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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

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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.


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