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Comment by Udo | Hacker News Reader

It would be at risk, yes, and probably more so than most space vehicles. Gladly our space junk hasn't yet reached the point where surprise cascades happen so we can track debris well in advance. But we'll be needing real countermeasures, active spacecraft that can clean up our orbits.

One interesting aspect of the space elevator remains its failure modes though. Cable failure, for any reason, is going to be a problem. We'll need a safety concept both for the ground as well as the payload if humans are going up there. I hope this isn't going to be one of these things where everybody ends up agreeing that it's not feasible to save lives in the event of catastrophic failures.

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"We'll need a safety concept both for the ground"

Think about it... without a working active stabilization system and a carefully engineered for re-entry shape, "stuff" that re-enters usually doesn't make it very far in the atmosphere before being turned into dust.

We have a long way to go until an elevator can be as large as a dino-killer asteroid. Existing designs have spectacular surface are to volume ratios, there won't be much left...

This is, however, a very serious problem on the moon or perhaps mars. And because the moon's gravity is so low compared to the earth, we're almost certain to have elevators all over the moon before the first one on the earth. A moon-vator is so small and light we don't even need new tech to pull it off, just drop a couple billion and we can do it within perhaps 3 years if done privately. Maybe 30 if NASA runs it.

So yeah, its a problem, but not for legacy earthlings.

The space elevator would be flung out to space, in the event of breakage.

Except for the part that comes down to Earth, depending on the place where the ribbon breaks.

I've seen simulations. Parts of it are ejected far away, but other parts whiplash to the ground with great energy. Luckily, a large part of the whip may burn out in atmosphere, but there's still a large remaining portion moving at hypersonic speeds that you need to worry about.

From what I've read of carbon tube based cables, most of the length would burn up in the atmosphere before reaching the ground.

Edit: VLM does have a point in that a falling cable might be a problem on Mars. The moon doesn't have enough rotation for a fixed skyhook (space elevator) anyway. :-) More advanced rotating elevators in a moon orbit, maybe. But it might be hard because of the moon mascons (the moon is lumpy internally with different density, which results in gravitational anomalies).

    The moon doesn't have enough rotation for a fixed skyhook (space elevator) anyway.

Actually, it does: http://en.wikipedia.org/wiki/Lunar_space_elevator

You're just fairly restricted in where you can put it- you can only put one near the sub-Earth and anti-Earth points, such that the cable passes through the L1 or L2 Lagrange points, rather than anywhere along the equator. The cable would have to much, much longer than for a space elevator on Earth, but the Moon's lower gravity actually makes the engineering challenges Not As Bad- i.e., we could build a lunar space elevator with current materials.

OK ok, but it is economical insanity.

On the moon, an electric rail launcher with 5-10G only needs to be a few km long.

(Iirc, 1G acceleration of four minutes is the launch velocity of the moon? ~ 2.4 km/s.)

Edit: This assumes local materials for the electric launcher. If everything is skipped from Earth, it might be different. But, consider -- if you need to launch so much stuff from the moon, you must already have the infrastructure to build things there.

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