A
reaction wheel (RW) is a type of
flywheel used primarily by
spacecraft for
attitude control without using fuel for
rockets or other reaction devices. They are particularly useful when the spacecraft must be rotated by very small amounts, such as keeping a telescope pointed at a star. They may also reduce the
mass fraction needed for fuel. This is accomplished by equipping the spacecraft with an electric motor attached to a flywheel which, when its rotation speed is changed, causes the spacecraft to begin to counter-rotate proportionately through
conservation of angular momentum. Reaction wheels can only rotate a spacecraft around its center of mass (see
torque); they are not capable of moving the spacecraft from one place to another (see
translational force). Reaction wheels work around a nominal zero rotation speed. However, external torques on the spacecraft may require a gradual buildup of reaction wheel rotation speed to maintain the spacecraft in a fixed orientation.