Aten asteroid
An Aten asteroid is a near-Earth asteroid with an orbit smaller than Earth's that crosses Earth's path.
An Aten asteroid has a semi-major axis less than 1 astronomical unit, so its orbit is on average smaller than Earth's, but its farthest point from the Sun reaches beyond about 0.983 astronomical units, Earth's closest distance to the Sun. Its path therefore crosses ours.
The group is named after 2062 Aten, discovered in 1976 by the American astronomer Eleanor Helin. Atens make up a smaller share of known near-Earth asteroids than Apollos, partly because they spend most of their time inside Earth's orbit, in the daytime sky, where they are hard to see.
Apophis, the asteroid that will pass about 32,000 kilometres from Earth's surface on 13 April 2029, is currently an Aten. Its close encounter with Earth will change its orbit, turning it into an Apollo. Such changes show how near-Earth orbits can shift through planetary encounters.
Related terms
Level: Enthusiast Teens and adults