617 Patroclus

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**617 Patroclus**
Discovery ^A

Discoverer	August Kopff
Discovery date	October 17, 1906
Alternate designations ^B	1906 VY; 1941 XC; 1962 NB
Category	Jupiter Trojan
Orbital elements ^C
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 (Ω)	44.37°
Argument of perihelion (ω)	307.81°
Mean anomaly (M)	149.94°
Physical characteristics ^D
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 temperature	110 K
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617 Patroclus is a binary minor planet made up of two similarly-sized objects orbiting their common centre of gravity. It is classified as a Trojan, sharing an orbit with Jupiter. It was discovered in 1907 by August Kopff, and was the second Trojan asteroid to be discovered. Recent evidence suggests that the objects are icy comets, rather than rocky asteroids.

1 Orbit
2 Binary
3 Composition
4 See Also
5 Notes
6 External links

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 conventions for the Trojan asteroids were not adopted until after Patroclus was named (similarly, the asteroid Hektor is the only Trojan character to appear in the Greek camp).

In 2001, it was discovered that Patroclus is a binary object, made up of two components of roughly similar 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 estimated^[1] that the two components orbit around their center of mass in 4.283±0.004 d at a distance of 680±20 km, describing a roughly circular orbit. Combining their observations with thermal measurements taken in November 2000, the team estimated the size of the components of the system. The slightly larger component, which measures 122 km in diameter, retains the name Patroclus. The smaller component, 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.

Because of the density of the components (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 composition. It is suspected that many Trojan asteroids are in fact small planetesimals captured in the Lagrange point of Jupiter-Sun system during the outer migration of the giant planets, 3.9 billion years ago. This scenario was proposed by A. Morbidelli and colleagues in a series of articles published in May 2005 in Nature journal.