near-Earth asteroid

near-Earth asteroid (NEA) Any asteroid belonging to the Apollo, Amor, or Aten groups. Such asteroids have perihelion distances of less than 1.3 AU. The term near-Earth objects (NEOs) is also used, allowing for the fact that some may be extinct short-period comets. Members of the Amor group cross the orbit of Mars but do not quite reach the Earth's orbit, while the Apollo and Aten groups do cross the Earth's orbit. The three groups are not completely separate, since planetary perturbations may cause an Amor asteroid to become an Apollo, and vice versa. Atens are the least common NEAs, while Apollos and Amors are found in roughly equal numbers. Only three NEAs are larger than 10 km in diameter: (1036) Ganymed, (433) Eros, and (4954) Eric, all members of the Amor group. Near-Earth asteroids have finite lifetimes (typically 10 million years); most will be destroyed by collision with one of the inner planets, and the remainder ejected from the Solar System. At least 100 000 NEAs larger than 100 m in diameter are estimated to exist, and perhaps 1000 larger than 1 km. At the start of 2007 over 4500 were known with new discoveries running at over 600 a year. See also potentially hazardous asteroid.