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Eclipses by and of Pluto's moon.

It was nearly seven years ago that James W. Christy of the Washington, D.C.-based U.S. Naval Observatory, looking at images of Pluto made through the USNO's big astrometric telescope in Flagstaff, Ariz., noted a large bulge on one side of the planet. Too far from earth to be photographed clearly, the bulge seemed much too high to be a mountain. Christy, together with colleague Robert Harrington, concluded that the images were almost surely showing Pluto to have a moon, circling the planet about every 6.4 days (SN: 7/15/78, p. 36). Yet not until now have astronomers been able to detect either object passing in front of the other -- eclipses.

Eclipses are important, because in distant Pluto's case, the images -- which by now have been made with a charge-coupled-device camera, speckle interferometry and other techniques--still leave a lot to be desired, so that even the objects' sizes and the separation between them remain uncertain. Eclipses, monitored with photometers rather than imaging systems, offer the chance to see how much the combined light reflected from planet and moon decreases as the satellite either blocks out a part of Pluto's surface or disappears behind it. An analysis of the changing "light curves" over a period of time, combined with spectral measurements that have indicated the presence of methane ice or frost (which can mean that smaller objects reflect a given amount of light), should greatly improve the calculations.

To observers on earth, however, Pluto and its moon--unofficially named Charon by Christy--eclipse each other only about every 124 years. This is because Pluto takes about 248 years to circle the sun, and Charon's orbit is so steeply inclined relative to earth's (about 112[deg.]) that eclipses can be seen only during the two occasions per 248 years when earth is at the intersection of the two orbit planes. This should enable observations over perhaps three to four years at a time, after which the chance is lost for more than a century. But one of those viewing opportunities--sought unsuccessfully by astronomers since 1982--seems to have just begun.

On Jan. 16, Edward Tedesco at the Palomar Observatory in California detected a dip in the light curve suggesting that Charon was grazingly passing in front of Pluto, but instrumental difficulties and other factors left the results uncertain. On Feb. 17, however, Richard Binzel at the University of Texas McDonald Observatory caught it clearly. Three days later another dip -- this time apparently from Charon's passing behind Pluto -- was detected from Mauna Kea Observatory by David Tholen of the University of Hawaii.

When the objects' diameters are better refined from additional observations, their calculated masses can be figured in to indicate their densities--a key clue to what they are made of. Some researchers have already speculated that Pluto and Charon my essentially be balls of ice (possibly frozen methane, even less dense than water ice). Ice is thought to have been a readily available structural material in the outer solar system and believed at the heart of most comets. In addition, the light curves' aid in establishing an orbit for Charon may win formal acknowledgment from the international Astronomical Union that Pluto's moon is now officially there.
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Title Annotation:unofficially named Charon
Author:Eberhart, Jonathan
Publication:Science News
Date:Mar 2, 1985
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