BIOREGENERATIVE LIFE-SUPPORT SYSTEMSPlacing a man in spacerequires a complete life-support system capable of supplying sufficient oxygen, food, and water and removing excess carbon dioxide, water vapor, and human body wastes. In addition, the oxygen, carbon dioxide, and pressure must be maintained at a suitable level. Any accumulated toxic products and noxious odors must be removed.In the spacecraft the human is confined in a restricted environment in which it is necessary to establish a balanced microcosm or closed ecological system. This is an enormous biological and bioengineering problem. Weight, size, simplicity of operation, and reliability particularly are important factors.For relatively short missions involving one or several astronauts, food, oxygen, and water can be stored and made available as required, and the various waste products can be stored. On longer missions, particularly those involving more than one astronaut, efficient chemical or biological regenerative systems will be required. Any regenerative system introduces a fixed cost in weight of processing equipment and energy requirements.Chemical, or partially regenerative, methods for providing breathing oxygen by the regeneration of metabolic products such as water vapor and carbon dioxide include the thermal decomposition of water and CO2, photolysis and radiolysis of water, electrolysis of fused carbonates and aqueous solutions, and the chemical reduction of CO2with H2, followed by electrolysis of the water formed. Chemical regenerative systems have been developed to remove excess carbon dioxide and water vapor from the atmosphere. Nonbiological regenerative systems are time limited by the amount of food, water, and oxygen that can be carried or recovered. These physical-chemical processes show greatpotential, but they also present many difficulties, including requirements for extremely high temperatures and considerable amounts of power, the formation of highly toxic materials, and high susceptibility to inactivation. None of the presently studied nonbiological processes can function as completely as a bioregenerative system. All these nonbiological systems have unrealistic supply requirements and produce unusable wastes. Consequently, for long planetary missions the bioregenerative systems, though also beset with problems, are potentially far superior to their physical and chemical counterparts.Table VIIIshows average daily metabolic data for a 70-kg astronaut. A man breathes about 10 cubic feet of air per minute, or 400 000 liters, daily. The expired air contains about 4 percent carbon dioxide. Man normally breathes air containing 0.03 percent CO2, but can withstand comfortably about 1.5 percent CO2. Anything in excess of 1.5 percent will produce labored breathing, headaches, and, if greatly exceeded, death. A man exhales about 1.1 pounds of water per day and this, in addition to water from perspiration and other sources, must be removed from the air.Table VIII.—Average Daily Metabolic Data for a 70-kg, 25-Year-Old Astronaut With Normal Spacecrew Activity[From[ref.173]]O2input, kg0.862CO2output, kg1.056Drinking water, liters2.5Food rehydrating water, liters1Caloric value of food, kcal3000Water output:Urine, liters1.6Respiration and perspiration, liters2.13Feces, kg0.09Total heal output, Btu11 100Two types of biological regenerative systems have been proposed. The photosynthetic closed ecological system was proposed as early as 1951. This involves the use of single-celled algae or higher plants, including floating aquatic and terrestrial plants, and requires the interaction of light energy with CO2and H2O to produce O2and plant cells. Another system, proposed in 1961, involves electrolysis of water into oxygen and hydrogen, and the concurrent use ofHydrogenomonasbacteria which take up hydrogen, some oxygen, carbon dioxide, and urine yielding water and bacterial cells.Table IX.—Requirements for Regenerative Life-Support SystemsSystemRequirements/1 man4Requirements/3 men (270 man-day mission)5Weight, kgPower, kWWeight, kgPower, kWPartial chemoregenerative73321.75LiOH1251.40NaOH1557.68CO2-H234.36Full bioregenerative—algae:Artificial illumination116610.4059125.00Solar illumination1031.70356.60Electrolysis-hydrogenomonas55.251292.60The values given intable IXindicate relative weights and powers required by various systems to provide the gaseous environment for manned space cabins. If one considers operating temperatures and hazards, other systems may offer advantages which offset the weight and power advantages of the hydrogen reduction of LiOH systems.Research is being conducted by NASA on life-support-system technology applicable to missions planned for 20 years in the future. Life-support systems include the requirements for supplying breathing gases, control of contaminants in the cabin atmosphere, water reclamation, food supply, and personal hygiene. The disciplines involved in such systems include biology and microbiology, cryogenic fluid handling at zero g, heat transfer, and thermal integration with other systems, such as power. The physiological, psychological, and sociological problems of the crew are also being considered.Photosynthetic SystemGreen plants contain chlorophyll which captures light energy thermodynamically required to convert carbon dioxide and water into carbohydrate which can subsequently be transformed into other foods such as protein and fat. During this process, carbon dioxide is consumed, and an approximately equal amount of oxygen gas is liberated. As a first approximation, photosynthesis is the reverse of the oxidative metabolism of animal life:OxidationC6H12O6+ 6O2———————> 6CO2+ 6H2O + heatPhotosynthesis6CO2+ 6H2O + light ———————> C6H12O6+ 6O2The photosynthetic process in plants and respiration during photosynthesis have been studied intensively, and several metabolic pathways have been elucidated. Mechanisms are being studied to explain the inhibitory effect of strong visible light on this process. This program may lead to the use of chloroplasts or chlorophyll without cells in future photosynthetic bioregenerative systems for long-term space travel.One of the prime considerations of a closed ecological system is that the environmental gases shall remain physiologically tolerable to all of the ecologic components. Ideally, a photosynthetic gas exchange organism should possess a high ratio of gas exchange to total mass (considering all equipment and material incidental to growth, harvesting, processing, and utilization); and a controllable assimilation rate to maintain steady-state gas composition. It should also be (1) amenable to confining quarters which may be imposed by inflexibility of rocket or space station design; (2) genetically and physiologically stable and highly resistant to anticipated stresses; (3) edible and capable of supplying most or all human nutritional requirements; (4) capable of utilizing raw or appropriately treated organic wastes; and (5) amenable to water recycling as demanded by other components of the ecosystem.Higher PlantsEfforts to utilize multicellular plants as photosynthetic gas exchangers have been somewhat neglected, since it has been assumed by many that algae would be more efficient. The familyLemnaceae(duckweeds) are small primitive aquatic plants with a minimum of tissue differentiation. Practically all of the cells of the plant contain chlorophyll and are capable of photosynthetic activity. They reproduce principally by asexual budding of parent leaflike fronds. They can be grown readily on moist surfaces ([ref.177]) on almost any medium suitable for the growth of autotrophic plants. With duckweeds the problems of gaseous exchange and harvesting are simplified and the volume of medium can be greatly decreased as compared with algae.Ney ([ref.177]) obtained a very high gas exchange rate with duckweeds. Using small cultures under controlled optimal conditions of temperature, light(600-1000 ft-c), and CO2, concentration, he estimated that 2.3 m2of frondal surface of duckweed, at a gas exchange rate of 10.8 liters m2/hrwould provide sufficient gas exchange for one man. This would produce about 25 grams of dry plant material per hour.A few nutritional studies have been carried out with duckweeds. Nakamura ([ref.178]) consideredWolffiaas a possible source of food for space travel and found that it contained carbohydrate 25-60 percent, protein 8-10 percent, fat 18-20 percent, minerals 6-8 percent (all dry weights), and vitamins B2, B6, and C, with C the most abundant.One of the desirable features of a duckweed system is that the gas exchange is direct between the atmosphere and the plant and does not require dissolving the respiratory gases in a bulky fluid system which introduces special engineering difficulties in zero- or low-gravity conditions.In the design of equipment for photosynthetic studies, careful consideration should be given to the material used in the construction of the unit. Most plastic materials are subject to photo-oxidative degradation, with CO as one of the products. When air is recirculated through plastic tubing and transparent rigid plastics in the presence of light, considerable quantities of CO are given off. With high-intensity illumination such as sunlight, a CO buildup of several hundred parts per million is not uncommon. Also, plant pigments such as the carotenoids and chlorophylls will react similarly when exposed to light of high intensity. If the plants die, then CO is released quite rapidly.At Colorado State University the responses of plants to high-intensity radiation (ultraviolet to infrared) are being studied. Plants from high mountaintops that are exposed to greater ultraviolet light are being studied for specialized adaptations. The effect of temperature on photosynthesis is being explored. Various plants are also being studied under germ-free conditions.Screening of higher plants for possible use in bioregenerative systems at Connecticut Agriculture Experiment Station resulted in the selection of corn, sugarcane, and sunflower. Under optimal conditions it has been shown that 100 to 130 ft2of leaf surface are required to support an astronaut.Plants considered as possible food sources include soybeans, peanuts, rice, and tomatoes, which can be combined with algae to give a well-balanced and reasonably varied diet. Hydroponic systems use large quantities of water, but progress is being made in reducing this.The possibility of using animals in the closed ecological system is open to question, particularly in the absence of gravity, and much work remains to be done on using plant materials as animal food and on the disposal of wastes. Animals which have been considered are crustaceans, fish, chickens, rabbits, and goats.AlgaeAlgae have the fastest growth rate and are among the most efficient plants for oxygen and food production. It has been amply demonstrated by Myers ([ref.179]) and other workers thatChlorellacan be used in a closed ecological system to maintain animals such as mice and a monkey. The use of algae for supplying O2and food, and for removing CO2and odors has been considered by many authors for use in spacecraft, space platforms, and for establishing bases on the Moon or Mars.Estimates of total efficiency are based on extrapolated laboratory data and vary widely, since many different types of data have been used as a basis for these estimates.The respired air containing about 4-5 percent CO2is bubbled into theChlorellaculture, at either atmospheric or increased pressure. Air containing a high percentage of oxygen and saturated with moisture is released from the algal system.The use of algae for several purposes might require from one to three separate algal systems. For food production,Chlorellaproduces 50 percent protein and 50 percent lipids in high-nitrogen media. In low-nitrogen media, it produces 85 percent lipids. Proper choice ofChlorellastrains and media will produce not only the necessary calories but also the necessary specific nutrients required. Certain strains are more effective in O2production, and others in the use of urine and other wastes.Some of the early estimates, usingChlorellagrown at 25° C, for supplying these requirements for a single man in space include the following: 168 kg of algal suspension ([ref.179]), 200 kg of algal suspension and 50 kg of equipment including pumps (refs.