⇔ 7 ⇔ VIEW TO THE FUTURE
In earlier chapters conservative projections are made on the possibilities of space colonization. The view is that the potentialities of the concept are substantial even if no advanced engineering can be employed in its implementation. Abandoning a restriction to near-term technology, this chapter explores long-term development in space, mindful of a comment made many years ago by the writer Arthur C. Clarke. In his view, those people who attempt to look toward the future tend to be too optimistic in the short run, and too pessimistic in the long run. Too optimistic, because they usually underestimate the forces of inertia which act to delay the acceptance of new ideas. Too pessimistic, because development tends to follow an exponential curve, while prediction is commonly based on linear extrapolation.
What might be the higher limits on the speed and the extent of the development in space? First, consider some of the benefits other than energy which may flow from space to the Earth if the road of space colonization is followed. To the extent that these other benefits are recognized as genuine, space colonization may take on added priority, so that its progress will be more rapid.
From the technical viewpoint, two developments seem almost sure to occur: progress in automation, and the reduction to normal engineering practice of materials technology now foreseeable but not yet out of the laboratory. Those general tendencies, in addition to specific inventions not now foreseen, may drive the later stages of space colonization more rapidly and on a larger scale than anticipated in the other chapters of this report.
BENEFITS NOT RELATED TO ENERGY
Space colonization is likely to have a large favorable effect on communication and other Earth-sensing satellites. Already communication satellites play an important role in handling telex, telephone, computer, and TV channels. They provide data-links and track airplanes and ships as well as rebroadcast TV to remote areas. In the future even more of these data-link applications can be expected. Not only will planes and ships be tracked and communicated with by using satellites, but trains, trucks, buses, cars, and even people could be tracked and linked with the rest of the world continuously. Currently, the main obstacle blocking direct broadcasting of radio and TV to Earth from orbit is the lack of low-cost power in space. SSPS's would produce such power. In addition, their platforms could be used to provide stability. Currently, up to 40 percent of the in-orbit mass of communication satellites consists of equipment used to provide power and maintain stability. Finally, colonists could carry out servicing and ultimately build some of the components for such satellites.
Space manufacturing such as growing of large crystals and production of new composite materials can benefit from colonization by use of lunar resources and cheap solar energy to reduce costs. Space manufactured goods also provide return cargo for the rocket traffic which comes to L5 to deliver new colonists and components for SSPS's.
Within the past half-century many of the rich sources of materials (high-grade metallic ores in particular) on which industry once depended have been depleted. As the size of the world industrial establishment increases, and low-grade ores have to be exploited, the total quantity of material which must be mined increases substantially. It is necessary now to disfigure larger sections of the surface of the Earth in the quest for materials. As both population and material needs increase, the resulting conflicts, already noticeable, will become more severe. After the year 2000 resources from the lunar surface or from deep space may be returned to the Earth. Much of the lunar surface contains significant quantities of titanium, an element much prized for its ability to retain great strength at high temperature, and for its low density. It is used in the airframes of high performance aircraft, and in jet engines. Given the convenience of a zero-gravity industry at L5, a time may come when it will be advantageous economically to fabricate glider-like lifting bodies in space, of titanium, and then to launch them toward the Earth, for entry into the atmosphere. The transportation of material to the Earth in this form would have minimum environmental impact, because no rocket propellant exhaust would be released into the biosphere in the course of a descent. (Some oxides of nitrogen would be formed as a result of atmospheric heating.) Titanium may be valuable enough in its pure form to justify its temporary fabrication into a lifting-body shape, and its subsequent retrieval in the ocean and towing to port for salvage and use. If such lifting-bodies were large enough, it might be practical to employ them simultaneously as carriers of bulk cargo, for example, ultra-pure silicon crystals zone-refined by melting in the zero-gravity environment of the L5 industries. It has been suggested that the traditional process of metal casting in the strong gravitational field of the Earth limits the homogeneity of casts because of turbulence due to thermal convection. Quite possibly, in space, casting can be carried out so slowly that the product will be of higher strength and uniformity than could be achieved on Earth. A titanium lifting-body might carry to the Earth a cargo of pure silicon crystals and of finished turbine blades.
RESEARCH IN DEEP SPACE
The foundries of the Earth fabricate heavy machinery in an intense gravitational field simply because there is no other choice. The ideal location for the construction of a very large object is almost certainly a zero-gravity region. The L5 colonies, furnished with abundant solar power, relatively conveniently located for access to lunar materials, and with zero gravity at their "doorsteps" will very likely become the foundries of space, manufacturing not only satellite power stations but also radio telescope antennas many kilometers in dimension, optical telescopes of very large size, and vessels intended for scientific voyages to points farther out in space. Research probes to the asteroids and to the outer planets could be built, checked out and launched gently from L5 colonies and with none of the vibration which attends their launch from the surface of the Earth. Once the principles of closed-cycle ecology have been worked out thoroughly, as they almost surely will be during the first few years of colonization, a vessel large enough to carry a "laboratory village" of some hundreds of people could be built at L5 and sent forth on an exploratory trip of several years. On Earth, villages of smaller size have remained stable and self-maintaining over periods of many generations, so there seems no reason why a trip of a few years in the spirit of one of Darwin's voyages could not be undertaken in deep space.
