Asteroids spotted just before hitting Earth

In 1908 an approximately 30-meter-diameter asteroid or comet entered the atmosphere and exploded above ground in Tunguska, Russia. The explosion knocked down approximately 80 million trees over an area of 2,150 square kilometers (830 square miles). This explosion has been estimated at 1,000 times more powerful than the explosion of the atomic bomb at Hiroshima. Had this taken place over a metropolitan area, that city would have been completely destroyed. Luckily, it happened in a remote area of Siberia, and nobody is thought to have been killed.