In its journey through space, the planet earth is continually colliding with asteroids (boulders comprised of compressed rocks and dust) and comets (compressed ice and dust). The earth meets a 100 tonne rock about a metre wide every day, a 1,000 ton body once a month, a 15,000 ton boulder once a year and a 100,000 ton behemoth every 20 years. The greatest danger during the past few thousand years has been fragments of the comet Encke. Each June 30 the earth passes right through this debris. Not only does massive destruction occur when a major boulder collides with the earth, but since the entry fireball mimics a nuclear explosion, there is a risk of detonating defence systems.
In 1908, a meteorite weighing a million tonnes fell into the atmosphere over the Tunguska region of Siberia, travelling at 72,000 mph. Its entry fireball was brighter than the sun and it exploded four miles up in a 12-megaton flash. 1,000 square miles of pine forest were destroyed and the local soil is still covered with tiny particles of glass and iron from the melted and vaporized boulder. A massive boulder of this size hits the earth on average every 100 years.