This Sunday, Pluto's shadow will be visible in Texas and California as it passes in front of a faint star. This is the first time such an occultation has been seen from North America. Naturally, astronomers are ready for this and will study how the star's light disappears and reappears to gain insight into why Pluto's atmosphere appears to be changing. (Pluto and moon Charon pictured, courtesy of NASA).
It was during a similar occultation, on June 9, 1988, that Pluto was found to be surrounded by tenuous wisps of nitrogen with an estimated surface pressure of just 50 microbars -- less than 1/100,000 the sea-level pressure on Earth.
But two subsequent occultations, witnessed in 2003, showed that the atmosphere is changing and perhaps getting colder. Pluto passed through perihelion, its closest to the Sun, in 1989, and theorists expect much of the gas to freeze out onto the supercold surface as the Sun becomes more distant and the temperature drops in the years ahead. "Occultations are the only way we can monitor the atmosphere of Pluto from Earth," notes William B. Hubbard (University of Arizona).
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Oh, man. It is going to be so awesome when New Horizons finally arrives and we get to make some direct observations of Pluto and Charon. It's just too bad we'll have to wait 8 years(!) for the probe to get out there.