JACK SPURLOCK, WILLIAM COOPER, PAUL DEAL, ANNITA HARLAN, MARCUS KAREL, MICHAEL MODELL, PAUL MOE, JOHN PHILLIPS, DAVID PUTMAN, PHILIP QUATTRONE, C.DAVID RAPER JR., ELLIOT SWAN, FREIDA TAUB, JUDITH THOMAS, CHRISTINE WILSON and BEN ZEITMAN
This paper describes the second phase of analyses that were conducted by the Life Support Systems Group of the 1977 NASA Ames Summer Study. This phase of analyses included a preliminary review of relevant areas of technology that can contribute to the development of closed life-support systems for space habitats, the identification of research options in these areas of technology, and the development of guidelines for an effective research program. The areas of technology that were studied included: (1) nutrition, diet and food processing; (2) higher plant agriculture; (3) animal agriculture; (4) waste conversion and resource recovery; and (5) system stability and safety. Results of these analyses, including recommended research options and criteria for establishing research priorities among these many options, are discussed in this paper. However, the companion paper that was also prepared by the Life Support Systems Group (entitled "Systems Engineering Overview for Regenerative Life-Support Systems Applicable to Space Habitats," by J. M. Spurlock and M. Modell) should be read first to provide a background on system closure requirements, potential problems associated with the operation of closed systems in the environment of a space habitat, and systems-engineering factors that affect technology-development planning and management for closed life-support systems.
INTRODUCTION AND OBJECTIVES
This paper was prepared by the Life-Support Systems Group of the 1977 NASA Ames Summer Study on Space Settlements and Industrialization Using Nonterrestial Materials. It summarizes the group's analyses and recommendations concerning the following issues;
The 1977 Summer Study was concerned principally with industrial production in space using nonterrestial materials, with housing and life support for an industrial work force in space, and with space settlements per se. The study planners reasoned that the earliest industrial activities in space probably will depend on "open-loop" life-support provisions, including resupply form Earth for food with perhaps some partial regeneration of atmospheric components and water. However, beyond this early transitional period, it is believed that long-term space settlements will require essentially completely regenerative, or "closed," life-support systems (CLSS). Therefore, the charter of the Life-Support Systems Group (LSSG) was to determine the requirements for achieving this closure, particularly the research that will be necessary over future years.
The efforts of the LSSG were in cooperation with the Bioenvironmental Systems Study Group (BSSG) of the Society of Automotive Engineers, Inc. (The BSSG is sponsored by the Office of Life Sciences, NASA Headquarters, a cosponsor of the 1977 NASA Ames Summer Study.) A major chartered responsibility of the BSSG is to develop recommendations concerning the scope of NASA's future work in regenerative life-support systems for space habitats, including the assignment of research and development priorities to most effectively advance the associated technology. Our joint findings, which must be regarded as interim results, are presented here and in a companion paper, "Systems Engineering Overview for Regenerative Life-Support Systems Applicable to Space Habitats." That paper summarizes criteria for closure of the life-support system for space habitats and factors to consider when assessing the relative benefits and costs of closure in comparison with nonregenerative options. In addition, it specifies a methodology for identifying, screening, and evaluating candidate components through an iterative procedure that involves scenario formulation and modeling, together with trade-off analyses.
As described in the companion report, there are many alternative components from which closed-system scenarios can be postulated. For virtually all of these alternatives, a significant amount of research would be required before any could meet the closure needs of a permanent space habitat. Nevertheless, certain scenarios might show more promise, thereby justifying research emphasis over other options. Presently, the available data base is not adequate to support a priori selections or exclusions from the possible options. In the near future, follow up studies will be carried out by the BSSG. Therefore, the review of relevant technology, characterization of research options, and structuring of priorities among these options (as discussed here) are ongoing, iterative processes.
The necessary provisions and accommodations required of life-support systems on manned space missions generally include adequate food, potable water, breathable air, waste disposal, personal hygiene, medical care, safety from identifiable hazards, and reasonable comfort. For this study, the related subtopical areas of regenerative life-support system analysis were designated to be: (1) the dietary basis - for human foodstuffs, (2) how foodstuffs are produced (e.g., grown), (3) how waste products are recycled back into forms that can be used as inputs (i.e., for food production as well as water and oxygen for humans), and (4) how to develop appropriate controls and protective systems to prevent cross-contamination among subsystems and to stabilize the operation of the system. These areas are discussed in detail in subsequent sections.
