A member of a group of asteroids whose orbits cross that of the Earth. They are named after the first of their kind, Apollo, discovered in 1932 by German astronomer Karl Reinmuth and then lost until 1973. Apollo asteroids are so small and faint that they are difficult to see except when close to the Earth (Apollo is about 2 km across).
Apollo asteroids can collide with the Earth from time to time. In December 1994 the Apollo asteroid 1994 XM1 passed within 100 000 km of the Earth, the closest observed approach of any asteroid. A collision with an Apollo asteroid 65 million years ago may have been one of the causes of the extinction of the dinosaurs. A closely related group, the Amor asteroids, come close to Earth but do not cross its orbit.
Subjects: Astronomy and Astrophysics.