121 Hermione

From Wikipedia, the free encyclopedia
Jump to navigation Jump to search
121 Hermione
121Hermione (Lightcurve Inversion).png
A three-dimensional model of 121 Hermione based on its light curve.
Discovered by James Craig Watson
Discovery date 1872-05-12
MPC designation (121) Hermione
Pronunciation /hɜːrˈm.əni/ hur-MY-ə-nee
Named after
1970 VE
Main belt Main belt (Cybele)
Orbital characteristics[2]
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
Inclination 7.604°
Known satellites S/2002 (121) 1
Physical characteristics
Dimensions 268×186×183 km[3]
254±4 × 125±9 km[4]
Mean radius
95 km[4]
Mass 5.38 ± 0.3 ×1018 kg[3]
5.4 ± 0.3×1018 kg[5]
Mean density
1.13 ± 0.3 g/cm³[3]
1.8 ± 0.2 g/cm³[5][6]
Equatorial surface gravity
0.022 m/s²[7]
Equatorial escape velocity
0.075 km/s[7]
0.2313 d (5.551 h)[8]
+10 ± 2°[5]
1.5 ± 2°
0.0482 ± 0.002[9]
Surface temp. min mean max
Kelvin ~152 231
Celsius -44°
C [10]

121 Hermione is a very large asteroid discovered in 1872. It orbits in the Cybele group[11] 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.[11]

Hermione is a Cybele asteroid and orbits beyond most of the main-belt asteroids.


Hermione was discovered by J. C. Watson on May 12, 1872, from Ann Arbor,[11] and named after Hermione, daughter of Menelaus and Helen in Greek mythology.

Physical properties[edit]

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.[4] 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.[5] 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.[5] 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.

S/2002 (121) 1
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
Main belt Main belt (Cybele)
Orbital characteristics[13]
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[4]
Mass ~1.6×1015 kg[14]
Equatorial escape velocity
~ 6 m/s


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.[citation needed]


  1. ^ Discovery Circumstances: Numbered Minor Planets, Minor Planet Centre
  2. ^ ASTORB orbital elements database, Lowell Observatory
  3. ^ a b c Jim Baer (2008). "Recent Asteroid Mass Determinations". Personal Website. Retrieved 2008-12-07. 
  4. ^ 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): 39–63. Bibcode:2006Icar..185...39M. doi:10.1016/j.icarus.2006.06.001. PMC 2600456Freely accessible. PMID 19081813. 
  5. ^ 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. 
  6. ^ Using the "snowman" shape model, which best matches the value of J2 implied from precession.
  7. ^ a b On the extremities of the long axis.
  8. ^ IAUC 8264
  9. ^ a b Supplemental IRAS minor planet survey Archived 2010-01-17 at WebCite
  10. ^ PDS node taxonomy database Archived 2010-01-17 at WebCite
  11. ^ a b c d e Linda T. Elkins-Tanton - Asteroids, Meteorites, and Comets (2010) - Page 96 (Google Books)
  12. ^ IAUC 7980 Archived 2006-05-01 at the Wayback Machine.
  13. ^ 121 Hermione and S/2002 (121) 1, orbit data website maintained by F. Marchis.
  14. ^ Assuming a similar density to the primary.

External links[edit]