Hermione ( minor planet designation: 121 Hermione) is a very large binary asteroid discovered in 1872. It orbits in the

Cybele group The Cybele asteroids (also known as the "Cybeles") are a dynamical group of asteroids, named after the asteroid 65 Cybele. Considered by some as the last outpost of an extended asteroid belt, the group consists of just over 2000 members an ...

in the far outer

asteroid belt The asteroid belt is a torus-shaped region in the Solar System, located roughly between the orbits of the planets Jupiter and Mars. It contains a great many solid, irregularly shaped bodies, of many sizes, but much smaller than planets, c ...

. As an asteroid of the dark C spectral type, it is probably composed of

carbonaceous Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon makes up ...

materials. In 2002, a small

moon The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...

was found to be orbiting Hermione.

Discovery

Hermione was discovered by J. C. Watson on 12 May 1872 from

Ann Arbor, Michigan Ann Arbor is a city in the U.S. state of Michigan and the county seat of Washtenaw County, Michigan, Washtenaw County. The 2020 United States census, 2020 census recorded its population to be 123,851. It is the principal city of the Ann Arbor ...

, in the United States, and named after

Hermione Hermione may refer to: People * Hermione (given name), a female given name * Hermione (mythology), only daughter of Menelaus and Helen in Greek mythology and original bearer of the name Arts and literature * ''Cadmus et Hermione'', an opera by ...

, daughter of

Menelaus In Greek mythology, Menelaus (; grc-gre, Μενέλαος , 'wrath of the people', ) was a king of Mycenaean (pre- Dorian) Sparta. According to the ''Iliad'', Menelaus was a central figure in the Trojan War, leading the Spartan contingent of th ...

and

Helen Helen may refer to: People * Helen of Troy, in Greek mythology, the most beautiful woman in the world * Helen (actress) (born 1938), Indian actress * Helen (given name), a given name (including a list of people with the name) Places * Helen, ...

Greek mythology A major branch of classical mythology, Greek mythology is the body of myths originally told by the Ancient Greece, ancient Greeks, and a genre of Ancient Greek folklore. These stories concern the Cosmogony, origin and Cosmology#Metaphysical co ...

Physical properties

left, Lightcurve-based 3D-model of Hermione The asteroid has a bi-lobed shape, as evidenced by

adaptive optics Adaptive optics (AO) is a technology used to improve the performance of optical systems by reducing the effect of incoming wavefront distortions by deforming a mirror in order to compensate for the distortion. It is used in astronomical tele ...

images, the first of which were taken in December 2003 with the

Keck telescope The W. M. Keck Observatory is an astronomical observatory with two telescopes at an elevation of 4,145 meters (13,600 ft) near the summit of Mauna Kea in the U.S. state In the United States, a state is a constituent political entit ...

. Of several proposed shape models that agreed with the images, a "snowman"-like shape was found to best fit the observed

precession Precession is a change in the orientation of the rotational axis of a rotating body. In an appropriate reference frame it can be defined as a change in the first Euler angle, whereas the third Euler angle defines the rotation itself. In othe ...

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 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 In astronomy, a rubble pile is a celestial body that is not a monolith, consisting instead of numerous pieces of rock that have coalesced under the influence of gravity. Rubble piles have low density because there are large cavities between the ...

Occultation An occultation is an event that occurs when one object is hidden from the observer by another object that passes between them. The term is often used in astronomy, but can also refer to any situation in which an object in the foreground blocks ...

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

Moon

satellite A satellite or artificial satellite is an object intentionally placed into orbit in outer space. Except for passive satellites, most satellites have an electricity generation system for equipment on board, such as solar panels or radioisotope ...

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 the

Marquis de Lafayette Marie-Joseph Paul Yves Roch Gilbert du Motier, Marquis de La Fayette (6 September 1757 – 20 May 1834), known in the United States as Lafayette (, ), was a French aristocrat, freemason and military officer who fought in the American Revolutio ...

to reach America to help the insurgents.

References

External links

*
121 Hermione and S/2002 (121) 1
', orbit data website maintained by F. Marchis. Includes adaptive optics images, orbit diagrams, and shape models.

datasheet, johnstonsarchive.net

Robert Johnston, johnstonsarchive.net *

' (Space.com, 3 October 2002) * * {{DEFAULTSORT:Hermione 000121 Discoveries by James Craig Watson Named minor planets 000121 000121 000121 000121 18720512