe task? Only power from the Orbiter can be used for testing. So the PET planned ahead. Three identical battery sets kept under simulated flight conditions were made available here on Earth. One battery set began a 155-day "coast" to be followed by a simulation of the experiments that the actual Probe will conduct on arrival at Jupiter. By beginning the activity before the actual Probe release in July, the PET will be able to address any anomalies that occur, just as you would prepare for a big event with a dress rehearsal or dry run presentation. A Flight Descent Antecedent Test (FDAT) confirmed that all is well. The other two sets will be used for anomaly response, if required. Ralph Reichert, Engineering Office Manager, said that ground testing indicated the batteries showed substantial margins in all areas.
A test of the accelerometers in the Atmospheric Structure Instrument (ASI) checked out fine. The Neutral Mass Spectrometer (NMS) ion pump removed excess argon gas, relieving the ionization chamber of that burden. The radio relay hardware (RRH) aboard the Orbiter was also exercised during this activity; it received the Probe's signal and passed it on to the CDS. The RRH did its job very well.
Are we surprised at these positive results? After launch of Galileo in October 1989 and again in December 1990, the Probe had successfully passed a System Functional Test (SFT), indicating all science instruments and engineering subsystems were functioning well. In November 1992, a full Mission Sequence Test (MST) reassured us that each unit received and executed the stored sequence commands as planned during the actual pre-entry and descent mission (except for irreversible events, for example, staging). The Abbreviated SFT (ASFT) conducted in December 1992 on Earth-2 approach focused on the battery set check and removal of argon from the NMS ionization chamber. The results were excellent, just as with the current testing.
Nevertheless, on April 18, Project Galileo Manager William J. O'Neil met at Ames Research Center with Probe Scientist Rich Young and the Probe Science Team; Probe Manager Marcie Smith and Deputy Probe Manager Charlie Sobeck (ARC); and Chief Pat Melia and the Probe Engineering Team, including the Hughes team led by Program Manager Bernie Dagarin. The teams made a careful evaluation of the checkout results. They studied the FDAT performance and the subsystem functionality shown in the ASFT to determine whether to follow their nominal plan or the alternative contingency mission. The contingency mission would have omitted the pre-entry science, that is, data on Jupiter's atmosphere for areas not to be reached by the Orbiter. Switches which detect deceleration would have deployed the parachute for descent so that available energy would be used primarily for the collection and relay of descent phase science data. As we go to press, the Project Manager has approved a full Probe mission.
To On My Own--At Last! The Galileo Atmospheric Probe Release
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