121 Hermione

Lore & Background

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

A satellite of Hermione was discovered in 2002 with the Keck II telescope. It is about 8 miles (13 km) in diameter. 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 Marquis de Lafayette to reach America to help the insurgents.

Reader's Guide

121 Hermione is significant as a very large binary asteroid in the Cybele group of the far outer asteroid belt. Its discovery in 1872 by J. C. Watson added to the catalog of early asteroid finds. The asteroid's dark C spectral type suggests it is composed of carbonaceous materials, common among primitive bodies. Its most notable feature is its bi-lobed, snowman-like shape, confirmed by adaptive optics images from the Keck telescope in December 2003. This shape, modeled as two overlapping spheres, allowed astronomers to determine the asteroid's density and porosity, indicating it may be a fractured solid body rather than a loose rubble pile. The discovery of its moon in 2002, S/2002 (121) 1, made possible an accurate mass determination and provided insights into the system's dynamics. The proposed name 'LaFayette' for the moon, though unofficial, reflects a historical connection. Three successful occultation observations, the last in February 2004, have further contributed to knowledge of its orbit and size. The asteroid remains a subject of study for understanding binary asteroid formation and the properties of primitive bodies in the outer belt.

Did You Know?

• It has a bi-lobed, snowman-like shape approximated by two overlapping spheres of radii 80 and 60 km.
• Its moon, S/2002 (121) 1, is about 13 km in diameter and was discovered in 2002 with the Keck II telescope.
• The asteroid's density is 1.8 ± 0.2 g/cm³, with a porosity of about 20%, suggesting it is a fractured solid body.