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121 Hermione

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121 Hermione is a very big asteroid of the dark C spectral type, orbiting in the Cybele group in the farther part of the main belt. As a C-type, it is probably made of carbonate.

Hermione was found by J. C. Watson on May 12, 1872, and named after Hermione, daughter of Menelaus and Helen in Greek mythology. Hermione is a Cybele asteroid and orbits beyond most of the main belt asteroids.

The asteroid has an odd shape, as evidenced by adaptive optics images, the first of which were taken in December 2003, with the Keck telescope.[1] Of many proposed shape models that agreed with the images, a "snowman"-like shape was found to best fit the seen precession rate of Hermione's moon.[2]

Observation of the moon's orbit has made possible a correct determination of Hermione's mass.[2] For the best-fit "snowman" model, the density is found to be 1.8 ± 0.2 g/cm³. This gives a porosity of the order of 20%. This could be because the main asteroids are fractured solid bodies, but not a rubble pile (an asteroid that has been broken up by a collision and pulled back together by gravity).

Occultations by Hermione have been successfully seen three times so far, the last time in February 2004.

Moon (S/2002 (121) 1)

A moon of Hermione was found in 2002 with the Keck II telescope. The moon is currently called S/2002 (121) 1. It has not yet been officially named, but the name "LaFayette" has been suggested by a group of astronomers. This is after the ship used in secret by the Marquis de Lafayette to reach America to help the Americans during the American Revolutionary War.

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 date28 September, 2002
Main belt (Cybele)
Orbital characteristics[4]
768 ± 11 km
Eccentricity0.001 ± 0.001
2.582 ± 0.002 d
22 m/s
Inclination3 ± 2°
with respect to Hermione pole
Satellite of121 Hermione
Physical characteristics
Dimensions12 ± 4 km [1]
Mass~1.6×1015 kg [5]
Equatorial escape velocity
~ 6 m/s

121 Hermione
Discovered byJames Craig Watson
Discovery dateMay 12, 1872
1970 VE
Main belt (Cybele)
Orbital characteristics[7]
Epoch December 31, 2006 (JD 2454100.5)
Aphelion588.328 Gm (3.933 AU)
Perihelion446.029 Gm (2.982 AU)
517.179 Gm (3.457 AU)
2347.854 d (6.43 a)
15.94 km/s
Known satellitesS/2002 (121) 1
Physical characteristics
Dimensions254±4 × 125±9 km [1]
Mean radius
95 km [1]
Mass5.4 ± 0.3×1018 kg [2]
Mean density
1.8 ± 0.2 g/cm³[2][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°[2]
1.5 ± 2°
0.0482 ± 0.002 [11]
Surface temp. min mean max
Kelvin ~152 231
Celsius -44°
C [12]
7.31 [11]


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