Hubble zooms around the Earth every 97 minu*tes, so focusing on a target can be difficult. Three onboard systems allow the telescope to remain fixed on an object: gyroscopes, the Fine Guidance Sensors that we talked about in the previous section, and reaction wheels.
The gyroscopes keep track of Hubble's gross movements. Like compasses, they sense its motion, telling the flight computer that Hubble has moved away from the target. The flight computer then calculates how much and in what direction Hubble must move to remain on target.
The flight computer then directs the reaction wheels to move the telescope.
Hubble's Fine Guidance Sensors help keep the telescope fixed on its target by sighting on guide stars. Two of the three sensors find guide stars around the target within their respective fields of view. Once found, they lock onto the guide stars and send information to the flight computer to keep the guide stars within their field of view. The sensors are more sensitive than the gyroscopes, but the combination of gyroscopes and the sensors can keep the HST fixed on a target for hours, despite the telescope's orbital motion.
The HST can't use rocket engines or gas thrusters to steer like most satellites do, because the exhaust gases would hover near the telescope and cloud the surrounding field of view. Instead, the HST has reaction wheels oriented in the three directions of motion (x/y/z or pitch/roll/yaw). The reaction wheels are flywheels, like those found in a clutch. When the HST needs to move, the flight computer tells one or more flywheels which direction to spin in and how fast, which provides the action force. In accordance with Newton's third law of motion (for every action there is an equal and opposite reaction), the HST spins in the opposite direction of the flywheels until it reaches its target.
