Near-term NASA Mars and lunar in situ propellant production: complexity versus simplicity

First, lunar ISPP is analyzed from aspects of lunar resources, near-term lunar processes, carbothermal process, polar ice, and reduction of iron oxides. There are basically 4 potential lunar resources: (1) Silicates in regolith containing typically >40% oxygen. (2) Regolith containing FeO for hydrogen reduction. FeO content may vary from 5% to 14%, leading to recoverable oxygen content in the 1 to 3% range. (3) Imbedded atoms in regolith from solar wind (typically parts per million). (4) Water ice in regolith pores in permanently shadowed craters near the poles (unknown percentage but possibly a few percent in some locations). NASA near-term plans for lunar ISPP appear to be based on H₂ and O₂ propellants. The carbothermal process (see Fig. 1) produces oxygen from lunar regolith. The plan is to have 2 ISPP modules, each operating independently in batch mode during a 7.4-month continuous sunlit period. The original plan called for each module to produce 8 tons of O₂ per year, but a scaled-down version to produce 3.5 tons per year was provided instead. As for polar ice, the author imagines a system in which the excavator/hauler make 1200 trips, delivering 416 kg of water-laden regolith per trip, while the regolith processing station tankers make 37 trips, delivering 275 kg of water per trip. However, it should be noted that nobody has a reliable estimate based on in situ observation. The hydrogen reduction system operates by reducing metal oxides, mainly iron oxide, within the lunar regolith. However, Initial modeling exercises for predicting the overall system mass and power requirements for various oxygen production mass rates using hydrogen reduction which are developed by NASA yield impressively large figures.

Fig. 1. Flowchart for carbothermal process.

Then, Mars ISPP is analyzed from aspects of Mars resources, electrolysis of atmospheric CO₂, reverse water gas synthesis (RWGS), and water-based Mars ISPP. Mars resources for ISPP include (a) the atmosphere containing ~95% CO₂ as an oxygen supply, (b) regolith containing minerals with water of hydration as a source of H₂O, and (c) water ice embedded in near-surface regolith at higher latitudes. The simplest and most straightforward approach to ISPP is electrolyzing CO₂ in the Mars atmosphere, splitting CO₂ into CO and O₂. The system is shown in Fig. 5. It seems likely that NASA could leverage the field of solid oxide electrolysis cell (SOEC) technology with a relatively small investment, by continuing to adapt advances in terrestrial SOEC technology to space applications. As for RWGS, the efficiency is highly influenced by reaction temperature. It remains to be seen how efficient and practical this system will be when further developed. At last, argue that a water-based Mars ISPP is preferred rather than a process produces both CH₄ and O₂, because for the near term with a minimum of complexity, bringing CH₄ to Mars is simpler than carrying water.

Fig. 5. The end-to-end CO₂ electrolysis system.

Finally, power for lunar and Martian ISPPs are discussed. Every form of ISPP is power hungry. Providing power for ISPP on the Moon or Mars is a major challenge. The power requirement for ISPP on Mars is roughly comparable to the power requirement for life support after the crew arrives. Thus, the mass, cost, and logistics of the power system is not attributable to ISPP. By contrast, the power requirements for lunar ISPP far exceed the power requirements for life support, and furthermore, the power dissipated in lunar ISPP is additive to power for life support, so the entire mass, cost, and logistics for lunar ISPP power is attributable to lunar ISPP, reducing the ROI. For Mars ISPP, recent studies concluded that use of solar power might be more feasible than previously thought. Compared to nuclear power, solar power might offer mass advantages. Nevertheless, a plan for use of multiple kilopower reactors appears less risky to us. In addition, recent research on Li-CO₂ batteries shows promise, and CO2 is readily available on Mars. But this does not appear to be near term. Power on the Moon can be derived from nuclear reactors or solar. Current thinking for lunar ISPP seems to be that solar concentrators would be constructed on a crater ridge and beamed down to a receiver within the permanently shadowed regions of the crater where the concentrated solar flux would be partly converted to electric power. A simpler approach might be a kilopower fission reactor and a tether with a copper wire to the water processing unit. An alternative approach is to beam power down from a satellite array. All in all, the author comes to the conclusion that despite the ongoing mission to return to the Moon, NASA might be best off bringing propellants to the Moon from Earth, while pursuing far more feasible Mars ISPP at a moderate level.