617 Patroclus

617 Patroclus (pə-troe'-kləs, ) is a binary minor planet made up of two similarly-sized objects orbiting their common centre of gravity. It is a Trojan, sharing an orbit with Jupiter. It was found in 1907 by August Kopff, and was the second Trojan asteroid to be found. Recent evidence suggests that the objects are icy comets, rather than rocky asteroids.

 Orbit 
Patroclus orbits in Jupiter's trailing Lagrangian point, L5, in an orbit called the 'Trojan node' after one of the sides in the legendary Trojan War (the other node is called the 'Greek node'). Patroclus is the only object in the Trojan camp to be named after a Greek character; the naming rules for the Trojan asteroids were not made until after Patroclus was named (similarly, the asteroid Hektor is the only Trojan character to appear in the Greek camp).

 Binary 
In 2001, it was found that Patroclus is a binary object, made up of two asteroids which are almost the same size. In February, 2006, a team of astronomers led by Franck Marchis measured accurately the orbit of the system using the Keck Laser guide star adaptive optics system. They thought that the two asteroids orbit around their center of mass in 4.283±0.004 d at a distance of 680±20 km, describing a close to circular orbit. Putting together their sightings with thermal measurements taken in November 2000, the team thought what the size of the asteroids of the system could be. The slightly bigger asteroids, which measures 122 km in diameter, continues to have the name Patroclus. The smaller asteroid, measuring 112 km, is now named Menoetius  (full name (617) Patroclus I Menoetius), after the legendary Patroclus's father. Its provisional designation was S/2001 (617) 1.

 What they are made of 
Because of the density of the asteroids (0.8 g/cm³) is less than water and about one third that of rock, the team of researchers led by F. Marchis suggest that the Patroclus system, previously thought to be a pair of rocky asteroids, is more similar to a comet in make up. It is thought that many Trojan asteroids are in fact small planetesimals captured in the Lagrange point of Jupiter-Sun system during the farther migration of the giant planets, 3.9 billion years ago. This scenario was suggested by A. Morbidelli and colleagues in a series of articles published in May 2005 in Nature journal.

 Notes 
  UC-Berkeley press release on cometary origin

References

 Other websites 
 Johnston's Archive: detailed orbital and physical data
 Keck Obs. press release Trojan Asteroid Patroclus: Comet in Disguise? 
 Patroclus and Menoetius web page
 Orbital simulation from JPL (Java) / Ephemeris

Category:Trojan asteroids