by D. Vogt

Created on: July 22, 2010

On the morning of June 30, 1908, something exploded over Tunguska, Russia, with the force of a large thermonuclear weapon. Nearly a thousand square miles of forest was knocked over by the explosion, people hundreds of miles away were knocked off their feet by the shock wave, and seismologists across the continent registered a moderate-sized earthquake. A century later, we are still not entirely sure what caused the massive explosion, although the most commonly accepted explanation is that a small asteroid burst apart several miles above the surface of the Earth.

- About the Tunguska Event -

At the time, the Tunguska region of Russia was only sparsely populated by the indigenous Evenki (whose earlier Europen name, Tungus, was given to the region which bears the name). Those nearby reported two or three separate explosions or thumps, followed by a number of lesser explosions.

Because Tunguska was so remote and the Russian government of the period so weak, initially there was little serious interest in the phenomenon. However, after the Russian Revolution, the new Soviet government sent investigators to the region to discover what had happened. Subsequent analyses showed that the primary explosion over Tunguska had the force of a large hydrogen bomb, probably about 15 megatonnes of TNT (fifteen million tonnes of TNT), and possibly as much as twice that. This would make one-third to one-half as large as the most massive nuclear detonation ever, the Tsar Bomba nuclear test conducted by the Soviet Union in the 1950s. Had such an explosion occurred over Europe instead of remote Siberia, it could have devastated one or more countries and set back European progress by decades or more.

Yet, most frustrating of all, no impact crater at Tunguska has ever been found. This means that, although the most commonly accepted explanation is that an asteroid burst over the site, it is impossible to rule out alternative theories.

- Asteroid Airburst -

Because of their extremely high velocities, even relatively small rocky objects (about 20-30 feet across) can produce large explosions when they enter Earth's atmosphere and the forces of air resistance exceed the cohesive force holding the objects together. Every year or so, one disintegrates in the upper atmosphere and releases the force equivalent to a small atomic bomb, similar to the one exploded over Hiroshima in 1945. A few times as large and the explosion becomes massive indeed, like the one at Tunguska. Asteroids several

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