As NASA plans for future human missions to the Moon and Mars, the concept of in-situ propellant production (ISPP) has gained significant attention. ISPP involves producing rocket propellants using resources available on the surface of these celestial bodies, potentially reducing the amount of propellant that needs to be transported from Earth. A recent paper discusses the implementation of ISPP systems and presents a range of challenges and complexities that must be carefully considered.
Lunar ISPP
Resources and Processes
The Moon offers several potential resources for ISPP, including silicates in the regolith, regolith containing iron oxide (FeO), solar wind-implanted atoms, and water ice in permanently shadowed craters near the poles. The two leading candidates for lunar ISPP are the carbothermal process and the extraction of water from polar ice deposits.
The carbothermal process involves a complex series of steps, including mining and transporting regolith, beneficiation, reactor operations, and waste disposal. This process requires high temperatures (around 1,650°C) and involves the use of a carbon reduction agent, such as methane. The end products are oxygen and carbon monoxide, which can be further processed to produce water and regenerate the reduction agent.
Extracting water from polar ice deposits is another potential approach to lunar ISPP. However, the physical state, distribution, and accessibility of these ice deposits remain uncertain. A significant ground-truth campaign would be necessary to locate and verify access to the ice. Once the ice is extracted, it would need to be melted and electrolyzed to produce hydrogen and oxygen propellants.
Challenges and Complexities
Lunar ISPP faces numerous challenges and complexities. Both the carbothermal process and polar ice extraction require autonomous mining, transportation, and processing of large quantities of regolith under harsh environmental conditions. These operations involve heavy machinery that consumes significant amounts of power, and the reliability and durability of these systems in the lunar environment are critical concerns.
Providing adequate power for lunar ISPP is another major challenge. Solar power is only available for a portion of the lunar day, and the power requirements for ISPP processes far exceed those for life support systems. Nuclear power is an alternative, but it introduces additional complexities and risks.
The logistics of lunar ISPP are also daunting. The processes involve numerous steps, each with its own set of challenges and potential failure points. Synchronizing the operations of excavators, haulers, reactors, and other components is a complex task that requires careful planning and coordination.
Martian ISPP
Resources and Processes
Mars offers several resources for ISPP, including the atmosphere (which is primarily composed of carbon dioxide), regolith containing hydrated minerals, and water ice embedded in near-surface regolith at higher latitudes. The most straightforward approach to Martian ISPP is the electrolysis of atmospheric carbon dioxide to produce oxygen.
The Mars Oxygen ISRU Experiment (MOXIE) successfully demonstrated this process on a small scale during the Mars 2020 mission. The system draws in Martian atmosphere, compresses it, and uses solid oxide electrolysis cells (SOECs) to split the carbon dioxide into oxygen and carbon monoxide. The oxygen is then collected and stored, while the carbon monoxide is vented back into the atmosphere.
Advantages and Challenges
Compared to lunar ISPP, Martian ISPP based on atmospheric carbon dioxide electrolysis has several advantages. The process is relatively simple and continuous, requiring minimal human intervention once the system is deployed and activated. The feedstock (atmospheric carbon dioxide) is readily available, and the system does not require complex mining or transportation operations.
However, Martian ISPP still faces challenges, particularly in terms of power requirements. The electrolysis process is energy-intensive, and a full-scale system would require a reliable, high-capacity power source. Nuclear power is a likely candidate, but it introduces its own set of challenges and risks.
Another potential approach to Martian ISPP involves the use of water and carbon dioxide as feedstocks for a process involving electrolysis and the Sabatier reaction. While this approach could produce both methane and oxygen propellants, it is more complex than direct carbon dioxide electrolysis and would require the transportation of water from Earth in the near term.
Comparison and Outlook
When comparing lunar and Martian ISPP, it becomes clear that Martian ISPP based on atmospheric carbon dioxide electrolysis has several advantages in terms of simplicity, reliability, and potential return on investment. Lunar ISPP, while potentially valuable in the long term, faces significant technical challenges and uncertainties that may limit its near-term feasibility and cost-effectiveness.
Despite the ongoing Artemis program focused on returning humans to the Moon, NASA may benefit from prioritizing the development of Martian ISPP technologies, particularly SOECs, which can leverage ongoing terrestrial investments in this field. By focusing on a simpler, more mature approach to ISPP, NASA could potentially reduce costs and risks while still making progress toward the ultimate goal of sustainable human presence on Mars.
Summary
In-situ propellant production is a promising concept for reducing the cost and complexity of future human missions to the Moon and Mars. However, the implementation of ISPP systems presents a range of challenges and complexities that must be carefully considered. Lunar ISPP, while potentially valuable in the long term, faces significant technical challenges and uncertainties related to resource extraction, processing, and power requirements. Martian ISPP based on atmospheric carbon dioxide electrolysis, on the other hand, offers a simpler, more reliable approach with potentially higher returns on investment. As NASA plans for the future of human space exploration, a balanced approach that prioritizes the development of key enabling technologies while carefully evaluating the feasibility and cost-effectiveness of different ISPP options will be essential for success.
