Astronomers swarm southern Ariz. to watch as Pluto blots out star

By: Associated Press | Saturday, March 17, 2007 6:54 PM PDT ∞

TUCSON, Ariz. -- Swarms of astronomers are expected to pack major observatories in Arizona this weekend hoping to see a rare "occultation" as Pluto crosses in front of a star and blots out its light.

Sunday morning's event is exciting for scientists because it will give them a better idea of the size and makeup of Pluto's atmosphere.

In an occultation -- not an eclipse, mind you -- the nearer object blots out the light and is backlit. If there is no atmosphere, it will blink out almost instantly, said Don McCarthy of the University of Arizona's Steward Observatory.

But in an object with an atmosphere, the backlighting will "bend the light and go out slowly," McCarthy said. How that light is bent and filtered can tell something about the composition of the atmosphere.

"The only thing better than an occultation is going there. But it's a heck of a lot cheaper to wait for one of these things to happen than send a spacecraft out there," said Lowell Observatory's Marc Buie, who has been studying Pluto since he was a University of Arizona graduate student in 1982.

A 1988 occultation taught scientists much about Pluto's atmosphere. The next event, 14 years later in 2002, astonished viewers.

"It had puffed up three times as much gas," Buie said. "Here (on Earth) the atmosphere and temperature is so stable. We suffer a variation of a few percent as storms come and go."

The event where Pluto blocks the light from a star in the constellation Sagittarius will only last a few minutes. It is only expected to be visible with telescopes in a narrow strip west from Texas to the California coast and south from Wyoming to somewhere in Mexico.

In addition to the Lowell Observatory in Flagstaff and the Steward Observatory's telescopes, other teams will be using the Smithsonian Institution's 6.5-meter telescope at the MMT Observatory on Mount Hopkins.

A team from the Massachusetts Institute of Technology has reportedly predicted that the event won't even be visible in Arizona, but Buie and McCarthy say they've also heard that the MIT observers have packed their portable observation equipment and plan to be in southern Arizona on Sunday.

"Healthy skepticism abounds," Buie said of the expected occultation.

On the Net:

University of Arizona astronomy department: www.as.arizona.edu/

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