Oddballs of the solar system, asteroids are battered chunks of rock and metal that have tumbled around the heavens since the Sun’s eight major planets (plus demoted Pluto) formed about 4.6 billion years ago. Astronomers have cataloged about a half-million asteroids, most in the gap between the orbits of Mars and Jupiter. About 7,000 known NEOs loop wildly among the inner planets, following paths that shift in response to gravity and the Sun’s heat. “Their orbits are all over the place,” says Paul Chodas of JPL. “They’re rebels.”
In the desert 175 miles north of Tucson, Meteor Crater is the scar where a boxcar-size hunk of iron slammed into Earth 50,000 years ago. The crater is nearly a mile wide and 550 feet deep, edged with layers of warped and shattered rock. The asteroid blew up with the energy of the largest hydrogen bombs ever detonated on Earth, vaporizing the desert and unleashing deadly supersonic winds for many miles. I visited the crater as night fell, and I felt keenly aware that fragments of the solar system can invade our cozy realm of Earth and Moon.
If a 100-foot-wide asteroid hit Earth, the shock wave from its explosion in the atmosphere could flatten trees and kill every large animal for hundreds of square miles. That’s just what happened in 1908 at Tunguska, Siberia. The odds are roughly one in ten that such a blast will occur in the next 40 years. An asteroid 500 feet across could destroy a metropolitan area or spawn massive tsunamis. Those impacts occur every 30,000 years, on average.
Hundreds of known NEOs are more than a mile wide. If an asteroid that big struck Earth, firestorms could produce worldwide clouds of soot that would block sunlight and plunge the planet into an “asteroid winter.” That happens every few million years, scientists estimate. Once every 100 million years or so, an even larger asteroid may cause a mass extinction; most scientists believe a six-mile-wide asteroid doomed the dinosaurs 65 million years ago.
Astronomers with the Catalina survey find new NEOs almost every night. They start by taking four pictures of the same patch of sky, with ten minutes between each exposure, and compare them on a computer screen. While background stars shine in the same place in each image, NEOs appear as four distinct dots along a straight line. The astronomers are skilled at ruling out man-made satellites, electronic sparks from cosmic rays and other streaking objects that could be mistaken for an NEO. “They look at everything with the human eye,” NASA’s Yeomans says. “They’ve been doing it for so long, and they’re so dedicated.”
Hill, who has used telescopes since he was a child during the Sputnik era, has been on the team since 1999. He has found more comets—22—than all but three other people in history. (Comets usually originate in the outer solar system and are less common in Earth’s neighborhood than asteroids.) During my visit to Mount Lemmon, he made a trumpeting noise just before he pointed out the first NEO to us. “I love what I do,” he says. “I would do this for free.”
The Catalina Sky Survey consists of nine astronomers using two modest telescopes in Arizona and one in Australia. The team refurbished a long-unused telescope at Mount Lemmon with a 60-inch mirror, small by modern standards. NASA provides $1 million per year—peanuts in astronomy circles. “We’re very careful and meticulous,” says Beshore, a former software engineer who directs the survey. “We get the numbers just right.”
As it happens, astronomers at the Catalina telescope in Australia and other sites around the world took pictures of the NEO after Hill discovered it the night of my visit, allowing the Minor Planet Center to calculate its orbit. By the next morning, the results had been posted online: the asteroid didn’t threaten Earth. I felt a bit let down; no worldwide scoop for me.
Before Beshore joined the survey in 2002, he was skeptical that he’d spot any hazardous asteroids. “Then I realized, my God, the sky is full of these things,” he says. “I have more perspective that yes, this could happen, we might get hit. It would be really satisfying to find an object and then do something about it.”
Don Yeomans often thinks about what that might be. Scale models of asteroids fill the windowsill of his office at JPL in Pasadena. He runs the lab’s NEO clearinghouse, which looks nothing like a Hollywood depiction of a planetary-defense headquarters. There are no wall-size display screens, no blinking panels or red telephones, just ordinary-looking offices. But the workers are well aware of their lofty mission. “We don’t let our guard down, even for a day,” Yeomans says. “It’s our job to monitor the inner solar system and make sure none of these objects gets close to the Earth.”