Integration of lessons from recent research for “Earth to Mars” life support systems

Development of reliable and robust strategies for long-term life support for planetary exploration must be built from real-time experimentation to verify and improve system components. Also critical is incorporating a range of viable options to handle potential short-term life system imbalances. This paper revisits some of the conceptual framework for a Mars base prototype which has been developed by the authors along with others previously advanced (“Mars on Earth^®”) in the light of three years of experimentation in the Laboratory Biosphere, further investigation of system alternatives and the advent of other innovative engineering and agri-ecosystem approaches. Several experiments with candidate space agriculture crops have demonstrated the higher productivity possible with elevated light levels and improved environmental controls. For example, crops of sweet potatoes exceeded original Mars base prototype projections by an average of 46% (53% for best crop) ultradwarf (Apogee) wheat by 9% (23% for best crop), pinto bean by 13% (31% for best crop). These production levels, although they may be increased with further optimization of lighting regimes, environmental parameters, crop density etc. offer evidence that a soil-based system can be as productive as the hydroponic systems which have dominated space life support scenarios and research. But soil also offers distinct advantages: the capability to be created on the Moon or Mars using in situ space resources, reduces long-term reliance on consumables and imported resources, and more readily recycling and incorporating crew and crop waste products. In addition, a living soil contains a complex microbial ecosystem which helps prevent the buildup of trace gases or compounds, and thus assist with air and water purification. The atmospheric dynamics of these crops were studied in the Laboratory Biosphere adding to the database necessary for managing the mixed stands of crops essential for supplying a nutritionally adequate diet in space. This paper explores some of the challenges of small bioregenerative life support: air-sealing and facility architecture/design, balance of short-term variations of carbon dioxide and oxygen through staggered plantings, options for additional atmospheric buffers and sinks, lighting/energy efficiency engineering, crop and waste product recycling approaches, and human factor considerations in the design and operation of a Mars base. An “Earth to Mars” project, forging the ability to live sustainably in space (as on Earth) requires continued research and testing of these components and integrated subsystems; and developing a step-by-step learning process.     

Crop productivities and radiation use efficiencies for bioregenerative life support

NASA’s Biomass Production Chamber (BPC) at Kennedy Space Center was decommissioned in 1998, but several crop tests were conducted that have not been reported in the open literature. These include several monoculture studies with wheat, soybean, potato, lettuce, and tomato. For all of these studies, either 10 or 20 m² of plants were grown in an atmospherically closed chamber (113 m³ vol.) using a hydroponic nutrient film technique along with elevated CO₂ (1000 or 1200 μmol mol⁻¹). Canopy light (PAR) levels ranged from 17 to 85 mol m⁻² d⁻¹ depending on the species and photoperiod. Total biomass (DM) productivities reached 39.6 g m⁻² d⁻¹ for wheat, 27.2 g m⁻² d⁻¹ for potato, 19.6 g m⁻² d⁻¹ for tomato, 15.7 g m⁻² d⁻¹ for soybean, and 7.7 g m⁻² d⁻¹ for lettuce. Edible biomass (DM) productivities reached 18.4 g m⁻² d⁻¹ for potato, 11.3 g m⁻² d⁻¹ for wheat, 9.8 g m⁻² d⁻¹ for tomato, 7.1 g m⁻² d⁻¹ for lettuce, and 6.0 g m⁻² d⁻¹ for soybean. The corresponding radiation (light) use efficiencies for total biomass were 0.64 g mol⁻¹ PAR for potato, 0.59 g DM mol⁻¹ for wheat, 0.51 g mol⁻¹ for tomato, 0.46 g mol⁻¹ for lettuce, and 0.43 g mol⁻¹ for soybean. Radiation use efficiencies for edible biomass were 0.44 g mol⁻¹ for potato, 0.42 g mol⁻¹ for lettuce, 0.25 g mol⁻¹ for tomato, 0.17 g DM mol⁻¹ for wheat, and 0.16 g mol⁻¹ for soybean. By initially growing seedlings at a dense spacing and then transplanting them to the final production area could have saved about 12 d in each production cycle, and hence improved edible biomass productivities and radiation use efficiencies by 66% for lettuce (to 11.8 g m⁻² d⁻¹ and 0.70 g mol⁻¹), 16% for tomato (to 11.4 g m⁻² d⁻¹and 0.29 g mol⁻¹), 13% for soybean (to 6.9 g m⁻² d⁻¹ and 0.19 g mol⁻¹), and 13% for potato (to 20.8 g m⁻² d⁻¹ and 0.50 g mol⁻¹). Since wheat was grown at higher densities, transplanting seedlings would not have improved yields. Tests with wheat resulted in a relatively low harvest index of 29%, which may have been caused by ethylene or other organic volatile compounds (VOCs) accumulating in the chamber. Assuming a higher harvest index of 40% could be achieved by scrubbing VOCs, productivity of wheat seed could have been improved nearly 40% to 15.8 g m⁻² d⁻¹ and edible biomass radiation use efficiency to 0.30 g mol⁻¹.     

