Momentum wheel

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Momentum wheels are a type of flywheel, mainly used for gyroscopic stabilization of spacecraft: momentum wheels have high rotation speeds (around 5000 rpm) and mass.

They are electric motor driven rotors made to spin in the direction opposite to that required to re-orient the vehicle. Since momentum wheels make up a small fraction of the spacecraft's mass and are computer controlled, they give precise control. Momentum wheels are generally suspended on magnetic bearings to avoid bearing friction and breakdown problems. To maintain orientation in three dimensional space a minimum of two must be used, with additional units providing single failure protection. See Euler angles.

They are distinct from reaction wheels, also used in spacecraft, which work around a nominal zero rotation speed.

For example momentum wheels are used in the Hubble Space Telescope for very precise pointing.