The LSSG recognizes the fact that the development of a closed regenerative life-support system is an enormously complicated biological problem, particularly with respect to the fundamental ecological issues that must be solved. However, time constraints imposed by the available work period for the 1977 Summer Study required that emphasis be placed on the analysis of problems that would be essentially unique to closed systems, specifically for space habitats. Certainly, the numerous unknowns concerning ecological stability of closed systems in general, which would be common to terrestrial as well as space habitats, must be addressed in significant depth before actual systems designs can be formulated and developed. The reader is cautioned not to conclude from material presented in this paper that such designs are a straightforward undertaking based on present knowledge and data.
BASIS FOR TECHNOLOGY REVIEW
The relevant technology was reviewed so as to delineate the research options. The principal steps in this review procedure were: (1) to identify key technology areas involved in closure of a life-support system for space habitats; (2) to assess current status of applicable developments for each area; and (3) to identify major technological issues and potential problems that must be addressed in each area.
The basis for identifying key technology areas in closure is illustrated by the qualitative input-output model in figure 1
(ref. 1). Generalized roles for plants and animals in closure linkages were assumed in the model. Figure 2 illustrates
loop-closing possibilities for direct and indirect connections among inputs and outputs for animal outputs and human,
plant, and animal inputs. The direct connections between outputs and inputs require little biochemical or
physicochemical preprocessing. The preprocesses (or transformations) required for indirect connections are denoted
in figure 2 by the encircled symbols T;. Such flow sheets for connections among all human-plant-animal inputs
and outputs (prepared by the BSSG) are useful in visualizing major categories of species subsystems required to
accomplish the transformations of indirect connections.
The perceived knowledge and the technological development needs for the major categories of species are discussed in the following sections as the basis for eventual comparison and evaluation of candidate components of a closed system.
NUTRITION, DIET, AND FOOD PROCESSING
The first criterion for a successful diet is that it provide all nutrients necessary for long-term maintenance of health and work efficiency. Diet determines the agriculture inputs which then flow through the food system, as shown in figure 3. At present, there is no certainty that all necessary components of a human diet have been identified (ref. 2). It is likely that unrecognized nutrients are contained in the mixed plant and animal diets that have evolved culturally among Earth populations. As engineered foods from a narrow range of plant and animal products are being substituted increasingly into a diet that lacks natural diversity, the danger of missing some trace element or an unidentified nutrient also increases. Thus, the development of diet for closed life-support systems must include long-term evaluation of proposed diets that are significantly different from conventional established diet patterns.
Nutritional requirements for CLSS could differ from those of similar terrestrial populations. Construction work might proceed in zero gravity (i.e., extravehicular or extra habitat activity), and even leisure hours might be spent in only some fraction of full gravity. Therefore, it is of concern that crewmen on long-duration missions such as Skylab showed losses of body mass, water, calcium, and electrolytes, continuing even after months in space (refs. 3, 4). More research under space conditions is necessary to establish whether changes can be normalized by compensatory diet adjustments. Precise information is needed on the individual daily requirements of nutrients for space-habitat personnel before an entirely adequate selection can be made of basic sources of these nutrients. These sources of foodstuffs must be known before the sophisticated existing technology of food processing can be fully utilized to optimize CLSS development.
In anticipating the foods necessary for adequate nutrition, attention must be given to the criteria used to evaluate the complexity of diet that will be acceptable for various periods of time. This is because it can be assumed that the longer the period of habitation in space the more complex will be the dietary requirements. Even in traditional military environments, dietary changes are a major psychological factor. Eating preferences show the need for individual flexibility in diet selection as tastes become jaded and preferences shift (ref. 5).
Diversity in diet requires not only an increase in the number of primary food sources, but also in the forms provided.
Processing converts raw materials into a variety of individual ingredients of cuisine, such as the many forms of corn
from starch, to meal, to fresh-on-the-cob corn, as in the American diet. Also, processing can produce analogs of
traditional ingredients from new materials (e.g., the various "synthetic" products of texturized soy protein). Similarly,
nonconventional protein sources such as leaf-protein concentrate, yeast, etc., may be incorporated into the diet
through processing into substitutes for traditional food forms (ref. 6). The processes required for such new food
sources must be selected and developed, and their complications of acceptance weighed against the problems in
expanded agricultural complexity. As new food sources are incorporated into the diet, research is needed to consider
the long-term medical and nutritional tolerances.
The conventional technology of food processing has, of course, been developed under terrestrial conditions (ref. 7). Conditions of gravity, atmospheric composition, and Coriolis force can affect the heat-and mass-transfer processes required to process and store food. Research on specific problems is necessary to anticipate and minimize the need for redesign of processing equipment. Furthermore, the existing processing technology must be adapted to the specific space habitat conditions.
