Hubble’s Gyros Cause a Stir

Rachel Osten (osten[at]stsci.edu) and Tom Brown (tbrown[at]stsci.edu)

We describe recent gyroscope behavior on Hubble which caused a short-term interruption to normal science operations. Hubble has returned to observing with three gyroscopes and is continuing to perform groundbreaking science.

The 2018 Volume 35, Issue 01 of the STScI Newsletter article discussed the status and behavior of two of Hubble’s gyroscopes, noting the increased level of jitter and large bias rate drifts on Gyro-2, and the sudden failure of Gyro-1 in April. Gyro-5 was the first of the six to fail, in March, 2014. Gyro-2 continued to exhibit occasional performance problems throughout the spring and summer which resulted in intermittent guide star acquisition and re-acquisition failures. During this time, the flight operations team at Goddard Space Flight Center and the planning and scheduling teams at STScI worked valiantly to implement mitigations to enable successful acquisitions and thus extend the observatory lifetime in 3-gyro mode. On October 5, Gyro-2 suffered a long-anticipated catastrophic failure.

Hubble's Gyro
Exploded view of a gyro.

The recovery plan involved bringing up Gyro-3, which had been held in reserve since 2011. The complement of remaining gyroscopes (3, 4, and 6) are of an enhanced design which should overcome some of the limitations of the previous generations of gyroscopes installed on the observatory. Their greatly enhanced expected lifetime significantly extends the overall observatory lifetime. The behavior of this gyro was not within performance limits when it was brought into the control loop, however, exhibiting much larger than expected gyro rate bias levels (essentially a large systematic error on the sensed motion of the vehicle). For the ensuing two+ weeks, several activities attempted to bring the gyro into acceptable levels of performance, including toggling between operational states, vehicle maneuvers, and a running restart of the gyroscope. A combination of these activities ultimately had the desired effect, bringing the rate bias levels into an acceptable range to be useable for normal science operations. All during this time Hubble’s instruments were protected and awaiting recovery. 

The observatory resumed normal science operations under three-gyro control on Saturday, Oct. 27 at 2:10 AM EDT.