Two estimates conflict on Charon's density.

Locked in a gravitational embrace 5.9 billion kilometers from the sun, Pluto and its moon, Charon, rank among the least explored bodies in the solar system. This week, astronomers reported two new estimates of the density of Charon.

Though the estimates differ -- one group calculates that Charon has about the same density as Pluto, while the other estimates it has significantly less -- the findings may ultimately shed light on the origin and composition of these distant denizens of our solar system.

George W. Null and his colleagues at NASA's Jet Propulsion Laboratory in Pasadena, Calif., base their findings on observations taken by the Hubble Space Telescope during four days in August 1991. Hubble resolved each member of the closely orbiting Pluto-Charon pair. Using the single star in their images as a reference point, the researchers tracked the center of mass of the Pluto-Charon system and calculated each body's mass and density.

Null's team, which includes William M. Owen Jr. and Stephen P. Synnott, puts Charon's density at 1.30 grams per cubic centimeter (gm/cm3), with an uncertainty of 0.23 gm/cm3. That number is about 60 percent of the density of Pluto. The astronomers presented their findings this week in Flagstaff, Ariz., at a conference on Pluto and Charon. They also detail their study in the June ASTRONOMICAL JOURNAL.

Another team, headed by Leslie A. Young of the Massachusetts Institute of Technology. calculates that Charon has a density similar to that of Pluto. The scientists observed the planet and its moon for six nights in early 1992, using the University of Hawaii's 2.2-meter telescope atop Mauna Kea. At the Flagstaff conference, Young and her colleagues reported that Charon has a density of 2.35 gm/cm3, with an uncertainty of 0.02 gm/ cm3.

Young notes that, unlike Hubble, their telescope could not separate the Pluto-Charon pair into its individual partners. However, she adds, the Hawaii telescope's larger field of view encompassed 10 stars, compared with Hubble% single reference star. Null says his team calculates that additional reference stars should make little difference in the accuracy of its density determination.

If Charon has a density similar to Pluto's - halfway between that of rock and ice - it would indicate that Charon is a mixture of the two kinds of materials, Young says. In contrast, she adds, Null's estimate suggests that Charon is mostly ice.

S. Alan Stern of the Southwest Research Institute in San Antonio, Texas, notes that if Charon is less dense than Pluto, it would support the theory that the satellite was created by a projectile that slammed into Pluto after the planet% denser material had sunk toward its core. The projectile would have gouged a Charon-size chunk from Pluto's low-density mantle, akin to the way astronomers believe our moon was gouged from Earth. He notes that if Pluto and Charon have the same density, a collision could still have created Charon, but it must have happened before the planet's denser material separated out.

Null and Young agree that more observations should settle the density debate. They won't have to wait long. This August, Null and his colleagues will again use Hubble to observe Pluto and Charon.