[meteorite-list] Tunguska-- the movie

[Jerry]

Right on the mark Darren. It speaks to many of the questions raised in the 
Mammoth thread in terms of frequency and potential effects of an airburst as 
well as impacts.
The footnote to Comet Levy/Shomaker may also reminds us that impactors don't 
necessarily travel alone. In tandem, some strike, some vaporize and maybe 
some or one airburst.
Jerry Flaherty
----- Original Message ----- 
From: "Darren Garrison" <cynapse at charter.net>
To: <meteorite-list at meteoritecentral.com>
Sent: Wednesday, December 19, 2007 9:42 AM
Subject: [meteorite-list] Tunguska-- the movie


Videos on the site.

http://www.sandia.gov/news/resources/releases/2007/asteroid.html

Sandia supercomputers offer new explanation of Tunguska disaster
Smaller asteroids may pose greater danger than previously believed

ALBUQUERQUE, N.M. - The stunning amount of forest devastation at Tunguska a
century ago in Siberia may have been caused by an asteroid only a fraction 
as
large as previously published estimates, Sandia National Laboratories
supercomputer simulations suggest.

"The asteroid that caused the extensive damage was much smaller than we had
thought," says Sandia principal investigator Mark Boslough of the impact 
that
occurred June 30, 1908. "That such a small object can do this kind of
destruction suggests that smaller asteroids are something to consider. Their
smaller size indicates such collisions are not as improbable as we had
believed."

Because smaller asteroids approach Earth statistically more frequently than
larger ones, he says, "We should be making more efforts at detecting the 
smaller
ones than we have till now."

The new simulation - which more closely matches the widely known facts of
destruction than earlier models - shows that the center of mass of an 
asteroid
exploding above the ground is transported downward at speeds faster than 
sound.
It takes the form of a high-temperature jet of expanding gas called a 
fireball.

This causes stronger blast waves and thermal radiation pulses at the surface
than would be predicted by an explosion limited to the height at which the 
blast
was initiated.

"Our understanding was oversimplified," says Boslough, "We no longer have to
make the same simplifying assumptions, because present-day supercomputers 
allow
us to do things with high resolution in 3-D. Everything gets clearer as you 
look
at things with more refined tools."

Sandia is a National Nuclear Security Administration laboratory.

The new interpretation also accounts for the fact that winds were amplified
above ridgelines where trees tended to be blown down, and that the forest at 
the
time of the explosion, according to foresters, was not healthy. Thus 
previous
scientific estimates had overstated the devastation caused by the asteroid,
since topographic and ecologic factors contributing to the result had not 
been
taken into account.

"There's actually less devastation than previously thought," says Boslough, 
"but
it was caused by a far smaller asteroid. Unfortunately, it's not a complete 
wash
in terms of the potential hazard, because there are more smaller asteroids 
than
larger ones."

Boslough and colleagues achieved fame more than a decade ago by accurately
predicting that that the fireball caused by the intersection of the comet
Shoemaker-Levy 9 with Jupiter would be observable from Earth.

Simulations show that the material of an incoming asteroid is compressed by 
the
increasing resistance of Earth's atmosphere. As it penetrates deeper, the 
more
and more resistant atmospheric wall causes it to explode as an airburst that
precipitates the downward flow of heated gas.

Because of the additional energy transported toward the surface by the 
fireball,
what scientists had thought to be an explosion between 10 and 20 megatons 
was
more likely only three to five megatons. The physical size of the asteroid, 
says
Boslough, depends upon its speed and whether it is porous or nonporous, icy 
or
waterless, and other material characteristics.

"Any strategy for defense or deflection should take into consideration this
revised understanding of the mechanism of explosion," says Boslough.

One of most prominent papers in estimating frequency of impact was published
five years ago in Nature by Sandia researcher Dick Spalding and his 
colleagues,
from satellite data on explosions in atmosphere. "They can count those 
events
and estimate frequencies of arrival through probabilistic arguments," says
Boslough.

The work was presented at the American Geophysical Union meeting in San
Francisco on Dec. 11. A paper on the phenomenon, co-authored by Sandia
researcher Dave Crawford and entitled "Low-altitude airbursts and the impact
threat" has been accepted for publication in the International Journal of 
Impact
Engineering.

The research was paid for by Sandia's Laboratory-Directed Research and
Development office.


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