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Asteroid approaching Earth. Computer artwork of an asteroid on a collision course with Earth, on a starfield. It is night over Europe (upper centre) and the eastern USA (upper left). City lights identify areas of major urbanisation. The asteroid could be travelling at several thousand kilometres an hour. It would be seen as a fireball once it entered the Earth's atmosphere. The effect of its impact on the ground or sea would be catastrophic. It is believed that the impact of a large asteroid may have caused a mass loss of life that included the extinction of the dinosaurs, 65 million years ago. (KEYSTONE/SCIENCE PHOTO LIBRARY/CHRIS BUTLER)