The values given in [table IX] indicate 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 System
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:
Oxidation
C6H12O6 + 6O2 ———————> 6CO2 + 6H2O + heat
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 Plants
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 family Lemnaceae (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 m2 of frondal surface of duckweed, at a gas exchange rate of 10.8 liters m2/hr would provide sufficient gas exchange for one man. This would produce about 25 grams of dry plant material per hour.