In 1994 the comet Shoemaker Levy-9 demonstrated the power of a cometary impact with a planet. The ‘target’ was Jupiter, but there have undoubtedly been many collisions between the Earth and comets or asteroids in the past. Traces of some craters survive; one off the Yucatan peninsula in Mexico seems to mark the impact of a body 10Km wide 65 million years ago. Although the theory is contested, many scientists believe that the dust cloud generated by this impact resulted in significant climatic change. The closest known approach to the Earth by any asteroid in historical times was within 170,000 Km less than half the distance of Moon, in 1991.