NASA's Science News blog explains why the asteroid that hit Russia's Ural Mountains on February 15th was confounding. It unexpectedly coincided with another rare event: a second asteroid that passed Earth about 17,200 miles above Indonesia. The events were unrelated. The report goes into how the impact was detected by sensors used to monitor nuclear explosions and testing on the planet. According to estimates, the energy of the asteroid's explosion was the equivalent of 470 kilotons of TNT--similar to the explosive potential of a modern nuclear warhead.