Trojan asteroid

Trojan asteroids have a mean orbital radius between 5.05 AU and 5.40 AU, and lie in elongated, curved regions around the two Lagrangian points 60° ahead and behind of Jupiter.

History

E. E. Barnard is now believed to have made the first observation of a Trojan asteroid, in 1904, but the significance of his observation was not noted at the time. It was believed to have been a sighting of 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 observed was not realised until an orbit was constructed for the Trojan (12126) 1999 RM₁₁, an object that was only (re)discovered in 1999. For failing to realise what he was looking at, Barnard's observation is now only a historical curiosity.

In February 1906, the German astronomer Max Wolf discovered an asteroid at the L₄ Lagrangian point of the Sun-Jupiter system, and named it 588 Achilles, after the mythical Achilles, one of the heroes of Homer's Iliad. The oddity of its orbit was realized within a few months, and before long, many other asteroids were discovered at this point (and the other triangular Lagrange point of the Sun-Jupiter system).

There are currently (July 2004) 1679 known Trojan asteroids —1051 at L₄ and 628 at L₅. There are undoubtedly many others too small to be seen with current instruments. As of October 1999, 170 had been numbered; by July 2004, that number had grown to 877. The largest of the Trojans is 624 Hektor, measuring 370×195 km.

Nomenclature

Following Wolf's lead these asteroids were given names associated with the Iliad -- in fact, those in the L₄ point are named after Greek heroes of the Iliad (the "Greek node"), and those at the L₅ point are named after the heroes of Troy (the "Trojan node"). Confusingly, the latter group are sometimes called Patroclean asteroids after the most prominent of those, even though Patroclus (the hero) was on the Greek side. However, 617 Patroclus (the asteroid) was the first discovered asteroid at the L₅ point, and was named before the Greece/Troy rule was devised. The Greek node also has one "misplaced" asteroid; 624 Hektor.

As the Iliad deals with the events of the Trojan War, the asteroids came to be collectively known as Trojan asteroids. Over time, this term has come to be more generally applied to any planetoidal body at the triangular Lagrangian point of any two bodies —besides Jupiter's Trojans, Mars and Neptune have one Trojan each, plus there are Trojan moons around Saturn (Telesto-Tethys-Calypso and Dione-Helene). Strictly speaking, the term Trojan applies only to those in the L₄ and L₅ points of the Sun-Jupiter system.

External Link

• Minor Planet Center's List of Trojan Minor Planets