Caption: Graphic of FUSE on-orbit. (Courtesy of FUSE project, JHU.)

More info on Reaction Wheel Anomaly. (Added Jan. 7, 2005.)

The FUSE satellite experienced a significant anomaly on Dec. 27, 2004, that has temporarily suspended science operations.

For reasons that are still under investigation, one of our two remaining reaction wheels (the one on the spacecraft "roll" axis) stalled early on December 27, 2004, placing the satellite into a safe mode. The satellite is now oriented with the telescopes roughly toward the anti-solar direction. The solar arrays are rotated toward the sun, maintaining a full charge on the batteries. The instrument doors are closed and the instrument is safe. Initial attempts to restart this reaction wheel have been unsuccessful. The JHU operations team is working to diagnose the problem, attempting to restart the errant wheel, and studying options for returning FUSE to science operations. We are working closely with the spacecraft manufacturer to meet these objectives.

You may recall that back in December 2001, FUSE experienced an 8-week downtime due to reaction wheel problems. (See Mission Status Report #52.) Ultimately, we were able to revise the flight software to compensate for the missing reaction wheels by substituting inputs from our magnetic torquer bars. (See Mission Status Report #55.) In the event that we cannot get the roll wheel restarted, we hope to be able to accomplish something similar using just a single reaction wheel, although our pointing capabilities are likely to be more restricted. It will take a month or two to carry out the wheel restart program and, if necessary, modify flight control software to determine how extensive or limited the scientific capabilities of FUSE would be with only one reaction wheel.

This is not how we wanted to start 2005! It will take time to develop and test work-arounds and to understand what our observing capabilities will be in the future. NASA Headquarters has delayed the announcement of the FUSE Cycle 6 proposal selections until scientific operations resume and any changes in the mission's scientific capabilities are known.

I will post further updates as they are warranted. Stay tuned.

Reported by: Bill Blair, FUSE Chief of Observatory Operations