Over the next several years, new telescopes will spot thousands of near-Earth asteroids and comets. If one is headed our way, will world leaders be ready to respond?

NEO Figure
Aswarm with asteroids. In 2000, there were more than 86,000 known asteroids. By 2007, there were nearly 380,000, including main-belt objects that don't approach Earth (green); objects that approach but do not cross Earth's orbit (yellow); and objects that cross Earth's orbit (red).

TIESHAN TEMPLE NATIONAL FOREST, CHINA--In the control room of XuYi Observatory, Zhao Haibin sits at a computer and loads the night sky over Jiangsu Province. A faint white dot streaks across a backdrop of pulsating stars. "That's a satellite," Zhao says. Elsewhere on the screen, a larger white dot lumbers from east to west. It's a main-belt asteroid, circling the sun between Mars and Jupiter.

On a ridge in this quiet, dark corner of southeastern China, about 100 kilometers northwest of Nanjing, XuYi's new 1-meter telescope espies a few dozen asteroids on a good night. Most are known to science. But since China's first telescope dedicated to asteroid detection saw first light early last year, Zhao's team has discovered more than 300 asteroids, including a near-Earth object (NEO), the class of asteroids and comets that could smash into our planet, if fate would have it.

China's asteroid hunters are the latest participants in a painstaking global effort to catalog NEOs. Close encounters with asteroids in recent years--and comet Shoemaker-Levy's spectacular death plunge into Jupiter in 1994--have spurred efforts to find the riskiest NEOs before they blindside us. Tracking potentially hazardous objects--NEOs passing within 0.05 astronomical units, or 7.5 million kilometers, of Earth's orbit--is essential for any attempt to deflect an incoming rock.

The first test of our planet's defenses could be Apophis, an asteroid the size of a sports arena that made the world sweat for a few days in December 2004, when calculations suggested as great as a 1 in 37 chance of an impact in 2029. Although further data ruled out that day of reckoning, another could be looming. In April 2029, Apophis will pass a mere 36,350 kilometers from Earth, inside the orbits of geostationary satellites. If it enters a keyhole--a corridor of space barely wider than the asteroid itself where gravitational forces would give it a tug--it will end up on a trajectory that would assure a collision 7 years later: on 13 April 2036, Easter Sunday. The odds of Apophis threading the needle are currently 1 in 45,000--but dozens of factors influence asteroid orbits. Researchers will get a better look during Apophis's next appearance in our neighborhood in 2012.

By then, a powerful new telescope for detecting asteroids and comets--the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS), expected to be up and running by summer--should have unmasked thousands more NEOs. An even grander project, the 8.4-meter Large Synoptic Survey Telescope (LSST), is expected to be operational in 2014.

The anticipated bumper crop of NEOs confronts society with urgent questions. In the next several years, with increasing rapidity, Pan-STARRS and its ilk will discover potentially dangerous NEOs. Currently, 168 NEOs have a chance of striking Earth in the next century, although the odds are minuscule. By 2018, the risky rock roster could swell more than 100-fold. Additional observations will allow astronomers to refine orbits, and in most cases, rule out a threat. For that reason, astronomers are debating when the public should be alerted to hazards, to minimize false alarms.

Eventually, an asteroid with our name on it will come into focus, forcing an unprecedented decision: whether to risk an interdiction effort. "The very concept of being able to slightly alter the workings of the cosmos to enhance the survival of life on Earth is staggeringly bold," says Russell Schweickart, chair of the B612 Foundation, a Sonoma, California, nonprofit that lobbies for NEO deflection strategies. We have the means to deflect an asteroid--indeed, "it's really the only natural hazard that we can possibly prevent," says NEO specialist David Morrison, an astrobiologist at NASA's Ames Research Center in Mountain View, California.

There is one "fatal missing element," says Schweickart, who in 1969 piloted the lunar module for the Apollo 9 mission: "There is no agency in the world charged with protecting the Earth against NEO impacts." He and others hope to change that.

