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The Themis or Themistian asteroid family is a Hirayama family (having similar orbital elements) of asteroids found in the outer portion of the asteroid belt, between the orbits of Mars and Jupiter. At a mean distance of 3.13 AU from the Sun, it is one of the more populous asteroid families. It consists of a well-defined core of larger asteroids and a surrounding region of smaller ones. This core group includes (and is named after) the asteroid 24 Themis, discovered on April 5, 1853 by Italian astronomer Annibale de Gasparis.
Asteroids in the Themis family share the following orbital elements:
- semimajor axes between 3.08 AU and 3.24 AU
- orbital eccentricities between 0.09 and 0.22
- orbital inclinations of less than 3°
The Themis family is one of the largest and longest-recognized dynamical families of asteroids, and is made up of C-type asteroids with a composition believed to be similar to that of carbonaceous chondrites (http://www.lpi.usra.edu/meetings/lpsc2004/pdf/1646.pdf). To date, the Themis family comprises approximately 535 known asteroids. The major ones include:
- Castillo-Rogez, C. and Schmidt, B. E. (2010). Geophysical Evolution of the Themis Family Parent Body.