In astronomy, a Trojan is an asteroid or moon that shares an orbit with a larger asteroid or moon, but does not collide with it.  The Trojan orbits within one of the Lagrangian points of stability ahead or behind the main body.

Usually the asteroids which accompany Jupiter around its orbit come to mind when you mention the Trojans.  The Jupiter asteroids were the first discovered, and are believed to be almost as numerous as those in the asteroid belt.  Since the Jupiter Trojans were discovered, scientist have found “Trojans” in the orbits of Mars, Neptune, and Saturn.

Discovered in Jupiter’s orbit in 1906 (588 Achilles was the first one), there have been 4,076 Jupiter Trojans found so far.  There are believed to be over a million Jupiter Trojans larger than 1 km in diameter.  As in the main asteroid belt, the Trojans form asteroid “families”.  Currently, most scientist believe the Trojans are “captured” Kuiper Belt Objects.  Sometimes the “capture” appears to imperfect.  The comet Shoemaker-Levy 9 is believed to have been one such imperfect capture.

Currently there are only four known Mars Trojans, and seven around Neptune.  There may be many more Trojans than those recorded so far; we’re just not in a position to see them yet.  Perhaps when New Horizons passes Neptune…

Martian Trojans, image by Andrew Buck, all rights reserved

There is some speculation that the Trojans are a source for new short-period comets and Centaurs.  Considering the four discovered around Mars, that puts them in our very near back yard.