Pluto, the space object formerly known as a planet, will get more attention than Britney's head Sunday morning when it briefly blocks out a star in the constellation Sagittarius.

Astronomers and planetary scientists — who will be swarming nearly all the major telescopes in Southern Arizona and at the Lowell Observatory in Flagstaff — hope it will tell them more about the atmosphere of the Guam of our solar system. It may also help members of NASA's New Horizons mission decide what to focus on when it passes by in 2015.

The event, called an "occultation"— don't call it an "eclipse," astronomers say — occurs when one space object passes between the viewer and another celestial body, blocking out its light.

Sunday's occultation — in this case Pluto blocking out the light of a star in the constellation Sagittarius — is expected to be visible for a few minutes in a narrow strip west from Texas to the California coast and south from Wyoming to somewhere in Mexico.

It's a big deal because "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 said he's been "studying Pluto since I was a grad student at the U of A in 1982."

When the nearer object, in this case Pluto, blots out the light, that closer object 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, if there is an atmosphere — and it's already known that Pluto has an atmosphere — the backlighting will "bend the light and go out slowly," McCarthy said.

And it not only goes out more slowly, showing that there is an atmosphere, but the way the light is bent and filtered can tell something about the composition of the atmosphere.

It was through an occultation in 1988 that Buie said much was learned about Pluto's atmosphere.

Then, Pluto scientists had to wait 14 years — until the next occultation in 2002 — to find that the atmosphere had changed drastically. "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."

That's not the only pressure involved, McCarthy said.

"I'm pretty nervous about it," McCarthy said. "The shadow only comes over for six minutes.

"You have to be pointing your telescope in the right place at the right time with everything functioning. It's 4 a.m. Sunday morning and Pluto is only 20 degrees above the horizon. That's low.

"You know how stars twinkle more when you look at them just above the horizon? (It's) because there's more" atmosphere to look through.

He and a team of astronomers will be using the university's and the Smithsonian Institution's 6.5-meter telescope at the MMT Observatory on Mount Hopkins in the Santa Rita Mountains.

McCarthy said they won't know whether they've captured good images until sometime later.

They'll be too busy snapping 10 frames a second on two cameras, one operating in the visible part of the spectrum and one in the infrared part.

"I think we have the second-largest telescope in Southern Arizona to look at this thing," he said.

"I don't want to screw it up because I'm on a large telescope and this is an Arizona tradition," a reference to Pluto's Arizona connection since its discovery by a Flagstaff astronomer in 1930.

The Massachusetts Institute of Technology's occultation crew has reportedly predicted that the event won't even be visible here. Buie and McCarthy have heard that. But they also have heard that the MIT observers, regular attendees at occultations worldwide, have packed their portable observation equipment and plan to be in Southern Arizona Sunday morning.

"Healthy skepticism abounds," Buie said, referring to the predictions of whether the Plutonic shadow will be cast over Arizona.

"Predicting where Pluto is going to be relative to that star with any accuracy is probably one of the hardest tasks out there in calculating occultations. ...

"One of the things that makes this so amazing is it's crossing one of the parts of the world that probably has more telescopes per square mile than anyplace in the world," Buie said.

"It doesn't happen (near observatories) very often. Normally, we have to travel. You just really can't chase this down with those small telescopes" used when they've chased occultations observable only from Mexico or other areas without major permanent telescopes. "We're going where the big telescopes are," he said. "That's why MIT, which has the prediction that it's not going to happen here, they're going to be here."

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Copyright 2007 Arizona Daily Star