Lunar Reconnaissance Orbiter Science Targeting Meeting (2009) 6031.pdf

LUNAR MINING: KNOWNS, UNKNOWNS, CHALLENGES, AND TECHNOLOGIES. L. S. Gertsch1, 1Missouri University of Science and Technology (Rock Mechanics and Explosives Research Center, 1006 King- shighway, Rolla, MO, 65409-0660, United States, [email protected]).

Introduction: Humans have mined rock and soil basis of access: surface methods and underground for more than 300,000 years [1]. The first mining methods. The choice between approaches is ordinarily technology was digging tools shaped from antler, made on economic grounds. For lunar operations, the bone, wood, and rock [2],[3]. Today’s mines rely on choice will be recast in terms of launch mass and ener- global positioning satellites, semi-automated machines, gy in the early stages, and probably in terms of energy and explosives. Lunar and planetary mines will use alone for later stages. technology derived from this background and adapted Surface mining methods. These are operations for new environments. All, however, use their tech- where personnel are not required to go underground; nologies for the same purpose: to access the desired usually (but not necessarily) all material from the orig- material, and separate it from the unwanted material inal ground surface to the bottom of the mine is re- with which it is mixed, using the least energy or eco- moved. Those that appear most applicable to the nomic expenditure possible. Moon consist of open pit mining, area (strip, or open- The mining cycle. The mining cycle includes: cast) mining, auger (highwall) mining, and dry dredg- • Resource assessment. ing. • Resource extraction. Open pit mining is best applied to thick, irregular • Resource beneficiation. deposits expressed on the surface or occurring at shal- • Mine closure low depths. It removes all material to create one or Mining unit operations. Every mining method more horizontal benches, each successively deeper and must achieve these unit operations. How they are covering less area than the preceding one. The un- achieved, and their difficulty, depend on the material wanted material is stored or disposed of in surface of interest, its surroundings, and numerous other envi- stockpiles nearby. ronmental characteristics. The unit operations are: Area mining is similar except that the unwanted • Fragmentation - breaking material from its in si- material is piled within previously mined areas. Over tu surroundings. time the pit, usually a trench, remains a constant size • Excavation - picking up and loading the frag- and area, and appears to march across the landscape. mented pieces. (Open pit mines, on the other hand, grow deeper • Material handling - hauling as needed. and/or larger over time.) Area mining is used in hori- • Beneficiation - onsite preparation of the mined zontally bedded deposits such as coal. material for further processing or for use. Modern auger mining occurs underground, but is Technologies: The technologies used in mining run entirely from the surface. Single or double augers and excavation consist of all the methods by which the (or other types of excavators) bore into the otherwise fundamental laws of physics can be harnessed in the unavailable highwalls left when coal seams are mined specific locale of interest to achieve the goals of the in mountainous terrain. project. Even on Earth, there is enough of a range of Dry dredging, in the form most applicable to lunar operational constraints that a correspondingly wide operations, is similar to surface versions of the slusher range of technologies has evolved. Expansion to a mining methods previously used in some underground nonterrestrial location with its significantly different mines. A slusher is a drag scraper. Here is an exam- constraints will involve re-examination of the motives ple of the close relationship between methodology and for the means, and re-engineering for effective opera- technology. tion there. It will also offer new opportunities for Underground mining methods. Underground technology development, as well as new difficulties. mines are accessed by passages through undesired Many of both are presently unforeseen. material to an orebody; mining occurs beneath the Mining technologies evolved to achieve the unit cover of overlying formations. The major types are operations as needed in the various stages of the min- unsupported, supported, and caving. Several methods ing cycle in various types of locations. Workable sys- have been used for underground extraction of near- tems of these are termed “mining methods,” which are surface, unconsolidated deposits and thus may be use- in turn classified in a variety of ways. This review ful initiation points for lunar regolith mining. More follows the classification of mining methods on the mining methods can be found in [4], with more de- tailed descriptions in [5]. Lunar Reconnaissance Orbiter Science Targeting Meeting (2009) 6031.pdf

