THE ORBITAL EVOLUTION OF THE ASTEROID EROS AND IMPLICATIONS FOR COLLISION WITH THE EARTH

THE population of asteroids that cross the Earth's orbit is responsibl e for most of the terrestrial impacts of kilometre-size objects, of wh ich there may be several per million years(1). About 150 Earth-crossin g asteroids are known, although many more are thought to exist(2). Ast eroids that come close to the Earth's orbit, but do not currently cros s it, may also pose a threat if they evolve onto Earth crossing orbits . The asteroid 433 Eros, with a diameter of similar to 22 km and a per ihelion of 1.13 AU (where 1 AU is the average distance of the Earth fr om the Sun), is the second-largest near-Earth asteroid(3). Here we rep ort a study of the dynamical evolution of Eros's orbit over a period o f two million years. We identify an orbital resonance with Mars that h as the potential to perturb Mars-crossing asteroids, such as Eros, ont o Earth-crossing orbits; of eight trial orbits that closely match Eros 's present orbital parameters, three become Earth-crossing on the time scale of our simulations, and one of these hits the Earth after 1.14 M yr. Although our simulations indicate no significant danger of a catas trophic impact by this large near-Earth asteroid during the next simil ar to 10(5) years, such a collision is likely in the far future.