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. Even if no name has been officially assigned to it yet, "LaFayette" was proposed by a group of astronomers in reference to the name of the fregate used in secret by the the Marquis de la Fayette to reach America to help the insurgents.
In December 2003, the adaptive optics of the Keck telescope were able to resolve the shape 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, the last time in February, 2004.
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