. "Experts still disagree whether the object wasa comet or an asteroid (see New Scientist, Science, 16 January) but onething is for certain, if such an object ever explodes over a denselypopulated city such as London, Edinburgh or Birmingham it will be adisaster of immense proportions.Tunguska-size impacts are, fortunately, rare but are probably morecommon than experts would have believed just a few" . . .