You may have heard that the Hubble Space Telescope recently had some issues that were resolved by a good old fashioned “reboot.” No, that’s not what happened.
Over the last week, NASA has made great strides towards solving the problem with a backup gyroscope on the NASA/ESA Hubble Space Telescope. The backup gyroscope was turned on after the spacecraft entered safe mode following a gyroscope failure on Friday, 5 October. However, when it was turned on the backup gyroscope incorrectly returned extremely high rotation rates. These high rotation rates have since reduced and are now within the expected range. Additional tests will be performed to ensure Hubble can return to science operations using this gyroscope.
A wheel inside the gyroscope spins at a constant rate of 19,200 revolutions per minute. This wheel is mounted in a sealed cylinder, called a float, which is suspended in a thick fluid. Electricity is carried to the motor by thin wires, approximately the size of a human hair, that are immersed in the fluid. Electronics within the gyroscope detect very small movements of the axis of the wheel and communicate this information to Hubble’s central computer. The gyroscopes have two modes — high and low. High mode is a coarse mode used to measure large rotation rates when the spacecraft turns across the sky from one target to the next. Low mode is a precision mode used to measure finer rotations when the spacecraft locks onto a target and needs to stay very still.
On 18 October, the Hubble operations team commanded a series of turns in opposite directions in an attempt to clear any blockage that may have caused the float to be off-center and produce the exceedingly high rotation rates. During each turn the gyroscope was switched from high mode to low mode to dislodge any blockage that may have accumulated around the float.
Following these maneuvers, the team noticed a significant reduction in the high rotation rates, allowing the rates to be measured in low mode for brief periods of time. On 19 October, the operations team commanded Hubble to perform additional maneuvers and gyroscope mode switches, which appear to have cleared the issue. Gyroscope rates now appear to be normal in both high and low mode. Hubble then executed additional maneuvers to make sure that the gyroscope remained stable within operational limits as the spacecraft moved. The team saw no problems and continued to observe the gyroscopes through the weekend to ensure that it remained stable.
The Hubble operations team plans to execute a further series of tests to evaluate the performance of the gyroscopes under conditions similar to those encountered during routine science observations, including moving to targets, locking on to a target, and performing precision pointing. After these engineering tests have been completed, Hubble is expected to return to normal science operations.