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 responsible for most of the terrestrial impacts of kilometre-size objects, of which there may be several per million years 1 . About 150 Earth-crossing asteroids are known, although many more are thought to exist 2 . Asteroids that come close to the Earth's orbit, but do not currently cross it, may also pose a threat if they evolve onto Earth-crossing orbits. The asteroid 433 Eros, with a dia-meter of ∼22 km and a perihelion of 1.13 AU (where 1 AU is the average distance of the Earth from the Sun), is the second-largest near-Earth asteroid 3 . Here we report a study of the dynamical evolution of Eros's orbit over a period of two million years. We identify an orbital resonance with Mars that has the potential to perturb Mars-crossing asteroids, such as Eros, onto Earth-crossing orbits; of eight trial orbits that closely match Eros's present orbital parameters, three become Earth-crossing on the timescale of our simulations, and one of these hits the Earth after 1.14 Myr. Although our simulations indicate no significant danger of a catastrophic impact by this large near-Earth asteroid during the next ∼10 5 years, such a collision is likely in the far future.