Industrial Support for Food-Processing Industry
Even if a minimal food-processing option is selected for space settlements, there will be demands to provide solvents, salts, emulsifiers, bleaching agents, antioxidants (for contingency storage requirements), and other chemicals to process raw agricultural products into usable foods (ref. 8). Without an in situ "chemical industry," many standard food-processing chemicals such as the organic solvents used to extract soybeans will not be available to a CLSS operation. Alternative methods for supplying or eliminating the need for these chemicals must be evaluated. The effects these food-processing alternatives have on the nutritional value of the resulting foods must also be evaluated (ref. 9).
Terrestrial experience in food service shows that processing, packaging, preparation, and waste disposal streams must all be matched for efficient operation. The extreme efficiency of some types of fast-food service (e.g., "hamburger-fries-shake" meals) suggests possible benefits in CLSS development. Cafeteria vs, home cooking vs vending systems should be considered. Equipment inventory and layout, especially if spatial conservation is important, must be tailored to specific scenarios. Diet modifications, methods of processing, and feeding patterns will have important effects on optimal storage, heating, dispensing, and equipment maintenance. A major problem of present-day food service industries is the lack of adequate equipment for intermediate-sized operations, that is, for 1000 to 10,000 people. Hence, methodology that will minimize spatial requirements for feeding a self-contained group of people must be developed (ref. 10).
The size and permanence of the cooking staff are also affected by the food-service component of a given scenario. If it
is determined that, for sound psychological reasons, the concept of choice and the principles of variation and change
should be built into a system from the beginning, one way to generate these options is to allow individuals to "cook"; that is, to choose from prepared or partially-prepared alternatives.
On Earth, there is renewed interest in water-saving and energy-saving methods of preparation because of various shortages. Research programs in these areas could be doubly rewarding if planning and design are initiated early. It is essential that food-preparation concepts that are deemed potentially appropriate for space use be tested in space environments since those conditions may affect heating, mixing, etc.
Micronutrients and Mineral Toxicity
In closed systems, there exists an added danger regarding control of various elements in metabolism. Since human requirements for trace minerals have not been adequately determined on Earth, and there is often a narrow tolerance between minimum requirement and toxicity, the artificial levels available in a space habitat might be critically difficult to adjust. A better grasp of requirements and tolerances, and an efficient monitoring system for metals such as copper, chromium, zinc, and cobalt, will be required.
Research and Development Options
Nutritional research and development should follow these steps:
HIGHER PLANT AGRICULTURE
Plant agriculture is included as an important option in planning CLSS because the photosynthetic process is a
conventional means of closing the life-support system. Photosynthesis recycles carbon dioxide, produced by human
metabolism, back into organic constituents
necessary for human nutrition and into oxygen necessary for respiration. However, in considering plant agriculture as an integral component of a CLSS, it must be recognized that higher plants are a complex and environmentally responsive system. With their thousands of interdependent physiological and morphological processes, they lack the engineered reliability of mechanical and physicochemical systems. Since higher plants are already being grown in controlled environments with both natural and artificial light, although not in a completely closed system, it can be assumed that the minimum technology for intensive plant culture already exists. However, as evidenced by the variability of reported growth and yields (as illustrated in table 1), there is little consistency in productivity. If the demands of a human population can be sustained by higher plant agriculture in a CLSS, then research and development emphasis must center on the technology and tolerable environmental limits for reliable production.
Plant growth occurs as a result of the processes of cell division and cell differentiation in the various meristematic centers and apices of the plant, and the capacity for production and translocation of photosynthetic products to support the expansion of these cells. The morphological and physiological processes are interactively influenced by such environmental parameters as temperature, quality and quantity of light, availability of water and mineral nutrients, atmospheric concentrations of carbon dioxide, oxygen, water vapor, gaseous pollutants, and the presence of certain toxic metal ions in the plant environment (ref. 11). If the technology of crop productivity is to be advanced to the state of reliability required for inclusion of higher plant agriculture in a CLSS, the morphological and physiological responses to all combinations of the environmental parameters must be precisely documented. This is an extensive task when one considers that the magnitude and nature of the responses change with plant age and species.
The only practical means of approaching this technological competence in assessing and predicting plant responses is to develop comprehensive mathematical models, which would be based on known physiological and morphological principles and would describe environmentally and genetically induced changes in plant growth and development. Modeling would be coordinated with selective experimentation so that the model could be adjusted to simulate the growth of various crop species
The development of a dynamic crop simulator is recommended for four reasons. First, it establishes a priority for experimental determination of plant responses to environment. The parameters of photosynthetically active radiance (PAR) and temperature form the basis of net carbon fixation; other environmental parameters are then incorporated as regulators of photosynthetic production and partitioning. Second, because predictive ability can be enhanced as the simulator is developed, experimental combinations of environmental variables can be truncated to the few levels necessary to establish and verify a response surface. Similar truncation of environmental combinations are possible for each new growth stage and each species adapted to the dynamic simulator. Third, on a longer-term basis, as the sensitivity of the crop simulator is increased by experimentation, it can become a feed-forward interactive system for controlling the plant environment. Thus, when coupled with the environmental and plant monitoring system, the crop simulator can project final yield from attained growth and suggest down-the-line environmental alternatives to optimize yield. The fundamental logic for such dynamic plant growth models is already being developed (refs. 12-14), including at least one model capable of multiple crop simulation (ref. 14). Finally, the crop simulator will possess the accuracy needed for final reiterations of whatever input-output model is used for sensitivity studies in developing the ultimate CLSS.
