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I've read references in both science and science-fiction articles to "asteroid mining." Is this a feasible thing to do? If so, how might it be accomplished, and what kind of valuable materials could we extract?

David S. McKay of the NASA Johnson Space Center (best known as the lead author on the paper describing evidence of possible ancient life in a Martian meteorite) has studied this idea extensively. He replies:

"Asteroid mining is a concept that has been around for decades. The basic notion is to extract material from near-earth asteroids, those having orbits that come near the earth--a group quite separate from the main belt asteroids, which orbit between Mars and Jupiter. Resources extracted from the asteroids could be exploited in space to support space flight, space stations or even a lunar base. The most useful material for these applications would likely be water, methane or other compounds that could be either processed into rocket fuel or utilized to replace expendable materials needed for life support. Some researchers have suggested that the metals in asteroids (iron, nickel and so on) might also be mined as raw material for the construction of structures in space.

"The other major reason to mine asteroids would be to bring strategic or precious metals back to the earth. The most promising metals to extract would include the rare (and costly) platinum and platinum-group precious metals as well as gold. Planetary astronomers believe the average asteroid should have much higher abundances of these metals than typical rocks on the earth or even on the moon. This expectation is based on the presumption that asteroids are either undifferentiated objects (in technical terms, they have a 'chondritic' composition in which all minerals are combined together), or differentiated objects (the minerals are not mixed together). If the asteroids are differentiated, they might even have concentrated these elements into accessible locations, such as metal-rich cores that were exposed by impacts.

"Most earlier asteroid-mining concepts required humans to visit the asteroids and mine them, but some of the newer ideas involve strictly robotic missions. One option would be simply to bring chunks of the asteroid back to the earth and crash them in some remote area where a processing plant would be set up. Other possibilities include dropping the asteroid chunks on the moon or processing materials on the asteroid itself, perhaps first bringing it into orbit around the earth. Recent economic analysis by Jeffrey S. Kargel of the U.S. Geological Survey (co-author of "Global Climatic Change on Mars" in the November 1996 issue of Scientific American) shows that it may be profitable to do this even if the price of precious metals is significantly depressed by a big new supply. Other experts in this field include David Kuck, a consulting geologist in Arizona who has developed concepts for robotic mining, and John Lewis of the University of Arizona, long an advocate of asteroid mining.

"The technology needed to visit near-earth asteroids is well in hand--the amount of rocket power and fuel needed to visit some of these bodies is less than it takes to go to the moon. The technology necessary to mine them and bring back useful material has not been developed, however. It is not clear how difficult and costly this would be, nor is it clear if the task could be done robotically or would require human supervision. The National Aeronautics and Space Administration has no plans to mine asteroids, although the agency does intend to explore asteroids with robotic probes and possibly eventually with human missions."

John S. Lewis of the University of Arizona adds another perspective:

"There are really two quite different answers to this question. The first addresses the usual, tacit assumption that we would seek out extraterrestrial resources for the purpose of importing them to the earth for industrial use here. In reality, there are few commodities that might be profitably imported in this way. The only traditional import--scientific samples--not only has a limited market, but demand for such samples is likely to continue to decrease as analytical techniques become more sensitive. The other major import-worthy category is precious and strategic materials, including platinum-group metals, (for high-temperature and corrosion-resistant alloys and coatings and industrial and automotive catalysts) and nonmetals such as gallium, germanium and arsenic (for making VLI computer chips). The latter may profitably be processed into large crystals in microgravity.

"The second type of answer, which is to my mind by far the more important, is the utilization of space resources both to defray the costs of large-scale space operations and to permit autonomous operations in space. Here the first commodity of interest is likely to be water from the subset of near-earth asteroids that are either C-type (carbonaceous) asteroids or extinct comet nuclei. Together these probably make up half or more of the near-earth asteroid population. That water would be used to make hydrogen and oxygen rocket propellants, and of course water and oxygen would also be made available for use in space habitats. The other nonreturnable resource of interest is almost certainly ferrous metals. Native 'stainless steel' alloys are very common in meteorites and asteroids, suggesting their large-scale use as structural materials in space.

"There are also some very interesting opportunities associated with the generation of electrical power from space resources. The options here include the construction of solar-power satellites in high earth orbits that would beam solar power down to the ground in the form of microwave energy. The retrieval of helium 3 (implanted by the solar wind) from the surface of the moon may be economically attractive as a source of clean fuel for fusion power reactors on the earth or for fusion on the moon with the power beamed down to the earth. Similarly, solar collectors may be built on the moon out of native lunar materials to send their power back to the earth.

"The construction of solar-power satellites could in principle be made much cheaper if the high-mass, low-tech components of the satellite are fabricated in space from asteroidal or possibly lunar materials. Looking farther afield, the helium 3 and deuterium contents of the giant planets are so vast that schemes for extraction and retrieval of fusion fuel from their atmospheres (especially Uranus and Neptune) could power the earth until the sun dies of old age.

"The most economical sources of space materials are those bodies that have the greatest richness of valued commodities and that are most accessible from the earth: these are the near-earth asteroids. All they lack is economically attractive amounts of helium 3.

"Near-future launch costs of about $600 per kilogram for the trip from earth's surface to orbit, combined with space ferries cycling between high-earth orbits and nearby asteroids (returning, over their lifetimes, 100 tons of materials for each ton of equipment launched from the earth), suggest future supplies of raw materials in near-earth space at a cost of a few dollars per kilogram. This is comparable to the cost of a house on the earth.

"These and other resource options are treated in detail in my recent book Mining the Sky: Untold Riches from the Asteroids, Comets, and Planets (Addison-Wesley, 1996) and in the technical volumes Resources of Near-Earth Space (by J. S. Lewis, M. S. Matthews and M. L. Guerrieri, University of Arizona Press) and Space Resources (edited by M. F. McKay, D. S. McKay and M. B. Duke, available at the U.S. Government Printing Office).

"The reported detection of polar ice on the moon by the Clementine spacecraft adds another interesting option: to build a base at a permanently illuminated site near the lunar pole.

"The bottleneck in such plans is the need to obtain detailed mineral characterization of near-earth asteroids and the lunar poles. Although funding for the discovery of near-earth asteroids and for small lunar and asteroidal spacecraft missions has recently been on the upswing, the level of funding is still 100 to 1,000 times smaller than that lavished on a single large military or civilian spacecraft (the Hubble Space Telescope, for example, or a radar or optical reconnaissance satellite)."

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