A much more likely problem than an asteroid hitting the earth is a "supervolcano" eruption. These happen WAY more frequently than giant asteroid hits, and they're just as bad. Yellowstone National Park is a supervolcano that erupts about every 600,000 years. Guess how long since it's erupted? 640,000 years.
What do we do when there's no sun hitting the earth's surface for six months?