Hubble has six main gyroscopes for pointing control.
The gyroscopes, or gyros, sense changes in the telescope's orientation and help its control system keep the observatory aimed precisely at astronomical targets. Hubble's six gyros are housed in three Rate Sensor Units, with two gyroscopes in each unit.
Six does not mean that all six must operate simultaneously. Hubble normally used three gyros for pointing and tracking, while the remaining units provided redundancy. By 2024, NASA had transitioned Hubble to a one-gyro operating mode after repeated failures, allowing the telescope to continue science operations with reduced redundancy.
Gyroscopes are only part of the pointing system. Hubble also uses sensors, reaction wheels, and other control hardware to determine and adjust its attitude. This is why the number of gyros installed should not be confused with the minimum number needed for normal operation.
The first servicing mission in December 1993 replaced four gyroscopes, and Servicing Mission 3A in 1999 replaced all six aging gyros.