[meteorite-list] Space Shuttle Shows 1908 Tunguka Explosion Was Caused by Comet

[Ron Baalke]

http://www.news.cornell.edu/stories/June09/TunguskaComet.html

A mystery solved: Space shuttle shows 1908 Tunguska explosion was caused
by comet

By Anne Ju 
Cornell Chronicle
June 24, 2009

The mysterious 1908 Tunguska explosion that leveled 830 square miles of
Siberian forest was almost certainly caused by a comet entering Earth's
atmosphere, says new Cornell 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 (June 24) by the journal
Geophysical Research Letters, published by the American Geophysical
Union, 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, the James A. Friend Family Distinguished 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 to black holes.

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 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 55 miles over the polar regions during the
summer months when the mesosphere is around minus 180 degrees Fahrenheit
(minus 117 degrees Celsius).

The space shuttle exhaust plume, the researchers say, resembled the
water vapor from the comet. 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 regions,
where they form the clouds after settling into the mesosphere. The
thermosphere is the layer of the atmosphere above the mesosphere.

Kelley and collaborators saw the noctilucent cloud phenomenon days after
the space shuttle Endeavour (STS-118) launched on Aug. 8, 2007. Similar
cloud formations had been 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, particularly Great
Britain -- more than 3,000 miles 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 vapor was injected into the atmosphere.

The scientists have attempted to answer how this water vapor traveled so
far without scattering and 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," 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 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, which is difficult to do by such traditional
means as sounding rockets, balloon launches and satellites, explained
Charles 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.

The paper is also co-authored by Clemson University physicist Miguel
Larsen, Ph.D. '79, a former student of Kelley. The work performed at
Cornell was funded by the Atmospheric Science Section of the National
Science Foundation.

On July 1, Kelley will give a lecture, "Two-dimensional Turbulence,
Space Shuttle Plume Transport in the Thermosphere, and a Possible
Relation to the Great Siberian Impact Event," at a plenary session of
the annual meeting of Coupling, Energetics and Dynamics of Atmospheric
Regions in Santa Fe, N.M.

The paper is available at:
http://www.agu.org/journals/gl/papersinpress.shtml.

##