Re: Reaction wheels
Reaction wheels and gyros only work to alter the orientation of a spacecraft while they are spinning. Slow them down to a stop to save electricity and wear and tear on the bearing etc. and the spacecraft settles back into its original orientation.
Reaction wheels will set a fixed orientation: assuming the probe is in a stable orientation spin up the wheels and the probe will begin rotating in response. Stop the wheels and that reaction is reversed stopping the rotation, in general in a new orientation.
What reaction wheels can't do in a sustainable manner is compensate for an externally induced pre-existing rotation, in the long term inevitable even in deep space, especially when slingshotting around. To correct that you need to eject mass, i.e. thrusters.