Space Life Support Research Articles

Self-Sustaining Living Habitats in Extraterrestrial Environments.

Standard definitions of habitability assume that life requires the presence of planetary gravity wells to stabilize liquid water and regulate surface temperature. Here, the consequences of relaxing this assumption are evaluated. Temperature, pressure, volatile loss, radiation levels, and nutrient availability all appear to be surmountable obstacles to the survival of photosynthetic life in space or on celestial bodies with thin atmospheres. Biologically generated barriers capable of transmitting visible radiation, blocking ultraviolet, and sustaining temperature gradients of 25-100 K and pressure differences of 10 kPa against the vacuum of space can allow habitable conditions between 1 and 5 astronomical units in the solar system. Hence, ecosystems capable of generating conditions for their own survival are physically plausible, given the known capabilities of biological materials on Earth. Biogenic habitats for photosynthetic life in extraterrestrial environments would have major benefits for human life support and sustainability in space. Because the evolution of life elsewhere may have followed very different pathways from that on Earth, living habitats could also exist outside traditional habitable environments around other stars, where they would have unusual yet potentially detectable biosignatures.

Recycling potential of Cupriavidus necator for life support in space: Production of SCPs from volatile fatty acid and urea mixture

The International Space Station currently requires four annual replenishments for food supply, a practice that won't be feasible for deep space missions due to the greater distances. Based on the design of closed ecological life support systems, two waste streams were identified: urea from the crew urine, volatile fatty acids (VFAs) from a first stage of anaerobic digestion of waste. The objective of this study was to assess the ability of bacterium Cupriavidus necator to produce single cell protein on urea and VFAs. Thus, the effect of carbon sources (glucose vs VFAs) and the dilution rate on the biomass composition was determined in continuous cultures. Complete transformation of the carbon source into protein-rich biomass was achieved up to 78 % cell dry weight (CDW). For both carbon sources, the protein content increased from 55.0 %CDW to 78 %CDW with a decrease in the dilution rate. Conversely, the nucleic acid and polyhydroxyalkanoate contents decreased with the dilution rate from 8.8 %CDW to 4.8 %CDW and 9.8 %CDW to 0.6 %CDW respectively. Working at a low dilution rate seems to be a good way to maximize protein content while minimizing unwanted nucleic acids and polyhydroxyalkanoates.

Effects of simulated microgravity on current generation of electrochemically active bacteria: Insights from case-control study using random positioning machine

Electrochemically active bacteria (EAB) exhibit promising prospects for space exploration and life support systems. However, the effects of the space environment on EAB are unclear. In this study, the effects of simulated microgravity on the current generation of mixed-culture EAB were illustrated, and the underlying mechanism was elucidated. The results demonstrated that the electrochemical activity of mixed-culture EAB was enhanced, which was mainly due to the enrichment of Geobacter and the increase in EAB biomass. Additionally, the genes and proteins of the biofilm changed obviously under simulated microgravity conditions, including: I) genes related to signal transfer, II) genes related to cell wall synthesis, and III) genes related to riboflavin synthesis. This study first revealed the enrichment in EAB abundance, the increase in EAB biomass, and the promotion of current generation under simulated microgravity.

Biofilm reactors for the treatment of used water in space:potential, challenges, and future perspectives

Water is not only essential to sustain life on Earth, but also is a crucial resource for long-duration deep space exploration and habitation. Current systems in space rely on the resupply of water from Earth, however, as missions get longer and move farther away from Earth, resupply will no longer be a sustainable option. Thus, the development of regenerative reclamation water systems through which useable water can be recovered from “waste streams” (i.e., used waters) is sorely needed to further close the loop in space life support systems. This review presents the origin and characteristics of different used waters generated in space and discusses the intrinsic challenges of developing suitable technologies to treat such streams given the unique constrains of space exploration and habitation (e.g., different gravity conditions, size and weight limitations, compatibility with other systems, etc.). In this review, we discuss the potential use of biological systems, particularly biofilms, as possible alternatives or additions to current technologies for water reclamation and waste treatment in space. The fundamentals of biofilm reactors, their advantages and disadvantages, as well as different reactor configurations and their potential for use and challenges to be incorporated in self-sustaining and regenerative life support systems in long-duration space missions are also discussed. Furthermore, we discuss the possibility to recover value-added products (e.g., biomass, nutrients, water) from used waters and the opportunity to recycle and reuse such products as resources in other life support subsystems (e.g., habitation, waste, air, etc.).

