On Friday, October 5th, 2018, NASA’s favorite Hubble Space Telescope lost its last remaining old-style gyroscope used to detect and measure the telescope’s pointing and rotation, leaving it with only two functional gyroscopes and forcing its operators to place it into safe mode. This is definitely not the first time the Hubble has lost a gyroscope. In fact, this is not even the first time the Hubble has been down to two gyroscopes! The Hubble Space Telescope has the unique honor of being the only satellite to be serviced and repaired in orbit. There have been not one, not two, but five Space Shuttle missions to service the telescope, three of which brought replacement gyroscopes for the aging telescope. Hubble, now in its 28th year of operation, has broken eighteen gyroscopes out of the twenty-two it has had on board throughout its lifetime. Two of those unbroken gyroscopes are on Hubble right now, and the other two were replaced during the last Space Shuttle service mission to the telescope. Fortunately, Hubble’s operators at NASA found a solution to the problem.
The Hubble Space Telescope has six on board gyroscopes. In order to point precisely at objects like these, Hubble needs three functioning gyroscopes; one for each axis of rotation (pitch, yaw, and roll). Hubble’s designers included six gyroscopes so that, if a gyroscope fails, there is a backup gyroscope that can be activated to take its place. On Friday, when the gyroscope failed, there were no backup gyroscopes ready to immediately take its place. Gyroscopes have lots of moving parts, so Hubble’s operators frequently test all gyroscopes, primary and backup, to make sure they are providing good data. The backup for the gyroscope that failed on Friday had been providing numbers that were too high, so Hubble operators continued to use the older gyroscope until the very end of its life. When the older gyroscope failed, operators had two options: figure out how to fix the errant new gyroscope, or rely on the significantly less accurate star trackers to provide rotation data for that axis. NASA scientists then decided to use two tried-and-true I.T. tricks at the same time to try and solve the gyroscope; turning it off and back on again, and wiggling it around a bit. NASA preformed a full reboot of all the telescopes systems, and used the Hubble’s reaction wheels to wiggle the gyroscope around a little bit. This plan worked flawlessly, and the Hubble Space Telescope continues to reach for the stars.