[FMO] Fwd: NEO News (12/19/07) Tunguska Revision & New Book
- From: Marco Langbroek <marco.langbroek@xxxxxxxxxx>
- To: FMO lijst <fmo@xxxxxxxxxxxxx>
- Date: Thu, 20 Dec 2007 08:24:52 +0100
-------- Originele bericht --------
Onderwerp: NEO News (12/19/07) Tunguska Revision & New Book
Datum: Wed, 19 Dec 2007 14:57:55 -0800
Van: David Morrison <david.morrison@xxxxxxxx>
Aan: David Morrison <david.morrison@xxxxxxxx>
NEO News (12/19/07) Tunguska Revision & New Book
Season's greetings and best wishes for a good new year!
The main story in this edition of NEO News concerns a proposed
downsizing of the energy of the 1908 Tunguska airburst, with
associated increase in the expected frequency of such impacts. Mark
Boslough of Sandia has generated supercomputer simulations of the
Tunguska atmospheric explosion. In part his models require less
energy in the explosion because he includes the substantial downward
momentum of the rocky impactor, rather then modeling it as a
stationary explosion. If this revision (down to an estimated energy
of 3-5 megatons, and a corresponding diameter of about 50 meters) is
correct, the expected frequency of such impacts changes, from once in
a couple of millennia to once in a few hundred years. If smaller
impactors can do the damage previously associated with larger ones,
of course, the total hazard from such impacts is increased.
The second item below is an announcement of publication of a new
multi-author book "Comet/Asteroid Impacts and Human Society."
David Morrison
===================================
NEW ESTIMATE OF TUNGUSKA IMPACTOR SIZE
Sandia: December 17, 2007
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."
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.
==================================
NEW BOOK ON IMPACTS AND HUMAN SOCIETY
During the first days of December 2004, a multidisciplinary workshop
was held in the town of La Laguna on the Canary isle of Tenerife with
the title "Comet/Asteroid Impacts and Human Society". This was funded
as part of a project with the same name by ICSU, the International
Council for Science. The driving force behind the project is the
realization of a need to support the development of both national and
international policies regarding the impact hazard. And the direct
goal of the workshop was to bring together experts on as wide a range
of topics as possible with a bearing on all issues from the
astronomical observations and dynamical theories to the down-to-Earth
aspects of disaster planning and information chains.
The papers presented at this workshop - both keynote and research
talks - have been refereed and are now available as a Springer volume
entitled "Comet/Asteroid Impacts and Human Society - an
Interdisciplinary Approach" (ISBN 978-3-540-32709-7), edited by Peter
Bobrowsky and Hans Rickman. They were the leaders of the project and
main organizers of the workshop, representing the International Union
of Geological Sciences and the International Astronomical Union,
respectively. The book has been prepared as a technical document
describing the state of knowledge in many different fields in a way
that should be understandable across all borders. It should not be
perceived as the end of an effort but rather as the beginning of
something new, i.e., a concerted effort to develop an
interdisciplinary scientific field and bring the knowledge to both
citizens and decision makers of society.
Hans Rickman
--
+++++++++++++++++++++++++++++++++++++++++++
NEO News (now in its fourteenth year of distribution) is an informal
compilation of news and opinion dealing with Near Earth Objects
(NEOs) and their impacts. These opinions are the responsibility of
the individual authors and do not represent the positions of NASA,
Ames Research Center, the International Astronomical Union, or any
other organization. To subscribe (or unsubscribe) contact
dmorrison@xxxxxxxxxxxxx For additional information, please see the
website http://impact.arc.nasa.gov. If anyone wishes to copy or
redistribute original material from these notes, fully or in part,
please include this disclaimer.
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