Oversimplification of Systems Engineering Goals, Processes, and Criteria in NASA Space Life Support

AbstractThis paper investigates the oversimplification of the inherently complex systems engineering process in space life support. The standard systems engineering process steps are described. The International Space Station (ISS) life support system is explained with its goals and performance criteria. Although it is not usually emphasized, the essential function of developing a hierarchy of systems and subsystems is to simplify the design process. The System Complexity Metric (SCM) shows how this divide‐and‐conquer approach also reduces the system complexity. The complete systems engineering process has many detailed steps. It is often simplified because of the effort required and the human limitations on working memory and decision span. Systems analysis demands slow, logical, and focused thinking but is often bypassed in favor of quick, intuitive, subconscious “gut feel.” A study of 100 system designs found examples of 12 specific mental mistakes, such as ignoring stakeholder needs, and these mistakes are essentially oversimplifications of the systems engineering process. An analysis of space life support goals, options, criteria, and processes found 11 examples of oversimplifications in systems engineering, such as neglecting safety and cost. All these 11 oversimplifications could be traced to one or more of the 12 previously identified mental mistakes or other well‐known ones, such as ignoring sunk costs. Oversimplification of the systems engineering process is rarely noticed but is a common and harmful problem. A study of failures in 50 different space systems found that problems in systems engineering caused failures and often led to errors in design, development, and test that further contributed to failure. It seems that more diligent systems engineering could prevent many project problems and failures, but projects seem to be more guided by “gut feel” based on tradition, authority, and consensus than on the logical, rational systems engineering approach.

The potential of direct air capture using adsorbents in cold climates.

Global warming threatens the entire planet, and solutions such as direct air capture (DAC) can be used to meet net-zero goals and go beyond. This study investigates using DAC in a 5-step temperature vacuum swing adsorption (TVSA) cycle with adsorbents' Li-X and Na-X, readily available industrial zeolites, to capture and concentrate CO2 from air in cold climates. From this study, we report that Na-X in cold conditions has the highest known CO2 adsorption capacity in air of 2.54mmol/g. This combined with Na-X's low CO2 heat of adsorption, and fast uptake-rate in comparison to other benchmark materials, allowed for Na-X operating in cold conditions to have the lowest reported DAC operating energy of 1.1 MWh/tonCO2. These findings from this study show the promise of this process in cold climates of Canada, Alaska, Greenland, and Antarctica to be part of the solution to global warming.

Food production in space from CO2 using microbial electrosynthesis

The current food method in space is launching prepackaged food which is costly and unsustainable. Alternatives include growing crops and microalgae single cell protein (SCP) using artificial light photosynthesis, which are energy inefficient. Prepackaged food and microalgae food were compared to microbial electrosynthesis of acetic acid (MES-AA). Since the dominant cost of a space mission is the cost of launching mass, components of a system were converted to an equivalent mass, including power, heat rejection, and volume. Three-year roundtrip crewed missions were evaluated for the International Space Station, the Moon, and Mars. The average Equivalent System Mass (ESM) of MES-AA is 1.38x and 2.84x lower than prepackaged food and microalgae SCP, respectively. The expected electricity-to-calorie conversion efficiency of MES-AA is 19.8 %, consuming 3.45 kW to fully feed five astronauts; diets would realistically include multiple foods. MES-AA has a higher energy efficiency than any currently investigated resilient food in space. MES-AA can provide diet diversity at a lower cost than customarily storing prepackaged food or growing crops in space. Producing food while contributing to closed loop life support in space can contribute to reducing global catastrophic risk and is relevant in off-grid communities, like in rural Alaska.

