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]

Contents

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
Named after Hermione
Minor planet
category		 Main belt (Cybele)
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
Dimensions 12 ± 4 km[2]
Mass ~1.6×1015 kg[6]
Equatorial escape velocity ~ 6 m/s
Absolute magnitude (H) 13.0[2]
121 Hermione
Discovery[7] and designation
Discovered by James Craig Watson
Discovery date 1872-05-12
Designations
Named after [[]]
Alternate name(s) 1970 VE
Minor planet
category		 Main belt (Cybele)
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°
Satellites S/2002 (121) 1
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]
Sidereal rotation
period		 0.2313 d (5.551 h)[12]
Axial tilt 73°
Pole ecliptic latitude +10 ± 2°[3]
Pole ecliptic longitude 1.5 ± 2°
Geometric albedo 0.0482 ± 0.002[13]
Surface temp.
   Kelvin
   Celsius
min mean max
~152 231
-44°
Spectral type C [14]
Absolute magnitude (H) 7.31[13]

References

  1. ^ a b c d e Linda T. Elkins-Tanton - Asteroids, Meteorites, and Comets (2010) - Page 96 (Google Books)
  2. ^ a b c d e F. Marchis et al. (2006). "Shape, size and multiplicity of main-belt asteroids I. Keck Adaptive Optics survey". Icarus 185 (1): 39. Bibcode 2006Icar..185...39M. doi:10.1016/j.icarus.2006.06.001. PMC 2600456. PMID 19081813. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=2600456. 
  3. ^ a b c d e 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. ^ a b c Jim Baer (2008). "Recent Asteroid Mass Determinations". Personal Website. http://home.earthlink.net/~jimbaer1/astmass.txt. Retrieved 2008-12-07. 
  10. ^ Using the "snowman" shape model, which best matches the value of J2 implied from precession.
  11. ^ a b On the extremities of the long axis.
  12. ^ IAUC 8264
  13. ^ a b Supplemental IRAS minor planet survey
  14. ^ PDS node taxonomy database

External links

120 Lachesis•121 Hermione• 122 Gerda
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