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Comment by rietta | Hacker News Reader

The experiment sounds great. But should this work and a real cable to orbit is built, what happens when the cable snaps!? Can such a structure be made failsafe?

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Kim Stanley Robinson [0] explores some of the failure modes when bad actors are involved. Bad actors with serious weapons. People on the satellite end get flung off into space, the cable wraps itself around the equator, and people on the elevator had better hope that there are nearby spacecraft to pick them up.

[0] https://en.wikipedia.org/wiki/Mars_trilogy

There are various simulations of what may happen. Here is one that demonstrates what may happen if the cable breaks at various points.

http://gassend.net/spaceelevator/breaks/

In summary, if the cable breaks at the base, the elevator will fly harmlessly into space. If it breaks at the counterweight, a large portion of the cable (several thousand miles long) will collide with the Earth’s equator.

The piece that falls to Earth ends up wrapping faster and faster, this causes centrifugal force on the tip, increasing the tension in the ribbon. Often the ribbon breaks on its way down and some fragments go flying out of Earth's gravity well. I didn't expect this at all.

The space elevator would become a space whip. I wonder whether this has a use case, like getting hardened unmanned space probes to speeds required for interstellar missions, or just much faster missions to other planets in the solar system.

Space elevators can be used this way because once the tether goes beyond geostationary orbit the tether is going above orbital velocity. Go far enough and its above escape velocity. So you just drag a probe out far enough and let go and it will escape the gravity well.

Harmless, except for the passengers.

Some proposals include spacing charges along the cable to break it into small sections and enable it to come down more safely. Of course, this means lining your vulnerable cable with explosives, which seems like a bit of a design smell.

Physics would tend towards "No."

Assuming a catastrophic break near the upper end of the cable (before the counterweight), there's not much you can do to keep it from coming down.

Perhaps unless you decide to detach the cable at the earth end? You might be able to react to a break faster than the cable can fall.

Even if you could react and cut the cable in 1 ms, if the counterweight is disconnected, the rest of the cable is coming down to Earth.

Depending on what it's made of, a break in the cable wouldn't be instantaneous, but happen in stages. Perhaps cut the earth end at the red alert stage up top.

That's what safeties are for! We already have a lot of engineering around making elevators safe. TLDR the elevator will clamp on to the rails to slow the free fall. Neat info below on snapping elevator cables and some of the things that would help you. https://science.howstuffworks.com/science-vs-myth/everyday-m...

What rails? They're not going to run more than one of these difficult-to-build structures up for each elevator. There will be a single cable, and the car will climb it. What happens if the cable snaps will depend on how far up the car is. Low enough, and it will descend to the surface on a builtin parachute. High enough, and it will perform some portion of an orbit, before descending on its parachute.

Presumably you could have redundant cables. But two almost adjacent cables would still be exposed to many single points of failure, especially any caused by a deliberate attack on the system.

And just a cable falling could be a significant disaster by itself.

Once you have a space elevator, it’s relatively easy to send up some rockets and demolition explosives that just sit on the cables at appropriate locations and stay there waiting for a disaster; should the cable be broken by malice, accident, or natural causes, the explosives can cut the falling cable into pieces small enough that the rockets can safely guide to the ocean (or orbit, whichever has the lowest delta-v).

The travel pods, likewise, can always have rockets on them whose sole purpose is to get the passengers to safely. Yes, it means you’re taking up more mass than you strictly need to, but the great thing about a space elevator is that the mass is no longer a hyper-critical parameter.

But yes, no rails here.

The rails? The car will travel over a stationary structure, not be pulled up by a pulley like a conventional elevator is. So it is failure of that stationary structure that we are talking about.

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