Space Power Resources, Manufacturing and Development Volume 10 Number 1 1991
SPACE POWER Published under the auspices of the Council for Social and Economic Studies EDITOR Andrew Hall Cutler, NASA Space Engineering Center, The University of Arizona ASSOCIATE EDITORS Roger A. Binot, European Space Agency, The Netherlands Eleanor A. Blakely, Lawrence Berkeley Laboratory, USA Richard Boudreault, Consultant, Montreal, Canada Lars Broman, SERC, Sweden Gay Canough, Extraterrestrial Materials, Inc., USA Lucien Deschamps, Paris, France Ben Finney, University of Hawaii, USA Josef Gitelson, Academy of Sciences, USSR Peter Glaser, Arthur D. Little, Inc., USA Praveen K. Jain, Northern Telecomm, Ottawa, Ont., Canada Dieter Kassing, ESTEC, The Netherlands Mikhail Ya. Marov, Academy of Sciences, USSR Gregg Maryniak, Space Studies Institute, USA Michael Mautner, University of Canterbury, New Zealand Makoto Nagatomo, ISAS, Japan Mark Nelson, Institute of Ecotechnics, USA John R. Page, University of New South Wales, Australia Geoffrey Pardoe, Brunel Science Park, UK Tanya Sienko, NASDA, Tsukuba, Japan Ray A. Williamson, OTA/US Congress, USA Space Power is a quarterly, international journal for the presentation, discussion and analysis of advanced concepts, initial treatments and ground-breaking basic research on the technical, economic and societal aspects of: large-scale spaced-based solar power, space resource utilization, space manufacturing, space colonization, and other areas related to the development and use of space for the benefit of humanity. Papers should be of general and lasting interest and should be written so as to make them accessible to technically educated professionals who may not have worked in the specific area discussed in the paper. Editorial and opinion pieces of approximately one journal page in length will occasionally be considered if they are well argued and pertinent to the content of the journal. Submissions should represent the original work of the authors and should not have appeared elsewhere in substantially the same form. Proposals for review papers are encouraged and will be considered by the Editor on an individual basis. Editorial Correspondence: Dr. Andrew Hall Cutler can be reached by telephone at (602) 322- 2997, by Facsimile at (602) 326-0938 and by mail at 4717 East Fort Lowell, Tucson, AZ 85712, USA. Dr. Cutler should be consulted to discuss the appropriateness of a given paper or topic for publication in the journal, or to submit papers to it. Questions and suggestions about editorial policy, scope and criteria should initially be directed to him, although they may be passed on to an Associate Editor. Details concerning the preparation and submission of manuscripts can be found on the inside back cover of each issue. Business correspondence including orders and remittances for subscriptions, advertisements, back numbers and offprints, should be addressed to the publisher: The Council for Social and Economic Studies, 6861 Elm Street, Suite 4H, McLean, Virginia 22101. The journal is published in four issues which constitute one volume. An annual index and titlepage is bound in the December issue. ISSN 0883-6272 © 1991, SUNSAT Energy Council
SPACE POWER Volume 10, Number 1, 1991 Stephen L. Gillett. Thoughts of an Economic Geologist 3 Lars Broman. On the Possibility of Space Generated Solar Electricity for the Antarctic 19 Andrew Hall Cutler. Comment on the Possibility of Space Generated Solar Electricity for the Antarctic 21 K. P. Bogus, G. Dudley, J. Haines, D. Kassing & D. O'Sullivan. The Space Power Programme of the European Space Agency 23 Louis O. Cropp, Donald R. Gallup & Albert C. Marshall. Mass and Performance Estimates for 5 to 1000 kW(e) Nuclear Reactor Power Systems for Space Applications 43 F. Carre, E. Proust, S. Chaudoume, P. Keirle, Z. Tilliette & B. Vrillon. Overview of CNES-CEA Joint Programme on Space Nuclear Brayton Systems 79 R. J. de Young, M. D. Williams, G. H. Walker, G. L. Schuster & J. H. Lee. A Lunar Rover Powered by an Orbiting Laser Diode Array 103
Dr. Terry Triffet Space Engineering Research Center 4717 East Fort Lowell Rd. / Tucson, Arizona 85712 University of Arizona / NASA Space Engineering Research Center for the Utilization of Local Planetary Resources Extraterrestrial Resources: Stepping Stones to the Stars Innovations in space technology are needed if our dream of settling other planets in the solar system is to become reality. The idea of "mining" and processing resources on another planet to support man's presence there and his travels in the solar system may help make this dream possible. The UA/NASA Space Engineering Research Center, a national center for space engineering, research, and educaton, seeks out the means to make habitation on Earth's moon and other planets affordable by using the resources found there. Research at the Center focuses on making useful products—water, fuels, building materials—from materials (and energy) that occur naturally in near-Earth space. Offering numerous research opportunities for students interested in the space resources field, the Center also serves as a meeting ground where government agencies and private sector companies can create strategies for establishing industries in space, and develop the hardware to do the job in the severe environments near-Earth planetary bodies present. Research at the Center emphasizes: * Investigating methods of producing oxygen and hydrogen for rocket propellant and other products from Lunar and Martian materials ♦ Developing artificially intelligent control and communication systems for remote materials-processing plants designed to operate autonomously ♦ Defining the conditions necessary to optimize the production of such materials while minimizing energy consumption ♦ Searching the skies and tracking near-Earth asteroids that may also provide valuable resources for space exploration and development Student Research: The Center offers abundant research opportunity, in which both graduate and undergraduate students participate extensively, as well as course work in numerous engineering and science departments. Ongoing areas of student research include: ♦ Fuel for space exploration. The Center is exploring ways to extract oxygen from materials found on the Moon, Mars, and asteroids that could then be used in the production of chemical propellants for spacecraft. ♦ Oxygen from Carbon Dioxide. The Center has produced a pilot plant engineering demonstration system: oxygen production from a simulated Martian atmosphere. ♦ Building Materials. After Lunar soil has been mined for its oxygen, for example, it can be processed into bricks, beams, metals and ceramic/composite materials from which habitable structures and a variety of useful products can be constructed. Education Programs: Students at the Center are also actively involved in reaching out to the surrounding community, providing elementary, middle, and high school students the opportunity to work on its experiments, a hands-on approach to student involvement directed toward attracting the nation's youth to technical careers. ■ Publications: The Center provides information services, at no charge, to interested individuals and organization. For a complete list of available publications and further infrirmatinn nn ontor rnntoot'
