A spinning wheel that can be moved to change the orientation of a body to which the wheel is attached. Reaction wheels are used in many satellites, including the Hubble Space Telescope, to allow precise pointing. By attaching an electric motor to a heavy wheel, and spinning the wheel one way quickly, a satellite rotates the other way slowly by conservation of angular momentum. This method can be made very precise, and it doesn't use up fuel.
The problem is that if there is some small continuous torque on a satellite, e.g., radiation pressure, the wheels end up spinning all the time, faster and faster, to counteract it. The answer to that is to have some way of dumping momentum. The usual technique, used by the Hubble Space Telescope, is utilizing a set of electromagnets that can be used to exert a weak torque against the Earths magnetic field.
Conservation of Angular Momentum
