The scene is Siberia; the date 30 June 1908.Suddenly a massive explosion rips apart the silence hanging over plains near the stony Tunguska River, and a huge pillar of smoke and fire rises to the sky.The thunder of the explosion is heard hundreds of kilometers away, and seismic tremors are registered all around the globe.Trees across an area of more than 2000 sq km are snapped like matchsticks and beneath the center of the explosion a forest fire rages.
This real catastrophic event was probably caused by a 30-m-wide asteroid exploding 10 km above the Earth's surface.Fortunately,the affected region was almost uninhabited so,although several reindeer herds fell victim to the explosion,only two people died.Had the asteroid exploded above a city it would have been a tragedy of major proportions.The force of the blast was comparable to 10-20 megatons of TNT-the equivalent to 50-100 times the explosive power of the Hiroshima atom bomb.
Astronomers have calculated that an event like the one at Tunguska involving an asteroid is likely to happen somewhere on Earth every couple of hundred years. This is because of the innumerable asteroids and comets that are travelling through our solar system,one of which could at some point crash into the Earth.
Are comets Dangerous
In Medieval times, the appearance of a large comet in the sky caused fear and alarm because they were considered to be the harbingers of plagues and wars. Even as recently as 1910, when Earth crossed the tail of Halley's Comet, many people feared that the world was about to end because astronomers had detected substances like Sulphur and cyanide in the comet's tail. Today we know that the matter in a comet's tail is much too thinly distributed to pose any kind of danger to us. A comet is only dangerous if it crosses Earth's path. In 1994, the comet Shoemaker-Levy could happen at any time. It is extremely unlikely that humankind would have survived if Shoemaker- Levy would have survived if Shoemaker- Levy had collided with the Earth rather than Jupiter.