How does Hubble Space Telescope (HST) re-orient itself; and what is an underlying mathematical description? http://en.wikipedia.org/wiki/Gyroscope" [Broken]
HST has tiny sensor gyroscopes which spin freely and so always point the same way it uses this to know which way is up.
For more accurate pointing it also star trackers built into some of the instruments.
It also has reaction wheels (also sometimes loosely called gyros) these heavy wheels spin like gyros and can be used to move the telescope. When the spin they tend to stay in the same plane and so when you push on them you move instead.
