Achilles was discovered on 22 February 1906, by the German astronomer Max Wolf at the Heidelberg-Königstuhl State Observatory 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 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–Jupiter System once every 11 years and 11 months. Its orbit shows an eccentricity of 0.15 and an inclination 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 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 , 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".
In the Tholentaxonomic scheme, Achilles is classified as a D-type asteroid with an unusual spectrum. Its V–Icolor index of 0.94 is typical for most larger Jupiter trojans .
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 . 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, Rozhen, Maidanak and Kharkiv observatories. 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, Robert Stephens, Stefano Mottola and Vincenzo Zappalà are mostly in good agreement.
According to the surveys carried out by the Infrared Astronomical Satellite, IRAS, the Japanese Akari satellite, and the NEOWISE mission of NASA's Wide-field Infrared Survey Explorer, the body's surface has a very low albedo in the range of 0.0328 to 0.043, making its absolute magnitude 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's name was suggested by Austrian astronomer Johann Palisa. It was named after Achilles, the legendary hero from Greek mythology and central figure in Homer's Iliad which tells the accounts of the Trojan War'. As an infant, Achilles was plunged in the River Styx by his mother Thetis ', thus rendering his body invulnerable excepting the heel by which he was held. He slew Hector, the greatest Trojan warrior. He was eventually killed by an arrow in the heel by Paris.