Lunar resources, when available, will have a profound impact on the cost of travel between low Earth orbit (LEO) and L5. Indeed, an ordinary chemical rocket, able to reload with liquid oxygen at L5, and to carry only hydrogen as a propellant component from the Earth, would perform as a LEO-to-L5 shuttle. For a trip from L5 out to the asteroids, it may be that eventually each exploratory ship will carry enough propellant for only a one-way trip, relying on the carbonaceous chondritic asteroids as inexhaustible "coaling stations" for hydrogen and oxygen, thereby making longer voyages possible.
ROCKET ENGINES FOR DEEP SPACE
For operations from Earth a rocket engine has to be compact and very strong, capable of withstanding high temperatures and pressures. For voyages between L5 and the asteroids there is no need for rapid acceleration, and in zero gravity there is no reason why an engine could not be many kilometers in length and quite fragile. One obvious candidate for a deep-space rocket engine is the "mass-driver" which would, presumably, be proven and reliable even before the first space colony is completed. For deep-space use a solar-powered mass-driver could be as much as 50 km in length, made with yard-arms and guy-wires, much like the mast of a racing vessel. Note that on the surface of the Earth, in one gravity, it is possible to build very lightweight structures (television towers) with a height of 500 m. For deep space an acceleration of 10⁻⁴ g would be sufficient, so it should be possible without excessive structure to build something much longer.
A mass-driver optimized for propulsion rather than for materials transport would have a lower ratio of payload mass to bucket mass than is baselined for the Moon. For a length of 50 km an exhaust velocity of as much as 8 km/s (in rocketry terms, a specific impulse of 800) should be possible without exceeding even the present limits on magnetic fields and the available strengths of materials. A mission to the asteroids, with an exhaust velocity that high, would require an amount of reaction mass only a little more than twice as large as the final total of payload plus engine.
A mass-driver with a length of 50 km could hardly be made in a miniature version; it would probably have a mass of some thousands of tonnes, a thrust of about 10,000 newtons, and would be suitable as the engine for a ship of several tens of thousands of tonnes total mass.
TRANSPORT
In the course of the first decades of colonization it seems likely that solar-cell powerplants for space vehicles will decrease in mass, ultimately becoming very light. It will not be economically reasonable to continue using rocket engines which exhaust hydrogen, scarce as it is on the Moon. The rocket engines of that period will very likely be solar-powered, and must exhaust as reaction mass some material that appears naturally as a waste-product from the processing industries in space; further, that material must not be a pollutant. One good candidate may be oxygen; it constitutes 40 percent by weight of the lunar soils.
At least two types of rocket engines satisfying these conditions seem good possibilities: the mass-driver, used with liquid or solid oxygen payloads for reaction mass, and the colloidal-ion rocket, which would accelerate electrically small micropellets having a ratio of charge to mass which is optimized for a particular mission. The mass-driver, as a rocket engine, only makes sense for large vehicles or loads; its length would be comparable to that of an SSPS, and its thrust would be several thousand newtons. The colloidal-ion rocket would have much lower thrust but could be compact.
When traffic between the Earth and the colonies becomes great enough, the most economical system may consist of a single-stage-to-orbit shuttle between Earth and LEO. Because there would be no need for the transport of large single structures, shuttles of that kind could be sized for optimum efficiency. From LEO to L5 the transport problem is entirely different, transit times are several days rather than a few hours, and high thrusts are not required. The most economical vehicles for that part of space may be large ships built at the colonies. These ships, mass-driver powered through solar energy, could carry a round-trip load of oxygen as reaction mass when they leave the colonies, and could then rendezvous with shuttles in low orbit. The outbound trip would be faster than the inbound.
THE ASTEROIDAL RESOURCES
The evidence is mounting that a substantial fraction, if not actually a majority, of the asteroids are made up of carbonaceous chondritic material. If so, the asteroids contain an almost inexhaustible supply of hydrogen, nitrogen and carbon. In energy (namely: in velocity interval squared) the asteroids are about as distant from L5 as is the surface of the Earth: the velocity change to either destination, from L5, is 10 to 11 km/s. This is about four times that between L5 and the Moon. For some time, then, it seems likely that the asteroidal mines will be exploited mainly for the "rare" elements rather than for those which can be obtained from the Moon. Ultimately, as industry shifts from L5 out toward the asteroids, lunar resources may be used less as materials are mined and used directly, without the necessity of prior shipping.