121 Hermione

121 Hermione 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.

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
LaFayette
Main belt (Cybele)
Orbital characteristics[5]
768 ± 11 km
Eccentricity 0.001 ± 0.001
2.582 ± 0.002 d
22 m/s
Inclination 3 ± 2°
with respect to Hermione pole
Satellite of 121 Hermione
Physical characteristics
Dimensions 12 ± 4 km[2]
Mass ~1.6×1015 kg[6]
Equatorial escape velocity
 ~ 6 m/s
13.0[2]
    121 Hermione

    A three-dimensional model of 121 Hermione based on its light curve.
    Discovery[7] and designation
    Discovered by James Craig Watson
    Discovery date 1872-05-12
    Designations
    Pronunciation /hərˈmaɪ.əniː/ hər-MY-ə-nee
    Named after
    		 Hermione
    1970 VE
    Main belt (Cybele)
    Orbital characteristics[8]
    Epoch December 31, 2006 (JD 2454100.5)
    Aphelion 588.328 Gm (3.933 AU)
    Perihelion 446.029 Gm (2.982 AU)
    517.179 Gm (3.457 AU)
    Eccentricity 0.138
    2347.854 d (6.43 a)
    15.94 km/s
    248.068°
    Inclination 7.604°
    73.209°
    296.215°
    Known satellites S/2002 (121) 1
    Physical characteristics
    Dimensions 268×186×183 km[9]
    254±4 × 125±9 km[2]
    Mean radius
    		 95 km[2]
    Mass 5.38 ± 0.3 ×1018 kg[9]
    5.4 ± 0.3×1018 kg[3]
    Mean density
    		 1.13 ± 0.3 g/cm³[9]
    1.8 ± 0.2 g/cm³[3][10]
    Equatorial surface gravity
    		 0.022 m/s²[11]
    Equatorial escape velocity
    		 0.075 km/s[11]
    0.2313 d (5.551 h)[12]
    73°
    Pole ecliptic latitude
    		 +10 ± 2°[3]
    Pole ecliptic longitude
    		 1.5 ± 2°
    0.0482 ± 0.002[13]
    Surface temp. min mean max
    Kelvin ~152 231
    Celsius -44°
    C [14]
    7.31[13]

      References

      1. ↑ 1.0 1.1 1.2 1.3 1.4 Linda T. Elkins-Tanton - Asteroids, Meteorites, and Comets (2010) - Page 96 (Google Books)
      2. ↑ 2.0 2.1 2.2 2.3 2.4 F. Marchis et al. (2006). "Shape, size and multiplicity of main-belt asteroids I. Keck Adaptive Optics survey". Icarus 185 (1): 39–63. Bibcode:2006Icar..185...39M. doi:10.1016/j.icarus.2006.06.001. PMC 2600456. PMID 19081813.
      3. ↑ 3.0 3.1 3.2 3.3 3.4 F. Marchis et al. (2005). "Mass and density of Asteroid 121 Hermione from an analysis of its companion orbit". Icarus 178 (2): 450. Bibcode:2005Icar..178..450M. doi:10.1016/j.icarus.2005.05.003.
      4. ↑ IAUC 7980
      5. ↑ 121 Hermione and S/2002 (121) 1, orbit data website maintained by F. Marchis.
      6. ↑ Assuming a similar density to the primary.
      7. ↑ Discovery Circumstances: Numbered Minor Planets, Minor Planet Centre
      8. ↑ ASTORB orbital elements database, Lowell Observatory
      9. ↑ 9.0 9.1 9.2 Jim Baer (2008). "Recent Asteroid Mass Determinations". Personal Website. Retrieved 2008-12-07.
      10. ↑ Using the "snowman" shape model, which best matches the value of J2 implied from precession.
      11. ↑ 11.0 11.1 On the extremities of the long axis.
      12. ↑ IAUC 8264
      13. ↑ 13.0 13.1 Supplemental IRAS minor planet survey
      14. ↑ PDS node taxonomy database

      External links