Marsden's team put Garradd's data on the center's Web page, a signal for astronomers to get more fixes. On Dec. 20, JPL produced its solution. Chance of impact was one in 2,500 -- nothing to get excited about. "Usually the probability goes down with more observations," Marsden said.
Not this time. On Dec. 23, the risk rose to one in 270, and rose steadily over Christmas and beyond. "We'd never had anything this big come this close, and we'd never predicted anything like it," Marsden said. "It was quite fantastic." The asteroid was 9 million miles away -- about as close as it would get this trip.
By Dec. 26, the impact probability had risen to one chance in 38. What the plotters needed was a "precovery," an overlooked observation from before Tholen's initial June fixes to yield a more precise orbital solution.
In Tucson, astronomers at the Spacewatch Project, at the University of Arizona's Lunar and Planetary Laboratory, started searching their archive. Spacewatch has been surveying the solar system for 20 years, and precovery is a specialty.
"We store [our images] on DVDs," Spacewatch leader Robert S. McMillan said. "If there's something that wasn't automatically sorted by our software, we can usually find it -- if we were looking in the right place at the right time."
They were. On Dec. 27, Spacewatch astronomers Jeffrey Larsen and Anne Descour found 2004 MN4 in a series of images taken March 15, more than three months before Tholen's sighting. They passed the word to JPL, which issued a news bulletin: "An Earth impact on 13 April 2029 can now be ruled out."
Since then, astronomers have continued to observe 2004 MN4 whenever possible, but most of the time it is obscured.
"It would be awfully nice to have information so we don't get surprised," said Schweickart, who advocates flying a small interceptor mission to plant a transponder on 2004 MN4 that would constantly radio its location, tagging it like a grizzly bear. "Our favorite little asteroid might provide enough reality here to provoke people. Maybe we should get serious."