617 Patroclus

617 Patroclus (pə-troe'-kləs, English pronunciation: /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.

617 Patroclus

Hubble Space Telescope image composite of Patroclus and its companion Menoetius, taken in 2018
Discovery [1]
Discovered by	A. Kopff
Discovery site	Heidelberg Obs.
Discovery date	17 October 1906
Designations
MPC designation	(617) Patroclus
Pronunciation	/pəˈtroʊkləs/[2]
Named after	Πάτροκλος Patroklos (Greek mythology)[3]
Alternative designations	1906 VY · 1941 XC 1962 NB
Minor planet category	Jupiter trojan [1][4][5] Trojan [6][7] · background [7]
Adjectives	Patroclean /pætrəˈkliːən/[8]
Orbital characteristics [4]
Epoch 23 March 2018 (JD 2458200.5)
Uncertainty parameter 0
Observation arc	19.15 yr (6,993 d)
Aphelion	5.9376 AU
Perihelion	4.4959 AU
Semi-major axis	5.2167 AU
Eccentricity	0.1382
Orbital period	11.92 yr (4,352 d)
Mean anomaly	170.39°
Mean motion	0° 4m 57.72s / day
Inclination	22.047°
Longitude of ascending node	44.354°
Argument of perihelion	308.15°
Known satellites	1 (Menoetius)[9]
Jupiter MOID	0.1966 AU
TJupiter	2.8360
Physical characteristics
Dimensions	127 km × 117 km × 98 km (primary only)[10]
Mean diameter	113±3 km (primary only)[10] 140.36±0.87 km[11] 140.85±3.37 km[12] 140.92±4.7 km[13] 143.14±8.37 km[14] 154 km[10]
Volume	1.36×106 km3[10]
Mass	(1.36±0.11)×1018 kg[14] 1.20×1018 kg[10]
Mean density	0.88±0.17 g/cm3[14][10]
Rotation period	40 h (at least; dated)[15] 102.8 h[16] 102 h[17] 103.02±0.40 h[18] 103.5±0.3 h[19]
Geometric albedo	0.047±0.003[11][12][13]
Spectral type	D (Tholen)[20] C0 (Barucci)[20] D (Tedesco)[20] U–B = 0.215±0.045[20] B–V = 0.710±0.050[21] V–R = 0.420±0.030[21] V–I = 0.830±0.020[21]
Absolute magnitude (H)	8.19[4][11][12][13][5]

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

1. ^ UC-Berkeley press release on cometary origin