One day you walk down to the local deli for lunch and on the way back to work you find a piano has fallen on the sidewalk, right where you pass every day. Gosh, you think, ten minutes later and I would have been standing right there. Close one.

You think that's close? Imagine how the earth feels.

In 1987 an asteroid measuring a whopping half a mile in diameter came by, missing our planet by a measly six orbital hours. An orbital hour is simply a measure of how far the earth moves around the sun in one hour. Had we been six hours ahead of where we are, or had the asteroid come by six hours later, hello enormous rock, and, quite possibly, goodbye everything else.

Oh come on, you say. Would a collision really have been that bad? After all, the earth has a diameter of 8000 miles, being hit by a half-mile wide asteroid would be like being tapped by a B-B.

In one sense, that is true. The earth itself would not be moved from its orbit, or anything that dramatic. However, we who live here would definitely have noticed. Calculations have shown that the explosion would have been equivalent to one thousand million tons of TNT going off all at once. A cloud of dust and debris would have been thrown up into the stratosphere, gradually darkening the skies and chilling the planet for months or even years. Many scientists believe just this kind of thing wiped out the dinosaurs, by changing the environment rapidly.