Pluto occults star P131.1

On August 21, 2003, Pluto drifted across a faint star in southern Ophiuchus. This series, taken with the University of Hawaii's 2.2-meter telescope on Mauna Kea, shows the motion of Pluto and Charon, its largest moon, with respect to the 16th-magnitude star known as P131.1.

Courtesy Jay M. Pasachoff, Bryce Babcock, David Ticehurst, and Steven Souza (Williams College).

Astronomers from California to Texas will be up early on Sunday morning, March 18th, when Pluto sweeps directly in front of a faint star. It's the first time an occultation by this distant world has been observable from North America. By carefully monitoring how the star's light disappears and reappears, researchers hope to gain insight into why Pluto's atmosphere seems to be changing.

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).

The results obtained this weekend should prove especially useful to mission planners for New Horizons, a NASA spacecraft scheduled to reach Pluto in July 2015.

In the predawn hours of March 18th, Pluto's shadow will sweep over major observatories (red stars) throughout western North America. Telescopes there will see the distant world cover a 15th-magnitude star for up to 6 minutes. Dots along the track's centerline are 1 minute apart. Click on map for a more detailed view.

Kelly Beatty

Sunday's cover-up involves a reddish 15th-magnitude star in Sagittarius that's been designated P445.3 by the occultation teams. Along the event's centerline, which runs from central California through Kansas, observers should see the star wink out for up to 6 minutes. Locations farther east will be compromised by twilight or daylight. This event puts some of astronomers' most powerful telescopes — among them Palomar Observatory in California, Kitt Peak in Arizona, and McDonald in Texas &mdash in Pluto's shadow.

But professional astronomers aren't the only ones readying for the event. Amateur observers equipped with 10-inch or larger telescopes and a sensitive detector have a shot of recording the brief disappearance of P445.3. For more details, see the Sky & Telescope AstroAlert issued on March 15th.

David Dunham sends an update March 17th: "For much more information, see my website at"


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