WHILE many scientists prepare for Voyager 2's Neptune encounter next month, a few dedicated observers have their eyes on Pluto. This smallest of planets reaches the perihelion of its orbit - the point closest to the sun - Sept. 5. It won't be there again until the year 2236.
It was closest to Earth May 4, a mere 4.3 billion kilometers (2.7 billion miles) away. This close-in swing brings Pluto 90 million km (56 million miles) inside Neptune's orbit. Until 1999, Neptune, not Pluto, is the most distant of the nine planets.
Scientists have been making the most of the opportunity. They are learning more about Pluto in a few years than was learned in all the preceding decades since Clyde Tombaugh discovered it in 1930. Among other things, their studies appear to have settled the old question of whether Pluto is a planet or an escaped moon of Neptune. It's a planet.
Also, some experts are roughing out maps of Pluto and its moon Charon. They hope to produce maps of at least a hemisphere on these bodies with the degree of detail we see on our own moon with the naked eye. As Richard P. Binzel of the Massachusetts Institute of Technology puts it, the maps may have about as much detail as Walt Disney sketched on Pluto, the cartoon dog.
A second rare event aids the research. Since December 1984, Pluto and Charon have been partially or fully eclipsing each other as Charon orbits Pluto. This will continue through October 1990 and won't recur for 124 years.
These eclipse studies enable analysts to refine the estimates of the sizes and masses of Pluto and Charon. The changes in surface brightness of each body, as it is eclipsed by the other, and changes in surface coloration provide data for the map makers. Dr. Binzel has also found signs of polar caps of frozen methane in the color data.
Best estimates now give Pluto's diameter as 2,284 km (1,419 miles) and Charon's diameter as 1,192 km (741 miles). They put Charon's orbital radius at 19,640 km (12,204 miles), making this one of the most tightly bound planet-moon pairs in the solar system. Charon orbits Pluto once every 6.4 days. Not only does it always present the same face to Pluto - as our moon does to Earth - but Pluto also always presents the same hemisphere to Charon. In other words, Charon remains more or less fixed in Pluto's sky.
Last September, William B. McKinnon of Washington University, St. Louis, and Steve Mueller of Southern Methodist University explained in the journal Nature that Pluto probably is three-fourths rock and one-fourth water ice with a little methane. Its density is between 1.84 and 2.14 times that of water.
That's more dense than the large icy moons of Jupiter and Saturn. So Pluto is unlikely to be an escaped moon of Neptune. But Pluto data do fit the computer model, developed by McKinnon and Mueller, that describes the structure of a rocky body formed directly from the primordial solar nebula.
Pluto, these scientists observe, ``is a new kind of world'' for astronomers to explore. It's intermediate between asteroids and planets. It is probably left over from the earliest days of the solar system. Detailed study of it would undoubtedly help scientists understand the planet-making process.
But Pluto is not on any space agency's planet itinerary. Voyager's Neptune flyby next month will be the last visit to an outer planet for an indefinitely long time. This makes Pluto's present orbital position a precious event for scientists. They won't have another such opportunity for Earth-based observations until the 23rd century.