NEO News (12/19/07) Tunguska Revision & New Book

From David Morrison...

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

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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.

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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

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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@arc.nasa.gov. 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.