The first inkling of the concept goes back as far as Konstantin Tsiolkovsky who, in his 1895 paper "Day-dreams of Heaven and Earth," imagined escaping from the Earth by climbing a high tower:
On the tower, as one climbed higher and higher up it, gravity would decrease gradually; and if it were constructed on the Earth's equator and, therefore, rapidly rotated together with the earth, the gravitation would disappear not only because of the distancefrom the centre of the planet, but also from the centrifugal force that is increasing proportionately to that distance. The gravitational force drops ... but the centrifugal force operating in the reverse direction increases. On the earth the gravity is finally eliminated at the top of the tower, at an elevation of 5.5 radii of the Earth (36,000 km).In 1962 the Convair Division of General Dynamics looked into the feasibility of building very high towers for use in astronomy, high altitude research, communications, and rocket launching. A limit was found of 6 km for steel towers and almost 10 km for aluminum ones. Today's graphite composites would extend that to about 40 km. However, this is still three orders of magntitude lower than geostationary orbit, so it's clear that no solid, rigid structure could reach to such an altitude. The solution – and the basis of the space elevator – is to let out a cable, as first realized by Artsutanov. For a fuller description of the history of the space elevator concept and a very user-friendly guide to its operation, the reader is directed to Arthur C. Clarke's 1981 paper on the subject, which can be found at the first of the external links listed below.
Magnetic levitation, or maglev, may provide the best means of propulsion for the space elevator. The greatest technological challenge is the cable itself because the weight of the structure dangling from geostationary orbit would place extraordinary demands on the material used to make it. For a cable of practical dimensions, with a minimum diameter of 10 cm, NASA estimates that it would need to be made of something 30 times stronger than steel and 17 times stronger than Kevlar. One possibility is carbon in the form of so-called nanotubes: tiny, hollow cylinders made from sheets of hexagonally arranged carbon atoms. At present nanotubes are extremely expensive and can be fabricated only in short lengths. However, it seems likely that production costs will fall dramatically in the future and that some will be found to bind nanotubes into a composite material like fiberglass.
External sitesArthur C. Clarke's 1981 paper "The Space Elevator"
NASA news ("Audacious and Outrageous: Space Elevators")
Related category ADVANCED PROPULSION CONCEPTS
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