[ref.180]and[ref.181]), and 100 kg of algal suspension and 50 cubic feet for equipment and gas exchange ([ref.182]). Using the blue-green algaSynechocystis, 600 kg of algal suspension would be required, according to Gafford and Craft. These estimates are based on preliminary studies, are quite high, and are not of real practical value.Other studies have indicated an extremely efficient algal system which offers a real potential for a practical and effective gas exchanger ([ref.183]). A thermophilic strain ofChlorellawith an optimum growth temperature of 39° C and an optimum temperature for photosynthesis of about 40° C can increase its cell mass 10 000-fold per day. When operating at one-half maximum efficiency, this alga produces 100 times its cell volume of oxygen per hour. Burk et al. ([ref.183]) state: "Future engineering development should lead to a space requirement, per adult person, of no more than 3 to 5 cubic feet of algal culture, equipment, and instrumentation for adequate purification of air." The requirements of this system would require additional energy in the form of light and of small amounts of nitrogenous and mineral material for the algae. The lightsource used by Burk et al. ([ref.183]) is a tungsten filament quartz lamp the size of a pencil, which has a long life, produces a luminous flux 5-10 times greater than sunlight on Earth, and operates at a 10-12 percent light efficiency.Research is being carried out on algal regenerative systems by about 40 or 50 laboratories in the United States. NASA is supporting several basic studies on photosynthesis, the physiology of algae, and engineering pilot-plant development. Much of the research on algae is being supported by the Air Force.Most algal studies have been carried out in small units and the data obtained have been used as a basis for extrapolating logistic values for the use of these organisms in manned space vehicles. Myers ([ref.179]) has shown that the quantity of algae necessary to support a man (with an assumed O2requirement of 625 liters per day) would yield about 600-700 grams dry weight of new cells per day. If algal growth in mass cultures could be maintained in a steady-state concentration of 2.5 gram dry weight per liter with such a growth rate as to yield 10 grams weight per liter per day, the volume of algal culture would be 60-70 liters and the total mass of the system would approximate 200-250 pounds.Using an 8-liter system, Ward et al. ([ref.176]) have produced algal concentrations of 5-7 grams of dry algae per liter with a high-temperature algal strain. The maximum growth rate observed with the culture was 0.375 gram dry weight per liter per hour, or 9 grams dry weight per liter per day. This was accomplished by using 1-centimeter layers of culture and a light intensity of 8000 foot-candles. The culture system consisted of a rectangular plastic chamber having an area of 0.5 square meter and illuminated on each side to an intensity of 4000 foot-candles (cool-white). To produce 25 liters of oxygen per hour, an area of 8.3 square meters (85 square feet) would be required.The major problem in large-scale production of algae is that of illumination. Conversion of electricity to light has an efficiency of only 10 to 20 percent. In addition, the maximum efficiency of light utilization byChlorellaalgae lies in the range of 18-22 percent. This results in a maximum efficiency of only 4 percent for photosynthetic systems. Another problem involved in conversion of electricity to light is the production of heat which has to be removed even with thermophilic algae. With a human demand of 600 liters of oxygen per day, the minimum electrical requirement becomes 4 kW. No large-scale culture has yet been managed at anything close to this minimum figure.Another problem is the poor penetration of light into concentrated cultures of algae. This necessitates construction of large tanks of only about ¼-inch thickness. This results frequently in fouling of the surfacesof the tank by algae and makes the removal of the excess algae difficult. Production of 1 liter of oxygen results in the production of 1 gram dry weight of algae. Although a small amount of CO is produced by some algae, it can probably be removed by catalytic oxidation. Other problems include mutation and genetic drift of the algae and the necessity for maintaining bacteria-free cultures. There are also difficulties in maintaining a sterile culture if urine is to be used as a nitrogen source. While there is a potential for using algae as food, more research is required before it can be determined what quantity and methods of processing can be used. Research and development on algae is much greater than on both the higher plants and the electrolysis-Hydrogenomonassystems together.The difference between the photosynthetic and electrolysis-chemosynthetic systems is the way electrical energy is made available to the organisms. In the photosynthetic system, electrical energy is converted to light which the algae or plants transform into chemical energy. In the chemosynthetic process, electrical energy is transformed into the chemical energy of hydrogen gas which is used by the bacteria. Both organisms use the chemical energy available to them to synthesize cell material with similar degrees of efficiency. The problem is to make the conversion of electricity to available chemical energy as efficient as possible.In photosynthetic systems much energy is lost in the conversion of electricity to light, a process only 10-20 percent efficient at best. When this is combined with the loss from the inefficient use of light by plants, an overall efficiency of about 4 percent is obtained. In the electrolysis-Hydrogenomonassystem, the two steps are very efficient. Electrolysis cells can operate at up to 85 percent efficiency and the overall efficiency can be up to seven times that of a photosynthetic system.
BIOREGENERATIVE LIFE-SUPPORT SYSTEMSPlacing a man in spacerequires a complete life-support system capable of supplying sufficient oxygen, food, and water and removing excess carbon dioxide, water vapor, and human body wastes. In addition, the oxygen, carbon dioxide, and pressure must be maintained at a suitable level. Any accumulated toxic products and noxious odors must be removed.In the spacecraft the human is confined in a restricted environment in which it is necessary to establish a balanced microcosm or closed ecological system. This is an enormous biological and bioengineering problem. Weight, size, simplicity of operation, and reliability particularly are important factors.For relatively short missions involving one or several astronauts, food, oxygen, and water can be stored and made available as required, and the various waste products can be stored. On longer missions, particularly those involving more than one astronaut, efficient chemical or biological regenerative systems will be required. Any regenerative system introduces a fixed cost in weight of processing equipment and energy requirements.Chemical, or partially regenerative, methods for providing breathing oxygen by the regeneration of metabolic products such as water vapor and carbon dioxide include the thermal decomposition of water and CO2, photolysis and radiolysis of water, electrolysis of fused carbonates and aqueous solutions, and the chemical reduction of CO2with H2, followed by electrolysis of the water formed. Chemical regenerative systems have been developed to remove excess carbon dioxide and water vapor from the atmosphere. Nonbiological regenerative systems are time limited by the amount of food, water, and oxygen that can be carried or recovered. These physical-chemical processes show greatpotential, but they also present many difficulties, including requirements for extremely high temperatures and considerable amounts of power, the formation of highly toxic materials, and high susceptibility to inactivation. None of the presently studied nonbiological processes can function as completely as a bioregenerative system. All these nonbiological systems have unrealistic supply requirements and produce unusable wastes. Consequently, for long planetary missions the bioregenerative systems, though also beset with problems, are potentially far superior to their physical and chemical counterparts.Table VIIIshows average daily metabolic data for a 70-kg astronaut. A man breathes about 10 cubic feet of air per minute, or 400 000 liters, daily. The expired air contains about 4 percent carbon dioxide. Man normally breathes air containing 0.03 percent CO2, but can withstand comfortably about 1.5 percent CO2. Anything in excess of 1.5 percent will produce labored breathing, headaches, and, if greatly exceeded, death. A man exhales about 1.1 pounds of water per day and this, in addition to water from perspiration and other sources, must be removed from the air.Table VIII.—Average Daily Metabolic Data for a 70-kg, 25-Year-Old Astronaut With Normal Spacecrew Activity[From[ref.173]]O2input, kg0.862CO2output, kg1.056Drinking water, liters2.5Food rehydrating water, liters1Caloric value of food, kcal3000Water output:Urine, liters1.6Respiration and perspiration, liters2.13Feces, kg0.09Total heal output, Btu11 100Two types of biological regenerative systems have been proposed. The photosynthetic closed ecological system was proposed as early as 1951. This involves the use of single-celled algae or higher plants, including floating aquatic and terrestrial plants, and requires the interaction of light energy with CO2and H2O to produce O2and plant cells. Another system, proposed in 1961, involves electrolysis of water into oxygen and hydrogen, and the concurrent use ofHydrogenomonasbacteria which take up hydrogen, some oxygen, carbon dioxide, and urine yielding water and bacterial cells.Table IX.