I can't remember the numbers, but there's is a substantial amount of world destroying asteroids in our system. Enough that, if they were able to reflect sun so we could see them in the night sky, it would look like we have a swarm of flies buzzing around our planet. It's lucky that we haven't been hit yet, and in fact, almost did a few years back. One skimmed through the top of our atmosphere and was actually captured on film. I'll try to find the link.
Basically, we are still here only by luck.
