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
Lightcurve-based 3D-model of Hermione
Discovery [1]
Discovered byJames Craig Watson
Discovery date12 May 1872
Designations
(121) Hermione
Pronunciation/hɜːrˈmaɪ.əniː/[2]
Named after
Hermione [3]
(Greek mythology)
1970 VE
main-belt · Cybele
AdjectivesHermionean /hɜːrmaɪ.əˈniːən/
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
Eccentricity0.1331
6.40 yr (2,338 d)
157.08°
0° 9m 14.4s / day
Inclination7.5975°
73.127°
298.18°
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]
73°
+10 ± 2°[7]
1.5 ± 2°
0.0482 ± 0.002[11]
C [12]
7.31[11]

    Discovery

    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.[3]

    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.[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
    Discovery[14]
    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
    Designations
    LaFayette
    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]
    13.0[6]

      Moon

      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.

      References
      1. "121 Hermione". Minor Planet Center. Retrieved 14 September 2018.
      2. Noah Webster (1884) A Practical Dictionary of the English Language
      3. 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.
      4. "JPL Small-Body Database Browser: 121 Hermione" (2018-05-25 last obs.). Jet Propulsion Laboratory. Retrieved 14 September 2018.
      5. Jim Baer (2008). "Recent Asteroid Mass Determinations". Personal Website. Archived from the original on 8 July 2013. Retrieved 7 December 2008.
      6. 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. 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. On the extremities of the long axis.
      10. IAUC 8264 Archived 9 July 2008 at the Wayback Machine
      11. 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. 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.

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