On this date in 1908, riders on the Trans-Siberian railroad witnessed a fireball as bright as the sun streaking across the sky. It exploded above the Tunguska River and flattened over a thousand square miles of forest and was seen and heard for 1,000 miles. Scientists believe a chunk of asteroid vaporized in the lower atmosphere. On June 17, 2002, a football field sized asteroid, traveling at 66,000 miles an hour, came within 75,000 miles of Earth, only noticed by scientists after it zipped by. Scientists continue to plot the orbits of asteroids, determining if they could be on a crash course with Earth. In the future, missiles will intercept objects heading our way. The next asteroid within 4.6 million miles of Earth occurs today.