Crop simulators can only be as precise as the data base underlying their development. Systematic data collection at frequent intervals during plant growth is required to adapt a crop species to the model. These data must include: (1) accurate monitoring of the physical and chemical environment, (2) detailed plant chemical analysis of inorganic nutrients and organic plant constituents of physiological importance, and (3) thorough descriptions of plant morphological development as functions of selected changes in environmental parameters. Such data sets, although being developed for a few crop species (refs. 15, 16), are not available in the literature. However, the controlled-environment research facilities required to generate an adequate data base for the crop simulator are available at the phytotron units of the Southeastern Plant Environment Laboratories at Duke University and North Carolina State University (ref. 17), the Environmental Research Laboratory at the University of Arizona (ref. 18), and the Biotron at the University of Wisconsin. Utilization of these existing facilities for fundamental research into plant growth dynamics can greatly expedite the development of a prototype crop simulator. They can also be used to develop cultural and nutrient delivery systems (aeroponics, artificial substrates, etc.) for higher plant agriculture in the CLSS.
Development of Completely Closed Facilities
The existing controlled-environment facilities are open systems in which some portion of the conditioned atmosphere is continually replaced from external sources. Hence, they do not have the capacity for research to address three critical questions about the tolerance limits of adapting higher plants to a CLSS for space habitats. The first of these questions concerns the long-term tolerance of higher plants to low gravity. There is no available research on the long-term response of plants to true low-gravity conditions; hence priority must be given to include on the earliest possible long-duration orbiting mission carefully controlled experiments to determine physiological and morphological effects of low gravity. (All effects of low gravity are not necessarily deleterious since a reduced gravitational force on plants may reduce the proportion of nonedible structural biomass.) Meanwhile, attention in terrestrial facilities should be given to the study of proposed hormonal involvement in geotropic responsiveness of plants (ref. 19) to develop chemical or genetical means for counteracting the deleterious effects of low gravity. A second limitation of existing controlled-environment facilities is an inability for long-term regulation of composition of the atmosphere, including toxicants, and changes in atmospheric pressure. Tolerance limits for both environmental parameters must be determined. Special growth rooms must be designed and built for such experimentation. A third concern is the tolerance level of plants to ionizing radiation. Although plants may have a greater tolerance than humans or animals, the long-term effects and resulting requirements for shielding must be determined in experiments in space environments.
Lighting for plants is viewed as a more critical research problem if natural, rather than artificial light is used as the source. Comparisons of growth over extended periods (ref. 20) have shown little difference between natural light, which had a mean daily PAR of 42 Einsteins m-2 day-1, and artificial light supplied at fluxes above 17 Einsteins m-2 day-1 . Furthermore, plants have been grown for several generations with artificial light sources (ref. 21) with no measurable effects on growth. Hence, the use of artificial light for plant growth appears to pose no serious or unknown constraints on crop productivity.
On the other hand, utilization of natural radiation with spectral characteristics unattenuated by the Earth's atmosphere poses questions of possible adverse or mutagenic effects on plants beyond the limits of current predictability. In addition, use of natural light poses problems of photoperiodic control necessary for reproductive development of some plant species (ref. 22). However, problems associated with the use of solar radiation in space may possibly be circumvented by the use of spectral shift and selective transmission of glazings to provide filtered light that approximates Earth radiation, If glazings are used, control of day length for photoperiodically responsive crops can be provided by a system of shutters.
A final question that must be resolved before higher plant agriculture can be included as an integral part of the CLSS is whether the plant agricultural system can maintain a specified level of productivity over long periods of time and several generations of plant culture An Earth-based model CLSS should be established as a final feasibility study in which all elements of the CLSS are present, and in which reliability of productivity and the interactive crop-environment simulator can be evaluated for several years.
A model evaluation of a CLSS, including linked man and plant systems, was attempted in BIOS-3 by the USSR on a modest scale (ref.23). They reported deterioration of crop productivity over a 6-month test, but because of their failure to adequately evaluate tolerance limits of plants to changes in the environment, they were unable to diagnose the cause of the partial failure. However, the USSR experience underscores the necessity of a model evaluation of closure in a life-support system.
