Hundreds of billions of them. Ranging from the size of a speck of dust to about the size of Pluto. Also, let's not forget the Kuiper Belt, which has about a trillion such rocks, covered in ice, that occasionally come into our solar system. However, due to Jupiter's immense gravity, it's very unlikely that we'll be hit by an asteroid or a comet. The closest one that scientists predict will come to us will come in the 2030's, and the odds of it hitting us are one in 18,000.
I love space, man.
Yeah, i know about that... i'm collaborating with EURONEAR project, as do some of my collegues at uni, in trying to find or redescover Near Earth Asteroids, but i was wondering more along the lines of hey, i am a superpower and i don't have nukes. I'll just threaten the world that i'll destroy it if they don't do as i say...