On December 27, 2004, while the world was focused on the Indian Ocean tsunami, a few astronomers were contemplating the possibility of an even deadlier disaster: that of a massive asteroid striking Earth. A fifth of a mile wide—heftier than the space rock that leveled a vast swath of Siberian forest in 1908—Near-Earth Asteroid 2004 MN4 had grabbed the attention of NASA scientists just before Christmas. They put the chance of an April 13, 2029, collision at 1 in 2,700 and two days later upped the odds to 1 in 165.