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, L<sub>5</sub>, 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

## 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

## Infobox (planet)

- **minorplanet**: yes
- **name**: 617 Patroclus
- **background**: #C2FFFF
- **image**: 617 Patroclus Hubble.jpg
- **image size**: 250
- **caption**: Hubble Space Telescope image composite of _Patroclus_ and its companion Menoetius, taken in 2018
- **discovery ref**:  
- **discovered**: 17 October 1906
- **discoverer**: August Kopff, A. Kopff
- **discovery site**: Heidelberg-Königstuhl State Observatory, Heidelberg Obs.
- **mpc name**: (617) Patroclus
- **alt names**: 1906 VY1941 XC
1962 NB
- **named after**: Patroclus, Πάτροκλος _Patroklos_

- **mp category**: Jupiter trojan

- **adjectives**: Patroclean
- **orbit ref**:  
- **epoch**: 23 March 2018 (Julian day, JD 2458200.5)
- **uncertainty**: 0
- **observation arc**: 19.15 Julian year (astronomy), yr (6,993 d)
- **aphelion**: 5.9376 Astronomical unit, AU
- **perihelion**: 4.4959 AU
- **semimajor**: 5.2167 AU
- **eccentricity**: 0.1382
- **period**: 11.92 yr (4,352 d)
- **mean anomaly**: 170.39Degree (angle), °
- **mean motion**: / day
- **inclination**: 22.047°
- **asc node**: 44.354°
- **arg peri**: 308.15°
- **jupiter moid**: 0.1966 AU
- **tisserand**: 2.8360
- **satellites**: 1 (Menoetius)
- **mean diameter**:

- **mass**:

- **rotation**:

- **spectral type**: D-type asteroid, D
C-type asteroid, C0
D-type asteroid, D
Asteroid color indices, U–B
Asteroid color indices, B–V
Asteroid color indices, V–R
Asteroid color indices, V–I
- **abs magnitude**: 8.19