NASA SP-413 — SPACE SETTLEMENTS — A Design Study 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.
NEW METHODS OF CONSTRUCTION Construction methods which are now only at the stage of laboratory test may be practiced only in the space environment. In zero gravity and with a good vacuum, it may be practical to form a shell by using concentrated solar heat to melt aluminum or another metal at the center of a thin form. Evaporation over a period of months or years would build up on the form a metal shell, for which the thickness at each point would be controlled by masking during the evaporation. This process would lend itself well to automation. Alternatively, or in addition, habitat sections could be constructed of fiber-composites. On Earth, the most familiar example of such a material is fiberglass, a mixture of glass threads in an organic matrix. Boron filaments are used in place of glass for high strength in aerospace applications. Glass fiber could easily be made from lunar materials. As a matrix, a silicon compound might be used in the space environment similar to a corresponding carbon-based organic. Such a compound might be attacked by the atmosphere if it were used on Earth, but could be quite stable in vacuum.
HABITAT DESIGN In the long run, as colony size approaches diameters of several kilometers and individual land areas of more than 100 km2, the cosmic-ray shielding provided by the colony land area, structure, and atmosphere becomes great enough so that no additional shielding need be added, allowing the development of large-size colonies earlier than can otherwise be justified on economic grounds. Mankind’s descendents who may live in space during the next century will probably be far more adventurous in their choice of styles of habitation than can now be projected, and in the spirit of this section, a relaxation of strict choices of physiological parameters seems permissible. The assumption that the retention of artificial gravity in the living habitat continues to be necessary may be rather conservative. This assumption is based on human nature. Most people do not keep in good physical condition by self-imposed exercise. Return to Earth, whether or not occurring, must remain an option with strong psychological overtones. To rule it out, as might be the case if bones and
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muscles were allowed to deteriorate too far by long habitation in zero gravity, would be to make of the colonists a race apart, alien to and therefore quite possibly hostile to those who remain on Earth. Habitation anywhere within a range of 0.7 to 1.0 g is assumed to be acceptable, and in the course of a normal day a colonist may go freely between home and zero-gravity work or recreation areas. As colony size increases, the rotation-rate criterion ceases to be a design limit. Atmospheric pressure is important to large colonies. With increasing experience in an environment of very large volume, with an abundant source of water, and with artifacts made for the most part of minerals rather than organics, fire protection is expected to be practical in an atmosphere having a total pressure of 36 kPa, of which half is oxygen. The oxygen at Denver, Colorado (which is 18 kPa), is normal for millions of human beings in that area. It is no great leap to assume an atmospheric mix of 50 percent oxygen, 50 percent nitrogen with appropriate amounts of water vapor. From the esthetic viewpoint, people might prefer an “open” nonroofed design habitat (sphere or cylinder) when it is available to one of the more mass-efficient roofed designs. It may be possible to get some better information about public preference after further exposure of the ideas to the public. Architectural design competitions could be a means to yield valuable new ideas. It seems certain that over a time-span of several decades new designs will evolve. Some may combine mass-efficiency, achieved by optimizing the shape of the pressure shell and the cosmic-ray shield, with visual effects which are tailored to meet the psychological needs of the colony’s people. The ways in which sunlight is brought into a habitat may be adjusted to suit psychological needs which we on Earth do not yet appreciate. Similarly, the degree of visual openness of a habitat may be separated from the structure itself; it is possible to divide an open geometry into visual subsections, and to provide visual horizons in a variety of ways, though a closed geometry cannot easily be opened. To estimate the total resources of land area which could ultimately be opened by space colonization requires a model. An example from what might be a class of geometries is the “Bernal sphere” discussed in chapter 4, which seems representative of possible designs of interest. As far as now known the Bernal sphere is more suitable than other geometries for the addition of passive cosmic-ray shielding, and for a given diameter it is far more efficient in mass than the cylinder geometries. Quite possibly, of course, other designs not yet thought of may be found more desirable in the long run. A spherical habitat of 900 m radius, rotating at 1 rpm, and containing an atmosphere with a total pressure of 36 kPa, with Earth-normal gravity at its “equator,” has a structural mass of 5 X 106 t if made of aluminum. Its habitable area is 6.5 km2 , in the form of a single connected region 1400 m wide and 5.6 km in circumference. It has agricultural areas of comparable size, and low-gravity regions for heavy assembly, and for recreation. Its stationary or slowly counter-rotating
Chapter 7 — View To The Future