Cosmic Tales - Adventures in Sol System

by T. K. F. Weisskopf

  In the story I make the assumption that cheap nanotube manufacture requires zero gravity, and so we have to build a large and expensive electromagnetic launch system to get the raw carbon up to a factory in geosynchronous Earth orbit. On the more reasonable assumption that, if we can mass produce nanotubes at all we can do it just fine on the ground, then turning the initial thin "leader line" into a robust elevator cable isn't too hard. NASA suggests small, automated bobbins, powered by ground-based lasers, climbing the cable from the ground up, adding strands as they go until a cable strong enough to support serious payloads is complete. These bobbins are kept busy after this, searching out damage caused by micrometeors and space debris and repairing it. Some estimates put the cost of installing such a system (given cheap manufacture of buckytubes) as low as five billion dollars over ten years.

  Interestingly enough, at a given strength-to-density ratio it actually takes less material to build a tower into space than it does to hang a cable from orbit. Cables get more press partly because they're a neater concept, but mostly because we can get far more strength out of the best materials in tension than we can in compression. The tallest building in the world, Toronto's CN tower, is over half a kilometer high using ordinary concrete, which is nowhere near even the practical limit. The theoretical maximum height for a steel tower is five kilometers, for aluminum it's fifteen kilometers. As with the cable we can get better results by tapering the structure, and by pressurizing the structural members we can effectively swap tensile strength for compressive strength. These techniques, using high-strength polyaramid composites as the material of choice, would allow a tower three thousand kilometers high. Using this as the base structure for a tensile cable that went the rest of the way to geosynchronous orbit would reduce the cable's mass one hundred and fifty times.

  However we build a skyhook, once it is complete the raw energy costs of transport (it could hardly be called launch) to geosynchronous orbit would be around $1.50 a kilogram, and the landed cost for a product returned from orbit would be only marginally higher. This radically changes the economics of space industry, and any slight advantage to be gained from space manufacturing would be well worth it, even for basic industrial processes. As an example, it takes about the same amount of energy to refine a kilogram of alumimum from bauxite (aluminum ore) as it does to beanstalk a kilogram to orbit. The energy available in an orbital solar furnace is basically free, so it would be almost cost effective to ship raw ore from Earth to orbit for smelting (not quite though, because we have to process about two kilos of bauxite for every one kilo of aluminum we get back). A product made using zero G, vast amounts of solar power or limitless vacuum doesn't have to get too much more sophisticated than aluminum ingots to be worth making in space—once a beanstalk is up and running.

  In addition to allowing cheap access to orbit, the beanstalk can be used to launch spacecraft for destinations around the solar system. Energy has to be used to raise a mass up the cable to geosynchronous orbit—but less and less as we get higher. At geosynchronous orbit a mass is effectively weightless. If we give it a little push downward it will fall to Earth. If we give it a little push upward it will fall into space. If the cable extends out to the point where its rotational velocity exceeds Earth's escape velocity then the mass can be launched on an orbit to anywhere in the solar system we like—with the proviso that we will have to expend some energy to change the plane of its orbit to intersect useful destinations like planets and asteroids. This isn't too arduous a demand, because except for space probes, we want what we've launched to come back, and we want to be able to maneuver and do useful things while we're out there too, so we require some sort of engine anyway. Read on.

