Trojan Asteroid

Asteroid (one type of small solar system body) sharing the orbit of a larger object. The term was first applied to asteroids moving in the orbit of Jupiter, but it (or the term ‘Trojan’) is now extended to bodies sharing the orbits of other planets and even the orbits of satellites.

The first Trojan asteroid to be recognized as such was 588 Achilles, observed in 1906 by the German astronomer Max Wolf, and soon realized to be moving in Jupiter's orbit and 60° ahead of the planet. By now over 1,000 asteroids have been identified as moving ahead of Jupiter, and over 1,000 as moving behind it.

The Trojan asteroids are so called because by custom they are all named after mythical characters from the Trojan War. Those that lead Jupiter are named after Greek heroes, those that trail the planet are named after heroes of Troy – with one exception in each camp, named before the convention had been fixed.

In the space around every body that orbits another body there are five points of gravitational stability called Lagrangian points. One of these lies 60° ahead of the body, the other 60° behind it. Trojans orbit these two points, but they can be at large distances. Jupiter's two groups of Trojan asteroids occupy clouds spread around a large part of Jupiter's vast orbit.

By the end of 2007, four Trojan asteroids were known to share the orbit of Mars, and six to share the orbit of Neptune. Some of the satellites of the gas giant Saturn orbit around the Lagrangian points of larger moons: for example, Telesto leads the satellite Tethys by 60° and Calypso trails Tethys by the same amount. However, it is certain that every body of substantial size would be found to have Trojan companions if observed with sufficiently sensitive instruments.