thumb|upright|right|A small reaction wheel viewed in profile
thumb|upright|A momentum/reaction wheel comprising part of a high-accuracy Conical Earth Sensor to maintain a satellite's precise attitude
A reaction wheel (RW) is an electric motor attached to a flywheel, which, when its rotation speed is changed, causes a counter-rotation proportionately through conservation of angular momentum. A reaction wheel can rotate only around its center of mass; it is not capable of moving from one place to another (translational force).
Reaction wheels are used primarily by spacecraft for three-axis fine attitude control, but can also be used for fast detumbling. Reaction wheels do not require rockets or external applicators of torque, which reduces the mass fraction needed for fuel. They provide a high pointing accuracy, and are particularly useful when the spacecraft must be rotated by very small amounts, such as keeping a telescope pointed at a star.
A reaction wheel is sometimes operated at a constant (or near-constant) rotation speed, to provide a satellite with a large amount of stored angular momentum. Doing so alters the spacecraft's rotational dynamics so that disturbance torques perpendicular to one axis of the satellite (the axis parallel to the wheel's spin axis) do not result directly in spacecraft angular motion about the same axis as the disturbance torque; instead, they result in (generally smaller) angular motion (precession) of that spacecraft axis about a perpendicular axis. This has the effect of tending to stabilize that spacecraft axis to point in a nearly-fixed direction, or a spare wheel carried in addition to a three axis configuration. Typically designers use "reaction control systems": arrays of small chemical rocket engines that fire as the wheels slow down to counter the torque the wheels are imparting on the spacecraft as they slow down. with the goal of landing on the Moon. Beresheet uses the low-energy transfer technique to save fuel. Since its fourth maneuver in its elliptical orbit, to prevent shakes when the amount of liquid fuel ran low, there was a need to use a reaction wheel.
The James Webb Space Telescope has six reaction wheels built by Rockwell Collins Deutschland.
LightSail 2 was launched on 25 June 2019, focused around the concept of a solar sail. LightSail 2 uses a reaction wheel system to change orientation by very small amounts, allowing it to receive different amounts of momentum from the light across the sail, resulting in a higher altitude.
Failures and mission impact
The failure of one or more reaction wheels can cause a spacecraft to lose its ability to maintain attitude (orientation) and thus potentially cause a mission failure. Recent studies conclude that these failures can be correlated with space weather effects. These events probably caused failures by inducing electrostatic discharge in the steel ball bearings of Ithaco wheels, compromising the smoothness of the mechanism. Supporting this hypothesis, newer reaction wheels have non-conducting ceramic bearings, and none have failed in this manner.
Two servicing missions to the Hubble Space Telescope have replaced a reaction wheel. In February 1997, the Second Servicing Mission (STS-82) replaced one after 'electrical anomalies', rather than any mechanical problem. Study of the returned mechanism provided a rare opportunity to study equipment that had undergone long-term service (seven years) in space, particularly for the effects of vacuum on lubricants. The lubricating compound was found to be in 'excellent condition'.
In 2004, during the mission of the Hayabusa spacecraft, an X-axis reaction wheel failed. The Y-axis wheel failed in 2005, causing the craft to rely on chemical thrusters to maintain attitude control.
From July 2012 to May 11, 2013, two out of the four reaction wheels in the Kepler space telescope failed. This loss severely affected Kepler ability to maintain a sufficiently precise orientation to continue its original mission. On August 15, 2013, engineers concluded that Kepler's reaction wheels cannot be recovered and that planet-searching using the transit method (measuring changes in star brightness caused by orbiting planets) could not continue. Although the failed reaction wheels still function, they are experiencing friction exceeding acceptable levels, and consequently hindering the ability of the telescope to properly orient itself. The Kepler telescope was returned to its "point rest state", a stable configuration that uses small amounts of thruster fuel to compensate for the failed reaction wheels, while the Kepler team considered alternative uses for Kepler that do not require the extreme accuracy in its orientation needed by the original mission. On May 16, 2014, NASA extended the Kepler mission to a new mission named K2, which uses Kepler differently, but allows it to continue searching for exoplanets. On October 30, 2018, NASA announced the end of the Kepler mission after it was determined that the fuel supply had been exhausted.
The NASA space probe Dawn had excess friction in one reaction wheel in June 2010. It was originally scheduled to depart Vesta and begin its two-and-a-half-year journey to Ceres on August 26, 2012; The loss of the reaction wheels limited the camera observations on the approach to Ceres.
On the evening of Tuesday, January 18, 2022, a possible failure of one of the Swift Observatory's reaction wheels caused the mission control team to power off the suspected wheel, putting the observatory in safe mode as a precaution. This was the first time a reaction wheel failed on Swift in 17 years. Swift resumed science operations on February 17, 2022.
Similar devices
A control moment gyroscope (CMG) is a related but different type of attitude actuator, generally consisting of a momentum wheel mounted in a one-axis or two-axis gimbal.