Editing Exterminatus (section)

===An Asteroid===
Really, all it takes to kill everything on a planet is a big enough rock traveling fast enough. Normally it's the cloud of dust that is kicked up into the atmosphere and blocks out the sun that does most of the work. Dinosaurs learned this the hard way. Of course, this doesn't really work too well on a forge or hive world which is already like that. For raw destructive force, however, the damage is a function of the speed and size of the asteroid. The former has some practical limits (though a civilization looking to weaponize this sort of Exterminatus could possibly bring the rock up to relativistic speeds), but the latter can be nearly unlimited.

A collision with a near planet-sized object would be more devastating than most ''fictional'' Exterminatus weapons, obliterating the target world entirely. There could be any number of so-called 'rogue' planets floating in the empty spaces between stars, ready to slide into the solar system and crash into Earth, assuming humanity fails its collective ''Save or Die'' roll for the week. They'd have to [[fail]] incredibly hard because the overwhelming chance is that the rogue body will end up into the Sun (or Jupiter as a distant second choice), but yeah. [[Just as planned|Shit happens, yo!]]

Also, to clarify something: once the asteroid rises above a certain size threshold --many times larger than the one that killed the dinosaurs, in fact-- then a collision with Earth would actually result in completely ''sterilizing'' the planet, as in everything down to the last microbe would die. So... sweet dreams. (though once they got that big we would almost certainly see it coming.)