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.