In parallel with a critical evaluation of a CLSS, other research should be undertaken to broaden the options in the ultimate system. This research should include novel concepts such as cultural or biochemical manipulation of plant growth to idealize the plant phenotype of CLSS agriculture. Crop plants possess a certain range of morphological plasticity that potentially can be used to breed plants with a more favorable ratio of edible/total biomass in the stable environments of a CLASS (ref. 21). Similarly, manipulation of environmental factors such as temperature, photoperiod, and partial pressure of oxygen during certain stages of plant development can shift the ratio of total/edible biomass (ref. 22). The environment not only affects the growth and development of the concurrent generation, but also can affect the viability of the seeds and thus that of several subsequent generations (refs. 24, 25)
Hence, the environmental requirements for plant propagation (e.g., seed production) must be considered separate from the environmental requirements for food crop production. Since traditional agriculture has been developed within the constraints of a virtually non-manipulable environment, the flexibilities of the plant system for optimization in controlled rolled-environment agriculture have neither been emphasized nor extensively exploited.
Research and Development Options
Considerable information is being gathered on the effects of environmental systems on higher plant growth and development. However, this research has been conducted within the context of terrestrial agricultural problems. Before the technological base and experimental systems can be applied to developmental problems in adapting higher plant agriculture to CLSS, three specific research requirements must be considered: (1) What are the tolerance limits of higher plants to reduced gravity and can the possible effects be countered by biochemical manipulation? (2) What are the tolerance limits of higher plants to lower atmospheric pressures and gaseous compositions different from those normally encountered in terrestrial systems? (3) What are the tolerance limits of higher plants to ionizing radiation and what amount of shielding is required for growth?
Research and development to resolve these questions are critical to a determination of the feasibility of including
higher plant agriculture as an integral part of the CLSS. The design of such experiments should be carefully controlled and coordinated so that they can be integrated into the terrestrial technology base.
Other pertinent options for research and development are:
There is some question concerning the need for a livestock component in a CLSS. This fundamental consideration relates to many aspects of CLSS design such as mass, area, and volume requirements, waste and atmospheric recycling, epidemiology, and nutritional adequacy of the diet provided for humans.
The question is difficult to resolve because of the complexity of the trade-offs involved. Several design studies have shown that the inclusion of animals in the food-production subsystem of a CLSS will increase area requirements substantially. Questions have been raised about the additional complexity of the total system, the need to provide additional plant-growing area for feed and forage production and special areas for animal production as well as the processing and storage requirements for animal products. The use of animals in a CLSS will also place additional burdens on the waste recycling and atmospheric regeneration systems. In particular grave concern has been expressed concerning the additional risk involved in using animals that may serve as vectors for human pathogenic organisms.
The primary justifications for including animals in a CLSS appear to be their nutritional value, dietary variety for humans, and the possibility that animal component will result in a more balanced, stable ecosystem with enhanced closure characteristics. Animal products do provide high-quality protein in a very acceptable form.
In addition, milk and milk products provide most of the essential minerals, such as calcium, in American diets. Since some animal production is possible by feeding them by-products and plant residues from plants grown for human consumption, limited meat and milk may be made available without additional plant agricultural requirements. The extent to which additional animal products should be included in the diet will be determined by the balance between the esthetic and nutritional value of these products and the cost in terms of plant-growth requirements to produce any additional feed required, spatial and equipment requirements for animal culture, etc.
The primary animal type most suited for converting inedible plant material into animal products will be ruminants that is, dairy cows for milk production and additional cattle for meat. Swine, chickens, and turkeys can be fed partially from table scraps and noncellulosic plant by-products, but most feed consumed by these animals could also be consumed directly by humans (i.e., competitive consumption).
Cellulosic materials such as plant roots, tops, brans, and hulls, can only be effectively used by ruminants. Microorganisms in the rumen of these animals allow them to digest these fibrous materials and convert them into fatty acids that are absorbed as a primary source of energy for their metabolism. Although the digestion processes are similar for both dairy and beef animals, the overall efficiency of converting feed energy into milk energy is several times greater than feed to beef because of the high rate of daily production possible (ref. 26).
The efficiency of both dairy and beef animals is influenced most by voluntary food intake and by the digestibility of the
diet. Because of the intense production rate that would be required by agriculture in a CLSS, these factors will
become increasingly important. Increasing from 45 to 75 percent the digestibility of plant residues that are high in
lignin and structural carbohydrates can reduce by 40 percent the amount of feed required per unit of milk produced. Of
even greater consequence is the concomitant reduction of 73 percent in undigested animal waste voided. Improved
digestibility would also permit substantially increased voluntary intake with a resulting gain in gross efficiency (e.g.,
increasing the intake of a growing steer from 5.0 to 7.2 kg/day would double the rate of edible meat produced).
