Cassini's Reaction Wheels

Cassini uses three electrically driven reaction wheels (also called momentum wheels) for routine control of the spacecraft's orientation. Three are mounted near the bottom of the spacecraft, mutually perpendicular to each other. The fourth reaction wheel is a spare that can be articulated into a position in order to take over from any one of the others in case of failure.

In principle, angular momentum is traded back and forth between the wheels and the whole spacecraft. When a motor accelerates a wheel, the spacecraft begins to rotate in the opposite direction. Decelerating the wheel has the effect of rotating the spacecraft back again. Driving the three wheels in various combinations, under control of the onboard attitude control computers, can point the spacecraft in any desired direction.

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Reaction Wheels in the Basics of Space Flight