By Senior Science Writer
updated 2/25/2004 1:49:09 PM ET 2004-02-25T18:49:09

A pair of comets that astronomers have been monitoring for several months could become plainly visible in the night sky this spring.

Each comet is currently visible in telescopes. Scientists can't say for sure how bright they will get, but there is some optimism that both might reflect enough sunlight to be visible to the unaided eye at the same time.

For viewers in the Northern Hemisphere, the odds look good for at least one interesting spectacle. It will be a few weeks, however, before firmer predictions can be made of the other comet.

"If they brighten as predicted, then both may be visible to the naked eye in late April and part of May," said Dan Green, an astronomer at the Harvard-Smithsonian Center for Astrophysics and director of the Central Bureau for Astronomical Telegrams, which monitors comet observations among other things.

It would be unusual for two comets to be visible in the sky at once. The objects are catalogued as C/2001 Q4 (NEAT) and C/2002 T7 (LINEAR).

They will likely be referred to commonly as Comets NEAT and LINEAR.

Out of the deep freeze
Comets are frozen balls of ice and dirt that typically originate in the far reaches of the solar system. They are leftovers of the formation process — essentially, frozen rocks that didn't get incorporated into planets.

Image: NEAT location
Space.com / Starry Night
A sky chart shows where Comet NEAT could be seen during mid-May. Click on the image to see a larger chart.
Now and then one is gravitationally booted to the inner solar system. As a comet nears the sun, solar radiation kicks up surface material, which then surrounds the comet in a coma, or head. Sometimes a tail forms, too. The heads and tails of gas and dust are visible because they reflect sunlight.

Comet NEAT was discovered by the Near Earth Asteroid Tracking program, or NEAT, in August 2001. At the time, it was a magnitude-20 object. On this scale of brightness used by astronomers, smaller numbers represent brighter objects. Very bright stars are magnitude 1 or 2. The dimmest objects visible under very dark skies are around magnitude 6.5.

NEAT could brighten to magnitude 1 or 2 in late April and remain that bright through mid-May, according to a statement issued Wednesday by the CfA.

Comet LINEAR was found by the Lincoln Laboratory Near Earth Asteroid Research program, or LINEAR, in October 2002. It may become visible to the naked eye in mid-March for experienced observers under dark skies.

Uncertain predictions
Astronomers think both comets are making their first trips through the inner solar system, so it is impossible to predict with certainty how bright they will become.

"Comets do a lot of things that are unpredictable," Green said.

Some comets break apart, losing any chance of great brilliance. Others experience sudden outbursts that can make them far brighter than expected. Others remain unimpressive for reasons not well understood.

Comet NEAT will first be visible from the Southern Hemisphere. It shows up for viewers north of the equator in early May.

Comet LINEAR will also be visible from the Southern Hemisphere, and depending on how much it brightens it might be seen just before sunrise in late April and early May by Northern Hemisphere observers.

Lowering expectations
Joe Rao, Space.com's Night Sky columnist, cautions that comets making their first loop around the inner solar system have a tendency to fall short of expectations.

"I think if prospective skywatchers in the Northern Hemisphere are looking for a good sky show they're going to have to pin their hopes on Comet NEAT," Rao says. "Comet LINEAR will unfortunately remain rather close to the sun and very low down near the eastern horizon all through April and early May, making it rather difficult to see."

Comet NEAT, on the other hand, "should vault high into the western evening sky during May, making it much easier to sight," Rao said. "Observers in the Southern Hemisphere will have an advantage in that both comets will be fairly high up and easy to spot."

Space.com will provide complete viewer's guides, with maps and updated reports on each comet's brightness predictions, beginning March 19.

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