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Around midnight on 6 October 2008,a whie do flied across he screenof Richard Kowalskis computer at anobservatory atop Mount Lemmon in

Arizona. Kowalski had seen hundreds of suchdos dring hree and a half years of scanningelescope images for aseroids ha migh hiEarh or come close. He followed he objechrogh he nigh and sbmied he coordi-naes, as sal, o he Minor Plane Cener inCambridge, Massachusetts, which keeps trackof asteroids and other small bodies. When the

sky began o brighen, he sh down he ele-scope, wen o he dorm down he monainand fell asleep.

The only hing ha had pzzled Kowalskiabo he midnigh blip was he Minor PlaneCenters response to his report. Its website postedhe discovery righ away b when he ried oadd more data, the system stayed silent.

Tim Spahr, the Minor Planet Centers direc-tor, found out why the following morning. Thecenres sofware compes orbis aomai-cally, b his aseroid was nsally close oEarth. The computer ran to me for help, saysSpahr. He did some quick calculations on Kow-

alskis data to figure out the path of the asteroid,which was now named 2008 TC3. As soon asI looked a i and did an orbi manally, i wasclear it was going to hit Earth, he says.

The brighness of 2008 TC3 sggesed iwas only a few meres across and, assmingi was a common rocky aseroid, wold prob-ably spli ino fragmens soon afer eneringhe amosphere. B safe as ha migh seem,Spahr had procedures to follow. He called Lind-ley Johnson, head of NASAs Near Earth Object

Observaions programme in Washingon DC,on his BlackBerry a number only to be usedin emergencies.

Hey Lindley, is Tim, said Spahr. Whywould I be calling you?

Johnsons response: Were going to get hit?Spahr also called asronomer Seve Ches-

ley of he Je Proplsion Laboraory (JPL) inPasadena, California, who at the time was hus-tling his kids out of the door for school. Chesleyhurried into the office, ran a program to calcu-lae he aseroids orbi and was asonded o

see 100% impact probability, he says. Id neverseen ha before in my life. Chesley calclaedha he aseroid wold hi Earhs amosphereless han 13 hors laer, a 2:46uthe nex day; he impacsie wold be norhern Sdan,where he local ime wold be5:46 a.m.. He sen his resls oNASA headqarers and heMinor Planet Center, which cir-culated an electronic bulletin toa worldwide network of astronomers. A groupcalled NEODys in Pisa, Ialy, also confirmedthat an impact was nearly certain.

Although several small objects such as 2008TC3 hit Earth each year, researchers had neverspoed one before i srck. Kowalskis dis-covery, therefore, provided a unique chance tostudy an asteroid and its demise in real time, ifastronomers could mobilize resources aroundthe world quickly enough.

Soon e-mails and phone calls were flyingacross he globe as scieniss raced o coordi-nae observaions of he incoming aseroid.IMPACT TONIGHT!!! wrote physicist Mark

Boslogh of Sandia Naional Laboraories inAlbqerqe, New Mexico, o colleages,inclding a Sandia engineer responsible formonitoring US government satellite data.

Countdown to impactPeer Brown, an asronomer a he Universiyof Wesern Onario in Canada who heard henews from JPL, ran o his local observaory,fired p he elescope and began racking heaseroid, which looked like a very small, fain,fast-moving streak, he says. Alan Fitzsimmons

a Qeens Universiy Belfas in Norhern Ire-land called wo of his colleages, who had jsarrived at the William Herschel Telescope at La

Palma on he Canary Islandsand were no schedled o sethe telescope until the next day.

Lisen gys, his is happen-ing, his is going o happenonigh, he old he research-ers, who arranged o borrowan hour of observing time from

another astronomer.All day, observations poured into the Minor

Plane Cener, which released new daa and

orbit calculations several times an hour. NASAnotified other government agencies, includingthe state and defence departments, and issueda press release ha afernoon saying ha hecollision cold se off a poenially brilliannaral fireworks display. Abo an horbefore impact, the asteroid slipped into Earthsshadow and o of view o opical elescopes.By hen, asronomers from 26 observaoriesworldwide had already captured and submittedabout 570 observations, allowing JPL to refineP.

Jenniskens

When an asteroid was spotted heading towards our planet last October, researchers rushed to

document a cosmic impact from start to finish for the first time. Roberta Kwok tells the tale.

Listen guys, this

is happening,

this is going to

happen tonight.

Alan Fitzimmon

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0

0.5

1.0

1.5

600 700 800 900 1,000

Reflectance

Wavelength (nm)

When the meteoroid was 121,100

kilometres from Earth, a telescope

in the Canary Islands measured

how much light the body reflected

at dierent wavelengths.

The little boulder 2008 TC3 went through a series of name changes during its

brief moment in the scientific spotlight. In space, the hunk of rock was called

an asteroid or meteoroid. After it hit Earths atmosphere, frictional heating

set it aglow and it became a meteor. The pieces that

fell to the ground are called meteorites. Here is

the 2008 TC3 biography, from the moment it

was discovered.

