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very red at wavelengths of less than 0.7 (m and a deep absorption band centred near 0.95 (m

other

any

any object not falling into one of the above classes

*Classes E, M, and P are spectrally indistinguishable at these wavelengths and require an independent albedo measurement for unambiguous classification.

the nickel-iron meteorites. Paradoxically, however, some M-class asteroids have spectral features due to the presence of hydrated minerals. D-class asteroids have low albedos and show reflectance spectra similar to the spectrum exhibited by a relatively new type of carbonaceous chondrite, represented by the Tagish Lake meteorite, which fell in January 2000.

P- and T-class asteroids have low albedos and no known meteorite or naturally occurring mineralogical counterparts, but they may contain a large fraction of carbon polymers or organicrich silicates or both in their surface material. R-class asteroids are very rare. Their surface material has been identified as being most consistent with a pyroxene- and olivine-rich composition analogous to the pyroxene-olivine achon-drite meteorites. The E-class asteroids have the highest albedos and have spectral reflectances that match those of the enstatite achondrite meteorites. V-class asteroids have reflectance properties closely matching those of one particular type of basaltic achondritic meteorite, the eucrites. The match is so good that some believe that the eucrites exhibited in museums are chips from the surface of a V-class asteroid that were knocked off during a major collision. The V class had been thought confined to the large asteroid Vesta and a few very small Earth-approaching asteroids until 2000, when asteroid (1459) Magnya—located at 3.15 AU from the Sun, compared with 2.36

AU for Vesta—was discovered also to have a basaltic surface.

Among the larger asteroids (those with diameters greater than about 25 km [15 miles]), the C-class asteroids are the most common, accounting for about 65 percent by number. This is followed, in decreasing order, by the S class, at 15 percent; the D class, at 8 percent; and the P and M classes, at 4 percent each. The remaining classes constitute less than 4 percent of the population by number. In fact, there are no A-, E-, or Q-class asteroids in this size range, only one member of the R and V classes, and between two and five members of each of the B, F, G, K, and T classes.

The distribution of the taxonomic classes throughout the asteroid belt is highly structured. Some believe this variation with distance from the Sun means that the asteroids formed at or near their present locations and that a detailed comparison of the chemical composition of the asteroids in each region will provide constraints on models for the conditions that may have existed within the contracting solar nebula at the time the asteroids were formed.

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