30th Largest Asteroid, 121 Hermione

121 Hermione is a large asteroid in the outer main asteroid belt. It is the 30th largest asteroid currently known.

Discovery

Hermione was discovered by James Craig Watson on May 12, 1872 from Ann Arbor, Michigan, USA.

Naming

Hermione the asteroid was named after Hermione, daughter of Menelaus and Helen in Greek mythology.

In Greek mythology, Menelaus was a legendary king of Mycenaean Sparta, the husband of Helen of Troy, and a central figure in the Trojan War.

Helen of Troy, also known as Helen of Sparta, was the daughter of Zeus and Leda (or Nemesis), step-daughter of King Tyndareus and wife of Menelaus. Her abduction by Paris brought about the Trojan War.

Stats

Diameter (mean): 190 km
Aphelion: 3.933 AU
Perihelion: 2.983 AU
Semi-major axis: 3.449 AU
Orbital Period: 6.41 years
Rotation period: 5.55 hrs
Date discovered: 1872.5.12
Class: C
Group: Cybele group
Type: Outer Main-belt Asteroid
Satellite: 1
(data from JPL Small-Body Database)

Orbit

Hermione is a Cybele asteroid and orbits beyond most of the main-belt asteroids.

Satellite

A satellite of Hermione was discovered in 2002 with the Keck II telescope.

Physical properties

Hermione is an asteroid of the dark C spectral type, and is probably composed of carbonaceous materials.

Hermione has a bi-lobed shape, as evidenced by adaptive optics images, the first of which were taken in December 2003, with the Keck telescope. 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. 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 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. For the best-fit "snowman" model, the density is found to be 1.8 ± 0.2 g/cm³, giving a porosity of the order of 20%, and possibly indicating that the main components are fractured solid bodies, but that the asteroid is rather not a rubble pile.

Star Occultation

Occultations by Hermione have been successfully observed three times so far, the last time in February 16, 2004, when Hermione occulted TYC 1905-00864-1 in Gemini.