—Requirements for Regenerative Life-Support SystemsSystemRequirements/1 man4Requirements/3 men (270 man-day mission)5Weight, kgPower, kWWeight, kgPower, kWPartial chemoregenerative73321.75LiOH1251.40NaOH1557.68CO2-H234.36Full bioregenerative—algae:Artificial illumination116610.4059125.00Solar illumination1031.70356.60Electrolysis-hydrogenomonas55.251292.60The values given intable IXindicate relative weights and powers required by various systems to provide the gaseous environment for manned space cabins. If one considers operating temperatures and hazards, other systems may offer advantages which offset the weight and power advantages of the hydrogen reduction of LiOH systems.Research is being conducted by NASA on life-support-system technology applicable to missions planned for 20 years in the future. Life-support systems include the requirements for supplying breathing gases, control of contaminants in the cabin atmosphere, water reclamation, food supply, and personal hygiene. The disciplines involved in such systems include biology and microbiology, cryogenic fluid handling at zero g, heat transfer, and thermal integration with other systems, such as power. The physiological, psychological, and sociological problems of the crew are also being considered.Photosynthetic SystemGreen plants contain chlorophyll which captures light energy thermodynamically required to convert carbon dioxide and water into carbohydrate which can subsequently be transformed into other foods such as protein and fat. During this process, carbon dioxide is consumed, and an approximately equal amount of oxygen gas is liberated. As a first approximation, photosynthesis is the reverse of the oxidative metabolism of animal life:OxidationC6H12O6+ 6O2———————> 6CO2+ 6H2O + heatPhotosynthesis6CO2+ 6H2O + light ———————> C6H12O6+ 6O2The photosynthetic process in plants and respiration during photosynthesis have been studied intensively, and several metabolic pathways have been elucidated. Mechanisms are being studied to explain the inhibitory effect of strong visible light on this process. This program may lead to the use of chloroplasts or chlorophyll without cells in future photosynthetic bioregenerative systems for long-term space travel.One of the prime considerations of a closed ecological system is that the environmental gases shall remain physiologically tolerable to all of the ecologic components. Ideally, a photosynthetic gas exchange organism should possess a high ratio of gas exchange to total mass (considering all equipment and material incidental to growth, harvesting, processing, and utilization); and a controllable assimilation rate to maintain steady-state gas composition. It should also be (1) amenable to confining quarters which may be imposed by inflexibility of rocket or space station design; (2) genetically and physiologically stable and highly resistant to anticipated stresses; (3) edible and capable of supplying most or all human nutritional requirements; (4) capable of utilizing raw or appropriately treated organic wastes; and (5) amenable to water recycling as demanded by other components of the ecosystem.Higher PlantsEfforts to utilize multicellular plants as photosynthetic gas exchangers have been somewhat neglected, since it has been assumed by many that algae would be more efficient. The familyLemnaceae(duckweeds) are small primitive aquatic plants with a minimum of tissue differentiation. Practically all of the cells of the plant contain chlorophyll and are capable of photosynthetic activity. They reproduce principally by asexual budding of parent leaflike fronds. They can be grown readily on moist surfaces ([ref.177]) on almost any medium suitable for the growth of autotrophic plants. With duckweeds the problems of gaseous exchange and harvesting are simplified and the volume of medium can be greatly decreased as compared with algae.Ney ([ref.177]) obtained a very high gas exchange rate with duckweeds. Using small cultures under controlled optimal conditions of temperature, light(600-1000 ft-c), and CO2, concentration, he estimated that 2.3 m2of frondal surface of duckweed, at a gas exchange rate of 10.8 liters m2/hrwould provide sufficient gas exchange for one man. This would produce about 25 grams of dry plant material per hour.A few nutritional studies have been carried out with duckweeds. Nakamura ([ref.178]) consideredWolffiaas a possible source of food for space travel and found that it contained carbohydrate 25-60 percent, protein 8-10 percent, fat 18-20 percent, minerals 6-8 percent (all dry weights), and vitamins B2, B6, and C, with C the most abundant.One of the desirable features of a duckweed system is that the gas exchange is direct between the atmosphere and the plant and does not require dissolving the respiratory gases in a bulky fluid system which introduces special engineering difficulties in zero- or low-gravity conditions.In the design of equipment for photosynthetic studies, careful consideration should be given to the material used in the construction of the unit. Most plastic materials are subject to photo-oxidative degradation, with CO as one of the products. When air is recirculated through plastic tubing and transparent rigid plastics in the presence of light, considerable quantities of CO are given off. With high-intensity illumination such as sunlight, a CO buildup of several hundred parts per million is not uncommon. Also, plant pigments such as the carotenoids and chlorophylls will react similarly when exposed to light of high intensity. If the plants die, then CO is released quite rapidly.At Colorado State University the responses of plants to high-intensity radiation (ultraviolet to infrared) are being studied. Plants from high mountaintops that are exposed to greater ultraviolet light are being studied for specialized adaptations. The effect of temperature on photosynthesis is being explored. Various plants are also being studied under germ-free conditions.Screening of higher plants for possible use in bioregenerative systems at Connecticut Agriculture Experiment Station resulted in the selection of corn, sugarcane, and sunflower. Under optimal conditions it has been shown that 100 to 130 ft2of leaf surface are required to support an astronaut.Plants considered as possible food sources include soybeans, peanuts, rice, and tomatoes, which can be combined with algae to give a well-balanced and reasonably varied diet. Hydroponic systems use large quantities of water, but progress is being made in reducing this.The possibility of using animals in the closed ecological system is open to question, particularly in the absence of gravity, and much work remains to be done on using plant materials as animal food and on the disposal of wastes. Animals which have been considered are crustaceans, fish, chickens, rabbits, and goats.AlgaeAlgae have the fastest growth rate and are among the most efficient plants for oxygen and food production. It has been amply demonstrated by Myers ([ref.179]) and other workers thatChlorellacan be used in a closed ecological system to maintain animals such as mice and a monkey. The use of algae for supplying O2and food, and for removing CO2and odors has been considered by many authors for use in spacecraft, space platforms, and for establishing bases on the Moon or Mars.Estimates of total efficiency are based on extrapolated laboratory data and vary widely, since many different types of data have been used as a basis for these estimates.The respired air containing about 4-5 percent CO2is bubbled into theChlorellaculture, at either atmospheric or increased pressure. Air containing a high percentage of oxygen and saturated with moisture is released from the algal system.The use of algae for several purposes might require from one to three separate algal systems. For food production,Chlorellaproduces 50 percent protein and 50 percent lipids in high-nitrogen media. In low-nitrogen media, it produces 85 percent lipids. Proper choice ofChlorellastrains and media will produce not only the necessary calories but also the necessary specific nutrients required. Certain strains are more effective in O2production, and others in the use of urine and other wastes.Some of the early estimates, usingChlorellagrown at 25° C, for supplying these requirements for a single man in space include the following: 168 kg of algal suspension ([ref.179]), 200 kg of algal suspension and 50 kg of equipment including pumps (refs.[ref.180]and[ref.181]), and 100 kg of algal suspension and 50 cubic feet for equipment and gas exchange ([ref.182]). Using the blue-green algaSynechocystis, 600 kg of algal suspension would be required, according to Gafford and Craft. These estimates are based on preliminary studies, are quite high, and are not of real practical value.Other studies have indicated an extremely efficient algal system which offers a real potential for a practical and effective gas exchanger ([ref.183]). A thermophilic strain ofChlorellawith an optimum growth temperature of 39° C and an optimum temperature for photosynthesis of about 40° C can increase its cell mass 10 000-fold per day. When operating at one-half maximum efficiency, this alga produces 100 times its cell volume of oxygen per hour. Burk et al. ([ref.183]) state: "Future engineering development should lead to a space requirement, per adult person, of no more than 3 to 5 cubic feet of algal culture, equipment, and instrumentation for adequate purification of air." The requirements of this system would require additional energy in the form of light and of small amounts of nitrogenous and mineral material for the algae. The lightsource used by Burk et al. ([ref.183]) is a tungsten filament quartz lamp the size of a pencil, which has a long life, produces a luminous flux 5-10 times greater than sunlight on Earth, and operates at a 10-12 percent light efficiency.Research is being carried out on algal regenerative systems by about 40 or 50 laboratories in the United States. NASA is supporting several basic studies on photosynthesis, the physiology of algae, and engineering pilot-plant development. Much of the research on algae is being supported by the Air Force.Most algal studies have been carried out in small units and the data obtained have been used as a basis for extrapolating logistic values for the use of these organisms in manned space vehicles. Myers ([ref.179]) has shown that the quantity of algae necessary to support a man (with an assumed O2requirement of 625 liters per day) would yield about 600-700 grams dry weight of new cells per day. If algal growth in mass cultures could be maintained in a steady-state concentration of 2.5 gram dry weight per liter with such a