Borderlands of Science

by Charles Sheffield

  The system has another use. If a mass is sent all the way out to the end of the cable and then released, it will fly away from Earth. An object released from 100,000 kilometers out has enough speed to be thrown to any part of the solar system. The energy for this, incidentally, is free. It comes from the Earth itself. We do not have to worry about the possible effects of that energy depletion. The total rotational energy of the Earth is only one-thousandth of the planet's gravitational self-energy, but that is still an incredibly big number.

  The converse problem needs to be considered: What about the effects of the Earth on the beanstalk?

  Earthquakes sound nasty. However, if the beanstalk is tethered by a mass that forms part of its own lower end, the situation will be stable as long as the force at that point remains "down." This will be true unless something were to blow the whole Earth apart, in which case we might expect to have other things to worry about.

  Weather will be no problem. The beanstalk presents so small a cross-sectional area compared with its strength that no imaginable storm can trouble it. The same is true for perturbations from the gravity of the Sun and Moon. Proper design will avoid any resonance effects, in which forces on the structure might coincide with natural forcing frequencies.

  In fact, by far the biggest danger that we can conceive of is a man-made one: sabotage. A bomb exploding halfway up a beanstalk would create unimaginable havoc in both the upper and lower sections of the structure. The descent of a shattered beanstalk was described, in spectacular fashion, in Kim Stanley Robinson's Red Mars (Robinson, 1993). My only objection is that in the process the town of Sheffield, at the base of the beanstalk, was destroyed.

  * * *

  8.22 Theme and Variations. We now offer three variations on the basic beanstalk theme. None needs any form of propellant or uses any form of rocket, and all could, in principle, be built today.

  The rotating beanstalk is the brainchild of John McCarthy and Hans Moravec, both at the time at Stanford University. Moravec and McCarthy termed the device a nonsynchronous skyhook, though I prefer rotating beanstalk. It is a strong cable, 8,500 kilometers long in one design, that rotates about its center of mass as the latter goes around the Earth in an orbit 4,250 kilometers above the surface. Each end dips into the atmosphere and back out about once an hour.

  The easiest way to visualize this rotating structure is to imagine that it is one spoke of a great wheel that rolls around the Earth's equator. The end of the beanstalk touches down like the spoke of a wheel, vertically, with no movement relative to the ground. Payloads are attached to the end of the beanstalk at the moment when it touches the ground. However, you have to be quick. The end comes in at about 1.4 gees, then is up and away again at the same acceleration.

  The great advantage of the rotating beanstalk is that it can be made with materials less strong than those needed for the "static" beanstalk. In fact, it would be possible to build one today with a taper factor of 12, using graphite whiskers in the main cable. There is of course no need for such a structure to be in orbit around the Earth. It could sit far out in space, providing a method to catch and launch spacecraft.

  The dynamic beanstalk has also been called a space fountain and an Indian rope trick. It is another elegant use of momentum transfer.

  Consider a continuous stream of objects (say, steel bullets) launched up the center of an evacuated vertical tube. The bullets are fired off faster than Earth's escape velocity, using an electromagnetic accelerator on the ground. As the bullets ascend, they will be slowed naturally by gravity. However, they will receive an additional deceleration through electromagnetic coupling with coils placed in the walls of the tube. As this happens, the bullets transfer momentum upward to the coils. This continues all the way up the tube.

  At the top, which may be at any altitude, the bullets are slowed and brought to a halt by electromagnetic coupling. Then they are reversed in direction and allowed to drop down another parallel evacuated tube. As they fall they are accelerated downward by coils surrounding the tube. This again results in an upward transfer of momentum from bullets to coils.

  At the bottom the bullets are slowed, caught, given a large upward velocity, and placed back in the original tube to be fired up again. We thus have a continuous stream of bullets, ascending and descending in a closed loop.

  If we arrange the initial velocity and the bullets' rate of slowing correctly, the upward force at any height can be made to match the total downward gravitational force of tube, coils, and anything else we attach to them. The whole structure will stand in dynamic equilibrium, and we have no need for any super-strong materials.

  The dynamic beanstalk can be made to any length, although there are advantages to extending it to geosynchronous height. Payloads raised to that point can be left in orbit without requiring any additional boost. However, a prototype could stretch upward just a few hundred kilometers, or even a few hundred meters. Seen from the outside there is no indication as to what is holding up the structure, hence the "Indian rope trick" label.

  Note, however, that the word "dynamic" must be in the description, since this type of beanstalk calls for a continuous stream of bullets, with no time out for repair or maintenance. This is in contrast to our static or rotating beanstalks, which can stand on their own without the need for continuously operating drive elements.

  8.23 The launch loop. As we have described the dynamic beanstalk, the main portions are vertical, with turnaround points at top and bottom. However, when the main portion is horizontal we have a launch loop.

