“On June 30, 1908, a small asteroid 330 feet (100 meters) in diameter exploded over the remote region of Tunguska in Siberia, devastating more than half a million acres of forest. One of the most recent close calls occurred on March 23, 1989, when an asteroid 0.25-mile (0.4- kilometer) wide came within 400,000 miles (640,000 kilometers) of Earth. Surprised scientists estimated that Earth and the asteroid — weighing 50 million tons and traveling at 46,000 miles/hour (74,000 kilometers/hour) — had passed the same point in space just six hours apart.”