Asteroids and comets in nearby space pose a constant threat to our planet. Can we avert catastrophe the next time around?

About 140 million years ago,a comet or an asteroid smashed into the Australian outback,blasting a hole in 14 miles in diameter.Today,Gosses Bluff is the two-mile-wide central remnant of the crater.The first sign of the threat was no more than a speck on a star-streaked telescope image.Just after 9 p.m. on June 18 2004,as twilight faded over Kitt Park Observatory in Arizona,David Tholen was scanning for asteroids on an astronomical blind spot,right inside Earth's orbit,where the sun's glare can overwhelm telescopes.Tholen,an astronomer from the university of Hawaii,knew that objects lurking there could sometimes veer toward Earth.He had enlisted Roy Tucker,an engineer and friend,and Rabrizio Bernardi,a young colleague at Hawaii,to help.As they started at a computer,three shots of the same swath of sky,made a few minutes apart,cycled onto the screen.Here's your guy,said Tucker,pointing at a clump of white pixels that moved from frame to frame.An estimate ten million rocky asteroids and ice-and-dirt comets pirouette in outer space,and once in a while their paths fatefully intersect our planet's.One such encounter took place a hundred miles from present-day Washington D.C.,where a 53 miles wide crater lies buried beneath Chesapeake Bay-the scar left when a two miles wide rock smashed into the seafloor 30 million years ago.More notorious is the titan,six miles in diameter,that barreled into the Gulf of Mexico around 65 million years ago,releasing thousands of times more energy than all the nuclear weapons on the planet combined.The whole earth burned that day.says Ed Lu,a physicist and former astronaut.Three quarters of all life-forms,including the dinosaurs,went extinct.




Astronomers have identified serveral hundred asteroid big enough to cause a planetwide disaster.None is on course to do so in our lifetimes.But the heavens teem with smaller,far more nerberous asteroid that could strike in the near future,with devastating effects.On June 30,1908,an object the size of 15-story-building fell in a remote part of Sibera called Tunguska.The object-an asteroid or a small comet-exploded a few miles before impact,scorching and blowing down trees across 800 square miles.The night sky was so bright with dust from the explosion.or icy clouds from the water vapor it blasted into the upper atmosphere,that for days people in Europe could read newspapers outdoors at night.On Tunguska's hundredth anniversary,it's unsettling to note that objects this size crash into Earth every few centuries or so.The next time the sky falls,we may be taken by surprise.The vast majoriity of these smallish bodies,capable of wiping a city off the map,are not yet on our radar screens.Ignorance is bliss,in that if you don't know about these things,you just go about your merry way,says Lu.Over the next decade,however sky surveys like Tholen's should begin filling that gap,cataloging asteroids by the thousands.Every couple of weeks,says Lu,we're going to be finding another asteroid with like a one-in-a-thousand chance of hitting the Earth.


The goal is not just to foretell the date and time of a potential catastrophe.The goal is to forestall it.With years or decades of warning,a spacecraft,using its own minuscule gravity,might nudge a threatening asteroid off course.For objects requiring a bigger kick,a kamikaze spacecraft or a nuclear bomb might do the job.Vexing dilemma would attend this showdown in space.How will governments decide to act?This is a class of problem that the world isn't set up to deal with,says physicist David Dearborn,an advocate of a nuclear strike against an incoming asteroid.Two facts are clear,Whether in 10 years or 500,a day of reckoning is inevitable.More heartening,for the first time ever we have the means to prevent a natural disaster of epic proportions.Everydays dozens of tons of detritus from other space-dust from comets,tiny shard of asteroid-burn up in the earth's upper atmosphere,leaving bright meteor trails at night.Most days a chunk or two of rock or metal,fist size or bigger,survives the fiery plunge.



Yet the odds of seeing a meteorite hit the ground,let alone being struck,are phenomenally low.Only one is knwon to hit a person.Around 1 p.m. on November 30 1954,a meteorite tore through the roof of a house near Sylacauga,Alabama,across the street from the Comet drive in Theatre.The rock,about the size of a softball,caromed off a console radio and clipped Ann Hodges as she snoozed on her couch,bruising her left hip and wrist.She was hospitalized to recover from the shock.Since then there have been spectacular near misses.On Auguest 10 1972,an object around 15 feet across and weighing 150 tons skipped off the upper atmosphere.Hundreds of eyewitnesses saw the glowing streak,dazzling on a sunny afternoon,as it traversed the sky from Utah to Alberta before whizzing back out into space.On March 22,1989,a rock as much as a thousand feet across came within a few hundreds thousands miles of Earth-an uncomfortably close shave.Erosion and vegetation have erased most of the scars left by impacts in the geologic past.Perhaps the best preserved lies about half an hour east of Flagstaff,Arizona.On a late autumn morning Carolyn Shoemarker and I pull off Interstate40 and wind through scrubby desert toward a low rise marking the rim of the crater.Fifty thousand years ago this was a forested plain inhabited by mammoths,giant ground sloths,and other ice age animals.Shoemaker,an asteroid expert with the Lowell Observatory in Flagstaff,image the day the sky fell.Suddenly,there's a terrific,brilliant light,she says.In a flash,a searing hot iron nickle mass,150 feet wide and weighing 300000 tons,tears into the Coconini sandstone,flinging boulders and molten iron for miles.A blast of wind more powerful than any earthly tornado scours the landscape.



All that's left of the catalysm now is a chasm three-fourths of a mile wide and 570 feet deep,fringed with Mormon tea bushes.At the turn of the 20th century,an engineer named Daniel Moreau Barringer was convinced that a massive iron meteorite lay beneath the crater and obtained the mining rights to the land.But after a series of shafts revealed nothing,many prominent geologists concluded that a volcanic eruption,not a meteorite,had formed the crater.Carolyn's husband,Gene,made Meteor Crater one of America's most recognizable landmarks.In the late 1950s he mapped the overturned rock around the crater and pointed out similarities to the Teapot Ess crater in Nevada,formed by a nuclear test.His date showed that Barringer was right.A meteorite had gouged the crater,although most of the iron had melted into tiny droplets.Several of Barringer's shafts can still be seen from the rim,along with a full size cutout of a waving astronaut-a nod to NASA,which once used the crater as a training ground.Some visitors whisper and point at Carolyn,and one man plucks up the courage to come over and request her autograph.Carolyn is famous in her own right.She discovered a comet that,in 1994,vividly demonstrated the cosmic threat we face.



In 1980,their children grown and out the door,Gene suggestes that Carolyn start a career as an asteroid hunter.I'm a morning person.She says.I had never stayed awake all night in my life.I don't know if I can do that.But she decided to give asteroid hunting a shot.Gene had access to the Palomar Observatory near San Diego.After a couple years,I learned how to discover things.She says,modestly.She has 32 comets and 367 asteroids to her credit.Some are more interesting than others.