asteroids
if a giant asteroid were to strike the earth that would be really bad. depending on the size and where it hit it could destroy all life on the planet in a global conflagration. that'd be pretty awesome to watch. like say from mars. if a large asteroid were to hit earth it'd mess things up real bad. but civilization would continue. we'd have to do a whole lot of rebuilding. fortunately there aren't very many such chunks of material flying around the solar system. and we're keeping a casual watch in the hopes of spotting one far enough in advance that we might be able to do something about it. occasionally there are reports about of a doomsday rock. they usually become near misses with more accurate tracking.