John S. Lewis
There are three types of possible asteroidal materials that appear to be attractive for exploitation:
Because some of the near-Earth asteroids are energetically more accessible than the Moon [require a round-trip total change in velocity (delta V) less than 9 km/sec (though the trip time would be measured in years, not days)], such an asteroid might be chosen as the source of any useful material, even if that material was also available on the Moon. Provided that the asteroid was minable, it might therefore be chosen as the source of bulk dirt needed for shielding in low Earth orbit (LEO) or elsewhere in near- Earth space.
And the near-Earth asteroids may offer materials that are rare or absent on the surface of the Moon. Some of them are spectrally similar to ordinary and carbonaceous chondrites. These meteorites contain free metals and volatiles at a concentration about 1 00 times that in the lunar soil. Thus, if an asteroid was found to have one of these compositions and to be accessible and minable as well, it would be a very attractive source of such needed materials.
An asteroid of the composition of an ordinary chondrite could be processed to provide very pure iron and nickel for use in structures in LEO. The principal byproducts would be cobalt, the platinum group metals, and other useful elements such as gallium, germanium, and arsenic. These are all materials of high value and utility in an industrial economy. Some might even be valuable and useful enough to merit being returned to the surface of the Earth (though the high cost of space transportation has ruled out economical return of gold and even diamonds, thus far).
Volatiles, such as water and carbon dioxide, obviously useful in any space settlement, could be found in an asteroid that resembles a carbonaceous chondrite or one that consists of the nucleus of a former comet. Water content by weight for these materials may range from 5 percent for C2 chondrites through 10 percent in C1 s to about 60 percent in typical cometary nucleus material. The abundance of organic matter in C1s is about 6 percent by weight, and nitrogen, sulfur, and chlorine are readily available. Attractive bonuses from C1s are that on the order of 10 percent of their weight may be magnetite** and about 2 percent is nickel-rich sulfides. As an alternative to returning asteroidal volatiles to LEO, the in situ extraction of water on an asteroid may be justifiable.
The Asteroid-Meteorite Relationship
Spectroscopic comparisons of asteroids with laboratory samples of meteorites show that the dominant minerals in meteorites are also the principal components of asteroid surfaces. Indeed, many asteroids have reflectance spectra that are identical with those of known classes of meteorites. See table 14. However, many asteroids appear not to belong to known classes of meteorites (although they are made of the same major minerals). Further, there is little relation between the abundance of meteorites of a given type and the abundance of asteroids of the corresponding spectral class. Of course, the large majority of the asteroids studied are in the asteroid belt, beyond the orbit of Mars, while the objects that fall on Earth must have very different orrbits It is instructive to note that the commonest class of meteorites falling on Earth.. the ordinary chondrites, is apparently absent in the asteroid belt, but at least one spectroscopic match for ordinary chondrites can be found among the small, poorly studied near-Earth asteroids.
We have known for many years that the Earth receives in the meteorites a biased sampling of the asteroid types as spectral reflectance classifies them. The main problem is that the most abundant meteorites (the ordinary chondrites, which comprise almost 3/4 of the meteorites we have found on Earth) have rare asteroidal analogs and the most abundant asteroids (spectral type S, which comprises about 1/3 of all the asteroids that have been classified and over 1/2 of the near-Earth asteroids that have been classified) have rare meteoritic analogs. (See figure 8 for the type distribution of the near- Earth asteroids.) The explanation for this mismatch is among the most intriguing subjects being addressed by meteoriticists and asteroid spectroscopists.
We know that asteroid discoveries are biased in favor of the brighter objects; that is, those that are large or close (in the inner as opposed to the outer belt) or have a high albedo. We know that meteorite finds are biased in favor of those that can survive atmospheric entry. There may be an accidental bias in the meteorite population: that IS, they could be the products of the fragmentation of only a few, unrepresentative parent asteroids. The ordinary chondrites could come from only parts of larger asteroids. These meteorites could come from somewhere other than the asteroid belt. There may be a time bias; comparison of the well- preserved meteorites found in the Antarctic with the more weathered meteorites found elsewhere (which presumably fell within the last 200 years) suggests that Antarctica may have sampled a different meteoroid population in the past than is being sampled by contemporary, non-Antarctic falls and finds.
Thus, although we must be aware that, as Lipschutz says, "the meteorites are an incomplete and unrepresentative sample of the asteroid belt" (and of intermediate parent bodies such as the near- Earth asteroids), the volume of data on the meteorites so far exceeds the volume of data on the near-Earth asteroids that we are compelled to assume for the time being that the meteorites are adequate representations of the near-Earth asteroid population.
In this paper, I will present a brief overview of the entire range of meteorite
compositions, with emphasis on the occurrence of interesting resources. I will
focus on materials useful in space, especially volatiles, metals, and raw"
dirt." Those few materials that may have sufficiently high market value
to be worth returning to Earth will also be mentioned.
* The editor acknowledges the critical help of John Wasson, for figure 11 and its interpretation; Lucy McFadden, for the spectral classifications of the near-Earth asteroids and other clarifications; and Michael Lipschutz, for the relationship between meteorites and asteroids and other information. Wasson's figure comes from his book Meteorites: Their Record of Early Solar-System History (New York: W. H. Freeman and Co., 1985), p. 29. McFadden and Lipschutz are the lead authors of two chapters in the 1989 book Asteroids II, ed. Richard P. Binzel, Tom Gehrels, and Mildred Shapley Matthews (Tucson: Univ. of Arizona Press). McFadden's "Physical Properties of Aten, Apollo and Amor Asteroids" is coauthored by David J. Tholen and Glenn J. Veeder. Lipschutz's "Meteoritic Parent Bodies: Nature, Number, Size and Relation to Present-Day Asteroids" is coauthored by Michael J. Gaffey and Paul Pellas.
**M. Hyman and M. W. Rowe, 1983, "The Origin of Magnetite in Carbonaceous Chondrites," abstract in Lunar & Planetary Sci. XIV (Houston: Lunar & Planetary Inst.), pp. 341-342.
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