Asteroid Taxonomy
There are at least five taxonomic classification systems used by various camps (see Tholen & Barucci). In Tholen's scheme,there are at least 14 asteroid classes, based on albedo and spectral characteristics. However, most asteroids fall into one of the following classes:

Class albedo spectrum surface mineralogy[a]
C low flat to slightly reddish hydrated silicates + carbon, organics
D low featureless, slightly red shortward of 5500 Å, very red longward of 5500 Å carbon, organic-rich silicates?
P low featureless, flat, intermediate between C and D carbon, organic-rich silicates?
M moderate featureless, flat metal (possible trace silicates)
S moderate moderate to strong absorption feature shortward of 7000 Å metal + olivine + pyroxene
[a] See Gaffey et al.

Birlan et al. performed a multivariate statistical analysis to determine asteroid reflection spectra in an automated manner, as opposed to the usual "by hand" approach of classification. One of the most interesting findings from this study is that infrared colors are very important in discriminating between the various asteroid types.

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