growth rate as to yield 10 grams weight per liter per day, the volume of algal culture would be 60-70 liters and the total mass of the system would approximate 200-250 pounds.Using an 8-liter system, Ward et al. ([ref.176]) have produced algal concentrations of 5-7 grams of dry algae per liter with a high-temperature algal strain. The maximum growth rate observed with the culture was 0.375 gram dry weight per liter per hour, or 9 grams dry weight per liter per day. This was accomplished by using 1-centimeter layers of culture and a light intensity of 8000 foot-candles. The culture system consisted of a rectangular plastic chamber having an area of 0.5 square meter and illuminated on each side to an intensity of 4000 foot-candles (cool-white). To produce 25 liters of oxygen per hour, an area of 8.3 square meters (85 square feet) would be required.The major problem in large-scale production of algae is that of illumination. Conversion of electricity to light has an efficiency of only 10 to 20 percent. In addition, the maximum efficiency of light utilization byChlorellaalgae lies in the range of 18-22 percent. This results in a maximum efficiency of only 4 percent for photosynthetic systems. Another problem involved in conversion of electricity to light is the production of heat which has to be removed even with thermophilic algae. With a human demand of 600 liters of oxygen per day, the minimum electrical requirement becomes 4 kW. No large-scale culture has yet been managed at anything close to this minimum figure.Another problem is the poor penetration of light into concentrated cultures of algae. This necessitates construction of large tanks of only about ¼-inch thickness. This results frequently in fouling of the surfacesof the tank by algae and makes the removal of the excess algae difficult. Production of 1 liter of oxygen results in the production of 1 gram dry weight of algae. Although a small amount of CO is produced by some algae, it can probably be removed by catalytic oxidation. Other problems include mutation and genetic drift of the algae and the necessity for maintaining bacteria-free cultures. There are also difficulties in maintaining a sterile culture if urine is to be used as a nitrogen source. While there is a potential for using algae as food, more research is required before it can be determined what quantity and methods of processing can be used. Research and development on algae is much greater than on both the higher plants and the electrolysis-Hydrogenomonassystems together.The difference between the photosynthetic and electrolysis-chemosynthetic systems is the way electrical energy is made available to the organisms. In the photosynthetic system, electrical energy is converted to light which the algae or plants transform into chemical energy. In the chemosynthetic process, electrical energy is transformed into the chemical energy of hydrogen gas which is used by the bacteria. Both organisms use the chemical energy available to them to synthesize cell material with similar degrees of efficiency. The problem is to make the conversion of electricity to available chemical energy as efficient as possible.In photosynthetic systems much energy is lost in the conversion of electricity to light, a process only 10-20 percent efficient at best. When this is combined with the loss from the inefficient use of light by plants, an overall efficiency of about 4 percent is obtained. In the electrolysis-Hydrogenomonassystem, the two steps are very efficient. Electrolysis cells can operate at up to 85 percent efficiency and the overall efficiency can be up to seven times that of a photosynthetic system.
BIOREGENERATIVE LIFE-SUPPORT SYSTEMSPlacing a man in spacerequires a complete life-support system capable of supplying sufficient oxygen, food, and water and removing excess carbon dioxide, water vapor, and human body wastes. In addition, the oxygen, carbon dioxide, and pressure must be maintained at a suitable level. Any accumulated toxic products and noxious odors must be removed.In the spacecraft the human is confined in a restricted environment in which it is necessary to establish a balanced microcosm or closed ecological system. This is an enormous biological and bioengineering problem. Weight, size, simplicity of operation, and reliability particularly are important factors.For relatively short missions involving one or several astronauts, food, oxygen, and water can be stored and made available as required, and the various waste products can be stored. On longer missions, particularly those involving more than one astronaut, efficient chemical or biological regenerative systems will be required. Any regenerative system introduces a fixed cost in weight of processing equipment and energy requirements.Chemical, or partially regenerative, methods for providing breathing oxygen by the regeneration of metabolic products such as water vapor and carbon dioxide include the thermal decomposition of water and CO2, photolysis and radiolysis of water, electrolysis of fused carbonates and aqueous solutions, and the chemical reduction of CO2with H2, followed by electrolysis of the water formed. Chemical regenerative systems have been developed to remove excess carbon dioxide and water vapor from the atmosphere. Nonbiological regenerative systems are time limited by the amount of food, water, and oxygen that can be carried or recovered. These physical-chemical processes show greatpotential, but they also present many difficulties, including requirements for extremely high temperatures and considerable amounts of power, the formation of highly toxic materials, and high susceptibility to inactivation. None of the presently studied nonbiological processes can function as completely as a bioregenerative system. All these nonbiological systems have unrealistic supply requirements and produce unusable wastes. Consequently, for long planetary missions the bioregenerative systems, though also beset with problems, are potentially far superior to their physical and chemical counterparts.Table VIIIshows average daily metabolic data for a 70-kg astronaut. A man breathes about 10 cubic feet of air per minute, or 400 000 liters, daily. The expired air contains about 4 percent carbon dioxide. Man normally breathes air containing 0.03 percent CO2, but can withstand comfortably about 1.5 percent CO2. Anything in excess of 1.5 percent will produce labored breathing, headaches, and, if greatly exceeded, death. A man exhales about 1.1 pounds of water per day and this, in addition to water from perspiration and other sources, must be removed from the air.Table VIII.—Average Daily Metabolic Data for a 70-kg, 25-Year-Old Astronaut With Normal Spacecrew Activity[From[ref.173]]O2input, kg0.862CO2output, kg1.056Drinking water, liters2.5Food rehydrating water, liters1Caloric value of food, kcal3000Water output:Urine, liters1.6Respiration and perspiration, liters2.13Feces, kg0.09Total heal output, Btu11 100Two types of biological regenerative systems have been proposed. The photosynthetic closed ecological system was proposed as early as 1951. This involves the use of single-celled algae or higher plants, including floating aquatic and terrestrial plants, and requires the interaction of light energy with CO2and H2O to produce O2and plant cells. Another system, proposed in 1961, involves electrolysis of water into oxygen and hydrogen, and the concurrent use ofHydrogenomonasbacteria which take up hydrogen, some oxygen, carbon dioxide, and urine yielding water and bacterial cells.Table IX.—Requirements for Regenerative Life-Support SystemsSystemRequirements/1 man4Requirements/3 men (270 man-day mission)5Weight, kgPower, kWWeight, kgPower, kWPartial chemoregenerative73321.75LiOH1251.40NaOH1557.68CO2-H234.36Full bioregenerative—algae:Artificial illumination116610.4059125.00Solar illumination1031.70356.60Electrolysis-hydrogenomonas55.251292.60The values given intable IXindicate relative weights and powers required by various systems to provide the gaseous environment for manned space cabins. If one considers operating temperatures and hazards, other systems may offer advantages which offset the weight and power advantages of the hydrogen reduction of LiOH systems.Research is being conducted by NASA on life-support-system technology applicable to missions planned for 20 years in the future. Life-support systems include the requirements for supplying breathing gases, control of contaminants in the cabin atmosphere, water reclamation, food supply, and personal hygiene. The disciplines involved in such systems include biology and microbiology, cryogenic fluid handling at zero g, heat transfer, and thermal integration with other systems, such as power. The physiological, psychological, and sociological problems of the crew are also being considered.Photosynthetic SystemGreen plants contain chlorophyll which captures light energy thermodynamically required to convert carbon dioxide and water into carbohydrate which can subsequently be transformed into other foods such as protein and fat. During this process, carbon dioxide is consumed, and an approximately equal amount of oxygen gas is liberated. As a first approximation, photosynthesis is the reverse of the oxidative metabolism of animal life:OxidationC6H12O6+ 6O2———————> 6CO2+ 6H2O + heatPhotosynthesis6CO2+ 6H2O + light ———————> C6H12O6+ 6O2The photosynthetic process in plants and respiration during photosynthesis have been studied intensively, and several metabolic pathways have been elucidated. Mechanisms are being studied to explain the inhibitory effect of strong visible light on this process. This program may lead to the use of chloroplasts or chlorophyll without cells in future photosynthetic bioregenerative systems for long-term space travel.One of the prime considerations of a closed ecological system is that the environmental gases shall remain physiologically tolerable to all of the ecologic components. Ideally, a photosynthetic gas exchange organism should possess a high ratio of gas exchange to total mass (considering all equipment and material incidental to growth, harvesting, processing, and utilization); and a controllable assimilation rate to maintain steady-state gas composition. It should also be (1) amenable to confining quarters which may be imposed by inflexibility of rocket or space station design; (2) genetically and physiologically stable and highly resistant to anticipated stresses; (3) edible and capable of supplying most or all human nutritional requirements; (4) capable of utilizing raw or appropriately treated organic wastes; and (5) amenable to water recycling as demanded by other components of the ecosystem.Higher PlantsEfforts to utilize multicellular plants as photosynthetic gas exchangers have been somewhat neglected, since it has been assumed by many that algae would be more efficient. The familyLemnaceae(duckweeds) are small primitive aquatic plants with a minimum