The greatest risk to Earth comes from comets, which have proved harder for NASA 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 -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. A new study of the Trojan asteroids that exist around the orbit of Neptune concludes that material from these may go on to become comets that could strike our planet.
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.
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 feeding 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 concludes that the main source of the Centaurs is the "scattered disc", part of the Kuiper belt of debris beyond Neptune.
Casey Kazan via http://www.newscientist.com