Well, probably nothing because we'd be dead. The impact of an asteroid that large would be like an atomic bomb approximately 30 times more powerful than Hiroshima's bomb.
Tomorrow an asteroid 1,300 ft long is going to come 1.3 lunar distances from us. Which is pretty crazy because asteroids don't come that close very often and to be an asteroid that large is just pretty amazing. It could be just a bit closer and we'd all be blown to smithereens.
anyways, read more about it here: