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Pluto's atmosphere is here today, gone tomorrow.

New Horizons Catches a Break

BOSTON--A NASA spacecraft is speeding toward Pluto for humanity's first close encounter with this planet, Kuiper Belt object, or dwarf planet--depending on your terminology. All the while, astronomers have been worrying: Will Pluto's tenuous atmosphere still be there when New Horizons arrives in 2015?

Pluto's atmosphere is a strange duck. Made up primarily of methane and nitrogen, it's present only when Pluto is close to the sun. As Pluto moves away from our star--as it has been doing since 1989--the temperature of its atmosphere drops. Eventually, it gets so cold that the methane and nitrogen solidify and fall to the surface as ice and snow.

That hasn't happened yet, and it likely won't before New Horizons gets there, astronomer Jay Pasachoff of Williams College in Williamstown, Massachusetts, told ScienceNOW during the American Association for the Advancement of Science (which publishes ScienceNOW) meeting here. Observations made in 2002 by astronomers at Williams and at the Massachusetts Institute of Technology in Cambridge found that Pluto's atmosphere was still expanding, even as the object sped away from the sun. What's more, Pasachoff says, data gathered during an occultation--when Pluto slides in front of a star, allowing astronomers to briefly examine the state of the atmosphere--last year show that the atmosphere's temperature remains steady. That means the cooling is proceeding much slower than some scientists had anticipated.

NASA science chief S. Alan Stern, who also is principal investigator of the New Horizons spacecraft mission, likens the situation to the warmth that increases and then lingers following the summer solstice on Earth. "The atmosphere certainly will be there for New Horizons," he says. That should allow the spacecraft to take detailed measurements.

Figuring out exactly when Pluto's atmosphere will collapse is going to require more observations. The next data are due from Pasachoff's team--one of three monitoring Pluto's atmosphere--in June, when an occultation of a particularly bright star by Pluto will be observable in southeastern Australia.

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