121 Hermione

121 Hermione
Lightcurve-based 3D-model of Hermione
Discovery [1]
Discovered by James Craig Watson
Discovery date 12 May 1872
Designations
MPC designation (121) Hermione
Pronunciation /hɜːrˈmaɪ.əni/
hur-MY-ə-nee
Named after
 Hermione [2]
(Greek mythology)
1970 VE
main-belt · Cybele
Orbital characteristics [3]
Epoch 23 March 2018 (JD 2458200.5)
Uncertainty parameter 0
Observation arc 145.96 yr (53,312 d)
Aphelion 3.9067 AU
Perihelion 2.9889 AU
3.4478 AU
Eccentricity 0.1331
6.40 yr (2,338 d)
157.08°
0° 9m 14.4s / day
Inclination 7.5975°
73.127°
298.18°
Known satellites S/2002 (121) 1
Physical characteristics
Dimensions 268×186×183 km[4]
254±4 × 125±9 km[5]
Mean radius
 95 km[5]
Mass 5.38 ± 0.3 ×1018 kg[4]
5.4 ± 0.3×1018 kg[6]
Mean density
 1.13 ± 0.3 g/cm³[4]
1.8 ± 0.2 g/cm³[6][7]
Equatorial surface gravity
 0.022 m/s²[8]
Equatorial escape velocity
 0.075 km/s[8]
0.2313 d (5.551 h)[9]
73°
+10 ± 2°[6]
1.5 ± 2°
0.0482 ± 0.002[10]
C [11]
7.31[10]

    121 Hermione is a very large binary asteroid discovered in 1872. It orbits in the Cybele group in the far outer asteroid belt.[12] 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.[12]

    Discovery

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

    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.[5] 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.[6] 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.[6] 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[13]
    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[14]
    768 ± 11 km
    Eccentricity 0.001 ± 0.001
    2.582 ± 0.002 d
    Inclination 3 ± 2°
    (with respect to Hermione pole)
    Satellite of 121 Hermione
    Physical characteristics
    Dimensions 12 ± 4 km[5]
    Mass ~1.6×1015 kg[15]
    13.0[5]

      Moon

      A satellite of Hermione was discovered in 2002 with the Keck II telescope.[12] It is about 8 miles (13 km) in diameter.[12] 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. 1 2 Schmadel, Lutz D. (2007). Dictionary of Minor Planet Names – (121) Hermione. Springer Berlin Heidelberg. p. 26. ISBN 978-3-540-00238-3. Retrieved 14 September 2018.
      3. ↑ "JPL Small-Body Database Browser: 121 Hermione" (2018-05-25 last obs.). Jet Propulsion Laboratory. Retrieved 14 September 2018.
      4. 1 2 3 Jim Baer (2008). "Recent Asteroid Mass Determinations". Personal Website. Retrieved 2008-12-07.
      5. 1 2 3 4 5 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.
      6. 1 2 3 4 5 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.
      7. ↑ Using the "snowman" shape model, which best matches the value of J2 implied from precession.
      8. 1 2 On the extremities of the long axis.
      9. ↑ IAUC 8264
      10. 1 2 Supplemental IRAS minor planet survey Archived 2010-01-17 at WebCite
      11. ↑ PDS node taxonomy database Archived 2010-01-17 at WebCite
      12. 1 2 3 4 5 Linda T. Elkins-Tanton - Asteroids, Meteorites, and Comets (2010) - Page 96 (Google Books)
      13. ↑ IAUC 7980 Archived 2006-05-01 at the Wayback Machine.
      14. ↑ 121 Hermione and S/2002 (121) 1, orbit data website maintained by F. Marchis.
      15. ↑ Assuming a similar density to the primary.

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