Celestial bomb

If a comet merely ten-thousandth the size of Hale-Bopp strikes the Earth, it would create an explosion ten times as powerful as all the nuclear weapons in existence at the height of the Cold War, according to a supercomputer simulation. David Crawford of Sandia National Laboratories, Livermore, California, has modelled the impact of a billion-tonne comet smacking into the ocean. During a test when the comet hit the virtual ocean, it vapourised nearly 500 cubic kilometres of ocean instantly and filled the atmosphere with enough vapour to darken skies. The test shows that the vapour may darken skies for even years, devastating world agriculture ( New Scientist , Vol 155, No 2090).