Asteroids are assigned a type based on spectral shape, color, and sometimes albedo. These types are thought to correspond to an asteroid's surface composition. For small bodies which are not internally differentiated, the surface and internal compositions are presumably similar, while large bodies such as 1 Ceres and 4 Vesta are known to have internal structure. A materials emission spectrum is the amount of electromagnetic radiation of each frequency it emits when it is heated (or more generally when it is excited). ... Colour is an important part of the visual arts. ... The albedo is a measure of reflectivity of a surface or body. ... 1 Ceres (seer-eez, Latin Cerēs) was the first asteroid to be discovered. ... 4 Vesta (ves-ta) is the third-largest asteroid in the Main belt, between 530 and 468 km in diameter. ...

Present-day classifications

The present-day classification was initiated by Clark R. Chapman, David Morrison, and Ben Zellner in 1975 (2) with three categories: C for dark carbonaceous objects, S for stony (silicaceous) objects, and U for those which did not fit into either C or S. This classification has since been expanded and clarified. C-type asteroids are carbonaceous asteroids. ... Approximately 17% of all known asteroids are of an S-type (for stony) composition. ...

A number of classification schemes are currently in existence (1), and while they strive to retain some mutual consistency, quite a few asteroids are sorted into different classes depending on the particular scheme. This is due to the use of different criteria for each approach. The two most widely used classifications are described below:

Tholen classification

The most widely used taxonomy for over a decade has been that of David J. Tholen, first proposed in 1984. This classification was developed from broad band spectra (between 0.31μm and 1.06μm) obtained during the Eight-Color Asteroid Survey (ECAS) in the 1980s, in combination with albedo measurements (3). The original formulation was based on 978 asteroids. The albedo is a measure of reflectivity of a surface or body. ...

This scheme includes 14 types with the majority of asteroids falling into one of three broad categories, and several smaller types:

• • C-group dark carbonaceous objects, including several sub-types:
    • B-type
    • F-type
    • G-type
    • C-type the remaining majority of 'standard' C-type asteroids. This group contains about 75% of asteroids in general.
  • S-type silicaceous (i.e. stony) objects. This class contains about 17% of asteroids in general.
  • X-group
    • M-type metallic objects, the third most populous group.
    • E-type differing from the M-type in albedo
    • P-type differing from the M-type in albedo
  • A-type a small category
  • D-type a small category
  • T-type a small category
  • Q-type for 1862 Apollo
  • R-type for 349 Dembowska
  • V-type for 4 Vesta

The three last types contained only a single asteroid having unique spectral properties. Objects were sometimes assigned a combined type such as e.g. CG when their properties were a combination of those typical for several types. C-type asteroids are carbonaceous asteroids. ... B-type asteroid is a subdivision of C-type asteroids distinguished spectrally by differences in the ultraviolet absorption. ... F-type asteroid is a subdivision of C-type asteroids distinguished spectrally by differences in the ultraviolet absorption and the lack of a water absorption feature at 3 micrometres. ... G-type asteroid is a subdivision of C-type asteroids distinguished spectrally by differences in the ultraviolet absorption. ... C-type asteroids are carbonaceous asteroids. ... Approximately 17% of all known asteroids are of an S-type (for stony) composition. ... M-type asteroids are metallic asteroids; they are moderately bright (albedo . ... E-type asteroids are inner-belt asteroids that contain enstatite. ... P-type asteroids have low albedo and a featureless reddish spectrum. ... A-type asteroids are inner-belt asteroids that have a strong, broad 1 µm olivine feature and a very reddish spectrum shortwards of 0. ... D-type asteroids have a very low albedo and a featureless reddish spectrum. ... T-type asteroids are rare inner-belt asteroids of unknown composition with dark, featureless and moderately red spectra, and a moderate absorption feature shortwards of 0. ... Q-type asteroids are inner-belt asteroids with a strong, broad 1 micrometre olivine and pyroxene feature, and a spectral slope that indiciates the presence of metal. ... 1862 Apollo is a Q-type asteroid, discovered by Karl Reinmuth in 1932, but lost and not recovered until 1973. ... R-type asteroids are moderately bright, inner-belt asteroids that are spectrally intermediate between the V and A-type asteroids. ... 349 Dembowska is a very large Main belt asteroid. ... The V-type asteroids or Vestoids are moderately bright, and very similar to the more common S-type, which are also made up of stony irons and ordinary chondrites. ... 4 Vesta (ves-ta) is the third-largest asteroid in the Main belt, between 530 and 468 km in diameter. ...