Top-slicing was used to mine placer gold from directly to government support. The latter controls the thick stream gravel deposits before the equipment for willingness of the mining industry to commit. modern high-volume methods was available and where Challenges: The unknowns listed above lead to contemporary methods such as hydraulic mining could engineering challenges expected during lunar mining not be used. Similar deposits are mined today either and excavating. by surface methods (above) or, if in permafrost, by Regolith in situ is tightly compacted and contains adaptations of methods developed for rock. The most varying densities of pebbles, rocks, and boulders. common of the latter approaches is room-and-pillar Even without the cementing effect of intergranular ice, (below). In the form most suitable for regolith mining, undisturbed regolith deposits require some force to top-slicing consists of fragment and excavate [9]. Mass usually provides the Room-and-pillar mines require the ore to be strong reaction force for surface mining, yet launched mass enough to support openings wide enough for the will be at a premium. Can the materials needed for the equipment to operate. This method leaves areas of ore pre-manufacturing stage of lunar presence be obtained (pillars) to support the overlying material, which itself from the more easily excavated upper layers? must be strong enough to bridge the gaps between the Digging becomes more difficult with depth, possi- pillars. bly plateauing below some critical depth. This, in ad- Longwall and shortwall are mechanized adapta- dition to the presence of oversized particles, will re- tions of top-slicing that also permit / require the over- quire development of techniques for real-time ahead- lying material to fail and fill the void created by re- of-the-face sensing and machine control. This is part moval of the ore. The main difference between these of the challenge of sufficient characterization of the and top-slicing is that, to date, the ore has had to be target material, which needs to be more complete than strong enough to support excavations without the sig- on Earth to offset the difficulty of the additional chal- nificant artificial support requirements of top-slicing. lenges of remote operation, maintenance, and repair. Knowns: The first target of lunar mining and con- Mining and excavation equipment is built to be ro- struction will be the regolith. That will expand to in- bust, because it must deal with significant -- and diffi- clude intact rock when operation depths exceed the cult-to-characterize -- ranges of material behavior. thickness of the regolith covering. This is true in any natural geologic material. Long- Many aspects of regolith behavior have been term operation of such equipment in the unfamiliar and measured from the perspective of soil mechanics, extreme environment of the Moon adds the difficulties summarized by [6]. The low number of sample sites, referred to above. and their bias toward mare locations, will be ameli- Any prototype technology, or old technology used orated by the much greater coverage and resolution of in a new way or a new place, requires significant de- the LRO mission. velopment and testing. NASA is familiar with this, but Feedstocks for the production of propellants, life- the greatest challenge will be whether humanity yet support gases, and basic structural materials probably has the political and financial will to carry the process will require only minimal beneficiation of excavated through well enough to encourage success. regolith prior to processing. References: Unknowns: The variability and spatial scale of [1] Verri G. et al. (2005) J Archaeological Science, 32, variability, of the regolith properties that affect mining 207–213. [2] Weisgerber B. and Pernicka E. (1995) and excavation are not well-characterized in three di- Prehistoric Gold in Europe: Mines, Metallurgy, and mensions. Regolith variability is controlled by its Manufacture, 159-182. [3] Russell M. (2000) Flint formation mechanisms, which are due primarily to Mines in Neolithic Britain. [4] Hartman H. and Mut- impact processes. Understanding the effects of these mansky J. (2002) Introductory Mining Engineering. processes will enhance understanding of the spatial [5] Hartman H.L. (1992) SME Mining Engineering distribution of material targets, and vice versa. It also Handbook, 2nd ed. [6] Heiken G.H. et al. (1991) Lunar will improve the development of effective excavation Sourcebook, A User’s Guide to the Moon. [7] Taylor techniques and equipment. L.A. et al. (2001) JGR, 106(E11), 27,985–27,999. [8] The fourth dimension -- time -- may be of some Crider D.H. and Vondrak R.R. (2003) JGR, 108(E7), importance to the determination of potential orebodies 5079. [9] Gertsch L.S. et al. (2006) GoldenRocks, and is being studied (e.g., [7], [8]). 42nd U.S. Symp on Rock Mechanics. Political and legal uncertainty are two unknowns unrelated to science or engineering that nevertheless affect the probability of success. The former speaks