Wake-up calls
Like any natural disaster, impacts occur periodically; gargantuan impacts are so rare that their frequency is hard to fathom. Every 100 million years or so, an asteroid or a comet a few kilometers or more in width--a titan like the rock thought to have wiped out the dinosaurs 65 million years ago--smacks Earth. "This is not just getting hit and killed," says Edward Lu, a former astronaut who now works for Google. "You're on the other side of the Earth and the atmosphere turns 500° hotter. Lights out."

Reassuringly, no doomsday asteroid identified thus far is on track to intersect Earth's orbit in the next century. Less reassuring, an unobserved, long-period comet from the Oort cloud could swoop in with little warning. Although the odds of this happening in anyone's lifetime are on the order of winning the Powerball lottery, a megaimpact's annualized fatality rate is likely to rival those of earthquakes or tsunamis, says Clark Chapman, an astronomer at the Southwest Research Institute in Boulder, Colorado.

Near-Earth asteroids tens to hundreds of meters in diameter are far more numerous-- there may be as many as 3 million in the solar system--and they cross Earth's path more frequently. The iconic Meteor Crater in northern Arizona was gouged by a 50-meter-wide hunk of iron and nickel 50,000 years ago. In 1908, a fireball scorched and flattened trees over 2100 square kilometers of taiga in Siberia's Tunguska region--the devastating footprint, many experts say, of a modest asteroid that exploded in midair.

Recent supercomputer modeling has downsized the Tunguska rock. An asteroid just a few dozen meters wide, fragmenting explosively with a yield of 3 to 5 megatons--a fraction of earlier estimates--could have done the trick, Mark Boslough and David Crawford of Sandia National Laboratories in Albuquerque, New Mexico, report in an article in press in the International Journal of Impact Engineering. If this is correct, the expected frequency of Tunguska-sized impacts changes from once every couple of millennia to once every couple of centuries. "Smaller objects may do more damage than we used to think," says Chapman.

Today the impact threat may seem obvious, but for decades it was largely ignored. Aerodynamicist Anatoly Zaitsev, director general of the Planetary Defense Center in Moscow, sounded the alarm in a landmark report delivered to Soviet leaders in 1986. "They just laughed," he says. Then on 22 March 1989, an asteroid several hundred meters across whizzed by Earth at about twice the distance to the moon; astronomers didn't spot Asclepius until it had already passed.

Asclepius was a shot across the bow, prompting the U.S. Congress to query NASA about whether the agency had a plan for the next killer asteroid. A parade of committees followed, after which Congress in 1998 ordered NASA to tally and track at least 90% of NEOs that are more than 1 kilometer wide. NASA launched the Spaceguard Survey, named after a survey in Arthur C. Clarke's 1972 novel Rendezvous with Rama. To date, Spaceguard and other efforts have identified more than 700 of an estimated 1000 or so NEOs in this category. Then in 2005, Congress called on NASA to expand the search by 2020 to cover 90% of NEOs at least 140 meters in diameter--the approximate minimum size to damage an area at least as large as a state or seaboard. NASA expects Spaceguard II to spot 21,000 potentially hazardous NEOs and forecasts a 1-in-100 chance that such a rock will hit Earth in the next 50 years.

The uncertainties are huge. Main-belt asteroids can knock into each other, turning a benign rock into a malignant projectile. And with only a fraction of NEOs having been identified so far, what we don't know can hurt us. Astronomer Brian Marsden, director emeritus of the International Astronomical Union's Minor Planet Center, the clearinghouse for asteroid and comet orbits, figuratively sums up the situation: "The ones to worry about are those that were discovered yesterday and have a very high probability of hitting us the day after tomorrow. Those, plus the ones we've never even seen yet!"

Drawing a bead
Night has fallen on an early December evening near Tieshan Temple, which, according to local lore, was the home of China's first monk. The sky above the national forest is pitch-black but overcast. On nights like this, asteroid hunters know how to kill time. In a chilly, cigarette smoke-filled lounge down the hall from XuYi's control room, Zhao and his colleagues play cards and sip from tall, clear plastic bottles packed with green tea leaves, hoping that the weather forecast is wrong and the skies will clear.

