Tunguska impact riddle solved at last

Scientists believe they have solved the century-old mystery of an explosion that devastated a vast region of Earth – thanks to the space shuttle.
Tunguska trees felled

They say the blast known as the Tunguska Event that flattened 830 square miles of forest in a thankfully remote region of Siberia in April 1908 was caused by a comet entering the atmosphere.

The findings by a team from Cornell University, New York, are based on a study of the exhaust plume left by the shuttle during a launch.

Scientists connected the two events because each has been followed around a day later by the appearance of rare but brilliant noctilucent clouds – clouds of ice particles in the highest reaches of the atmosphere that are visible at night.

Previous speculation had ranged from a crumbly asteroid to a passing alien spacecraft – but a comet has been a favourite candidate due to the lack of an impact crater. It seems the explosion happened above Siberia before any impact could occur.

But expeditions that eventually reached the region showed the incredible result with trees blown to the ground. The blast is estimated to have been equivalent ten to 15 million tons of TNT. Bright glows were seen later over Europe and Asia but no traces of the impacting body have ever been found. Italian scientists recently suggested that a nearby lake could be the impact site but this has not been generally accepted.

The researchers believe that the massive amount of water vapour spewed into the atmosphere by the comet’s icy nucleus was caught up in energetic, swirling eddies. The process, called two-dimensional turbulence, explains why the noctilucent clouds formed a day later many thousands of miles away.

The space shuttle exhaust plume resembled the comet’s action, say the team. A single launch injects 300 metric tons of water vapour into the Earth’s thermosphere, and the water particles have been found to travel to the Arctic and Antarctic regions, where they also form noctilucent clouds.

Professor Michael Kelley, who led the new research, said: “It’s almost like putting together a 100-year-old murder mystery. The evidence is pretty strong that the Earth was hit by a comet in 1908.”

The team had to explain how the vapour travelled so far without dispersing. They suggest that this can be explained by extremely energetic eddies that travel rapidly through the upper atmosphere, or mesosphere.

Picture: Trees lie flattened by the blast above Tunguska in 1908.

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