Oversimplification of Systems Engineering Goals, Processes, and Criteria in NASA Space Life Support

AbstractThis paper investigates the oversimplification of the inherently complex systems engineering process in space life support. The standard systems engineering process steps are described. The International Space Station (ISS) life support system is explained with its goals and performance criteria. Although it is not usually emphasized, the essential function of developing a hierarchy of systems and subsystems is to simplify the design process. The System Complexity Metric (SCM) shows how this divide‐and‐conquer approach also reduces the system complexity. The complete systems engineering process has many detailed steps. It is often simplified because of the effort required and the human limitations on working memory and decision span. Systems analysis demands slow, logical, and focused thinking but is often bypassed in favor of quick, intuitive, subconscious “gut feel.” A study of 100 system designs found examples of 12 specific mental mistakes, such as ignoring stakeholder needs, and these mistakes are essentially oversimplifications of the systems engineering process. An analysis of space life support goals, options, criteria, and processes found 11 examples of oversimplifications in systems engineering, such as neglecting safety and cost. All these 11 oversimplifications could be traced to one or more of the 12 previously identified mental mistakes or other well‐known ones, such as ignoring sunk costs. Oversimplification of the systems engineering process is rarely noticed but is a common and harmful problem. A study of failures in 50 different space systems found that problems in systems engineering caused failures and often led to errors in design, development, and test that further contributed to failure. It seems that more diligent systems engineering could prevent many project problems and failures, but projects seem to be more guided by “gut feel” based on tradition, authority, and consensus than on the logical, rational systems engineering approach.

Microbiomes of air dust collected during the ground-based closed bioregenerative life support experiment "Lunar Palace 365"

BackgroundUnderstanding the dynamics of airborne microbial communities and antibiotic resistance genes (ARGs) in space life support systems is important because potential pathogens and antibiotic resistance pose a health risk to crew that can lead to mission failure. There have been few reports on the distribution patterns of microbiomes and ARGs in space life support systems. In particular, there have been no detailed investigations of microbiomes and/or antibiotic resistance based on molecular methods in long-term confined bioregenerative life support systems (BLSSs). Therefore, in the present study, we collected air dust samples from two crew shifts, different areas, and different time points in the "Lunar Palace 365" experiment. We evaluated microbial diversity, species composition, functional potential, and antibiotic resistance by combining cultivation-independent analyses (amplicon, shot-gun sequencing, and qPCR).ResultsWe found that the bacterial community diversity in the Lunar Palace1 (LP1) system was higher than that in a controlled environment but lower than that in an open environment. Personnel exchange led to significant differences in bacterial community diversity, and source tracking analysis revealed that most bacteria in the air derived from the cabin crew and plants, but no differences in microbial function or antibiotic resistance were observed. Thus, human presence had the strongest effect on the succession of microbial diversity in the BLSSs.ConclusionsOur results highlight that microbial diversity in BLSSs is heavily influenced by changes in crew and is unique from other open and controlled environments. Our findings can be used to help develop safe, enclosed BLSS that meet the requirements of human survival and habitation in outer space. In addition, our results can further enhance our understanding of the indoor air microbial community and effectively maintain a safe working and living environment, including plant growth.

USE OF METHANE IN CLOSED-LOOP LIFE SUPPORT SYSTEMS FOR SPACE MISSIONS

The paper discusses the use of methane (generated in the process of oxygen recovery from carbon dioxide released by the crew during its hydrogenation in the Sabatier reaction, with subsequent extraction of 61% of oxygen through electrolysis of the resultant water) in a regenerative life support system for crews on space missions. It demonstrates that the methane resulting from Sabatier reaction can be used both for pyrolysis in order to return the resulting hydrogen into this reaction so as to extract 100% of oxygen from carbon dioxide, and for producing food protein for life support in space. The use of methane pyrolysis was enabled by new technologies which allowed lowering the process temperature down to 500–700°C and obtaining the easy-to-remove carbon. It provides recommendations for designing space systems for methane pyrolysis. The paper makes the case for use of the existing processes for industrial production of protein from methane using methanotrophic bacteria in the production of food protein for space food rations, determines the balance of a closed-loop methanotrophic reaction, provides calculation basis and recommendations for designing space systems for methanotrophic production of food protein. Development of a system for food protein production from methane will enable its use as one of the systems for providing food on the Moon and Mars, as well as a backup system in space transportation missions. Key words: space missions, crew life support, СО2 hydration, methane pyrolysis, methanotrophic bacteria, food protein.

Design of an Artificial Intelligence-Based Commercial Photobioreactor for Optimal Algae Growth in Space Life Support

Processing carbon dioxide (CO2) to generate oxygen (O2) is one of the challenges in developing life support systems for human presence in space. Algae is known for growing its biomass through photo...

