The Science of Discworld Revised Edition

we’ve used it as a metaphor for processes like life.
    The idea of a space elevator was originated by the Leningrad engineer Y.N. Artsutanov in 1960, in an article in
Pravda
. He called it a ‘heavenly funicular’ and calculated that it could lift 12,000 tons per day into orbit. The idea came to the attention of Western scientists in 1966, thanks to John Isaacs, Hugh Bradner, and George Backus. These scientists weren’t interested in getting into space: they were oceanographers – the only people seriously interested in hanging things on long cables. Except that they wanted to hang them down into the ocean bottoms, not up into space. The oceanographers were unaware of the earlier Russian work, but Artsutanov’s anticipation quickly became known to Western scientists too. The astronaut and artist Alexei Leonov published a painting of a space elevator in action in 1967.
    Such a simple but mostly impractical idea is likely to occur to lots of people, but wouldn’t become widely known
because
it looks impractical with current technology, and that means that it will be re-invented independently by many people. In 1963 the science-fiction author Arthur C. Clarke considered suspending a lower satellite by cable from a geosynchronous one, as a way to increase the number of effectively geo- synchronous satellites for communication purposes. Later he realized that the same method would lead to the space elevator, an idea that he developed in his novel
The Fountains of Paradise
. In 1969 A.R. Collar and J.W. Flower also considered suspending a lower satellite by cable from a geosynchronous one And in 1975 Jerome Pearson suggested an ‘orbital tower’ that was essentially the same idea.
    You can, of course, suspend more than one cable – once you’ve got
one
space elevator you can lift everything else that you need into space at low cost, so why not go the whole hog? Charles Sheffield’s
The Web Between the Worlds
envisages a whole ring of space elevators round the equator. This is what the wizards have found. Ironically, because human civilization has taken such a short time to develop, on evolutionary timescales, the wizards missed us …
    Is the space elevator anything other than a wild fantasy? Could one really be built in the near future? In 2001 two NASA teams carried out feasibility studies, and both concluded that the space elevator is technologically possible. Just. David Smitherman, who led one team, reckons it could be in place by 2100.
    The main problem is the cable. The tension in the cable is lowest near the ground, and highest at the top, because each section of cable has to support only the weight of cable below it. So the cable should be made thin at the bottom, and thicker towards the top. The big question is: which material has enough tensile strength? Steel won’t do: a steel cable 4 inches (10 cm) wide at the bottom would have to be 2.5 trillion miles (4 trillion km) across at the top. (This is an engineer’s way of saying ‘don’t use steel’: it’s too heavy and the stress goes up extremely fast as the cable gets longer.) Kevlar would be more practical: the top would then need to be only 1600 metres across – just over a mile wide. But even this is not practical enough.
    For the size to be acceptable, the cables tensile strength needs to be at least 62.5 gigapascals – 30 times stronger than steel and 17 times stronger than Kevlar. Such materials do exist: the best known is the carbon nanotube, a molecule of carbon shaped like a hollow cylinder and related to the famous molecule buckminsterfullerene, which is made from 60 atoms of carbon and is shaped like a soccer ball. The tensile strength of a carbon nanotube is at least 130 gigapascals, more than twice as strong as necessary. The only snag is, that right now the longest carbon nanotubes we can make extend for only a few millionths of a metre. But if that could be increased to 4 millimetres, then the nanotubes could be embedded in a composite material with the necessary strength.
    A second problem is the base. The higher the bottom of the cable is off the ground, the more material is saved at the top, where most of the mass is. This is why the cable in our story has a huge ‘etiolated whelk’ at its base. The NASA study concluded that a tower at least 6 miles (10 km) high would be best. It could be built on a mountaintop, to reduce the height needed, but if the cable were to snap, the main debris would then fall on land. So a tower