Asteroid Won't Hit Earth

Doomsday asteroid watchers can relax. A reanalysis of the orbit of a large asteroid headed for a close encounter with Earth in 30 years (ScienceNOW, 11 March) predicts no chance of a collision. Archival images of the asteroid, identified this morning, and a more sophisticated analysis of the orbit suggest the object is likely to whiz by at a safe 900,000 km from Earth, more than twice as far away as the moon.

The new analysis, by Donald Yeomans and Paul Chodas of the Jet Propulsion Laboratory (JPL) in Pasadena, California, takes careful account of the uncertainty in available observations of the asteroid. By projecting the effect of those uncertainties forward 30 years, the astronomers calculated an "error ellipsoid," a region of space where the asteroid is likely to be during its nearest pass on 26 October 2028. The program, says Yeomans, produces a cigar-shaped ellipse that comes near, but does not overlap Earth. As astronomers gather more observations and the uncertainty decreases, the ellipse will get smaller--and more distant from Earth.

Images of the asteroid taken at the Mt. Palomar Observatory in 1990--and pulled from archives today by Eleanor Helin at JPL--allowed Brian Marsden of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, to refine his prediction as well. Astronomers use snapshots of asteroids and comets to estimate orbits, and the more snapshots, the more accurate the estimate. Although Marsden's earlier calculations, which drew widespread media attention today, allowed for a chance of a collision, he now agrees that the odds are nil.

Other astronomers decry the mixed signals being sent to the public. "I would give us a D-" at communicating to the public, says Richard Binzel of MIT. "It reflects horribly on our credibility." Clark Chapman of the Southwest Research Institute in Boulder, Colorado, agrees. As the pace of discovery of asteroids increases, he says, "we will have more near-misses like this one. A real one might get lost in crying wolf."

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