It hasn't happened yet, as far as we know, so we can safely keep ignoring it forever and spending like fifty bucks a year on detection programs.

Is the part that makes ya drink.

Not true. Apparently, we're about to drill for core samples in the crater of the one that whacked the dinosaurs. Should prove interesting to see what happens in the short term after such an event. I was surprised how quickly things came back around Mt. St. Helens after the big eruption. Not that there isn't a big moonscape still, but life fills the niches quickly.