[pharmablogger]

unless of course the aseroid was made up of plutonium

Plutonium in asteroids and the like, found today, is pretty low in radiation; since asteroids formed in our solar system at the same time as the Earth did, only Pu-244 would be around in any decent amount (as it has the longest half-life). But this, in turn, would mean it's level of radioactivity is really low.

Basically, asteroids just aren't radioactive enough to worry us - the fact that they're travelling at ludicrous speeds, with a shed load of mass, is though...