Reaction wheel
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.
Reaction wheels are used primarily by spacecraft for three-axis attitude control, and do not require rockets or external applicators of torque, which reduces the mass fraction needed for fuel.
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.
This has the effect of tending to stabilize that spacecraft axis to point in a nearly-fixed direction,[2]: 362 allowing for a less-complicated attitude control system.
These methods include magnetorquers (better known as torque rods), which transfer angular momentum to the Earth through its planetary magnetic field requiring only electrical power and no fuel.
Since its fourth maneuver[6] in its elliptical orbit, to prevent shakes when the amount of liquid fuel ran low, there was a need to use a reaction wheel.
These events probably caused failures by inducing electrostatic discharge in the steel ball bearings of Ithaco wheels, compromising the smoothness of the mechanism.
[14] 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.
[15][16][17][18] 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.
It was originally scheduled to depart Vesta and begin its two-and-a-half-year journey to Ceres on August 26, 2012;[22] however, a problem with another of the spacecraft's reaction wheels forced Dawn to briefly delay its departure from Vesta's gravity until September 5, 2012, and it planned to use thruster jets instead of the reaction wheels during the three-year journey to Ceres.
[23] 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.
