‘The Threat from Space’ considers the threat of asteroids and comets colliding with Earth. Potential impacts of Near Earth Asteroids, with almost circular orbits, have been identified, but the threat from comets, which follow strongly elliptical paths, is uncertain. The Mexican asteroid impact 65 million years ago is thought to have wiped out two-thirds of all species living at the time. The Earth will be hit again at some point in the future, but how will it affect us? This will depend upon three things: the size of the object, how quickly it is travelling, and whether it hits the land or the ocean.