WASHINGTON (Reuters) - Poor Pluto. NASA (news - web sites) has stopped work on a robotic mission to this distant, mysterious planet, the only one in the solar system not yet explored by earthly spacecraft.

If work does not resume by the end of this year, planetary astronomers said on Friday they fear the mission will lose its place on NASA's space launch schedule in 2004.

That could delay the craft's expected arrival at Pluto and its moon Charon by seven years, and by that time, the distant little planet's tenuous atmosphere could have started to freeze as Pluto moves into a winter lasting more than 100 years.

The National Aeronautics and Space Administration stressed that the so-called Pluto-Kuiper Express mission being put together by the Jet Propulsion Laboratory in Pasadena, California, was being ``rethought and replanned,'' not scrapped.

"The mission will be deferred until they can replan it for what's affordable," NASA spokesman Don Savage said in a telephone interview.

Originally budgeted at $350 million a year ago, the mission as currently envisioned would now cost more than $500 million to complete, Savage said, "and that's just not affordable."

NASA's chief of space science, Ed Weiler, ``would like to see some way for them to do the mission by 2020 when the atmosphere will still be there, not frozen out yet,'' Savage said. Pluto, the most distant planet from the Sun, was only discovered in 1930 and takes 248 years to make one solar orbit, so scientists have never observed its winter and do not know exactly what to expect, said Ellis Miner, a spokesman for the American Astronomical Society.

Get There Before Atmosphere Freezes

The society's planetary scientists expressed ``major concerns'' over stopping work on the Pluto mission, and Miner said that any substantial delay might mean astronomers would not be able to observe the planet's atmosphere.

Pluto came closest to the Sun in 1989 and has been moving away ever since. Even at its closest, it is still vastly distant: 30 times Earth's distance from the Sun, or about 2.8 billion miles.

"As Pluto moves out (away from the Sun), the amount of sunlight that it gets is decreasing rapidly," Miner said by telephone. "At some point the temperature will be cold enough that the atmosphere will basically snow out onto the surface and all that will be left is a very tenuous trace atmosphere and it may be difficult to detect."

A planet's atmosphere is often the key to finding out how it formed, and with an eccentric planet like Pluto, this could be important. Astronomers have inspected the atmospheres of every other planet except Pluto.

Pluto has always been a bit of an oddball among planets.

It is small and craggy where the other planets in the outer solar system are big and gassy; it is less than half the size of any other planet; its orbit tilts up from the solar system plane and is the only one to cross the orbit of another planet -- Neptune; and its moon, Charon, is larger in proportion to it than any other planet's moon.

There was a move afoot last year to reclassify it as a minor planet, instead of a major one, but it kept its major planet standing.