A reaction wheel is a device used for making precise adjustments in a spacecraft’s orientation. It operates through the principle of conservation of angular momentum—accelerating a reaction wheel brings about a proportional response by the rest of a spacecraft. In this way, a spacecraft’s orientation can be controlled to the same high level of precision as reaction wheels themselves. Momentum wheels, on the other hand, are used to stabilize a spacecraft in motion.
In an isolated system, the total amount of angular momentum must remain the same. Angular momentum is analogous to linear momentum, except that it concerns spinning motion rather than linear motion. If an ice skater pushes off another stationary ice skater, they both move apart to conserve the net linear momentum. Similarly, if part of a system begins to spin in one direction, the rest of the system must spin in the opposite direction. Otherwise, the total angular momentum would change.
In the case of a spacecraft, a reaction wheel can be spun to induce a response from the rest of the spacecraft. Without this induced response, the total angular momentum would change. The final rotation rate of the spacecraft will typically not be the same as the speed of the reaction wheel, however. This is because angular momentum must be conserved, not angular speed. The spacecraft will be moving slower if it is much more massive than the reaction wheel.
Reaction wheel technology is generally well-developed. Metal wheels have been used for over a century on trains. The electronics and equipment to make fine adjustments to reaction wheels, likewise, have many terrestrial applications. Small rocket thrusters, on the other hand, have a limited history of development and refinement.
Occasionally, reaction wheels need to be “unloaded” if they are spinning too fast. In an unpredictable mission, engineers may not know in which direction a spacecraft will need to be pointed; therefore, a method of slowing down reaction wheels without inducing a response is necessary. Rocket thrusters can be fired to counter the response of slowing down a reaction wheel.
A momentum wheel also uses the principal of conservation of angular momentum. Its purpose in a spacecraft, however, is to maintain a certain orientation rather than change it. Various forces on a spacecraft—including air friction and varying gravitational and magnetic fields—can produce torques that disturb a spacecraft’s orientation. This is experienced on the Hubble Space Telescope, which depends on a stable orientation to focus on distant objects. When spun rapidly, momentum wheels can resist such disturbances and help maintain a steady orientation.