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August 03, 2010

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Astronomers catch a “shooting star”

March 25, 2009
Courtesy SETI Institute
and World Science staff

As­ter­oid 2008 TC3 has a hum­drum name but an unusual dis­tinc­tion: it’s the first space rock to have been spot­ted be­fore it made a fiery ren­dez­vous with our plan­et, as­tro­no­mers say.

It streaked in­to the skies over north­ern Su­dan in the early morn­ing of Oct. 7, then burst at a high 37 km (23 miles) over the Nu­bi­an Des­ert. It was thought to have fully dis­in­te­grat­ed in­to dust. But a me­te­or as­tron­o­mer with the Moun­tain View, Calif.-based SETI (Search for Ex­tra­ter­res­tri­al In­tel­li­gence) In­sti­tute, Pe­ter Jen­niskens, thought oth­er­wise. 

Work­ing with phys­i­cist Mauwia Shad­dad of the Uni­ver­s­ity of Khar­toum, Su­dan, and stu­dents and staff from the uni­ver­s­ity, he col­lect­ed nearly 280 pieces of the as­ter­oid, strewn over miles of des­ert. Nev­er be­fore had me­te­orites been col­lect­ed from such a high-altitude ex­plo­sion, ac­cord­ing to as­tro­no­mers.

As it turns out, the as­sem­bled rem­nants are un­like an­y­thing in our me­te­orite col­lec­tions, and may be an im­por­tant clue in un­rav­el­ing the early his­to­ry of the so­lar sys­tem, said Jen­niskens. “This was an ex­tra­or­di­nary op­por­tun­ity, for the first time, to br­ing in­to the lab ac­tu­al pieces of an as­ter­oid we had seen in space,” said Jen­niskens, lead au­thor on an ar­ti­cle in the cur­rent issue of the research jour­nal Na­ture on the anal­y­sis.

Pick­ed up by Ari­zon­a’s Catalina Sky Sur­vey tel­e­scope on Oct. 6, the truck-sized as­ter­oid ab­ruptly ended its 4.5 bil­lion year so­lar sys­tem od­ys­sey only 20 hours af­ter disco­very, when it broke apart in the Af­ri­can skies. The in­com­ing rock was tracked by sev­er­al groups of as­tro­no­mers, in­clud­ing a team at the La Pal­ma Ob­serv­a­to­ry in the Ca­nary Is­lands that was able to meas­ure sun­light re­flected by the ob­ject.

Stud­y­ing the re­flected sun­light gives clues to the min­er­als at the sur­face of these ob­jects. As­tro­no­mers group the as­ter­oids in­to clas­ses, and at­tempt to as­sign me­te­orite types to each class. But their abil­ity to do this is of­ten frus­trat­ed by dust on the as­ter­oids.

Jen­niskens and the SETI In­sti­tute’s Jan­ice Bish­op meas­ured the re­flec­tion prop­er­ties of the me­te­orite. The pair found that both the as­ter­oid and its me­te­oritic re­mains re­flected light in much the same way, si­m­i­lar to the known be­hav­ior of so-called F-class as­ter­oids.

“F-class as­ter­oids were long a mys­tery,” Bish­op not­ed, as as­tro­no­mers had nev­er been able to ac­tu­ally hold a spec­i­men. “The good cor­re­spond­ence be­tween tel­e­scop­ic and lab­o­r­a­to­ry meas­urements for 2008 TC3 sug­gests that small as­ter­oids don’t have the trou­ble­some dust lay­ers, and may there­fore be more suita­ble ob­jects for es­tab­lish­ing the link be­tween as­ter­oid type and me­te­orite prop­er­ties,” he added. “That would al­low us to char­ac­ter­ize as­ter­oids from afar.”

“2008 TC3 could serve as a Ro­set­ta Stone, pro­vid­ing us with es­sen­tial clues to the pro­cesses that built Earth and its plan­etary sib­lings,” said Rocco Mancinelli, a mi­cro­bi­al ecolo­g­ist at the in­sti­tute and mem­ber of the re­search team. “In the dim past, as the so­lar sys­tem was tak­ing shape, small dust par­t­i­cles stuck to­geth­er to form larg­er bod­ies, a pro­cess of ac­cu­mula­t­ion that even­tu­ally pro­duced the as­ter­oids. Some of these bod­ies col­lid­ed so vi­o­lently that they melted through­out.”

2008 TC3 is an in­ter­me­diate case, hav­ing been only par­tially melted, ac­cord­ing to re­search­ers. The re­sult­ing ma­te­ri­al pro­duced what’s called a polymict ure­ilite me­te­orite.

