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S. J. Bus, R. P. Binzel, T. H. Burbine (MIT)
We present a feature-based asteroid taxonomy that has been derived from the spectral reflectance properties of 1190 asteroids measured during the second phase of the Small Main-belt Asteroid Spectroscopic Survey (SMASSII). These CCD spectra were obtained between 1993 and 1997 using the 2.4-m and 1.3-m telescopes of the Michigan-Dartmouth-MIT (MDM) Observatory on Kitt Peak, and cover the wavelength interval of 0.44 to 0.92 \mum.
Our intent in developing this taxonomy is to maintain the overall structure and integrity of the system defined by Tholen (PhD. Thesis 1984), while taking advantage of the substantially greater information contained in CCD spectra of asteroids. In particular, we observe a finite number of absorption features that combine to form an apparent continuum of spectral types, with previously recognized classes such as B-, D-, A-, and Q-types defining end-member spectral types that encompass this continuous distribution. The classification system we propose attempts to account for this continuum, and produces classes whose variance in spectral reflectances are consistent with the range of variance we have observed among many of the asteroid dynamical families.
We find that cluster analysis techniques previously used in asteroid taxonomy are, by themselves, insufficient to accurately define boundaries between spectral types, and therefore we rely on a combination of quantitative techniques, such as principal component analysis (PCA), the quantification of spectral slopes, and the presence or absence of spectral features. A total of 26 spectral classes are identified among the main-belt asteroids we have sampled. The primary aspects of this classification system are: 1) K-types (Bell 1988, {\it Meteoritics} 23, 256) are recognized, and we introduce a new "L-class" that, while spectrally similar to K-types, have spectral slopes that are steeper. 2) S-types are divided into 6 subclasses, reflecting the continuous variation in spectral reflectances found between the S- and A-, S- and K-, S- and L-, S- and R-, and S- and Q-types. 3) C-types are subdivided into 6 classes based on the presence or absence of a UV absorption feature and a broad 0.7 \mum (Vilas and Gaffey 1989, {\it Science} 246, 790) absorption band. 4) The X-types (E-, M-, and P-types) are subdivided, not based on albedo as in the Tholen taxonomy, but rather on subtle variations in spectral features (see also, Burbine {\it et al.}, these proceedings).