Life Support Trade Studies Involving Plants 2001-01-2362
Plants can be grown in space to support human life, providing food, and regenerating water and air. Various groups have demonstrated that plants can support human life on the ground, and that plants can grow in space. One would suppose that plants are also able to support human life in space, though obviously it would be a good idea to demonstrate that ability before committing to a mission requiring bioregeneration. However, plant growth in space requires that we provide the necessary conditions for growth, and this might require not only providing water and fertilizer as we do in terrestrial agriculture, but also a controlled environment and lighting. This would make crops much more costly than we are accustomed to on Earth, where the majority of crops are grown outside and where natural sunlight is generally adequate. On the other hand, providing food, air, and water in space by any other means is also costly.
The real question is whether and under what conditions it is cost effective to grow plants for life support in space. There are also intangible benefits in growing crops, and these need to be considered in designing a mission, though it is difficult to assess their importance. For simplicity, this paper uses a quantitative approach, and ignores, for now, intangible benefits. Cost effectiveness can be calculated for identified products and for a given mission where factors such as mission duration, equipment design, and mission equivalencies can be specified.
Equivalent system mass (ESM) has been calculated for a number of plant products both for supply from Earth and for local production on Mars. A dimensionless number is then obtained by dividing the supply ESM by the local-production ESM. This “goodness” number will be greater than one when local production is cost-effective. Very different values of goodness are obtained according to the plant product for which the value is estimated. Thus, fresh tomatoes turn out to be worth more than dried tomatoes. This is probably realistic, as fresh tomatoes would cost more to ship, even if they could arrive in pristine condition (which, of course, is not the case). Furthermore, drying locally grown tomatoes would cost more in equipment, energy, and heat rejection, and probably in crew time, than the fresh product, so locally produced dried tomatoes would cost more per kg of usable product than fresh tomatoes.
A correction can be applied to account for benefits of water and air regeneration, based on the reduced water and air systems that are needed when plants are grown. Thus, air regeneration would not be needed if the degree of food closure exceeds about 50%. However, gas storage, air circulation, and other functions would still be needed. With water, if it is accepted that plants can be used for producing potable water (and not everybody agrees with this due to concerns over pathogens and contaminants), functions such as storage and distribution would still be needed, but water closure would be reached at about 20% food closure. For simplicity, this correction has not been applied here
Goodness is considered for a number of crops products and a number of assumptions, using a baseline of terrestrial growth-chamber productivity, Mars surface missions of variable durations, and established equivalencies.