Lunar Resources: Thoughts of an Economic Geologist STEPHEN L. GILLETT SUMMARY From terrestrial experience, the most important characteristics of a potential ore deposit are its grade and tonnage; i.e., the degree of concentration of the desired material, and the total amount of the material. Despite - even because of - the high cost of access to space, these characteristics are likely to be extremely important for space-derived resources as well, because such resources, at least in the near term, will be high volume commodities. Hence, it will probably be cost-effective to do a great deal of additional investigation of the Moon before committing to a specific, large-scale mining process. Local concentrations of common lunar elements are highly probable, and even anomalous concentrations of rare elements cannot be ruled out. Introduction It has long been noted that because of the high cost of access to space from Earth, other sources of raw materialswill be necessary for construction of extensive space facilities [e.g., Lewis & Lewis, 1987], The Moon is an obvious source of extraterrestrial raw materials: it is always nearby, it has a shallow gravity well, it is airless, and its geologic framework is already roughly known. Much speculation about lunar resources, however, appears to lack the perspective developed from centuries of exploring for resources on Earth. For example, the observation that average lunar regolith is about 75% "demandite" [Waldron & Criswell, 1982] is merely equivalent to stating that the Moon is made of common rocks. Such rocks are not mined on the Earth for their elements; the economics of such a separation would be horrendous, and this will almost certainly be true on the Moon as well. Indeed, terrestrial ore materials are not split into all their constituents; to be sure, some by-products are recovered, but even so the cost of extracting all elements remains prohibitive. Such other elements are more cheaply won from their own ores. The concept of obtaining all resources from common rocks also overlooks the value of a planet-sized body as a resource base. The welter of natural processes on a planet-sized body ensures that in rare cases a fortuitous combinations of circumstances will have caused anomalous concentrations of valuable elements that can be exploited - and as will be seen, the concentration of a desired material is one of the most vital parameters in determining whether a deposit is economic. To be sure, the Moon does not have the vast array of on-going geologic processes that the Earth does, in particular those in which water is involved. Nonetheless, the Moon is a good deal more complicated than is generally recognized in the non-specialist MacKay School of Mines, University of Nevada, Reno.
literature; common elements are certainly significantly more concentrated in some places, and even deposits of rare elements cannot be excluded on the basis of present knowledge [e.g., Gillett, 1983, 1990; Haskin, 1983; Binder, 1988], In this paper, I will first review what makes a deposit mineable on the Earth; then I will apply these constraints to the Moon and discuss their implications. Fundamental Constraints An "ore" is any natural material from which a substance, usually a metal, can be won at a profit. It thus is an economic term only; mode of formation, mineralogy or chemistry are irrelevant. Three factors enter into whether a geologic deposit is an ore: (1) Concentration and amount (or "grade" and "tonnage", as the mining industry terms them). How much of the desirable element is there, and how much other stuff is it dispersed in? In a general way, of course, this double constraint is obvious; but it seems to be not so well recognized how vital the degree of concentration is. (2) Contrast of properties. This constraint is often overlooked by non-geologists: how easily (i.e., cheaply) the desired element(s) are separated depends strongly on their chemical and physical state in the prospective ore material. Partly, but only partly, this is a question of the energy needed to break the chemical bonds that must be broken. In fact, physical processes ("mineral dressing") are generally used first to concentrate ("beneficiate") the ore mineral(s) before any chemical separation is carried out, because physical processes are much cheaper. Thus, ease of separation also strongly depends on any contrast in properties between the ore mineral(s) and non-ore minerals ("gangue") in the prospective ore, the grain sizes of the ore mineral(s), and the chemical state of the ore minerals themselves. Last, the very toughness of the rock - how difficult it is to crush - is a major cost determinant. (3) Location. This constraint is again obvious, but paradoxically, it is somewhat less important than commonly thought. To be sure, ceteris paribus, the more convenient to markets and transportation infrastructure, the more valuable a potential ore deposit is, and certainly the overwhelming cost of transport to LEO is the major motivation for the use of extraterrestrial resources. Nonetheless, this cost is a two-edged sword, because it makes the capital and maintenance costs for resource extraction also extremely high. Each of these general constraints, and the trade-offs between them, is discussed in detail below. Grade and Tonnage Almost anything, if large enough, convenient enough, and pure enough, can be a valuable resource. Limestone, for example, is an abundant rock type on the Earth's surface. It is also in demand by the megaton for concrete manufacture: limestone is largely calcite, CaCO, and the carbonate is pyrolyzed in a kiln to lime:
