An artist's impression of the explosion over Tunguska in 1908, believed to have been caused by a comet.
Shuttle shines light on comet impact
1 Jul 2009
The mysterious explosion in 1908 that levelled over 2,000 square kilometres of Siberian forest was almost certainly caused by a comet entering the Earth's atmosphere, says new Cornell University research. The conclusion is supported by an unlikely source—the exhaust plume from the NASA space shuttle launched a century later.
The research, accepted for publication by the journal Geophysical Research Letters, connects the two events by what followed each about a day later—brilliant, night-visible clouds, or noctilucent clouds, that are made up of ice particles and only form at very high altitudes and in extremely cold temperatures.
"It's almost like putting together a 100-year-old murder mystery," said Michael Kelley, Professor of Engineering at Cornell, who led the research team. "The evidence is pretty strong that the Earth was hit by a comet in 1908." Previous speculation had ranged from comets to meteors.
< The explosion at Tunguska in 1908, flattened over 2,000 square kilometres of forest.
Following the 1908 explosion, known as the Tunguska Event, the night skies shone brightly for several days across Europe, particularly Great Britain—more than 4,800 kilometres away.
Kelley said he became intrigued by the historical eyewitness accounts of the aftermath, 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 following launch. In both cases, water vapour was injected into the atmosphere.
The researchers contend that the massive amount of water vapour spewed into the atmosphere by the comet's icy nucleus was caught up in swirling wind eddies with tremendous energy by a process called two-dimensional turbulence, which explains why the noctilucent clouds formed a day later many thousands of miles away.
Noctilucent clouds are the Earth's highest clouds, forming naturally in the mesosphere at about 90 kilometres over the polar regions during the summer months, when the mesosphere is around minus 117 degrees Celsius.
Kelley and collaborators saw noctilucent cloud phenomena days after the space shuttle Endeavour (STS-118) launched on August 8, 2007. Similar cloud formations had been observed following launches in 1997 and 2003.
The space shuttle's exhaust plume, the researchers say, resemble the comet's action. A single space shuttle flight injects 300 tonnes of water vapour into the Earth's upper atmosphere, and from there the water particles have been blown all the way to the Arctic and Antarctic regions, where they form the clouds.
Drifting smoke plumes over the Kennedy Space Centre after the launch of a space shuttle.
The scientists have attempted to answer how this water vapour travelled so far without scattering and diffusing, as conventional physics would predict.
"There is a mean transport of this material for tens of thousands of kilometres in a very short time, and there is no model that predicts that," Kelley said. "It's totally new and unexpected physics."
This "new" physics, the researchers contend, is tied up in counter-rotating eddies with extreme energy. Once the water vapour got caught up in these eddies, it travelled very quickly—close to 300 feet [100 metres] per second.
Scientists have long tried to study the wind structure in the upper regions of the atmosphere, which is difficult to do by such traditional means as small rockets, balloon launches and satellites, 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," Seyler said.
Adapted from information issued by Anne Ju, Cornell Chronicle.
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