Backward green comet makes one-time only visit

WASHINGTON — An odd, greenish backward-flying comet is zipping by Earth this month, as it takes its only trip toward the sun from the farthest edges of the solar system. The comet is called Lulin, and there's a chance it can be seen with the naked eye — far from city lights, astronomers say. But you'll most likely need a telescope, or at least binoculars, to spot it.

The best opportunity is just before dawn one-third of the way up the southern sky. It should be near Saturn and two bright stars, Spica and Regula.

On Monday at 10:43 p.m. EST, it will be 38 million miles from Earth, the closest it will ever get, according to Donald Yeomans, manager of NASA's Near Earth Object program.

The story behind the comet is more intriguing than its appearance — the greenish tinge may be hard for many to discern. The color comes from a type of carbon and cyanogen, a poisonous gas.

Lulin was discovered by a Chinese teenager two years ago. It still has many of its original gases — gases that are usually stripped away as comets near the sun. Unlike most comets viewable from Earth, this one hasn't been this close to the sun before, Yeomans said.

While all the planets and most of the other objects in the solar system circle the sun counterclockwise, Lulin circles clockwise, said NASA astronomer Stephen Edberg. And thanks to an optical illusion, from Earth it appears as if the comet's tail is in the front as the comet approaches Earth and the sun.

"It essentially is going backwards through the solar system," he said.

It came from the outskirts of the solar system, 18 trillion miles away. Once it's made the journey around the sun, Lulin will gain enough speed to escape the solar system, Edberg said.

"If you are interested in comets, make sure you see it," he said. "But it's not going to be a real great blast for the general public."

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On the Net:

NASA on Comet Lulin: http://science.nasa.gov/headlines/y2009/04feb_greencomet.htm