Cosmic Tales - Adventures in Sol System

by T. K. F. Weisskopf

  The Economics of Space Mining

  In fifty years space flight has moved from a starry-eyed dream to a multibillion-dollar industry with hundreds of geosurvey and communications satellites now circling the globe. However, these profit-based efforts are all ultimately based on providing services to the earth's surface, exploiting the big picture that only space can provide. To date interplanetary travel has remained the domain of scientific research, which has to meet less stringent economic tests than purely commercial enterprises. Making the jump to the commercial exploitation of the solar system depends on the economic viability of the venture, and the numbers are daunting.

  As an example, Boeing charges $19,000 per kilogram for a payload delivered to orbit on its Delta IV booster, which makes anything at all delivered to orbit literally worth its weight in solid platinum (spot price the day I wrote this: $20,500 per kilogram). Boeing intends to make money putting up satellites so we can assume these figures represent the real costs of launch. Take mission costs into account and the reality is stark. If there were pure platinum just floating around in low Earth orbit, it simply wouldn't be worth our while to go collect it—at least not with Boeing. The heavily subsidised Space Shuttle doesn't make money for the government but taking advantage of government largesse is a routine business practice. In addition the Shuttle can put a 22,000-kilo payload on the runway from orbit—something the Delta IV can't do. At NASA's current figure of $300,000,000 per launch that gives us a transportation cost of $13,600 per kilogram. Now it's worth flying to fill the cargo hold with platinum—at least until Congress catches on that we're milking the taxpayer, but pure gold (at just over $10,000 per kilo) is still a losing proposition, and this is just low Earth orbit. Contrast this with the transportation cost of $5 per metric ton required to ship iron ore from mines in the Canadian arctic and Brazil to the ore terminals in Rotterdam and the economic obstacles facing an asteroid-mining venture are made very clear.

  This doesn't necessarily slam the door on space manufacturing, because some products can have tremendous value packed into very little mass. My tenth-of-a-gram contact lenses are a hundred dollars each, making them worth a cool million dollars a kilo. This cost has nothing to with the price of plastic, a little to do with the computer-controlled lathe that custom cuts them to fit my eyes, and everything to do with the skill of my optometrist and the tremendous research and development effort that went into proving them safe and effective. Microchips (just the thin thumbnail of silicon, not the bulky black pin-package you plug into your computer) can be worth tens of millions per kilo, and some drugs can be worth billions.

  The orbital manufacture of products like this is worth pursuing, although there may be very few that absolutely require zero gravity or limitless hard vacuum to manufacture. However space mining seems to be out of the running before we get started because no raw material is even close to valuable enough. Space mining means going to the asteroids, and even assuming a tenfold price reduction for a ticket to low Earth orbit, getting out to the asteroids is going to cost more. Mars Global Surveyor cost $85,000 a kilo to launch to the red planet, plus another $200,000 a kilo for the hardware and support, and Mars Observer was five times more than this—and it was lost due to error. These are just observation missions—the equivalent to prospecting in a space mining operation. Getting launch costs way down is only part of the equation, we also have to radically slice mission costs. This has some promise—a lot of that $200,000 a kilo is due to the fact that space probes are one-of-a-kind creations of tremendous sophistication, and making a simpler probe can save a lot of money. The Japanese Muses-C asteroid sample-and-return probe (which is asteroid prospecting) cost just $33,000 per kilogram, plus mission costs. However, the unavoidable fact is, high technology costs a lot of money. NASA tried to escape this equation with its Faster-Better-Cheaper program and wound up with a string of spectacular failures (only one in three Mars missions has been successful). This isn't only true in space. The B2 bomber is worth (or costs, which is not quite the same thing) its 153 tons in solid platinum and it's never going to orbit.

  With these figures in mind the hundred-billion-dollar figure mentioned in the story for a running asteroid-mining infrastructure is probably very conservative. The International Space Station is a thirty-five—billion-dollar effort and represents only a faltering first step in the direction of the infrastructure required. However, the size of the investment required is not an insurmountable obstacle. Regardless of the capital costs involved the key measure of any investment is its ability to turn a profit, which ultimately hinges on supply and demand. It is this economic reality which is likely to prevent asteroid mining from getting off the ground.

  In the story I make the assumption that metals will become increasingly scarce and therefore expensive in the future as supplies are mined out, but current trends do not support that assumption. Despite tremendous increases in demand, industrially useful metals have gotten steadily cheaper and more readily available throughout all of history and this curve has accelerated drastically in our century. A factor which Watson overlooks in his enthusiasm for asteroid mining is that if buckytube composites can be made cheap enough to make a space elevator possible they will entirely replace steel as a structural material. The trend of replacing metals with composites is already well established in the aerospace and automotive industries, and fiberglass is now being used instead of steel in reinforced concrete structures.

