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Close call for earth ahead?

The asteroid Toutatis is named for a Gaulish god in the popular French cartoon series "Asterix.' The mortal acquaintances of Toutatis constantly are worrying that the sky will fall someday. Naming an asteroid after Toutatis is whimsical, but nonetheless appropriate, because astronomers can not give any absolute guarantee that the two-mile-diameter asteroid will not strike the Earth someday. In fact, it passed within 10 lunar distances (about 2,-200,000 miles) on Dec. 8, 1992, which certifies the asteroid as the celestial object whose orbit regularly brings it closest to the Earth and the moon. Toutatis makes a complete orbit of the sun once every four years and, in 2004, will come within 1,000,000 miles of the Earth. There will be two other closer encounters after that - in 2008 and 2012 - then the near approaches will end for a few decades.

Toutatis is unlike most of the more than 20,000 asteroids in the solar system in that it has been kicked into an elliptical orbit by the gravitational force of Jupiter, explain Art Whipple and Peter Shelus of the University of Texas at Austin's Department of Astronomy. Rather than orbiting quietly in a circular motion between Mars and Jupiter, as the vast majority do, it swings outward toward Jupiter once every four years, then loops inside the orbit of the Earth two years later. it is this elliptical movement that brings the asteroid within the gravitational affraction of Mars, the Earth, and Venus, and those additional gravitational tugs make it difficult for astronomers to determine exactly where the asteroid will be the next time it comes around.

Since an unperturbed body in orbit follows the identical path throughout eternity, the task of computing whether the Earth and Toutatis ever will meet normally would be a straightforward mathematical calculation. However, Toutatis is by no means unperturbed. Therefore, predictions can not be made very far into the future.

Whipple likens the motion of the asteroid to a leaf floating in a river. In still waters, the direction the leaf will take is fairly easy to predict, but, in rapids, its motion becomes more erratic. Mathematically, the leaf in rapid waters is similar to Toutatis in the midst of the inner planets - small errors in prediction are compounded, and the observer is at a loss to forecast a future position with any degree of accuracy.

This is why the orbit of Toutatis is considered to be chaotic. By chaos, the scientists do not mean that the motion is totally random. in fact, the asteroid's future path is perfectly orderly and predetermined. However, the nature of the gravitational attraction is such that standard mathematical tools can not be used to calculate its future path exactly, because the slightest inaccuracies in the positions entered into these equations will snowball.

Whipple and Shelus maintain that the likelihood of the asteroid striking the Earth is small, given astronomical scales, and that a miss is as good as a mile. Even if the human race is not in imminent danger of extinction from such a collision, the issue of chaotic orbits presents interesting problems to keep specialists in the field busy. Understanding the elliptical orbits of objects like Toutatis could shed light on the origin of the moon's craters and, perhaps indirectly, the manner in which cosmic debris came together billions of years ago to form the solar system.

Whipple points out that the current thinking among astronomers is that an asteroid only can stay in a planet-crossing orbit for a certain period of time. "This can't have gone on for more than a few tens of thousands of years. We think that eventually all planet-crossing asteroids either hit something or are ejected from the solar system."
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Title Annotation:possible collision with asteroid Toutatis
Publication:USA Today (Magazine)
Date:Jun 1, 1993
Words:622
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