Ronald Brak

Because not everyone can be normal.

Friday, September 30, 2005

Junking the Space Elevator

I think a space elevator is a really cool idea. Unfortunately, cool is not the same as practical. While I am certain that we will eventually be able to make cables of such strength they could theoretically be suspended from orbit, I'm far from sure we can make a space elevator cable that will be able to withstand the inevitable damage it will sustain.

Any cable that is put under tension stores up energy. If a steel cable that is being used to tow a truck snaps, it can whip around with enough force to kill someone. The cable of a space elevator will be under incredible tension. If a single fibre breaks the shockwave of energy released could destroy adjacent fibres and set off a chain reaction that snaps the entire cable. The easiest way to avoid this problem might be to use a "mesh bag" design in which one strand breaking won't propagate enough energy to other strands to destroy them.  There are people who think this design will work.  I have no idea if they are right. But even if we can make a cable robust enough to survive if one fibre snaps, it might not take long for it to be severely damaged or destroyed by impacts with space junk or meteorites.

A space elevator cable can be moved to avoid collisions with larger objects that can be tracked by radar. However there is no way to avoid the many smaller pieces of junk ranging from centimetres across down to particles the size of a speck of dust. Dangerous impacts don’t happen all the time, otherwise satellites wouldn’t last as long as they do, however the space shuttle suffers numerous impacts with tiny pieces of space junk and tiny meteorites on every mission, which typically lasts less than two weeks. Space shuttle windows are replaced on average every other mission as a result of impacts. One impact left a hole about half a centimetre deep. If the space shuttle windows which are only about a couple of square meters in area are hit so often, then a space elevator cable, which in one design will be a a thin ribbon about a metre across, can expect to be hit often as well.

Although only about 2,000 kilometres of cable will pass through areas with significant space junk, a square meter of cable can expect to be hit more often and with particles travelling at greater velocities than a square meter of space shuttle. This is because the space shuttle and space junk all orbit from east to west, even objects in high latitude orbits. When the space shuttle orbits there will also be a portion of space junk that will be in a similar orbit and so will have a low relative velocity. However, only the part of the space elevator cable at 36,000 km will actually be in orbit. In lower orbit, the cable will almost be standing still compared to the space junk, and many pieces will collide with it at speeds in excess of 10,000 kilometres per hour (2.8 kilometres per second). These impact speeds are fast enough to pit or chip diamond, although I am unsure of what the exact effects would be, as I lack a small bore rifle with which to take pot shots at my store of diamonds.

If we can’t make a cable that won’t destroy itself if even one fibre breaks, then it won’t be possible to build a space elevator. Impacts would soon destroy it even if it could be deployed intact. A ribbon like, meter wide cable could receive an impact like the one that put a half centimetre deep hole in the space shuttle window every couple of days. A significantly larger impact could be expected every month and so on.

So if we can build a a cable that can survive multiple minor impacts for an extended period of time, then it will need to be either regularly repaired or replaced as a result of damage from these impacts. If it is to be replaced that will greatly increase the cost of using space elevators. Repair would be difficult unless we had robot climbers that could manipulate carbon molecules in such a way as to perform flawless repairs in material under incredible amounts of tension. However, if we had robots that could do that, we probably wouldn’t need a space elevator to haul cargos up from earth as robots with such brilliant ability to manipulate matter could probably make whatever we want from materials found in space.


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