of tissue differentiation. Practically all of the cells of the plant contain chlorophyll and are capable of photosynthetic activity. They reproduce principally by asexual budding of parent leaflike fronds. They can be grown readily on moist surfaces ([ref.177]) on almost any medium suitable for the growth of autotrophic plants. With duckweeds the problems of gaseous exchange and harvesting are simplified and the volume of medium can be greatly decreased as compared with algae.Ney ([ref.177]) obtained a very high gas exchange rate with duckweeds. Using small cultures under controlled optimal conditions of temperature, light(600-1000 ft-c), and CO2, concentration, he estimated that 2.3 m2of frondal surface of duckweed, at a gas exchange rate of 10.8 liters m2/hrwould provide sufficient gas exchange for one man. This would produce about 25 grams of dry plant material per hour.A few nutritional studies have been carried out with duckweeds. Nakamura ([ref.178]) consideredWolffiaas a possible source of food for space travel and found that it contained carbohydrate 25-60 percent, protein 8-10 percent, fat 18-20 percent, minerals 6-8 percent (all dry weights), and vitamins B2, B6, and C, with C the most abundant.One of the desirable features of a duckweed system is that the gas exchange is direct between the atmosphere and the plant and does not require dissolving the respiratory gases in a bulky fluid system which introduces special engineering difficulties in zero- or low-gravity conditions.In the design of equipment for photosynthetic studies, careful consideration should be given to the material used in the construction of the unit. Most plastic materials are subject to photo-oxidative degradation, with CO as one of the products. When air is recirculated through plastic tubing and transparent rigid plastics in the presence of light, considerable quantities of CO are given off. With high-intensity illumination such as sunlight, a CO buildup of several hundred parts per million is not uncommon. Also, plant pigments such as the carotenoids and chlorophylls will react similarly when exposed to light of high intensity. If the plants die, then CO is released quite rapidly.At Colorado State University the responses of plants to high-intensity radiation (ultraviolet to infrared) are being studied. Plants from high mountaintops that are exposed to greater ultraviolet light are being studied for specialized adaptations. The effect of temperature on photosynthesis is being explored. Various plants are also being studied under germ-free conditions.Screening of higher plants for possible use in bioregenerative systems at Connecticut Agriculture Experiment Station resulted in the selection of corn, sugarcane, and sunflower. Under optimal conditions it has been shown that 100 to 130 ft2of leaf surface are required to support an astronaut.Plants considered as possible food sources include soybeans, peanuts, rice, and tomatoes, which can be combined with algae to give a well-balanced and reasonably varied diet. Hydroponic systems use large quantities of water, but progress is being made in reducing this.The possibility of using animals in the closed ecological system is open to question, particularly in the absence of gravity, and much work remains to be done on using plant materials as animal food and on the disposal of wastes. Animals which have been considered are crustaceans, fish, chickens, rabbits, and goats.AlgaeAlgae have the fastest growth rate and are among the most efficient plants for oxygen and food production. It has been amply demonstrated by Myers ([ref.179]) and other workers thatChlorellacan be used in a closed ecological system to maintain animals such as mice and a monkey. The use of algae for supplying O2and food, and for removing CO2and odors has been considered by many authors for use in spacecraft, space platforms, and for establishing bases on the Moon or Mars.Estimates of total efficiency are based on extrapolated laboratory data and vary widely, since many different types of data have been used as a basis for these estimates.The respired air containing about 4-5 percent CO2is bubbled into theChlorellaculture, at either atmospheric or increased pressure. Air containing a high percentage of oxygen and saturated with moisture is released from the algal system.The use of algae for several purposes might require from one to three separate algal systems. For food production,Chlorellaproduces 50 percent protein and 50 percent lipids in high-nitrogen media. In low-nitrogen media, it produces 85 percent lipids. Proper choice ofChlorellastrains and media will produce not only the necessary calories but also the necessary specific nutrients required. Certain strains are more effective in O2production, and others in the use of urine and other wastes.Some of the early estimates, usingChlorellagrown at 25° C, for supplying these requirements for a single man in space include the following: 168 kg of algal suspension ([ref.179]), 200 kg of algal suspension and 50 kg of equipment including pumps (refs.[ref.180]and[ref.181]), and 100 kg of algal suspension and 50 cubic feet for equipment and gas exchange ([ref.182]). Using the blue-green algaSynechocystis, 600 kg of algal suspension would be required, according to Gafford and Craft. These estimates are based on preliminary studies, are quite high, and are not of real practical value.Other studies have indicated an extremely efficient algal system which offers a real potential for a practical and effective gas exchanger ([ref.183]). A thermophilic strain ofChlorellawith an optimum growth temperature of 39° C and an optimum temperature for photosynthesis of about 40° C can increase its cell mass 10 000-fold per day. When operating at one-half maximum efficiency, this alga produces 100 times its cell volume of oxygen per hour. Burk et al. ([ref.183]) state: "Future engineering development should lead to a space requirement, per adult person, of no more than 3 to 5 cubic feet of algal culture, equipment, and instrumentation for adequate purification of air." The requirements of this system would require additional energy in the form of light and of small amounts of nitrogenous and mineral material for the algae. The lightsource used by Burk et al. ([ref.183]) is a tungsten filament quartz lamp the size of a pencil, which has a long life, produces a luminous flux 5-10 times greater than sunlight on Earth, and operates at a 10-12 percent light efficiency.Research is being carried out on algal regenerative systems by about 40 or 50 laboratories in the United States. NASA is supporting several basic studies on photosynthesis, the physiology of algae, and engineering pilot-plant development. Much of the research on algae is being supported by the Air Force.Most algal studies have been carried out in small units and the data obtained have been used as a basis for extrapolating logistic values for the use of these organisms in manned space vehicles. Myers ([ref.179]) has shown that the quantity of algae necessary to support a man (with an assumed O2requirement of 625 liters per day) would yield about 600-700 grams dry weight of new cells per day. If algal growth in mass cultures could be maintained in a steady-state concentration of 2.5 gram dry weight per liter with such a growth rate as to yield 10 grams weight per liter per day, the volume of algal culture would be 60-70 liters and the total mass of the system would approximate 200-250 pounds.Using an 8-liter system, Ward et al. ([ref.176]) have produced algal concentrations of 5-7 grams of dry algae per liter with a high-temperature algal strain. The maximum growth rate observed with the culture was 0.375 gram dry weight per liter per hour, or 9 grams dry weight per liter per day. This was accomplished by using 1-centimeter layers of culture and a light intensity of 8000 foot-candles. The culture system consisted of a rectangular plastic chamber having an area of 0.5 square meter and illuminated on each side to an intensity of 4000 foot-candles (cool-white). To produce 25 liters of oxygen per hour, an area of 8.3 square meters (85 square feet) would be required.The major problem in large-scale production of algae is that of illumination. Conversion of electricity to light has an efficiency of only 10 to 20 percent. In addition, the maximum efficiency of light utilization byChlorellaalgae lies in the range of 18-22 percent. This results in a maximum efficiency of only 4 percent for photosynthetic systems. Another problem involved in conversion of electricity to light is the production of heat which has to be removed even with thermophilic algae. With a human demand of 600 liters of oxygen per day, the minimum electrical requirement becomes 4 kW. No large-scale culture has yet been managed at anything close to this minimum figure.Another problem is the poor penetration of light into concentrated cultures of algae. This necessitates construction of large tanks of only about ¼-inch thickness. This results frequently in fouling of the surfacesof the tank by algae and makes the removal of the excess algae difficult. Production of 1 liter of oxygen results in the production of 1 gram dry weight of algae. Although a small amount of CO is produced by some algae, it can probably be removed by catalytic oxidation. Other problems include mutation and genetic drift of the algae and the necessity for maintaining bacteria-free cultures. There are also difficulties in maintaining a sterile culture if urine is to be used as a nitrogen source. While there is a potential for using algae as food, more research is required before it can be determined what quantity and methods of processing can be used. Research and development on algae is much greater than on both the higher plants and the electrolysis-Hydrogenomonassystems together.The difference between the photosynthetic and electrolysis-chemosynthetic systems is the way electrical energy is made available to the organisms. In the photosynthetic system, electrical energy is converted to light which the algae or plants transform into chemical energy. In the chemosynthetic process, electrical energy is transformed into the chemical energy of hydrogen gas which is used by the bacteria. Both organisms use the chemical energy available to them to synthesize cell material with similar degrees of efficiency. The problem is to make the conversion of electricity to available chemical energy as efficient as possible.In photosynthetic systems much energy is lost in the conversion of electricity to light, a process only 10-20 percent efficient at best. When this is combined with the loss from the inefficient use of light by plants, an overall efficiency of about 4 percent is obtained. In the electrolysis-Hydrogenomonassystem, the two steps are very efficient. Electrolysis cells can operate at up to 85 percent efficiency and the overall efficiency can be up to seven times that of a photosynthetic system.
