Cosmic Trojans may sneak comets towards Earth
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.