Deep Impact: The Sequel

A year ago, astronomers announced that asteroid 1997 XF11 might hit the earth in October 2028, then retracted their prediction 24 hours later. Now a new "doomsday asteroid" is generating a lot of excitement--and some anger.

The source of the upheaval, called 1999 AN10, was discovered on 13 January by an automated search camera in Socorro, New Mexico, operated by the Massachusetts Institute of Technology and the U.S. Air Force. Andrea Milani of the University of Pisa in Italy and his colleagues, who analyzed its orbit, concluded that the asteroid will pass very close to the Earth in August 2027, and there is an extremely remote possibility that Earth's gravity will alter its orbit in such a way that it will slam into the planet 12 years later, in 2039.

Milani decided not to call in the press, but he did put up a preprint of a paper about 1999 AN10 on his Web site on 6 April--where it was soon found by Benny Peiser, the moderator of a mailing list on "neocatastrophism." Peiser concluded that Milani and his colleagues were trying to hide something. "Instead of informing the interested public about their potentially explosive findings, the authors have hidden away their results on an obscure Web page," he wrote in a release posted on 13 April. The next day, the story hit newspaper headlines and Internet news services.

Astronomers reacted furiously. Because of the extremely small probability of the impact (only one in a billion), "the 1999 AN10 matter has no relevance to the 'man-on-the-street' whatsoever," according to Clark Chapman of the Southwestern Research Institute in Boulder. Indeed, the chance of AN10 colliding with our planet is no bigger than the chance of the Earth being hit by an undetected asteroid within the next hour. By not drawing attention to the object, "Milani did the right thing," says David Morrison of NASA's Ames Research Center.

Moreover, says Donald Yeomans of NASA's Jet Propulsion Laboratory, chances are high that the small probability of an impact will dwindle to zero in a few months' time. "More data will soon be arriving when the asteroid once again becomes observable in June." says Yeomans.

Nevertheless, astronomers need proper guidelines on how to deal with predictions, probabilities, and the press, says Morrison, and they will discuss the issue at an international meeting in Turin, Italy, next June. "We're still in a learning process," says Morrison. "The community did a much better job [this time] than in the case of 1997 XF11."