Placing a man in spacerequires a complete life-support system capable of supplying sufficient oxygen, food, and water and removing excess carbon dioxide, water vapor, and human body wastes. In addition, the oxygen, carbon dioxide, and pressure must be maintained at a suitable level. Any accumulated toxic products and noxious odors must be removed.
In the spacecraft the human is confined in a restricted environment in which it is necessary to establish a balanced microcosm or closed ecological system. This is an enormous biological and bioengineering problem. Weight, size, simplicity of operation, and reliability particularly are important factors.
For relatively short missions involving one or several astronauts, food, oxygen, and water can be stored and made available as required, and the various waste products can be stored. On longer missions, particularly those involving more than one astronaut, efficient chemical or biological regenerative systems will be required. Any regenerative system introduces a fixed cost in weight of processing equipment and energy requirements.
Chemical, or partially regenerative, methods for providing breathing oxygen by the regeneration of metabolic products such as water vapor and carbon dioxide include the thermal decomposition of water and CO2, photolysis and radiolysis of water, electrolysis of fused carbonates and aqueous solutions, and the chemical reduction of CO2with H2, followed by electrolysis of the water formed. Chemical regenerative systems have been developed to remove excess carbon dioxide and water vapor from the atmosphere. Nonbiological regenerative systems are time limited by the amount of food, water, and oxygen that can be carried or recovered. These physical-chemical processes show greatpotential, but they also present many difficulties, including requirements for extremely high temperatures and considerable amounts of power, the formation of highly toxic materials, and high susceptibility to inactivation. None of the presently studied nonbiological processes can function as completely as a bioregenerative system. All these nonbiological systems have unrealistic supply requirements and produce unusable wastes. Consequently, for long planetary missions the bioregenerative systems, though also beset with problems, are potentially far superior to their physical and chemical counterparts.
Table VIIIshows average daily metabolic data for a 70-kg astronaut. A man breathes about 10 cubic feet of air per minute, or 400 000 liters, daily. The expired air contains about 4 percent carbon dioxide. Man normally breathes air containing 0.03 percent CO2, but can withstand comfortably about 1.5 percent CO2. Anything in excess of 1.5 percent will produce labored breathing, headaches, and, if greatly exceeded, death. A man exhales about 1.1 pounds of water per day and this, in addition to water from perspiration and other sources, must be removed from the air.
O2input, kg
0.862
CO2output, kg
1.056
Drinking water, liters
2.5
Food rehydrating water, liters
1
Caloric value of food, kcal
3000
Water output:
Urine, liters
1.6
Respiration and perspiration, liters
2.13
Feces, kg
0.09
Total heal output, Btu
11 100
Two types of biological regenerative systems have been proposed. The photosynthetic closed ecological system was proposed as early as 1951. This involves the use of single-celled algae or higher plants, including floating aquatic and terrestrial plants, and requires the interaction of light energy with CO2and H2O to produce O2and plant cells. Another system, proposed in 1961, involves electrolysis of water into oxygen and hydrogen, and the concurrent use ofHydrogenomonasbacteria which take up hydrogen, some oxygen, carbon dioxide, and urine yielding water and bacterial cells.
System
Requirements/1 man4
Requirements/3 men (270 man-day mission)5
Weight, kg
Power, kW
Weight, kg
Power, kW
Partial chemoregenerative
7332
1.75
LiOH
125
1.40
NaOH
155
7.68
CO2-H2
34
.36
Full bioregenerative—algae:
Artificial illumination
116
610.40
591
25.00
Solar illumination
103
1.70
356
.60
Electrolysis-hydrogenomonas
55
.25
129
2.60
The values given intable IXindicate relative weights and powers required by various systems to provide the gaseous environment for manned space cabins. If one considers operating temperatures and hazards, other systems may offer advantages which offset the weight and power advantages of the hydrogen reduction of LiOH systems.
Research is being conducted by NASA on life-support-system technology applicable to missions planned for 20 years in the future. Life-support systems include the requirements for supplying breathing gases, control of contaminants in the cabin atmosphere, water reclamation, food supply, and personal hygiene. The disciplines involved in such systems include biology and microbiology, cryogenic fluid handling at zero g, heat transfer, and thermal integration with other systems, such as power. The physiological, psychological, and sociological problems of the crew are also being considered.
Photosynthetic SystemGreen plants contain chlorophyll which captures light energy thermodynamically required to convert carbon dioxide and water into carbohydrate which can subsequently be transformed into other foods such as protein and fat. During this process, carbon dioxide is consumed, and an approximately equal amount of oxygen gas is liberated. As a first approximation, photosynthesis is the reverse of the oxidative metabolism of animal life:OxidationC6H12O6+ 6O2———————> 6CO2+ 6H2O + heatPhotosynthesis6CO2+ 6H2O + light ———————> C6H12O6+ 6O2The photosynthetic process in plants and respiration during photosynthesis have been studied intensively, and several metabolic pathways have been elucidated. Mechanisms are being studied to explain the inhibitory effect of strong visible light on this process. This program may lead to the use of chloroplasts or chlorophyll without cells in future photosynthetic bioregenerative systems for long-term space travel.One of the prime considerations of a closed ecological system is that the environmental gases shall remain physiologically tolerable to all of the ecologic components. Ideally, a photosynthetic gas exchange organism should possess a high ratio of gas exchange to total mass (considering all equipment and material incidental to growth, harvesting, processing, and utilization); and a controllable assimilation rate to maintain steady-state gas composition. It should also be (1) amenable to confining quarters which may be imposed by inflexibility of rocket or space station design; (2) genetically and physiologically stable and highly resistant to anticipated stresses; (3) edible and capable of supplying most or all human nutritional requirements; (4) capable of utilizing raw or appropriately treated organic wastes; and (5) amenable to water recycling as demanded by other components of the ecosystem.
Green plants contain chlorophyll which captures light energy thermodynamically required to convert carbon dioxide and water into carbohydrate which can subsequently be transformed into other foods such as protein and fat. During this process, carbon dioxide is consumed, and an approximately equal amount of oxygen gas is liberated. As a first approximation, photosynthesis is the reverse of the oxidative metabolism of animal life:
OxidationC6H12O6+ 6O2———————> 6CO2+ 6H2O + heatPhotosynthesis6CO2+ 6H2O + light ———————> C6H12O6+ 6O2
OxidationC6H12O6+ 6O2———————> 6CO2+ 6H2O + heatPhotosynthesis6CO2+ 6H2O + light ———————> C6H12O6+ 6O2
OxidationC6H12O6+ 6O2———————> 6CO2+ 6H2O + heatPhotosynthesis6CO2+ 6H2O + light ———————> C6H12O6+ 6O2
Oxidation
Oxidation
Oxidation
C6H12O6+ 6O2———————> 6CO2+ 6H2O + heat
Photosynthesis
Photosynthesis
Photosynthesis
6CO2+ 6H2O + light ———————> C6H12O6+ 6O2
The photosynthetic process in plants and respiration during photosynthesis have been studied intensively, and several metabolic pathways have been elucidated. Mechanisms are being studied to explain the inhibitory effect of strong visible light on this process. This program may lead to the use of chloroplasts or chlorophyll without cells in future photosynthetic bioregenerative systems for long-term space travel.
One of the prime considerations of a closed ecological system is that the environmental gases shall remain physiologically tolerable to all of the ecologic components. Ideally, a photosynthetic gas exchange organism should possess a high ratio of gas exchange to total mass (considering all equipment and material incidental to growth, harvesting, processing, and utilization); and a controllable assimilation rate to maintain steady-state gas composition. It should also be (1) amenable to confining quarters which may be imposed by inflexibility of rocket or space station design; (2) genetically and physiologically stable and highly resistant to anticipated stresses; (3) edible and capable of supplying most or all human nutritional requirements; (4) capable of utilizing raw or appropriately treated organic wastes; and (5) amenable to water recycling as demanded by other components of the ecosystem.
