6 ms·

If one end is tethered to the moon, and the other end is in geostationary orbit...you're gonna have a bad day. The moon is not in geostationary orbit. Not an o

If one end is tethered to the moon, and the other end is in geostationary orbit...you're gonna have a bad day. The moon is not in geostationary orbit. Not an orbital scientist but doesn't seem that will work out very well.

deleted 7y ago
[deleted]
empath75 7y ago
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).
nabla9 7y ago
Other end is not 'in geostationary orbit' it's at the same height from the earth as earth's geostationary orbit.
Majestic121 7y ago
It would be an issue if you expect it to always end up to the same place, but if you don't care where the elevator ends on the Earth side, because you know it will be in your area at regular intervals, it can be dealt with. I'm not sure why they picked geostationnary orbit distance if they don't look for locked location though, it seems the risk of colliding with objects would be greater at this height
- extropy 7y ago
  Since the tether will be moving a lot slower than sattelites in geostationary orbit, and the risk of collision, I assume stopping just short of geostationary is the closest that will be deemed reasonably safe.
otroqueccorre 7y ago
The article mentions: Future moon travelers will still have to ride a rocket, though, to fly up to the elevator’s dangling point, and then transfer to a robotic vehicle, which would climb up the cable all the way up to the moon. So in the end, we will need a rocket anyway.
excalibur 7y ago
It will actually work out better than the alternative, because the moon is tidally locked. An elevator anchored to the moon will always be pointed toward the earth, an elevator anchored to the earth will rarely be pointed toward the moon.

deleted 7y ago

[deleted]

empath75 7y ago

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).

nabla9 7y ago

Other end is not 'in geostationary orbit' it's at the same height from the earth as earth's geostationary orbit.

Majestic121 7y ago

It would be an issue if you expect it to always end up to the same place, but if you don't care where the elevator ends on the Earth side, because you know it will be in your area at regular intervals, it can be dealt with. I'm not sure why they picked geostationnary orbit distance if they don't look for locked location though, it seems the risk of colliding with objects would be greater at this height

extropy 7y ago
Since the tether will be moving a lot slower than sattelites in geostationary orbit, and the risk of collision, I assume stopping just short of geostationary is the closest that will be deemed reasonably safe.

otroqueccorre 7y ago

The article mentions: Future moon travelers will still have to ride a rocket, though, to fly up to the elevator’s dangling point, and then transfer to a robotic vehicle, which would climb up the cable all the way up to the moon. So in the end, we will need a rocket anyway.

excalibur 7y ago

It will actually work out better than the alternative, because the moon is tidally locked. An elevator anchored to the moon will always be pointed toward the earth, an elevator anchored to the earth will rarely be pointed toward the moon.