Hermione (minor planet designation: 121 Hermione) is a very large binary asteroid discovered in 1872. It orbits in the Cybele group in the far outer asteroid belt.[11] 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.[11]
 121 Hermione and its moon | |
| Discovery[1] | |
|---|---|
| Discovered by | James Craig Watson |
| Discovery date | 12 May 1872 |
| Designations | |
| (121) Hermione | |
| Pronunciation | /hɜːrˈmaɪ.əniː/[2] |
Named after | Hermione[3] (Greek mythology) |
| A872 JA; 1970 VE | |
| main-belt · Cybele | |
| Adjectives | Hermionean /hɜːrmaɪ.əˈniːən/ |
| Orbital characteristics[4] | |
| Epoch 23 March 2018 (JD 2458200.5) | |
| Uncertainty parameter 0 | |
| Observation arc | 145.96 yr (53,312 d) |
| Aphelion | 3.9067 AU |
| Perihelion | 2.9889 AU |
| 3.4478 AU | |
| Eccentricity | 0.1331 |
| 6.40 yr (2,338 d) | |
| 157.08° | |
| 0° 9m 14.4s / day | |
| Inclination | 7.5975° |
| 73.127° | |
| 298.18° | |
| Known satellites | S/2002 (121) 1 |
| Physical characteristics | |
| Dimensions | 268 × 186 × 183 km[5] (254 ± 4) × (125 ± 9) km[6] |
Mean radius | 95 km[6] |
| Volume | (3.0±0.4)×106 km3[7] |
| Mass | (5.381±5%)×1018 kg[7] |
Mean density | 1.8 ± 0.2 g/cm3[7][a] |
Equatorial surface gravity | 0.022 m/s2[b] |
Equatorial escape velocity | 0.075 km/s[b] |
| 0.2313 d (5.551 h)[8] | |
| 73° | |
Pole ecliptic latitude | +10 ± 2°[7] |
Pole ecliptic longitude | 1.5 ± 2° |
| 0.0482 ± 0.002[9] | |
| C[10] | |
| 7.31[9] | |
Discovery
Hermione was discovered by J. C. Watson on 12 May 1872 from Ann Arbor, Michigan, in the United States,[11] and named after Hermione, daughter of Menelaus and Helen in Greek mythology.[3]
Physical properties
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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.[6] 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.[7] 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 a distance of 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.[7] For the best-fit "snowman" model, the density is found to be 1.8 ± 0.2 g/cm3, giving a porosity on the order of 20%, and possibly indicating that the main components are fractured solid bodies, rather than the asteroid being a rubble pile.
Occultations by Hermione have been successfully observed three times so far, the last time in February 2004.
| Discovery[12] | |
|---|---|
| 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 | |
| LaFayette | |
| main-belt · Cybele | |
| Orbital characteristics[13] | |
| 768 ± 11 km | |
| Eccentricity | 0.001 ± 0.001 |
| 2.582 ± 0.002 d | |
| Inclination | 3 ± 2° (with respect to Hermione pole) |
| Satellite of | 121 Hermione |
| Physical characteristics | |
| Dimensions | 12 ± 4 km[6] |
| Mass | ~1.6×1015 kg[c] |
| 13.0[6] | |
Moon
A satellite of Hermione was discovered in 2002 with the Keck II telescope.[11] It is about 8 miles (13 km) in diameter.[11] 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.[14][6]
Notes
References
External links
- 121 Hermione and S/2002 (121) 1, orbit data website maintained by F. Marchis. Includes adaptive optics images, orbit diagrams, and shape models.
- (121) Hermione, datasheet, johnstonsarchive.net
- Asteroids with Satellites, Robert Johnston, johnstonsarchive.net
- Tally of Asteroids Harboring Moons Grows Beyond 30 (Space.com, 3 October 2002)
- 121 Hermione at AstDyS-2, Asteroids—Dynamic Site
- 121 Hermione at the JPL Small-Body Database
