Let’s take a closer look at that asteroid that struck at the end of the Cretaceous. A 10-kilometer-wide rock might not seem all that bad, considering it was going up against our 13,000-kilometer-wide planet. But the asteroid was probably moving about 20 kilometers per second, or around 45,000 miles per hour. At such speeds, it carried the energy of a thousand billion (a trillion) tons of TNT.