which is an essential raw material for Portland cement. Despite its ubiquity, though, not just any limestone deposit will do; the purity of the deposit is extremely important. Lime to be used in cement manufacture must contain little magnesia (MgO) [e.g., Boynton, 1966, p. 98], but the double carbonate dolomite (Mg3Ca(CO3)) is a ubiquitous impurity in limestone, and indeed commonly a rock-forming mineral in its own right. Other common impurities such as phosphates, fluorite, and minerals containing heavy metals can also severely affect the quality of the cement [e.g., Bye, 1983, p. 16], Economic separation of such impurities is impossible, particularly at the scale that would be required. We thus arrive at the somewhat paradoxical result that the purity of a deposit of a substance that is both common and required in high volume is especially important! This is an important point that will be stressed again and again. Another example is elemental Si, for manufacture of IC chips. Although Si is the second most common element in Earth's crust, and silicates are dominant components of most rock types, Si is not recovered from just any rock; relatively rare deposits of nearly pure silica (SiO2, most commonly occurring as the mineral quartz) are exploited instead. The salient point is that separation of a desired element or compound is expensive. Not only are capital costs high, but maintenance costs are extremely high. Rocks are hard, and handling them in volume is rough on machinery, particularly in the enormous volumes necessary for resource extraction. It is highly cost-effective to let Mother Nature do as much of the separation as possible, and hence it is highly cost-effective to invest a great deal in seeking high-concentration deposits first. Searching for mineable deposits is called "exploration" for a reason! Obviously, the necessary concentration of a desired element for an economic deposit is a function of the element's value. As examples we may take iron versus gold. Traditionally, iron ores are essentially pure oxides, either magnetite (Fe2O2) or hematite (Fe2O2). These are reduced with C (charcoal or coke) at high temperature to yield Fe metal and CO, a process whose basic form goes back to antiquity. The weight percentage Fe in such ores is about 70%. Since World War II, lower-grade rocks containing iron silicates - "taconite" - have been exploited by the development of new beneficiation procedures that first separate the Fe oxides from the silicates [e.g., Park & MacDiarmid, 1970, p. 405]. Still, an iron ore has —30% Fe and an enormous volume. By contrast, gold ores can contain only a few parts per million Au [e.g., Percival et al., 1988]; the high value of the gold, of course, makes such low concentrations economic. Other metals occurring with the gold may - or may not - be economically recoverable as by-products. In the Fortitude deposit near Battle Mountain, Nevada, for example, Cu is not economically recoverable although it is much more abundant than gold in the deposit [Wotruba et al., 1988]. It is commonly pointed out that, because of the high costs of transport to space,
even common elements on the Moon are "worth more than their weight in gold." This is misleading. As discussed at length below, these high transport costs also apply to any capital and maintenance equipment required to put those elements in a useful form. Hence the high cost cut both ways, so that a large concentration of the desired element in potential lunar raw materials remains extremely important. Intermediate cases exist, of course; but these examples nevertheless indicate that the sources for high-volume commodities must be as pure as possible. Such commodities, of course, are the ones required for space development. Contrast Not only must the desired element be sufficiently concentrated, it must be amenable to extraction. The nature of the chemical and physical state of the potential ore material versus the host is another fundamental determinant of separation costs. For example, many base metals, although relatively rare, occur as sulfides. Commonly these sulfides are dispersed - "disseminated" - in a much larger volume of silicate rock. Examples of disseminated sulfide deposits arc "porphyry copper" bodies, the source of much of the world's Cu. Such a deposit consists of a granitic igneous body in which copper and other sulfides are dispersed, mostly in the upper part. In the main part of the orebody, copper typically amounts to ~0.4% [e.g., Gordon et al., 1987, p. 38], To mine such a deposit, the rock is mined in bulk and crushed; then after crushing, the sulfide grains are easily separated from the silicate gangue by froth flotation. That is, the contrast in physical properties between sulfide and silicate permits their efficient separation. Were the Cu dispersed instead in a separate sulfide mineral, it probably could not be extracted profitably because the physical properties would be too similar for efficient beneficiation. Chrysocolla (a copper silicate) ores, for example, must be leached. Another example is the "Carlin-type" gold deposits of Nevada. Typical "sulfide ore" in such deposits consists of disseminated sulfides and minor arsenides, antimonides, etc., in which a trace of gold is present. Parts of such orebodies typically contain "oxidized ore" zones, however, and these are more valuable because they are easier to work with (Percival et al., 1988], In these zones, the sulfides have been oxidized by subsequent geologic events. As gold has low affinity for oxygen, the oxidation has caused the gold to segregate into microscopic grains of native metal. The concentration of gold is the same, but it is now easier to process. (Even so, new technology still had to be developed, as the native gold grains are far too small to be recovered with conventional extraction processes). The costs of winning an element from its ore are, of course, ultimately constrained by its oxidation potential, which determines the minimum energy that must be expended. For certain metals such as Al this is indeed a major part of the cost. Indeed, of the common metals in the crust, only Fe has been known from antiquity, because it has by far the lowest oxidation potential; hence it can be
reduced with primitive technology by using carbon. As discussed, though, even Fe is not separated from ordinary rock but from ore (Other long-known metals, such as Cu, are easier to reduce but rare. Due to their chemistry, however, they become concentrated into sulfide deposits - yet another example of the value of natural concentration processes in a planet!). In addition, however, the costs of crushing the ore to separate the ore mineral in the first place is always a major part of the cost! Many people, including the present author, have commented that the pervasive comminution of the lunar regolith by meteorite bombardment is an advantage for mining operations in that, at least initially, no such rock crushing will be required. However, this "natural milling" is compromised by (a) the poor size sorting in the regolith, and (b) the pervasiveness of impact-generated glass. The latter is more important, because it complicates beneficiation: small, heterogeneous grains become cemented together by glass, so that a clean separation becomes more difficult. The smaller and rarer the grains, the more such glass cementation will be a problem; much native Fe, for example, is intimately associated with silicate glass [Morris, 1980]. It's appropriate here to digress on the structure and composition of common rocks. As