Momentum Wheel

A momentum wheel is a motor-driven flywheel operated at high bias speed to store a large, roughly constant angular momentum aboard a spacecraft. The stored momentum makes the vehicle gyroscopically stiff: like a spinning top, it resists disturbance torques about the axes perpendicular to the wheel, turning slow drifts into a stable, predictable precession that the control system corrects gently. This distinguishes it from its sibling the reaction wheel, which idles near zero and changes speed to exchange momentum for steering; a momentum wheel's job is stability, with small speed and gimbal adjustments providing fine control around the bias. The architecture — "momentum-bias" — carried a generation of geostationary communications satellites: one wheel aligned with the orbit-normal axis stiffened the spacecraft against solar-pressure torques while pitch control came free from the wheel's speed trim. Modern designs increasingly favour zero-bias sets of four reaction wheels for agility, but momentum bias persists where steadfast nadir pointing matters more than slewing — and the boundary blurs, since the same hardware can run in either regime.