[mcherm]

Yes, the article uses those words. But an understanding of elementary physics tells us that momentum is conserved in an isolated system. Spending the reaction wheels can cause the outer frame of the satellite to rotate in a certain fashion but it cannot possibly alter the overall momentum or angular momentum of the entire satellite unless it somehow interacts with an external force.

[misnome]

Right, the reaction wheels help if you want to maintain orientation of the satellite under external forces that want to (gently!) rotate it. And eventually your reaction wheel saturates and you can't get more useful rotation out of it.

This is where thrusters are useful, because you can use the thrusters to give you a "source of force" while you spin the reaction wheels back down. I believe this is where the original parent's question is coming from - if you have no thrusters, how do you avoid saturation?

Presumably, they've worked out a way to accurately predict the external forces and gently operate against them while spinning the wheels down in a way that doesn't make the satellite spin faster.

[snovv_crash]

I guess the motors are lossy and some of it gets converted to heat if they speed the wheels up and down

[WJW]

That won't solve the conservation of momentum problems though, it'll just use more energy to get to the same state. If you could demonstrate controlling the rotation of a spacecraft just by using more inefficient motors you would upend a large portion of modern physics and could probably go and collect your Nobel prize within the week.

[snovv_crash]

Ok, read TFA now. It sounds like they need to dump energy from the reaction wheels. Maybe they can do this by inducing a spin, rotating 180 on a different axis then cancelling the spin?

[FeepingCreature]

You can't cheat conservation of momentum, no matter how cleverly you move your spacecraft. That's why all the solutions involve expelling reaction mass. (Or, in the case of solar wind, interacting with an external force.)