already mentioned, most rocks consist largely of silicate minerals, and silicates are difficult to deal with. In part this is due to the high energy of the Si-O bond, and in part to the high oxidation potential of the metals commonly found in silicates. High oxidation potential is only part of the problem, however. Silicates are in general "giant molecule" structures based on a tetrahedron of four oxygen atoms surrounding a single Si atom. These tetrahedra can share vertices to make complex ring (beryl, tourmaline), chain (pyroxenes, amphiboles), and sheet structures (micas, clay minerals), and even 3-dimensional networks (silica minerals, feldspars, feldspathoids, zeolites). To preserve charge balance, metal ions fit into interstices in such silicate frameworks. (In the 3-D frameworks, replacement of Si atoms by Al gives the structure a net negative change, which is balanced by included metal ions. A fully polymerized 3-D structure with no Al substitution is just silica, SiO2). A further complication is that the identity of the interstitial metal ions is not critical; if nearly the same size, many can substitute free for each other. Ferrous iron (Fe2) and Mg2 commonly substitute for one another in geologic materials, for example. Such substitution introduces a great deal of compositional variety into silicate minerals. Thus, extracting elements from silicates is hampered not just by the high energy of the metal-oxygen bonds but by the polymerization of the silicate backbone; for sheer kinetic reasons, the molecules resist being broken up. Polymerization persists even in molten silicates; the viscosity of lavas is largely a consequence of their silica content, which determines their degree of polymerization: the more silica, the more viscous. It is hardly surprising, then, that raw silicates are seldom economic sources of elements on Earth, even though they are the reservoir of most material in the planet's crust. Only a few rare metals (e.g., Li, Be, Zr) are routinely extracted from silicates. Furthermore, beneficiation of such silicate ores is difficult because
the properties of the silicate ore mineral differ little from those of the silicate gangue. It is informative to contrast the terrestrial sources of two common metals with high oxidation potential: Al and Mg, the third and eighth most common elements in the crust, respectively. Both are essential constituents of many minerals, including some extremely common rock-forming silicates, and thus they are abundant in ordinary rock. Neither is recovered from ordinary rock, however. Aluminum ore is bauxite, a mixture of hydrated aluminum oxides that occurs as a soil in certain humid tropical areas. In such areas, abundant rainfall and warm temperatures cause extremely pervasive and deep surface weathering. Under such conditions, common rocks are ultimately rendered down to their least soluble constituent: aluminum. Other metals are removed as ions in solution, and even the silicate framework is eventually broken down, with the silica also leached away. Here again, processes on the Earth have concentrated Al naturally, and it turns out to be vastly more cost-effective to seek out those concentrated deposits rather than to mine ordinary rock. Bauxite is an iron-free form of laterite; if iron is also present in the original rocks, it becomes oxidized to the ferric state and remains behind as well, since it's also insoluble. (Such ferric iron accounts for the deep red color of most tropical soils.) To highlight a point of this paper, the reader should note that laterite, which contains both Fe and Al, is an economic source of neither. (Locally, however, aluminum-free laterites can be iron ores.) Although it is also abundant in silicates in ordinary rock, most magnesium is recovered from salt deposits or seawater. In either case, Mg in aqueous solution is concentrated and precipitated as the chloride, which is then electrolyzed. In this case, the ease of handling magnesium ion in solution has proved to be the critical economic factor. A little Mg is still made by reduction of the oxide with ferrosilicon, but the oxide is derived by calcination of dolomite, not from a silicate [e.g., Rosenqvist, 1983], Location The location of a potentially economic deposit is important, but typically less so than the constraints above. We can broadly divide economic deposits into "opportunistic" vs. "logistic" deposits, depending on the increasing importance of location. Like all such divisions, this one is not hard and fast but is nonetheless useful. Opportunistic Opportunistic deposits include those whose sheer size is so overwhelming that they are worth mining despite the extensive transport and infrastructure costs. This category also contains those commodities whose value is so great that they are worth seeking for themselves. One example is Au, as shown by the "gold rushes" from antiquity to present-day Nevada. The metal is sufficiently valuable that it's worth
seeking out even in remote areas. (Even here, though, technology has been a factor; the deposits targeted by the 1849 California gold rush contained native metal in macroscopic amounts, which could be extracted by extremely primitive means. By contrast, Carlin-type gold in Nevada requires an extremely expensive crushing and extraction technology.) Logistic These are commodities for which transport costs are high, and thus nearby locations have a decided advantage. They are bulk commodities: they have very high volume and low unit cost, so that the tradeoffs of transportation costs versus quality become significant. Little beneficiation or extraction is also required. One example is aggregate for making concrete. For large projects, such as roads or dams, local sources of aggregate are generally found and used. Even then they must be tested and evaluated first. In modern times, no metals on Earth fall into the "logistic" category; it is always cheaper to ship metal rather than mine it locally. There is some historical precedent for local extraction of bulk metal, however, when transportation was still extremely expensive. For example, low-grade siderite (FeCO,) deposits, worked on a small scale by hand, furnished much of America's iron in Colonial times [e.g., Park & MacDiarmid, 1970, p. 407]. They are not now economic because they are far too small and low-grade. It also should be noted that even in ancient times, it was worthwhile bringing rare metal long distances; e.g., Sn from Britain to the eastern Mediterranean. Finally, it should be noted that when logistic deposits are worked, only the high-volume material that requires little processing is extracted; the deposits are not separated into all their constituents. Intermediate cases exist, of course; a porphyry copper deposit that might be economic in Chile, where a mining and transportation