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.
Figure 3-1 — A rotating system (used to illustrate Coriolis force).
<!-- image -->ATMOSPHERE: LESS IS ENOUGH
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 ($p\text{O}_2$) 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 $p\text{O}_2$ 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 $p\text{CO}_2$ be less than 0.4 kPa (3 mm Hg). At the same time the $\text{CO}_2$ 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° C and a relative humidity of about 40 percent. This criterion implies a partial pressure of water vapor ($p\text{H}_2\text{O}$) of $1.0 \pm 0.33$ kPa ($7.5 \pm 2.5$ 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.
FOOD AND WATER
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.¹ 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.
¹ Sweet, Haven, Florida Technological University, Orlando, Fla., personal communication, July 1976.
COMBINED ENVIRONMENTAL STRESSES: PROBABLY NOT SERIOUS
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.
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.
ENVIRONMENTAL DESIGN TO REDUCE STRESS
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.