Higher PlantsEfforts to utilize multicellular plants as photosynthetic gas exchangers have been somewhat neglected, since it has been assumed by many that algae would be more efficient. The familyLemnaceae(duckweeds) are small primitive aquatic plants with a minimum of tissue differentiation. Practically all of the cells of the plant contain chlorophyll and are capable of photosynthetic activity. They reproduce principally by asexual budding of parent leaflike fronds. They can be grown readily on moist surfaces ([ref.177]) on almost any medium suitable for the growth of autotrophic plants. With duckweeds the problems of gaseous exchange and harvesting are simplified and the volume of medium can be greatly decreased as compared with algae.Ney ([ref.177]) obtained a very high gas exchange rate with duckweeds. Using small cultures under controlled optimal conditions of temperature, light(600-1000 ft-c), and CO2, concentration, he estimated that 2.3 m2of frondal surface of duckweed, at a gas exchange rate of 10.8 liters m2/hrwould provide sufficient gas exchange for one man. This would produce about 25 grams of dry plant material per hour.A few nutritional studies have been carried out with duckweeds. Nakamura ([ref.178]) consideredWolffiaas a possible source of food for space travel and found that it contained carbohydrate 25-60 percent, protein 8-10 percent, fat 18-20 percent, minerals 6-8 percent (all dry weights), and vitamins B2, B6, and C, with C the most abundant.One of the desirable features of a duckweed system is that the gas exchange is direct between the atmosphere and the plant and does not require dissolving the respiratory gases in a bulky fluid system which introduces special engineering difficulties in zero- or low-gravity conditions.In the design of equipment for photosynthetic studies, careful consideration should be given to the material used in the construction of the unit. Most plastic materials are subject to photo-oxidative degradation, with CO as one of the products. When air is recirculated through plastic tubing and transparent rigid plastics in the presence of light, considerable quantities of CO are given off. With high-intensity illumination such as sunlight, a CO buildup of several hundred parts per million is not uncommon. Also, plant pigments such as the carotenoids and chlorophylls will react similarly when exposed to light of high intensity. If the plants die, then CO is released quite rapidly.At Colorado State University the responses of plants to high-intensity radiation (ultraviolet to infrared) are being studied. Plants from high mountaintops that are exposed to greater ultraviolet light are being studied for specialized adaptations. The effect of temperature on photosynthesis is being explored. Various plants are also being studied under germ-free conditions.Screening of higher plants for possible use in bioregenerative systems at Connecticut Agriculture Experiment Station resulted in the selection of corn, sugarcane, and sunflower. Under optimal conditions it has been shown that 100 to 130 ft2of leaf surface are required to support an astronaut.Plants considered as possible food sources include soybeans, peanuts, rice, and tomatoes, which can be combined with algae to give a well-balanced and reasonably varied diet. Hydroponic systems use large quantities of water, but progress is being made in reducing this.The possibility of using animals in the closed ecological system is open to question, particularly in the absence of gravity, and much work remains to be done on using plant materials as animal food and on the disposal of wastes. Animals which have been considered are crustaceans, fish, chickens, rabbits, and goats.
Efforts to utilize multicellular plants as photosynthetic gas exchangers have been somewhat neglected, since it has been assumed by many that algae would be more efficient. The familyLemnaceae(duckweeds) are small primitive aquatic plants with a minimum of tissue differentiation. Practically all of the cells of the plant contain chlorophyll and are capable of photosynthetic activity. They reproduce principally by asexual budding of parent leaflike fronds. They can be grown readily on moist surfaces ([ref.177]) on almost any medium suitable for the growth of autotrophic plants. With duckweeds the problems of gaseous exchange and harvesting are simplified and the volume of medium can be greatly decreased as compared with algae.
Ney ([ref.177]) obtained a very high gas exchange rate with duckweeds. Using small cultures under controlled optimal conditions of temperature, light(600-1000 ft-c), and CO2, concentration, he estimated that 2.3 m2of frondal surface of duckweed, at a gas exchange rate of 10.8 liters m2/hrwould provide sufficient gas exchange for one man. This would produce about 25 grams of dry plant material per hour.
A few nutritional studies have been carried out with duckweeds. Nakamura ([ref.178]) consideredWolffiaas a possible source of food for space travel and found that it contained carbohydrate 25-60 percent, protein 8-10 percent, fat 18-20 percent, minerals 6-8 percent (all dry weights), and vitamins B2, B6, and C, with C the most abundant.
One of the desirable features of a duckweed system is that the gas exchange is direct between the atmosphere and the plant and does not require dissolving the respiratory gases in a bulky fluid system which introduces special engineering difficulties in zero- or low-gravity conditions.
In the design of equipment for photosynthetic studies, careful consideration should be given to the material used in the construction of the unit. Most plastic materials are subject to photo-oxidative degradation, with CO as one of the products. When air is recirculated through plastic tubing and transparent rigid plastics in the presence of light, considerable quantities of CO are given off. With high-intensity illumination such as sunlight, a CO buildup of several hundred parts per million is not uncommon. Also, plant pigments such as the carotenoids and chlorophylls will react similarly when exposed to light of high intensity. If the plants die, then CO is released quite rapidly.
At Colorado State University the responses of plants to high-intensity radiation (ultraviolet to infrared) are being studied. Plants from high mountaintops that are exposed to greater ultraviolet light are being studied for specialized adaptations. The effect of temperature on photosynthesis is being explored. Various plants are also being studied under germ-free conditions.
Screening of higher plants for possible use in bioregenerative systems at Connecticut Agriculture Experiment Station resulted in the selection of corn, sugarcane, and sunflower. Under optimal conditions it has been shown that 100 to 130 ft2of leaf surface are required to support an astronaut.
Plants considered as possible food sources include soybeans, peanuts, rice, and tomatoes, which can be combined with algae to give a well-balanced and reasonably varied diet. Hydroponic systems use large quantities of water, but progress is being made in reducing this.
The possibility of using animals in the closed ecological system is open to question, particularly in the absence of gravity, and much work remains to be done on using plant materials as animal food and on the disposal of wastes. Animals which have been considered are crustaceans, fish, chickens, rabbits, and goats.
AlgaeAlgae have the fastest growth rate and are among the most efficient plants for oxygen and food production. It has been amply demonstrated by Myers ([ref.179]) and other workers thatChlorellacan be used in a closed ecological system to maintain animals such as mice and a monkey. The use of algae for supplying O2and food, and for removing CO2and odors has been considered by many authors for use in spacecraft, space platforms, and for establishing bases on the Moon or Mars.Estimates of total efficiency are based on extrapolated laboratory data and vary widely, since many different types of data have been used as a basis for these estimates.The respired air containing about 4-5 percent CO2is bubbled into theChlorellaculture, at either atmospheric or increased pressure. Air containing a high percentage of oxygen and saturated with moisture is released from the algal system.The use of algae for several purposes might require from one to three separate algal systems. For food production,Chlorellaproduces 50 percent protein and 50 percent lipids in high-nitrogen media. In low-nitrogen media, it produces 85 percent lipids. Proper choice ofChlorellastrains and media will produce not only the necessary calories but also the necessary specific nutrients required. Certain strains are more effective in O2production, and others in the use of urine and other wastes.Some of the early estimates, usingChlorellagrown at 25° C, for supplying these requirements for a single man in space include the following: 168 kg of algal suspension ([ref.179]), 200 kg of algal suspension and 50 kg of equipment including pumps (refs.[ref.180]and[ref.181]), and 100 kg of algal suspension and 50 cubic feet for equipment and gas exchange ([ref.182]). Using the blue-green algaSynechocystis, 600 kg of algal suspension would be required, according to Gafford and Craft. These estimates are based on preliminary studies, are quite high, and are not of real practical value.Other studies have indicated an extremely efficient algal system which offers a real potential for a practical and effective gas exchanger ([ref.183]). A thermophilic strain ofChlorellawith an optimum growth temperature of 39° C and an optimum temperature for photosynthesis of about 40° C can increase its cell mass 10 000-fold per day. When operating at one-half maximum efficiency, this alga produces 100 times its cell volume of oxygen per hour. Burk et al. ([ref.183]) state: "Future engineering development should lead to a space requirement, per adult person, of no more than 3 to 5 cubic feet of algal culture, equipment, and instrumentation for adequate purification of air." The requirements of this system would require additional energy in the form of light and of small amounts of nitrogenous and mineral material for the algae. The lightsource used by Burk et al. ([ref.183]) is a tungsten filament quartz lamp the size of a pencil, which has a long life, produces a luminous flux 5-10 times greater than sunlight on Earth, and operates at a 10-12 percent light efficiency.Research is being carried out on algal regenerative systems by about 40 or 50 laboratories in the United States. NASA is supporting several basic studies on photosynthesis, the physiology of algae, and engineering pilot-plant development. Much of the research on algae is being supported by the Air Force.Most algal studies have been carried out in small units and the data obtained have been used as a basis for extrapolating logistic values for the use of these organisms in manned space vehicles. Myers ([ref.179]) has shown that the quantity of algae necessary to support a man (with an assumed O2requirement of 625 liters per day) would yield about 600-700 grams dry weight of new cells per day. If algal growth in mass cultures could be maintained in a steady-state concentration