National Geographic News
Photo: Dwarf planet Pluto and its moons

Pluto and its three known moons—Charon, Nix, and Hydra—as seen by the Hubble Space Telescope.

Photograph courtesy NASA/ESA/H. Weaver (JHU/APL)/A. Stern (SwRI)/HST Pluto Companion Search Team

Brian Handwerk

for National Geographic News

Published June 22, 2011

On Thursday tiny Pluto will cause a bright star to fade in the nighttime sky, and an army of astronomers is fanning out across the Pacific to capture the rare event.

The unprecedented sky show involves what scientists call an occultation—when an object passes directly in front of a star, as seen from Earth, causing the star to dim temporarily.

Starting at 11:15 UT on June 23, Pluto and its largest moon Charon will both occult a very bright star. Just a few days later, beginning at 14:18 UT on June 27, Pluto and its smaller moon Hydra will each pass in front of a different bright star.

"We've never had an event like this one we're seeing now," said team member Leslie Young of the Southwest Research Institute in Boulder, Colorado. "We're getting two bright stars, both brighter than Pluto itself—as seen from Earth—occulting Pluto just about four days apart."

The pair of unusual celestial events will provide astronomers with a Pluto "weather report" and may shed light on other mysteries ahead of a visit to the dwarf planet in 2015 by NASA's New Horizons spacecraft.

For instance, by reading the starlight that filters through Pluto's atmosphere during the occultations, scientists will be able to take more exact measurements of Pluto's size and the temperature and density of its atmosphere.

Amateur stargazers across Australia and the Pacific will also be scanning the skies during both events, and Young says their data will be welcome.

The project's wiki planning page features finder charts, schedules, local occultation timings, and other useful information for those hoping to take in a truly historic view of Pluto and its moons.

"All you really need is an 11-inch telescope with video capability and some way of timing exactly when things happen," Young said.

Double Pluto Events Like "Christmas in June"

Occultations of Pluto were very rare before 2002. But since then Pluto has moved in its tilted orbit so that it's now entering the star-rich central plane of our Milky Way galaxy.

That means Pluto has been been passing between Earth and other stars more often.

Still, an occultation with a particularly bright background star happens, on average, just once every two or three years, and time is running out to learn as much as possible about Pluto before the New Horizons spacecraft arrives in the dwarf planet's home region, the Kuiper belt.

"It's really Christmas in June, the totality of this event," said Alan Stern, principal investigator of the New Horizons mission.

The hitch is that there's always the chance a patch of bad weather could obscure the occultation entirely.

It's also key that at least some of the observers get the best possible vantage point, which is found within the narrow shadow cast on Earth by Pluto and its moons.

"We don't know Pluto's position in the sky perfectly, or the star's position in the sky perfectly, so there is always some uncertainty about exactly where the shadow will cross," team member Young explained.

To make sure they get what they need from the occultations, Young and colleagues have coordinated a massive campaign that's sent astronomers and mobile observatories to several exotic locales.

Team members are even now setting up base camps in eastern Pacific sites such as Hawaii, California, and Mexico, as well as in western Pacific countries including the Marshall Islands, Nauru, Indonesia, and the Philippines.

The dozen or so telescopes out in the field includes two 14-inch mobile units funded by the National Geographic Society's Committee for Research and Exploration, which are stationed in Nauru and the Marshall Islands. (The National Geographic Society owns National Geographic News.)

"We're going to countries that we've never been to before and working out practicalities—from customs to connectivity of telephones and the Internet—and trying to find out how it works to observe in those countries," Young said.

"But we work with local observers, both professionals and amateurs, and that helps a lot."

(Get daily field updates and pictures from team members in the Philippines and the Marshall Islands.)

Getting Pluto's Weather Report

If all goes as planned, the team will come home with valuable new data on the small and largely mysterious world.

"Whenever you have a bright-star [occultation], you get a detailed look at the weather report on Pluto," Young said. "We can sample the atmosphere at the scale of a kilometer or better and get details of waves on Pluto, the jet stream on Pluto, and turbulence on Pluto, which is astonishing."

The dwarf planet Pluto and its moons have been moving away from the sun since 1989 on their 248-year orbit. Teams should therefore be able to see any seasonal changes in atmospheric pressure and temperature that have occurred since previous observations.

"We expect Pluto's atmosphere to change as it moves away from the sun," Young said.

"It's made of nitrogen, just like Earth's. But unlike Earth, Pluto is so cold that nitrogen is also frozen on the surface. So eventually, as Pluto moves away from the sun, the atmosphere will cool down and snow out, and it will be ten or a hundred or perhaps a thousand times less dense than it is now."

(Related: "Pluto Has 'Upside Down' Atmosphere.")

Because two occultations are occurring one after the other, scientists will also have an unprecedented opportunity to see what a day looks like in the life of Pluto.

"Pluto's day is 6.5 times longer than the Earth day, so with occultations just about four Earth days apart, we can see how the dynamics of weather on Pluto change from one part of the day to the next," Young said.

Astronomers Jane Greaves of the University of St. Andrews in the U.K. said she is excited about the occultation because the bright starlight could reveal more about the thin upper layers of Pluto's atmosphere.

"Occultations over the past couple of decades have been mysterious, with [Pluto's] atmosphere sometimes staying stable and sometimes swelling up for no obvious reason," said Greaves, who is not part of the current occultation field team.

"Adding to this sequence will hopefully help show if and why Pluto has such erratic weather."

(Related: "Pluto Has Toxic Carbon Monoxide in Its Atmosphere.")

Pluto's Orbital Mysteries

The June 23 occultation will also reveal precisely where Pluto and Charon are relative to one another in the sky.

The data should help astronomers solve an orbital mystery that was revealed by the discovery of the two smaller moons Nix and Hydra almost six years ago.

"We once thought that Pluto and Charon orbited in simple ellipses. But, with the discovery of the small moons, we found they all pull on each other and affect each other in subtle ways," Young said.

Pinpointing the orbits more exactly will reveal the objects' masses—and will help astronomers plan observation strategies for the New Horizons spacecraft.

"Knowing the orbits better will help us refine where we aim our cameras—you don't want an image with half of an object out of the frame," explained NASA's Stern.

"Knowing the orbits more accurately will also make us more efficient, so we have time to do more things—and we're only going to be there once."

(Related: "Pluto is the Biggest Dwarf Planet After All?")

The June 27 Hydra occultation is especially exciting, team member Young added, because the small moon has been an elusive target for the past six years. With this round of observing, her team is hopeful that they'll finally be able to pinpoint Hydra's size.

"It's been very hard to do, because until recently we didn't know the orbit of Hydra relative to Pluto very well, and because the object is so small that we have a very small chance of being in Hydra's shadow" to observe it from the proper place on Earth, Young said.

Recent work with Hubble Space Telescope images has narrowed Hydra's position relative to Pluto to within tens of kilometers. With that information in hand, observations from the June 23 occultation can be analyzed in time to pinpoint where in Australia to place a telescope to be in Hydra's shadow four days later.

"By the time [astronomer] Marc Buie starts driving out of Alice Springs two days before the second occultation, we'll be able to tell him and his army of amateurs where to go."

Even with all the planning, NASA's Stern added, part of the allure is that no one's quite sure exactly what they'll see during the Pluto occultations.

"That's the great thing about astronomy," Stern said. "Sometimes when you unwrap a present, you really get a surprise gift."

(The Pluto occultation research is supported by the National Geographic Society's Committee for Research and Exploration, NASA's Planetary Astronomy program, NASA's New Horizons Mission, and the Southwest Research Institute.)

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