The structure, mass, and shape of the habitat are sensitive to the choice of design criteria. Rather substantial savings in structural mass, and hence in cost and construction time, can be obtained by deviating from Earthlike conditions. Because the physiological effects of appreciable deviations from some of the terrestrial conditions are unknown, the living conditions in space are designed to be similar to those on Earth despite additional costs. The treatment of weightlessness is an example of this conservative approach.
An outstanding feature of space is the absence of the sensation of weight. In vessels moving freely in orbit objects exhibit weightlessness; they are said to be in "free fall," or subject to "zero gravity" or "zero g." Weightlessness is a major potential resource of space, for it means humans can perform tasks impossible on Earth. Large masses do not require support, and their movement is restricted only by inertia. Structures can be designed without provision for support against the forces of gravity; in free space there is no such thing as a static load. Although these opportunities are only beginning to be explored, it seems likely that weightlessness will permit novel industrial processes (refs. 1, 2). Moreover, in free space, levels of pseudogravity can be produced and controlled over a wide range of values. This capability should foster the development of manufacturing processes not possible on Earth. Despite these potentially important commercial advantages of life in free fall, possible physiological consequences are of concern.
On Earth, gravity subjects everyone continuously and uniformly to the sensation of weight. Evolution occurred in its presence and all physiology is attuned to it. What happens to human physiology in the absence of gravity is not well understood, but experience with zero g is not reassuring. In all space flights decalcification occurred at a rate of 1 to 2 percent per month (ref. 3), resulting in decreased bone mass and density (ref. 4). There is no evidence to suggest that the rate of calcium resorption diminishes even in the longest Skylab mission of 89 days (ref. 5). Longer exposures could lead to osteoporosis and greatly reduced resistance to fracture of bones on minor impact. Moreover, because the body presumably draws calcium from the bones to correct electrolyte imbalances (ref. 4), it is clear that in zero g over many weeks and months a new equilibrium in the cellular fluid and electrolyte balance is not achieved. Furthermore, hormone imbalances also persist. In the later stages of some missions suppression of steroid and other hormone excretions were noted, together with reduction of norepinephrine output (ref. 3), unstable protein and carbohydrate states (ref. 5), indications of hypoglycemia, and unusual increases in secondary hormone levels with corresponding increases in primary hormones (private communication from J. V. Danellis, NASA/Ames Research Center). The medical problems on returning to Earth from zero g are also significant. Readaptation to 1 g has been almost as troublesome as the initial changes due to weightlessness. Following even the relatively short missions that have been flown to date astronauts have experienced increases of 10-20 beats/min in heart rate, decreased cardiac silhouette, changes in muscle reflexes, venous pooling, and leucocytosis (refs. 3-5). Although changes in physiology have been reversible, it is not known whether this will be so after prolonged weightlessness. Vascular changes, such as reduction in the effectiveness of veins or variations in the pattern of response of mechano-receptors in the walls of blood vessels, or changes such as decrease in the effectiveness of the immune system, or the manifestation of differences in fetal development (especially possible inhibitions of the development of the balance mechanism of the inner ear), may become irreversible.
From present knowledge of the effects of weightlessness on physiology it seems appropriate to have at least some level of gravity acting on humans in space most of the time. Levels below the Earth normal (1 g) are not considered because there is no data on the effects of long-term exposure to levels of gravity between zero and one. Consequently because short term excursions into weightlessness reveal the complexity of the resulting physiological phenomena, and because the study group decided to be cautious in the absence of specific information, a criterion for safe permanent habitation is adapted - that the residents should live with the same sensation of weight that they would have on the Earth's surface, namely 1 g. Some variation about this figure is inevitable and so it is specified that humans permanently in space should live between 0.9 g and 1 g. This choice of a 10 percent variation is arbitrary, but also maintains conditions as Earth-like as possible.
