The habitability challenge of early spaceflight was to sustain the individual for very brief periods of time. More recently, the goal has been to maintain a few individuals for several months within some acceptable limits of discomfort. For the future, the challenge will be to provide not only a sufficient, but an enriching environment for groups of people for periods of a year and more.
Within the vastness of space, areas in which the astronaut can sleep, work, recreate, etc., are necessarily limited. An important question is how limited these spaces can be before they begin to interfere with a human's normal functioning. Since all areas in a spacecraft are "primary" (i.e., areas in which the individual spends a large amount of time), high density in space is likely to result in physiological and psychological effects. These effects could show themselves in specific biomedical responses, in general health problems, and in performance decrements. Therefore, volume considerations must command continued attention from a general habitability standpoint.
Although we now have a reasonable understanding of the spatial needs necessary to sustain a person for brief periods, we have only limited understanding of what constitutes adequate space over long durations. Nor do we understand how the number of people sharing an area affects each individual's requirements. To date, there is not even agreement on whether large crews require more or less area per person than small crews. When the direction of this effect is ascertained, we can begin to explore the details of the relationship between crew size and spatial needs.
Several questions emerge regarding how the space that is available should be allocated. Is private space necessary and if so, how should it be structured? Should public areas be multipurpose, or used for specific activities? If particular areas are used for specific activities, how can these areas be set apart or "marked" in order to provide maximal psychological distance? And, although astronauts are provided with a 360° world, they continue to operate as if they lived in a modified two dimensional world. If astronauts can learn to use all available surfaces, how does this learning take place and what are the time considerations? Such research would benefit not only the space effort but environmental psychology in general since space, by exposing individuals to the totally new experience of 0 g, offers a  unique opportunity to examine how unfamiliar environments are mastered.
There are numerous environmental factors which singularly are of only minor importance, but which together determine the quality of life in a spacecraft. Although few of these factors have yet posed serious problems, certain questions suggest themselves as deserving attention in extended spaceflight.
Food emerges as an important habitability consideration in many confinement situations. Various studies indicate that an even greater emphasis is placed on food and on mealtime generally when individuals are confined. It will be important to determine what role food consumption, as a diversionary or social activity, will play in long duration spaceflight. Also, it will be important to determine how the use of food can be influenced by other environmental factors. For instance, it is likely that the greater the satisfaction with work and leisure activities, and the more prevalent the opportunity for social interactions, the less will be the interest in food. A psychophysical question was suggested by the astronauts of Skylab. These crewmembers observed that food which had seemed adequately seasoned prior to flight tasted bland in space. This observation raises the question of whether taste and/or odor thresholds shift in weightlessness, a possibility that should be investigated.
Hygienic facilities in space habitats are still only marginally adequate. When designs reach a higher level of utility it will be possible to evaluate hygienic facilities in terms of more demanding requirements such as acceptability over long durations, ability to support physically active space travelers, etc.
Temperature and humidity can also be expected to affect the adjustment to space living. Cold temperatures can influence an individual's ability to perform manipulative tasks as well as cause general discomfort. High temperatures can result in both decreased performance levels generally and aggressive displays. Since individuals differ in what they consider acceptable temperatures, space travelers may have to consider temperature preferences along with other features of compatibility.
We know that decor assumes more or less importance depending on the conditions of confinement. Decor is found to be more important when crews are composed of both men and women and where there is little work required; decor is less important with all male  crews or where there is meaningful work to be performed. Although people differ in what they consider desirable, there seems to be an almost universal aversion to environmental uniformity, along with some evidence that, over time, people seek greater complexity in the environment. In the closed environment of space, it will be necessary to test tolerances to both differences and sameness in design. Flexibility is likely to be fundamental to any spacecraft design. However, even this recognized value will have its limits, since a living environment must provide an opportunity for stability as well as for adventure. Lighting offers an opportunity for rapid environmental shifts. Just how lighting can and should be used, both from a design and a performance perspective, requires further investigation.
