Mission to Pluto edges closer to a 2006 liftoff

Go-ahead to complete a design study means exploration of the solar system's most remote planet is back on the drawing board - for now

Alan Stern hunts for ancient relics.

No picks, shovels, or dusting brushes fill his tool kit, however. His lost city of Troy is Pluto - the only planet in the solar system that a spacecraft has yet to visit.

Little wonder then that when the National Aeronautics and Space Administration recently hired his team to help flesh out its concept for a mission to Pluto, Dr. Stern was delighted.

"It's the dream of a lifetime," says Stern, lead scientist for the Pluto-Kuiper Belt Mission, which he describes as "the first mission to the solar system's last planet."

Since its discovery in 1930, Pluto has been "an enigmatic spit of an ice ball" in the solar system's planetary line-up, says Stern, director of the space-science program at the Southwest Research Institute in Boulder, Colo. But Pluto's importance rose dramatically after 1992, with the discovery of a belt of planet wannabes beyond Neptune called the Kuiper Belt.

Pluto and its moon, Charon, are the largest objects in the Kuiper Belt region, which scientists believe served as the nursery for fledgling planets. In this area, planets still in the formation stages sometimes smashed together in spectacular collisions, halting their "normal" path of development. Indeed, says Stern, Pluto and Charon are viewed as chunks of a larger planet whose growth ended when it collided with another large body.

In Pluto and the Kuiper Belt, Stern says, "we have this equivalent of an archaeological dig into the history of the solar system." It's the "only place we have," he adds, that can give scientists a window on that period of time.

Beyond Pluto's role as a planetary pot shard, its scientific allure stems from the fact that so little is known about its basic physical characteristics compared with the rest of the solar system's major bodies. Charon was discovered only in 1978, allowing researchers to get a better handle on the mass and density of the Pluto-Charon system.

Powerful lens, a distant surface

Seen from Earth, Charon and Pluto appear to sometimes hide each other as they orbit. By carefully tracking these disappearances and reappearances, or occultation, astronomers have been able to map rough differences in brightness across the faces of both bodies.

Using the Hubble Space Telescope and ground-based observatories, "we're starting to get enough information to point to a particular place on the surface and detect differences," says Marc Buie, an astronomer at the Lowell Observatory in Flagstaff, Ariz., where Pluto was discovered. For example, observations have pinpointed a spot on the surface unusually rich in carbon-monoxide ice.

This could be a carbon-monoxide deposit bared by an impact crater, he says, or the result of cryovulcanism - the frosty equivalent of volcanic activity on Earth.

"What the heck is going on at the surface? There's no way of knowing without a mission to the planet," he says.

As currently envisioned, the Pluto-Kuiper Belt mission would launch in 2006 and fly by Pluto and Charon before 2020. Timing is important, because planetary alignments will be most favorable during this period for using Jupiter's gravity to sling the spacecraft toward Pluto.

The timing also is urgent because Pluto is heading for the solar system's outskirts. The planet's orbit traces an oval around the sun, and Pluto has just finished its closest approach. Astronomers estimate that by 2020, much of the planet's atmosphere will have frozen and fallen to the surface, preventing scientists from getting a handle on Pluto's tenuous envelope of gas and how it interacts with the solar wind, a stream of charged particles constantly flowing from the sun.

"This is a very ambitious mission," acknowledges Colleen Hartman, director of NASA's solar-system exploration division.

For all the scientific interest, the mission's finances may be as ephemeral as Pluto's atmosphere. The project is funded only through the end of the current $30 million mission-design study, a condition that Dr. Hartman says "is very unusual for a planetary mission."

Until November 2000, CalTech's Jet Propulsion Laboratory (JPL) in Pasadena spearheaded the project. But cost estimates ballooned to nearly $1 billion, threatening to become one of the Battlestar Galacticas that former NASA administrator Daniel Goldin fought to ground in favor of smaller, more frequent missions.

Last fall, NASA directed JPL to stop work on the project. When President Bush submitted his fiscal 2002 budget, a mission to Pluto was nowhere to be found.

Saved by a science buff

But a trip to Pluto quickly became the people's mission. When the project was canceled, Stern says, "a 19-year-old space buff set up a website and triggered an avalanche of 10,000 letters to Congress in two weeks flat."

Groups such as the Planetary Society, headquartered in Pasadena, threw their weight behind a Pluto mission. When the smoke of budget battle cleared, Congress had added $30 million to NASA's budget to keep a Pluto mission alive.

But it's a one-year reprieve. The project's progress beyond blueprints hinges on a favorable review of the design, regulatory approval to use plutonium-driven generators for electrical power, and availability of money.

If planners must go back to Capitol Hill, Stern says, they will be armed. This week, a NASA planetary science advisory committee said that a Pluto mission should be given the highest priority in NASA's planetary exploration program.

"The space program been lacking the kind of first-time exploration that was popular with Apollo, Voyager, and Viking," Stern says, adding that the Pluto mission can fill that void.

The world of Pluto

One trip around the sun takes Pluto 248 years; a season lasts 62 years.

Pluto is half the size of the second-smallest planet, Mercury. Pluto's moon, Charon, is half the size of the planet.

Pluto's composition is unknown, but its density indicates that it is probably a mixture of 70 percent rock and 30 percent water ice.

Pluto and Mercury are the only planets that have elliptical, not circular, orbits.

In Roman mythology, Pluto is the god of the underworld. The planet got its name - after suggestions such as Atlas, Cronus, Minerva, Artemis, Vulcan, and Perseus - perhaps because it's so far from the sun that it is in perpetual darkness.

At Pluto's farthest point from the sun, sunlight takes seven hours to travel the 4.6 billion miles. Sunlight reaches Earth in eight minutes.

Scientists estimate surface temperatures on Pluto can reach minus 400 degrees F.

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