ROSAT Status Report A
Feb 6 1991
This note is to bring you up to date on an event which occurred onboard ROSAT recently. Apparently, on January 26, the attitude control system on ROSAT had a problem which resulted in the telescope tumbling for several orbits as it was trying to go into safemode. During this period, the telescope axis came close to the sun, close enough to destroy the thin window in front of PSPC-A and one of the WFC filters.
The good news is:
ROSAT is still functioning despite having scanned across the sun. It is currently a stable attitude and in safemode. It will remain in safe mode until the operations team had implemented on-board procedures which will prevent the recurrence of complete loss of control. Already implemented as part of the safing procedure is a command to move the opaque filter in front of the PSPC if it is in the focal plane, or move the HRI out of the focal plane if it is in.
Although PSPC-A has been lost, PSPC-B (the backup) is functioning. It has actually been turned on and tested, although it is still out of the focal plane.
The WFC is still operating. The ruined filter was redundant.
A checkout of the HRI out of the focal plane earlier this week demonstrated that it is functioning. All quantities that are measureable outside of the focal plane, such as background, are consistent with previous in-flight measurements.
The Attitude Measurement and Control System (AMCS) computer whose failure initiated the crisis is functioning once again. Thus redundancy in that subsystem has been preserved.
The X-axis gyro, which had been switched out of the control loop in December because of excessive drift rates, is back within spec. As a result of reduced spacecraft power during the crisis it had spun down.
The bad news is:
PSPC-A is lost. PSPC-B was never turned on in orbit, and thus it has not been calibrated using cosmic X-ray sources.
The WFC survey filter is lost.
No one really knows why the spacecraft attitude went bad. Most discussion centers around soft upsets in the CPUs on board. Operations have been suspended until the problem is better understood. That means that pointed observations, scheduled to commence on Feb. 7, will be delayed for at least a few days. Once the decision is reached to resume normal operations, a few days will be spent performing checkouts of the spacecraft and instruments. No replanning is expected: the timeline will be picked up at the point when observations begin. Missed observations will be attached at the end of the AO1 observing season.
In summary, although there is certainly cause for concern, the prospects for successful pointed phase observations are still excellent. We will be sure to send out another email bulletin when there is significant news to report. In the meantime, continue work on proposals for the next observation cycle.
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