In 1994, the Hubble Space Telescope took pictures of Pluto, then 2.7 billion miles from Earth. The top photo is the Hubble image; the large one is a computer-generated image of Pluto.

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Pluto, the smallest and farthest of the solar system's planets, is still the only planet not yet visited by a spacecraft.

In the 72 years since Clyde W. Tombaugh, an astronomer at the Lowell Observatory in Flagstaff, Ariz., spotted Pluto as a point of light on one of his photographic plates, Pluto has largely remained just that: a point of light. Ground-based telescopes are not powerful enough to pick out any details of its surface.

But with ingenuity, astronomers have learned a lot just by staring intently at the point of light. Specific colors in the light point to the composition of its surface and thin atmosphere. By measuring how it brightened and dimmed during eclipses with its moon Charon, scientists have even created a rough map of its surface.

"We know quite a bit in general," said Dr. John A. Stansberry, an astronomer at the Steward Observatory in Tucson, "but we don't know a lot in particular."

In the public limelight, Pluto has recently been the subject of a custody battle between those who say it should be counted among the Kuiper Belt objects — a ring of icy rocks beyond the orbit of Neptune that failed to coalesce into a larger planet — and those who attack any efforts to diminish Pluto's status as the ninth planet. That debate boils down to what is a planet and what is not, a seemingly straightforward concept with no precise definition.

Pluto, together with the thousands of Kuiper Belt objects, represents a "fossil remnant" of the planet-making process, said Dr. S. Alan Stern, director of space studies at the Southwest Research Institute in Boulder, Colo. "Here we have, for some reasons we do not understand, a place where accretion was halted in midstep."

Pluto also appears to be a close cousin of Triton, one of the large moons of Neptune, and some hypothesize that other, large Pluto-like bodies remain hidden farther out in the Kuiper Belt.

Despite its distance from the Sun — 2.9 billion miles and growing — Pluto is not just an unchanging, frozen iceball. Its polar caps wax and wane over the decades. There are suspicions that as the atmosphere thins, the winds will accelerate to a couple hundred miles per hour.

Planetary scientists desperately want a closer look at Pluto, and NASA's off-and-on plans to send a mission there are on again, at least for the moment.

After canceling its own efforts in September 2000, NASA last November chose a team led by the Southwest Research Institute and the Johns Hopkins University's Applied Physics Laboratory to design, build and operate a reconnaissance mission to Pluto at a cost of $546 million, including projected inflation. Continuing on to explore a Kuiper Belt object would cost an additional $73 million.

As recently as a decade ago, Pluto was regarded as a single oddball at the fringe of the solar system of little scientific interest. The first Kuiper Belt object was seen in 1992 and hundreds more have been cataloged since in that vast expanse of unexplored real estate.

After years of exploring the inner solar system and the gas giants, "This is a mission to reconnoiter the third geographic region of the solar system," said Dr. Stern, the mission's principal investigator. Pluto, he said, has moved in scientific importance "from footnote to center stage."

Called New Horizons, the spacecraft is expected to be launched in January 2006 and will take a decade to travel to its destination to explore Pluto, Charon and a couple of the Kuiper Belt bodies.

Congress provided $30 million in this year's budget for preliminary design work, but the administration's budget plans for 2003 and beyond currently do not include any money for actually building and flying the mission.

"I'm glad it's getting started," said Dr. Marc W. Buie, an astronomer at Lowell. "I think it's a travesty it takes this long to get to this point." He added, "On the other hand, we can't kick a gift horse in the mouth."

Dr. Richard P. Binzel, a professor of planetary sciences at the Massachusetts Institute of Technology, said the uncertainty was familiar to Pluto researchers.

"The analogy a lot of us give is it's like Lucy holding the football," he said, referring to the "Peanuts" comic strip character who inevitably jerks away the football as Charlie Brown tries to kick it.

"Once again," Dr. Binzel said, "the football is there."

New Horizons may be the last chance for a while. After 2007, Jupiter will move out of position for providing a spacecraft with a convenient, no-cost gravitational catapult to Pluto; a direct trajectory would probably be slower and require some new, and therefore more risky, propulsion technology.

Scientists also worry that with further delays, there will be much less to see when the spacecraft finally gets there. Pluto is on the outbound leg of its elliptical orbit, and as temperatures, now ranging between minus-395 degrees and minus-370 degrees Fahrenheit, get colder, its already tenuous atmosphere may literally freeze solid and fall to the ground. Computer models differ on when the freeze-out will occur, but most expect Pluto's air to be gone by 2020.

Dr. Stansberry of the Steward Observatory, however, is not sure. His calculations indicate that the crystal structure of the nitrogen ice on the surface will change to a different shape that will not radiate heat as easily. "It just cools off less quickly after that point," he said. The atmosphere will thin, but not fully collapse.

But if it does freeze, it will remain frozen until after 2200, when Pluto's 248-year orbit will again come close enough to the Sun for it to be heated back into vapor.