The decision to provide 1 g to the colonists means they must reside in a rotating environment; the most feasible way to generate artificial gravity. However, in a rotating system there are forces acting other than the centrifugal force which supplies the pseudogravity. Thus, although the inhabitant at rest in the rotating system feels only the sensation of weight, when he or she moves, another force, called the "Coriolis force," is felt. The Coriolis force depends upon both the speed of motion and its direction relative to the axis of rotation. The direction of the force is perpendicular to both the velocity and the axis of rotation. Thus if the person in figure 3-1 jumps off the mid-deck level of the rotating torus to a height of 0.55 m (21.5 in.), because of Coriolis force he would not come straight down, but would land about 5.3 cm (more than 2 in.) to one side. At low velocities or low rotation rates the effects of the Coriolis force are negligible, as on Earth, but in a habitat rotating at several rpm, there can be disconcerting effects. Simple movements become complex and the eyes play tricks: turning the head can make stationary objects appear to gyrate and continue to move once the head has stopped turning (ref. 6).
This is because Coriolis forces not only influence locomotion but also create cross-coupled angular accelerations in the semicircular canals of the ear when the head is turned out of the plane of rotation. Consequently motion sickness can result even at low rotation rates although people can eventually adapt to rates below 3 rpm after prolonged exposure (ref. 6).
Again a design parameter must be set in the absence of experimental data on human tolerance of rotation rates. Although there has been considerable investigation (refs. 7-20) of the effects of rotating systems on humans the data gathered on Earth do not seem relevant to living in space. Earth-based experiments are not a good approximation of rotation effects in space because most tests conducted on Earth orient the long axis of the body parallel to the axis of rotation. In space these axes would be mutually perpendicular. Also on Earth a spinning laboratory subject still has Earth-normal gravity acting as a constant reference for the mechanism of the inner ear.
Although most people can adapt to rotation rates of about 3 rpm, there is reason to believe that such adaptation will be inhibited by frequent, repeated changes of the rate of rotation. This point is important because colonists living in a rotating system may also have to work in a non-rotating environment at zero g to exploit the potential benefits of weightlessness. For a large general population, many of whom must commute between zero g and a rotating environment, it seems desirable to minimize the rotation rate. There is a lack of consensus in the literature and among experts who have studied the problem on the appropriate upper limit for the rotation rate (refs. 21-28). For the conditions of the space colony a general consensus is that not more than several rpm is acceptable, and for general population rates significantly greater than 1 rpm should be avoided. Therefore, 1 rpm is set as the upper limit of permissible rotation rate for the principal living quarters of the colonists, again reflecting the conservative design criteria.
To maintain life processes adequately the human organism requires an atmosphere of acceptable composition and pressure. The atmosphere of the space habitat must contain a partial pressure of oxygen (pO2) sufficient to provide high enough partial pressure within the alveoli of the lungs (~13.4 kPa or ~100 mm Hg) for good respiration yet low enough to avert losses in blood cell mass and large changes in the number and distribution of micro-organisms, such as the growth of "opportunistic" bacteria (refs. 4, 29). The value of pO2 at sea level on Earth is 22.7 kPa (170 mm Hg) which sustains the needed oxygen in the blood. The range of tolerable variation is large and not well defined, but for general populations deviations of more than 9 kPa (70 mm Hg) in either direction seem unwise (ref. 30).
The presence of an inert gas in the colony's atmosphere is desirable since it would prevent an unusual form of decompression from occurring in the body's chambers and sinuses, while providing a greater safety margin during either accidental pressure drops or oxygen dilution by inert gases (ref. 31). Although several other gases have been used for this purpose, there are several reasons why nitrogen appears the most reasonable candidate for the colony. For example, since nitrogen constitutes almost 80 percent of the Earth's atmosphere, it is not surprising to find that some organisms require the gas for normal development (ref. 31). Further, with time, denitrifying bacteria will release nitrogen gas into the atmosphere, thereby resulting in the eventual accumulation of significant quantities. Finally, the inclusion of nitrogen-fixing plants in the colony's life support system means that the gas level can be biologically maintained by the conversion of nitrogen gas into protein. Thus the inevitable presence and the various benefits of nitrogen gas dictate its inclusion in the atmosphere, perhaps at a level of 26.7 kPa (~200 mm Hg).
