[meteorite-list] Small Asteroids = Small Threat?

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:21:06 2004
Message-ID: <200307171550.IAA16987_at_zagami.jpl.nasa.gov>

http://skyandtelescope.com/news/article_1005_1.asp

Small Asteroids = Small Threat?
By J. Kelly Beatty
Sky & Telesscope
July 16, 2003

Every time a rocky body whizzes past Earth, asteroid specialists brace
themselves for a frenzied burst of "what-if" doomsday questions from the
news media. But while the risk of the interloper striking our planet is
usually vanishingly small, the longer-term prospects for all such near-Earth
asteroids are poorly known. Current thinking holds that objects 50 to 75
meters across, comparable to the one whose airburst devastated the Tunguska
region of Siberia in 1908, probably come crashing down every 1,000 years on
average. Most researchers think that objects twice that size, even though
they arrive less often, probably strike the ground intact - wreaking havoc
locally if one should hit land and triggering an ocean-spanning tsunami if
it makes a splash.

But Philip A. Bland (Imperial College London) and Natalya A. Artemieva
(Institute for Dynamics of Geospheres, Moscow) argue that the threat from
small asteroids has been greatly exaggerated. Writing in July 17th's Nature,
they conclude that stony asteroids up to several hundred meters across
rarely make it to the ground but instead almost always break up high in the
atmosphere.

Their finding is based more than 1,000 computer-simulated collisions with
bodies up to 1 kilometer in diameter. In the past, most theorists have
treated cosmic projectiles as giant blobs of liquid racing downward through
Earth's atmosphere. But Bland and Artemieva believe that even sizable stony
objects should break into successively smaller pieces while still very high
up. This fragmentation model, first introduced in 1980 and refined recently
by Artemieva and others, implies that a stony impactor will be reduced to
rubble long before it reaches ground level.

"Our simulations predict atmospheric fragmentation for much larger objects
than previously thought," the two researchers conclude. They predict that a
220-meter-wide asteroid, the size considered most likely to create a tsunami
at least 5 meters high when it strikes the open ocean, actually makes it all
the way to the ground only once in 170,000 years on average - an interval 50
times longer than previously estimated. A fragment 3 to 5 meters across, big
enough to create a 100-meter crater, also strikes Earth infrequently, only
every 3,000 to 4,000 years.

Artemieva points out that their "separated fragment" model doesn't apply to
comets or loosely bound piles of rocky material, which behave more like
fluids as they disintegrate in the atmosphere. But it's a particularly good
fit to iron meteoroids, which will remain intact longer but should still
break apart if no more than about 50 meters across. "I think the most
intersting advantage of this model is the possibility to predict
strewnfields," she says, which are clusters of meteorites dropped during a
single event like the 1947 Sikhote-Alin fall in eastern Siberia.

Dynamicist Erik Asphaug (University of California at Santa Cruz) comments
that if Bland and Artemieva are right, "certainly the tsunami hazard becomes
trivial in comparison with other natural catastrophes." But Clark R. Chapman
(Southwest Research Institute) warns that we shouldn't lose sight of the
potential damage from Tunguska-like airbursts. "Atmospheric breakup and
explosion in the lower atmosphere may be even more damaging than if the
object hit the ground intact," he points out.

If the threat from small asteroids were truly reduced so dramatically,
astronomers would be forced to rethink their plans to build telescopes
capable of detecting hundreds of thousands of Earth-threatening asteroids in
the 100- to 500-meter size range. One such effort, dubbed the Panoramic
Survey Telescope and Rapid Response System (Pan-STARRS), got the U.S. Air
Force's go-ahead for development last October. "A 200-meter body carries
roughly 1,000 megatons of kinetic energy," observer David C. Jewitt
(University of Hawaii), a Pan-STARRS participant. "Personally, I want to
know as much as possible about any 1,000-megaton explosion on Earth - no
matter where it might occur!"
Received on Thu 17 Jul 2003 11:50:08 AM PDT