588 Achilles is a large

Jupiter Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but ...

trojan asteroid In astronomy, a trojan is a small celestial body (mostly asteroids) that shares the orbit of a larger body, remaining in a stable orbit approximately 60° ahead of or behind the main body near one of its Lagrangian points and . Trojans can sha ...

of the

Greek camp This is a list of Jupiter trojans that lie in the Greek camp, an elongated curved region around the leading Lagrangian point (), 60 ° ahead of Jupiter in its orbit. All the asteroids at Jupiter's point have names corresponding to participants ...

. Achilles was the first Jupiter trojan to be discovered, and was discovered by

Max Wolf Maximilian Franz Joseph Cornelius Wolf (21 June 1863 – 3 October 1932) was a German astronomer and a pioneer in the field of astrophotography. He was the chairman of astronomy at the University of Heidelberg and director of the Heidelberg-K� ...

at the

Heidelberg Observatory Heidelberg (; Palatine German: ') is a city in the German state of Baden-Württemberg, situated on the river Neckar in south-west Germany. As of the 2016 census, its population was 159,914, of which roughly a quarter consisted of students. ...

in 1906. Wolf named the minor planet after the legendary hero

Achilles In Greek mythology, Achilles ( ) or Achilleus ( grc-gre, Ἀχιλλεύς) was a hero of the Trojan War, the greatest of all the Greek warriors, and the central character of Homer's ''Iliad''. He was the son of the Nereid Thetis and Peleus, k ...

from Greek mythology. The dark

D-type asteroid D-type asteroids have a very low albedo and a featureless reddish Asteroid spectral types, spectrum. It has been suggested that they have a composition of organic-rich silicates, carbon and anhydrous silicates, possibly with water ice in their inte ...

measures approximately in diameter which makes it one of the 10 largest Jupiter trojans. It has a

rotation period The rotation period of a celestial object (e.g., star, gas giant, planet, moon, asteroid) may refer to its sidereal rotation period, i.e. the time that the object takes to complete a single revolution around its axis of rotation relative to the ...

of 7.3 hours and possibly a spherical shape.

Discovery

Achilles was discovered on 22 February 1906, by the German astronomer

at the

Heidelberg-Königstuhl State Observatory Heidelberg-Königstuhl State Observatory (german: Landessternwarte Heidelberg-Königstuhl) is a historic astronomical observatory located near the summit of the Königstuhl hill in the city of Heidelberg in Germany. The predecessor of the curre ...

in southern Germany. It was the first discovery of a Jupiter trojan, although had been observed as two years previously. This body, however, remained unconfirmed as the observation period was not long enough to calculate an orbit. August Kopff, a colleague of Wolf at Heidelberg, then discovered

617 Patroclus 617 Patroclus ( ) is a large binary Jupiter trojan asteroid. It is a dark D-type asteroid and a slow rotator, due to the 103-hour orbital period of its two components. It is one of five Jovian asteroids targeted by the ''Lucy'' space probe, and ...

eight months after Achilles, and, in early 1907, he discovered the largest of all Jupiter trojans,

624 Hektor 624 Hektor is the largest Jupiter trojan and the namesake of the Hektor family, with a highly elongated shape equivalent in volume to a sphere of approximately 225 to 250 kilometers diameter. It was discovered on 10 February 1907, by astronome ...

Orbit and classification

Achilles orbits the Sun at a distance of 4.4–6.0 AU in the

Lagrangian point In celestial mechanics, the Lagrange points (; also Lagrangian points or libration points) are points of equilibrium for small-mass objects under the influence of two massive orbiting bodies. Mathematically, this involves the solution of th ...

of the Sun–

System once every 11 years and 11 months (4,343 days;

semi-major axis In geometry, the major axis of an ellipse is its longest diameter: a line segment that runs through the center and both foci, with ends at the two most widely separated points of the perimeter. The semi-major axis (major semiaxis) is the long ...

of 5.21 AU). Its orbit shows an eccentricity of 0.15 and an

inclination Orbital inclination measures the tilt of an object's orbit around a celestial body. It is expressed as the angle between a Plane of reference, reference plane and the orbital plane or Axis of rotation, axis of direction of the orbiting object ...

of 10 degrees from the plane of the ecliptic. Achilles is the first known example of the stable solution of the three-body problem worked out by French mathematician

Joseph Lagrange Joseph-Louis Lagrange (born Giuseppe Luigi Lagrangia1006 Lagrangea is named. After the discovery of other asteroids with similar orbital characteristics, which were also named after heroes from the Trojan War ''(see below)'', the term "Trojan asteroids" or "Jupiter trojans" became commonly used. In addition, a rule was established that the point was the "

", whereas the point was the "

Trojan camp This is a list of Jupiter trojans that lie in the Trojan camp, an elongated curved region around the trailing Lagrangian point, 60° behind Jupiter. All the asteroids at the trailing point have names corresponding to participants on the Troj ...

", though not before each camp had acquired a "spy" (Hektor in the Greek camp and Patroclus in the Trojan camp).

