Asteroid 1950 DA. Photo: NASA/JPL

Applying unprecedented refinements to the analysis of celestial hazards, NASA astronomers have identified a potential close encounter with Earth more than eight centuries in the future by an asteroid two-thirds of a mile (one kilometer) wide.

What will most likely be a miss, even without preventive measures, will come on March 16, 2880, said Jon Giorgini, a senior engineer at NASA's Jet Propulsion Laboratory, Pasadena, Calif. Odds for a collision are at most one in 300, and probably even more remote, based on what is known about the asteroid so far. Still, that makes this space rock, named 1950 DA, a greater hazard than any other known asteroid.

"This is not something to worry about," said Giorgini, leader of a team reporting about the asteroid in the April 5 edition of the journal Science. "We're showing that searches with optical telescopes and follow-up observations with radar telescopes can provide us centuries of advance notice about potential close encounters of asteroids with Earth. That's plenty of time to consider the options -- 35 generations, in fact."

"This report is a success story for our efforts to identify potential troublemakers," said JPL's Dr. Don Yeomans, manager of the NASA Near Earth Object Program. "Radar observations are helping us push predictions 5 to 10 times further into the future."

This report differs from previous ones about other asteroids' Earth-impact potential. Estimates of impact risks in earlier cases came from a few nights' optical observations of newly found asteroids. Astronomers soon ruled out the possible impacts after a few more observations narrowed uncertainties about the asteroids' orbits. The current orbit of 1950 DA has been mapped with great accuracy using precise radar data and a 51-year span of optical data. Uncertainty about how close it will come to Earth in 2880 stems from gaps in knowing physical details of the asteroid that could subtly alter its course over the centuries.

Illustrations show postions of Earth and asteroid 1950 DA on April 5, 2002. Photo: NASA/JPL

"How close 1950 DA will approach Earth turns out to depend on the asteroid's physical attributes -- it's size, shape and mass, and how it spins, reflects light and radiates heat into space," Giorgini said. These things are unlikely to be known any time soon. The way the asteroid radiates energy absorbed from the Sun back into space has the biggest potential effect, he said. Releasing heat in one direction nudges the asteroid in the opposite direction. The resulting acceleration is tiny, but over the centuries acts like a weak rocket and could make the difference between a hit and a miss. cAsteroid 1950 DA was discovered from Lick Observatory, Mount Hamilton, Calif., in 1950. It faded from view for five decades then was found from Lowell Observatory in Arizona in 2000. Astronomers used large dish antennas of NASA's Deep Space Network site at Goldstone, Calif., and the Arecibo Observatory in Puerto Rico to examine the asteroid with radar when it passed at a distance 21 times farther away than the Moon in March 2001.

"Once an asteroid is discovered, radar is the most powerful way to find its exact orbit and, apart from sending a spacecraft, the only way to see what it looks like," said JPL's Dr. Steve Ostro, who led the radar observations of 1950 DA.

Giorgini refined calculations of future orbits by including factors such as the push from sunshine and the potential gravitational tug from 7,000 other asteroids and nearby stars. Effects of each small influence on the asteroid's movement could be amplified by 15 gravitational tugs during close approaches to Earth and Mars -- none of which have any chance of an impact -- prior to 2880. "It's like predicting a 15-bank shot in a pool game," Giorgini said. "We know the cue stroke extremely well because it is right now and we can measure it. But at each future bank, small variations accumulate and change the next bounce, which changes the following one and so on. What we've done is find the range of changes possible due to tilt, imperfections and fuzz on the table, the bounce of the cushions, and wind blowing across the room. We need to know more about the 'cue ball' to really be sure of how the last three banks in 2809, 2840 and 2860 will line things up for 2880."

If future generations' studies of 1950 DA indicate it ought to be diverted to prevent a collision, the subtle influences that its physical properties have on its motion might be manipulated to advantage. For example, Giorgini suggested, its surface could be dusted with chalk or charcoal to alter the way it reflects light, or a spacecraft propelled with a solar sail could collapse its reflective sail around the asteroid. In any event, determining asteroids' physical properties will be important for long-term calculations of impact hazards.

In addition to Giorgini, Ostro and Yeomans, authors of the report include Dr. Lance Benner, Dr. Paul Chodas, Dr. Steven Chesley, Dr. Myles Standish, Dr. Ray Jurgens, Randy Rose and Dr. Alan Chamberlin, all of JPL; Dr. Scott Hudson, Washington State University, Pullman; Dr. Michael Nolan, Arecibo Observatory; Dr. Arnold Klemola, Lick Observatory; and Dr. Jean-Luc Margot, California Institute of Technology, Pasadena.

Arecibo Observatory is operated by the National Astronomy and Ionosphere Center at Cornell University, Ithaca, N.Y., under an agreement with the National Science Foundation. NASA's Office of Space Science, Washington, D.C supported the radar observations. JPL is managed for NASA by the California Institute of Technology.