Article: "Small Asteroids Pose Big New Threat"

A supercomputer simulation of a fireball that might be expected from an asteroid exploding in Earth's atmosphere, as pointed out by Sandia National Laboratories researcher Mark Boslough. Credit: Randy Montoya

From the abstract...

"Computational Modeling of Low-Altitude Airbursts," Boslough, M. [Sandia National Laboratories], American Geophysical Union 2007 Fall Meeting, San Francisco, CA, December 10-14, 2007.

New simulations of airbursts in the Earth's lower atmosphere from hypervelocity asteroid impacts suggest that a re-evaluation of the impact hazard is necessary to properly account for the enhanced damage potential relative to point-source approximations. The intent of these simulations was to explore the phenomenology associated with low-altitude airbursts and to determine whether the altitude of maximum energy deposition can be used as a reasonable estimate of the equivalent height of a point explosion. The simulations suggest that this not a good approximation, because the center of mass of an exploding projectile is transported downward in the form of a high-temperature jet of expanding gas. The jet descends by a significant fraction of the burst altitude before its velocity becomes subsonic. The time scale of this descent is similar to the time scale of the explosion itself, so the jet simultaneously couples both its translational and its radial kinetic energy to the atmosphere. Because of this downward flow, larger blast waves and stronger thermal radiation pulses are felt at the surface than would be predicted for a nuclear explosion of the same yield at the same height. For impacts with a kinetic energy above some threshold, the hot jet of vaporized projectile (the descending "fireball") makes contact with the Earth's surface, where it expands radially. During the time of radial expansion, the fireball can maintain temperatures well above the melting temperature of silicate minerals, and its radial velocity can exceed the sound speed in air. Surface materials can ablate by radiative/convective melting under these conditions, and then quench rapidly to form glass after the fireball cools and recedes. One possible example of an airburst glass is the Libyan Desert Glass of western Egypt. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under Contract DE-AC04-94AL85000.

From the article...

Now new supercomputer simulations suggest "the asteroid that caused the extensive damage was much smaller than we had thought," Boslough [Mark Boslough, a physicist at Sandia National Laboratory in Albuquerque, N.M.] said. Specifically, he and his colleagues say it would have been a factor of three or four smaller in mass and perhaps 65 feet (20 meters) in diameter.

The simulations run on Sandia's Red Storm supercomputer — the third fastest in the world — detail how an asteroid that explodes as it runs into Earth's atmosphere will generate a supersonic jet of expanding superheated gas. This fireball would have caused blast waves that were stronger at the surface than previously thought.

At the same time, previous estimates seem to have overstated the devastation the event caused. The forest back then was not healthy, according to foresters, "and it doesn't take as much energy to blow down a diseased tree than a healthy tree," Boslough said. In addition, the winds from the explosion would naturally get amplified above ridgelines, making the explosion seem more powerful than it actually was. What scientists had thought to be an explosion between 10 and 20 megatons was more likely only three to five megatons, he explained.

All in all, the researchers suggest that smaller asteroids may pose a greater danger than previously believed. Moreover, "there are a lot more objects that size," Boslough told SPACE.com.

NASA Ames Research Center planetary scientist and astrobiologist David Morrison, who did not participate in this study, said, "If he's right, we can expect more Tunguska-sized explosions — perhaps every couple of centuries instead of every millennia or two." He added, "It raises the bar in the long term — ultimately, we'd like to have a survey system that can detect things this small."

Boslough and his colleagues detailed their findings at the American Geophysical Union meeting in San Francisco on Dec. 11. A paper on the phenomenon has been accepted for publication in the International Journal of Impact Engineering.

"Small Asteroids Pose Big New Threat"
Charles Q. Choi
19 December 2007
SPACE.com

Link: Space.com Article

"Computational Modeling of Low-Altitude Airbursts," Boslough, M. [Sandia National Laboratories], American Geophysical Union 2007 Fall Meeting, San Francisco, CA, December 10-14, 2007.

"Terrestrial Impact Cratering: New Insights Into the Cratering Process From Geophysics and Geochemistry," C. Koeberl [University of Vienna], M. Boslough [Sandia National Laboratories], American Geophysical Union 2007 Fall Meeting, San Francisco, CA, December 10-14, 2007.

Link: American Geophysical Union 2007 Fall Meeting