Physical characteristics

Spectral type

In the

Tholen Tholen () is a 25,000 people municipality in the southwest of the Netherlands. The municipality of Tholen takes its name from the town of Tholen, which is the largest population center in the municipality. The municipality consists of two peninsu ...

taxonomic scheme, Achilles is classified as a

with an unusual spectrum (DU). Its V–I color index of 0.94 is typical for most larger Jupiter trojans ''(see table below)''.

Photometry

Achilles's

of 7.3 hours is somewhat shorter than that of most other large Jupiter trojans but close to that of 911 Agamemnon, 3451 Mentor and 3317 Paris, which are similar in size ''(see table below)''. Its low brightness amplitude is indicative of a rather spherical shape. From July 2007 until September 2008, coordinated photometric observations were carried out by astronomers at

Simeiz Simeiz ( uk, Сімеїз, russian: Симеи́з, crh, Simeiz) is a resort town, an urban-type settlement in Yalta Municipality in the Autonomous Republic of Crimea, a territory recognized by a majority of countries as part of Ukraine and incorp ...

(Crimea), Rozhen (Bulgaria), Maidanak (Uzbekistan) and

Kharkiv Kharkiv ( uk, wikt:Харків, Ха́рків, ), also known as Kharkov (russian: Харькoв, ), is the second-largest List of cities in Ukraine, city and List of hromadas of Ukraine, municipality in Ukraine. Analysis of the obtained lightcurves determined a period of hours with a brightness amplitude of 0.02–0.11 magnitude (). Alternative period determinations by Cláudia Angeli (7.0 h),

Robert Stephens Sir Robert Graham Stephens (14 July 193112 November 1995) was a leading English actor in the early years of Britain's Royal National Theatre. He was one of the most respected actors of his generation and was at one time regarded as the natur ...

(7.312 h),

Stefano Mottola This is a list of minor-planet discoverers credited by the Minor Planet Center with the discovery of one or several minor planets (such as near-Earth and main-belt asteroids, Jupiter trojans and distant objects). , the discovery of 612,011 numb ...

(7.32 h) and Vincenzo Zappalà (12 h) are mostly in good agreement ().

Diameter and albedo

According to the surveys carried out by the Infrared Astronomical Satellite,

IRAS The Infrared Astronomical Satellite (Dutch: ''Infrarood Astronomische Satelliet'') (IRAS) was the first space telescope to perform a survey of the entire night sky at infrared wavelengths. Launched on 25 January 1983, its mission lasted ten mo ...

, the Japanese Akari satellite, and the NEOWISE mission of NASA's

Wide-field Infrared Survey Explorer Wide-field Infrared Survey Explorer (WISE, observatory code C51, Explorer 92 and SMEX-6) is a NASA infrared astronomy space telescope in the Explorers Program. It was launched in December 2009, and placed in hibernation mode in February 2011, ...

, the body's surface has a very low

albedo Albedo (; ) is the measure of the diffuse reflection of sunlight, solar radiation out of the total solar radiation and measured on a scale from 0, corresponding to a black body that absorbs all incident radiation, to 1, corresponding to a body ...

in the range of 0.0328 to 0.043, making its

absolute magnitude Absolute magnitude () is a measure of the luminosity of a celestial object on an inverse Logarithmic scale, logarithmic Magnitude (astronomy), astronomical magnitude scale. An object's absolute magnitude is defined to be equal to the apparent mag ...

of approximately 8.57 correspond to a diameter of 130.1 to 135.5 kilometers. Achilles is the 6th largest Jupiter trojan according to IRAS and Akari, and the 4th largest based on NEOWISE data:

Naming

This

minor planet According to the International Astronomical Union (IAU), a minor planet is an astronomical object in direct orbit around the Sun that is exclusively classified as neither a planet nor a comet. Before 2006, the IAU officially used the term ''minor ...

's name was suggested by Austrian astronomer

Johann Palisa Johann Palisa (6 December 1848 – 2 May 1925) was an Austrian astronomer, born in Troppau, Austrian Silesia, now Czech Republic. He was a prolific discoverer of asteroids, discovering 122 in all, from 136 Austria in 1874 to 1073 Gel ...

. It was named after

, the legendary hero from

Greek mythology A major branch of classical mythology, Greek mythology is the body of myths originally told by the Ancient Greece, ancient Greeks, and a genre of Ancient Greek folklore. These stories concern the Cosmogony, origin and Cosmology#Metaphysical co ...

and central figure in Homer's ''

Iliad The ''Iliad'' (; grc, Ἰλιάς, Iliás, ; "a poem about Ilium") is one of two major ancient Greek epic poems attributed to Homer. It is one of the oldest extant works of literature still widely read by modern audiences. As with the ''Odysse ...

'' which tells the accounts of the

Trojan War In Greek mythology, the Trojan War was waged against the city of Troy by the Achaeans (Greeks) after Paris of Troy took Helen from her husband Menelaus, king of Sparta. The war is one of the most important events in Greek mythology and has ...

''(also see 5700 Homerus and 6604 Ilias)''. As an infant, Achilles was plunged in the River Styx by his mother Thetis ''(also see 17 Thetis)'', thus rendering his body invulnerable excepting the heel by which he was held. He slew Hector (''see also

''), the greatest Trojan warrior. He was eventually killed by an arrow in the heel by Paris (''see 3317 Paris'').