The level of carbon dioxide should be maintained below the OSHA standard (ref. 32), which specifies that pCO2 be less than 0.4 kPa (3 mm Hg). At the same time the CO2 levels will be high enough to permit maximum rates of photosynthesis by crop plants. Trace contaminants should be monitored and controlled to very low levels.
Finally, it is desirable to maintain a comfortable relative humidity and temperature. Various sources (ref. 30) suggest a range of temperatures around 22 degrees C and a relative humidity of about 40 percent. This criterion implies a partial pressure of water vapor (pH2O) of 1.0 +/- 0.33 kPa (7.5 +/- 2.5 mm Hg).
A major consequence of these various criteria is that human life can be safely and comfortably supported at a pressure well below that of a normal Earth atmosphere (ref. 31). The grounds for choosing a particular value are discussed in chapter 4.
Humans living in space must have an adequate diet; and food must be nutritious, sufficiently abundant, and attractive. There must be enough water to sustain life and to maintain sanitation. A diet adequate for a reasonable environmental stress and a heavy workload requires about 3000 Cal/day. It should consist of 2000 g of water, 470 g dry weight of various carbohydrates and fats, 60 to 70 g dry weight of proteins, and adequate quantities of various minerals and vitamins(l) .The importance of the psychological aspects of food should not be neglected. The variety and types of food should reflect the cultural background and preferences of the colonists.
While very little is known about physiological response to individual environmental stresses, even less is known about combined effects. The long-term, cumulative, interactive effects of biodynamic factors (hypogravity, Coriolis forces), atmospheric factors (composition, pressure, temperature), radiation and electromagnetic factors (illumination quality and periodicity, magnetic field strength), temporo-spatial factors, and other environmental factors could be additive. (ref.33)
It seems probable that if a substantial effort is made to provide reinforcing stimuli for maintaining biological rhythm (solar spectral and intensity distribution) (ref. 34) and diurnal periodicity (ref. 35), adequate nutrition, and a pleasant living environment, the problems of combined environmental stress would prove minimal.
To satisfy the physical needs of people in a way consistent with the goals described in chapter 1, habitable environments have to be created with maximum efficiency and minimum mass. Unless design criteria are carefully set, such environments may be so artificial or so crowded as to exert damaging psychological stresses on the inhabitants. The psychological needs are discussed more fully in appendix A. Moreover, the extreme novelty of the surroundings or the sense of isolation of living in space may be stressful. It is the task of the architectural (ref. 36) and environmental designer to reduce such stresses by shaping and interrelating structures and surroundings to meet the psychological, social, cultural and esthetic needs of the colony's inhabitants while also satisfying their vital physiological needs.
Diversity and Variability
Environmental psychologists and behavioral scientists (refs. 37-39) have pointed out that variety, diversity, flexibility and motivation can make apparently deficient environments quite satisfactory to their inhabitants. It is important that space colonists become meaningfully involved in their environment. This can result from there being a planned complexity and ambiguity (ref. 38), that is, the design of the habitat must not be so complete as to be sterile; it must avoid motel banality. The ideal is to build a setting that provides individuals and groups alternate ways of satisfying their goals, thus giving them freedom of choice. Attaining such an ideal is greatly facilitated by the large size of the habitat which frees from limitations planned for in the small interiors of space stations.
In particular the interior should have a general plan so that the finishing and details can be left to the choice of the colonists themselves. Emphasis in the design of living area of a space colony should not be on specifics, but on the range of options. Colonists need access to both large and small, private and community spaces, to long vistas and short ones, but with a flexible, manipulatable architecture. They need to be able to exploit and to change these spaces according to individual wants. The initial design must permit the colonists to reshape the interior by developing and altering the spaces.
