Three groups of such asteroids
Aten, Apollo, and Amor 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, whereas only about half the Amors are Earth crossers and then only part of the time. 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 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 {{link=hydrogen bomb">hydrogen bomb s, 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 dinosaur s, at the end of the Cretaceous Period .