  Imagine a closed loop of evacuated tube through which runs a continuous, rapidly moving metal ribbon. The tube has one section that runs from west to east and is inclined at about 20 degrees to the horizontal. This leads to a 2,000-kilometer central section, 80 kilometers above the Earth's surface and also running west to east. A descending west-to-east third section leads back to the ground, and the fourth section is one at sea level that goes east to west and returns to meet the tube at the lower end of the first section.

  The metal ribbon is 5 centimeters wide and only a couple of millimeters thick, but it travels at 12 kilometers a second. Since the orbital velocity at 80 kilometers height is only about 8 kilometers a second, the ribbon will experience a net outward force. This outward force supports the whole structure: ribbon, containing tube, and an electromagnetic launch system along the 2,000 kilometer upper portion of the loop. This upper part is the acceleration section, from which 5-ton payloads are launched into orbit. The whole structure requires about a gigawatt of power to maintain it. Hanging cables from the acceleration section balance the lateral forces produced by the acceleration of the payloads.

  Although the launch loop and the dynamic beanstalk both employ materials moving through evacuated tubes, they differ in important ways. In the dynamic beanstalk the upward transfer of momentum is obtained using a decelerating and accelerating particle stream. By contrast, the launch loop contains a single loop of ribbon moving at constant speed and the upper section is maintained in position as a result of centrifugal forces.

  8.24 Space colonies. I can imagine some readers at this point saying, all this talk of going to space and traveling in space, and no mention of space colonies except those on the surface of planets. There are hundreds and hundreds of stories about self-sufficient colonies in space.

  There are indeed, and during the 1970s I read many of them with pleasure and even wrote some myself. One of the most fruitful ideas involved "L-5 colonies." "L-5" describes not a type of colony, but a place. In the late eighteenth century, the great French mathematician Joseph Louis Lagrange studied the problem of three bodies orbiting about each other. This is a special case of the general problem of N orbiting bodies, and as mentioned in the previous chapter, no exact solution is known for N greater than 2. Lagrange could not solve the general 3-body problem, but he could obtain useful results in a certain case, in which one of the bodies is very small and light compared with the other two. He found
that there are five places where the third body could be placed, and the gravitational and centrifugal forces on it would exactly cancel. Three of those places, known as L-1, L-2, and L-3, lie on the line joining the centers of the two larger bodies. The other two, L-4 and L-5, are at the two points forming equilateral triangles with respect to the two large bodies, and lying in the plane defined by their motion about each other.

  The L-1, L-2, and L-3 locations are unstable. Place a colony there, and it will tend to drift away. However, the L-4 and L-5 locations are stable. Place an object there, and it will remain. There are planetoids, known as the Trojan group, that sit in the L-4 and L-5 positions relative to Jupiter and the Sun.

  The Earth-Moon system also has Lagrange points, which in the case of the L-4 and L-5 points are equidistant from Earth and Moon. In the 1970s, an inventive and charismatic Princeton physicist, Gerard O'Neill, proposed the L-5 location as an excellent place to put a space colony (L-4 would actually do just as well). The colonies that he designed were large rotating cylinders, effective gravity being provided by the centrifugal force of their rotation. Within the cylinder O'Neill imagined a complete and self-contained world, with its own water, air, soil, and plant and animal life. Supplies from Earth or Moon would be needed only rarely, to replace inevitable losses due to small leaks.

  The idea was a huge success. In 1975 the L-5 Society was formed, to promote the further study and eventual building of such a colony.

  What has happened since, and why? Gerard O'Neill is dead, and much of his vision died with him. The L-5 Society no longer exists. It merged with the National Space Institute to become the National Space Society, which now sees its role as the general promotion of space science and space applications.

  More important than either of these factors, however, is another one: economic justification. The prospect of a large self-sufficient space colony fades as soon as we ask who would pay for it, and why. Freeman Dyson (Dyson, 1979, Chapter 11) undertook an analysis of the cost of building O'Neill's "Island One" L-5 colony, comparing it with other pioneering efforts. He made his estimate not only in dollars, but in cost in man-years per family. He decided that the L-5 colony's per family cost would be hundreds of times greater than other successful efforts. He concluded "It must inevitably be a government project, with bureaucratic management, with national prestige at stake, and with occupational health and safety regulations rigidly enforced." All this was before the International Space Station, whose timid builders have proved Dyson exactly right: "The government can afford to waste money but it cannot afford to be responsible for a disaster."

  The L-5 colony concept has appeal, and the technology to build the structure will surely become available. But it is hard to see any nation funding such an enterprise in the foreseeable future, and still harder to imagine that industrial groups would be interested.

  The L-5 colony—regrettably, because it is such a neat idea—is part of what I like to call false futures of the past, projections made using past knowledge that are invalidated by present knowledge.

  I believe there will certainly be space colonies in the future. Write stories about them by all means. But don't make them rotating cylinders at the L-5 location. Those stories have already been written.

  8.25 Solar power satellites. While in skeptical mode, let me say a few words about another concept of initial high appeal, the Solar Power Satellite. This was proposed in the 1960s by Peter Glaser, and like the L-5 colonies it had its heyday in the 1970s and early 1980s. Proponents of the idea believed (and believe) that it can help to solve Earth's energy problems.

