Last week, the signal was sent too early to the Progress to fire the thrusters ( rockets ) and the reaction wheels fought that move,
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Observations were halted after one of the spacecraft's four " reaction wheels " began to fail, causing Cassini to switch to a different maneuvering system.
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Newman and Massimino, during the second walk, are to switch out the second solar array and replace one of the telescope's four reaction wheel assemblies.
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On May 15, 2013, NASA announced the spacecraft had been crippled by failure of a reaction wheel that keeps it pointed in the right direction.
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:Great swooping turns in space to face an enemy rather than just rotating with thrusters or reaction wheels would would be be very wasteful of fuel.
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Since the reaction wheel is a small fraction of the spacecraft's total mass, easily controlled, temporary changes in its speed result in small changes in angle.
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A simple implementation of this can be the application of the proportional control for nadir pointing making use of either momentum or reaction wheels as actuators.
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JWST also has six reaction wheels for attitude control, which are spinning wheels that allow the orientation to be changed without necessarily using propellant to change momentum.
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The excursion would have permitted the replacement of one of the telescope's four reaction wheels, which move the barrel of the telescope from one celestial target to another.
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Also changed out was one of four Reaction Wheel Assembly units that use spin momentum to move telescope toward a target and maintain it in a stable position.
