There's a chap in Wales who monitors space for potential Extinction Level Events (ELEs), which is things like an asteroid or comet heading for Earth.
An asteroid the size of a cricket pitch would take out a city and an asteroid 10 km wide would cause global mass extinction.
Such an object is certain to head our way too. They've impacted before and it was one such object that wiped out the dinosaurs. Of course we don't know when it'll happen — it might be tomorrow or it might be in a million years — but it will happen.
If we see it early enough we should be able to deflect it off course. Bruce Willis got rid of that last one.