617 Patroclus

617 Patroclus

Discovery
Discovered by	August Kopff
Discovery date	October 17, 1906
Designations
Named after	Patroclus
Alternative names	1906 VY; 1941 XC; 1962 NB
Minor planet category	Jupiter trojan
Adjectives	Patroclean
Orbital characteristics
Epoch 2455800.5 (JD 2011-Aug-27.0)
Aphelion	5.9481823 AU
Perihelion	4.4878688 AU
Semi-major axis	5.2180256 AU
Eccentricity	0.1399297
Orbital period	11.9197598 a (4353.6923 d)
Mean anomaly	331.77002°
Inclination	22.05276°
Longitude of ascending node	44.36649°
Argument of perihelion	307.90775°
Physical characteristics
Dimensions	143.14 ± 8.37 km
Mass	(1.36 ± 0.11) × 1018 kg
Mean density	0.88 ± 0.17 g/cm3
Rotation period	> 4.283±0.004 days
Albedo	0.047
Temperature	110 K
Spectral type	P-type
Absolute magnitude (H)	8.19

617 Patroclus (/pəˈtroʊkləs/ pə-TROH-kləs) is a binary minor planet made up of two objects of similar size orbiting their common centre of gravity. It is a Jupiter trojan. It was discovered in 1906 by August Kopff, and was the second trojan to be discovered. Its binary nature was discovered in 2001; the name Patroclus now refers to the larger of the two components, whereas its slightly smaller companion body has been named Menoetius (/mᵻˈniːʃəs/ mə-NEE-shəs, official designation (617) Patroclus I Menoetius). Recent evidence suggests that the objects are icy like comets, rather than rocky like most asteroids.

Orbit

Patroclus orbits in Jupiter's trailing Lagrangian point, L₅, in an area called the 'Trojan node' after one of the sides in the legendary Trojan War (the other node, at the L₄ point, 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 Jupiter trojans were not adopted until after Patroclus was named (similarly, the asteroid Hektor is the only Trojan character to appear in the Greek camp).

Binary system

In 2001, it was discovered that Patroclus is a binary object, made up of two components of roughly similar size. In 2006, accurate measurements of the orbit from the Keck Laser guide star adaptive optics system were reported. It was estimated that the two components orbit around their center of mass in 4.283±0.004 d at a distance of 680±20 km in a roughly circular orbit. Combining these observations with thermal measurements taken in 2000, the sizes of the components of the system were estimated. The slightly larger component, which measures 141 km in diameter, retains the name Patroclus. The smaller component, measuring 112 km, is now named Menoetius, after the legendary Patroclus's father. Its provisional designation was S/2001 (617) 1.

Composition

Because the density of the components (0.8 g/cm³) is less than water and about one third that of rock, it was suggested 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 Jupiter trojans are in fact small planetesimals captured in the Lagrange point of Jupiter–Sun system during the 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.

Exploration

Patroclus is a proposed target for Lucy, a mission to several asteroids and Jupiter trojans.

The largest Jupiter trojans

Trojan	Diameter (km)
624 Hektor	225
911 Agamemnon	167
1437 Diomedes	164
1172 Äneas	143
617 Patroclus	141
588 Achilles	135
1173 Anchises	126
1143 Odysseus	126
Source: JPL Small-Body Database, IRAS data

See also

• Asteroid moon

External links

• Keck Obs. press release Trojan Asteroid Patroclus: Comet in Disguise?
• Patroclus and Menoetius web page
• Orbital simulation from JPL (Java) / Ephemeris