Hi, Darren, List
Where I read this, I just walked around saying
"Ah-ha! Ah-ha!" for about five minutes. I watched
their eight movies and with particular interest, the
cross-sectional ones that show the "curly tongues
of fire" extending down. I am irresistibly reminded
of all those old medieval woodcuts that show "curly
tongues of fire" extending from a fireball in the sky,
you know the ones -- the ones we tend to dismiss as
the fanciful exaggerations of an ignorant age. Just
because someone doesn't understand something
doesn't mean that they can't be a good observer of it.
And, as this simulation shows, apparently there are
things we still don't understand. Imagine, the notion
that we don't already know everything worth knowing...
Secondly, I thought about all the arguments about
whether ANY meteoritic phenomenon can cause a
fire. We've certainly had lots of such arguments here
on the List. Again, it's the old descriptions of "columns
of fire" descending from the heavens that we dismiss;
and again, these simulations greatly resemble "columns
of fire" descending from the heavens!
There was a thread here on the List some time back
about a suggestion (not mine) that the many simultaneous
destructive fires (over 11 states and Canada) on October
8, 1871, had been ignited some meteoritic phenomenon.
That is the date of the Great Chicago and Peshtigo fires
http://en.wikipedia.org/wiki/Peshtigo_Fire
among others. Frankly, the evidence and witness reports
strongly suggest some unknown aerial mechanism, but
I was at a loss to account for how very small high-velocity
cometary fragments could cause fires. [Note the date: the
peak hours of the Draconids or "Giacobinids" from the
rapidly disintegrating Comet Giacobini-Zinner, which is
above the horizon all day.]
Well, here's the mechanism I could never find.
More than that, the Sandia simulation shows another
effect argued about on this List. In the simulation that
shows sections of internal vortexes, you can see toroidal
rings of plasma forming and rotating around their own
central axis and surrounding the "column of fire," which
would then behave as the huge toroidal windings of an
immense electromagnet, the rotating plasma rings being
in effect currents of charged particles. You can also see
how they resist being pinched shut, how they force the material
inside, along the axis, up and out of the "plasma tube" so
formed. In a big impact, that "tube" would extend out
the top of the atmosphere and be evacuated, and the
"vacuum" would collect surface materials from around
the crater, suck them up, and eject them from the planet into
space at escape velocity plus. This is how, say, chunks
of Mars could get started toward the Earth without being
violently shocked and even shattered by the impact, a long-
mysterious question without a good answer. I think Sandia
may have found just such a mechanism in a larger scale
version of their Tunguska-sized model.
I think, for example, about all the evidence that EP
(Grondine) collected about the destruction the French
town of Bazas in the year 580 by "fire from the sky."
The objections to his impact interpretation are the lack
of ground evidence: craters, fragments, and so forth.
An airburst with a thermal pulse is usually offered as
the mechanism, but it's really hard to get an airburst
low enough to get a hot thermal pulse without getting
a crater along with it and big blast effects; it's like a
kind of balancing act, just enough without too much,
and it's not convincing.
But the Sandia simulations show that those ancient
descriptions of "tongues of fire from the heavens" were
almost certainly literally true! The folks at Sandia made
(rightly) the point that even a small impactor can be much
more destructive than we presently think, but secondarily
it changes the way we must evaluate the historical record.
I wonder how many years (decades) it will take for
this lesson to sink in?
Sterling K. Webb
----------------------------------------------------------------------
----- Original Message -----
From: "Darren Garrison" <[EMAIL PROTECTED]>
To: <meteorite-list@meteoritecentral.com>
Sent: Wednesday, December 19, 2007 8:42 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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