Hubble troubleshooting seems to have shaken the problem loose
After about 28 years of diligently studying the skies, Hubble's future was thrown into question a few weeks ago when one of its gyroscopes failed. NASA has been working to return the telescope to working order, and now it looks like all systems are go for normal science operations to resume.
The failed gyro caused the spacecraft to enter safe mode on October 5, and at first NASA wasn't too worried – the instrument had looked on the verge of breaking down for the last year or so. But the real issues began when the team tried to boot up the backup gyros. The last remaining one was also faulty, incorrectly reporting extremely high rotation rates.
NASA's first attempt to fix the problem was exactly what you'd expect – the team turned the gyro off and on again. On October 16, a "running restart" was conducted where the gyro was turned off for one second, and turned back on before the wheel inside had stopped spinning. Unfortunately, that didn't help.
The next idea was that the issue may be a blockage in the thick fluid surrounding the instrument, which could knock it off-kilter. This gyro had, after all, been off for almost eight years, which is plenty of time for a blockage to build up.
So, on October 18 the Hubble team instructed the telescope to turn back and forth, essentially trying to shake the blockage out. During each turn, the gyro was made to switch modes, from "high" – used when the spacecraft is making large turns from one target to another – to "low", which is used for more precise measurements when Hubble needs to stay locked onto a target.
And that seems to have worked. The team noticed the erroneous high rotation rates seemed to drop, and after a few more maneuvers and mode switching, gyro rates seem to be back to normal in both high and low mode. After a few more tests and observations, the gyro appears to be stable.
The next steps are to test out the gyro in the kinds of tasks it would do in regular operations, like moving to targets, precisely pointing at them and locking on. If all goes well, NASA is cautiously optimistic that Hubble should be back up and running as normal in the near future.