To date, olfactory adaptation in spaceflight has apparently kept ahead of olfactory buildup. However, odors may present problems in future flights, especially when humans and experimental animals share the same living area, and when new crewmembers are rotated into the closed environment. We need to determine tolerance levels for various odors and to understand more fully how odors work in concert.
Noise problems are among the more serious habitability concerns of long duration spaceflight. We have some understanding of the biomedical, performance, and other changes associated with various noise stimuli in a changing terrestrial environment. However, we know little of these effects when, as in space, noise stimuli are virtually unremitting. A complicating factor in understanding the effects of noise is that an individual's response can be influenced by certain mediating factors. For instance, noise can cause little disruption if the individual believes that the noise is necessary, or that the person causing the noise is acting out of necessity or in a considerate manner.
Tolerance levels to various noise stimuli appropriate to space need to be determined, taking into consideration both adaptation effects and individual differences. We also need to establish how spacecraft noise sources can be adjusted or varied to provide maximum relief. Another important issue concerns the relationship of noise to speech communication, especially when atmospheric pressure is low. There are also questions of how sound, and especially music, might be used to enrich the space environment and, perhaps, to increase productivity. As with issues of decor and temperature, there are further questions of personal preferences and of crew compatibility.
 The area of recreation and exercise is only now emerging as a research field. Here the approach must be to observe how people behave, both in their normal lives and in confinement; to suggest explanations for these behaviors, testing explanations where possible; and to relate these findings to the needs of extended spaceflight. We know that people in confinement adopt an extreme work orientation and exhibit little interest in leisure activity. It has been suggested that a more balanced approach will occur with time and with experience in confinement. This hypothesis needs testing. If situations of long duration confinement can be observed, or if confinement studies are conducted in which the same participants are used more than once, special note should be made of any changes in the use of leisure time.
An overall goal for long duration spaceflight will be to foster the kinds of leisure activities which will contribute to the general health of the individual. Kleiber (1980) has found some support for the hypothesis that individuals who are actively engaged in their free time are psychologically healthier than more passive individuals. Yet, when confined subjects engage in recreational pursuits, they opt for the passive or noninteractive variety, probably to a greater extent than they do in their everyday lives. An important question for longduration spaceflight is why leisure pursuits tend to the passive and/or noninteractive variety. Once the basis of this choice is understood (e.g., reduced morale, fear of conflict, preference for the familiar, etc.), we can determine whether the choice of pastime is likely to contribute to psychological and relational problems in extended spaceflight, and if so, how alternative activities can be encouraged. For instance, can a matching of the individual's interest to recreational provisions reverse the trend to passive use of leisure? For those activities which cannot easily be transferred to space, what kinds of activities can be substituted satisfactorily? Can a different (more heterogeneous) mix of individuals lead to more active involvement? Can activities such as the tending of gardens or competing with oneself circumvent the occasion for interpersonal conflict while maintaining the individual in an active mode? And, how can the benefits of passive recreation be maintained or increased? Reading is a favorite pastime in confinement. In space, electronic media will replace the written page for leisure reading as well as for work related tasks. It must be determined how present or planned information systems, and especially visual displays, will meet the demanding requirements of space.
 With reference to exercise, we need to assess the kind and extent of activity that is necessary to maintain an individual at peak physical and psychological health during prolonged weightlessness. We must then determine how to motivate astronauts to perform the needed exercise. If an exercise program is externally imposed, it will probably be maintained for only a short period of time. For long term spaceflight the motivation to exercise must come from the space travelers themselves. We have little data on what motivates individuals to exercise even under ordinary circumstances. The following hypotheses are offered for consideration:
Some data suggest that cognitive strategies are very important in maintaining an exercise commitment (Morgan, 1978). Although these findings are based on methods employed by extraordinary athletes, the strategies deserve testing on others as well. Space offers new opportunities for space sports, but we don't know if such sports will be of general interest. It is likely that exercise facilities, like recreational facilities, must cater to the special interests of individuals. We need to determine if, by addressing the interests of the individuals, and by providing them with a means of increasing skills and tracking progress, we can improve the likelihood that an exercise program will be maintained.
A critical habitability concern for long duration spaceflight is the need for privacy. Privacy is a balance between forces to affiliate and forces to withdraw. This balance helps an individual define himself and his relationship to others. An important question is just what this balance implies in space.
