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Growing Crops in a CELSS
The capability to produce human food on a continuing basis is the single most limiting component of human life support in space that presently cannot be accomplished by physico-chemical means. Efficient crop production utilizing hydroponic culture and recycled solid, liquid, and gaseous wastes under conditions of mass closure will negate the need for resupply of food, oxygen, and water and will help make a CELSS sustainable with respect to power, labor, and materials. By selecting species and cultivars of food crops that will contribute substantially to the creation of a balanced vegetarian diet for humans, and by defining efficient crop production protocols and optimizing environments for CELSS candidate species, the power, labor, mass, and volume requirements for crop production in a Controlled Ecological Life-Support System (CELSS) will be minimized and productivity and value added will be maximized.
For long-duration manned space missions, controlled environment agriculture (CEA) is the most likely system for growing crops. Plants are maintained in growth chambers where environmental conditions, like lighting, nutrient supply, temperature and humidity, can be strictly controlled by computer. It is unlikely that crops will be grown using sunlight, especially during surface operations. On Mars, for example, sunlight is only available for half of each rotation period. More light is required for optimal growth of many plant species. In addition, the sun can be obscured for months at a time by the giant dust storms on Mars. This could put the crew in jeopardy. CEA crops will probably be grown using some form of hydroponics, whether this be directly in liquid media, having the roots in contact with a thin film of liquid, having the roots in contact with a porous pipe that carries media, or in some porous matrix like vermiculite. Growth media containing the minerals needed for plant growth will likely be formulated from minerals recovered from the waste stream supplemented by stowed materials. Water transpired by the plants can be collected and used as drinking water for the crew.
As basic responses of the CELSS candidate species to photoperiod, planting density, and mineral nutrition are characterized, and suitable hydroponic production systems are developed, research will evolve toward phasic control of crop development, utilizing discrete periods of CO2 enrichment to appropriate levels, different photon fluxes and spectra of plant-growth lighting, etc. In addition to defining the limits of productivity using these environmental variables, those phases of crop development during which plants are insensitive to optimizing environments will be identified, thereby paving the way to save power and expensive resources. Auto-optimization programs can be developed that will seek the highest photosynthetic rate possible at the time based upon computer-driven manipulation of environmental variables. Plants can be maintained under non-limiting mineral nutrition during optimization periods by a combination of automated pH/conductivity control and careful hydroponics management. Using power consumption rates measured for different growth chamber functions, yield rates per unit power consumption can be used to identify the best compromise between cost and yield. The most sustainable suite of optimized environmental conditions likely will not drive the highest possible yield rates of edible biomass, but rather the most obtainable for the least power input (for lighting, cooling, etc.). In addition, quantum efficiency of crop canopies, water-use efficiency, and total power consumption during crop production can be determined for optimized environments. Measurement of instantaneous net photosynthesis rate (mmol m-2 s-1), transpiration rate (mmol m-2 s-1), and proximate composition (i.e., protein, carbohydrate, fat, water, ash) of various plant parts will assist in calculating photosynthetic quotient, caloric partitioning, and cultural manipulation of food content under different growth environments.
The protocols, technologies, and information generated from higher plant biomass production research has direct application to the fledgling CEA industry as well as to the CELSS program. There has not been a consistent funding source for research and development of CEA due to lack of profitability in most sectors of the industry, and the venture capital that has been available has been applied directly toward greenhouse crop production. As optimizing environments and phasic growth requirements of CELSS crops become better defined, their implementation within crop canopies will be better visualized as automated and/or robotic crop production systems. These technologies, and the scientific data and information generated from crop production research using highly controlled, modified environments, will be just as valuable for CEA on Earth as for life support in space. Anticipated breakthroughs in plant-growth lighting technologies over the next two decades and/or hoped for progress in finding a cleaner, cheaper source of electrical power (other than hydroelectric) will help make CEA a viable growth industry, and many computerized control algorithms to be developed for optimization of CO2 and lighting at different stages of crop development will have immediate application to the CEA industry for commercial crops analogous to candidate species.