by Wesley Johnson, NASA Glenn Research Center, firstname.lastname@example.org
One of the key philosophies for sustained manned exploration is In-Situ Resource Utilization, first mentioned within a Cold Facts article  where I explored possible cryogenic uses in space exploration. ISRU has expanded further and today involves hardware and operations that harness and utilize local in-situ resources to create products and services for robotic and human exploration. A large percentage of the mass needed for any transportation system is fuel and oxidizer, making propellant available at the exploration site a good starting place for the use of resources. The topic was explored within ISRU research, and the moon and Mars remain the initial two bodies in discussion, especially in regards to manned space exploration missions.
Each of these surfaces has two primary methods for producing rocket propellant, one that is unique to a given surface and another that’s held in common. The options include extraction of oxygen from the Martian atmosphere, extraction of oxygen from lunar regolith (soil minus the organic compounds that we have on Earth) and extraction of water from water-ice deposits on either surface.
Method on Mars
Extraction of oxygen from the Martian atmosphere is theoretically the easiest method, simply because gas is the only input and it can be freely delivered to a reactor. On Mars, carbon dioxide is the main component of the atmosphere, in this case greater than 95 percent by mass.
Simple electrolysis of carbon dioxide yields carbon monoxide and oxygen, with oxygen being essential for any transportation element. Oxygen generally makes up between 65 and 80 percent of the combined mass of fuel and oxidizer in a reaction. Within Earth’s atmosphere, most vehicles pull oxygen out of the air and only carry fuel, but this would not be possible on Mars since it doesn’t have free oxygen. Oxygen there will probably have to be liquefied for more compact storage and could be used for things other than rocket propellant, including oxygen for breathable air, rover propellant or a power source. The fuel for this type of architecture must be carried up since only oxygen is being generated.
The upcoming Mars Oxygen In-Situ Resource Utilization Experiment—or MOXIE—will demonstrate the production, but not liquefaction, of oxygen from the Mars atmosphere. It is currently scheduled for launch on the NASA rover being prepared for 2020. The Martian atmosphere also contains significant amounts of nitrogen and argon that could be collected if uses were found for them.
Method on the Moon
The regolith is a good place to start examining oxygen production on the moon. I like to say that the moon is a big rust ball. Mars is too, though on the moon most of the ground-up powder we call regolith is made of oxidized metals such as iron-oxide, aluminum-oxide and silicon-oxides, among others.
By taking advantage of the concentration of oxidized metals, processes such as hydrogen reduction can be performed on the powdery soil to produce oxygen. This process generally takes place at much higher temperatures than electrolysis, and requires some sort of mechanism on the reactor to feed and drain the regolith at a reasonable rate. For instance, processing a football field-sized area that is eight centimeters deep could perhaps produce around 10 metric tons of oxygen. This technique could start annual production that could reasonably fuel a human lander.
Method for Both
It is known that water is present in various underground glaciers on Mars and trapped at the bottom of craters on the poles of the moon, although the quantity is not currently well known. This water could be mined and electrolyzed into hydrogen and oxygen constituents that could then be used as oxidizer and fuel for any spacecraft.
On the moon, it would simply be a matter of producing hydrogen and oxygen propellants. On Mars, however, the electrolysis could be combined with a Sabatier reaction, using carbon dioxide from the atmosphere along with water mined to produce methane fuel along with excess oxygen. The decision on whether to do this is directly related to the type of fuel needed by the architecture and the amount of power that can be provided.
Hydrogen produces more energy and requires lower production rates, but it requires more power to liquefy than methane. American-designed rockets tend to use five to six times more oxygen by mass than hydrogen. However, the stoichiometric ratio between them is eight times more oxygen by mass. Thus, for water electrolysis, there will be leftover oxygen that can either be vented or used for another purpose.
Significant uncertainties remain no matter what method is chosen. All of the processes require large quantities of power, something that is especially challenging during long lunar nights (or 14 Earth days). This has generated considerable interest in the lunar poles, where some places stay light all the time and thus require fewer batteries when using solar power. Nuclear power is another possible solution that NASA has been investigating, but flying significant sized nuclear reactors will have to address public safety concerns. Additionally, specifications will need to be created for the fluids that are produced, especially concerning contaminates that may be unique to each location and require special purification and cleaning of the processing plant.
Once the rocket propellant has been produced, it could be used to refuel landers to launch again, or to fill propellant depots (gas stations) in various orbits that could be used to refuel passing spacecraft heading to other destinations. Either way, the production and use of oxygen (and possibly fuel) is an interesting first step in using resources that can be found locally on nearby celestial bodies. Once this first step is completed, it will be pivotal in allowing companies to investigate other uses of these local resources. ■
 T. Polsgrove, et al., “Mars Ascent Vehicle Design for Human Exploration,” American Institute of Aeronautics and Astronauctics, 2015.
 G.B. Sanders, “Comparison of Lunar and Mars In-Situ Resource Utilization for Future Robotic and Human Missions,” presented at the 49th AIAA Aerospace Sciences Meeting, 2011.
 https://www.lpi.usra.edu/publications/books/lunar_stratigraphy/chapter_6.pdf, Table 6.2
 S. Li, et al., “Direct Evidence of Surface Exposed Water Ice in the Lunar Polar Regions,” Proceedings of the National Academy of Sciences, 2018.