The removal of constraints on voluntary feed intake and rate and extent of digestion of cellulosic materials has high priority in the development of an animal agriculture compatible with the requirements of the space settlement.
Aquatic animal resources- As suggested, the animal component in space agriculture may perform the functions of a biological recycling agent, converting a portion of useless plant wastes to useful protein and releasing the remaining plant material as egesta. The egesta from the terrestrial animal component of a CLSS must be recycled eventually to plant nutrients and carbon dioxide in order to regenerate the food-production cycle. The same process must be performed for unconsumed plant material and for human wastes. Conventionally, this recycling can be accomplished by the use of some form of physicochemical oxidation process. Another potential option exists, however, to further augment protein production in a CLSS by incorporating an aquatic production process as a supplementary organic recycling system to handle a portion of the wastes generated by humans, animals, and plants.
Aquatic recycling processes have been the subject of considerable research effort in recent years (refs. 27-30). This
research has demonstrated that such processes can purify effluents to acceptable standards while producing
valuable, high-protein biomass as a by-product. The possible role of aquatic processes for food production and waste
recycling in space applications merits additional investigation.
Small animal resources- The possible role of small animals for food production in space settlements should be examined. In many ways, they offer substantial production advantages over the large ruminants. Some small animals that should be considered include milk goats, rabbits, ducks, and geese.
Small animals have an advantage in production systems because they reproduce more often and reach maturity more quickly. They have a more efficient overall feed/protein conversion ratio because they are maintained only through the exponential growth phase; therefore less biomass is used in maintenance. For many small animals, feed requirements need only be marginally competitive with the human diet, consisting predominantly of table scraps, vegetable by-products, and small supplements of grain or forage. Lastly, small-animal production components may enhance the safety, stability, and reliability by increasing the species diversity of any animal production subsystem.
Diseases of domestic animals are an important consideration because they may dramatically disrupt the food production process. Also, in some instances, there exists a direct danger to the human population from disease organisms that may infect both human and animal hosts. Terrestrial experience in high-density, confined animal husbandry has documented the need to consider epidemiology and to safeguard animal populations by appropriate sanitation and disease-control measures. At times, the routine administration of antibiotics is recommended. Procedures and facilities should be designed to isolate disease outbreaks. All livestock products must be routinely inspected to ensure their safety for human consumption.
Research and Development Options
The currently recognized limitations to the use of total available plant material by animals must be removed. Voluntary intake of some residues may be limited by use of chemical components that reduce palatability. To remove this limitation:
WASTE CONVERSION AND RESOURCE RECOVERY
The primary objectives of waste processing in a recycling life-support system are to remove waste materials before they build up to toxic levels and to convert them into life-sustaining inputs. In a CLSS, the gaseous, solid, and liquid outputs of food-producing and processing operations must be dealt with in addition to human wastes. Waste can be processed using physicochemical techniques, by bioregeneration, or by some combination of these two approaches.
Current Physicochemical Alternatives
Among waste management concepts, the following are relevant to current research objectives.
Water reclamation technology deals primarily with the processing of lightly contaminated water (humidity or condensate, wash water) or highly contaminated water (urine, fecal water). Ongoing research and development activities are focusing on improved reliability of these processes.
Handling and either storing or treating solid wastes (food waste, wet waste, and fecal matter) probably will require extensive research and development effort. Four processing approaches have been investigated to date for manned spacecraft (ref. 32). One is a dehydration process using space vacuum to achieve a dried, stabilized mass. This method was adapted for use on Skylab and is being developed for the Space Shuttle. The other three approaches involve oxidation. Two of these combine concentrated waste water with solid wastes, the wet oxidation (ref. 33) and the RITE process (ref. 34) , respectively. Wet oxidation uses an aqueous slurry contacted with oxygen at elevated temperatures and pressures to produce a stabilized sterile ash and sterile water that requires further treatment. RITE combines solid waste and liquid residues and processes the evaporated water (vapor) by catalytic oxidation. The remaining solid wastes are then pyrolized, incinerated, and their gases removed (for subsequent processing), leaving a sterile solid ash. The other oxidative process involves dry incineration of solid waste which produces a sterile ash after pyrolysis and subsequent oxidation of its vapors (ref. 35) .
Any oxidative process offers potentially feasible approaches to solid-waste management. Each has been operated in the batch mode, but the problems of transporting wastes to the processing units for a continuous operation have not been solved. Significant data are lacking on the design and performance of catalytic oxidation units (ref. 32) . In addition, applications in CLSS will require specific processes for the selective removal and separation of mineral constituents in the solid-ash residue from the oxidation of the wastes. These minerals are vital nutrient components for humans, plants, and animals, and they must be made available in acceptable form as input constituents.
