Space Elevator

[Nathan]

I not an expert on Lagrange points, but the way I see is like this:

L1, L2 (other side of the Moon), and L3 (other side of the Earth) should all lie in a straight line. If an object is on the imaginary line, but not located at a Lagrange point it will either fall towards the Earth or Moon, or fly off into outer space, but it won't deviate from the line.

[edit]Just to preempt any possible correction. The above is not actually correct. The coriolis effect will cause the asteroid to move off the line denoted to the L1 and L3 Lagrangian points. But for the amount of variation we're talking about (60km-ish) the effect would be tiny.[/edit]

In the case of a Moon elevator, it should be positioned Earthward of L1. This will mean that the asteroid will try to fall towards the Earth. However, the cable, pulling in the opposite direction, will balance the attraction of the Earth and keep the asteroid stationary. The more massive the asteroid is the closer it can be to L1 while still being able to resist the cable's pull. However, it can never be situated at L1 (unless it is infintely massive).

 

[3dilettante]

That sounds reasonable with what I know about orbital mechanics (pretty much nothing).

I guess it's just the 58,000 km thing that gets me. Just making the thing, then maintaining it, and then hoping nothing goes wrong along the entire length.

Who's going to clamber along a 58,000km ribbon to make sure it's in tip top shape?
Maybe little robots, but then there's going to be some poor sap who has to fix the robots.

Then there's the chance of micrometeorite damage. If the cable had a cross section width of a meter (just a round number), it would present a target of 58 square kilometers to get hit by space debris. I don't know the odds of an impact in lunar orbit, but the elevator is going to be there for a long time, and I'd hate to see it get severed and send an asteroid coasting towards earth.

We'd need some kind of self-healing or massively redundant system to prevent problems.

 

[Nathan]

I fully agree. It might be theoretically possible, but practically it's a complete nightmare. A meteorite strike would be disasterous. Even a minute meteor would cause a significant surface flaw that would seriously compromise the strengh of the cable. I suppose some sort of protective coating could be added, but I think 58,000Km of it would be prohibitively heavy for a structure that's most likely already operating with a very low factor of safety.

1 metre diameter is far too much. I would think it would be around 1mm if the 6800kg quoted weight for the cable is correct. But this just makes me wonder even more. How could 200kg be moved at hundreds of kilometres per hour on a 1mm cable? I was assuming that the cable would be anchored to the Moon, but perhaps it isn't. (or two cables are required) :?

 

[MPI]

Ponder this: how much rope could you haul down an infinite abyss?

A lot!

Well, say that you haul out 100'000 metres of rope. How much would 100'000 metres of rope weigh?

Say it's a light rope, 1kg/10m. That's 10 tons of rope. The weight of the entire rope will stress a cross-section at the top end.

There are roughly 36'000'000 metres to geosynchronous orbit. Now g decreases from ~9.8 towards 0 at geosynchronous orbit, but I think you get the idea of the problem. Then factor in dynamic stress... Hoo, boy.