LYCOS RETRIEVER 
Charon: Pluto-Charon
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Astronomers announced last week that they used a stellar occultation to measure the size of Charon, Pluto's largest moon, and confirm that the world does not have an atmosphere. Several teams of astronomers in South America observed Charon as it passed in front of a distant star on July 10, 2005, temporarily blocking the light from the star. Analysis of those observations found that the moon has a radius of 606 kilometers, with a margin of error of only eight kilometers. The lightcurve from the occultation ... led researchers to put an upper limit of one microbar on the surface pressure of any atmosphere, consistent with expectations that the moon did not have any substantial atmosphere. The results add additional credence to the hypothesis that the Pluto-Charon system formed through the collision early in the history of the solar system. The results were published in the January 5 issue of the journal Nature.
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Any final doubts were erased when Pluto and Charon entered a five-year period of mutual eclipses between 1985 and 1990. This occurs when the Pluto-Charon orbital plane is edge-on as seen from Earth, which only happens at two intervals in Pluto's 248-year orbital period. It was fortuitous good luck that one of these intervals happened to occur so soon after Charon's discovery.
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More information and images of Pluto, Charon, and the Edgeworth-Kuiper Belt are available at NASA's Pluto-Kuiper Belt Mission and Planetary Photojournal. Fact sheets on Pluto and the Centaur object Chiron are ... available from NASA's National Space Science Data Center.
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The center of mass (barycenter) of the Pluto-Charon system lies outside either body. Since neither object truly rotates around the other, and they are comparable in terms of mass, it has been argued that Charon should not be considered to be a satellite of Pluto. Instead, it has been suggested that they form dual dwarf planets, following the re-classification of Pluto.
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