CURRENT AND FUTURE WAYS TO CLOSED LIFE SUPPORT SYSTEMS: VIRTUAL MELISSA CONFERENCE, GHENT (B) (3-5 /11/2020). A REVIEW

The 3-days international virtual MELiSSA conference (November 3 to 5, 2020) about the closed life support systems was organized by the University of Ghent and IDDUP in collaboration with ESA/ESTEC (European Space Agency) and MELiSSA Foundation, and welcomed around 150 papers. The first day addressed the welcomed contributions of the 5 major space agencies, the plants & photosynthetic microorganisms used for life support in space, the simulation of planetary ecosystems as well as MELiSSA inspired circular economy and education. The second day focused on Ground demonstration and analogue testing, Photobioreactors and photosynthetic microorganisms characterization, Modelling and Systems design, Organic wastes process and Metabolome where a real breakthrough was observed as well as in human microbiome in closed environments. The last day addressed: 1) the complex problematics of urine as a major source of nutrients, 2) Terrestrial synergies, and 3) some Physical, Chemical and Microbial contaminants linked to space research.

Biosphere 2’s Lessons about Living on Earth and in Space

Biosphere 2, the largest and most biodiverse closed ecological system facility yet created, has contributed vital lessons for living with our planetary biosphere and for long-term habitation in space. From the space life support perspective, Biosphere 2 contrasted with previous BLSS work by including areas based on Earth wilderness biomes in addition to its provision for human life support and by using a soil-based intensive agricultural system producing a complete human diet. No previous BLSS system had included domestic farm animals. All human and domestic animal wastes were also recycled and returned to the crop soils. Biosphere 2 was important as a first step towards learning how to miniaturize natural ecosystems and develop technological support systems compatible with life. Biosphere 2’s mostly successful operation for three years (1991-1994) changed thinking among space life support scientists and the public at large about the need for minibiospheres for long-term habitation in space. As an Earth systems laboratory, Biosphere 2 was one of the first attempts to make ecology an experimental science at a scale relevant to planetary issues such as climate change, regenerative agriculture, nutrient and water recycling, loss of biodiversity, and understanding of the roles wilderness biomes play in the Earth’s biosphere. Biosphere 2 aroused controversy because of narrow definitions and expectations of how science is to be conducted. The cooperation between engineers and ecologists and the requirement to design a technosphere that supported the life inside without harming it have enormous relevance to what is required in our global home. Applications of bioregenerative life support systems for near-term space applications such as initial Moon and/or Mars bases, will be severely limited by high costs of transport to space and so will rely on lighter weight, hydroponic systems of growing plants which will focus first on water and air regeneration and gradually increase its production of food required by astronauts or inhabitants. The conversion of these systems to more robust and sustainable systems will require advanced technologies, e.g., to capture sunlight for plant growth or process usable materials from the lunar or Martian atmosphere and regolith, leading to greater utilization of in situ space resources and less on transport from Earth. There are many approaches to the accomplishment of space life support. Significant progress has been made especially by two research efforts in China and the MELiSSA project of the European Space Agency. These approaches use cybernetic controls and the integration of intensive modules to accomplish food production, waste treatment and recycling, atmospheric regeneration, and in some systems, high-protein production from insects and larvae. Biosphere 2 employed a mix of ecological self-organization and human intervention to protect biodiversity for wilderness biomes with a tighter management of food crops in its agriculture. Biosphere 2’s aims were different than bioregenerative life support systems (BLSS) which have focused exclusively on human life support. Much more needs to be learned from both smaller, efficient ground-based BLSS for nearer-term habitation and from minibiospheric systems for long-term space application to transform humanity and Earth-life into truly multiplanet species.

Investigation of a biosystem based on Arthrospira platensis for air revitalisation in spacecrafts: Performance evaluation through response surface methodology

