Dark, distant, named for the jealous Roman god of the dead and sheathed in frozen gases, Pluto has stalked the edge of the solar system for eons -- the only planet never visited by a spacecraft -- in an orbit around the sun that takes centuries to complete.
So tiny and strange is Pluto, smaller than Earth's moon and rotating at a weird angle, some scientists have questioned its long-held status as the ninth planet.
Now, after years of on-and-off effort, NASA has received preliminary funding from Congress for a mission to fling a spacecraft 3 billion miles or so to take a look.
The voyage is to take about 10 years.
"It's marvelous," said Andrew Cheng, 50, the mission's project scientist, of the Johns Hopkins University Applied Physics Laboratory in Laurel, Md. "It's about the most exciting thing that we've done in a long, long time."
The "New Horizons" mission, which would be managed by the Applied Physics Lab, also calls for the spacecraft to plunge beyond Pluto, into the recently discovered Kuiper Belt.
There, some scientists say,, resides the frozen refuse of the solar system's creation, which may include fledgling comets and weird "planetoids" as big as, or perhaps bigger than, Pluto.
The outer planet -- glimpsed through telescopes as a shadowy golden orb -- still is one of the solar system's great secrets, 71 years after its discovery by Clyde Tombaugh.
"This has been something we've worked on for 10 years," said Alan Stern, 44, the mission's lead investigator and director of the Southwest Research Institute's Department of Space Studies in Boulder, Colo. "When I started, I was 30 years old. This will be my life's work. This is my contribution. It's so gratifying that we're finally going to get to do this."
Congress provided $30 million for the next phase of study and development by the Applied Physics Lab -- where the spacecraft is to be built -- the Boulder institute and other agencies working on the project. NASA said further work on the $488 million enterprise is contingent on, among other things, continued funding from Congress.
But Stern vowed that the mission could be done successfully. "I know we know how to do it," he said. "We've been given the keys, at least temporarily."
2006 launch
The 1,000-pound New Horizons spacecraft, according to the plan, would be launched from Cape Canaveral atop a rocket in January 2006 and hurtle into space at about 70,000 mph.
The first leg of the trip, dubbed "Cruise 1," would take it to Jupiter, which it would reach in about 14 months.
Plans call for it to study the giant gas planet and perhaps one of its moons and then, using Jupiter's gravity as a booster, dash into "Cruise 2," the almost decade-long journey to Pluto. For much of that time, the spacecraft, with its onboard nuclear power source, would be in hibernation, being awakened for only about 50 days each year until it neared Pluto.
Back on Earth, the New Horizons team would stand down for much of the time. Some may retire, or even "expire," as mission director Robert Farquhar, 69, of the physics lab, joked.
"I'll be 65 when we get out there," Cheng said, "but hope to be going strong. I don't plan to retire then. ... It takes a long time. The solar system's a big place."
For Pluto, though, a decade is a trifle, a small fraction of an orbit so vast that the planet still has not completed the 248.5-year circuit it began when Benjamin Franklin flew kites in Philadelphia.
Pluto is so small and distant that it is now generally described as a "Kuiper Belt object," associated with the huge realm of recently discovered miniature planets or ice dwarfs that float beyond its orbit. Pluto and its comparatively large moon, Charon, named for the figure in classical mythology who ferried the dead over the River Styx, "are sort of the king and queen of the Kuiper Belt," Cheng said.
Some experts believe that Pluto no longer qualifies as a planet. Two years ago, New York's American Museum of Natural History opened a space center that excluded Pluto as a planet and listed a solar system family of just eight.
Uproar ensued.
"It is a silly debate," Cheng said. "We call it a planet. It's always been a planet, since it was discovered. Let's not waste time."
As the spacecraft neared Pluto and Charon, it would target the two with an array of cameras and sensors. Data and pictures should begin pouring back months before the craft makes its closest pass, 6,000 miles from Pluto, between 2016 and 2018.
When that happens, Stern said, "it'll just be breathtaking. We will, at closest approach, see objects the size of office buildings."
Back on Earth, if all goes well, aging scientists may be permitted a high-five or two, if they're still up to it.