In confinement, many of the accustomed mechanisms for privacy regulation are either lacking in the environment, or inappropriate to the situation. Characteristically, confined individuals  respond to this unfamiliar circumstance by first exceeding normal privacy limits, and then by withdrawing from others in the group. Although withdrawal is an effective form of privacy regulation, it is destructive to group cohesion and ultimately to mission goals. For extended spaceflight, alternative means must be found both to protect privacy and to foster effective group functioning.
To meet the individual's need for privacy, we must understand what this need entails and which events or conditions most threaten privacy in space. Is there a specific need for physical or psychological separation? Can privacy needs be met through personal possessions (other than space), through individualistic clothing, or by some architectural arrangement/design?
Confined individuals have shown a penchant for oversharing personal information, suggesting that this is an area requiring particular attention for spaceflight. Generally, it has been observed that friends commit fewer privacy violations than new acquaintances. Similarly, we know that when one is with friends, space limits are perceived as less pressing than when one is with strangers. If these findings are verified in the confinement of space, it suggests that established relationships would transfer well to spaceflight. Preliminary data indicate that women adjust more favorably to a crowded environment than men. However, there is also the suggestion that, over time, women suffer more from crowding than men. These studies require verification and extension. Perceptions of crowding have been found to relate to certain personality characteristics. We need to explore more fully just which personality characteristics are most predictive of spatial demands and how these characteristics relate to other requirements of spaceflight. An interesting question concerns the possibility of training individuals for privacy regulation in space. The goal of this effort would be to foster both group cohesion and adequate levels of personal separation. Ideally, privacy violations would be avoided; practically, one would seek to reduce privacy violations to some manageable level and, at the same time, to provide options (other than withdrawal) for dealing with the violations that do occur. We need to determine whether, and to what extent, training could accomplish these ends.
It is necessary to distinguish among the many issues that relate to privacy, such as personal space, control over information about oneself, etc. And, in order to deal with a particular situation, it is important to understand what a person means when he implies that his privacy is violated. For instance, if a person complains of being  "crowded,'' he or she may be expressing a need for personal space (in which case barriers would help) or a need for more space generally (in which case barriers would compound the problem.) Privacy violation can occur as a result of a single event or condition, or because of a compilation of events and conditions. Privacy violations in space are likely to result from multiple events or conditions, and this compounding effect needs to be taken into account.
In addressing privacy issues, it is important to appreciate the entire situation in which the space traveler must function. In attempting to deal with a problem of too much exposure, one must guard against creating a larger problem by cutting off the individual's access to others. One must also not overlook the costs associated with establishing and maintaining privacy. These costs arise because of conflicts between the needs of the individual and the needs of other individuals or of the larger society. If such conflicts occur in space, the victory of the individual will not be taken easily.
Various environmental stressors can combine to trigger behavior which is different in kind from the response to individual stressors. And, in some cases environmental stressors can exert negative aftereffects such as decreased performance or aggressive behavior (Cohen, 1980). For spaceflight, we must consider the environmental effects over long periods of time which include not only the flight itself but pre and post flight adjustment.
One of the more promising findings from environmental research is that many of the negative effects of environmentally induced stress can be ameliorated if the stressor occurs in a predictable and controllable fashion. Stokols (1979) points out also the importance of understanding the salience of the event to the individual. A better understanding is needed of how predictability and control counter the effects of stress (Stokols, 1979). However, the identification of mitigating variables such as these does indicate that it is possible to devise methods to help individuals deal with stressful environments, and that research explicating such variables can have an influence on training strategies.
Another method of reducing the environmental stress of space is to match, as far as possible, the individual with the environment. Although the conditions of space will be demanding, they will also present challenges which for some individuals could offset the inconveniences and disruptions inherent in the environment. If conditions do not tax or exceed the individual's adaptive resources, then for this  individual they do not constitute an environmental stressor (Stokols, 1978a). When attention have been given to the individual and to the environment, it is probable that further attention to the relationship between the two, i.e., the person environment fit, will be required.