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121 Hermione (pronounced /hɚˈmaɪəni/ hər-mye'-ə-nee) is a very large asteroid of the dark C spectral type, orbiting in the Cybele group in the far outer main belt. As a C-type, it is probably composed of carbonaceous materials.
Hermione 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.
The asteroid has a bi-lobed shape, as evidenced by adaptive optics images, the first of which were taken in December 2003, with the Keck telescope [1]. Of several proposed shape models that agreed with the images, a "snowman"-like shape was found to best fit the observed precession rate of Hermione's satellite [2]. In this "snowman" model, the asteroid's shape can be approximated by two partially overlapping spheres of radii 80 and 60 km, whose centers are separated by 115 km. A simple ellipsoid shape was ruled out.
Observation of the satellite's orbit has made possible an accurate determination of Hermione's mass [2]. For the best-fit "snowman" model, the density is found to be 1.8 ± 0.2 g/cm³, giving a porosity of the order of 20%, and possibly indicating that the main components are fractured solid bodies, but that the asteroid is rather not a rubble pile.
Occultations by Hermione have been successfully observed three times so far, the last time in February, 2004.
Satellite (S/2002 (121) 1)
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A satellite of Hermione was discovered in 2002 with the Keck II telescope. The satellite is currently designated S/2002 (121) 1. It has not yet been officially named, but "LaFayette" has been proposed by a group of astronomers in reference to the frigate used in secret by the Marquis de Lafayette to reach America to help the insurgents.
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