Zhao has worked at Purple Mountain Observatory, which operates XuYi, since graduating from Nanjing University in 1996. He has a comet named after him, but his biggest thrill came last spring, when he found an NEO.

On most nights, the telescope is pointed away from the sun, toward main-belt asteroids outside Earth's orbit. More elusive objects between Earth and the sun can be discerned in the right conditions. With a clear sky and a new moon, just after nightfall or before sunrise, Zhao aims the telescope at a 60° angle to the sun, where faint NEOs, like a crescent or gibbous moon, reflect sunlight in phases. During the telescope's first year, his team got fewer than a dozen opportunities to gaze sunward. One was 7 May, when they scored their NEO.

Tonight, just after midnight, the clouds have dispersed enough for viewing. Zhao's team swings into action, pointing the telescope at a 2-degree-square patch of sky. As dawn breaks, they will e-mail the data to Purple Mountain's Nanjing headquarters for analysis.

Zhao's team is working fast to stake NEO claims before Pan-STARRS, the first Spaceguard II facility, starts gobbling up the heavens. The telescope on Mount Haleakala on Maui Island, Hawaii, has a charge-coupled device camera with 1.4 billion pixels--the highest resolution in the world--that acquires images every 30 seconds.

Pan-STARRS, which saw first light last August, will usher in a new paradigm in observational astronomy (Science, 12 May 2006, p. 840). "It's a set of surveys that will be analyzed in a wealth of different ways," says Kenneth Chambers, an astronomer with the Institute for Astronomy (IfA) at the University of Hawaii, Manoa, who is leading a consortium of 300 scientists whose institutions have paid for first crack at Pan-STARRS gold. Some will map the Milky Way or look for distant quasars. Others will hunt for asteroids. "The astronomical community is not ready for the fire hose of data that's going to hit them," Chambers says.

Once Pan-STARRS begins taking data in earnest this summer, NEO finds should come thick and fast. According to IfA astronomer Robert Jedicke, who led development of the software that will cull NEOs from the data deluge, Pan- STARRS will be 10 times more effective at spotting NEOs than all current surveys combined. "Are there many more objects like Apophis out there? This is something that Pan-STARRS will answer," says IfA Director Rolf-Peter Kudritzki.

Magnificent feats of detection are also expected from LSST, which will have 24 times greater survey power than Pan-STARRS. Like its Hawaiian rival, the $389 million project has broad science objectives, including studying dark energy and dark matter and mapping the Milky Way. Unlike Pan-STARRS, LSST data will be available immediately to any researcher. Construction is expected to begin in 2011 at Cerro Pachón, Chile.

When completed, LSST will cover the entire available sky every 4 nights with a 3.2-billion-pixel camera. Project scientists have teamed up with Google, Microsoft, and others to develop algorithms for processing the masses of data. After 10 years of operation, LSST should have plotted rough orbits for 82% of potentially hazardous NEOs larger than 140 meters, with only the risk assessments requiring human input, says LSST Director J. Anthony Tyson, a physicist at the University of California, Davis.

Funding is not assured. Tyson has lined up $45 million so far from private sources, including two gifts announced in January that will help pay for the mirror: $20 million from Charles Simonyi, chief executive of Intentional Software, and $10 million from Microsoft's Bill Gates. The tycoons, says Tyson, "are excited about the LSST acting as a peripheral device for the Internet and thus bringing the universe to everyone's computer." Much of LSST's construction funds are expected from the U.S. National Science Foundation, which will hold a Major Research Equipment and Facilities Construction review on the project this autumn.

Gauging risks
In the early 1990s, as astronomers intensified their search for NEOs, IfA's David Tholen upbraided colleagues for turning a blind eye to asteroids lurking inside Earth's orbit. He was concerned that an inner-orbit NEO at its farthest point from the sun could hit our planet. "For years, I wanted to do something about that," says Tholen. But he lacked the means. "Other folks had great cameras. I was envious." In 1997, he finally got time on a decent telescope. Aiming it low on the horizon just after nightfall or before dawn, his group over 3 years discovered four asteroids in this blind spot--including a whopper that is 5 kilometers wide.