Know­ing the na­ture of F-class as­ter­oids could con­ceivably pay off in pro­tect­ing Earth from dan­ger­ous im­pactors, re­search­ers said. That 2008 TC3 blew up very soon af­ter hit­ting the at­mos­phere in­di­cates it was frag­ile. It weighed an es­ti­mat­ed 80 tons, of which only some 5 kg (11 lb.) has been reco­vered. If at some fu­ture time we disco­ver an F-class as­ter­oid that’s, say, sev­er­al kilo­me­ters or miles wide—one that could wipe out species—then we’ll know its make­up and can de­vise ways to ward it off. Hit­ting such a frag­ile as­ter­oid with an atom­ic bom­b, as Bruce Wil­lis did in the 1998 mov­ie Ar­ma­ged­don, would merely turn it in­to a deadly swarm of shot­gun pel­lets.

* * *

Asteroid 2008 TC3 has a humdrum name but an exciting distinction: it’s the first space rock to have been spotted before it crashed into our planet, astronomers say. It streaked into the skies over northern Sudan in the early morning of October 7, 2008, then burst at a high 37 km (23 miles) over the Nubian Desert. It was thought to have fully disintegrated into dust. But a meteor astronomer with the Mountain View, Calif.-based SETI (Search for Extraterrestrial Intelligence) Institute, Peter Jenniskens, thought otherwise. Working with physicist Mauwia Shaddad of the University of Khartoum, Sudan, and students and staff from the university, he collected nearly 280 pieces of the asteroid, strewn over miles of desert. Never before had meteorites been collected from such a high-altitude explosion, according to astronomers. As it turns out, the assembled remnants are unlike anything in our meteorite collections, and may be an important clue in unraveling the early history of the solar system, said Jenniskens. “This was an extraordinary opportunity, for the first time, to bring into the lab actual pieces of an asteroid we had seen in space,” comments Jenniskens, the lead author on an article in the journal Nature that describes the recovery and analysis of the remnants. Picked up by Arizona’s Catalina Sky Survey telescope on 6 October, 2008, the truck-sized asteroid abruptly ended its 4.5 billion year solar system odyssey only 20 hours after discovery, when it broke apart in the African skies. The incoming rock was tracked by several groups of astronomers, including a team at the La Palma Observatory in the Canary Islands that was able to measure sunlight reflected by the object. Studying the reflected sunlight gives clues to the minerals at the surface of these objects. Astronomers group the asteroids into classes, and attempt to assign meteorite types to each class. But their ability to do this is often frustrated by dust on the asteroids. Jenniskens and the SETI Institute’s Janice Bishop measured the reflection properties of the meteorite. The pair found that both the asteroid and its meteoritic remains reflected light in much the same way, similar to the known behavior of so-called F-class asteroids. “F-class asteroids were long a mystery,” Bishop noted, as astronomers had never been able to actually hold a specimen. “The good correspondence between telescopic and laboratory measurements for 2008 TC3 suggests that small asteroids don’t have the troublesome dust layers, and may therefore be more suitable objects for establishing the link between asteroid type and meteorite properties,” he added. “That would allow us to characterize asteroids from afar.” “2008 TC3 could serve as a Rosetta Stone, providing us with essential clues to the processes that built Earth and its planetary siblings,” said Rocco Mancinelli, a microbial ecologist at the institute and member of the research team. “In the dim past, as the solar system was taking shape, small dust particles stuck together to form larger bodies, a process of accumulation that eventually produced the asteroids. Some of these bodies collided so violently that they melted throughout.” 2008 TC3 turns out to be an intermediate case, having been only partially melted, according to researchers. The resulting material produced what’s called a polymict ureilite meteorite. The meteorites from 2008 TC3, now called “Almahata Sitta,” are anomalous ureilites: very dark, porous, and rich in highly cooked carbon. This new material may serve to rule out many theories about the origin of ureilites. In addition, knowing the nature of F-class asteroids could conceivably pay off in protecting Earth from dangerous impactors, researchers said. That 2008 TC3 blew up very soon after hitting the atmosphere indicates it was fragile. It weighed an estimated 80 tons, of which only some 5 kg (11 lb.) has been recovered. If at some future time we discover an F-class asteroid that’s, say, several kilometers or miles wide—one that could wipe out species—then we’ll know its makeup and can devise appropriate strategies to ward it off. Hitting such a fragile asteroid with an atomic bomb, as Bruce Willis did in the 1998 movie Armageddon, would merely turn it into a deadly swarm of shotgun pellets.