Hacker News Reader

Comment by barbegal | Hacker News Reader

What they don't mention is that orbiting the L2 point is unstable requiring constant manoeuvres to keep the satellite in position. You can think of it like going around the top of a hill. The top of the hill is relatively flat so it didn't require much energy to overcome gravity but after some time you end up rolling down the hill unless you apply some force to keep you in position.

View on HN

> You can think of it like going around the top of a hill

I always wondered: a ball is not stable on a hill since you have to balance it on the top, but a ring is. Could a large ring-size space-station be stable at these points by having them at their centre?

EDIT: looking here: https://www.youtube.com/watch?v=JZj9zINN3e0 it looks like more of a saddle. So maybe a space station of 2 parts tethered together with their centre-of-mass at L2? Since space is frictionless (as opposed to an actual saddle surface) I guess there will still need to be station-keeping, but only in on direction: maybe my allowing synchronised "ballast" weights to fall in/outwards.

I think the metaphor only sort-of works when you represent the object as a point. The farther you are from an unstable point of equilibrium, the harder you're getting pushed away from it. So if your station gets nudged, the farther part will push the station away harder than the closer part. Overall it probably changes nothing about how much effort is required to keep the station in place.

I thought that the definition of the Lagrange point was that it required no energy to remain in that position. Or does that happen because the Earth and the moon wobble a bit and therefore the exact position of the Lagrange point shifts?

I don't know much about Lagrange points specifically, but it sounds like this one is an "unstable equilibrium". Even if you could park on exactly the right point, any tiny force (changing gravitational pull of other planets, dust impact, even light pressure from the sun eventually) would nudge you off of it and you would start accelerating away from it.

L1-3 are unstable, L4 and L5 are not.

https://en.wikipedia.org/wiki/Lagrange_point#Point_stability

L4 and L5 are stable (see Jupiter's asteroid collection), but the others involve essentially trying to balance on a infinitesimal pinpoint, with any perturbation causing one to "fall off" and move away.

Yeah the stable point is a literal geometric point. And there's a whole solar system's worth of little gravitational disturbances going on.

James Webb space telescope will be at L2 point. My point is that station keeping is probably not that hard.

JWST won’t sit directly at L2. Instead it will orbit around that point in a “halo orbit”.

Reply on news.ycombinator.com