ASTRONOMERS HOLD HIGH HOPES COMET WILL ZOOM IN CLOSE TO EARTH.

Byline: Tony Knight Daily News Staff Writer

Astronomers are excited about a comet that will pass close to Earth later this month and could turn out to be the brightest in 20 years.

There's no way to tell just how bright Comet Hyakutake will be until it gets here. But Griffith Park Observatory astronomer John Mosley said its tail could stream a quarter of the way across the night sky.

The last time this comet passed Earth was about 20,000 years ago, he said.

Urban dwellers might find that the bright city lights will wash out the faintly glowing comet tail. But the comet should be dazzlingly visible to the unaided eye in outlying areas, Mosley said.

"If you're out in a dark location, it will probably stretch across the sky, and you'll tell your grandchildren," he said. "If you're in Van Nuys, you probably won't see it."

In recent weeks, the comet could be seen approaching Earth from low in the southeastern sky, but don't go out looking for it tonight because the moon will be too bright.

As the moon wanes and moves to another part of the sky Monday and Tuesday, the Comet Hyakutake will be visible in the constellation Libra to the southeast, Mosley said.

Using a star map usually found in a world atlas or in popular astronomy magazines, viewers should be able to find the comet about half-way between the star Spica in the constellation Virgo and the very bright star Antares in the constellation Scorpius, Mosley said.

"Right now it looks through binoculars like a fuzz ball about the size of the moon," Mosley said.

The comet, which was discovered Jan. 30 by amateur Japanese astronomer Yuji Hyakutake, will climb gradually higher in the sky until it is almost directly overhead March 25 when it makes its closest approach to Earth - a scant 9.5 million miles away.

"It's going to be wonderfully placed," Mosley said. "Early in the evening it will be almost directly overhead. And then it moves past the North Star, and it will be in the northwest for about a month."

The comet's tail will grow longer in April as it approaches the sun.

Comets are hunks of dirt and ice with elongated orbits that take them from the outer solar system to near the sun. They grow long tails as material streams away from their rocky cores driven by solar radiation.

The core of Comet Hyakutake is about 10 miles across, making it a large comet, Mosley said.

But comets can be deceptive, and this one could turn out to fizzle like the highly touted Kohoutek that got a lot of media attention but didn't put on much of a show in 1973.

Before Hyakutake was spotted, Earth astronomers were touting the arrival of Comet Hale-Bopp, which they said could be one of the brightest ever seen. Hale-Bopp was discovered last summer and is due to arrive near the sun in March 1997.

CAPTION(S):

MAP

Map COMET HYAKUTAKE Graphic shows the location of Comet Hyakutake in the evening sky over Los Angeles from March 20 to April 2. The comet will appear on this track at 9:30 p.m. March 20 -- and a few minutes each night through April 2, when it can be seen at 8:40 p.m. Source Griffith Observatory