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617 Patroclus

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617 Patroclus
Discoverer August Kopff
Discovery date October 17, 1906
1906 VY; 1941 XC;
1962 NB
Category Jupiter Trojan
Orbital elements
Epoch ???
Eccentricity (e) 0.138
Semi-major axis (a) 5.228 AU
Perihelion (q) 4.506 AU
Aphelion (Q) 5.950 AU
Orbital period (P) 4365.7 d (11.95 a)
Mean orbital speed  ???
Inclination (i) 22.03°
Longitude of the
ascending node
Argument of
Mean anomaly (M) 149.94°
Physical characteristics
Dimensions 122 km and 112 km
Mass 1.36×1018 kg
Density 0.8 g/cm³
Surface gravity  ???
Escape velocity  ???
Rotation period >4.283±0.004 days
Spectral class P-type
Absolute magnitude 8.19
Albedo (geometric) 0.047
Mean surface
110 K

617 Patroclus (pə-troe'-kləs, how to say: /pəˈtroʊkləs/) 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.


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).


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[1] 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.


  1. ^  UC-Berkeley press release on cometary origin

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