Space / Meteors, Asteroids, & Comets

In 2004, as a massive tsunami roiled through the Indian Ocean killing hundreds of thousands of people, a dozen or so scientists quietly confronted an impending disaster potentially even more lethal. They had inside intelligence that a chunk of rock and metal, roughly 1,300 feet wide, was hurtling toward a possible collision with the most populated swath of Earth—Europe, India, and Southeast Asia. Furiously crunching numbers on their computers, the researchers put the odds of impact in the year 2029 at exactly those of hitting the number in a game of roulette: 1 in 37.

“We usually deal with one chance in a million,” recalls Steven Chesley at NASA’s Jet Propulsion Laboratory in Pasadena, California. “This was absolutely extraordinary—I didn’t expect to see anything like it in my career.” By the end of the day on December 27, 2004, to the relief of the observers, archival data turned up trajectory information that rendered the odds of a collision nil. Nonetheless, in 2029 the asteroid, dubbed Apophis—derived from the Egyptian god Apep, the destroyer who dwells in eternal darkness—will zoom closer to Earth than the world’s communications satellites do. And April 13, 2036, it will return—this time with a 1-in-45,000 chance of hitting somewhere on a line stretching from the Pacific Ocean near California to Central America.

Because Apophis was discovered during one of the world’s greatest natural disasters, the worries about the impact went largely unnoticed. But that tense day, December 26, 2004, stunned the small group of astronomers who dutifully detect and plot trajectories of hundreds of thousands of the millions of chunks of rock whizzing around the solar system. Though too small to end civilization—unlike the asteroid that may have doomed the dinosaurs—Apophis could pack a punch comparable to a large nuclear weapon. Traveling at 28,000 miles per hour, it would heat up as it passed through Earth’s atmosphere, turning the dark rock into a fiery sun as it arced across the sky. Then it would either explode just aboveground—as one most likely did in 1908, leveling a vast forest in the Tunguska region of Siberia—or gouge a crater 20 times its size. “If it hit London, there would be no London,” says Apollo 9 astronaut Rusty Schweickart, who had closely followed the discussion of the potential 2029 impact. Slamming into the ocean, Apophis could create a tsunami dwarfing the one that killed more than 200,000 people around Indonesia.

Apophis is one of millions of asteroids roaming the solar system. None are known to pose an immediate threat, but some are bona fide civilization stompers. A monster rock discovered just this year, with the prosaic name of 2007PA8, is more than two miles across, large enough to wipe out most of humanity. Fortunately, the odds that it will hit are essentially zero. Smaller asteroids are less deadly but much more common. Planetary scientists now estimate that 150-foot-wide space rocks, comparable to the one that hit Tunguska, strike only once every thousand years or so. For a brief time in 2004, just months before the Apophis scare, astronomers feared that a 150-foot-wide asteroid was just days away from racing into the atmosphere. Fast-paced observations allowed them to calculate a more exact orbital path, which took it far from Earth.

After a number of false starts, such potential close calls have finally caught the attention of the U.S. Congress. At the request of lawmakers, scientists are struggling to pinpoint 90 percent of all seriously life-threatening asteroids by 2020 in order to assure at least some warning. The European Space Agency is contemplating a mission to test ways to push such an object off a threatening trajectory, the first serious attempt at developing a planetary defense.

But a group of astronauts, led by Schweickart, also wants their respective countries and the United Nations to prepare for avoiding a hit. “We’re living in a shooting gallery,” he warns. “We’ve evolved to the point where we can do something about this threat. We can either close our eyes as we cross the street and not know what we’ve dodged, or we can open our eyes and act accordingly.”

Amid fears about global warming, terrorism, disease, and nuclear proliferation, the threat of rocks from space may seem more the province of bad Hollywood movies than front-page news. Even professional astronomers have long dismissed asteroids as undistinguished flotsam and jetsam, would-be planets that circle the sun endlessly in a belt between Mars and Jupiter. Their derision left the field of asteroid hunting largely to amateurs and eccentrics.

Only recently have researchers glimpsed the dangers lurking in our deceptively quiet neighborhood. “Impacts are a fact of life in the universe, but when we look up, it’s not what we see,” says Carolyn Shoemaker, who, together with her late husband, Gene, pioneered ways of spotting asteroids and comets. It was geologists who first noticed the evidence of huge impact craters on Earth that had formed long after the solar system settled into its present form, prompting biologists to speculate on whether those collisions dramatically altered life’s evolution. Later, using new technologies on the ground as well as robotic spacecraft, scientists like Shoemaker started to track, catalog, and closely examine the objects.

With each new sighting, asteroids turn out to be far more varied, unruly, and bizarre than astronomers dreamed. Many have companions. Some are rubble heaps held together only loosely by their own gravity. Others are extremely dense nickel-iron objects. Their colors can range from a deep dark chocolate to a glinty white. Even the old distinction between comets (dirty snowballs) and asteroids (hard rocks) has become blurred. Some comets eventually turn into asteroids as they burn off their ice and lose their tails while traveling through the warm inner solar system. And comets—which mostly reside in the solar system’s far fringes—pop up occasionally in the asteroid belt. They may even be directly responsible for life on Earth. Donald Yeomans, who calculates the orbits for near-Earth objects at NASA’s Jet Propulsion Laboratory, says that comets flung out from that belt pummeled our planet shortly after its formation and could have left behind water, possibly creating the conditions that allowed Earth to become a cradle for life.