Uh-oh. An impact flattened 1600 square kilometers of Siberian forest in 1908, but astronomers still don't know what to do when they see one coming.

An Asteroid! Quick, Do Something!

Dick writes about Earth and planetary science for Science magazine.

It was all over in 6 hours, but the commotion triggered last month when a threatening asteroid popped up still has astronomers buzzing. The incident demonstrated that researchers have little idea how they should respond to the detection of an object that may hit Earth within days. "Nobody was in charge," says planetary scientist Clark Chapman of the Southwest Research Institute in Boulder, Colorado. "Things worked out right, but it was more or less good luck."

As Chapman explained today to an impact hazard meeting, NASA funds a search for potential civilization killers--objects 1 kilometer and larger--that are almost certain to be detected years if not decades before impact (Science, 19 September 2003, p. 1647). The International Astronomical Union (IAU) has a formal process through which discoveries of these large near-Earth objects would be evaluated over the weeks and months following discovery. But "the system isn't designed to search for imminent impacts," notes Chapman.

On the night of 12-13 January, an automated telescope picked up a fast-moving asteroid some tens of meters wide. As per routine, the Minor Planet Center (MPC) in Cambridge, Massachusetts, posted a notice on a public Web page. An amateur astronomer realized that the predicted path of the asteroid, now dubbed 2004AS1, implied an imminent impact, news he passed to a Web chatroom. From there, a semiretired professional astronomer alerted other professionals, triggering a flurry of orbital calculations into the night at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California, and back at MPC. Calculations at JPL were giving 2004AS1 a 25% chance of hitting somewhere in the Northern Hemisphere in a few days.

That was more than enough to set astronomer David Morrison of NASA's Ames Research Center in Mountain View, California, wondering, "Who do I call?" Morrison is chair of the IAU working group on near-Earth objects, but this "was an event none of us was prepared for," he says. A 25% chance seemed like enough to prompt a call to somebody, perhaps soon. Brian Marsden, director of MPC, saw the risk differently. "There was an enormous range of possibilities," he says. "Under these circumstances, we have to be sure its going to hit us" before calling anyone in authority. Lacking a plan, no one knew what to do.

Late that night, a Colorado amateur astronomer averted an embarrassing false alarm by failing to find 2004AS1 on its predicted collision course. A day later, European professionals relocated it, and MPC put out a formal announcement confirming it wouldn't be hitting anything. That hasn't resolved matters of risk perception, but all agree on the need to figure out how to respond to future alarms.

Related sites
The 2004 Planetary Defense Conference
NASA's impact hazard site

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