By Maggie Fox

LIVERPOOL, England (Reuter) - Astronomers tired of trying to get a quick and unfulfilling peek at comets when they make their rare visits to Earth's neighborhood revealed plans Tuesday to chase one down and take closer look.

A team of international scientists told the National Astronomy Meeting in Liverpool Tuesday they hope to learn more about why comets form their dramatic tails when they near the sun on their rare visits to the neighborhood of Earth. But the rendezvous of spacecraft and comet will not take place until the year 2012.

The astronomers have named their mission Rosetta, after the stone with a multi-lingual inscription that helped experts decipher ancient Egyptian hieroglyphics. The comet that Rosetta will chase is a frequent visitor called Wirtanen.

While last month's Comet Hyakutake, one of the brightest this century, is thought to pass near the Earth once every 20,000 years, the less spectacular Wirtanen appears every five years.

Astronomers are keen to study comets because they are a physical link to the beginnings of the universe and can tell a lot about how it evolved. ``Comets are in fact the most primitive matter that we can analyze in the solar system,'' said Colin Pillinger of the Open University in Milton Keynes, England, who is helping to design some of the instruments aboard the landers.

``If you get the cometary matter, you get the most primitive matter ... in the chain of evolutionary links to interstellar material,'' said Gerhard Schwehm, coordinator of the mission.

Astronomers already know a lot about Wirtanen and think they can chase it down.

``What we want to do is actually go into orbit around the comet,'' Schwehm, who works at the Dutch space agency ESTEC, said. Even an object as small as a comet has enough gravity to do this, he said.

Rosetta will stay with the comet as it approaches the sun and warms up, its ice turning directly into gas in a process that creates a comet's long, brilliant tail.

But because so little is known about comets, the landers and instruments must be carefully designed. Comets are believed to be balls of ice and dust -- but no one even knows how solid they are.

``Some people say you will sink in because it is like loose snow,'' Schwehm said. ``Others say it has a crust. You have to design a lander that can cope with a soft surface or hard.''

And then there is the small size of the lander. ``It is only one metre (three feet) across,'' Schwehm said. ``If you land in a crack ...''

Astronomers will also have to be patient. Rosetta will be not be launched until 2003 and will orbit the sun twice, getting

``gravity boosts'' from the Earth and Mars before heading across the asteroid belt to meet Wirtanen. It will not come close to the comet until 2012 -- nine years later.

Schwehm is philosophical about having to wait more than 15 years to learn Wirtanen's secrets. ``It's the only chance we have to get data from the comet,'' he said. ``There have been other missions where you had to wait a long time.'' He said scientists applying to take part in the $1 billion project were being asked to give details about who will take over from them. ``This goes up to the retirement age of most of us,'' he smiled.

Rosetta will be launched as part of the U.S. space agency NASA's Cassini mission, which will orbit Saturn for four years

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