  Even without this trend we won't be running out of metals any time soon. Refining has become much more efficient, mines can now reach miles down into the earth's crust and out under the oceans, and improvements in prospecting techniques have meant proven and untapped reserves are larger now than they have ever been. Earth's mineral resources are vast and we have barely begun to flirt with seafloor mining. Although this is certainly an expensive high-technology venture, it would still be cheaper than asteroid mining and it quadruples the area available for exploration. Furthermore, metals are an infinitely recycleable resource, and even much of what is "thrown away" can be recovered. Any urban landfill amounts to a high-grade ore deposit in terms of metal content per tonne and some companies are beginning to explore "secondary mining" of landfills as a serious option. There is already a small but thriving industry which raises sunken Second World War warships for the steel in their armor plating, valued for certain scientific applications because it is free of the trace radioactive contamination that has filled the world since Hiroshima. Unlike a space-mining operation, recycling programs are cheap, low risk and require no technological breakthroughs.

  Buckytubes

  Holmes' key enabling technology, buckytubes, is here today, although it's currently confined to the lab. Carbon materials are the strongest in nature, both because of the tremendous strength of the -carbon-carbon covalent bond and because carbon likes to arrange itself in triangles and hexagons, which are the stablest geometric structures possible. Diamond (tetrahedronal) and graphite (hexagonal) are the most familiar forms and both are repeating crystal lattices of arbitrary size. The edges of the crystal lattice have dangling bonds (like the edge of a cut piece of chicken wire), which are usually taken up by hydrogen atoms. The discovery of C60 by Kroto and Smalley—named Buckminsterfullerene for its structural resemblance to the geodesic dome invented by Buckminster Fuller—has triggered a revolution in carbon chemistry. Unlike the infinitely repeating lattices of diamond and graphite, each molecule of C60 is complete unto itself, twelve pentagons, each surrounded by five hexagons, with carbon atoms at the vertices—an atomic-scale soccerball, commonly called a buckyball. C60 is tremendously stable both chemically and physically and boasts a host of superlative characteristics. Because of its icosahedral structure it is the roundest molecule possible. Fully fluorinated as C60F60 it is the most stable molecule known to exist and may be the most stable molecule which can exist. Compressible by up to seventy percent without breaking down, it is the most resilient molecule, and in this state it is twice
as hard as diamond. Because of this it is capable of withstanding impacts of 15,000 mph unscathed. Buckyballs are normally insulators, but doped with potassium they become superconductors. Fullerenes in general are variations on the carbon geodesic theme, always with twelve pentagons but with more or fewer hexagons in the mix. A buckytube is simply a buckyball with a long, winding spiral of hexagons between two half-buckyball end caps. Because of the strength of the bonds and the rigidity of the hexagonal structure, buckytubes are the stiffest structures ever made. There is no theoretical limit on the length of a buckytube, although current techniques produce samples only a few microns long. When you consider that carbon-carbon composites made with graphite already demonstrate strength-to-weight ratios ten or more times better than steel, the structural potential of buckytubes becomes clear. Buckytube composites can reasonably be expected to have a strength-to-weight ratio a thousand times better than steel.

  In addition to their physical properties buckytubes display interesting electrical properties. By varying the pitch of the spiral, buckytubes can be made as semiconductors or as conductors as good as copper. Doping them with compounds like tantalum carbide makes them into submicron-sized superconducting wires, and packing them with potassium-doped buckyballs achieves the same effect, and if the buckytube is sized properly to fit the buckyball such packing would probably also serve to increase their already phenomenal stiffness and boost their compressive strength as well. There is some indication that it may be possible to build packed buckytubes like this that superconduct above the temperature of liquid nitrogen—the point at which superconductors can move from an expensive, specialized laboratory item to a low-cost industrial item. Further, it seems that their high-current carrying capacity may be preserved in the superconducting state.

  Fullerenes clearly have tremendous potential, but commercial realization of those potentials involves overcoming a number of hurdles, one of the major ones being that the mass production of fullerenes is not simple. High-purity samples of high-quality buckytubes go for $14,000 a kilogram and up. If zero gravity turned out to be the key to mass production, buckytube composites would be worth building in orbit even with today's launch costs, although the demand would not be very high at that price.

  Carbon nanotubes make possible a tremendous range of new technologies. The ground-to-space "beanstalk" space-elevator system and the electromagnetic launcher discussed in the story are two of the most dramatic examples, but, like fiberglass and graphite epoxy composites, they would find their way into everything from aircraft to sports equipment. If it does turn out to be possible to make them into cheap high-temperature superconductors they will have an impact on civilization akin to the introduction of plastic. Their primary use will be in superconducting power grids, power-storage rings, but the most exciting possibility then becomes high-field superconducting magnets, usable not only in fusion drives and fusion reactors, but maglev trains, high-efficiency generators and motors and a host of other applications.

