Could you imagine if we ever had a giant asteroid or comet come in at a low angle, instead of a perpendicular one, where we didn't know if it would actually crash or just cut through our atmosphere? A large one just cutting through our atmosphere would cause severe effects but the uncertainty of not knowing whether it would hit land or just barely skim by would be enough to start a panic.- especially if it passed by at a low angle over a large continent. Chances of that are probably extremely low in our lifetimes, but who really knows. I think all the large and even moderate sized ones have been mapped though and none are in a position to come near us for the next 100 years at least.