  The Carbon-Catalyzed Fusion Drive

  Mass drivers can get materials into space and beanstalks can get people into orbit, but to move around the solar system a drive of some kind is necessary. Fusion drives are a staple of science fiction for the simple reason that chemical fuels simply lack the energy content required to make an interplanetary, let alone interstellar, civilization possible. However, the reality is that controlled fusion has proven a tremendously difficult genie to get out of its bottle even for simple power generation, an easier problem to solve than a space drive. The first serious attempt at developing a fusion drive was Project Orion, which envisioned propelling a ship through the simple mechanism of kicking a nuclear bomb out the back and detonating it, repeating as necessary. This is hardly a graceful technique, but despite its seeming insanity it is not, a priori, unworkable. Calculation and experiment showed that a large "pusher plate" close to a nuclear explosion would experience a considerable acceleration—hardly surprising. More surprising is the fact that, given a thin protective layer of ablatable graphite, both the plate and a spacecraft on the other side of it could survive the detonation unscathed, could travel in a reasonably predictable direction and that a not—impossibly-large system of shock absorbers could make the ride survivable by people. Project Orion envisioned putting payloads measuring thousands of tons into orbit and saw a manned trip to Mars by 1965 and a trip to Saturn by 1970. However a bomb-powered spacecraft would inevitably generate a tremendous amount of radioactive fallout, and the accident risk involved with a ship carrying several thousand nuclear "pulse units" into orbit needs no elaboration. After the investment of several million dollars the project was shelved with the surface test-ban treaty. More advanced concepts have been put forward, including the British Interplanetary Society's project Daedelus which proposed laser-imploded microfusion capsules as the drive source. NASA's Variable Specific Impulse Magnetoplasma Rocket (VASIMR) plasma drive uses many of the techniques relevant to a fusion drive and the Gas Dynamic Mirror (GDM) project (currently shelved to pay for shuttle refurbishment) was aimed at creating a true fusion drive. A primary stumbling block to the GDM drive is the availability of sufficiently strong magnetic fields. Beyond this problems to be solved are not small, given that even fusion power production still has large technological hurdles to clear. Fission power plants are now a fifty-year-old technology, but fission rockets require too much shielding and operate at too low a temperature to be practical, even ignoring the specter of accident and widespread radiological contamination. The same concerns have left atomic cars and airplanes on 1950s drawing boards. Fission fuel contains millions of times more energy, kilo for kilo, than chemical fuel, but the realities of harnessing it confine its use to large power plants.

  Fusion happens whenever atomic nuclei get close enough together that the short range but powerful attraction of the strong nuclear force can overcome the weaker but longer-ranged electrostatic repulsion of the positive protons in the nucleus. Fused nuclei are more stable together than apart, and the stability difference shows up as energy that can then be put to work. The key variables are temperature and -pressure—the nuclei have to be moving fast and be close enough together to make them fuse, and it takes a lot of both. Fusion bombs use no less a force than a fission bomb to heat and compress their fuel, but power generation and space drives (Project Orion excepted) require less violent methods. Adding to the headaches, the easiest fusion reaction to ignite is the deuterium-tritium reaction, and this gives off a lot of neutrons which require a lot of shielding, and which generate a tremendous waste disposal problem by slowly rendering the reactor itself radioactive. In addition tritium, also radioactive, must be made in a nuclear reactor at tremendous expense, generating more waste in the process (tritium is roughly eighty times the cost of plutonium, weight for weight). Current reactor concepts cleverly use the neutron flux from the fusion reactor itself to make more tritium, solving some of the neutron contamination problem at the same time, but it would be nice if we could tap into fusion power without having to deal with radioactive waste at all, and preferably without using exotic and expensive isotopes either.

  The primary fusion pathways in the sun (the proton-proton chain, which burns normal light hydrogen and the carbon-nitrogen-oxygen catalysis cycle, or CNO c
ycle, which burns light hydrogen using carbon -12 as a catalyst) are aneutronic—fortunately for us, as any large-scale neutron flux from the sun would certainly bake our planet sterile. The sun uses nothing more than gravity to produce a stable, long-lived, high-flux fusion output without too many nasty side effects. In a star temperatures of ten million degrees and densities of 100 gm/cm3 are required for the p-p chain and sixteen million degrees and densities of over 150 gm/cm3 for the CNO cycle.