SMASS Classification

This is a more recent taxonomy introduced by Schelte J. Bus and Richard P. Binzel in 2002, based on the Small Main-Belt Asteroid Spectroscopic Survey (SMASS) of 1447 asteroids (4). This survey produced spectra of a far higher resolution than ECAS, and was able to resolve a variety of narrow spectral features. However, a somewhat smaller range of wavelengths (0.44μm to 0.92μm) was observed. Also, albedos were not considered. While attempting to keep to the Tholen taxonomy as much as possible given the differing data, asteroids were sorted into the 24 types given below. The majority of bodies fall again into the three broad C, S, and X categories, with a few unusual bodies categorized into several smaller types:

• • C-group of carbonaceous objects including:
    • B-type largely overlapping with the Tholen B and F types.
    • C-type the most 'standard' of the non-B carbonaceous objects
    • Cg Ch Cgh somewhat related to the Tholen G type
    • Cb transition objects between plain C and B types.
  • S-group of silicaceous (stony) objects including:
    • A-type
    • Q-type
    • R-type
    • K-type a new category
    • L-type a new category
    • S-type the most 'standard' of the S group
    • Sa, Sq, Sr, Sk, and Sl transition objects between plain S and the other types in the group.
  • X-group of mostly metallic objects including:
    • X-type the most 'standard' of the X group including objects classified by Tholen as M, E, or P-type.
    • Xe, Xc, and Xt transition types between plain X and the appropriately lettered types.
  • T-type
  • D-type
  • Ld-type: a new type with more extreme spectral features than the L-type
  • O-type
  • V-type

A significant number of small asteroids were found to fall in the O, Q, and V types, which were represented by only a single body in the Tholen scheme. In this Bus and Binzel SMASS scheme only a single type was assigned to any particular asteroid. C-type asteroids are carbonaceous asteroids. ... B-type asteroid is a subdivision of C-type asteroids distinguished spectrally by differences in the ultraviolet absorption. ... B-type asteroid is a subdivision of C-type asteroids distinguished spectrally by differences in the ultraviolet absorption. ... F-type asteroid is a subdivision of C-type asteroids distinguished spectrally by differences in the ultraviolet absorption and the lack of a water absorption feature at 3 micrometres. ... C-type asteroids are carbonaceous asteroids. ... G-type asteroid is a subdivision of C-type asteroids distinguished spectrally by differences in the ultraviolet absorption. ... Approximately 17% of all known asteroids are of an S-type (for stony) composition. ... A-type asteroids are inner-belt asteroids that have a strong, broad 1 µm olivine feature and a very reddish spectrum shortwards of 0. ... Q-type asteroids are inner-belt asteroids with a strong, broad 1 micrometre olivine and pyroxene feature, and a spectral slope that indiciates the presence of metal. ... R-type asteroids are moderately bright, inner-belt asteroids that are spectrally intermediate between the V and A-type asteroids. ... The spectrum of K-type asteroids resembles CV and CO meteorites. ... Approximately 17% of all known asteroids are of an S-type (for stony) composition. ... T-type asteroids are rare inner-belt asteroids of unknown composition with dark, featureless and moderately red spectra, and a moderate absorption feature shortwards of 0. ... D-type asteroids have a very low albedo and a featureless reddish spectrum. ... The V-type asteroids or Vestoids are moderately bright, and very similar to the more common S-type, which are also made up of stony irons and ordinary chondrites. ... Q-type asteroids are inner-belt asteroids with a strong, broad 1 micrometre olivine and pyroxene feature, and a spectral slope that indiciates the presence of metal. ... The V-type asteroids or Vestoids are moderately bright, and very similar to the more common S-type, which are also made up of stony irons and ordinary chondrites. ...

End Notes

These classification schemes are expected to be refined and/or replaced as further research progresses.

There is also an alphabetical list of the asteroid types. Asteroids are classified into one of a list of asteroid types, according to their spectral properties: A-type asteroids B-type asteroids C-type asteroids D-type asteroids E-type asteroids F-type asteroids G-type asteroids K-type asteroids L-type asteroids M-type asteroids P-type asteroids Q...

References

1. Planetary Data System Small Bodies Node: Asteroid Taxonomies
2. C. R. Chapman, D. Morrison, and B. Zellner Surface properties of asteroids: A synthesis of polarimetry, radiometry, and spectrophotometry, Icarus, Vol. 25, pp. 104 (1975).
3. D. J. Tholen Asteroid taxonomic classifications in Asteroids II, pp. 1139-1150, University of Arizona Press (1989).
4. S. J. Bus and R. P. Binzel Phase II of the Small Main-belt Asteroid Spectroscopy Survey: A feature-based taxonomy, Icarus, Vol. 158, pp. 146 (2002).