To these ends a building system must be developed which is fairly flexible, light weight, easily mass produced, capable of fast efficient erection, and yet allows a variety of spaces to evolve. It must also provide a sufficient esthetic quality, both materially and spatially. These criteria suggest a system that is built from modular components; that is, panels and structural elements that are uniform in size but when stacked or laid horizontally allow any combinations of shapes to be achieved.
There are ways to offset the undesirable effects of artificiality other than by facilitating individual variation. One is to provide large-scale vistas, that is, to make the habitat large enough to lessen the sense of its being manmade. To this end it may be desirable to limit a colonist's view so that the entire structure cannot be seen in a single scan by designing it so that some parts are always out of sight of others. Natural objects might also be simulated, but such simulation is usually recognized as being false. It then exaggerates the sense of artificiality, although it is possible to represent the natural environment by miniature design with a high degree of perfection and satisfaction, as in Japanese gardens. A better idea is to provide contact with the actual environment of space. Convenient access to regions of zero gravity and to views of the Earth, the Moon, and stars would provide stimuli taking full advantage of life in space; it also would provide panoramic vistas, long lines of sight, and awareness of reality beyond the human scale.
On a smaller scale the artificiality of the interior would be reduced by the presence of live, growing things such as vegetation for eating or for decoration, children playing and exhibiting the chaos of youth, or animals such as pets or livestock. Living things should be provided as an integral part of the interior architecture of the colony. The desire here is to have an environment that is not completely regimented. To that end, it might be desirable to have some random variation in the climate, but the politics of producing fluctuations in temperature and humidity are probably best left to the colonists themselves.
Space Needs Within the Colony
To design a human habitation in space, a criterion must be set for area available, most conveniently expressed in terms of area per person. The amount of area allocated per person has two important consequences: it determines the population density of habitation on which depends the sense of crowding; it limits services and facilities provided to the inhabitants.
A brief survey of the literature (refs. 40-43) indicates that there should be at least 40 m^2 of projected area per inhabitant. Projected area means area projected onto the largest plane perpendicular to the direction of the pseudogravity. Thus a three-story house with 60 m^2 of floor space occupies only 20 m^2 of projected area. Actual usable area can be made larger than projected area by constructing levels within a habitat, or several stories within a building. As table 3-1 shows, 40 m^2 per person is rather less than the area per person in most U.S. cities, although it is more than in some small French villages. It is an important task of space colony architects to organize this space to minimize the sense of crowding, while still providing needed services.
TABLE 3-1 (gif format)
| Location | Per capita area, m^2 /person |
|---|---|
| Boston, Mass | 185.8 |
| Chicago,Ill | 171.2 |
| El Paso,Tex. | 950.5 |
| Jersey City, N.J. | 150.1 |
| New York, N.Y. | 98.3 |
| Manhattan Borough, N.Y. | 38.2 |
| San Francisco, Calif. | 164.3 |
| St.Paul, France | 27 |
| Vence, France | 46.2 |
| Rome, Italy | 40.0 |
| Columbia, Md. | 503 |
| Soleri's Babel IIB | 15.1 |
| Space Colony | >= 40 |
Based upon experience with Earth cities, the needs of a community of 10,000 for living area and volume are categorized and a land-use plan is developed together with quantitative estimates of the volumes and areas needed. Some of the major spaces that must be provided are:
TABLE 3-2 (gif format)
| Space use | Surface area required, m^2/person | No. of levels |
Projected area, m^2 | Estimated height, m | Volume, m^3 |
|---|---|---|---|---|---|
| Residential | 49 | 4 | 12 | 3 | 147 |
| Business: Shops Offices | 2.3 1 | 2 3 | 1.0 .33 | 4 4 | 9.2 4.0 |
| Public and semipublic; Schools Hospital | 1 .3 | 3 1 | .3 .3 |
3.8 5 | 3.8 1.5 |
| Assembly (churches, community halls | 1.5 | 1 | 1.5 | 10 | 15 |
| Recreation and entertainment | 1 | 1 | 1 | 3 | 3 |
| Public open space | 10 | 1 | 10 | 50 | 500 |
| Service industry | 4 | 2 | 2 | 6 | 24 |
| Storage | 5 | 4 | 1 | 3.2 | 16 |
| Transportation | 12 | 1 | 12 | 6 | 72 |
