Sunday is International Asteroid Day, commemorating the Earth's largest recorded asteroid impact while focusing on the real danger of asteroids that could collide with Earth. In 1908, a powerful asteroid struck the Podkamennaya Tunguska River area in a remote Siberian forest of Russia. Six years ago, an asteroid entered Earth's atmosphere over Chelyabinsk, Russia. It exploded in the air, releasing 20 to 30 times more energy than that of the first atomic bombs and generating brightness greater than the sun. It damaged more than 7,000 buildings and injured more than 1,000 people. The shock wave broke windows 58 miles away....