Arthrospira platensis is featured as a promising microalgae candidate for the development of the biosystems for air revitalisation in spacecrafts and life support in space. An enhanced configuration of a sparged type photobioreactor (PBR), containing 5 L of A. platensis culture, which was equipped with an external LED lighting tube around the reactor, was used in this study. The PBR was operated under dynamic conditions (0.5 L/min) with synthetic air containing CO2 (400, 900, 1400 ppm) and other gas traces (NO2 1 ppm, SO2 2.5 ppm, acetic acid vapours 1 ppm), at various light intensities (1.5, 2.5, 3.5 klux), according to an experimental design. The removal of gas traces (NO2, SO2 and acetic acid vapours) was below the detection limit (e.g. above 90% removal efficiency), while the removal of CO2 ranged between 69% and 85%, depending on the initial CO2 concentration and the light intensity. Thus, the system is able to roughly decrease the contaminant concentration from 1 ppm to below 0.1 ppm for NO2, 2.5 ppm to below 0.1 ppm for SO2, 1 ppm to below 1 ppb for acetic acid vapours and from 1400 ppm to 370 or from 400 ppm to 60 ppm for CO2. The system performance was thus subject to mathematical modelling and optimization in terms of CO2 removal efficiency and CO2 elimination capacity, which were also corroborated with the power consumption for illumination.

Current Progression: Application of High-Throughput Sequencing Technique in Space Microbiology.

During a spaceflight, astronauts need to live in a spacecraft on orbit for a long time, and the relationship between humans and microorganisms in the closed environment of space is not the same as on the ground. The dynamic study of microorganisms in confined space shows that with the extension of the isolation time, harmful bacteria gradually accumulate. Monitoring and controlling microbial pollution in a confined environment system are very important for crew health and the sustainable operation of a space life support system. Culture-based assays have been used traditionally to assess the microbial loads in a spacecraft, and uncultured-based techniques are already under way according to the NASA global exploration roadmap. High-throughput sequencing technology has been used generally to study the communities of the environment and human on the ground and shows its broad prospects applied onboard. We here review the recent application of high-throughput sequencing on space microbiology and analyze its feasibility and potential as an on-orbit detection technology.

Electrochemistry for Space Life Support

Electrochemical technology plays a key role in maintaining habitable environments for human space exploration. Historically, oxygen generation through electrolysis has been a key electrochemical technology for space life support. However, emerging electrochemical technologies are changing the way we process carbon dioxide and urine to supply oxygen and fresh water. Similarly, electrochemical production of hydrogen peroxide can support in-situ resource utilization to produce disinfectants for sanitary needs. These technologies will support space exploration that relies upon closed loop living and is increasingly independent of ground support, opening opportunities for human exploration of the Moon and Mars.

Potatoes as a Crop for Space Life Support: Effect of CO2, Irradiance, and Photoperiod on Leaf Photosynthesis and Stomatal Conductance.

Potatoes (Solanum tuberosum L.) have been suggested as a candidate crop for future space missions, based on their high yields of nutritious tubers and high harvest index. Three cultivars of potato, cvs. Norland, Russet Burbank, and Denali were grown in walk-in growth rooms at 400 and 800 µmol m−2 s−1 photosynthetic photon flux (PPF), 12-h L/12-h D and 24-h L/0 h D photoperiods, and 350 and 1,000 ppm [CO2]. Net photosynthetic rates (Pnet) and stomatal conductance (gs) of upper canopy leaves were measured at weekly intervals from 3 through 12 weeks after planting. Increased PPF resulted in increased Pnet rates at both [CO2] levels and both photoperiods, but the effect was most pronounced under the 12-h photoperiod. Increased [CO2] increased Pnet for both PPFs under the 12-h photoperiod, but decreased Pnet under the 24-h photoperiod. Increased PPF increased gs for both [CO2] levels and both photoperiods. Increased [CO2] decreased gs for both PPFs for the 12-h photoperiod, but caused only a slight decrease under the 24-h photoperiod. Leaf Pnet rates were highest with high PPF (800), elevated [CO2] (1,000), and a 12-h photoperiod, while growing the plants under continuous (24-h) light resulted in lower leaf photosynthetic rates for all combinations of PPF and [CO2]. The responses of leaf photosynthetic rates are generally consistent with prior published data on the plant biomass from these same studies (Wheeler et al., Crop Sci. 1991) and suggest that giving more light with a 24-h photoperiod can increase biomass in some cases, but the leaf Pnet and overall photosynthetic efficiency drops.