infrastructure already exists, and where climatic constraints arc moderate, may not be economic in the Canadian Arctic Archipelago. Nonetheless, for metals the grade and tonnage of a deposit are usually the most important economic considerations. Resources for space development obviously fall into the "logistic" category. At least in the near term, space resources will not be the subject of opportunistic exploitation in the sense of being exported to Earth, except conceivably precious metals from asteroidal nickel-iron [e.g., Kuck, 1979; Lewis & Nozette, 1983]. This immediately implies that (1) near-term space resources will be high-volume commodities; and (2) they should require absolutely minimum processing - i.e., their grade should be as high as possible. Implications for Lunar Resource Development What has all this to do with the Moon? I believe that such constraints as described above will apply to space resources, despite the very high access costs. Such costs also drive the cost of the capital investment - and the maintenance costs! - necessary to extract lunar resources, so to a significant degree the costs are self
limiting. As it becomes cheaper to lift material into LEO, lunar sources of low-volume elements that require significant capital investment for their extraction become much less attractive. For the Moon, these constraints imply: (1) Initial space-based resources will be high-volume commodities. (2) Concentration of the desired element(s) in prospective orebodies is extremely important. Even on the Moon, therefore, it will be highly cost-effective to seek out the most concentrated sources of desired materials before committing to an extraction facility. Thus, I believe it is highly unlikely that ordinary regolith will be scooped up at random and separated into its components, as implied by some lunar development scenarios [e.g., Waldron & Criswell, 1982; Binder, 1990]. It has been pointed out that average lunar regolith is about 75% nonfuel "demandite", where "demandite" is defined as a substance containing all elements in the ratios currently demanded by industry. Such an analysis in my opinion is not useful for evaluating lunar resources. It contains two fallacies: (1) Separation is expensive, especially with tightly bound compounds like silicates. As noted, common terrestrial rocks are also mostly "demandite", but they are not economic sources of raw materials. (2) More subtly, "demand" is a function of availability. As prices of a commodity increase, substitutes become attractive in more and more of its applications [e.g., Gordon et al., 1987, pp. 60-76], Designs of space structures will be optimized accordingly. For example, a solar power satellite (SPS) design can be optimized for fabrication from lunar materials [Space Research Associates, 1985]. Again, the high cost of access to space is a two-edged sword: it makes it imperative to "live off the land", but it also makes capital and maintenance expenditures extremely expensive, such that facilities need to be as small and simple as possible. This has been emphasized by Haskin [1985], for example. Such considerations lead to the following general strategy for lunar resource exploration and exploitation. (1) Identify a resource and possible process(es) for extracting it. Such identification can also include tradeoffs with possible by-products (e.g., if ilmenite is to be reduced for O2, Fe might be an attractive by-product.) This constrains the potential ore minerals and thus the host rock types (or regolith types; the lithology of the regolith reflects the underlying bedrock fairly faithfully [e.g., Papike et al., 1982]). (2) Identify possible lunar geologic settings rich in the potential ore minerals, and target them for further examination. Again, terrestrial experience indicates it will be highly cost-effective to do lots of looking first before committing to a mine. Such a procedure is of course iterative, as new processes are developed for
beneficiation or extraction, and as new data on lunar mineralogy are acquired. Conceivably, new opportunities could arise with new knowledge, for example if concentrations of rare elements were discovered, such as in a carbonate or chloride deposit [Gillett, 1990]. Nonetheless, terrestrial experience suggests that a major part of optimizing the extraction of even common elements will consist of finding areas where they are exceptionally concentrated already. Tentative suggestions for exploration for sources of some bulk lunar commodities are presented below. Oxygen In many ways this is the easiest commodity, as oxygen is the most common element in the Moon's crust (as it is in the Earth's), being an essential constituent of silicates. Nonetheless, as would be expected, oxygen is not equally easy to extract from all lunar materials. Pyrolysis of undifferentiatedregolith [e.g., Agosto, 1983; Steurer & Nerad, 1983] has the virtue of being simple and of requiring little beneficiation; any silicate or oxide will work, at least to some degree. However, it has the disadvantage of being inefficient and extremely energy intensive. Moreover, at the extreme temperatures needed to pyrolyze silicates, many other volatile species besides oxygen are driven off and must be separated. Last, handling such high-temperature material, especially in the quantities needed for high-volume extraction, is likely to be difficult. Oxygen is also an automatic by-product of electrolysis of molten silicates or oxides, and thus would be a very attractive by-product of such a process [e.g., Haskin & Lindstrom, 1979; Steurer, 1982; du Fresne & Schroeder, 1983], As is pyrolysis, however, magma electrolysis is energy intensive and requires the routine handing of extremely hot material in large volumes. Ilmenite reduction is another possibility. Ilmenite, a double oxide of Fe and Ti (FeTiO3), is a common accessory mineral in mare basalts, especially in the "high-Ti" basalts, which can contain more than 10% TiO [e.g., Taylor, 1982, p. 286-287]. Furthermore, its physical properties are different enough from common silicates that beneficiation may be possible even under anhydrous conditions [Agosto, 1985], although achieving high ilmenite concentrations is evidently difficult [e.g., Vaniman & Heiken, 1990; Oder & Taylor, 1990], because many ilmenites are bound up in polycrystalline aggregates. At moderately elevated temperatures ilmenite reacts with H2 to yield water and a mixture of Fe and TiO2 - both potentially useful by-products, as noted below. The water can then electrolyzed to yield O2 and recover the H2 [e.g., Gibson & Knudsen, 1985; Gibson et al., 1990]. Alternatively, C may be the reductant of choice [Cutler & Krag, 1985]. The disadvantages of such processes include the initial need to import C or H2, and the ongoing need to recycle them with hivh efficiencv.