  For the sun, which is held together by gravity and isn't going anywhere in a hurry, temperature and pressure are the end of the story, but for an artificial fusion reactor a third critical variable enters the picture—confinement time. This is the length of time the system can hold the fusing plasma together against the expansion caused by the fusion energy. This expansion lowers the pressure and temperature and eventually stops the reaction. In order for fusion to occur efficiently at a given temperature the plasma particles have to be held together long enough for a significant number of fusion events to occur. Lawson's criterion, the product of particle density and confinement time, serves as a figure of merit for fusion reactors, and successive generations of research machines have striven to come ever closer to the magic point of breakeven, where the reactor produces as much power as it consumes, and ignition, where the plasma burn becomes self-sustaining, requiring no further energy input from outside. (The third magic point, profitability, the point where a fusion reactor produces power for something like a nickel per kilowatt hour, is nowhere on the horizon.) The oldest and best-explored fusion scheme, the toroidial magnetic tokamak, uses low pressures (a few atmospheres), long confinement times (a few seconds to half an hour) and temperatures of hundreds of millions of degrees to achieve fusion. The more recent inertial confinement fusion approach uses intersecting laser beams to compress a tiny fuel pellet to pressures more than ten million times higher. Fusion occurs so quickly in this regime that the fuel pellet's own inertia holds it together for the nanoseconds necessary for the reaction to complete. A third fairly recent development is magnetic target fusion, which magnetically implodes a thin-walled aluminum cylinder to achieve a fusion regime intermediate between these two. MTF achieves, for a few microseconds, magnetic field strengths of 500 Tesla.

  Given that all of these techniques are barely capable of generating fusion with deuterium-tritium it may seem needlessly ambitious to plan a drive around the CNO cycle. Even granted that we want aneutronic fusion, the proton-proton chain is available at much lower pressures and temperatures. However, the CNO cycle's reaction rate goes up no less than 350 percent for every 10 percent increase in temperature, rising as the sixteenth power of temperature, compared to the proton-proton chain's reaction rate which goes up as the fourth power of temperature. This means that, given the ability to reach this extreme regime, it should be possible to reach a point where the CNO cycle will become very efficient.

  The trick of course is reaching the CNO operating point at all, and whether this is possible is an open question—even buckytube supermagnets might not do the job. However if they do, the system might work like this.

  Imagine a hollow tube ringed with magnetic coils. A small stream of hydrogen and carbon plasma is fed in at one end, and a current is run through it to generate its own magnetic field. A magnetic wave is sent down the tube by successively energizing rings of magnets. This wave accelerates the ionized gases down the tube, and if the wave amplitude is made to increase down the tube while the wavelength decreases, the gases will also be compressed and heated as they travel. At some point, possibly with further energy inputs, the plasma begins to fuse. This tremendously increases its energy content—temperature and pressure soar and it begins to expand, still fusing. Forward and outward expansion are limited by the incoming magnetic wave, still increasing in amplitude, and the reaction forces of fusion particles moving against this wave provide thrust for the ship. Wave amplitude decreases and wavelength increases past the fusion point, allowing the plasma to expand and stop fusing in that direction, focusing it into a coherent jet. Finally the plasma exits the rear of the tube through another magnetic field which uses the stream of charged particles to generate electrical power to run the system. Behind the tube is a reaction chamber into which water can be sprayed to trade exhaust velocity for thrust for planetary launch. Once in orbit we turn off the water and use the jet by itself to accelerate.

