For the better part of a decade, astronomers concerned about Earth getting smacked by an asteroid have searched the night skies to find potential civilization-busters. Their quarry: near-Earth objects at least 1 kilometer across.
The possibility of such an impact has spawned Hollywood movies and real-world estimates of 1 billion fatalities and global environmental catastrophe. But researchers have long warned that smaller objects can also blindside us and wreak havoc. Some military analysts even warn that such a collision could be mistaken for a nuclear attack and trigger a response.
Now, researchers are laying plans to begin tracking the movements of such smaller objects, perhaps as early as 2008. Their efforts raise intriguing questions about how scientists report risks to the public.
Last week, NASA released a report from a 12-member scientific panel outlining the technological options for the search, aimed at detecting 90 percent of potentially hazardous objects (PHOs) between 140 meters and 1 kilometer across. Researchers suggest the search could be completed in seven to 20 years, and could be implemented using today's telescopes, cameras, and automation technologies.
Since researchers hope to wrap up current search efforts - aimed at 1 kilometer and larger hazards - by 2008, "now is the right time to begin looking at the next steps," says Richard Binzel, a planetary scientist at the Massachusetts Institute of Technology in Cambridge, Mass.
While movies such as "Armageddon" and "Deep Impact" have spun cinematic yarns out of some of the biggest collisions one might imagine, researchers point to the smaller - and more numerous - objects that can pose significant hazards. For example, some 50,000 years ago, an iron meteor estimated at 60 meters in diameter smashed into Arizona, gouging out a crater 1 kilometer (0.62 miles) across and 200 meters deep.
In 1908, a 50-meter object exploded in the skies over Siberia, flattening trees for 25 kilometers in every direction. By comparison, the Mt. Saint Helens eruption in 1980 flattened trees up to 30 kilometers away. Though Lilliputian by cosmic standards, such objects can carry the punch of 10- to 15-megaton hydrogen bombs, equal to some of the largest nuclear tests and thousands of times more powerful than the atomic bombs dropped on Japan.
The new search would aim at far more destructive objects in the 140 meters to 1 kilometer range. While there are only an estimated 1,100 near-Earth asteroids larger than a kilometer across, these smaller objects could number more than 20,000. More objects raise the prospect of more alerts - and heightens the challenge of communicating risk to the public.
Late last month, for example, astronomers discovered a space rock they dubbed 2003 QQ47. Initial calculations suggested it had a small but potentially worrisome probability of striking Earth in 2014. After additional observations over the next 24 hours, astronomers revised the estimate, eliminating the object as an immediate threat. But the initial announcement triggered Armageddon-like headlines, touching off a heated debate among key members of the asteroid-hunting community about the hazard scale they devised to communicate such risks.
Such alerts, followed by stand-downs, can create the public impression of astronomers "crying wolf," acknowledges Dr. Binzel, who devised and is revising one of the scales used. Yet the alternative - spotting a problem, then sitting on the information until additional observations come in - leaves the community vulnerable to accusations of hiding information.
"It's a known problem," he sighs, adding that it's better to be open about the data and the process used to collect and analyze it than to appear guilty of a cover-up.
Asteroid and comet hazards are estimated based on the likelihood that an object presents a danger sometime during the next 100 years. Thus, if the new survey detects these smaller potential hazards well in advance of a likely collision, "there's an excellent chance" they could be deflected, says Donald Yeomans, an astronomer at NASA's Jet Propulsion Laboratory in Pasadena, Calif., and vice-chairman of the panel that produced the report. The team estimates the cost of the new search effort between $236 million and $397 million.