An object larger than about 150 meters that can approach the Earth to within the distance of 4.6 million miles (7.5 million kilometers or 19.5 times the distance to the moon) is termed as a potentially hazardous object.
" Asteroids have us in our sight. The dinosaurs didn't have a space program, so they're not here to talk about this problem. We are, and we have the power to do something about it. I don't want to be the embarrassment of the galaxy, to have had the power to deflect an asteroid, and then not, and end up going extinct."
-Neil deGrasse Tyson