As humanity looks to expand its presence in the solar system, the ability to grow sustainable food crops in space will be essential for supporting long-duration crewed missions. While current methods rely on resupplying prepackaged food from Earth, this approach is not feasible for missions to more distant destinations like Mars. Using local planetary resources to grow crops in situ is a promising alternative.
Asteroids are an abundant resource in the solar system that could potentially be used for space agriculture. In particular, carbonaceous chondrite asteroids, which are rich in carbon and have undergone aqueous alteration, may contain bioavailable nutrients and hydrated minerals that could support plant growth. However, very little research has been done to date on the suitability of asteroid regolith as a plant growth substrate.
Evaluating Simulated Asteroid Regolith
To address this knowledge gap, a recent study evaluated the ability of simulated CI carbonaceous asteroid regolith to sustain plant growth. A CI asteroid regolith simulant was developed based on the mineralogy and chemistry of the Orgueil CI carbonaceous meteorite. The simulant primarily consists of serpentine clay minerals, magnetite, and organic carbon compounds.
Three common crop species were tested: lettuce (Latuca sativa), radish (Raphanus sativus), and pepper (Capsicum annuum). Seeds were planted in mixtures of the CI simulant with peat moss, a common organic soil amendment, in varying ratios. A nutrient-rich commercial topsoil was used as an Earth-based control. The crops were grown under controlled environmental conditions in a growth chamber.
Plant Growth Results
The results showed decreased germination rates, plant height, leaf area, and biomass as the proportion of CI simulant increased compared to the peat moss. No germination occurred in 100% CI simulant. Radishes were the most negatively impacted of the three species tested.
Soil analyses revealed that the simulant was very low in essential plant nutrients like nitrogen, phosphorus and potassium compared to the peat moss and topsoil. The simulant also had a high pH, low cation exchange capacity, and was classified as a silt-based soil prone to compaction and crusting. These properties likely contributed to the poor plant growth by impeding root development and water/nutrient uptake.
Interestingly, the lettuce and pepper plants were less affected by increasing simulant concentrations than the radishes in terms of height and biomass. This suggests some crops may be more tolerant of the harsh conditions in asteroid regolith than others. Careful selection of crop species will be important for optimizing growth.
Comparison to Other Extraterrestrial Simulants
When compared to lunar and Martian regolith simulants that have been previously studied for plant growth, the CI asteroid simulant had some similarities and differences:
- The CI simulant had higher concentrations of most plant nutrients than a lunar simulant, but still much lower than a typical soil.
- Its pH fell between that of a lunar and Martian simulant.
- Plants were able to germinate in 100% lunar and Martian simulant, but not the 100% CI simulant. This is likely due to the method of simulant production.
Lunar and Martian simulants are often produced from terrestrial volcanic or basaltic source materials to mimic the spectral and chemical properties of actual regolith. In contrast, the CI simulant was designed to more closely match the mineralogy of a CI meteorite, using multiple mineral phases. As a result, the CI simulant may be more “inert” and lack the incipient fertility of the single-source simulants.
Improving Asteroid Regolith for Agriculture
The results of this study indicate that raw, unaltered CI asteroid regolith is a poor medium for growing crops, primarily due to its lack of nutrients and adverse physical properties. However, the researchers suggest several potential approaches for enhancing its fertility:
Adding Organic Matter
Mixing the regolith with organic matter like compost or green manure cover crops could help alleviate compaction, improve water retention, and provide a slow-release nutrient source as the organic matter decomposes. Dead plant matter recycled from previous harvests could be used for this purpose to “condition” the regolith over time.
Supplementing with Nutrients
The regolith could be supplemented with essential plant nutrients to make up for its natural deficiencies. A hydroponic-type system delivering a balanced nutrient solution, as is used in current space crop production, may be effective. Alternatively, organic or mineral fertilizers could be added to the regolith substrate itself.
Selecting Crops and Optimizing Planting
Crops should be screened for their tolerance to the unique stresses of growing in asteroid regolith. Traits like drought resistance, ability to grow in compacted soils, and low nutrient requirements would be desirable. Planting depth and seeding density may also need to be adjusted to account for the poor structure and crusting tendency of the regolith.
Inoculating with Microbes
Inoculating the regolith with beneficial soil microbes could help improve its fertility over time. Bacteria and fungi play key roles in natural soil ecosystems, fixing nitrogen, solubilizing nutrients, building soil structure, and supporting healthy plant growth. Inoculating sterile regolith with a microbial consortium could help establish a living soil that can sustain crops.
Future Research Directions
This initial study provides a foundation for further research into using asteroid regolith for space agriculture. Some key areas for future work include:
- Evaluating the growth of a wider variety of crop species and cultivars in simulated asteroid regolith
- Studying the impacts of different organic and inorganic amendments on regolith fertility and plant growth
- Developing and testing regenerative systems for recycling biomass and nutrients in an asteroid regolith substrate
- Examining the effects of microgravity and reduced pressure on plant-regolith interactions using clinostats or other microgravity simulators
- Characterizing the microbial communities that develop in asteroid regolith-based growth systems over time
Implications for Space Exploration
The ability to use asteroid regolith as a local, in situ resource for growing crops would greatly improve the sustainability and self-sufficiency of long-duration space missions. Asteroids are abundant throughout the solar system, with over 1 million known asteroids in the main asteroid belt between Mars and Jupiter alone. Near-Earth asteroids, which pass close to Earth’s orbit, are also common, with over 20,000 documented to date.
Being able to tap into this vast reservoir of material to support human life in space would reduce the need for resupply missions from Earth and enable longer-range exploration. Carbonaceous asteroids in particular are rich in water and organic compounds in addition to regolith, making them promising candidates for supporting space settlements.
However, significant technological and engineering challenges remain in mining and transporting asteroid material for use. Robotic missions would be needed to survey candidate asteroids up close to assess their resource potential. New spacecraft and equipment would have to be developed for extracting and processing asteroid regolith in the difficult microgravity conditions. The regolith would then need to be delivered to a crewed spacecraft, space station, or surface habitat for use in crop production.
Despite these hurdles, utilizing asteroid resources offers immense potential for expanding humanity’s reach in the solar system. In addition to supporting space agriculture, mined asteroids could provide material for constructing space habitats, producing rocket propellant, and generating oxygen and other life support consumables. Metals and rare earth elements could also be extracted for in-space manufacturing or possibly returned to Earth.
Summary
The study of simulated asteroid regolith as a plant growth medium is an important first step in evaluating the potential of this abundant space resource for supporting crop production. Although the raw CI carbonaceous asteroid simulant proved to be a poor substrate for growing crops, several strategies for improving its fertility were identified, such as adding organic matter, supplementing nutrients, selecting appropriate crops, and inoculating with beneficial microbes.
Further research is needed to optimize asteroid regolith-based growing systems and to understand how plants respond to the unique conditions of the space environment. Overcoming the challenges of mining and utilizing asteroid material will also require significant technological development and space infrastructure.
Ultimately, the ability to grow crops in asteroid-derived soils would open up new possibilities for sustainable human habitation beyond Earth. In combination with other space resources like solar energy, asteroid regolith could help provide the raw materials needed to support a long-term human presence in space. As we continue to explore our solar system, making use of the vast resources available on asteroids may prove key to making space a new home for humanity.
