The average distance between asteroids (and their nearest neighbours) in the asteroid belt is about 8 times the earth-moon distance. Most of these asteroids hav

The average distance between asteroids (and their nearest neighbours) in the asteroid belt is about 8 times the earth-moon distance. Most of these asteroids have roughly the same orbit, so they have almost no relative velocity. So to achieve a collision we have to dramatically change the velocity of one of them in order to hit the other in a reasonable time frame. But this doesn’t take into account the masses. Asteroids vary in size from about 1 metre across to the dwarf planet Ceres, at 1000 km across. A giant asteroid is unlikely to have other giant asteroids nearby. If one giant asteroid is on a collision course with earth, what is the likelihood we’ll find a similar-enough sized one within a reasonable enough distance to be able to make them collide? Extremely remote.