In addition to the physicochemical methods reviewed previously, there are various bioregenerative systems that use microorganisms, aquatic vascular plants, aquatic animals, and other organisms either singly or in combination (cf. section on Animal Agriculture). This area of technology has received very little research attention for spacecraft applications. Most of the technological developments to date are the products of research and applicational practice relating to municipal waste processing. There is an enormously large body of literature that must be screened eventually to identify any approaches that might offer options feasible for use in CLSS applications.
Research and Development Options
The design of waste processing components for CLSS applications must be integrated with the design of the entire life-support system because it must interface with other components and subsystems to satisfy a complex, interrelated set of input and output requirements (fig. 2). In general, however, the following options now seem to be key data-base expanders that can provide an important basis for the identification, design, and development of appropriate concepts.
SYSTEM STABILITY AND SAFETY
Research options associated with system stability and safety must emphasize those areas of contamination, safety; health, and reliability that are uniquely associated with the design and environmental features of CLSS scenarios; for example, confinement with minimum ecological buffering against hazards. Various possible areas of concern, from which research options might be derived, and a preliminary list of options that should be given further consideration during the early phases of CLSS scenario analysis, are discussed below.
The microbial component of the habitat can reside in several areas: humans, agriculture, water supply, food
subsystems, atmosphere, waste processors, and some structural materials of the habitat. The relationships among
these are very complex and interdependent. These components form inescapable elements of the ecosystem in the
form of pathogens, symbionts, and commensals with humans, plants, and domestic animals.
Compared to terrestrial plant, animal, and soil ecosystems, the habitat will provide a simplified, largely synthetic environment. New biological relationships will undoubtedly form. Also, in new or drastically altered environments, biological change is likely to be abrupt, dramatic, and unpredictable (ref. 36).
Human, animal, and plant pathogens can, in part, be controlled in the habitat by the conventional techniques of
quarantine, screening for carriers, and immunization. However, some diseases arc so common and the carriers so
difficult to identify that exclusion from the habitat may not be possible. Soviet investigators observed major microbial
changes in enclosed ecosystems intended to provide human life support (ref. 23). Species composition and
population numbers changed throughout the time these simulations were in operation and possibly degraded the
performance of some components in the systems. Habitat design may involve problems similar to those encountered
in hospital design, but with contamination problems intensified by the impossibility of bringing in "outside" air from a
huge external pool and using it as a sink for contaminants. Rigorous microbiological security of the water supply will also be necessary because of the ease with which organisms can reproduce
in the aquatic environment and the readiness with which they might be transmitted (refs. 23, 37, 38).
One of the most common methods of microbial control is the use of chemical biocides (e.g., chlorine in water supplies, bacteriocides, fungicides, and algacides in industrial plumbing and tankage, pesticides in agriculture, chemical disinfectants in hospitals and homes, and preservatives in foods). However, the hazards associated with the use of any toxic substance in the space habitat must be thoroughly understood before its application can be rationally contemplated.
The fragile, synthetic environment of a habitat will probably be highly sensitive to a large array of hazardous organic and inorganic compounds. Since habitats will not have large areas of sediments or volumes of atmosphere to dilute toxic compounds, special measures must be taken to avoid environmental contamination. These measures may include prohibiting the use of many chemicals. Even though prohibited, toxic inorganics could accumulate in the food chain from the processing of nonterrestrial materials that contain trace metals. The severity of this problem increases as the ecological system approaches closure.
The current Environmental Protection Agency (EPA) list of possibly dangerous compounds includes some 30,000 substances. To analyze the effects, transport, and fates of toxic materials in the Earth environment, considerable resources are being committed by EPA, the National Institute of Health, the Food and Drug Administration, the National Cancer Institute, and various industries. A comparable effort is required for the space habitat. Every product, either transported from Earth or synthesized in space, must be evaluated in terms of manufacture, use, and disposal or recycling.
Biological systems are difficult to monitor. Sensitive detectors for the rapid measurement of pH, temperature, and a
variety of compounds such as H2S, SO2, and CO2, and sensors for toxic organic compounds, heavy metals, and
microorganisms have been developed for on-line control. However, much labor-intensive analysis is involved using routine microbiological techniques and modern analytical chemistry techniques, such as combined gas chromatography/mass spectrometry, electron microprobe, or atomic absorption spectrometry. Because biological transformations occur very rapidly, computer compatible monitoring systems must be developed for real-time analysis of trace quantities of organic, inorganic, and biological components. These computer systems should include anticipatory problem-predictive capabilities so that control measures can be implemented while there is still time to avert calamity.
