Reaction Wheel
A spinning flywheel used for precision attitude control of a spacecraft without expending propellant.
Explanation
Reaction wheels operate on conservation of angular momentum. When the motor speeds up or slows down the wheel, the spacecraft body rotates in the opposite direction. Three or four reaction wheels (one per axis plus a redundant spare) provide full three-axis attitude control. The key advantage over thruster-based control is precision: reaction wheels can make minute adjustments to a fraction of an arcsecond, which is critical for pointing telescopes, laser communications terminals, and Earth-observation instruments. Reaction wheels cannot absorb unlimited momentum. Over time, external torques from solar radiation pressure, gravity gradients, and magnetic fields cause momentum to build up. When wheels reach their saturation speed (typically 3,000-6,000 RPM), operators perform a momentum desaturation maneuver using thrusters or magnetorquers to spin down the wheels. Reaction wheel failures are a common spacecraft anomaly — bearing friction, lubrication degradation, and vibration can shorten wheel life.
Why It Matters
Reaction wheels enable the pointing accuracy required for high-resolution imaging, spectroscopic measurements, and laser communications. Without them, most precision space missions would be impossible.
Concept Map
How Reaction Wheel connects to other glossary terms:
Frequently Asked Questions
Why use reaction wheels instead of thrusters?
Reaction wheels provide much finer pointing precision and do not consume propellant. Thrusters are used for large maneuvers and momentum desaturation.
What happens when a reaction wheel fails?
Satellites with a fourth redundant wheel can continue operating. Otherwise, the satellite may lose fine-pointing capability or enter safe mode. Several famous missions (Kepler, Hayabusa) suffered reaction wheel failures.
Sources
Last updated: July 1, 2026