First, authors briefly concluded the natural resources of the Moon. The main natural resources of the Moon include mineral resources, water/ice resources, volatiles, and solar energy. Minerals of the lunar crust can be categorized as silicate minerals, oxide minerals, native metals, sulfide minerals, and phosphate minerals, as represented in currently collected samples from Apollo and Chang’E missions. Among these, silicate minerals are the most abundant components, making up more than 90% of the volume of most lunar rocks; oxide minerals are the second most abundant, which are particularly concentrated in mare basalts, which may account for up to 20% of the volume of these rocks; several native metal minerals have been identified including nickel, copper, kamacite, molybdenum, Cr, cerium, rhenium, and zinc. Most of the water resources, except for the very few volatile (OH/H2O) resources that have been found, are in the form of mixed water ice in the polar lunar soils, where the total mass of water contained within the uppermost meter of permanently shadowed regions (PSRs) was approximately about 2.9 × 1012 kg. As for the volatiles implanted by solar wind in the lunar regolith, the 3He isotope, which can be used for nuclear fusion reactors to produce electrical power, is very rare in lunar soils. The radiant energy flux contributes an average of about 1.1×1013 ergs/cm2 per year to the lunar surface; in some polar regions, nonpermanently shaded areas can provide an unlimited supply of solar energy.
Then, authors review the ISRU techniques on the Moon.
As for the in situ water replenishment, there are two methods. (1) In the thermal extraction of water from icy lunar soils, the ices in the frozen soils were heated to sublimate or evaporate, and then the water vapor was captured within the tent and collected by cold traps. (2) In the water production by hydrogen reduction of lunar soils, the main reaction materials are the ilmenite (FeTiO3) in the lunar soils and the basic reaction principle is FeTiO3 (s) + H2 (g) → Fe (s) + TiO2 (s) + H2O (g). The water extraction method would be more suitable for lunar bases in the polar regions, while the water production method would have a wider application prospect due to the widespread of lunar soils.
As for the in situ oxygen preparation methods, there also are two different methods. (1) In the electrolytic reduction method, metal elements in the lunar regolith are deposited at the cathode, while oxygen arises at the anode, via molten regolith electrolysis or molten salt electrolysis. (2) The vacuum thermal decomposition method directly heats the raw materials under vacuum conditions to decompose the oxide in lunar regolith into oxygen elements and other constituent elements. However, the electrolytic reduction method has high requirements for the inert electrode or requires the replenishment of fresh salt from Earth, while the vacuum thermal decomposition method requires heat-resisting material to sustain the reaction.
As for refining silicon, metal, and fiber from lunar soils, various methods have been investigated on Earth, such as (1) heating the lunar regolith in the presence of reductants or the typically carbothermic reduction of silicates at the heating temperature of about 2,000 °C for silicon refining, (2) Carbothermal reduction, hydrogen reduction high-temperature molten salt electrolysis, and vacuum thermal decomposition for metal refining, and (3) a 2-step method for the preparation of continuous lunar soil fibers. However, there are still several technical problems to be solved before the in situ application of these technologies on the Moon.
As for the in situ lunar construction techniques, there are two kinds of means, according to the different construction ways and materials. (1) Additive manufacturing techniques of lunar soils, is referred to the process of joining materials to make objects from 3D models. At present, various proofs of concept combined with the 3D printing technology and lunar regolith as raw materials have been proposed, such as contour crafting (selected by NASA) and D-Shape method (developed by ESA). (2) In the sintering techniques of lunar soils, the sintering process heats the fine grains of metallic or ceramic material to the melting temperature to form a fixed shape. In this process, microwave and concentrated solar light heating methods have been developed for lunar soil sintering, respectively.
As for the in situ energy conversion and utilization techniques, various schemes have been proposed for the energy utilization of lunar regolith, and numerical simulation and experimental research are implemented to optimize the schemes. The key issues to further promote the development of lunar energy utilization include (a) the process of regolith to improve its heat storage property, (b) the optimization of the material parameters to improve the heat-to-electricity conversion property, and (c) the engineering design of the energy utilization payload adapted to the extreme environment of the lunar surface.
As for artificial ecosystem for lunar life support, there have been varied related research. In China, the China Astronaut Research and Training Center established a small ecosystem in 2011 and completed a short-term crewed closed experiment with 2 people for 30 days. Later, a large Earth-based experimental facility was built with the cooperation of other institutes, and then they conducted the “Space 180” experiment in 2016, involving 4 crew members, lasting 180 days. Moreover, series of experiments of plant units were implemented in the “Lunar Palace 1” and “Lunar Palace 365”.
Ultimately, authors propose a 3-step development plan for lunar ISRU technologies in the next decade. Recommendations of roadmaps for future ISRU technology on the Moon are shown in Fig. 13. In the first stage, basic scientific problems, technical issues, and technological solutions relating to the lunar ISRU should be sufficiently investigated. In addition, on this basis, the exploration and usability assessment of mineral resources should be first completed before 2027. The second stage, it will focus on the development of experimental payloads that can adapt to the lunar environment. Then, the in situ experiments on lunar surfaces could be directly carried out by payload experiments of lunar exploration missions around 2030. In the last stage, the facilities for lunar ISRU would be constructed, and the pilot-scale replenishment of survival matters for medium-term lunar surface residence and scientific activities would be realized around 2035. Finally, around 2040, large-scale replenishment of survival matters for long-term lunar missions and future Martian missions will be realized. In this way, the cost, reliability, and security of future lunar exploration missions and other deep-space missions will be improved.
