Asteroids are not quite so common as some people think. Yes, there are thousands of rocks in the sky. But only some of them are proper asteroids. If you take out a telescope, you will find that under sixty percent of astronomical bodies contain any plant or animal life, and even fewer are inhabited.

That number does grow, however. Every once in a great while, an asteroid will reach conditions appropriate to habitation, spinning the three-millionth mote of astronomical dust into itself that pushes it over some barrier, and when that happens, poof! - an inhabitant may very well appear.