Mission Status Report #42 Star Date: February 24, 2001
FUSE Temporarily Out of Science Mode
The FUSE satellite is temporarily out of data taking mode while we are troubleshooting a problem with one of the spacecraft's so-called "reaction wheels." There are four such devices on FUSE, and normally all four wheels are used in the control loop to point FUSE from place to place in the sky and to hold it steady in position while observing. For normal operations at least three of the four wheels must be operating.
On February 16, 2001, one of the reaction wheels drew excessive current and took itself out of the control loop automatically. After a brief time, we were back on-line performing science, controlling in three-wheel mode.
A similar event had occurred the previous August with one of the other wheels. Analysis at that time indicated this rapidly spinning device was encountering excess friction on the inside of the housing, probably from a small trapped gas bubble under Kapton tape (used for cushioning from vibration). (See Mission Status Report #36 from September 2000.) This wheel was successfully brought back into service in September 2000. After spinning the wheel rapidly for some time, the wheel apparently burned through the bubble and the current returned to normal. All signs are that the event on February 16 was similar in nature.
On Friday February 23, 2001, we began an operation to attempt recovery of the problemmatic wheel. This activity is still ongoing as of this writing and will continue at least through this weekend. Since we cannot obtain science data while in this troubleshooting mode, we have had to temporarily halt data acquisition. Early next week I will provide an update of our situation.
Reported by: Bill Blair, Chief of Observatory Operations
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