Caption: The FUSE satellite is shown at left, at NASA/GSFC in August 1998. The spacecraft is the gold box toward the bottom and the science payload (telescopes and spectrographs) make up the silver and gold structure above. The solar panels are shown in the "stow" position. The enlargement at right shows a peek inside the spacecraft box, which is mostly empty space! The two working reaction wheels (marked RWA) are indicated, as is one of the gyro packages called an "Inertial Reference Unit." (Figure courtesy JHU.)

FUSE experienced a potentially serious bump in the road this week, when one of the remaining two reaction wheels (on the roll axis) temporarily stopped spinning. The anomaly occurred Tuesday morning local time, Dec. 17, and it placed the satellite into a "Safe Pointing Mode" configuration. However, after about an orbit, the wheel was spinning again, and shortly thereafter the spacecraft was commanded back into it's normal "Inertial Science Mode." As a precaution, and to allow time to study the situation further, the satellite was slewed to a safe, stable pointing location, and the wheel performance has been monitored for three days now, with no further evidence of problems of any kind.

This being the case, we have returned to the science timeline today (12/20/02) after making several adjustments to the operating system. The most significant of these changes involves keeping a "bias" (enhanced rate of spin) on the reaction wheels to keep momentum on the wheels and prevent slow rotation speeds. To accommodate this change, the rate we allow FUSE to slew (move from one place to another) has been reduced from 4 degrees/minute to 1.5 degrees per minute, at least temporarily (again, to be conservative, but still get back to performing science). These changes will be transparent to users, and the data quality will be unchanged from previously.

Reported by: Bill Blair, Chief of Observatory Operations