The little green men are innocent

Posted by Emma Woollacott

Ithaca, NY - In a blow to conspiracy theorists everywhere, new evidence has shown that the 1908 Tunguska explosion was caused not by an alien spaceship but by a comet.

The explanation for the mysterious explosion, which leveled 830 square miles of Siberian forest, is supported by an unlikely source: the exhaust plume from a NASA space shuttle launched a century later.

The research connects the two events through what followed each about a day later: brilliant, night-visible clouds known as noctilucent clouds, which are made up of ice particles and only form at very high altitudes and in extremely cold temperatures.

The researchers contend that the massive amount of water vapor spewed into the atmosphere by the comet's icy nucleus was caught up in swirling eddies with tremendous energy by a process called two-dimensional turbulence, which explains why the clouds formed a day later many thousands of miles away.

The space shuttle exhaust plume, the researchers say, resembled the comet's action. A single space shuttle flight injects 300 metric tons of water vapor into the Earth's thermosphere, and the water particles have been found to travel to the Arctic and Antarctic, where they form the clouds after settling into the mesosphere.

Kelley and collaborators saw the noctilucent cloud phenomenon days after the space shuttle Endeavour launched in August 2007. Similar cloud formations were observed following launches in 1997 and 2003.

Following the 1908 explosion, known as the Tunguska Event, the night skies shone brightly for several days across Europe. Kelley said he became intrigued by historical eyewitness accounts, and concluded that the bright skies must have been the result of noctilucent clouds. The comet would have started to break up at about the same altitude as the release of the exhaust plume from the space shuttle.

The scientists have attempted to answer how the vapor traveled so far without diffusing, as conventional physics would predict.

"There is a mean transport of this material for tens of thousands of kilometers in a very short time, and there is no model that predicts that," said Michael Kelley, the Cornell professor who led the team.  "It's totally new and unexpected physics."

The answer, the researchers contend, is tied up in high-energy, counter-rotating eddies. Once the water vapor got caught up in these eddies, the water traveled very quickly - close to 300 feet per second.

Scientists have long tried to study the wind structure in these upper regions of the atmosphere, explained Charlie Seyler, Cornell professor of electrical engineering and paper co-author. "Our observations show that current understanding of the mesosphere-lower thermosphere region is quite poor," he said.

The research will be published in Geophysical Research Letters.

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