Space Elevators

Another, perhaps longer-term, solution to the problem of reducing the cost of access to orbit is the space elevator. This is the idea of having a cable stretching from Earth's surface to a point beyond geostationary orbit (Fig. 10.9). The cable is anchored to Earth's surface at the equator, and the length of the cable is such that the forces due to Earth's rotation, which tend to fling the cable outward away from Earth, are exactly countered by the weight of the cable, tending to cause it to fall toward Earth. In this way, there is always tension in the cable, so that it will stay erect above the anchor point. Some kind of elevator vehicle can then climb the cable like a beanstalk to carry payloads to orbit. Once the elevator reaches geostationary height— around 36,800 km (22,200 miles)—the payload becomes weightless, so that launching it into orbit is just a matter of gently nudging it out of the elevator. Such a structure has the potential to deliver crew and cargo to orbit at a fraction of the cost incurred by using rocket-powered launchers.

Although this arrangement sounds unlikely, the theory is sound and such a structure is possible in principle. However, there are all sorts of practical difficulties that put its construction beyond our reach at present. The main one is that we currently do not have a material that is light enough and strong enough to withstand the tension in the cable, which reaches a maximum at geostationary orbit altitude. For example, the requirement

Figure 10.9: A diagram of the space elevator (not to scale). The cable is likely to be tapered, being fattest at geostationary height where the tension is greatest.

exceeds the ability of steel to sustain such a structure by a factor of the order of 100. Materials scientists are working to meet this challenge, but unfortunately it looks like it will be some time before the space elevator solves the problem of low-cost access to Earth orbit.

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