Sandia supercomputers offer new explanation of Tunguska disaster

[Dave Monroe]

FOR IMMEDIATE RELEASE
December 17, 2007

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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