4179 Toutatis

An asteroid 4.5 km in diameter passed by Earth a couple days ago. It wasn't really a near miss, since it was seven million kilometers away, but it was "close" in astronomical terms. It's a regular orbit-crosser for us, so it will almost certainly hit the Earth at some distant point in the future, but definitely not in our lifetimes.

Unless, of course, those dastardly aliens make a minor adjustment in its orbit...

In all seriousness, the threat is very real. We're not talking about a nuclear war, which a viable human breeding population would survive. An event on the scale of the Shoemaker-Levy comet impact on Jupiter a few years back would turn the Earth into a very close approximation of Hell. Not only would humans not survive it, you wouldn't WANT to survive it.