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 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 cosmic-ray shield has a mass of 6 X 10⁷ t, about 5.5 times larger than that of the torus design. Assuming a population density equal to that of the baseline design, such a sphere can support a population of about 140,000 people. However, it is probable that habitats of such a size would be settled with much lower population densities, so as to permit additional "wild" areas and parkland.
AUTOMATION AND PRODUCTIVITY
As the space community produces increased revenue, the standard of affluence is expected to increase. Increased use of automation and adjustment of levels of employment may permit the construction of habitats with a greater amount of area per person. Also, esthetic considerations will have greater impact on habitat design and architecture as habitat construction continues and per-capita wealth increases.
If automation permits a moderate increase of productivity to a value of 100 t/person-year, which is twice the value now appropriate for processing and heavy industries on Earth, the large Bernal sphere could be built for an investment of 50,000 man-years of labor. That is equivalent to the statement that 12 percent of the maximum population of one such sphere, working for 3 yr could duplicate the habitat. Automation is much better suited to the large scale, repetitious production operations needed for the habitat shell than to the details of interior architecture and landscape design. It seems quite likely, therefore, that the construction of new habitats will become an activity for specialists who supply closed shells, ready for interior finishing, to groups of prospective colonists.
LIMITS TO GROWTH
From the viewpoint of economics, the logical site for colony construction is the asteroid belt itself. The construction equipment for colony-building is much smaller in mass than the raw materials it processes. Logically the optimum method is to construct a new habitat near an asteroid, bring in its population, and let them use the colony during the period of about 30 yr it takes to move it by a colloidal-ion rocket to an orbit near L5. They may, however, prefer to go the other way, to strike out on their own for some distant part of the solar system.
At all distances out to the orbit of Pluto and beyond, it is possible to obtain Earth-normal solar intensity with a concentrating mirror whose mass is small compared to that of the habitat.
If the asteroids are ultimately used as the material resource for the building of new colonies, and if by constructing new colonies near asteroids relatively little reaction mass is wasted in transportation, the area of land that is made available on the space frontier can be estimated. Assuming 13 km² of total area per person, it appears that space habitats might be constructed that would provide new lands with a total area some 3,000 times that of the Earth. For a very long time at least mankind can look toward resources so nearly inexhaustible that the current frustration of limits to growth can be replaced by a sense of openness and the absence of barriers to further human development.
Size of an Individual Habitat
The structural shell which contains the forces of atmospheric pressure and of rotation need not, in principle, itself be rotated. In the very long term, it may be possible to develop bearings (possibly magnetic) with so little drag that a structural shell could be left stationary while a relatively thin vessel containing an atmosphere rotates inside it. Such a bearing cannot be built now, but does not seem to violate any presently known laws of physics. An invention of that kind would permit the construction of habitats of truly enormous size, with usable areas of several thousand km².
Even in the absence of a "frictionless" bearing, the size possibilities for an individual habitat are enormous. As an example, a large titanium sphere seems technically feasible of a diameter of 20 km. It would contain an atmosphere at about 18 kPa pressure of oxygen and be rotated to provide Earth normal gravity at its equator. The usable land area is several hundred km², comparable to the size of a Swiss canton or to one of the English shires.
The Speed of Growth
It may be that the residents of space, enjoying a rather high standard of living, will limit their population growth voluntarily, to zero or a low value. Similar populations, on Earth, underwent a transition of that kind in passing from an agrarian to an affluent industrial society. Economic incentives for having a substantial workforce in space may, however, drive the rapid construction of new industries and new habitations there. An upper limit to the speed of growth of space colonization is estimated by assuming 3 yr for the duplication of a habitat by a workforce equivalent to 12 percent of a habitat's population. Only 56 yr are required at this rate for the construction of communities in space adequate to house a population equal to that of the Earth today.
The Decrease of Population Density With Time
Here on Earth it seems impossible for the population to increase without a corresponding increase in crowding because economics force concentrations into cities. It is expected that with the passage of time the population density must almost certainly decrease, irrespective of the total number of colonists. It is fundamental to the colonization idea that productivity can continue to grow in the colonies. As a consequence there is a continued growth of energy usage per person. As an example, suppose that there is a real (noninflationary) productivity growth rate of 2.5 percent per year and a 1:1 relationship between this productivity growth rate and the increase in energy usage. That implies a growth of a factor of 24 in total energy usage over a 128-yr period (1976 to 2104).
SOME ECONOMIC CONSIDERATIONS
Space colonization appears to offer the promise of near-limitless opportunities for human expansion, yielding new resources and enhancing human wealth. The opening of new frontiers, as it was done in the past, brings a rise in optimism to society. It has been argued that it may also enhance the prospects of peace and human well-being. Just as it has been said that affluence brings a reduction in the struggle for survival, many have contended that expansion into space will bring to human life a new spirit of drive and enthusiasm.