Reaction Wheel

A reaction wheel is an electrically driven flywheel that provides precise attitude control by exploiting conservation of angular momentum. When the motor speeds up the wheel, the satellite rotates in the opposite direction. Three reaction wheels mounted along orthogonal axes (plus typically a fourth for redundancy) allow full 3-axis control without consuming propellant. The trade-off: reaction wheels gradually accumulate angular momentum from external torques (solar radiation pressure, gravity gradient), requiring periodic "momentum dumping" using magnetorquers or thrusters. The Hubble Space Telescope and Kepler space telescope both experienced mission-critical reaction wheel failures.