"Long before it's in the papers"
December 21, 2007

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Small asteroids can do great damage, study predicts

Dec. 21, 2007
World Science staff

An as­ter­oid whose fiery encounter with our atmo­sphere in­cin­er­at­ed a swath of Si­be­ri­an for­est, in 1908, was only a frac­ti­on as large as pre­vi­ously thought, new re­search sug­gests. That would mean such dev­as­tat­ing events are more com­mon than es­ti­mat­ed be­fore—and might hap­pen eve­ry two to three cen­turies, if a re­cent NASA anal­y­sis is cor­rect.

Re­search­er Mark Bos­lough points out de­tails of a "fire­ball" that a sim­u­la­tion shows would re­sult from an as­ter­oid burst­ing in the at­mos­phere. (Pho­to by Randy Mon­toya)

An asteroid whose fiery encounter with our sky incinerated a swath of Siberian woods, in 1908, was only a fraction as large as previously thought, new research suggests. That would mean such devastating events are more common than estimated before—and would happen every two to three centuries, if a recent NASA analysis is correct. Small asteroids, like any rocks, outnumber than big ones. Thus events like the Siberian blast “are not as improbable as we had believed,” said Mark Boslough of Sandia National Laboratories in New Mexico, principal investigator of the new study. “We should be making more efforts at detecting the smaller ones.” The June 30, 1908 explosion over Tunguska, Siberia, flattened some 60 million trees over a vast area of unpopulated forest. Boslough anc colleagues ran supercomputer simulations re-enacting what would happen when an asteroid explodes in the atmosphere, as is believed to have occurred then. The simulations—which the U.S. government laboratory contends are better than earlier ones—show the blast shooting a hot fireball downward faster than sound. Compared to an explosion that stays where it started, Boslough said, this causes stronger blast waves and blinding light pulses, or thermal radiation, at the ground level. “Our [previous] understanding was oversimplified,” said Boslough. Now, “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.” The research offers one encouraging revelation, Boslough said: the blast caused “less devastation than previously thought,” because some extraneous factors may have amplified the damage. For one, foresters believe the affected woodland was unhealthy at the time. Nonetheless, Boslough and colleagues said, the finding that the asteroid was relatively small should prompt a new look at current asteroid-detection efforts. What really determines an asteroid’s destructive potential, he wrote in an email, is its kinetic energy, or mass times velocity squared. But the new findings indicate that for a given speed—estimated to have been more than five times that of sound, in the Tunguska case—the asteroid needed to be only one-third to one-fourth as heavy as previously estimated. “If the White House were made out of a solid rock, it would have just about the right mass,” Boslough wrote; so would a stone ball 30 to 40 meters (33 to 44 yards) wide. The necessary size also depends on the object’s precise makeup, added Boslough, who with colleagues drew worldwide attention in the 1990s by correctly predicting that comet Shoemaker-Levy 9 would create a fireball visible from Earth when it hit Jupiter. The new Tunguska simulations indicate that an incoming asteroid is compressed by resistance from Earth’s atmosphere. As it pushes deeper, the growing resistance blasts it apart, leading to the downward flow of hot gas. Because this enhances the destruction, the blast probably needed to release only three to five megatons of power, the researchers said—not the previously estimated to 10 to 20. Such a re-evaluation of the power would mean Tunguska-type events happen around three times as often as previously believed, according to a 2003 report by NASA’s Near-Earth Object Science Definition Team. Specifically, the report said, such revised estimates would suggest Tunguska-type events happen every 250 years or so, rather than every 600 to 1000. Boslough’s work was presented at the American Geophysical Union meeting in San Francisco on Dec. 11, and accepted for publication in the International Journal of Impact Engineering.