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A near-spherical carbonaceous asteroid averaging some 198km in diameter, the surface of Themis is covered by a layer of water ice (and its interior may also contain further reserves of ice). Themis was discovered in the year 1853, and by 1875 its orbit was understood sufficiently that perturbations of that orbit caused by Jupiter could be used to help measure the mass of that giant planet.
Themis represents a remnant of a much larger object that at one time travelled around the outer edge of the Asteroid Belt. In a cataclysmic collision, approximately eighty per cent of the mass of this original object was broken away. The result was a dispersed cloud of nearly five thousand objects orbiting through the Belt, collectively known as the Themis or Themisitan family. Themis itself lies at the heart of a core group of several hundred larger asteroids, while the rest of the family consists of thousands of smaller fragments.
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