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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 Shoemaker and I pull off Interstate 40 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, imagines the day the sky fell. “Suddenly, there’s a terrific, brilliant light,” she says. In a flash, a searing-hot iron-nickel mass, 150 feet wide and weighing 300,000 tons, tears into the Coconino 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 cataclysm 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 data 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 suggested 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 didn’t know if I could 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.”

On March 25, 1993, Carolyn, Gene, and David Levy, an amateur astronomer, were at Palomar for their scheduled observation time. Snow was falling, and the night promised to be long and boring. Carolyn killed time by studying a batch of overexposed film from the previous night. Many frames were worthless. On one of the last images, however, she came across a smudge. “I said, ‘It looks like a squashed comet.’ ” The team asked astronomers at Kitt Peak to take a look. It then occurred to Carolyn that her squashed comet might be a broken comet. Confirmation came that same night when Kitt Peak spotted a string of comet shards traveling together.

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