Gyroscopes, or gyros, measure rates of motion when Hubble is changing its pointing from one target to another, and they help control the telescope's pointing while scientists are observing targets.
The gyroscopic function is achieved by a wheel inside each gyro that spins at a constant rate of 19,200 rpm on gas bearings. This wheel is mounted in a sealed cylinder, which floats in a thick fluid (about the thickness of 10W-30 motor oil). 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 gyro detect very small movements of the axis of the wheel and communicate this information to Hubble's central computer.
Gyros have limited lifetimes and need to be replaced periodically.
Previously, Hubble needed three of the six gyros to conduct science. However, after substantial changes to Hubble's pointing control algorithms, only two gyros are now needed. In the fall of 2005, Hubble began operating in 2-gyro mode. According to senior Hubble scientist David Leckrone at Goddard, "Hubble science on two gyros is indistinguishable from the superb science we have become accustomed to over the years."
Hubble has a total of six gyroscopes grouped in pairs inside three Rate Sensor Units (RSUs). Approximately the size of a toaster, the RSU is what astronauts change when they replace gyros, so gyros are always replaced two at a time. During Servicing Mission 4, astronauts will replace all six of Hubble's gyroscopes.