The reaction wheels is powered by electricity from the solar panels. The reason (as I understands it) to power down the reaction wheels is to minimize the wear on bearings and motors. Using the attitude jets for turning is just another way of aiming the craft. What they have done is simply slowed down the speed it turns. A small acceleration and a small brake takes less fuel than a larger acceleration and a harder brake. It just takes longer time.

For a solar powered spacecraft energy is free, reaction mass is expensive. The hydrazine and xenon is basically the same thing, to xenon energy is added from solar power while hydrazine contains it in it's chemical structure.

/Göran

Steve Dunklee wrote:
This might be a stupid question but I was wondering if a future craft similar 
to this could use the ion propulsion unit to power the reaction wheels. And if 
the rection wheels or gyros are powered up to full speed before the launch if 
it could reduce the amount of energy neeeded? In addition a high powered maser 
could be used for attitude adjustment from earth or space withought using the 
energy aboard the craft. If designed correctly the craft could be repowered for 
attitude adjustment indefinately from earth instead of having to launch the 
energy from earth. Cheers Steve


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