Even so, oxygen-extraction processes need to be advanced to secure a source of oxygen for prolonged missions. In addition, each of these techniques requires additional research to develop the most energy-efficient extraction processes. Research should also investigate the possibility of extracting hydrogen from the solar wind.129
Materials synthesis methods that have been proposed can be designed to utilize materials extracted from regolith or the regolith itself.130 It may also be feasible to use materials discarded from prior missions. The separation of oxides into oxygen and metallic elements takes place during oxygen extraction; the metallic elements can be a very important by-product. Silicon can be used for solar cells. Aluminum, titanium, and iron can be used for structural components. In addition, regolith compounds can be the raw materials for combustion synthesis reactions used for the fabrication of strategic components or welding/joining of structural components. Reactive elements, such as aluminum, that are extracted from the regolith or from scrapped spacecraft bodies and components can be used as a fuel for combustion synthesis reactions to fabricate and repair materials and components on the planetary surface. Alternatively, regolith separated into appropriate minerals could be fused into structural construction blocks using a solar concentrator or solar furnace.
The most accessible and abundant source of energy on the surface of the Moon or Mars is solar power. Since there is an abundance of silicate materials in the regolith, silicon-based photovoltaics and solar concentrators will be able to utilize solar energy for power, providing that essential processes, such as extracting silicon from the regolith, can be developed.131 The efficiency of solar cells has increased rapidly in recent years, which has improved their ability to power space missions.132 The current trade-off for space applications is weight for efficiency.133 Efficient solar cells are not lightweight, and lightweight solar cells are not efficient. Investigations into organic photovoltaics are underway in an effort to achieve a better efficiency-to-weight ratio for space applications.134
It may be possible to fabricate materials for high-efficiency, low-operating-temperature solid-oxide fuel cells from the regolith, which would provide a supplementary planetary energy source. Much of the current research on fuel cells is very diverse in regard to the materials being investigated. Many of these systems may be applicable to space exploration missions, but the specific characteristics required for the rigors of space travel are not at present being investigated.135 System performance at low temperatures and pressures should be studied in greater depth in order to produce a fuel cell capable of operating efficiently in the vacuum of space.
There is also a great need to find creative technologies that use the regolith to fabricate and, in some cases, repair components needed for life support, construction, energy generation and storage, forming tools, and other purposes.
The relationship between the processing of a material and the resulting structure is, in many cases, not well understood. For example, many materials are processed using approaches that involve a change in phase, such as a transition from a liquid or vapor to a solid. Density changes and the influence of temperature and concentration on the density of a fluid or vapor lead to buoyancy-driven convection or sedimentation on Earth, which frequently masks processes under study. Thus the study of a wide variety of phenomena related to processing that involves transformation from a liquid or a vapor to a solid can benefit greatly from experiments in reduced gravity. The objective of the experiments and associated ground-based modeling would be to gain a deeper understanding of processes underlying transformations, which would thus improve processes used on Earth.136 A second objective would be to provide insight into the effects of reduced gravity on processes of importance for ISRU applications and materials repair during spaceflight.
Materials Synthesis and Processing
The properties of many materials are directly related to the phases that nucleate from a liquid during processing. Thus, nucleation is an essential, but poorly understood, aspect of materials processing. Microgravity experiments, in which liquids can be held and solidified without a container, thus removing the effects of walls, can shed new light on the nucleation process. In addition, convection due to compositional inhomogeneities that accompany the formation of nuclei can be avoided in microgravity. It is thus possible to cool liquids far below their melting points.