Hubble Steering

The HST must remain fixed on a target while it takes an image, which could take up to several hours depending upon which instrument is being used by an observer. Bear in mind that the HST is moving around the Earth every 97 minutes, so focusing on a target is like keeping sight of a small object on the shore from the deck of a boat that is rapidly moving along the coast, bobbing up and down in the waves. To remain fixed on an object, the HST has three on-board systems:

• Gyroscopes - sense small to large motions
• Reaction wheels - move the telescope
• FGS - sense fine motion

The gyroscopes keep track of the gross movements of the HST. Like a compass, they sense the motion of the HST, telling the flight computer that the HST has moved away from the target. The flight computer then calculates how much and in what direction the HST must move to remain on target. The flight computer then directs the reaction wheels to move the telescope.

The HST cannot have 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.

As mentioned above, the FGS help keep the telescope fixed on its target by sighting on guide stars. Two of the three FGS 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 FGS are more sensitive than the gyroscopes; but the combination of gyroscopes and FGS can keep the HST fixed on a target for hours despite the telescope's orbital motion.