Earth-crossing asteroidasteroid whose path around the Sun
crosses Earth’s orbit. Two groups of such asteroids—Aten and
Apollo asteroids—are distinguished by the size of their orbits
and how closely they approach the Sun. The Atens and Apollos
cross Earth’s orbit on an almost continuous basis. Astronomers
have mounted searches for objects that closely approach Earth,
partly to determine whether they may collide with the planet,
since early detection might make it possible to avert a
catastrophe. According to some estimates, 1,000–2,000 Earth-
crossing asteroids larger than 0.6 mi (1 km) may exist. Impacts
of 0.6-mi-size asteroids are believed to occur a few times every
million years. Such a collision would deliver the explosive
force of several hydrogen bombs, possibly resulting in global
climate disturbances or huge tidal waves. The impact of an
object about 6 mi (10 km) in diameter is thought to have caused
a massive extinction of species, including the dinosaurs, at the
end of the Cretaceous Period (65.5 mil