| Mech. subsystem Communication distr. switching equipment for 2800 families | 0.5 | 1 | 0.5 | 4 | .2 |
| Waste and water treatment and recycling | 4 | 1 | 4 | 4 | 16 |
| Electrical supply and distribution | .1 | 1 | .1 | 4 | .4 |
| Miscellaneous | 2.9 | 3 | 1 | 3.8 | 11.2 |
| Subtotals | 94.2 | - | 46.6 | - | 823.3 |
| Agricultural space requirements (a) Plant growing areas | 44 | 3 | 14.7 | 15 | 660 |
| Animal areas | 5 | 3 | 1.7 | 15 | 75 |
| Food processing collection, storage, etc. | 4 | 3 | 1.3 | 15 | 60 |
| Agricultural drying area | 8 | 3 | 2.7 | 15 | 120 |
| Totals | 155.2 | - | 67.0 | - | 1738.3 |
(a) Agricultural space requirements are provided for completeness and convenience. The details are explained in the succeeding chapters.
NOTE: the areas and volumes arrived at are approximations for use in establishing mass estimates and aiding in the structural design of the entire habitat enclosure.
Trade is Essential
It has been empirically demonstrated that selfs Efficiency grows with size in modern high-energy societies. For communities of 10,000 people there is little hope of achieving self-sufficiency as measured by lack or absence of trade. There have been studies of sociology, economics, and geography which indicate the degree to which various specialities can be sustained. Colin Clark, one of the world's distinguished students of economic organizations, reports (ref. 45) that cities need populations of 100,000 to 200,000 in order to provide "an adequate range of commercial services....". Moreover, populations of 200,000 to 500,000 are required to support broadly-based manufacturing activity.
A small settlement in space, of less than 100,000 people, would necessarily require continuing support from Earth. There is little possibility that such a settlement can be sustained without a steady and sizable movement of materials and information between Earth and the colony. Because of high demands on material productivity, ordinary business services such as banking, insurance, bookkeeping, inventory control, and purchasing would very likely remain on Earth. Management of the transportation system, and sales and delivery of products would be Earth based. The highly technological and specialized services of medicine, higher education and even of those branches of science and engineering not used in the day-to-day life of the colony would come from Earth. A community of 10,000 cannot conceivably support a large research university or a large medical center. Communities of this size on Earth do not encompass much social and cultural variety, and their major productive activities are usually limited in kind and number. To point up the lack of diversity that may reasonably be expected, consider how many and what variety of religious organizations and sects might be expected in a space colony of size 10,000.
Economies of scale for communities suggest an optimal size well above that of the early settlement in space.
Isolation: Offset by Transportation and Communications
While the small size of the colony exaggerates its dependence on Earth, the vastness of space and the long times and expense of travel tend to isolate the colonists from the home planet. A good design must attempt to anticipate and offset the effects of such isolation.
Travel from the colony to Earth is expensive, and takes a number of days. Given the need for trade and for the importation of many items of high technology that a small community cannot supply itself, the transportation system is exceedingly important. It seems essential to provide for "return migration" of persons and families to Earth who strongly wish for it, although it might be necessary to devise ways to discourage commuting.
The difficulty of physically transporting goods or people leads to a strong emphasis on electronic communication. Direct lines of sight from the colony to the Earth make radio, television, and facsimile transmission easy. Many of the special services mentioned earlier could be supplied remotely, for example, accounting, education, and even many medical diagnostic services can be performed electronically. Consequently a colony in space is expected to have highly developed electronic communications for commerce, education, entertainment and community activities. These should be designed to be easily accessible to the members of the colony, probably with two-way capability and linked to computers. The network is of such importance that it should have a high degree of redundancy for reliability, and be designed to assure privacy. It is likely that both the physical and social organization of the community will be shaped around the communications system.
