Congressional Testimony of John S. Lewis: Lunar Science & Resources: Future Options
Chairman Rohrabacher, members of Congress, ladies and gentlemen: It is my pleasure to offer some remarks concerning the role of the Moon and its mineral resources in the future pursuit of lunar science and exploration. I shall also address the potential role of lunar resources in support of wider human exploration of the Solar System, and their potential economic importance to Earth.
The resources of nearby space can be exploited for two major purposes; either for local use in space, or for return to Earth. The most immediate utility of lunar materials lies in their use to support manned and unmanned activities on the lunar surface and to facilitate the return of astronauts and scientific samples to Earth. The easiest such scheme to implement would be that requiring the least-complex handling and processing. Even raw, unprocessed lunar surface material can be readily utilized to provide radiation shielding for lunar camps and base modules.
Most lunar resource utilization schemes, however, entail both the beneficiation (extraction and enrichment) of specific ores and their chemical processing. A variety of extraction schemes and target ores have been suggested. It is premature to select the “best” targets for extraction, but it is already clear that as many as a dozen different proposed schemes for extraction of oxygen from lunar minerals may be practical. Oxygen makes up 90% of the mass of the high-performance hydrogen/oxygen propellant combination. Even if hydrogen propellant for use on the Moon were imported from Earth, the energy requirements for delivery of propellants to the Moon would be reduced by a factor of ten. In any return to the Moon, it would be a serious oversight to ignore the great benefits of lunar oxygen production for providing both life-support materials for crews on the Moon and propellants for lunar-surface mobility and return to Earth.
Demonstration, and even practical use, of oxygen extraction technologies need not wait for human presence on the Moon. Lunar-derived propellants can play valuable roles in providing mobility for unmanned missions on the lunar surface (using brief rocket firings to hop over obstacles) and in returning scientific samples to Earth. It is possible to envision small processing units, dealing with kilogram quantities, on automated, unmanned spacecraft carrying out mineral extraction and processing at a low level of complexity. Such an experiment might, for example, react hydrogen gas brought from Earth with lunar minerals containing iron oxides to extract water vapor from them, leaving behind a residue containing high-purity iron metal. Electricity generated from sunlight by means of photovoltaic cells would then be used to separate water into hydrogen and oxygen by electrolysis. The hydrogen would be recycled, and the oxygen accumulated for use. Successful demonstration of this process would encourage scaling the equipment up to ton quantities to serve the need of a manned expedition or base, and would as a bonus permit recovery of ton quantities of ultra-pure iron metal from the extraction residue. That metal, which exhibits the strength and corrosion resistance of stainless steel, could be used for fabrication of beams and girders, nuts and bolts, wire and cables, and even the shells of habitat modules. Technologies for the extraction and purification of iron could be based on the gaseous carbonyl (Mond) process, which has over a century of industrial use on Earth.
One especially interesting lunar resource is the sunlight that impinges on its surface. Prof. David Criswell of the University of Houston has proposed that solar cell “farms” deployed on the lunar surface could collect vast amounts of solar power, convert that power into electricity, and beam that power back to Earth as microwave beams. Criswell argues that the installation cost of such a system could be slashed dramatically by fabricating its principal components on the Moon from lunar materials. A variety of chemical schemes for manufacturing solar cells, wire, and other system components have already been explored, and small-scale testing of these processes could be started at an early date. It is my opinion that the brightest prospect for profitable export of any commodity from the Moon to Earth is power from such a Lunar Solar Power Station.
Export of actual lunar materials for use elsewhere, especially on Earth, has rarely been suggested because of two major deterring factors. First, the overall composition of the lunar surface is strikingly similar to that of the slag discarded in metal smelting operations on Earth. Few native lunar materials may be of sufficient abundance, accessibility, and value to merit their extraction and export. Second, the gravity field of the Moon, although substantially less than that of Earth (an escape velocity of about a quarter of Earth’s and a surface gravity about one sixth of Earth’s) is still quite substantial. Given the propulsion requirements for escape from the Moon and return to Earth, and the complete absence of ready-to-use propellants on the Moon, the cost of retrieval of lunar materials is certain to be very high, rendering the return of almost any lunar-derived product to Earth prohibitively expensive. Of the materials known or suspected to exist on the Moon, only one appears to offer any hope of economic benefit: that is the isotope helium-3, a potential fuel for fusion reactors. I have reviewed Prof. Swindle’s testimony on this subject and concur with his assessment that many questions (such as the actual abundance, distribution, and recoverability of helium-3 on the Moon and the feasibility of commercial fusion reactors) need to be answered before we can conclude that helium-3 extraction from the Moon is economically sensible. A renewed program of lunar exploration must address these scientific and technical unknowns. Much of the needed research can be done by unmanned missions.
A second lunar resource, ice from permanently deep-frozen crater bottoms near the lunar poles, has also sometimes been suggested as appropriate for export either to the lunar equator or to space stations or vehicles off the Moon. I regard this suggestion with deep skepticism because of the immense technical difficulty of mining steel-hard and highly abrasive permafrost under conditions of permanent darkness, at the bottom of steep and rugged craters, at temperatures so low that most metals in the mining equipment are as brittle as glass. Further, the location of the hydrogen-bearing deposits (almost certainly dominated by water ice) at the poles is the most remote from sensible locations for a lunar base of any place on the Moon.
The lunar ice deposits are of great scientific interest for the stories they can tell about comet and asteroid bombardment of the lunar surface. Scientific investigation of these deposits need not, and arguably should not, involve human presence. With such composition data in hand, and with a greatly improved knowledge of the extent, concentration, and purity of the lunar ice, a more realistic assessment of the utility of these deposits could be made.
Specifically, the use of lunar-derived propellants, whether oxygen extracted from iron-bearing minerals such as ilmenite and olivine or hydrogen and oxygen made from polar ice, to support expeditions to Mars makes no logistic sense. The Moon is not “between” Earth and Mars; it is a different destination, poorly suited to function as a support base for travel to Mars. Water extraction from the martian moons Phobos and Deimos or from near-Earth asteroids may offer great advantages to Mars-bound expeditions, more profound than lunar water could even if the Moon had no gravity to fight. In any location, development of extraction and fabrication technologies should, like low-cost space launch services, be conducted as a commercial endeavor.
There are clear advantages to the use of lunar resources in support of both manned and unmanned activities on the Moon. Direct benefits to Earth from lunar resource exploitation are certainly conceivable, but will remain conjectural until substantial further research on in situ fabrication of solar cells and on the abundance and distribution of helium-3 and polar ice has been done on the Moon.