A fast-moving meteoroid

close to Earth was spotted

by the Catalina Sky Survey

on Mount Lemmon in

Arizona. Orbital calculations

suggested it would hit the

planet in 20 hours.

When the meteoroid broke apart,

it left behind clouds of hot dust,

observed by the Meteosat-8

weather satellite.

Ron de Poorter, a

KLM pilot flying

at an altitude of

10,700 metres

over Chad, saw

three or four short

pulses of light

beyond the

horizon as the

meteoroid flared

through the sky.

A photograph captured clouds left behind after

the fireball disappeared.

A search team combed

the desert multiple times

and recovered some 280

meteorites.

is prediced collision ime o 2:45:28 ut, giveor take 15 seconds.

As the countdown progressed, Jacob Kuiperfreed. Kiper, an aviaion meeorologis onthe night shift at the Royal Netherlands Mete-orological Insie in De Bil, had seen ane-mail abo he incoming aseroid. And hewas worried ha no one wold see he explo-

sion in the sparsely populated Nubian Desert.Wih less han 45 mines lef, Kiper real-

ized he cold noify Air France-KLM heairline o which he roinely issed weaherrepors which probably had planes flyingover Africa. About ten minutes later, pilot Ronde Poorter received a message print-out in thecockpi of KLM fligh 592, fly-ing norh from Johannesbrgo Amserdam. The messagegave he laide and longi-de of he prediced aseroidimpac. De Poorer calclaedthat he would be a distant 1,400

kilomeres from he collision.Sill, a he appoined ime heand his co-pilot dimmed the instrument lightsand peered northeast.

Far above the plane, asteroid 2008 TC3 hit theop of he amosphere a abo 12,400 meresper second. The collision heated and vaporizedhe oside of he rock, ripping maerial fromis srface. The impac of rock aoms wih airmolecles creaed a brillian flash ha li hedeser below. Less han 20 seconds afer 2008TC3 entered the atmosphere, calculations sug-ges, pressre on he rock riggered a series ofexplosions ha shaered i, leaving a rail of

hot dust.From the cockpit of his plane, de Poorter saw

flickerings of yellowish-red ligh beyond hehorizon, like distant gunfire. The flash woke astation manager at a railway outpost in Sudan.In a village near he Egypian border, peoplereturning from morning prayers saw a fireballha brighened and flared o, according oaccounts collected later by researchers.

Elecronic eyes wached, oo. US govern-

ment satellitesspotted the rock when it was 65kilometres above the ground. Moments later, itwas picked up by a European weather satellite,which cagh wo ds clods and ligh fromhe fireball. An array of microbaromeers inKenya normally sed o monior for nclearexplosions deeced low-freqency sond

waves from he blas, whichBrown laer calclaed woldbe eqivalen o abo 12kiloonnes of TNT, roghlyone-tenth the size of the atomicbomb dropped on Hiroshima.

Tracking of he fireballs ra-

jectory by US satellites showedha JPL accraely predicedhe objecs locaion wihin a few kilomeresand a few seconds. We have never had scha concree affirmaion ha all he machineryworks, says Chesley.

B for Peer Jenniskens, an asronomer ahe SETI Insie in Monain View, Califor-nia, the spectacular light show was not enough.For weeks afer he aseroid hi, Jenniskens,who sdies meeor showers, waied o hearwheher someone had fond he fallen mee-ories. No news emerged. Somebody neededto do something, he says.

Jenniskens flew o Sdan in early Decem-ber and me wih Mawia Hamid Shaddad,

an asronomer a heUniversiy of Kharom who had alreadyobained picres of he fireballs rail fromlocals. Togeher, hey drove norh from Khar-toum to the border town of Wadi Halfa, asking

villagers where the fireball had exploded in thesky. These eyewitness accounts convinced Jen-niskens that the rock had disintegrated high in

the atmosphere in good agreement with USsaellie daa and ha any fragmens weremost likely to be found southwest of Station 6,a tiny railroad outpost in the Nubian Desert.

Desert searchOn 6 December 2008, Jenniskens and Shaddadse o wih a grop of 45 sdens and safffrom the University of Khartoum to scour thearea. Team members lined up about 20 metresapart over a kilometre-wide strip, facing a seaof sand and gravel inerspersed wih hills,rocky ocrops and dry winding riverbeds.Flanked by wo pairs of cars and railed by a

camera crew from news nework Al Jazeera,he line of searchers began marching slowlyeas, like he eeh of a massive comb beingdragged through the desert.

Towards the end of the day, a car approachedJenniskens wih news ha a sden mighhave fond a meeorie. I remember hink-ing, oh no, no again, says Jenniskens, whohad already fielded several false alarms. Still,he jmped in he car and drove o he s-dent, who presented him with a small squarefragment, about a centimetre and a half acrosswih a hin, glassy oer layer. The srfaceresembled the crust that meteorites form after

being meled and solidified, and he rocksdeep black color sggesed i was freshly

nAsA/Univ.A

rizonA;FUllMoonPhotogrAPhy;A.F

it

zsiMMons,Q

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.Jenniskens

We have never

had such concrete

affirmation all the

machinery works.

steve Cheley

402

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