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MARS OBSERVER MISSION STATUS
From: David Farber <farber () central cis upenn edu>
Date: Fri, 27 Aug 1993 15:53:28 -0500
PUBLIC INFORMATION OFFICE JET PROPULSION LABORATORY CALIFORNIA INSTITUTE OF TECHNOLOGY NATIONAL AERONAUTICS AND SPACE ADMINISTRATION PASADENA, CALIF. 91109. (818) 354-5011 MARS OBSERVER MISSION STATUS August 26, 1993 2:30 p.m. Pacific Daylight Time Communications with the Mars Observer spacecraft have not yet been restored, one and a half days past its planned insertion into orbit around Mars. Mission controllers at JPL continued through the night and morning with efforts to re-establish the necessary radio link with the spacecraft by cycling the various elements of the communications system. At 2:37 p.m. Pacific Daylight Time today, the continued execution of the command loss timer subroutine will try to position the spacecraft for optimum pointing and will switch antennas to try to restore communications. Project officials still do not believe that the propulsion tanks leaked or exploded at the time of their pressurization. The pressure in the tanks at launch was 285 pounds per square inch absolute (psia). As propellants were used during the three trajectory correction maneuvers, the pressure was reduced to about 167 psia. Pressurizing the tanks would have raised the pressure to 264 psia. Any greater pressure would require a failure of both of the in-series pressure regulators. The burst pressure specification of the tanks is 465 psia and actual test data ruptured tanks at 678 psia. Flow restrictions limit the rate at which the pressure can increase. Analysis indicates that the probability that the pressure in the tanks would increase to the burst level within the 9 minutes that the radio transmitter was off is less than 0.1%. Project officials are systematically evaluating the most probable sources of the cause of the spacecraft's failure to communicate. One such source which has been receiving considerable attention is the potential failure of the spacecraft's central clock, whose official name is the "redundant crystal oscillator," or RXO for short. Proper operation of this device is required for operation of the spacecraft's central computers, which sequence the events on the spacecraft. The first hypothesis for the lack of communications pointed to the failure of the central computer to turn the transmitter back on. Failure of the central clock would prevent the central computer from doing its job. After sending commands to turn on the transmitter, switching to the backup clock was the next action taken by mission controllers. The central clock has been the focus of investigation because it contains transistors which have failed in other spacecraft applications using this type of clock. The launch of the NOAA-I spacecraft was delayed at the end of June 1993 when it was discovered that its RXO had failed. A subsequent investigation revealed that the RXO failure was caused by the failure of a 2N3421 transistor. Two of these transistors are used in each of the redundant halves of the RXO. Transistors from the same manufacturing lot as those in the NOAA-I RXO are installed in the Mars Observer RXO, making the reliability of Mars Observer's RXO suspect. The transistors fail when a weld between a gold-plated post and an aluminum wire breaks. This potential problem was discovered when Mars Observer was only 55 days away from Mars after the spacecraft had been in flight for over nine months. Because of the way that these transistors are used in the RXO, Mars Observer would be susceptible to losing its central clock function if one particular transistor in each half of the RXO failed. There is no alternative source of the central clock function in Mars Observer, and should the loss of this function occur, it would be a non-recoverable situation. The RXO, on its primary side, was working perfectly immediately before the pressurization activity. The last time the backup side of the RXO was tested was in a launch GO/NO-GO test on launch day, when it was also found to be working perfectly. Project officials were not, at first, concerned about the NOAA-I RXO failure because it would take a failure of two of these transistors to cause the loss of the central clock function. The spacecraft is not designed to automatically protect itself against more than a single failure in any piece of hardware. The restoring of the spacecraft's transmitter and the spacecraft's failure to act on ground commands could be tied to the loss of the central clock function. Project officials now surmise that one explanation for the loss of communications could result from the failure of the crucial transistor in each half of the RXO, or its failure to autonomously switch to the backup side. Then there would be no central timing function. This failure could have been induced by the shock of the pressurant valves operating during the propulsion tank pressurization event on Aug. 21, after which communications were not restored. [Ron Baalke, Jet Propulsion Lab, M/S 525-3684 Telos Pasadena, CA 91109 baalke () kelvin jpl nasa gov]
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