  A solar power satellite, usually written as SPS, has three main components. First, a large array of photoreceptors, kilometers across, in space. Each receptor captures sunlight and turns it to electricity. The most usual proposed location is in geosynchronous orbit, though some writers prefer the Earth-Moon L-4 location. The second component is a device that converts electricity to a beam of microwave radiation and directs it toward Earth. The third component is a large array on the surface of the Earth, usually known as a rectenna, that receives the microwave radiation and turns it into electricity for distribution nationally or internationally.

  The SPS has some great virtues. It can be placed where the Sun is almost always visible, unlike a ground-based solar power collector. It taps a power source that will continue to be steadily available for billions of years. It contributes no pollution on Earth, nor does it generate the waste heat of other power production systems. It does not depend on the availability of fossil or nuclear fuels.

  Of course, the SPS cannot be built without a powerful in-space manufacturing capability, something that is lacking today. We are having trouble putting modest structures, such as the International Space Station, into low orbit. It is likely that we will not be able to build an object as large as the proposed SPS for another century or more.

  But when a century has passed, we are likely to have much better energy-raising methods, such as controlled fusion. Admittedly, progress on fusion has been slow—we have been promised it for fifty years—but it, or some other superior method, will surely come along. A fusion plant (or, for that matter, a fission plant) in orbit would have all the advantages of SPS, and none of the disadvantages. Sunlight is a highly diffuse energy source unless you get very close to the Sun. As we pointed out in Chapter 5, the history of energy use shows a move in the direction of more compact power sources—oil is more intense and compact than water or wind, nuclear is more compact and intense than chemical. The other problem is that the Sun, unlike our future fusion reactors, was not designed to fit in with human energy uses and needs. I put the question the other way round: Why build a kilometers-wide array, delicate and cumbersome and vulnerable to micrometeor damage, when you can put the same power generating capacity into something as small as a school bus? Admittedly, we don't have controlled fusion yet—but we also can't build an SPS yet.

  However, the real killer argument is not technological, but economic. Suppose you launch SPS to serve, say, the continent of Africa. You still have the problem, who will pay for the energy? Economists distinguish two kinds of demand: real demand: the need for food of starving people with money to buy it; and other demand: the need for food of starving people without money. Regrettably, much demand for energy is in nations with no resources to pay for it.

  In spite of this economic disconnect, many people have suggested that an SPS would be great for providing energy to Africa, where energy costs are high. Suppose that you put SPS is geostationary orbit and beam down, say, 5 gigawatts. That's the power delivered by a pretty substantial fossil fuel station. Now, you could also generate that much energy by building a dam on the Congo River, where it drops sharply from Kinshasa to the Atlantic. So ask yourself which you would prefer if you were an African. Would you like SPS, providing power from a source over which you had no control at all—you couldn't even get to visit it. Or would you prefer a dam, which in spite of all its defects, sits on African soil and is at least in some sense under your control? SPS has to compete not only from an economic point of view, but from a social and political point of view.

  I think it fails on all those counts. Like the L-5 colony, SPS is part of a false future. It is not surprising to find Gerard O'Neill arguing that the sale of electricity generated by an SPS at L-5 would pay for the colony in the breathtakingly short period of twenty-four years. When we want to do something, all our assumptions are optimistic.

  There are still SPS advocates. A recent NASA study suggested that a 400 megawatt SPS could be built and launched for five billion dollars. Do I believe that number? Not in this world. We all know that paper studies often diverge widely from reality. NASA's original estimated cost to build the International Space Station was eight billion dollars. Over the years, the station has shrunk in size and the costs have risen to more than 30 billion dollars. Projects look a lot easier before you get down to doing them. Recall the euphoria for nuclear power plants in the 1940s, "ele
ctricity too cheap to meter." And that was for something we had a lot more experience with than the construction of monster space structures.

  Certainly, we hope and expect that the cost of sending material to space will go down drastically in the next few generations. We also will become increasingly unwilling to pollute the Earth with our power generation. But frequent space launches have their own effects on the environment of the upper atmosphere. If there is ever an SPS, which I doubt, it will more likely make little use of Earth materials and depend on the prior existence of a large space infrastructure.

  I feel sure that will come—eventually. By that time the idea of power generation plants near population centers will be as unacceptable as the Middle Ages habit of allowing the privy to drain into the well. However, I want to emphasize that our solutions to the problems of the future can be expected to work no better than two-hundred-year-old solutions to the problems of today. We can propose for our distant descendants our primitive technology as fixes for their problems. But I don't believe that they will listen.

  TABLE 8.1

  Strength of materials.

  Material

  Density

  Tensile strength

  Support length

  (gms/cc)

  (kgms/sq.cm.)

  (kms)

  Lead

  11.4

  200

  0.18

  Gold

  19.3

  1,400

  0.73

  Aluminum

  2.7

  2,000

  7.40

  Cast iron

  7.8

  3,500

  4.50

  Carbon steel