121 Hermione

121 Hermione is a very large binary asteroid discovered in 1872. It orbits in the Cybele group in the far outer asteroid belt.^[11] 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.^[11]

121 Hermione

121 Hermione and its moon
Discovery[1]
Discovered by	James Craig Watson
Discovery date	12 May 1872
Designations
MPC designation	(121) Hermione
Pronunciation	/hɜːrˈmaɪ.əniː/[2]
Named after	Hermione[3] (Greek mythology)
Alternative designations	A872 JA; 1970 VE
Minor planet category	main-belt · Cybele
Adjectives	Hermionean /hɜːrmaɪ.əˈniːən/
Orbital characteristics[4]
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
Semi-major axis	3.4478 AU
Eccentricity	0.1331
Orbital period (sidereal)	6.40 yr (2,338 d)
Mean anomaly	157.08°
Mean motion	0° 9m 14.4s / day
Inclination	7.5975°
Longitude of ascending node	73.127°
Argument of perihelion	298.18°
Known satellites	S/2002 (121) 1
Physical characteristics
Dimensions	268 × 186 × 183 km[5] (254 ± 4) × (125 ± 9) km[6]
Mean radius	95 km[6]
Volume	(3.0±0.4)×106 km3[7]
Mass	(5.381±5%)×1018 kg[7]
Mean density	1.8 ± 0.2 g/cm3[7][a]
Equatorial surface gravity	0.022 m/s2[b]
Equatorial escape velocity	0.075 km/s[b]
Sidereal rotation period	0.2313 d (5.551 h)[8]
Axial tilt	73°
Pole ecliptic longitude	1.5 ± 2°
Pole ecliptic latitude	+10 ± 2°[7]
Geometric albedo	0.0482 ± 0.002[9]
Spectral type	C[10]
Absolute magnitude (H)	7.31[9]

Discovery

Hermione was discovered by J. C. Watson on 12 May 1872 from Ann Arbor, Michigan, in the United States,^[11] and named after Hermione, daughter of Menelaus and Helen in Greek mythology.^[3]

Physical properties

Lightcurve-based 3D-model of Hermione

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[12]
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
Alternative designations	LaFayette
Minor planet category	main-belt · Cybele
Orbital characteristics[13]
Semi-major axis	768 ± 11 km
Eccentricity	0.001 ± 0.001
Orbital period (sidereal)	2.582 ± 0.002 d
Inclination	3 ± 2° (with respect to Hermione pole)
Satellite of	121 Hermione
Physical characteristics
Dimensions	12 ± 4 km[6]
Mass	~1.6×1015 kg[c]
Absolute magnitude (H)	13.0[6]

Moon

A satellite of Hermione was discovered in 2002 with the Keck II telescope.^[11] It is about 8 miles (13 km) in diameter.^[11] 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.^[14][6]

Notes

1. Using the "snowman" shape model, which best matches the value of J2 implied from precession.
2. On the extremities of the long axis.
3. Assuming a similar density to the primary.