The computerized monitoring systems may be augmented with sensitive biological indicator species; the coal miner's canary is the classical example. It may be possible to develop genetic strains that are sensitive to specific toxic conditions. These biological sensors might provide more efficient early warning against specific or synergistic toxic effects than could either electrical or mechanical monitoring systems.
Considerable interest exists in developing indices of pathologies for ecosystems comparable to those used by physicians. For example, infrared scanners might be used to determine changes in respiration patterns in controlled agricultural systems or shifts in the ratio of chlorophyl/carotenoid pigments could be monitored to indicate changes in algal populations.
Some environmental characteristics might be clustered to the extent that only one component may be measured. The other components may then be predicted with reasonable confidence. By perturbing synthetic ecosystems with known toxins, these diagnostic methods can be evaluated.
In a large space habitat with a terrestrial atmosphere, many of the conventional techniques of fire protection can be used. However, a priority must be placed on fire prevention because even a limited fire could have a devastating effect. An uncontrolled or smoldering fire would pose a serious threat to the structural integrity of the habitat because of the extensive heat and pressures produced. The constraints of habitat design have two important consequences in fire situations: egress will be limited and few items on board will be expendable.
Atmospheric contamination will occur from toxic, corrosive, and flammable residuals of combustion and pyrolysis. Therefore, fire safety will depend on careful material selection. Attention must be given to fire resistance, toxicity, and smoke evolution properties of all proposed materials. Fire detection and rapid automatic control will be the key issues. Combustion products from sustained burning could overwhelm even the most conservatively designed life-support system. This problem normally does not occur on Earth because the atmosphere provides a means of rapid dispersal. In addition, many conventional flame-retardant materials and extinguishing agents are themselves toxic (e.g., halogenated organics) and would pose an added threat to the life-support system.
The wide spectrum of radiation in the space environment will pose a serious health hazard to man and to the biological components of the space ecosystem (as discussed in the companion paper prepared by the LSSB). The shielding requirements for all candidate biological subsystems should be investigated further. Design shortcomings must not compromise the safety and well-being of humans functioning in space.
Current EPA noise standards for community and industrial areas are 70 and 90 dB, respectively. In the enclosed structure of a space habitat, the propagation of sound and vibration throughout the hull will pose a unique problem. In addition, the echo effect could be severe. Skylab astronauts experienced both these difficulties and also complained that differential heating of the spacecraft was a major source of noise (ref. 39).
Clearly, a comprehensive design strategy must be developed to limit noise pollution aboard a space habitat. This capability is represented by state-of-the-art submarine technology: the use of acoustic material to combat echos, and specially designed equipment mounts and foundation structures to isolate sources of vibration (refs. 40, 41). Mechanical oscillation can be dissipated as heat if the foundation structure is properly damped and its resonant modes avoided. In a space habitat, an alternative means to combat the problem of noise would be to physically separate industrial and community living areas.
Gravitational and Rotational Problems
The only practical means for providing pseudogravity to a space habitat is to rotate the habitat structure. Greater
economy is reached at higher rotation rates (ref. 42); however, it has been shown that living in a rapidly rotating
system can cause severe physiological disturbances due to Coriolis force (refs. 43, 44).
Upon head rotation, out of the
plane of vehicle rotation, semicircular canal fluids will undergo cross-coupled
accelerations. This can result in motion sickness and disorientation (ref.43). Vestibular physiologists have determined
that most people can adapt to rotational rates up to 4 rpm (ref. 43). Although adverse symptoms usually disappear
after a few days, some individuals are less tolerant. To avoid rotation-rate sensitivity as a constraint in crew selection,
a maximum of 1 rpm has been adopted as an engineering guideline for large habitats (~106 inhabitants) (J. Billingham,
NASA Ames Research Center, personal communication, July 1977).
The syndrome of space motion sickness, on exposure to zero gravity, is well known from Apollo and Skylab experience. Susceptible astronauts have required 2 to 5 days to adjust to zero gravity, during which they suffered from effects similar to motion sickness. This illness has been thought to result from conflicts that arise between perceptions of the visual and vestibular systems. It is not yet known what the effect will be of moving between the rotating and zero-gravity habitat regions (ref. 43). Functioning in the two environments will require two modes of vestibular operation. Conceivably, workers involved in extravehicular or zero-gravity activities may have to reside in zero gravity for additional periods to minimize vestibular symptoms.
Research and Development Options
Early emphasis should be placed on the investigations fisted below.
CRITERIA FOR ESTABLISHING RESEARCH PRIORITIES
A principal conclusion of this study is that it is premature to prescribe priorities for resource allocation among the research
options presented here. However, a procedure was developed to assign priorities in a carefully planned manner. The major
elements of this recommended procedure are summarized below.
Table of Contents