121 Hermione

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.[13] 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.[13]

121 Hermione
121Hermione (Lightcurve Inversion).png
Lightcurve-based 3D-model of Hermione
Discovery [1]
Discovered byJames Craig Watson
Discovery date12 May 1872
(121) Hermione
Named after
(Greek mythology)
1970 VE
main-belt · Cybele
AdjectivesHermionean /hɜːrm.əˈnən/
Orbital characteristics[4]
Epoch 23 March 2018 (JD 2458200.5)
Uncertainty parameter 0
Observation arc145.96 yr (53,312 d)
Aphelion3.9067 AU
Perihelion2.9889 AU
3.4478 AU
6.40 yr (2,338 d)
0° 9m 14.4s / day
Known satellitesS/2002 (121) 1
Physical characteristics
Dimensions268×186×183 km[5]
254±4 × 125±9 km[6]
Mean radius
95 km[6]
Mass5.38 ± 0.3 ×1018 kg[5]
5.4 ± 0.3×1018 kg[7]
Mean density
1.13 ± 0.3 g/cm³[5]
1.8 ± 0.2 g/cm³[7][8]
Equatorial surface gravity
0.022 m/s²[9]
Equatorial escape velocity
0.075 km/s[9]
0.2313 d (5.551 h)[10]
+10 ± 2°[7]
1.5 ± 2°
0.0482 ± 0.002[11]
C [12]


Hermione was discovered by J. C. Watson on 12 May 1872 from Ann Arbor, Michigan, in the United States,[13] and named after Hermione, daughter of Menelaus and Helen in Greek mythology.[2]

Physical propertiesEdit

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.

S/2002 (121) 1
Discovered byW. J. Merline,
P. M. Tamblyn,
C. Dumas,
L. M. Close,
C. R. Chapman,
F. Menard,
W. M. Owen,
and D. C. Slater
Discovery date2002-09-28
main-belt · Cybele
Orbital characteristics[15]
768 ± 11 km
Eccentricity0.001 ± 0.001
2.582 ± 0.002 d
Inclination3 ± 2°
(with respect to Hermione pole)
Satellite of121 Hermione
Physical characteristics
Dimensions12 ± 4 km[6]
Mass~1.6×1015 kg[16]


A satellite of Hermione was discovered in 2002 with the Keck II telescope.[13] It is about 8 miles (13 km) in diameter.[13] 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. ^ "121 Hermione". Minor Planet Center. Retrieved 14 September 2018. CS1 maint: discouraged parameter (link)
  2. ^ a b Schmadel, Lutz D. (2007). "(121) Hermione". Dictionary of Minor Planet Names – (121) Hermione. Springer Berlin Heidelberg. p. 26. doi:10.1007/978-3-540-29925-7_122. ISBN 978-3-540-00238-3.
  3. ^ Noah Webster (1884) A Practical Dictionary of the English Language
  4. ^ "JPL Small-Body Database Browser: 121 Hermione" (2018-05-25 last obs.). Jet Propulsion Laboratory. Retrieved 14 September 2018. CS1 maint: discouraged parameter (link)
  5. ^ a b c Jim Baer (2008). "Recent Asteroid Mass Determinations". Personal Website. Archived from the original on 8 July 2013. Retrieved 7 December 2008. CS1 maint: discouraged parameter (link)
  6. ^ 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 2600456. PMID 19081813.
  7. ^ 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): 450–464. Bibcode:2005Icar..178..450M. doi:10.1016/j.icarus.2005.05.003.
  8. ^ Using the "snowman" shape model, which best matches the value of J2 implied from precession.
  9. ^ a b On the extremities of the long axis.
  10. ^ IAUC 8264 Archived 9 July 2008 at the Wayback Machine
  11. ^ a b Supplemental IRAS minor planet survey Archived 2009-08-17 at the Wayback Machine
  12. ^ PDS node taxonomy database Archived 2009-08-05 at the Wayback Machine
  13. ^ a b c d e Linda T. Elkins-Tanton (2010). Asteroids, Meteorites, and Comets. Infobase Publishing. p. 96. ISBN 978-1-4381-3186-3.
  14. ^ IAUC 7980 Archived 2006-05-01 at the Wayback Machine
  15. ^ 121 Hermione and S/2002 (121) 1, orbit data website maintained by F. Marchis.
  16. ^ Assuming a similar density to the primary.

External linksEdit