Governance and Social Order
Distance and isolation also affect the governance and social order. Whether space colonization is a unilateral effort on the part of the United States or a cross-national enterprise, it will most likely be sponsored by a public or quasipublic organization with a bureaucratic structure which permeates the early settlement. The sense of isolation may stimulate the organizational development of communities away from the organizational form of the sponsor as the interests and life circumstances of a rapidly growing population change and develop. The form of governance depends very much on the preferences of the settlers, in much the same way as allowances for individual choice have been emphasized in other considerations of life in space.
Maintenance of order and of internal as well as external security initially falls to the Earth-based sponsoring organizations and then to the organized community which is expected to rise early in the colony's history. The small size of the settlement, combined with a rather precarious manufactured environment, may emphasize a concern for internal security. Any individual or small group could, in prospect, undertake to destroy the entire colony by opening the habitat to surrounding space, by disrupting the power supply, or by other actions which have few corresponding forms in Earth-based settings. Whatever organizational form the colonists evolve, it must be able to assure the physical security of the habitat and its supporting systems, and this need for security may infringe upon other desirable features of the colony and its operation.
Physiological Criteria
The basic physiological criteria are summarized in table 3-3.
TABLE 3-3 (gif format)
| Psuedogravity | 0.95 +/- 0.5 g |
| Rotation rate | <= 1 rpm |
|
Radiation exposure for the general population | <= 0.5 rem/yr |
| Magnetic field density |  < |
| Temperature | 23 +/- 8 degrees C |
| Atmospheric composition pO2 | 22.7 +/- 9 kPa (170 +/- 10 mm Hg) |
| p(inert gas; most likely N2) | 26.7 kPa (200 < pN2 < 590 mm Hg) |
| pCO2 | < 0.4 kPa |
| pH2O | 1.00 +/- 0.33 kPa (7.5 +/- 2.5 mm Hg) |
Environmental Design Criteria
The criteria for environmental design are both quantitative and qualitative. Quantitative criteria are summarized in table 3-4.
TABLE3-4 (gif format)
| Population: men, women, children | 10,000 |
| Community and residential projected area per person,m^2 | 47 |
| Agriculture, projected area per person, m^2 | 20 |
| Community and residential, volume per person, m^3 | 823 |
| Agriculture, volume per person, m^3 | 915 |
Desirable qualitative criteria are summarized in table 3-5.
TABLE 3-5 (gif format)
| Long lines of sight |
| Large overhead clearance |
| Noncontrollable unpredictable parts of the environment; for example, plants, animals, children, weather |
| External views of large natural objects |
| Parts of interior out of sight of others |
| Natural light |
| Contact with the external environment |
| Availability of privacy |
| Good internal communications |
| Capability of physically isolating segments of the habitat from each other |
| Modular Construction of the habitat of the structures within the habitat |
| Flexible internal organization |
| Details of interior design left to inhabitants |
Organizational Criteria
The organizational criteria have to do with both physical organization and social and managerial organization. In the first category is the criterion that the components of the colony be located so that the resources of space can be effectively exploited: solar energy, matter in the Moon or asteroids and on Earth, high vacuum of space and possibilities of pseudogravity variable from 0 g to more than 1 g. The colony must be provided with a transportation system that is capable of sustaining close contact with Earth, and with extensive electronic communications.
The organization of the governance of the colony is less restrained by specific criteria than are other aspects of life in space. Nevertheless, the organization must be such as to permit comfortable life under crowded conditions far from other human communities. Moreover, the organization must facilitate a high degree of productivity, foster a desirable degree of diversity and heterogeneity, and maintain the physical security of the habitat.
The next chapter considers a number of alternative ways of meeting these criteria. From the various alternatives the particular design is then selected and justified.
