You may have heard of the close call we had in 1987 when a half-mile wide asteroid missed the earth by a measly six orbital hours. Had that asteroid hit the ground, it would've probably kicked up enough dust to darken the skies for months or years, making the planet dangerously cold. If it hit the water, vast tidal waves would have resulted.
However, it didn't hit us, so, are we safe now?
Not really. Let's look at the facts.
We currently know where about one percent of the millions of asteroids in our solar system are. That's trouble, because the first thing you need to survive an asteroid collision is knowledge of when it's coming. In cosmic terms, the earth is hit all the time by asteroids. We are aware of over 150 impact craters on earth's surface. That's only a tiny fraction of the hits we've taken, though, because most craters are filled in or worn away across the years.
In an hour the earth collects about a ton of material from so-called "micrometeorites." These are mostly dust-sized, but about once every two hours we encounter a rock the size of your fist. Most of these burn up, or land harmlessly in the ocean. The biggies, say, roughly a mile wide, come about once in a million years.
However, that's just a statistical number; it's no guarantee that one won't hit us tomorrow. In fact, astronomers recently spotted a mile-wide asteroid whose course will bring it dangerously close to earth in about thirty years. Scientists have only recently begun work on strategies for diverting such threatening rocks from their courses.