If we blow up an as­ter­oid, it might pull back to­gether

The Charlotte Observer (Sunday) - - News - BY ROBIN GE­ORGE AN­DREWS New York Times

Faced with the prospect of a siz­able as­ter­oid head­ing to­ward Earth and caus­ing dooms­day, hu­man­ity has come up with var­i­ous re­sponses.

Hol­ly­wood may reckon that the best way to de­stroy an er­rant space rock is with nu­clear weapons. This is rarely the pre­ferred op­tion of ex­perts, but us­ing some sort of space­craft sys­tem to smash an as­ter­oid into small, harm­less pieces is seen as a real-world pos­si­bil­ity. A new study – look­ing at a gi­gan­tic space rock-on­space rock clash – hints at how ut­terly in­ef­fec­tive this type of as­ter­oid as­sas­si­na­tion at­tempt may be.

Us­ing com­puter mod­els, sci­en­tists sim­u­lated a 4,000-foot as­ter­oid smash­ing into a 15.5-mile as­ter­oid at 11,200 mph. Im­me­di­ately after col­lid­ing, the large as­ter­oid cracked con­sid­er­ably, with de­bris flow­ing out­ward like a cas­cade of Ping-Pong balls. De­spite some deep frac­tures, the heart of the as­ter­oid was not com­pre­hen­sively dam­aged.

As time went on, the grav­i­ta­tional pull of the as­ter­oid’s re­silient core was able to pull back ejected shards. It seems that as­ter­oids don’t just ab­sorb mind-bog­gling amounts of dam­age, but, as pre­vi­ous work has hinted, they also are able to re­build them­selves.

Charles El Mir, who stud­ies as­ter­oid an­ni­hi­la­tion at Johns Hop­kins Univer­sity and is the pa­per’s lead au­thor, said his find- ings “could be in­ter­preted as an ar­gu­ment against ‘blow­ing up’ an as­ter­oid as a de­fen­sive strat­egy.”

As­ter­oid col­li­sions and de­mo­li­tions have been sim­u­lated many times in re­cent decades. Ear­lier stud­ies sug­gested that large as­ter­oids are full of in­ter­nal scars be­cause of their vi­o­lent his­tory, and that a fast enough im­pact would com­pletely shat­ter them.

The new study, pub­lished this month in the jour­nal Icarus, tried a dif­fer­ent sim­u­la­tion.

K.T. Ramesh, di­rec­tor of the Hop­kins Ex­treme Ma­te­ri­als In­sti­tute, said that Andy Tonge, a for­mer grad­u­ate stu­dent, had de­vel­oped a com­pu­ta­tional model that looked at how ma­te­ri­als like bul­let­proof vests re­spond to im­pacts. Re­al­iz­ing that Tonge’s model could sim­u­late as­ter­oid im­pact events, the team­merged it with an­other model that also repli­cated the ef­fects of a large as­ter­oid’s grav­i­ta­tional field.

This hy­brid model al­lowed them to more real­is­ti­cally see how an as­ter­oid re­sponds to be­ing hit by a pow­er­ful pro­jec­tile. It cap­tured pre­vi­ously miss­ing but vi­tal small-scale de­tails, in­clud­ing where frac­tures would ap­pear and pre­cisely how they would spread.

Michele Ban­nis­ter, a plan­e­tary as­tronomer at Queen’s Univer­sity Belfast, de­scribed the re­search as “a nice up­grade on­mod­el­ing the com­plex phys­i­cal re­al­i­ties” of the so­lar sys­tem’s enig­matic rocky mon­sters.

The study has limita- tions. Both as­ter­oids are mod­eled as sim­ple, non­ro­tat­ing chunks of rock, whereas real as­ter­oids are far more vari­able. In ad­di­tion, the larger as­ter­oid, de­spite fea­tur­ing a start­ing col­lec­tion of cracks, did not have a his­tory of mul­ti­ple im­pacts as true as­ter­oids would. A large space rock smash­ing into a hu­mon­gous space rock also dif­fers from amis­sile on­slaught, or an atomic bomb ex­plod­ing on or be­neath an as­ter­oid’s sur­face while a pop­u­lar rock band plays.

The study doesn’t rule out us­ing pro­jec­tiles to de­stroy an in­com­ing as­ter­oid, El Mir said. But, he added, shat­ter­ing a large as­ter­oid may end up caus­ing more prob­lems than it solves. Turn­ing a can­non­ball into shot­gun-shell frag­ments could still re­sult in Ar­maged­don if the shards strike Earth.

NASA’s Plan­e­tary De­fense Co­or­di­na­tion Of­fice, which keeps an eye on as­ter­oids and comets that will one day pass close to Earth, in­stead sug­gests chang­ing a space rock’s tra­jec­tory by giv­ing it a small nudge well in ad­vance of reach­ing our world. NASA and oth­ers aim to test this strat­egy in 2022 with the Dou­ble As­ter­oid Redi­rec­tion Test, in which a space­craft will de­lib­er­ately crash into the smaller mem­ber of a bi­nary as­ter­oid sys­tem in an at­tempt to change its or­bit around the larger body.

Ul­ti­mately, the choice be­tween de­flec­tion and de­struc­tion largely de­pends on how quickly an in­com­ing as­ter­oid is spot­ted.

“A suc­cess­ful de­flec­tion be­comes more dif­fi­cult to ex­e­cute as warn­ing time de­creases,” said Me­gan Bruck Syal, a plan­e­tary de­fense re­searcher at the Lawrence Liver­more Na­tional Lab­o­ra­tory. “For the short­est warn­ing times, ro­bust dis­rup­tion and dis­per­sal of the frag­ments may be the only vi­able op­tion to pre­vent the im­pact.”

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