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)[change | change source]

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
Discovery and designation [3]
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 28 September, 2002
Category Main belt (Cybele)
Orbit [4]
Longest distance from the center of its orbital path
("semi-major axis")
768 ± 11 km
How long it takes to complete an orbit 2.582 ± 0.002 d
Average speed 22 m/s
Angle above the reference plane
3 ± 2°
with respect to Hermione pole
What it orbits 121 Hermione
Size and other qualities
Measurements 12 ± 4 km [1]
Mass ~1.6×1015 kg [5]
Escape velocity ~ 6 m/s
True brightness
("absolute magnitude")
121 Hermione
Discovery and designation [6]
Discovered by James Craig Watson
Discovery date May 12, 1872
Other names 1970 VE
Category Main belt (Cybele)
Orbit [7]
Reference date December 31, 2006 (JD 2454100.5)
Longest distance from the Sun 588.328 Gm (3.933 AU)
Shortest distance from the Sun 446.029 Gm (2.982 AU)
Longest distance from the center of its orbital path
("semi-major axis")
517.179 Gm (3.457 AU)
How long it takes to complete an orbit 2347.854 d (6.43 a)
Average speed 15.94 km/s
Mean anomaly 248.068°
Angle above the reference plane
Natural things which orbit it S/2002 (121) 1
Size and other qualities
Measurements 254±4 × 125±9 km [1]
Average radius 95 km [1]
Mass 5.4 ± 0.3×1018 kg [2]
Average density 1.8 ± 0.2 g/cm³[2][8]
Surface gravity 0.022 m/s²[9]
Escape velocity 0.075 km/s[9]
Angle at which it turns
(in relation to its orbit)
How much light it reflects 0.0482 ± 0.002 [10]
Surface temp. Min. Avg. Max.
Kelvin ~152 231
Celsius -44°
Spectral type C [11]
True brightness
("absolute magnitude")
7.31 [10]

References[change | change source]

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