of 2.5 gram dry weight per liter with such a growth rate as to yield 10 grams weight per liter per day, the volume of algal culture would be 60-70 liters and the total mass of the system would approximate 200-250 pounds.Using an 8-liter system, Ward et al. ([ref.176]) have produced algal concentrations of 5-7 grams of dry algae per liter with a high-temperature algal strain. The maximum growth rate observed with the culture was 0.375 gram dry weight per liter per hour, or 9 grams dry weight per liter per day. This was accomplished by using 1-centimeter layers of culture and a light intensity of 8000 foot-candles. The culture system consisted of a rectangular plastic chamber having an area of 0.5 square meter and illuminated on each side to an intensity of 4000 foot-candles (cool-white). To produce 25 liters of oxygen per hour, an area of 8.3 square meters (85 square feet) would be required.The major problem in large-scale production of algae is that of illumination. Conversion of electricity to light has an efficiency of only 10 to 20 percent. In addition, the maximum efficiency of light utilization byChlorellaalgae lies in the range of 18-22 percent. This results in a maximum efficiency of only 4 percent for photosynthetic systems. Another problem involved in conversion of electricity to light is the production of heat which has to be removed even with thermophilic algae. With a human demand of 600 liters of oxygen per day, the minimum electrical requirement becomes 4 kW. No large-scale culture has yet been managed at anything close to this minimum figure.Another problem is the poor penetration of light into concentrated cultures of algae. This necessitates construction of large tanks of only about ¼-inch thickness. This results frequently in fouling of the surfacesof the tank by algae and makes the removal of the excess algae difficult. Production of 1 liter of oxygen results in the production of 1 gram dry weight of algae. Although a small amount of CO is produced by some algae, it can probably be removed by catalytic oxidation. Other problems include mutation and genetic drift of the algae and the necessity for maintaining bacteria-free cultures. There are also difficulties in maintaining a sterile culture if urine is to be used as a nitrogen source. While there is a potential for using algae as food, more research is required before it can be determined what quantity and methods of processing can be used. Research and development on algae is much greater than on both the higher plants and the electrolysis-Hydrogenomonassystems together.The difference between the photosynthetic and electrolysis-chemosynthetic systems is the way electrical energy is made available to the organisms. In the photosynthetic system, electrical energy is converted to light which the algae or plants transform into chemical energy. In the chemosynthetic process, electrical energy is transformed into the chemical energy of hydrogen gas which is used by the bacteria. Both organisms use the chemical energy available to them to synthesize cell material with similar degrees of efficiency. The problem is to make the conversion of electricity to available chemical energy as efficient as possible.In photosynthetic systems much energy is lost in the conversion of electricity to light, a process only 10-20 percent efficient at best. When this is combined with the loss from the inefficient use of light by plants, an overall efficiency of about 4 percent is obtained. In the electrolysis-Hydrogenomonassystem, the two steps are very efficient. Electrolysis cells can operate at up to 85 percent efficiency and the overall efficiency can be up to seven times that of a photosynthetic system.
Algae have the fastest growth rate and are among the most efficient plants for oxygen and food production. It has been amply demonstrated by Myers ([ref.179]) and other workers thatChlorellacan be used in a closed ecological system to maintain animals such as mice and a monkey. The use of algae for supplying O2and food, and for removing CO2and odors has been considered by many authors for use in spacecraft, space platforms, and for establishing bases on the Moon or Mars.
Estimates of total efficiency are based on extrapolated laboratory data and vary widely, since many different types of data have been used as a basis for these estimates.
The respired air containing about 4-5 percent CO2is bubbled into theChlorellaculture, at either atmospheric or increased pressure. Air containing a high percentage of oxygen and saturated with moisture is released from the algal system.
The use of algae for several purposes might require from one to three separate algal systems. For food production,Chlorellaproduces 50 percent protein and 50 percent lipids in high-nitrogen media. In low-nitrogen media, it produces 85 percent lipids. Proper choice ofChlorellastrains and media will produce not only the necessary calories but also the necessary specific nutrients required. Certain strains are more effective in O2production, and others in the use of urine and other wastes.
Some of the early estimates, usingChlorellagrown at 25° C, for supplying these requirements for a single man in space include the following: 168 kg of algal suspension ([ref.179]), 200 kg of algal suspension and 50 kg of equipment including pumps (refs.[ref.180]and[ref.181]), and 100 kg of algal suspension and 50 cubic feet for equipment and gas exchange ([ref.182]). Using the blue-green algaSynechocystis, 600 kg of algal suspension would be required, according to Gafford and Craft. These estimates are based on preliminary studies, are quite high, and are not of real practical value.
Other studies have indicated an extremely efficient algal system which offers a real potential for a practical and effective gas exchanger ([ref.183]). A thermophilic strain ofChlorellawith an optimum growth temperature of 39° C and an optimum temperature for photosynthesis of about 40° C can increase its cell mass 10 000-fold per day. When operating at one-half maximum efficiency, this alga produces 100 times its cell volume of oxygen per hour. Burk et al. ([ref.183]) state: "Future engineering development should lead to a space requirement, per adult person, of no more than 3 to 5 cubic feet of algal culture, equipment, and instrumentation for adequate purification of air." The requirements of this system would require additional energy in the form of light and of small amounts of nitrogenous and mineral material for the algae. The lightsource used by Burk et al. ([ref.183]) is a tungsten filament quartz lamp the size of a pencil, which has a long life, produces a luminous flux 5-10 times greater than sunlight on Earth, and operates at a 10-12 percent light efficiency.
Research is being carried out on algal regenerative systems by about 40 or 50 laboratories in the United States. NASA is supporting several basic studies on photosynthesis, the physiology of algae, and engineering pilot-plant development. Much of the research on algae is being supported by the Air Force.
Most algal studies have been carried out in small units and the data obtained have been used as a basis for extrapolating logistic values for the use of these organisms in manned space vehicles. Myers ([ref.179]) has shown that the quantity of algae necessary to support a man (with an assumed O2requirement of 625 liters per day) would yield about 600-700 grams dry weight of new cells per day. If algal growth in mass cultures could be maintained in a steady-state concentration of 2.5 gram dry weight per liter with such a growth rate as to yield 10 grams weight per liter per day, the volume of algal culture would be 60-70 liters and the total mass of the system would approximate 200-250 pounds.
Using an 8-liter system, Ward et al. ([ref.176]) have produced algal concentrations of 5-7 grams of dry algae per liter with a high-temperature algal strain. The maximum growth rate observed with the culture was 0.375 gram dry weight per liter per hour, or 9 grams dry weight per liter per day. This was accomplished by using 1-centimeter layers of culture and a light intensity of 8000 foot-candles. The culture system consisted of a rectangular plastic chamber having an area of 0.5 square meter and illuminated on each side to an intensity of 4000 foot-candles (cool-white). To produce 25 liters of oxygen per hour, an area of 8.3 square meters (85 square feet) would be required.
The major problem in large-scale production of algae is that of illumination. Conversion of electricity to light has an efficiency of only 10 to 20 percent. In addition, the maximum efficiency of light utilization byChlorellaalgae lies in the range of 18-22 percent. This results in a maximum efficiency of only 4 percent for photosynthetic systems. Another problem involved in conversion of electricity to light is the production of heat which has to be removed even with thermophilic algae. With a human demand of 600 liters of oxygen per day, the minimum electrical requirement becomes 4 kW. No large-scale culture has yet been managed at anything close to this minimum figure.
Another problem is the poor penetration of light into concentrated cultures of algae. This necessitates construction of large tanks of only about ¼-inch thickness. This results frequently in fouling of the surfacesof the tank by algae and makes the removal of the excess algae difficult. Production of 1 liter of oxygen results in the production of 1 gram dry weight of algae. Although a small amount of CO is produced by some algae, it can probably be removed by catalytic oxidation. Other problems include mutation and genetic drift of the algae and the necessity for maintaining bacteria-free cultures. There are also difficulties in maintaining a sterile culture if urine is to be used as a nitrogen source. While there is a potential for using algae as food, more research is required before it can be determined what quantity and methods of processing can be used. Research and development on algae is much greater than on both the higher plants and the electrolysis-Hydrogenomonassystems together.
The difference between the photosynthetic and electrolysis-chemosynthetic systems is the way electrical energy is made available to the organisms. In the photosynthetic system, electrical energy is converted to light which the algae or plants transform into chemical energy. In the chemosynthetic process, electrical energy is transformed into the chemical energy of hydrogen gas which is used by the bacteria. Both organisms use the chemical energy available to them to synthesize cell material with similar degrees of efficiency. The problem is to make the conversion of electricity to available chemical energy as efficient as possible.
In photosynthetic systems much energy is lost in the conversion of electricity to light, a process only 10-20 percent efficient at best. When this is combined with the loss from the inefficient use of light by plants, an overall efficiency of about 4 percent is obtained. In the electrolysis-Hydrogenomonassystem, the two steps are very efficient. Electrolysis cells can operate at up to 85 percent efficiency and the overall efficiency can be up to seven times that of a photosynthetic system.