Iron As pointed out by many people, native Fe exists in the lunar regolith and is a potentially attractive resource. (The native Fe is not all meteoritic, as is sometimes asserted; about a third is indigenous to the lunar rocks, and another third results from solar-wind reduction of lunar FeO [Morris, 1980].) Although the concentration is low [—0.5%; Morris, 1980], beneficiation may be relatively easy, even under anhydrous conditions, because of the large contrast in physical properties between metallic Fe and silicates. Moreover, enhanced concentrations of native Fe may occur locally. However, much of this Fe occurs as extremely small grains (—4-33 nm) bound up in agglutinate glass, and as noted earlier separating the glass will be difficult. Alternatively, Fe may be a byproduct of ilmenite reduction for oxygen; the residue after reaction with H2 is an intimate mixture of metallic Fe and TiO2. Separating this mixture economically is challenging, but Lewis & Lewis [1987] suggest that extraction of Fe as the carbonyl may be useful. (Note that, although ilmenite is an ore of Ti on the Earth, it is not an ore of Fe!) Direct electrolysis of silicates or oxides is yet another alternative source. It is less attractive on the one hand because separating the cations that plate out at the cathode is a problem, as all silicates contain at least one cation (Si) in addition to Fe [e.g., Colson & Haskin, 1990; McCullough & Mariz, 1990], Depending on the cell conditions, various ferrosilicon alloys containing a few tenths of a percent of other metals (Ti, Cr) result. On the other hand, electrolysis itself is conceptually simple, although it remains to be seen whether conceptual simplicity will translate into operational simplicity. Moreover, ferrosilicon itself may be a useful reductant for other processes, such as reduction of Mg from olivine [Kuck, 1979], Aluminum The abundant aluminum mineral in the lunar crust is anorthite (CaAl2Si2Og), one of the feldspar minerals. It is the dominant constituent of the rock anorthosite, which makes up most of the lunar highlands. Thus, not only is Al abundant in the crust, but an aluminum-rich mineral is dominant over most of the lunar surface. Nonetheless, extracting aluminum from anorthite is difficult, especially under anhydrous conditions: not only is extraction energy-intensive, due to the high energy of the Al-O bond, but anorthite is a complex, highly polymerized silicate in which other cations are present to further hinder separation, regardless of whether wet-chemical [e.g., Waldron, 1983a], electrolytic, pyrolitic, or more exotic chemical techniques [e.g., Steurer, 1982, p. 88; Waldron, 1983b] techniques are used. Because of these difficulties, extraction of lunar Al will probably not be carried out at first. Titanium Ti will undoubtedly be extracted from ilmenite; the question is when. The titania residuum resulting after the extraction of oxygen and iron is an obvious feedstock.
However, Ti is difficult to separate from its oxide, especially under anhydrous conditions, both because it is refractory and because of the high strength of the Ti-O bond [e.g., du Fresne, 1983], Therefore, like aluminum, Ti will probably also not be extracted initially. Ilmenite is also the main Ti ore mineral on Earth; it is concentrated in certain placer deposits. Terrestrial smelting processes rely on abundant water, however, and thus are irrelevant for lunar conditions. As noted, Ti is also a possible by-product from electrolysis. Silicon As in Earth's crust, Si is the second most common element, after O, in the lunar crust. Unlike Earth, though, large deposits of essentially pure silica (SiO2) are unknown; the element nearly always occurs in more complex silicates. Thus, it is most likely that Si will be recovered as a by-product from electrolytic, pyrolytic, or chemical extraction of other elements, in one of the processes described above. Discovery of deposits of nearly pure silica could change this assessment, however. Crystals of cristobalite (one of the SiO2 polymorphs) up to 0.5 mm across occur in many mare basalts (D. Vaniman, personal comm., 1990), but separating such crystals economically is a challenge. Rare elements The conventional assessment that the Moon can be a source only of common elements is most probably premature. Although the Moon does not have the bewildering variety of geologic processes, many involving liquid water, that the Earth does, it nonetheless is a much more complicated body than sometimes recognized. The protracted igneous activity the Moon underwent during its early history may well have led to local concentrations of rare elements; a number of magmatic processes can form ores even under anhydrous conditions [Gillett, 1990]. Seeking such deposits is an additional rationale for further lunar exploration. It also should be noted that sources of potential reagents such as halogens or carbonates could make practical other extraction processes for metals [cf. Steurer, 1982, p. 88 ff]. Conclusions Terrestrial mining experience has much to offer in guiding the search for economic sources of materials off Earth. Fundamental lessons from this experience include: (1) the recoverable grade of a potential ore deposit is its most important economic characteristic, and (2) it is seldom cost-effective to commit to a mine without a great deal of exploration and metallurgical testing first. More generally, the value of a planet as a source of resources stems from the large-scale chemical fractionation caused by geologic processes. In effect, in local areas nature has carried out a great deal of the necessary separation already, rhe