Cowpeas and pinto beans: Performance and yields of candidate space crops in the laboratory biosphere closed ecological system

An experiment utilizing cowpeas (Vigna unguiculata L.), pinto beans (Phaseolus vulgaris L.) and Apogee ultra-dwarf wheat (Triticum sativa L.) was conducted in the soil-based closed ecological facility, Laboratory Biosphere, from February to May 2005. The lighting regime was 13 h light/11 h dark at a light intensity of 960 μmol m⁻² s⁻¹, 45 mol m⁻² day⁻¹ supplied by high-pressure sodium lamps. The pinto beans and cowpeas were grown at two different planting densities. Pinto bean production was 341.5 g dry seed m⁻² (5.42 g m⁻² day⁻¹) and 579.5 dry seed m⁻² (9.20 g m⁻² day⁻¹) at planted densities of 32.5 plants m⁻² and 37.5 plants m⁻², respectively. Cowpea yielded 187.9 g dry seed m⁻² (2.21 g m⁻² day⁻¹) and 348.8 dry seed m⁻² (4.10 g m⁻² day⁻¹) at planted densities of 20.8 plants m⁻² and 27.7 plants m⁻², respectively. The crop was grown at elevated atmospheric carbon dioxide levels, with levels ranging from 300–3000 ppm daily during the majority of the crop cycle. During early stages (first 10 days) of the crop, CO₂ was allowed to rise to 7860 ppm while soil respiration dominated, and then was brought down by plant photosynthesis. CO₂ was injected 27 times during days 29–71 to replenish CO₂ used by the crop during photosynthesis. Temperature regime was 24–28 °C day/deg 20–24 °C night. Pinto bean matured and was harvested 20 days earlier than is typical for this variety, while the cowpea, which had trouble establishing, took 25 days more for harvest than typical for this variety. Productivity and atmospheric dynamic results of these studies contribute toward the design of an envisioned ground-based test bed prototype Mars base.     

Carbon dioxide dynamics of combined crops of wheat,cowpea, pinto beans in the Laboratory Biosphere closed ecological system

A mixed crop consisting of cowpeas, pinto beans and Apogee ultra-dwarf wheat was grown in the Laboratory Biosphere, a 40 m³ closed life system equipped with 12,000 W of high pressure sodium lamps over planting beds with 5.37 m² of soil. Similar to earlier reported experiments, the concentration of carbon dioxide initially increased to 7860 ppm at 10 days after planting due to soil respiration plus CO₂ contributed from researchers breathing while in the chamber for brief periods before plant growth became substantial. Carbon dioxide concentrations then fell rapidly as plant growth increased up to 29 days after planting and subsequently was maintained mostly in the range of about 200–3000 ppm (with a few excursions) by CO₂ injections to feed plant growth. Numerous analyses of rate of change of CO₂ concentration at many different concentrations and at many different days after planting reveal a strong dependence of fixation rates on CO₂ concentration. In the middle period of growth (days 31–61), fixation rates doubled for CO₂ at 450 ppm compared to 270 ppm, doubled again at 1000 ppm and increased a further 50% at 2000 ppm. High productivity from these crops and the increase of fixation rates with elevated CO₂ concentration supports the concept that enhanced CO₂ can be a useful strategy for remote life support systems. The data suggests avenues of investigation to understand the response of plant communities to increasing CO₂ concentrations in the Earth’s atmosphere. Carbon balance accounting and evapotranspiration rates are included.     