Жизнеобеспечение экипажа пилотируемого космического объекта, проблемы управления

Life support for first manned spaceflights was based on supplies of consumables. Crew life support systems based on supplies of water and oxygen, in spite of their simplicity, are extremely inefficient in orbital space missions and are unfeasible in deep space missions because of mass and volume constraints. Therefore, there are currently developed and are to be used on space stations the life support systems that are based on chemical and physical regeneration of water and oxygen extracted from human waste. In view of further advances in long-duration orbital stations, and the prospects of establishment of planetary outposts and deep space exploration, the problem of constructing an automated system for controlling a suite of regenerative LSS becomes urgent. The complexity of solving the problem of constructing an efficient control system in this case owes to the existence of a large number of effectiveness criteria. The paper proposes a system of consolidated global efficiency criteria, which allows to break up this problem into a series of sub-problems of optimization in order to solve this problem. The proposed criteria are longevity, cost, comfort. The paper presents a series of specific examples of using the proposed principles with necessary generalizations. Key words: space life support systems, atmosphere revitalization equipment, automated control system, global generalized efficiency criteria, longevity, cost, comfort.

High density long-term cultivation of Chlorella vulgaris SAG 211-12 in a novel microgravity-capable membrane raceway photobioreactor for future bioregenerative life support in SPACE

Hybrid life support systems are of great interest for future far-distant space exploration missions to planetary surfaces, e.g. Mars, planned until 2050. By synergistically combining physicochemical and biotechnological algae-based subsystems, an essential step towards the closure of the carbon loop in environmental control and life support systems (ECLSS) shall be accomplished, offering a wide beneficial potential for ECLSS through the utilization of oxygenic photosynthesis: O2 and potential human food can be formed in-situ from CO2 and water. The wild type green alga Chlorella vulgaris strain SAG 211-12 was selected as model microorganism due to its photoautotrophic growth, high biomass yield, cultivation flexibility and long-term cultivation robustness. The current study presents for the first time a stable xenic long-term processing of microalgae in a novel microgravity capable membrane raceway photobioreactor for 188 days with the focus on algal growth kinetics and gas evolution. In particular, culture homogeneity and viability were monitored and evaluated during the whole cultivation process due to their putative crucial impact on long-term functionality and efficiency of a closed cultivation system. Based on a specially designed cyclic batch cultivation process for SAG 211-12, a successive biomass growth up to a maximum of 12.2 g l−1 with a max. global volumetric productivity of 1.3 g l−1 d−1 was reached within the closed loop system. The photosynthetic capacity was assessed to a global molar photosynthetic quotient of 0.31. Furthermore, cultivation parameters for a change from batch to continuous processing at high biomass densities and proliferation rates are introduced. The presented µgPBR miniature plant and the developed high throughput cultivation process are planned to be tested under real space conditions within the PBR@LSR project (microgravity and cosmic radiation) aboard the International Space Station with an operation period of up to 180 days to investigate the impact on long-term system stability.

Cultivar-Specific Performance and Qualitative Descriptors for Butterhead Salanova Lettuce Produced in Closed Soilless Cultivation as a Candidate Salad Crop for Human Life Support in Space.

Plant production is crucial for space journeys self-autonomy by contributing to the dietary intake necessary to sustain the physical and psychological well-being of space colonists, as well as for contributing to atmospheric revitalization, water purification and waste product recycling. Choosing the appropriate cultivar is equally important as the species selection, since cultivar influences the obtained fresh biomass, water use efficiency (WUE), growing cycle duration, qualitative features and postharvest performance. Two differently pigmented butterhead Lactuca sativa L. (red and green Salanova) cultivars were assessed in terms of morphometric, mineral, bioactive and physiological parameters. The experiment was carried out in a controlled environment growth chamber using a closed soilless system (nutrient film technique). Red Salanova registered a biomass of 130 g at harvest, which was 22.1% greater than green Salanova, and a water uptake of 1.42 L during the full growing period corresponding to WUE of 91.9 g L−1, which was 13.8% higher than that of green Salanova. At harvest, green Salanova had accumulated more P, K, Ca, Mg and 37.2% more nitrate than red Salanova, which however had higher relative water content, leaf total and osmotic potential and higher SPAD index. Red Salanova also exhibited at harvest around two-fold higher lipophilic antioxidant activity and total phenols, and around six-fold higher total ascorbic acid levels. These latter characteristics improved the antioxidant capacity of red Salanova enabling it to use light more efficiently and deliver better overall performance and yield than green Salanova. Moreover, the higher phenolics and total ascorbic acid contents of red Salanova constitute natural sources of antioxidants for enriching the human diet and render it an optimal candidate cultivar for near-term missions.