121 Hermione

121 Hermione is a very big asteroid of the dark C spectral type, orbiting in the Cybele group in the farther part of the main belt. As a C-type, it is probably made of carbonate. Hermione was found 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 an odd shape, as evidenced by adaptive optics images, the first of which were taken in December 2003, with the Keck telescope. Of many proposed shape models that agreed with the images, a "snowman"-like shape was found to best fit the seen precession rate of Hermione's moon. Observation of the moon's orbit has made possible a correct determination of Hermione's mass. For the best-fit "snowman" model, the density is found to be 1.8 ± 0.2 g/cm³. This gives a porosity of the order of 20%. This could be because the main asteroids are fractured solid bodies, but not a rubble pile (an asteroid that has been broken up by a collision and pulled back together by gravity). Occultations by Hermione have been successfully seen three times so far, the last time in February 2004. Moon (S/2002 (121) 1) A moon of Hermione was found in 2002 with the Keck II telescope. The moon is currently called S/2002 (121) 1. It has not yet been officially named, but the name "LaFayette" has been suggested by a group of astronomers. This is after the ship used in secret by the Marquis de Lafayette to reach America to help the Americans during the American Revolutionary War. S/2002 (121) 1 Discovery 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 28 September, 2002 Designations Minor planet category Main belt (Cybele) Orbital characteristics 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 Physical characteristics Dimensions 12 ± 4 km Mass ~1.6×1015 kg Equatorial escape velocity ~ 6 m/s Absolute magnitude (H) 13.0

121 Hermione Discovery Discovered by James Craig Watson Discovery date May 12, 1872 Designations Alternative designations 1970 VE Minor planet category Main belt (Cybele) Orbital characteristics Epoch December 31, 2006 (JD 2454100.5) Aphelion 588.328 Gm (3.933 AU) Perihelion 446.029 Gm (2.982 AU) Semi-major axis 517.179 Gm (3.457 AU) Eccentricity 0.138 Orbital period 2347.854 d (6.43 a) Average orbital speed 15.94 km/s Mean anomaly 248.068° Inclination 7.604° Longitude of ascending node 73.209° Argument of perihelion 296.215° Known satellites S/2002 (121) 1 Physical characteristics Dimensions 254±4 × 125±9 km Mean radius 95 km Mass 5.4 ± 0.3×1018 kg Mean density 1.8 ± 0.2 g/cm³ Equatorial surface gravity 0.022 m/s² Equatorial escape velocity 0.075 km/s Sidereal rotation period 0.2313 d (5.551 h) Axial tilt 73° Pole ecliptic latitude +10 ± 2° Pole ecliptic longitude 1.5 ± 2° Geometric albedo 0.0482 ± 0.002 Surface temp. min mean max Kelvin ~152 231 Celsius -44° Spectral type C Absolute magnitude (H) 7.31

Other websites

• 121 Hermione and S/2002 (121) 1, orbit data website maintained by F. Marchis. Includes adaptive optics images, orbit diagrams, and shape models.
• Data on (121) Hermione from Johnston's archive (maintained by W. R. Johnston)
• Tally of Asteroids Harboring Moons Grows Beyond 30 (Space.com, 3 October 2002)