Thursday, November 21, 2002

Yahoo has a piece (via the almighty Instapundit ) on space elevators. Basically, feasibility seems not to be an issue at this point: it is a case of getting the logistics and funding right. This is actually very exciting. We are really not in an environment to raise money for such a thing at the moment, however. Large sums have been lost on unviable space projects like Iridium and Globalstar recently, so I cannot imagine the private sector looking at something like this soon. But in ten or twenty years, who knows. (I have given a quick summary of how space elevators theoretically work at the end of this posting, which you may want to jump to if you are not familiar with the idea).

In the medium term, I cannot see this not happening. Although the fixed costs of such an elevator would be enormous, once it was there the marginal cost of taking a kilogram of payload into space would drop by several orders of magnitude, and suddenly space would be accessible for normal rather than astronomical sums of money. Middle class space tourism might become possible, and a huge number of applications we haven't yet thought of will suddenly be affordable. The infrastructure will be in place, and people will then be able to find their own applications for it. Space travel has been limited at this point because the cost of lifting payloads is simply too immense. The dreams of the science fiction writers of the 1940s and 1950s appeared to suddenly come true in the 1960s, and then it all stopped, I think largely due to this one factor. There was no Moore's Law. The cost of chemical rockets remained immense.

However, on the sadder side of today's climate, we need to think about the other question, which is how vulnerable such a construction would be. Kim Stanley Robinson's three novels about the future colonization of Mars seem to have quite a lot of relevance to the present crisis, and they have some here, too. Towards the end of Red Mars the colonists of Mars fight a war in which they attempt to obtain independence from Earth. A space elevator (which is much easier to build on a small planet like Mars) has been built earlier in the novel, and one of the first things the colonists do in their war is to disconnect the cable of the elevator from its anchor, in order to prevent large numbers of troops from being landed on Mars. The cable then falls to Mars. As it does so, Mars rotates, and so the cable heats up and wraps itself around the planet as it falls, eventually burning a groove all the way around the equator of the planet.

The question is, if an elevator was built on Earth, would there be a danger of such a thing happening, either through accident or war or terrorism? Such an elevator would have a cable long enough to wrap itself around the earth once, which could do quite a lot of damage. The cable would be built to withstand somewhat more than its maximum tension, so it seems unlikely that it would actually break in an accident, so the question is what sorts of shocks it could withstand. Its tension would be so great that (say) colliding an aircraft with it would be most unlikely to break it. It would likely slice the aircraft into two like a breadknife, but it probably wouldn't break. In any event, if the cable were to break at an altitude that an aircraft could reach (only about 15km) up, the bulk of the cable (and its anchor) would simply drift slowly up into space. A few kilometres of cable below the break would fall into the ocean, and even if there was some land in its path, it wouldn't be moving fast enough to do much damage at the time it hit. The issue is whether the cable could be broken or disconnected in space. Obviously it would be very necessary to put all sorts of precautions at the point at which the cable is anchored. It seems to me that it would be possible to make this pretty invulnerable. The question then, is how would it be possible to break the cable. The only answer seems to be that you would have to melt it. That is, explode a large bomb at the top of the cable or somewhere along it. It seems possible to build a carbon nanotube that would survive up to in excess of 5000 degrees Celcius . Most chemical explosions achieve a lot less than this. The question is can such a cable withstand a short, sharp, physical shock from such an explosion. I don't know the answer, but given the tensile strength of the substance I am guessing the answer is reasonably good. It is worth bearing in mind that any explosion near the cable is likely to be short lived, as you are in space and there is therefore no oxygen for a fire, so a WTC like affair where the structure is weakened by a prolongued fire is not likely to be possible. This all suggests that the cable is likely to able to withstand most forms of chemical explosion reasonably well. It is going to be very hard for terrorists to bring the thing down without a nuke, and it is fairly easy to prevent terrorists from brinking a nuke onto the elevator. (Just monitor for radiation). So, actually I think it would be pretty safe. I'm reassured.

(If you are not familiar with the concept of a space elevator, the basic point is that an object orbiting at the so called Clarke orbit - 36000 km above the equator - will stay stationary over the same point. (Most communications satellites do this). If you elongate the object so that its centre of mass remains at this altitude, you can have an object in such a stationary orbit even though most of the object is not at this altitude. Extend a very strong cable to the earth's surface from such an orbit and at the some time build a counterweight higher up, and bingo, you have an elevator cable into space. (You have also created a freestanding structure that is held up by the sky). Some proposals involve capturing a small asteroid as a counterweight to anchor the cable to, but this sounds perhaps too hard to me. I suspect we will have to provide our own counterweight. Many of us became familiar with this idea when Arthur C Clarke wrote the novel The Fountains of Paradise about the idea in 1980, although the idea is around 80 years old. The key issue was always that nobody knew how to build a cable strong enough to not break. Well, now we do.)

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