Device Description
Reaction wheels are devices which aim a spacecraft in different directions without firing rockets or jets. 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 rotated increasingly fast causes the spacecraft to spin the other way in a proportional amount by 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.
Momentum wheels (used in the Hubble Space Telescope) are a different type of actuator, mainly used for gyroscopic stabilization of spacecraft: momentum wheels have high rotation speeds (around 6000 rpm) and mass.
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