THE Trojan horse of legend held in its belly the men and means to help sack ancient Troy. Now it appears another type of Trojan could endanger every life on Earth. So says a study of the Trojan asteroids that exist around the orbit of Neptune: material from these may go on to become comets that could strike our planet.

Around three-quarters of the impact risk to Earth comes from near-Earth asteroids, about 1000 of which are being tracked by sky surveys. The rest of the risk comes from comets, which have proved harder to keep tabs on.

Many comets swing into the inner solar system every 200 to 300 years. The origin of such so-called "short-period comets" is unknown but the immediate source is thought to be the Centaurs. These are a collection of an estimated million icy objects more than 1 kilometre across on elliptical orbits that come closest to the sun between the orbits of Jupiter and Neptune.

Only about 250 of these Centaurs have been imaged by telescopes. All are on unstable orbits, and have a big chance of receiving a gravitational boost when their orbit brings them near Jupiter or one of the other giant planets. Such perturbation could redirect them into the inner solar system - and possibly towards Earth. As a wayward Centaur approaches the sun, its heat begins to evaporate the icy contents, resulting in a cometary tail.

Previous simulations of the Centaurs suggest something must be feeding them with extra material - each object will orbit for about 3 million years before it hits a planet, falls into the sun, is ejected from the solar system or simply disintegrates. "The population decays and it is being replenished from somewhere," says Jonathan Horner at the University of Durham, UK.

In a paper to appear in the International Journal of Astrobiology, Horner and Patryk Sofia Lykawka of Kinki University in Osaka, Japan, suggest that the source of this replenishment is the Neptunian Trojans - asteroids orbiting the sun on roughly the same path as Neptune. They calculate that one out of the six known Trojans has a 50 per cent chance of migrating to become a Centaur over the next 600 million years. Since there is reason to believe there may be as many as 10 million undiscovered Neptunian Trojans wider than 1 kilometre, the pair conclude that these could be topping up the Centaurs.

Hal Levison of the Southwest Research Institute in Boulder, Colorado, argues that to maintain the stock of known kilometre-sized short-period comets, the number of Trojans would have to be as high as a billion. He thinks this is unlikely because so many objects of such a size would collide and fragment to smaller dimensions. "I'm dubious," he says. Levison reckons the main source of the Centaurs is the "scattered disc", part of the Kuiper belt of debris beyond Neptune.