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I think it’s at geostationary orbit height.

mkl 7y ago
I don't really understand how it can stay at geostationary orbit height without being at geostationary orbit speed. Can someone explain?
- Majestic121 7y ago
  Well it's anchored on the moon at the other side
- extropy 7y ago
  It's dangling down from the moon. Going at moons speed - 28 days per revolution instead of geostationary 1 day. So less centrifugal force form rotation that is compensated by the cable strain.
- brians 7y ago
  The end is at that height. Its center of mass is much higher, and it's rigid enough to hold together. It's also not orbiting—its other end is nailed to the lunar surface.
- sathackr 7y ago
  Well I started thinking about it more. If it's orbiting at the speed of the moon, it's going slower than geostationary orbit speed. So its tendency will be to fall towards the earth. But since it's attached to the moon, this won't happen. Not sure how much station keeping would be needed to hold this in place. Also will have to deal with collision risks with objects in geostationary orbit. Though I don't think geostationary orbit is a particularly important factor. Could probably be a few thousand Km above geostationary orbit without any significant adjustment to their plan.
  - mcv 7y ago
    Crossing orbits full of satellites is going to be an even bigger problem with an Earth-based elevator. LEO is much more crowded than Geo. But it would get really interesting if we were to first build this moon-based elevator, and later once the tech is good enough, also an Earth-based one. Because that one is not going to extend merely to geostationary orbit, but well past it; the center of gravity is going to be in geostationary orbit. So over a distance of 36,000 km, there will be two space elevators zipping past each other once per day.
  - jacobush 7y ago
    The tether is thin. The orbit is kinda wide at that altitude. It's not going to hit anything, even if you tried.
    - amalcon 7y ago
      Surely, for this to be useful, we need to be able to hit it if we try. Agreed that anything not trying to intercept should be able to avoid that by not trying.
      - jacobush 7y ago
        Haha touche! I meant even if you tried with the tether to swipe something. But we should be able to hit it with a rocket
- yk 7y ago
  It is not a point. Consider two masses connected by an rope, one in an higher orbit. The entire contraption will move along their common center of mass, and there will be tension on the rope. The same thing applies for an lunar elevator, one end is somewhere close to geostationary orbit, the other is the moon, and the entire system moves as a combination of internal tension and the movement of the center of mass.
- nabla9 7y ago
  The tether is hanging from the moon, not orbiting earth. There is tension in the tether. It's not in free fall.
- cwmma 7y ago
  because it's attached to the moon, which is preventing it from falling into the earth which it would otherwise do at that altitude going that slowly.
- miahi 7y ago
  And the Moon orbit is not a perfect circle, the Earth-Moon distance varies quite a bit (about 40000km between perigee and apogee), so to actually keep the cable at the same earth altitude you will need to change its length quite a bit (seems like around 125km/hour).

mkl 7y ago

I don't really understand how it can stay at geostationary orbit height without being at geostationary orbit speed. Can someone explain?

Majestic121 7y ago
Well it's anchored on the moon at the other side
extropy 7y ago
It's dangling down from the moon. Going at moons speed - 28 days per revolution instead of geostationary 1 day. So less centrifugal force form rotation that is compensated by the cable strain.
brians 7y ago
The end is at that height. Its center of mass is much higher, and it's rigid enough to hold together. It's also not orbiting—its other end is nailed to the lunar surface.
sathackr 7y ago
Well I started thinking about it more. If it's orbiting at the speed of the moon, it's going slower than geostationary orbit speed. So its tendency will be to fall towards the earth. But since it's attached to the moon, this won't happen. Not sure how much station keeping would be needed to hold this in place. Also will have to deal with collision risks with objects in geostationary orbit. Though I don't think geostationary orbit is a particularly important factor. Could probably be a few thousand Km above geostationary orbit without any significant adjustment to their plan.
- mcv 7y ago
  Crossing orbits full of satellites is going to be an even bigger problem with an Earth-based elevator. LEO is much more crowded than Geo. But it would get really interesting if we were to first build this moon-based elevator, and later once the tech is good enough, also an Earth-based one. Because that one is not going to extend merely to geostationary orbit, but well past it; the center of gravity is going to be in geostationary orbit. So over a distance of 36,000 km, there will be two space elevators zipping past each other once per day.
- jacobush 7y ago
  The tether is thin. The orbit is kinda wide at that altitude. It's not going to hit anything, even if you tried.
  - amalcon 7y ago
    Surely, for this to be useful, we need to be able to hit it if we try. Agreed that anything not trying to intercept should be able to avoid that by not trying.
    - jacobush 7y ago
      Haha touche! I meant even if you tried with the tether to swipe something. But we should be able to hit it with a rocket
yk 7y ago
It is not a point. Consider two masses connected by an rope, one in an higher orbit. The entire contraption will move along their common center of mass, and there will be tension on the rope. The same thing applies for an lunar elevator, one end is somewhere close to geostationary orbit, the other is the moon, and the entire system moves as a combination of internal tension and the movement of the center of mass.
nabla9 7y ago
The tether is hanging from the moon, not orbiting earth. There is tension in the tether. It's not in free fall.
cwmma 7y ago
because it's attached to the moon, which is preventing it from falling into the earth which it would otherwise do at that altitude going that slowly.
miahi 7y ago
And the Moon orbit is not a perfect circle, the Earth-Moon distance varies quite a bit (about 40000km between perigee and apogee), so to actually keep the cable at the same earth altitude you will need to change its length quite a bit (seems like around 125km/hour).