Impacts on the Earth

Abstract

Only within recent years have we realized the threat to the Earth posed by the impacts of comets and asteroids. In 1908, at the time of the Tunguska event, no asteroids had been discovered in orbits that brought them close to the Earth. Of all the objects in space, only comets seemed capable of colliding with the Earth, but there was little appreciation of the danger they represented. At that time comets were commonly thought to be made of insubstantial stuff, rather like a loose pile of sand or gravel. The idea that comets might have solid, rocky nuclei was not to emerge until 1950, and only in 1986 did the first spacecraft come close enough to a comet (Halley’s Comet) to photograph its solid nucleus.