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Trojan asteroids share an orbit with Jupiter.

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The Trojan asteroids are a large group of objects that share the orbit of the planet Jupiter around the Sun. Trojan asteroids have a coordinate system that is fixed on Jupiter. Trojan asteroids appear to orbit one of the two Lagrangian points of stability, L4 and L5, that lie 60º ahead of and behind Jupiter in its orbit. Trojan asteroids have semi-major axes between 5.05 AU and 5.40 AU, and lie in elongated, curved regions around the two Lagrangian points. Trojan asteroids are called 'Trojans' because of a convention whereby they are named after characters from the Trojan War.

Trojan asteroids.
Image of the Trojan asteroids (coloured green) in front of and behind Jupiter along its orbital path. Also shown is the main asteroid belt between the orbits of Mars and Jupiter (white) and the Hilda family of asteroids (brown).

The term 'Trojan' is sometimes used to refer to other Small solar system bodies that have similar relationships to other major bodies: for example, there are Mars Trojans and Neptune trojans. However, the term 'Trojan asteroid' by itself refers only to the Jupiter Trojans.

History of Trojan asteroids.

E. E. Barnard made the first observation of a Trojan asteroid, in 1904, but the significance of his observation was not noted at the time. Barnard believed he had sighted the recently discovered Saturnian satellite Phoebe, which was only two arc-minutes away in the sky at the time, or possibly even a star. The identity of the point of light Barnard had observed was not realised until an orbit was constructed for the Trojan (12126) 1999 RM11, an object (re)discovered in 1999. Since he failed to realise what he was looking at, Barnard's observation is now only a historical curiosity.

The first true discovery of a Trojan occurred in February 1906, when the German astronomer Max Wolf discovered an asteroid at the L4 Lagrangian point of the Sun-Jupiter system, and named it 588 Achilles. The oddity of its orbit was realized within a few months, and before long, many other asteroids were discovered at this point (and at the other triangular Lagrange point of the Sun-Jupiter system).

As of August 2005, the number of known Trojan asteroids is 1108 at L4 and 718 at L5. There are undoubtedly many others too small to be seen with current instruments. (By October 1999, 170 had been numbered; by July 2004, that number had grown to 877.) The largest of the Trojans is 624 Hektor, measuring 370195 km.

Nomenclature of Trojan asteroids.

Wolf named the first known Trojan after Achilles, the hero of Homer's epic poem The Iliad, which depicts the Trojan War. Following Wolf's lead, subsequently discovered asteroids in Jupiter's Lagrangian points were given names associated with the Iliad and the group as a whole were called 'Trojans'. Those in the L4 point are named after Greek heroes (the "Greek node" or "Achilles group"), and those at the L5 point are named after the heroes of Troy (the "Trojan node"). Confusingly, 617 Patroclus, the first discovered asteroid at the L5 point, was named before the Greece/Troy rule was devised, and a Greek name thus appears in the Trojan node; the Greek node also has one "misplaced" asteroid, 624 Hektor, named after a Trojan hero. Even more confusingly, the Trojan node is sometimes called the 'Patroclean asteroids' after its most prominent member, even though Patroclus was Greek.

Originally, the term "Trojan" applied only to asteroids sharing Jupiter's orbit; however, planetoidal bodies have been discovered at the Lagrangian points of Mars and Neptune as well, and are also referred to as "Mars Trojans" and "Neptune Trojans" respectively.

Composition of Trojan asteroids.

A team from the Keck Observatory in Hawaii announced in 2006 that they had measured the density of the binary Trojan asteroid 617 Patroclus as being less than that of water ice, suggesting that the pair, and possibly all the Trojan objects, more closely resemble comets or Kuiper belt objects in size and composition-water ice with a layer of dust-than they do the main belt asteroids.

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