SIGHTINGS


NASA Says Its Calculations
Put Earth Out Of Harm's Way
By Alan Boyle, MSNBC
3-12-98



March 12 - NASA's Jet Propulsion Laboratory says Earth will dodge a mile-wide bullet in the year 2028. The laboratory's projections indicate that our planet is outside the "error ellipsoid" that forms the bull's-eye for Asteroid 1997 XF11 - but that will need to be confirmed by further observations and calculations.
 
 
AN ALERT about the asteroid's close approach created a media sensation Wednesday when it was issued by the International Astronomical Union,s Central Bureau for Astronomical Telegrams. Even if the asteroid passes harmlessly past Earth in 2028 - which always has been considered the likeliest scenario - astronomers say this closest of close calls will help society prepare for future threats.
 
"Most likely this will fizzle, but the next one may not," said Tom Gehrels, a leader of the Spacewatch Project at the University of Arizona. "So this is a good exercise."
 
Asteroid 1997 XF11 was discovered just last December by Jim Scotti, a senior research specialist for the Spacewatch Project, which keeps track of asteroids and comets. The chunk of space rock is thought to be about a mile wide and is currently about 150 million miles away from Earth. It circles the sun every 1.7 years; thus, astronomers will have ample opportunities to track its orbit over the next few years.
 
As the observations mount up, scientists will be able to generate more precise projections of the asteroid's orbit. The latest projections indicate that the most likely path would come within tens of thousands of miles of Earth on Oct. 26, 2028. But the key factor in the predictions is the error ellipsoid - a football-shaped area of probability surrounding the asteroid's most likely course.
 
Wednesday's alert indicated that Earth was within the error ellipsoid, meaning that based on the limited amount of data gathered so far, there was a very small chance that the asteroid would strike the planet. Such an impact would be the equivalent of an atomic-bomb blast, setting off a global catastrophe. Scientists believe the impact of a larger asteroid 65 million years ago caused the extinction of the dinosaurs and many other species.
 
But the Jet Propulsion Laboratory said its projections put Earth outside the error ellipsoid, meaning that there was no chance of a collision. Thursday afternoon's forecast indicated that the asteroid would probably not come closer than 600,000 miles. Calculations earlier in the day showed that the greatest margin of error would still put the asteroid 50,000 miles away from Earth.
 
The difference between a hit and a miss is very slight in astronomical terms, and small adjustments in the asteroid's mass or velocity would lead to different conclusions.
 
"Different calculations give slightly different numbers on the miss distance, and that,s understandable due to the fact that there are uncertainties in the measurements that astronomers make," said Paul Chodas, an astronomer at the Jet Propulsion Laboratory. "You'll get different predictions depending on whether you disbelieve those measurements."
 
Stephen Maran, a spokesman for the American Astronomical Society, said the different projections pointed up the need for more observations.
 
"We certainly need better tracking data on the asteroid," Maran said. "Independent experts of very high reputation are getting different results."
 
Gehrens said it was clear that the chances of an asteroid collision in 2028 were very slim at best. But he said he "was a bit taken aback that they made such a firm statement at JPL" that a collision was out of the question.
 
The primary purpose of Wednesday's alert, Gehrens said, was to enlist more astronomers for the observing effort - by turning large telescopes toward XF11 and reviewing past sky images for traces of the asteroid that may not have been noticed before. He said any questions about the asteroid,s path would be settled in the next few years.
 
The asteroid is currently too faint to be seen with the naked eye, and even if it passes mere tens of thousands of miles from Earth in 2028, it would probably look much like a star, satellite or comet in the night sky.
 
CHICKEN LITTLE?
 
 
Reaction to the alert has ranged from skepticism about a "sky-is-falling" false alarm to fatalism about personal retirement plans. On MSNBC's Space News bulletin board, many Internet correspondents focused on the need for closer monitoring and the formulation of an official plan to avoid collisions with passing comets and asteroids.
 
There are several programs that monitor near-Earth objects, including the Spacewatch Project as well as the Near-Earth Asteroid Tracking project, operated by NASA and the Air Force. But the plans to avert a projected asteroid strike are still largely in the sci-fi arena - ranging from blasting objects out of the sky with lasers or nuclear weapons to nudging them into different orbits using rocket interceptors.
 
The more dramatic methods will be illustrated in two Hollywood movies due to be released in the next few weeks. "Deep Impact" tells the story of an impending comet collision, while "Armageddon" focuses on an asteroid threat.
 
On a more down-to-earth note, Rep. Dana Rohrabacher, R-Calif., chairman of a House subcommittee on space and aeronautics, urged President Bill Clinton to restore funding for Clementine 2, an asteroid interceptor experiment that was axed last year in a line-item veto.
 
"This project would have been a low-cost proof of concept for any future attempt to protect our planet from an asteroid collision," Rohrabacher said in a written statement.


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