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121 Hermione is a very large and dark Main belt asteroid. It is probably composed of carbonaceous materials.
It was discovered by J. C. Watson on May 12, 1872 and named after Hermione, daughter of Menelaus and Helen in Greek mythology.
Hermione is a Cybele asteroid and orbits beyond most of the Main belt asteroids.
A companion of Hermione was found in 2002 from the Keck II telescope. The satellite measures about 18 km in diameter and orbits at a distance of about 800 km. One revolution around the primary takes 1.63 days to complete. The satellite was designated as S/2002 (121) 1 but no official name has been assigned to it yet.
The adaptive optics-enhanced Keck telescope was able to resolve the disc of Hermione. The bi-lobated shape of Hermione is easily perceived from the images.
The satellite discovery made possible an accurate measurement of mass and density of the asteroid. Early results indicate a very porous body, but further studies have cast doubt on this.
Occultations by Hermione have been successfully observed three times so far, last time in February, 2004.
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