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Comment by SeanLuke | Hacker News Reader

I don't understand why the Telescope requires fuel at all, except to change its orbit position. Couldn't it just rotate counterweights via electric motors, powered by solar cells?

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The first reason is that it needs to reach its destination... IIRC the JWST needed to accelerate itself to avoid damage on its initial orbital insertion (rather than relying on the launch vehicle). It also needs to stop once it reaches its destination. Electric propulsion doesn't provide enough thrust for this particular mission. Electric is better suited for very slow acceleration buildups rather than quick corrections.

It does actually have reaction wheels to control attitude without propellant, but it needs fuel to unload the momentum. It's possible the JWST could get into a situation where electricity wouldn't unload the momentum fast enough.

https://en.wikipedia.org/wiki/Spacecraft_bus_(James_Webb_Spa...

https://jwst-docs.stsci.edu/jwst-observatory-hardware/jwst-m...

But its orbit isn't perfect or perfectly stable: Other forces than just the Sun-Earth system act on it, albeit very weakly. Some include photonic pressure (photons from the sun pushing on one side, not the other), the moon, a few scattered particles (space isn't perfectly empty), etc. And the fact that the JWST itself isn't a point mass means that it is tugged at by gravity slightly differently in different places. Then add to all of these the most important fact that L2 is an unstable equilibrium (a local potential maximum, like a hill), and the telescope is more like a dancer on its toes needing to right itself by throwing something every now and then.

All these things necessitate expending some fuel for station keeping.

Yes it does exactly this. It uses reaction wheels (circular counterweights) for rotation generally, but when they can no longer spin faster it will have to use propellant to adjust its angular momentum back to zero.

In addition, it uses propellant for orbit position keeping (station keeping) as you wrote.

AIUI (I'm no expert, someone will no doubt correct me if I'm wrong), the L2 Lagrange point is not stable. If you were perfectly at the L2 point the forces would be balanced and you'd stay there, but you'd have to perfectly at that point with zero perturbations, which is impossible.

It's like standing a sharp pencil on its tip - the slightest disturbance causes an accelerating fall, so it always falls. In the case of L2 any object placed there will always drift away with accelerating velocity.

To counteract this you need thrust, and that needs reaction mass. You can't do it with gyros etc, you have to chuck something out in the opposite direction to the way you want to push.

Edit: Incidentally (again not an expert) this sounds like maybe a useful quality of L2. L4 and L5 are stable which I assume means they're filled up with space garbage and odd socks, which maybe would be a problem for putting things there?

It needs propulsion to get into orbit. On its current trajectory, without some delta-v, it would zip right by L2. Once in orbit, it needs propulsion to stay there.

It also needs propulsion for attitude control. It does have reaction wheels, but once those are saturated you have to spin them back down. The only way to do that without transferring all that momentum back to the rest of the spacecraft is to use propulsion to counteract it.

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