617 Patroclus

617 Patroclus /pəˈtrkləs/ 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 moon – later named Menoetius, and slightly smaller than its primary – was discovered. It was the first discovery of a binary asteroid among the Jupiter trojans.[9]

617 Patroclus
617 Patroclus Hubble.jpg
Hubble Space Telescope image composite of Patroclus and its companion Menoetius, taken in 2018
Discovery [1]
Discovered byA. Kopff
Discovery siteHeidelberg Obs.
Discovery date17 October 1906
(617) Patroclus
Named after
Πάτροκλος Patroklos
(Greek mythology)[2]
1906 VY · 1941 XC
1962 NB
Jupiter trojan[1][3][4]
Trojan[5][6] · background[6]
AdjectivesPatroclean /pætrəˈklən/[8]
Orbital characteristics[3]
Epoch 23 March 2018 (JD 2458200.5)
Uncertainty parameter 0
Observation arc19.15 yr (6,993 d)
Aphelion5.9376 AU
Perihelion4.4959 AU
5.2167 AU
11.92 yr (4,352 d)
0° 4m 57.72s / day
Known satellites1 (Menoetius)[9]
Jupiter MOID0.1966 AU
Physical characteristics
Dimensions127 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]
Volume1.36×106 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]
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]
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]


Patroclus orbits in Jupiter's trailing Lagrangian point, L5,[9] 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 systemEdit

Hubble images of Patroclus and Menoetius orbiting each other, from May to June 2017
Artist's conception of Patroclus and Menoetius orbiting around their center of mass, occasionally eclipsing one another
Artist's impression of the Patroclus-Menoetius binary system
Plot of the results of the multi-chord stellar occultation by 617 Patroclus and Menoetius
Discovery date2001
Pronunciation/mɪˈnʃəs/ mi-NEE-shəs
Named after
Menoetius (Greek mythology)
AdjectivesMenoetian /mɪˈnʃən/[22])
Orbital characteristics
680±20 km[9]
664.6 km[10]
102.8 h
Satellite ofPatroclus
Physical characteristics
Dimensions117 km × 108 km × 90 km[10]
Mean diameter
104±3 km[10]

In 2001, it was discovered that Patroclus is a binary system, made up of two components with of roughly similar size.[9][23][24] 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.[25]

It was estimated[26] that the two components orbit around their center of mass in 4.283±0.004 days at a distance of 680±20 km in a roughly circular orbit.[9] 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,[9] refined by later measurements from the Keck Observatory to approximately 122 km and 112 km for each partner,[27] and a co-orbital period of 103.5±0.3 hours (4.3125±0.0125 days).[25][19]

On 21 October 2013, both bodies occulted a magnitude 8.8 star as observed by a team of 41 observers stationed across the USA. Observation data put the orbital distance of 664.6 km, and give a size for the slightly larger component, which retains the name Patroclus with overall volume equivalent to a 113 km sphere, with the smaller component now named Menoetius with a volume equivalent to a 104 km diameter sphere.

Physical characteristicsEdit


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/).[16][17][18][19] 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 albedoEdit

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.[11][13] 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]

The largest Jupiter trojans
Trojan Diameter (km)
624 Hektor 225
617 Patroclus 140
911 Agamemnon 131
588 Achilles 130
3451 Mentor 126
3317 Paris 119
1867 Deiphobus 118
1172 Äneas 118
1437 Diomedes 118
1143 Odysseus 115
Source: JPL Small-Body Database, NEOWISE data


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.[25] 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.[28]


Artist's impression of the Lucy spacecraft flying past the Patroclus-Menoetius system

Patroclus is a proposed target for Lucy, a mission to several asteroids, mostly Jupiter trojans.[29] The mission's targets with their flyby dates are:[30][31]

  • 52246 Donaldjohanson — 20 April 2025: 4 km diameter C-type asteroid in the inner main-belt, member of the ~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.


This minor planet was named after the legendary Greek hero Patroclus. Friend of Achilles, he was killed by Hector during the Trojan War. (See 588 Achilles and 624 Hektor.) The 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]

In Greek and thus in Latin, Patroclus has all short vowels. Thus the expected English pronunciation would be with stress on the 'a'. However, Alexander Pope shifted the stress to the first 'o', /pəˈtrkləs/,[7] for metrical convenience in his verse translation of Homer, and this irregular pronunciation has become established in English.

Patroclus and its moon Menoetius are the only objects in the Trojan camp to be named after Greek rather than Trojan characters. 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).


  1. ^ Buie (2015). Volume equivalent diameters based on derived ellipsods are: Patroclus: 113 km and Menoetius: 104 km, while for the combined system, a mean-diameter of 154 km is given. Measured by asteroid occultation. Alternative observations gave a combined diameter of 140 kilometers. Summary figures for (617) Patroclus at the LCDB.


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External linksEdit