value of lunar fractionation is severely compromised, however, by basing mining and extraction scenarios on analyses of ordinary country rock at the handful of sites directly sampled in the Apollo and Luna missions, and indirectly sampled through the lunar meteorites found in Antarctica. It has been said that the Apollo missions carried out "prospecting"; this is true, but not in the sense generally implied. In terrestrial mineral exploration, samples are collected widely, including many from ordinary rock that could not possibly be ore. Such sampling is carried out to establish a geologic context, to find where the ore might be. The Apollo and Luna missions have only begun to establish such a context for the Moon. Last, although the Moon does not have the vast range of ongoing geologic processes that the Earth does, it is nonetheless a planet-sized body that underwent protracted igneous activity in its early history. It is almost certain that exceptional concentrations of common elements occur locally on the Moon, and anomalous deposits of rare elements may exist as well. From an economic geology standpoint, much further exploration and pilot process testing is necessary before committing to a large-scale lunar mining operation. Acknowledgments Dave Vaniman and Dave Kuck are thanked for their many useful comments in their reviews of the manuscript. References Agosto, W. N., 1983 Solar furnace extraction of volatiles, metals, and ceramics from nonterrestrial materials (summary), in Space Manufacturing 1983, edited by J. D. Burke and A. S. Whitt, pp. 273-274, Advances in the Astronautical Sciences, 53, Univelt, Inc., San Diego, 1983 (proceedings of a conference held May 9-12, 1983 at Princeton University). Agosto, W. N„ 1985 Electrostatic concentration of lunar soil minerals, in Lunar Bases and Space Activities of the 21st century, edited by W. W. Mendell, pp. 453-464, Lunar & Planetary Institute, Houston, 1985. Binder, A. B., 1988 Lunar resources: What is known and expected, in Engineering, Construction, and Operations in Space, pp. 48-54, American Society of Civil Engineers, 1988. (proceedings of a conference held August 29-31, 1988, Albuquerque, New Mexico). Binder, A. B., 1990 LLOX - Metal production via NaOH electrolysis, in Engineering, Construction, and Operations in Space II, pp. 339-346, American Society of Civil Engineers, 1990. (proceedings of a conference held April 23-26, 1990, Albuquerque, New Mexico).
Boynton, R. S., 1966 Chemistry and Technology of Lime and Limestone, Wiley-Interscience. Bye, G. C., 1983 Portland Cement: Composition, Production, and Properties, Pergamon, 1983. Colson, R. O., & L. A. Haskin, 1990 Lunar oxygen and metal for use in near-Earth space: magma electrolysis, in Engineering, Construction, and Operations in Space II, pp. 187-196, American Society of Civil Engineers, 1990. (proceedings of a conference held April 23-26, 1990, Albuquerque, New Mexico). Cutler, A. H., & P. Krag, 1985 A carbothermal scheme for lunar oxygen production, in Lunar Bases and Space Activities of the 21st century, edited by W. W. Mendell, pp. 559-570, Lunar & Planetary Institute, Houston. du Fresne, E., 1983 Investigation into metals separation, in Research on the Use of Space Resources, edited by W. F. Carroll, pp. 5-1 - 5-29, Jet Propulsion Laboratory (JPL) Publication 83-36, California Institute of Technology, Pasadena, CA, March 1, 1983. du Fresne, E., and J. E. Schroeder, 1983 Magma electrolysis, in Research on the Use of Space Resources, edited by W. F. Carroll, pp. 3-1 - 3-22, Jet Propulsion Laboratory (JPL) Publication 83-36, California Institute of Technology, Pasadena, CA, March 1, 1983. Gibson, M. A., and C. W. Knudsen, 1985 Lunar oxygen production from ilmenite, in Lunar Bases and Space Activities of the 21st century, edited by W. W. Mendell, pp. 543-550, Lunar & Planetary Institute, Houston. Gibson, M. A., C. W. Knudsen, & A. Roeger, III, 1990 Development of the Carbotek process for lunar oxygen production, in Engineering, Construction, and Operations in Space II, pp. 357-367, American Society of Civil Engineers. (Proceedings of a conference held April 23-26, 1990, Albuquerque, New Mexico). Gillett, Stephen L., 1983 Lunar ores, in Space Manufacturing 1983, edited by J. D. Burke and A. S. Whitt, pp. 277-296, Advances in the Astronautical Sciences, 53, Univelt, Inc., San Diego, (proceedings of a conference held May 9-12, 1983 at Princeton University). Gillett, Stephen L., 1990 Lunar ores from magmatic processes: A speculative assessment, in Engineering Construction, and Operations in Space II, pp. 88-97, American Society of Civil Engineers, 1990. (proceedings of a conference held April 23-26, 1990, Albuquerque, New Mexico). Gordon, R. B., T. C. Koopmans, W. D. Nordhaus, & B. J. Skinner, 1987 Toward a New Iron Age?, Harvard University Press, 1987.