Also, more and more of the planet is entering extended darkness, the same way that Antarctica plunges into a six-month-long night every year. Because Pluto's axis is almost keeled over on its side, the darkness will envelop most of its southern hemisphere, putting it out of view for decades.

This history of Pluto science is a mix of hide-and-seek and fortuitous observation.

Tombaugh spent years searching for a hypothesized ninth planet to explain small perturbations in the orbits of Uranus and Neptune, and then on Feb. 18, 1930, he found Pluto. Tombaugh's discovery turned out to be more the product of chance and persistence than deduction: the perturbation calculations were incorrect, and Pluto, once thought to be larger than Earth, turned out to be smaller than Earth's Moon — too small to account for the perturbations, in any case.

In June 1978, while taking photographs to better map Pluto's orbit, James W. Christy, an astronomer at a United States Naval Observatory telescope in Flagstaff, noticed that in one image, Pluto's point of light seemed to have a bump on it. He went back and looked at other photographs and Pluto seemed to have a bump in many of those images, too, and the bump seemed to be moving around.

The most plausible explanation for the bump was that it was a moon circling Pluto, and he named it Charon, after the ferryman in Greek myth who transports the dead across the river Styx.

In 1985, Dr. Binzel, then a graduate student at the University of Texas, confirmed the moon hypothesis when he observed an eclipse of Charon passing in front of Pluto. For the next five years, Charon's orbit faced Earth almost edge-on, and Pluto and Charon took turns passing in front of each other.

With each eclipse, Charon passed over a different strip of Pluto. By measuring the dip in brightness during the eclipse, scientists could deduce the brightness of the hidden piece. "It was like scanning the disk of Pluto for us," Dr. Binzel said.

With that data, astronomers constructed a map of Pluto's surface. "We derived a map of one hemisphere of Pluto that is considerably better than what is possible to do even with the Hubble Space Telescope," said Dr. Buie of the Lowell Observatory.

What they saw were blotchy regions of light and dark with especially bright landscape at the poles, especially the south pole. (In 1994, Hubble took the first picture that could directly show some of Pluto's features — still blurry blotches.)

That was a surprise because that meant there had to be weather and seasons. It frosts on Pluto.

"To get something so bright, it's got to be very fresh because space is a dirty place, and bright places turn dark," Dr. Binzel said.

Because different molecules absorb different colors of light, more detailed analysis of which wavelengths of light dimmed during the eclipses revealed some of what is on the surfaces of Charon and Pluto. "The difference tells you what's on the two pieces," Dr. Buie said. "What's there and what went away."

That turned out to be nitrogen, methane and carbon monoxide frost on Pluto, water frost on Charon.

The duration of the eclipses — each lasted a couple of hours — also allowed accurate calculation of the relative sizes of Pluto and Charon. (The Hubble image further nailed down the sizes. Pluto is 1,440 miles wide. Charon is 790 miles wide. By comparison, the diameters of Earth and the Moon are 7,900 miles and 2,160 miles.)

Another fortuitous alignment of the night sky occurred in 1988, when Pluto passed in front of a distant star. Again, astronomers examined the colors of starlight that were absorbed by Pluto's atmosphere and found its air was nitrogen with a bit of methane.

In the past decade, there have been no more eclipses between Pluto and Charon and no more fortuitous alignments of stars. Astronomers, however, continue to learn more, if only to form more questions.

Pluto has been getting dimmer over the decades because the bright south pole has been moving out of sight. But with the north pole coming into view, "I should already be seeing Pluto getting brighter again," said Dr. Buie. Instead, after the brightness leveled during the 1980's, Pluto has started fading again.

"I don't have an explanation for that yet," Dr. Buie said. "I don't know if it's just darker in the north pole area." Another possibility is that the bright frost has not yet formed.

Another puzzling feature is that the color signature for carbon monoxide gas on Pluto's surface appears and disappears on a 6.4-day cycle — the length of a day on Pluto. Dr. Buie hypothesizes that the carbon monoxide ice is located at a single site, possibly a meteor impact within the past few million years that exposed material from deep underground. Consulting the map of Pluto, he said he had a good candidate for the impact site.

That is speculation, but it gives planners for the New Horizons mission a site at which they should definitely point their cameras when the spacecraft arrives, probably in 2016.

New Horizons will carry a camera with a long-range telephoto lens that will start taking pictures months before the flyby, a second camera that will take pictures during the flyby, and an instrument to analyze atoms and molecules escaping from Pluto's atmosphere.

Its antenna will take part in an experiment that will also analyze the atmosphere. Transmitters on Earth will broadcast a strong signal to the spacecraft, which will measure how the radio waves are altered by the passage through the atmosphere and then radio back the results. That will tell not only the composition, but the temperature and density of gases.

But first, there is the question of whether the spacecraft will be built. After waiting a decade for a Pluto mission, Pluto researchers are still waiting.

"Oh boy, that's a loaded question," Dr. Binzel said. "I'm as optimistic as I've ever been that the Pluto mission will become a reality. It's time we finished the basic reconnaissance of the solar system, and that's what this mission does."