Pointing Primer




Pointing Primer  
Hubble's Pointing Control System
Holding a 12-ton orbiting telescope steady on a tiny target is no mean feat. This task falls to Hubble's pointing control system, which is made up of Fine Guidance Sensors, reaction wheels, and gyroscopes.  
Gyros and Reaction Wheels

Although Hubble doesn't "visit" celestial objects — it never leaves its orbit — it does need to point itself to different directions to see different objects. But there are no rockets on Hubble, because rockets would fill the space near the telescope with contaminating jet propellant residue.

Therefore, Hubble uses some very basic physics to turn itself around and look at different parts of the sky. Located on the telescope are six gyroscopes (which, like a compass, always point in the same direction) and four free-spinning steering devices called reaction wheels.

The gyroscopes sense when the telescope needs to be repointed. When they "tell" Hubble that it needs to turn itself, a computer gives a command to give the reaction wheels a "push" or "spin."

According to Newton's Third Law of Motion, every action has an equal and opposite reaction. Therefore, as Hubble accelerates its reaction wheels in one direction, Hubble's reaction is to rotate in the opposite direction.

Since the rotation axes of the four reaction wheels point in different directions, Hubble is able to use combinations of them to point itself toward any location in the sky.

Looking upward through the underside of the telescope's midsection.


Lockheed Martin

"For every action, there is an equal and opposite reaction."

Sir Isaac Newton

A reaction wheel

Fine Guidance Sensors

The three curved shapes make up the field of view of the three Fine Guidance Sensors.


Before Hubble can make an observation, it must find a pair of "guide stars" located alongside the observational target. To find these directional beacons, mission planners refer to an immense catalog containing the sky "addresses" for 15 million stars.

Hubble's Fine Guidance Sensors — its "targeting" devices — help aim the telescope by locking onto those guide stars and measuring the telescope's position relative to the target.