Shining light on Pluto and its moon. Astronomers get a close-in look at the solar system's outermost planet

PLUTO is making its closest approach to the sun. Because it comes this way only once every 248 years, astronomers - starved for details about the tiny planet - are trying to make the most of the astronomical event. Unlike the other planets, Pluto has been in the textbooks for less than two generations. Its small size and vast distance from Earth make studying the solar system's outermost planet a daunting task.

That study is vital, not only because scientists know so little about Pluto compared with other planets, but also because what they're learning has the potential to change notions of how the planets beyond Mars formed.

Planetary scientists say they expect the next two years to shed more light on some of the most basic details involving Pluto and its moon, Charon. ``The main issues we're groping with are diameters, densities, atmosphere, and other properties,'' says Richard P. Binzel, of the Planetary Science Institute in Tucson, Ariz.

To glean some of this information, astronomers are taking advantage of the fact that, as seen from Earth, Charon is passing directly in front of and behind Pluto.

By measuring changes in the amount of reflected light from these so-called mutual events and by using Charon's orbital radius, for example, University of Hawaii astronomer David J. Tholen calculates that Pluto's diameter is 2,330 kilometers (1,456 miles) and Charon's is 1,225 km. He says Charon's orbital radius is 19,800 km. These numbers, he says, update those he and colleagues presented at a recent meeting of the American Astronomical Society in Austin, Texas. And they'll continue to shift as astronomers make more observations.

The sizes find their way into density calculations that in turn give clues to the system's overall composition. The average density for the Pluto-Charon system is about twice that of water, says Dr. Binzel, indicating that they contain ``at least some rocky material. Right now that's a mystery. It's a higher density than we would have expected'' for an outer planet.

He says that because Pluto may be similar to one of Neptune's moons, Triton, astronomers are eagerly awaiting Voyager 2's flyby of Neptune and Triton in August 1989. If Triton's density turns out to be significantly lower than Pluto's, ``then Pluto will be an anomaly'' among the outer planets, he says. If Triton's density turns out to be as high as Pluto's, he adds, Pluto won't be the only outer-planet oddity. This may force scientists to rethink their ideas about how the outer planets formed.

Pluto and Charon's mutual events also allow scientists to gather spectra that reveal the chemical makeup of the surfaces of the two bodies. Recent observations have confirmed that Pluto's surface is largely methane ice, with some water ice as well. Charon's surface is largely water ice.

The issue of the density of the planet's atmosphere ``is one of the more interesting problems'' confronting Pluto-watchers, says Dr. Tholen.

Evidence is mounting that the atmosphere is much less dense than many scientists once thought. Images collected in 1983 from the Infrared Astronomical Satellite revealed evidence of polar icecaps made of methane. ``A thick atmosphere would lead to uniform temperatures'' over the planet, says Tholen. ``You wouldn't get polar caps.''

At its farthest point from the sun, some 50 astronomical units (more than 4 billion miles) away, Pluto is so cold that its atmosphere is locked up as methane ice.

As the planet approaches the sun and the ice begins to vaporize, the atmosphere begins to expand with enough velocity to escape Pluto's extremely weak gravity, says astronomer Laurence M. Trafton of the University of Texas at Austin. Wisps of that methane escape into space: Dr. Trafton says that over the course of the present age of the solar system, Pluto has lost about 0.2 percent of its mass. But he adds that most of the tenuous gas remains captured by the combined gravities of Pluto and Charon, enveloping them in a common - though extremely thin - atmosphere.

This ``halo'' phenomenon has not yet been observed, Trafton says, adding that the Hubble Space Telescope, when it is launched, should be able to observe the extent of this methane halo if it exists.

An additional way to check on Pluto's atmosphere, as well as its size, is to watch the planet as it slips in front of a distant star.

Scientists expect one such occultation to occur in June, somewhere over the northern Pacific. A team from the Massachusetts Institute of Technology has tied down telescope time on the National Aeronautics and Space Administration's Kuiper Airborne Observatory to chase the fleeting eclipse.

Ted Durham, a research scientist in MIT's Earth, Atmospheric, and Planetary Science Department and one of the participants in the effort, says the key problem is pinning down the track of Pluto's shadow during the eclipse. ``We know the time to within 10 minutes, which determines the longitude,'' he says. ``But a 10-minute uncertainty in latitude means we could be off by 12,000 miles.'' Between now and the eclipse, he and his colleagues hope to refine that significantly.

As the star slips behind Pluto, astronomers will be watching to see how rapidly the starlight disappears. If Pluto has a dense atmosphere, the star's light will fade gradually. If there is little or no atmosphere, the star will vanish abruptly. The technique is also expected to yield valuable information about temperature and pressure.

Dr. Durham says the stellar occultation can also be used to ``nail down Pluto's diameter'' more accurately than relying solely on the mutual events between Charon and Pluto. The principle is the same - watch the change in the combined light as the star slips behind Pluto. But because the stellar occultation takes 10 minutes, versus a couple of hours for a Pluto-Charon eclipse, the resulting changes in light are much sharper, leading to more-precise calculations.

Another question that nags at some scientists is whether Charon is Pluto's only moon. All the other outer planets have several moons. Trafton says that the relatively small volume of space taken up by the Pluto-Charon system leaves room for more. He and other astronomers will be looking for additional moons.

Ultimately, many planetary scientists would like to see a probe sent to Pluto. ``There's so much we can learn and so much we can't'' from ground-based observations, says Tholen. ``There'll still be controversy surrounding Pluto after the last [Pluto-Charon] mutual event in 1990. The only way to settle it is to go there.''