Probably more than a 100...since this one still didn't leave any tangible evidence it seems.
But ok, add one or two more zeroes and you're right. The principle counts regardless. Should a big enough hump of space rock come hurtling down upon us and we didn't see it in time...
^Aye. "Simply" nuking it with a big missile is a lot harder to do than it is to say.
In fact they're still thinking on what the hell they could do should the big one come our way.
Yet I have a lot of trust in those monitoring programs that keep a looksy out for the things. Unless it's a really really sneaky meteor chances are we would find it years before it should actually collide with Earth.