The scientific literature about beanstalks, in all its different versions (we’ll get to those later), has grown steadily over the past twenty years. Now there exist many varieties of proposed forms, for use in a variety of places, ranging from Earth to Mars to the Lagrange points of the Earth-Moon system. This book uses what I will term the “standard beanstalk,” a structure which extends from the surface of the Earth up into space, and stands in static equilibrium.
To understand how any beanstalk is possible, even in principle, we begin with a few facts of orbital mechanics. A spacecraft that circles the Earth around the equator, just high enough to avoid the main effects of atmospheric drag, makes a complete revolution in about an hour and a half. A spacecraft in a higher orbit takes longer, so for example if the spacecraft is 1,000 kilometers above the surface, it will take about 106 minutes for a complete revolution about the Earth.
If a spacecraft circles at a height of 35,770 kilometers above the Earth’s equator, its period of revolution will be 24 hours. Since the Earth takes 24 hours to rotate on its axis (I am ignoring the difference between sidereal and solar days), the spacecraft will seem always to hover over the same point on the equator. Such an orbit is said to be geostationary. A satellite in such an orbit does not seem to move relative to the Earth. It is clearly a splendid place for a communications satellite, since a ground antenna can point always to the same place in the sky; most of our communications satellites in fact inhabit such geostationary orbits.
A 24-hour circular orbit does not have to be geostationary. If the plane of its orbit is at an angle to the equator, it will be geosynchronous, with a 24-hour orbital period, but it will move up and down in latitude and oscillate in longitude during the course of one day. The class of geosynchronous orbits includes all geostationary orbits.
All geostationary orbits share the property that the gravitational and centrifugal forces on an orbiting object there are exactly equal. If by some means we could erect a long, thin pole vertically on the equator, stretching all the way to geostationary orbit and beyond, then every part of the pole below the height of 35,770 kilometers would feel a net downward force because it would be moving too slowly for centrifugal acceleration to balance gravitational acceleration. Similarly, every element of the pole higher than 35,770 kilometers would feel a net upward force, since these parts of the pole are traveling fast enough that centrifugal force exceeds gravitational pull.
The higher that a section of the pole is above geostationary height, the greater the total upward pull on it. So if we make the pole just the right length, the total downward pull from all parts of the pole below geostationary height will exactly balance the total upward pull from the parts above that height. The pole will then hang free in space, touching the Earth at the equator but not exerting any downward push on it.
How long does such a pole have to be? If we were to make it of uniform material along its length, and of uniform cross section, it would have to extend upward for 143,700 kilometers, in order for the upward and downward forces to balance exactly. This result does not depend on the cross-sectional area of the pole, nor on the material of which the pole is made. However, it is clear that in practice we should not make the pole of uniform cross section. The downward pull the pole must withstand is far greater up near geosynchronous height than it is near the surface of the Earth. At the higher points, the pole must support the weight of more than 35,000 kilometers of itself, whereas near Earth it supports only the weight hanging below it. Thus the logical design will be tapered, with the thickest part at geostationary altitude where the pull is greatest, and the thinnest part down at the surface of the Earth.
We now see that “pole” is a poor choice of word. The structure is being pulled, everywhere along its length, and all the forces at work on it are tensions. We ought to think of the structure as a cable, not a pole. It will be of the order of 144,000 kilometers long, and it will form the load-bearing cable of a giant elevator which we will use to send materials to orbit and back.
The structure will hang in static equilibrium, rotating with the Earth. It will be tethered at a point on the equator, and it will form a bridge to space that replaces the old ferry-boat rockets. It will revolutionize traffic between its end points, just as the Golden Gate Bridge and the Brooklyn Bridge have made travel between their end points a daily routine for hundreds of thousands of people.
That is the main concept. Now we have to worry about a number of “engineering details.”