Effects of lighting and air movement on temperatures in reproductive organs of plants in a closed plant growth facility

Temperature increases in plant reproductive organs such as anthers and stigmas could cause fertility impediments and thus produce sterile seeds under artificial lighting conditions without adequately controlled environments in closed plant growth facilities. There is a possibility such a situation could occur in Bioregenerative Life Support Systems under microgravity conditions in space because there will be little natural convective or thermal mixing. This study was conducted to determine the temperature of the plant reproductive organs as affected by illumination and air movement under normal gravitational forces on the earth and to make an estimation of the temperature increase in reproductive organs in closed plant growth facilities under microgravity in space. Thermal images of reproductive organs of rice and strawberry were captured using infrared thermography at air temperatures of 10–11 °C. Compared to the air temperature, temperatures of petals, stigmas and anthers of strawberry increased by 24, 22 and 14 °C, respectively, after 5 min of lighting at an irradiance of 160 W m⁻² from incandescent lamps. Temperatures of reproductive organs and leaves of strawberry were significantly higher than those of rice. The temperatures of petals, stigmas, anthers and leaves of strawberry decreased by 13, 12, 13 and 14 °C, respectively, when the air velocity was increased from 0.1 to 1.0 ms⁻¹. These results show that air movement is necessary to reduce the temperatures of plant reproductive organs in plant growth facilities.     

Gas exchange rates of potato stands for bioregenerative life support

Plants can provide a means for removing carbon dioxide (CO₂) while generating oxygen (O₂) and clean water for life support systems in space. To study this, 20 m² stands of potato (Solanum tuberosum L.) plants were grown in a large (113 m³ vol.), atmospherically closed chamber. Photosynthetic uptake of CO₂ by the stands was detected about 10 DAP (days after planting), after which photosynthetic rates rose rapidly as stand ground cover and total light interception increased. Photosynthetic rates peaked ca. 50 DAP near 45 μmol CO₂ m⁻² s⁻¹ under 865 μmol m⁻² s⁻¹ PPF (average photosynthetic photon flux), and near 35 μmol CO₂ m⁻² s⁻¹ under 655 μmol m⁻² s⁻¹ PPF. Short term changes in PPF caused a linear response in stand photosynthetic rates up to 1100 μmol m⁻² s⁻¹ PPF, with a light compensation point of 185 μmol m⁻² s⁻¹ PPF. Comparisons of stand photosynthetic rates at different CO₂ concentrations showed a classic C₃ response, with saturation occurring near 1200 μmol mol⁻¹ CO₂ and compensation near 100 μmol mol⁻¹ CO₂. In one study, the photoperiod was changed from 12 h light/12 h dark to continuous light at 58 DAP. This caused a decrease in net photosynthetic rates within 48 h and eventual damage (scorching) of upper canopy leaves, suggesting the abrupt change stressed the plants and/or caused feedback effects on photosynthesis. Dark period (night) respiration rates increased during early growth as standing biomass increased and peaked near 9 μmol CO₂ m⁻² s⁻¹ ca. 50 DAP, after which rates declined gradually with age. Stand transpiration showed a rapid rise with canopy ground cover and peaked ca. 50 DAP near 8.9 L m⁻² d⁻¹ under 860 μmol m⁻² s⁻¹ PPF and near 6.3 L m⁻² d⁻¹ under 650 μmol m⁻² s⁻¹ PPF. Based on the best photosynthetic rates from these studies, approximately 25 m² of potato plants under continuous cultivation would be required to support the CO₂ removal and O₂ requirements for one person.     

New concepts for the avoidance or utilisation of methane in life support systems

Due to high resupply costs, especially for long-duration stays in space habitats beyond low earth orbit, future manned space missions will require life support systems (LSS) with a high degree of regenerativity. Possible ways to overcome the waste of resources and to save on resupply mass are therefore of major interest for the development of next generation environmental control and life support systems.