Haskin, Larry A., 1983 Material resources of the Moon (abs.), 14th Lunar and Planetary Science Conference-Special Sessions, March 16-17, pp. 23-24. Haskin, L. A., and D. J. Lindstrom, 1979 Electrochemistry of lunar rocks, in 4th Princeton/AIAA Conference on Space Manufacturing Facilities, Princeton, May 14-17, 1979. Haskin, L. A., 1985 Toward a spartan scenario for use of lunar materials, in Lunar Bases and Space Activities of the 21st century, edited by W. W. Mendell, pp. 435-444, Lunar & Planetary Institute, Houston. Kuck, D. L., 1979 Near-Earth extraterrestrial resources, Fourth Princeton/AIAA Conference on Space Manufacturing Facilities, AIAA #79-1377, Princeton, NJ, May 14-17, 1979. Lewis, J. S., & R. A. Lewis, 1987 Space Resources: Breaking the Bonds of Earth, Columbia University Press. Lewis, J. S., and S. Nozette, 1983 Extraction and purification of iron-group and precious metals from asteroidal feedstocks, in Space Manufacturing 1983, edited by J. D. Burke and A. S. Whitt, pp. 351-354, Advances in the Astronautical Sciences, 53, Univelt, Inc., San Diego, 1983 (proceedings of a conference held May 9-12, 1983 at Princeton University). McCullough, E. D., & C. L. Mariz, 1990 Lunar oxygen production via magma electrolysis, in Engineering, Construction, and Operations in Space II, pp. 347-356, American Society of Civil Engineers, 1990. (proceedings of a conference held April 23-26, 1990, Albuquerque, New Mexico). Morris, R.V., 1980 Origin and size distribution of metallic iron particles in the lunar regolith. Proc. 11th Lunar Planet. Sci. Conf., 1697-1712. Oder, R. R., & L. A. Taylor, 1990 Magnetic beneficiation of highland and hi-Ti mare soils: magnet requirements, in Engineering, Construction, and Operations in Space II, pp. 133-142, American Society of Civil Engineers, 1990. (proceedings of a conference held April 23-26, 1990, Albuquerque, New Mexico). Papike, J. J., S. B. Simon, and J. C. Laul, 1982 The lunar regolith: Chemistry, mineralogy, and petrology, Rev. Geophys. Space Phys., 20, 761-826. Park, C. F., Jr., & R. A. MacDiarmid, 1970 Ore Deposits, Freeman. Percival, T. J., W. C. Bagby, & A. S. Radtke, 1988 Gold-silver deposits in sedimentary rocks, in Bulk Mineable Precious Metal Deposits of the Western United States, edited by R. W. Schafer, J. J. Cooper, & P. G. Vikre, pp. 11-34, Geological Society of Nevada, Reno, NV.
Space Research Associates, Inc., Redmond, WA, 1985 Solar power satellite built of lunar materials, study conducted for Space Studies Institute, Princeton, September. Steurer, W. H., 1982 Extraterrestrial materials processing. Jet Propulsion Laboratory (JPL) Publication 82-41, California Institute of Technology, Pasadena, CA. Steurer, W. H., and B. A. Nerad, 1983 Vapor phase reduction, in Research on the Use of Space Resources, edited by W. F. Carroll, pp. 4-1 - 4-29, Jet Propulsion Laboratory (JPL) Publication 83-36, California Institute of Technology, Pasadena, CA, March 1, 1983. Sundqvist, T., 1983 Principles of Extractive Metallurgy, 2nd ed., McGraw-Hill. Taylor, S. R., 1982 Planetary Science: A Lunar Perspective, Lunar and Planetary Institute, Houston. Vaniman, D. T., & G. H. Heiken, 1990 Getting lunar ilmenite: From soils or rocks?, in Engineering, Construction, and Operations in Space II, pp. 107-116, American Society of Civil Engineers, 1990. (proceedings of a conference held April 23-26, 1990, Albuquerque. New Mexico). Waldron, R. D„ 1983 Laboratory investigation of HF acid leach process for refining lunar materials: Preliminary results (abstract), in Space Manufacturing 1983, edited by J. D. Burke and A. S. Whitt, pp. 335-336, Advances in the Astronautical Sciences, 53, Univelt, Inc., San Diego, 1983a (proceedings of a conference held May 9-12, 1983 at Princeton University). Waldron, R. D., 1983 Non-electrolytic route to oxygen and metallic elements from lunar soil, in Space Manufacturing 1983, edited by J. D. Burke and A. S. Whitt, pp. 297-314, Advances in the Astronautical Sciences, 53, Univelt, Inc., San Diego, 1983b (proceedings of a conference held May 9-12, 1983 at Princeton University). Waldron, Robert D., and David R. Criswell, 1982 Lunar utilization, in Space Industrialization, Vol. Il, edited by Brian O'Leary, pp. 1-54, CRC Press. Wotruba, P. R., R. G. Benson, & K. W. Schmidt, 1988 Geology of the Fortitude gold-silver skarn deposit, Copper Canyon, Lander County, Nevada, in Bulk Mineable Precious Metal Deposits of the Western United States, edited by R. W. Schafer, J. J. Cooper, & P. G. Vikre, pp. 159-171, Geological Society of Nevada, Reno, NV.
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On the Possibility of Space Generated Solar Electricity for the Antarctic LARS BROMAN* The Antarctic is still largely an unpopulated continent. Pursuant to the Antarctic Treaty of December first, 1959, the area south of 60 degrees latitude has been declared an international preserve for scientific research. An increasing number of research stations have been established, and a small amount of tourism has recently begun. Even a sparse population of researchers need energy for survival. The use of fossil fuels to generate it creates environmental hazards in a climate so cold that biodegradation is all but non-existent. The accidental spread of radioactivity form a nuclear plant would be just as unacceptable a risk as that of serious liquid fuel spills. * Lars Broman, Solar Energy Research Center, University College of Falun/Borlange, P.O. Box 10044, S-781 10 Borlange, Sweden.
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