The latest asteroid scare was explained in a paper by Chapman, the Colorado researcher, and presented at last week's Planetary Protection Conference in Garden Grove, California. Chapman called the episode a "nine-hour crisis."
Early on the morning of January 14, Chapman decided not to issue a public warning about the asteroid threat. He had made his decision after an amateur astronomer in Colorado had snapped a picture of a blank patch of sky where the asteroid would have been if it had been on the collision course.
That same night, the powerful LINEAR telescope spotted the asteroid again. This time, it showed that 2004 AS1 was actually 20 million miles (32 million kilometers) away—not two million miles (3.2 million kilometers), as initially feared—and not on a collision course with Earth at all.
The problem was that the original prediction was based on only four images taken over a one-hour period. With such limited data, there were many possible orbits the asteroid could theoretically be on.
In fact, within hours of the initial prediction, Marsden changed the information on the Web site. The site now suggested the asteroid could actually be moving away from Earth—an orbit that still fit the original, limited observations.
"I was quite appalled when I found out that they were planning to call the President" without further observations, Marsden said.
Planet Killers
Asteroid flaps are fairly common. It's easy to see how mistakes can be made. The Minor Planet Center, a clearing house for asteroid observations, receives up to 15,000 new sightings a day from the LINEAR telescopes alone.
The Center is part of a (U.S.) 3.5-million dollar-a-year NASA program, called Spaceguard, that locates asteroids in Earth's neighborhood. This program focuses on the estimated 1,200 to 1,500 larger asteroids in our area that are over 1 kilometer (0.6 mile) wide and could be potential planet killers, like the one that probably killed the dinosaurs some 65 million years ago.
"The probability of a major asteroid impact into Earth is very low at any given time," said Bruce Betts, director of projects at the Planetary Society in Pasadena, California.
Asteroid impacts are one of the only natural disasters that can be predicted and, in theory, stopped. Unlike the small 2004 AS1, a large asteroid headed for Earth would likely be detected years, or maybe even decades, before hitting us, allowing scientists to figure out ways to deal with it.
"More tracking and research are critical to know if one of the objects out there has Earth's name on it," Betts said. "Although we have nothing ready right now to divert an approaching asteroid or comet, various methods are under consideration, for example, setting off nuclear bombs near the object to change its orbit."
Exploding Above
Smaller asteroids may actually pose a greater problem.
"We're more likely to be hit by a small object," Marsden said. "The larger ones clearly do more damage … but I think we can attend to [them]. The smaller ones are both much more numerous and harder to find."
Smaller asteroids can appear without notice and could potentially hit Earth in hours or even minutes. An asteroid that is 100 feet (30 meters) across—the original estimate of the 2004 AS1—would probably explode in the atmosphere with the force of a one-megaton bomb, setting off hurricane winds.
"You don't want that sort of thing going off over Los Angeles or New York," Marsden said.
NASA's Johnson admits there is no concerted effort at the moment to find smaller asteroids. "All NASA has been authorized to do is this observation program to establish what the population of large near-Earth objects is," he said. "There are no emergency plans beyond that."
Because asteroid impacts are extremely rare, Johnson says, there is no consensus on the part of the government that the issue is a top priority.
"Personally, I believe it is something that some effort should be spent on, but it needs to be done in a measured and well-thought-out way," Johnson said.
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