The Trojan asteroids are a large group of objects that share the orbit of the planet Jupiter around the Sun. Viewed from a coordinate system that is fixed on Jupiter, they appear to orbit one of the two Lagrangian points of stability, L4 and L5, that lie 60° ahead of and behind Jupiter in its orbit.

As originally defined, Trojan asteroids have a semi-major axis between 5.05 AU and 5.40 AU, and lie in elongated, curved regions around the two Lagrangian points. The term is sometimes used to refer to minor bodies with similar relationships to other major bodies, in which case the name of the major body is included, e.g. the Neptunian Trojan 2001 QR322.

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.


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 SunJupiter 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).

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 370×195 km.


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" or "Achilles group"), 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. 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. Also there are Trojan moons around Saturn (TelestoTethysCalypso and HeleneDionePolydeuces).

Trojan asteroids in this broader sense may also have played a key role in the formation of the Moon, for which the leading theory states that it formed from the debris of a collision between Earth and a Mars-sized planet very early in the history of the Solar system. Because the collision must have hit Earth sideways and not too hard (otherwise it would have led to full destruction of both objects), it is believed that this hypothetical planet, dubbed Theia, formed from planetesimals that settled into the Lagrangian point L4 of the Sun-Earth system before some cause sent the body veering out of its orbit slowly, onto a path of eventual collision with the young Earth.

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External links

The minor planets
Vulcanoids | Near-Earth asteroids | Main belt | Jupiter Trojans | Centaurs | Damocloids | Comets | Trans-Neptunians (Kuiper belt · Scattered disc · Oort cloud)
For other objects and regions, see: asteroid groups and families, binary asteroids, asteroid moons and the Solar system
For a complete listing, see: List of asteroids. See also Pronunciation of asteroid names and Meanings of asteroid names.
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