617 Patroclus

617 Patroclus (/pəˈtroʊkləs/ pə-TROH-kləs), provisional designation 1906 VY is a binary Jupiter trojan approximately 140 kilometers (87 miles) in diameter.^[a] It was discovered on 17 October 1906, by astronomer August Kopff at the Heidelberg Observatory in Germany. The asteroid was named after Patroclus from Greek mythology.^[1] It was the second trojan to be discovered and the only member of the Trojan camp named after a Greek character.^[2] The dark D-type asteroid is also slow rotator and one of the largest Jupiter trojans. Patroclus is one of five Jovian asteroids targeted by the Lucy space probe to be visited in 2033. In 2001, a minor-planet moon – later named Menoetius, and slightly smaller than its primary – was discovered. It was the first discovery of binary asteroid among the Jupiter trojans.^[7]

Orbit[edit]

Patroclus orbits in Jupiter's trailing Lagrangian point, L₅,^[7] in an area called the Trojan camp after one of the sides in the legendary Trojan War (the other node, at the L₄ point, is called the "Greek camp").

It orbits the Sun at a distance of 4.5–5.9 AU once every 11 years and 11 months (4,353 days). Its orbit has an eccentricity of 0.14 and an inclination of 22° with respect to the ecliptic.^[3] The asteroid's observation arc begins at the discovering Heidelberg Observatory in November 1906, about 3 weeks after its official discovery observation.^[1]

Binary system[edit]

In 2001, it was discovered that Patroclus is a binary system, made up of two components with a minor-planet moon of roughly similar size.^[7][20][21] It is one of 18 binary Trojan asteroids known to exist. In 2006, accurate measurements of the orbit from the Keck Laser guide star adaptive optics system were reported.^[22]

It was estimated^[23] that the two components orbit around their center of mass in 7000428300000000000♠4.283±0.004 days at a distance of 7005680000000000000♠680±20 km in a roughly circular orbit.^[7] Combining these observations with thermal measurements taken in 2000, the sizes of the components of the system were estimated at 106 km and 98 km, with an equivalent whole-system diameter of 145 km^[7], refined by later measurements from the Keck Observatory to approximately 122 km and 112 km for each partner^[24], and a co-orbital period of 7002103500000000000♠103.5±0.3 hours (7000431250000000000♠4.3125±0.0125 days)^[22][17].

More recent direct optical measurements using stellar occlusion data pin the orbital distance closer to 664.6 km, and give a size for the slightly larger component, which retains the name Patroclus, as 124.6x98.2 km and overall volume equivalent to a 113 km sphere, with the smaller component, now named Menoetius (/mɪˈniːʃəs/ mi-NEE-shəs; official designation (617) Patroclus I Menoetius) after the legendary father of Patroclus, measuring 117.2x93.0 km with a volume equivalent to a 104 km diameter sphere^[8]. It was previously known by the provisional designation S/2001 (617) 1.

Physical characteristics[edit]

Lightcurves[edit]

Since 1989, several rotational lightcurves of Patroclus have been obtained from photometric observations. Analysis of the best rated lightcurves gave a rotation period between 102.8 and 103.5 hours with a brightness amplitude of less than 0.1 magnitude (U=2/3/).^{[14][15][16][17]} A low brightness variation typically indicates that a body has a nearly spheroidal shape. Its long rotation period makes it a slow rotator.

Diameter and albedo[edit]

According to the surveys carried out by the Infrared Astronomical Satellite IRAS and NASA's Wide-field Infrared Survey Explorer with its subsequent NEOWISE mission, the Patroclus system has an effective combined size between 140.36 and 140.92 kilometers in diameter and its surface has an albedo of 0.047.^[9][11] The Collaborative Asteroid Lightcurve Link adopts the results obtained by IRAS, that is, an albedo of 0.0471 and a diameter of 140.92 kilometers based on an absolute magnitude of 8.19.^[4]

Composition[edit]

Recent evidence suggests that the objects are icy like comets, rather than rocky like most asteroids. In the Tholen classification, Patroclus is a dark P-type asteroid.^[4]

Because the density of the components (0.88 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.^[22] It is suspected that many Jupiter trojans are in fact small planetesimals captured in the Lagrange point of the 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.^[25]

Exploration[edit]

Patroclus is a proposed target for Lucy, a mission to several asteroids, mostly Jupiter trojans.^[26] The mission's targets with their flyby dates are:^[27][28]

• 52246 Donaldjohanson — 20 April 2025: 4 km diameter C-type asteroid in the inner main-belt, member of ~130Myr old Erigone family;
• 3548 Eurybates — 12 August 2027: 64 km diameter C-type Jupiter Trojan in the Greek camp at L₄, largest member of the only confirmed disruptive collisional family in the Trojans;
• 15094 Polymele — 15 September 2027: 21 km diameter P-type Trojan at L₄, likely collisional fragment;
• 11351 Leucus — 18 April 2028: 34 km diameter D-type slow rotator Trojan at L₄;
• 21900 Orus — 11 November 2028: 51 km diameter D-type Trojan at L₄;
• 617 Patroclus — 2 March 2033: P-type binary Trojan. The primary, Patroclus, has a mean diameter of 113 km and its companion, Menoetius, has a diameter of 104 km. The pair orbit at a separation of 680 km. The binary resides in the Trojan camp at L₅.

Naming[edit]

This minor planet was named after the Greek hero Patroclus from Greek mythology. Friend of Achilles, he was killed by Hector during the Trojan War (see also (588) and (624)). The minor planet's name was proposed by Austrian astronomer Johann Palisa. The official naming citation was mentioned in The Names of the Minor Planets by Paul Herget in 1955 (H 65).^[2]

Patroclus is the only object in the Trojan camp to be named after a Greek rather than a Trojan 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).

Notes[edit]

References[edit]

External links[edit]

• Keck Obs. press release Trojan Asteroid Patroclus: Comet in Disguise?
• Patroclus and Menoetius web page
• Asteroids with Satellites, Robert Johnston, johnstonsarchive.net
• Asteroid Lightcurve Database (LCDB), query form (info)
• Dictionary of Minor Planet Names, Google books
• Discovery Circumstances: Numbered Minor Planets (1)-(5000) – Minor Planet Center
• 617 Patroclus at AstDyS-2, Asteroids—Dynamic Site
  • Ephemeris · Observation prediction · Orbital info · Proper elements · Observational info
• 617 Patroclus at the JPL Small-Body Database
  • Close approach · Discovery · Ephemeris · Orbit diagram · Orbital elements · Physical parameters