"Long before it's in the papers"
January 28, 2015


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)

As with ordi­nary rocks, small as­ter­oids out­num­ber 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 in 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 about 2,000 square km (500,000 acres) of un­pop­u­lated for­est. 

Bos­lough and col­leagues ran su­per­com­puter si­m­ul­a­tions to re-enact what hap­pens when an as­ter­oid bursts in the sky, as is be­lieved to have oc­curred then. The sim­ul­a­tions—which the U.S. go­vernment lab­o­r­a­to­ry con­tends are the best to date—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,” he said. 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 ex­tra­ne­ous fac­tors likely am­pli­fied the dam­age. For one, for­esters be­lieve the af­fect­ed wood­land was un­healthy at the time. None­the­less, Bos­lough and col­leagues said, the most im­por­tant find­ing was the small size of the as­ter­oid, and this 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 this 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 on aver­age, 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.

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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.