Why the world still awaits its first space elevator

https://www.economist.com/blogs/economist-explains/2018/01/economist-explains-21

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What could possibly be difficult about building a 36,000km railway to space?

Jan 30th 2018by T.S.

FOR decades engineers and science-fiction writers have dreamed of lifts capable of carrying things into orbit from the Earth’s surface. Konstantin Tsiolkovsky, a Russian scientist, suggested the idea in 1895, inspired by the Eiffel Tower. And in 1979 Arthur C. Clarke wrote an entire novel, “The Fountains of Paradise”, about the construction of such a space elevator. Thanks to SpaceX and other private spaceflight companies, rocket launches have fallen in price in recent years. Each launch of the Falcon Heavy (猎鹰重型运载火箭), which will become the beefiest rocket in the skies when sent up by SpaceX next week, costs around $90m. But whisking satellites, space probes and even people into orbit on a giant elevator might be cheaper, more reliable and more civilised than using giant fireworks—if one could be built. Unfortunately, the technical challenges are formidable (艰难的).

The basic idea of a space elevator is to run a fixed cable from a point on the Earth’s equator to a space station directly overhead, in geostationary orbit (地球静止轨道) (that is, at an altitude of 36,000km). Objects at that altitude circle the planet once a day, so they have the useful characteristic of appearing to hover over a fixed spot. Cargo-carrying vehicles can then be run up and down the cable. They need to be powered on the way up, but can reclaim energy as gravity helps them on the way down. These vehicles would have to be quite large to carry people: even if they moved at 500kph (千米/小时), the trip in each direction would take three days. And building a 36,000km-long high-speed railway on Earth would be hard enough. Building a vertical one into space would be much more difficult.

The chief obstacle is that no known material has the necessary combination of lightness and strength needed for the cable, which has to be able to support its own weight. Carbon nanotubes (碳纳米管) are often touted (被吹捧的) as a possibility, but they have only about a tenth of the necessary strength-to-weight (强度重量比) ratio and cannot be made into filaments (细丝) more than a few centimetres long, let alone thousands of kilometres. Diamond nanothreads, another exotic form of carbon, might be stronger, but their properties are still poorly understood. Even if a suitable material could be found, the part of the cable within the atmosphere would be subject to weather disturbances, and the vehicles running up and down it could also cause dangerous oscillations (振动). Anchoring it to a moveable, seagoing platform might help, but keeping the cable steady would still be a tall order. A further worry is collisions: there are thousands of satellites and other items in orbit around the Earth, from an altitude of around 2,000km upwards. Any impact with the cable could cause disaster.

True believers in space elevators continue to look for ways around these problems, but they may be insurmountable (不能克服的). The idea refuses to die, however, possibly because of its elegance and simplicity. Perhaps the dream will be realised, just not on Earth. Building a space elevator between the moon’s surface and lunar orbit (to transport things such as visiting tourists or material mined on the moon) would be far easier, because of the weaker gravity and lack of atmosphere. Anyone hoping to take a space elevator into orbit from Earth, however, faces a long wait.