The vast bulk of asteroids—millions of individual objects ranging from 560-mile-wide Ceres to pea-size pieces of space shrapnel—reside in a broad zone between the orbits of Mars and Jupiter, the legendary asteroid belt. If pulled together, all this material would form a mass smaller than Earth’s moon, but the immense gravitational force of Jupiter prevents the bits from coalescing into a solid planet. When the rocks approach Jupiter, the occasional asteroid can find itself pushed out of the procession and into deep space; some spin out beyond Pluto’s orbit, while others fall toward the sun, each with its own unique orbit. Some even find a home around other planets. Mars’s two moons, Phobos and Deimos—along with several of Jupiter’s and Saturn’s satellites—may be captured asteroids.
What most interests and worries scientists like Chesley and Yeomans, however, are near-Earth asteroids—those with orbits disconcertingly close. Members of this class apparently ushered the dinosaurs off the evolutionary stage 65 million years ago and left a three-quarter-mile-wide hole in the Arizona desert less than 50,000 years ago. A few scientists think a near-Earth asteroid on a bull’s-eye path might even have reshaped human history (see “Did a Comet Cause the Great Flood?”). Somewhere in space, one of their kind is orbiting its way to an inevitable rendezvous with Earth: The question isn’t if we will be struck again, but when. There are scattered reports of deaths by meteorites through recorded history, like a Chinese chronicle asserting that thousands died during a 1490 meteor shower. One prediction is indisputable: With growing populations comes greater risk. Had the 1908 impact in Siberia landed in an urban area, for example, it would have been as devastating as the 2004 Indian Ocean tsunami.
Yet it wasn’t until the early 1970s that anyone seriously pursued how to track these potentially deadly objects. A few pioneers like the Shoemakers began to catalog the faint smudges on the glass plates they used to photograph the night sky. University of Arizona astronomer Tom Gehrels revolutionized that work by turning to charge-coupled devices, or CCDs—electronic light detectors, now common in cameras—to gather much better data than was possible using plates. In 1992, NASA set up the first formal effort to detect near-Earth asteroids.
The race was on, and it swept up a new generation of scientists, like Tim Spahr. As a graduate student at the University of Florida in Gainesville in 1996, he and fellow student Carl Hergenrother noticed an asteroid the length of two football fields heading almost directly toward Earth. Further calculations showed that the object, named 1996 JA1, would pass by at less of a distance than the moon is from Earth, spawning the first widespread media coverage of an asteroid threat. “It’s the reason I have my job,” says Spahr, now director of the Minor Planet Center run by the Smithsonian Institution and Harvard University in Massachusetts. “And it changed everything.”
Just two weeks after Spahr’s asteroid whizzed by, researchers at the MIT Lincoln Laboratory, given the task by the military of spotting enemy spy satellites, unveiled a novel approach for monitoring large areas of the sky using sophisticated software. The MIT group found nearly 50 asteroids within a couple of months—far faster than their competitors. “Soon the other surveys were getting their butts kicked,” recalls Spahr, who joined one of two University of Arizona teams rushing to incorporate the latest technology into their efforts.
The sudden popular interest had some embarrassing side effects. Hollywood went to work on a series of moderately ludicrous disaster movies—Deep Impact, Armageddon, and Asteroid (featuring the other guy from The Terminator). But there was also tangible progress. In 1998, Congress ordered NASA to spot all near-Earth asteroids two-thirds of a mile in diameter and larger—the ones that scientists say could wipe out civilization—by 2008. Meanwhile, the world’s space agencies began to bring their expertise to bear on the problem. Just a few months before Spahr’s discovery, NASA launched NEAR (for near-Earth asteroid rendezvous), a spacecraft to visit the near-Earth asteroid Eros. (Gene Shoemaker died while the probe was en route, and NASA renamed it NEAR-Shoemaker.) Arriving in 2000, the probe orbited for a year before controllers crashed it into Eros—but not before it sent back tens of thousands of detailed images of the 20-mile-wide banana-shaped asteroid. Eros’s surface—boulder strewn, heavily cratered, strangely smooth—was a geologic puzzle.
The Japanese got in the game too. In 2005, the Japanese probe Hayabusa hovered a dozen miles away from a lumpy asteroid named Itokawa, then collected some material in anticipation of a 2010 return to Earth. Radio contact with the probe was lost for a while, however, and it is uncertain whether it will return to Earth.
By 2005, lawmakers in Washington asked NASA what it would take to be able to spot 90 percent of near-Earth asteroids more than 460 feet in diameter by 2020. Astronomers say that incoming asteroids smaller than that would have a regional, rather than global, effect; ones that are less than about 180 feet are likely to disintegrate in the atmosphere. “There aren’t very many huge objects, so you don’t get hit by them very often,” Spahr says. “But as you get smaller, there are more and more.”
Finding smaller asteroids requires a whole new level of technology, and NASA is struggling to find ways to deliver the information Congress requested. Some astronomers have proposed a satellite orbiting near Venus that could easily spot asteroids that are hard to see from the ground. But NASA is in a budget crunch, so space-rock hunters are pinning their hopes on two earthbound projects. The National Science Foundation plans early in the next decade to build the Large Synoptic Survey Telescope with a 28-foot mirror. Meanwhile, aided by pork-barrel money won by a Hawaiian senator, work is under way on Pan-STARRS, a set of telescopes to be sited on Mauna Kea that will cover most of the sky every few nights down to a dim 24th order of magnitude. When these telescopes see light within the next few years, the result will be dramatic. David Morrison, a leading impact researcher at NASA’s Ames Research Center in California, predicts a hundredfold increase in discoveries, which will make Schweickart’s shooting gallery metaphor more believable. “We tend to find one scary asteroid a year,” Morrison says. “Soon it will be one a week!”
The details of Apophis’s discovery in 2004 showcase the evolving art of asteroid detection. Roy Tucker, David Tholen, and Fabrizio Bernardi spotted the object while trolling the skies at the University of Arizona’s Steward Observatory on Kitt Peak. Fans of the TV series Stargate SG-1, the three astronomers named the asteroid after an alien intent on destroying Earth. Science fiction, however, quickly took a turn toward reality show. At the Minor Planet Center—which absorbs asteroid data from all over the world—Spahr’s colleague Kyle Smalley took a closer look at the object’s path. “All the main-belt asteroids move across the sky in a procession,” he says. “And this thing was stepping out of line in this parade, so it caught my eye. It was obvious this thing was coming close to Earth.”