X-type asteroid

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The X-group of asteroids collects together several types with similar spectra, but probably quite different compositions.

Tholen classification[edit]

In the Tholen classification the X-group contains the types:

Since in this scheme the albedo is crucial in discriminating between the above types, some objects for which albedo information was not available were assigned an X-type. An example of this is 50 Virginia.

SMASS classification[edit]

The SMASS classification does not use albedo, but several spectral types are distinguished on the basis of spectral features which were too subtle to be visible in the broad-band ECAS survey used for the Tholen scheme. The X-group contains the types:

  • core X-type containing the asteroids with the most "typical" spectra
  • Xe-type of asteroids whose spectra contain a moderately broad absorption band around 0.49 μm. It has been suggested that this indicates the presence of troilite (FeS). There is some correlation between this group and the Tholen E-type.
  • Xc- and Xk-type asteroids, which contain a broad convex spectral feature in the range 0.55 μm to 0.8 μm (i.e. increased flux in this range). These spectra tend to be intermediate between the core X-type and the C and K-type.

Apart from the Xe-type, there is no significant correlation between the split into these SMASS types and the Tholen E, M, and P-types. All the types in the X-group contain a mixture of asteroids classified as either type E, M, or P.

See also[edit]


  1. 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).