Basic Asteroid Spectral Types

There are three major asteroid spectral types: C, S, M (carbonacious, stony, metal) and U for unclassified  others. This is based on early study using polarimetry, radiometry, and spectrophotometry by Clark R. Chapman, David Morrison, and Ben Zellner for ~100 asteroids published in 1975 and further refined by Bowell et al 1978 and others (see taxonomy data in PDS).  In this C-S-M taxonomy, most asteroids are either C-type or S-type.

• C: Carbonaceous
• S: Stony or silicate or silicaceous
• M: Metallic
• U: Undetermined or rare

C-type

• 76%
• Most common
• Extremely dark (albedo 0.03)
• Similar to carbonaceous chondrite meteorites
• Approximately the same chemical composition as the Sun minus hydrogen, helium and other volatiles
• Predominate the main belt, especially the outer half of the main belt between 2.77 and 3.0 AU
• Largest C-type asteroids include (1) Ceres, (2) Pallas, (10) Hygiea, (511) Davida

S-type:

• 16%
• Relatively bright (albedo 0.10 - 0.22);
• Metallic nickel-iron mixed with iron- and magnesium-silicates.
• Predominate the inner main belt
• Many S-type objects have diameters of 100–200 km
• Examples: (15) Eunomia, (3) Juno

M-type

• 5% - Most of the rest
• Third most common asteroid type.
• Bright (albedo 0.10-0.18)
• Pure nickel-iron or mixed with small amounts of stone
• Examples: (16) Psyche, (21) Lutetia, (22) Kalliope, (216) Kleopatra

U-type

• 3%
• For unclassified or rare such as E-type or enstatite achondrites
• Examples (4) Vesta, (1566) Icarus, (162) Laurentia, (267) Tirza

References:

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)
http://adsabs.harvard.edu/abs/1975Icar...25..104C

http://www.observeasteroids.com/images/spectra2.gif

http://www.psi.edu/pds/archive/asteroids.html

The Asteroids, Chapman, C. R.; Williams, J. G.; Hartmann, W. K.; Annual review of astronomy and astrophysics. Volume 16. (A79-14551 03-88) Palo Alto, Calif., Annual Reviews, Inc., 1978, p. 33-75
 http://adsabs.harvard.edu/abs/1978ARA%26A..16...33C

Bowell, E.; Chapman, C. R.; Gradie, J. C.; Morrison, D.; Zellner, B., Taxonomy of Asteroids Icarus vol. 35, Sept. 1978, p. 313-335: http://adsabs.harvard.edu/abs/1978Icar...35..313B

Neese, C., Ed., Asteroid Taxonomy. EAR-A-5-DDR-TAXONOMY-V5.0. NASA Planetary Data System, 2005 : http://www.psi.edu/pds/resource/taxonomy.html

Other classifications:

• Tholen classification: Tholen, D. J. 1989.  Asteroid taxonomic classifcations.  In  Asteroids II (R. P. Binzel, T. Gehrels, and M. S. Matthews, Eds.),  pp. 1139-1150.  Univ. of Arizona Press, Tucson.  [THOLEN1989]  
• SMASS classification: S. J. Bus, F. Vilas, and M. A. Barucci Visible-wavelength spectroscopy of asteroids in Asteroids III, pp. 169, University of Arizona Press (2002)
• Barucci, M. A., M. T. Capria, A. Coradini, and M. Fulchignoni 1987. Classification of asteroids using G-mode analysis.  Icarus 72, 304-324. [BARUCCIETAL1987] 
• Tedesco, E. F., J. G. Williams, D. L. Matson, G. J. Veeder, J. C.Gradie, and L. A. Lebofsky 1989.  A three-parameter asteroid taxonomy.  Astron. J. 97, 580-606. [TEDESCOETAL1989]                  
• Howell, E. S., E. Merenyi, and L. A. Lebofsky 1994. Classification of asteroid spectra using a neural network.  J. Geophys. Res. 99, 10847-10865. [HOWELLETAL1994]                       
• Xu, S., R. P. Binzel, T. H. Burbine, S. J. Bus 1995.  Small main-belt asteroid spectroscopic survey: Initial Results. Icarus 115, 1-35. [XUETAL1995]                                                                                        
• Bus, S.J. and R.P. Binzel 2002.  Phase II of the small main-belt asteroid spectroscopic survey:  A feature-based taxonomy.  Icarus 158, 146-177. [BUSETAL2002]                                                                  
• Lazzaro, D., C.A. Angeli, J.M. Carvano, T. Mothe-Diniz, R. Duffard,   and M. Florczak, S3OS2: The visible spectroscopic survey of 820 asteroids, Icarus 172, 179-220, 2004. [LAZARROETAL2004]"