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.

## 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)

## Infobox (planet)

- **name**: S/2002 (121) 1
- **bgcolour**: #A0FFA0
- **minorplanet**: yes
- **discoverer**: W. J. Merline, P. M. Tamblyn,
C. Dumas, L. M. Close,
C. R. Chapman, F. Menard,
W. M. Owen, and D. C. Slater
- **discovered**: 28 September, 2002
- **mp category**: Main belt (Cybele asteroid, Cybele)
- **semimajor**: 768 ± 11 kilometer, km
- **eccentricity**: 0.001 ± 0.001
- **period**: 2.582 ± 0.002 day, d
- **avg speed**: 22 m/s
- **satellite of**: 121 Hermione
- **inclination**: 3 ± 2°
with respect to Hermione pole
- **dimensions**: 12 ± 4 km
- **mass**: ~1.6 kg
- **escape velocity**: ~ 6 m/s
- **abs magnitude**: 13.0


## Infobox (planet)

- **bgcolour**: #FFFFC0
- **minorplanet**: yes
- **name**: 121 Hermione
- **discoverer**: James Craig Watson
- **discovered**: May 12, 1872
- **alt names**: 1970 VE
- **mp category**: Main belt (Cybele asteroid, Cybele)
- **epoch**: December 31, 2006 (Julian day, JD 2454100.5)
- **semimajor**: 517.179 Giga, Gmetre, m (3.457 Astronomical unit, AU)
- **perihelion**: 446.029 Gm (2.982 AU)
- **aphelion**: 588.328 Gm (3.933 AU)
- **eccentricity**: 0.138
- **period**: 2347.854 day, d (6.43 Julian year (astronomy), a)
- **inclination**: 7.604degree (angle), °
- **asc node**: 73.209°
- **arg peri**: 296.215°
- **mean anomaly**: 248.068°
- **avg speed**: 15.94 km/second, s
- **satellites**: S/2002 (121) 1
- **dimensions**: 254±4 × 125±9 km
- **mean radius**: 95 km
- **mass**: 5.4 ± 0.3 kilogram, kg
- **density**: 1.8 ± 0.2 g/cubic centimetre, cm³
- **surface grav**: 0.022 m/s²
- **escape velocity**: 0.075 km/s
- **sidereal day**: 0.2313 day, d (5.551 hour, h)
- **axial tilt**: 73°
- **pole ecliptic lat**: +10 ± 2°
- **pole ecliptic lon**: 1.5 ± 2°
- **temp name1**: Kelvin
- **mean temp 1**: ~152
- **max temp 1**: 231
- **temp name2**: Celsius
- **max temp 2**: -44°
- **albedo**: 0.0482 ± 0.002
- **spectral type**: C-type asteroid, C
- **abs magnitude**: 7.31