#3 – Asteroid
It may seem like the stuff of movies (we’re looking at you, Armageddon and Deep Impact) but smaller comets and asteroids have already hit or passed close to this planet in the past. If we are hit by a larger celestial body, it’s going to do a lot of damage to the planet and possibly cause floods, firestorms, and huge sun-blocking dust clouds. An asteroid of sufficient size could cause the end of humanity as we know it; that’s probably what got the dinosaurs, after all.