Achilles ( minor planet designation: 588 Achilles) is a large

Jupiter Jupiter is the fifth planet from the Sun and the 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 slightly less than one-thousandth t ...

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 sh ...

of the Greek camp. Achilles was the first Jupiter trojan to be discovered, and was discovered by Max Wolf 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 student ...

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 Pele ...

from Greek mythology. The dark

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

measures approximately in diameter which makes it one of the 10 largest Jupiter trojans. It has a rotation period of 7.3 hours and possibly a spherical shape.

Discovery

''Achilles'' was discovered on 22 February 1906, by the German astronomer Max Wolf 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 August Kopff (February 5, 1882 – April 25, 1960) was a German astronomer and discoverer of several comets and asteroids. Kopff studied and worked in Heidelberg, getting his PhD there in 1906 and he then joined the Humboldt University of Berli ...

, a colleague of Wolf at Heidelberg, then discovered 617 Patroclus eight months after ''Achilles'', and, in early 1907, he discovered the largest of all Jupiter trojans, 624 Hektor.

Orbit and classification

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

Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...

–

System once every 11 years and 11 months (4,343 days; semi-major axis of 5.21 AU). Its orbit shows an

eccentricity Eccentricity or eccentric may refer to: * Eccentricity (behavior), odd behavior on the part of a person, as opposed to being "normal" Mathematics, science and technology Mathematics * Off-Centre (geometry), center, in geometry * Eccentricity (g ...

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 reference plane and the orbital plane or axis of direction of the orbiting object. For a satellite orbiting the Eart ...

of 10 degrees from the plane of the

ecliptic The ecliptic or ecliptic plane is the orbital plane of the Earth around the Sun. From the perspective of an observer on Earth, the Sun's movement around the celestial sphere over the course of a year traces out a path along the ecliptic agains ...

. ''Achilles'' is the first known example of the stable solution of the

three-body problem In physics and classical mechanics, the three-body problem is the problem of taking the initial positions and velocities (or momenta) of three point masses and solving for their subsequent motion according to Newton's laws of motion and Newton's ...

worked out by French mathematician Joseph Lagrange in 1772, after whom the minor planet 1006 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 " Greek camp", whereas the point was the " Trojan camp", 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'' rotation period 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 (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

lightcurve In astronomy, a light curve is a graph of light intensity of a celestial object or region as a function of time, typically with the magnitude of light received on the y axis and with time on the x axis. The light is usually in a particular freq ...

s determined a period of hours with a brightness amplitude of 0.02–0.11

magnitude Magnitude may refer to: Mathematics *Euclidean vector, a quantity defined by both its magnitude and its direction *Magnitude (mathematics), the relative size of an object *Norm (mathematics), a term for the size or length of a vector *Order of ...

(). 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 natu ...

(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 Vincenzo Zappalà (born 1945) is an Italian astronomer and discoverer of several main-belt asteroids. He is credited by the Minor Planet Center with the discovered of 9 minor planets. All of his discoveries he made at ESO's Chilean La Silla Ob ...

(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 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 201 ...

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 201 ...

, the body's surface has a very low

albedo Albedo (; ) is the measure of the diffuse reflection of 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 that refl ...

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 astronomical magnitude scale. An object's absolute magnitude is defined to be equal to the apparent magnitude that the object would have if it ...

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 ''mino ...

's name was suggested by Austrian astronomer Johann Palisa. 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 Greeks, and a genre of Ancient Greek folklore. These stories concern the origin and nature of the world, the lives and activities ...

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 '' Odys ...

'' 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 ...

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