  Electromagnetic Space Launchers

  The concept of the mass driver has been around for a long time. The idea is simple—put a magnetic projectile into a long tube made of consecutive rings of magnetic coils, and switch the coils so that magnetic forces accelerate the projectile down the tube. Make the tube long enough and the field strengths high enough and you can launch something right into space. To date coil guns have been limited to the laboratory although, as with the quarter squisher, there is a core group of enthusiasts who build working models with impressive and occasionally dangerous performance. A full-scale mass driver for space launch would be a tremendous piece of machinery—the launch tube would have to be many kilometers long and the coils would require the rapid and precise switching of vast amounts of electrical power. In the story (and in most systems given serious study) the launch tube runs up a mountain. This is not necessary to get the launch vehicle into space—a perfectly horizontal trajectory would work just as well, given a muzzle velocity equal to escape velocity plus a bit, but serves to reduce the amount of atmosphere it has to punch through. This is important because atmospheric heating at speeds of eleven or more kilometers per second is considerable. An ablative heat shield on the launch vehicle is a given, and it may be desirable to put a vacuum in the launch tube to both save energy and reduce heat loads in the thickest part of the atmosphere. The vacuum would be maintained by a thin, frangible membrane over the far end of the tube, which the vehicle would simply shatter on its way through.

  However, running the tube up a mountainside might not be the best solution. This is more difficult to build than a horizontal launcher from a construction standpoint, and also requires that energy be put into turning the launch vehicle and its carrier as they travel down the tube. In addition a primary drawback of a mass driver is that it can only insert launch vehicles into a limited subset of all possible orbits because it is inherently unsteerable—and the orbits it can launch have a perigee at the altitude of the launch point, which is probably lower than we'd like. Using aerodynamic surfaces on the launch vehicle increases the range of possible orbits considerably, and a horizontal launch would give the vehicle more time in the atmosphere to effect course changes. Engines on the launch vehicle would also be necessary to effect orbit changes (at a mininum to brake for re-entry) but they wouldn't have to be large.

  One of the beauties of the mass driver is its very high efficiency. Almost all the power put into the coils can be recovered, save that actually transferred to the launch vehicle. Launch costs for a mass driver would be quite low, tens of dollars per kilogram launched right out of earth orbit, but the accelerations are -tremendous—the details depend on the launch parameters, but three hundred and fifty gravities is not an unreasonable figure. Bulk cargoes wouldn't be a problem, but mechanical and electronic systems would have to be specifically designed to take the load. Humans simply couldn't take the trip, but there is another way. . . .

  The Skyhook

  The Soviet scientist Y.N. Artsutanov first advanced the concept of a cable suspended from orbit in 1960, and it has been seriously studied since. Commonly called a beanstalk (after the fairytale plant that let Jack climb to the clouds) it is, in theory, very simple. A cable stretches from the ground right up to the sky, kept under tension by the centripetal acceleration of a counterweight at its far end, defying gravity just the way a child's ball whirled at the end of a string does. The snag is getting the ball high enough for the whirling of earth's rotation to be strong enough to overcome its gravity—and this height is geosynchronous Earth orbit, 22,000 miles up. This is where the center of mass of the system has to orbit, so we can either put a lot of mass, like an asteroid, a little above the 22,000-mile mark, or simply spin more cable out into space above this point. This second option is attractive, since we can then take payloads even higher if we want to, and use the slingshot effect to launch them for destinations around the solar system. The cable has to be able to support a transport vehicle, its associated hardware, and of course its own weight. A cable capable of supporting twenty tons, the shuttle's payload, is no earth-shattering achievement, and any cable can be made to support its own weight simply by tapering it—the bottom of the cable supports hardly any weight at all and so can be very thin, the top of the cable supports the entire thing and must be thicker. By choosing an appropriate degree of taper any tensile material can support any length of itself, and with a little more effort can support some additional weight, like the tracks and so on of the transport vehicle. From a theoretical perspective the beanstalk is completely possible.

  Looking a little deeper, the hook in the skyhook is contained in the phrase "an appropriate degree of taper." The stronger the material the less it needs to be tapered, but if we consider carbon-fiber epoxy resin composites, the strongest material we can currently build things with, the appropriate taper (about two and a half thousandths of a degree) for a cable just a millimeter in diameter at the ground produces a cable two kilometers in diameter at geosync
hronous Earth orbit and massing sixty trillion metric tons. This is clearly impractical. However, if we consider a cable made of carbon nanotubes at a strength-to-weight ratio a thousand times better than steel, then a self—supporting cable would be just over a tenth of a millimeter at ground level and just over a quarter millimetre in orbit, and weigh in at around ten metric tons. This could be spooled up and flown in the shuttle's cargo bay (the shuttle can't reach geosynchronous orbit of course, so the spool would need its own booster as well). This is a much more practical solution.