  Exploring the detailed physics of this system is far beyond the scope of this discussion. In a story it's easy to gloss over details like plasma disruption and wall interactions, but these are critical problems for real-world reactors. This may be the drive of the future; it may, like Project Orion and the atomic airplane, be possible but impractical; or it may be simply impossible. Assuming it is possible then the magnetic wave system means the thrust would be continuous and very controllable. This is the fusion equivalent of a jet engine with constant, or nearly constant, combustion, as opposed to a pulsed fusion drive (Project Orion being the extreme example) which, like a piston engine, produces power in discrete bursts. This steady, throttled power is exactly what we're looking for, and this drive is easily adaptable to the fusion ramjet concept which gathers its fuel from the interstellar medium as it travels. It should be noted though that the fusion ramjet requires solving even larger problems than a "simple" fusion drive, including the generation and control of a magnetic funnel about the size of Jupiter. Even buckytube supermagnets are probably inadequate to this task, but a field too small to serve as an interstellar fuel scoop could still serve well as an unconfined magnetic particle shield. As discussed above, some sort of particle shield is absolutely essential for manned interplanetary flight, with or without a fusion drive system, and supermagnets like this are a primary enabling technology. Solving the shield problem like this would open up the solar system to large-scale exploration and completely revolutionize the commercialization of space. Even if we don't want to colonize Mars or set up an asteroid-mining industry the scientific benefits alone would be incalculable.

  A Deep-Space Radiation Shield

  The single largest obstacle to manned interplanetary space flight is not efficient drives or long-term life support, it is the radiation that floods interplanetary space. This comes in two major varieties—the relatively soft protons and electrons streaming out from the sun (the solar wind), and the very hard interstellar cosmic rays, some of which have the impact of a major league fastball packed into a single heavy nucleus. Earth's magnetic field and atmosphere protect us from most of this radiation and in low earth orbit the doses are tolerable. Once you get beyond the Van Allen belts the situation changes drastically. The Apollo flights were possible only because they were short enough that the radiation risk was considered acceptable. Any long—duration flight, such as one to Mars or the asteroids, would be out of the question without shielding. There are five basic possibilities for shielding a spacecraft—mass, plasma volumes, electrostatic fields, confined magnetic fields and unconfined magnetic fields. None of these are perfect solutions, particularly against the heavy ion component of the cosmic ray background, however, the one with the most promise is unconfined magnetic shielding—this is what works for the Earth, after all. The Earth's magnetic field is tenuous here on the surface, but the dynamo that drives it is the entire iron inner core of the planet and the total energy in the field is tremendous. The 1000 Tesla field mentioned in the story is about twenty times the strongest constant field yet produced on Earth (about 45T, at the National High Magnetic Field research facility at Florida State University). Given the material properties of the buckytube components used in the story, this is a conservative estimate, and 5000T might not be out of the question. A 1000T field is starting to get within reach of the required field strength for a reasonable shield, although much depends on the details of the design and on the characteristics of the cosmic radiation profile, both of which are sketchily understood right now. One interesting side effect of a magnetic radiation shield is that it would capture part
icles from the solar wind and produce its own miniature version of Earth's Van Allen belts. The drag caused by this capture would exert a small but steady force on the bubble which would over time produce tremendous velocities, and magnetic sails like this have been considered along with light sails as a potential interplanetary drive mechanism. There are important differences though—a light sail uses photon pressure from the sun acting on a huge reflective sheet and can be positioned to "tack" a spacecraft up or down in the sun's gravity well. A magnetic sail of this type would only be able to accelerate away from the sun, like a surfer riding a wave in to shore. A ship which used a magnetic shield and a light sail would be able to accelerate rapidly out from Earth but would still be able to make it back down the gravity well on the return leg of the mission. This raises the rather beautiful image of space captains laying courses balancing the solar wind and photon pressure just as clipper captains balanced trade winds and ocean currents.

  BLOOD'S A ROVER

  This story of a girl and her racoon is an excerpted episode from a new novel, Beyond Infinity. Greg was one of the earliest supporters of the "swoosh" concept for this anthology, and I thank him for his enthusiasm. In From Dawn to Decadence Jaques Barzun wrote: "The great advantage for science of an aimless universe is that it frees the imagination." Greg has spent his professional career as a physicist looking to explain the universe, and it clearly has done nothing to impair his imagination. Rather the reverse, as Barzun implies. Prepare your mind to spread its wings and fly along with Greg as he takes us on a tour of the future. And watch for another new story by Greg in the next Cosmic Tales anthology: Adventures in Far Futures.

  Gregory Benford