121 Hermione

121 Hermione ( /hərˈmaɪ.əniː/ hər-MY-ə-nee) is a very large asteroid discovered in 1872. It orbits in the Cybele group^[1] in the far outer asteroid belt. As an asteroid of the dark C spectral type, it is probably composed of carbonaceous materials. In 2002, a small moon was found to be orbiting Hermione.^[1]

Discovery

Hermione was discovered by J. C. Watson on May 12, 1872 from Ann Arbor,^[1] and named after Hermione, daughter of Menelaus and Helen in Greek mythology.

Orbit and moon

Hermione is a Cybele asteroid and orbits beyond most of the main-belt asteroids.

A satellite of Hermione was discovered in 2002 with the Keck II telescope.^[1] It is about 8 miles (13 km) in diameter.^[1] The satellite is provisionally 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.^{[citation needed]}

Physical properties

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.^[2] 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.^[3] 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.^[3] 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.

S/2002 (121) 1 Discovery[4] and designation Discovered by W. J. Merline, P. M. Tamblyn, C. Dumas, L. M. Close, C. R. Chapman, F. Menard, W. M. Owen, and D. C. Slater Discovery date 2002-09-28 Designations Named after Hermione Minor planet category Main belt (Cybele) Orbital characteristics[5] Semi-major axis 768 ± 11 km Eccentricity 0.001 ± 0.001 Orbital period 2.582 ± 0.002 d Average orbital speed 22 m/s Inclination 3 ± 2° with respect to Hermione pole Satellite of 121 Hermione Proper orbital elements Physical characteristics Dimensions 12 ± 4 km[2] Mass ~1.6×1015 kg[6] Equatorial escape velocity ~ 6 m/s Absolute magnitude (H) 13.0[2]