Technical
Reaction Wheel
A spinning flywheel used to control a spacecraft's orientation by exchanging angular momentum, without expending propellant.
A reaction wheel changes a spacecraft's attitude by spinning up or down: by conservation of angular momentum, accelerating the wheel rotates the spacecraft in the opposite sense. A set of wheels provides three-axis control.
Wheels offer fine, propellant-free pointing but can saturate, requiring periodic "desaturation" using thrusters or magnetorquers. They are a core actuator within the attitude control subsystem.