Riding high, Tholen won a grant for a more intensive search campaign. But his team struggled with technical glitches, and by their final year of funding in 2004, he says, "we hadn't found a single asteroid." He redoubled his efforts, booking time at observatories around the world. In June 2004, he was juggling nights on two telescopes. Then in the early evening of the 18th, at Kitt Peak National Observatory near Tucson, Arizona, Tholen, Roy Tucker, and Fabrizio Bernardi hit pay dirt: They got a first glimpse of Apophis.

"Apophis demonstrated that we know very little about the region of space near Earth," says Boris Shustov, director of the Institute of Astronomy in Moscow. Anxiety will mount when Apophis chugs back into range in 2012. Ironically, the best instrument for refining the asteroid's orbit--the world's most powerful planetary radar at Arecibo Observatory in Puerto Rico--may be switched off in 2011, the victim of budget cuts. Even without Arecibo, optical measurements almost certainly will reduce or rule out the impact risk. For that reason, NASA has no plans to send a probe to Apophis, and the European Space Agency has shelved a mission (see sidebar, p. 1329).

A chilling reassessment of Apophis could change the political landscape fast.

Suppose that observations forecast a 1-in-1000 impact risk in 2036. "That risk is really low, but if it hits, it's really bad," says Lu. "How much is it worth to us to have peace of mind?"

The "threshold of pain," as Lu calls it, may depend on who would be affected--and what resources they have. Based on current calculations, the line where Apophis might hit--the so-called risk corridor--runs from Kazakhstan through Siberia, over the northern Pacific, and across Costa Rica, Colombia, Venezuela, and the south Atlantic. Who would mount and pay for a deflection mission? All countries along the corridor? Just Russia, vulnerable to a direct hit, or the United States, vulnerable to a towering tsunami? The United Nations? What if a mission failed, deflecting Apophis to another point on the risk corridor, converting an "act of God" into an act of humankind? Who would be liable?

As experts grapple with these questions, some are trying to rouse political leaders. With outside advice, the Association of Space Explorers, an organization of astronauts and cosmonauts based in Houston, Texas, is drafting an NEO Deflection Decision Protocol to present to the U.N.'s Committee on the Peaceful Uses of Outer Space in 2009. "Apophis should unite our efforts to deal with the threat," says Shustov, who is leading an effort to develop Russia's first national R&D program on NEO hazards.

Shustov's nightmare is that leaders will drag their feet until the threat of a direct hit becomes real. But an asteroid need not impact to cause chaos. Each year, military satellites detect several 1-kiloton explosions of asteroids in the upper atmosphere, and every several years, a much larger explosion of 10 kilotons or more, says Sandia's Boslough. "They are quite frightening to people on the ground." A bus-size meteoroid would explode in the stratosphere with the energy of a small atomic bomb, producing a blinding flash much brighter than the sun, says Chapman. "Military commanders in a region of tension might regard it as the hostile act of an enemy and retaliate," he says. A 25-kiloton airburst occurred over the Mediterranean Sea on 6 June 2002. Imagine, Chapman says, "if that had happened instead in the vicinity of Kashmir, where tensions between India and Pakistan were elevated."

While this scenario may argue for giving NEO sightings wide publicity, some experts think that detailed predictions--particularly risk corridors--should be withheld from the public. They want to avoid a "Chicken Little" phenomenon of repeatedly sounding alarms that are later downgraded or called off. NASA has not released Apophis's risk corridor in 2036. (The B612 Foundation provided the diagram above.) "We do not generally release these kinds of diagrams when they relate to future and ongoing risk assessments," says Steven Chesley, an NEO specialist at NASA's Jet Propulsion Laboratory in Pasadena, California.

Others believe in full disclosure. "People don't like secrecy. It breeds distrust," says Chapman. "When the facts are finally revealed, people wonder whether to believe them and wonder about what else might be still under wraps." NEO impact forecasts, he says, should be treated like hurricane forecasts, allowing people to respond.

Like the first hurricane of the season, the first test of our planetary defenses may be an asteroid whose name starts with the letter "A."