There actually is a rock heading our way, but they've pretty much ruled out its hitting us, although it will come fairly close (quite a bit closer than the Moon, which isn't all that far itself). But even if it did hit, it wouldn't wipe out the planet, or even knock us back to the Stone Age. It isn't big enough.

Eventually a big rock will hit us though. 'Eventually' might be a long time, but still, one WILL hit us, which is why I'm a big proponent of establishing human colonies elsewhere than this planet. Odds are that we'll do ourselves in though, rather than have some natural disaster cause our end. Nuclear war, climate change, destructive nanobots turning the world into sludge, some crazy deadly virus we cook up, something like that.