NASA's first mission to the solar system's last planet is now in a race against time.
Tuesday, the clock starts as the New Horizons spacecraft prepares to hurtle from its Florida launchpad on a 10-year, 3 billion-mile journey to where the roan world of Pluto tracks a frigid course through the borders of the solar system.
No one knows if the planet's tenuous but intriguing atmosphere will even be there when the spacecraft arrives: It could freeze and fall out of the sky as the planet wheels away from the warmth of the sun on its oblong orbit. No one knows if the probe will be able to see the quarter of the planet that will be consigned to permanent shadow. And no one knows if Pluto will even be considered a planet.
What is certain, however, is that Pluto will draw back the curtain on a corner of the solar system only recently discovered and still barely understood. Just as the Voyager and Pioneer probes opened the giant planets beyond Jupiter to humankind, New Horizons will strike out into a mysterious realm where Pluto is not an odd runt of a world, but the king of perhaps hundreds of similar would-be planets.
"We've learned that the view of four inner rocky planets and four outer gas giants and one misfit Pluto is wrong," says Alan Stern, lead scientist for the mission. "Now we understand Pluto's context."
It turns out that Pluto is gatekeeper to the Kuiper Belt - a 2 billion-mile wide band of cosmic rubble beyond Neptune that scientists say will be an archaeological trove for New Horizons. Unlike most of the material in the solar system, which has been worked over by planetary formation, the odds and ends of the Kuiper Belt were too cold and distant to congeal into a large planet. As a result, Pluto and its Kuiper Belt brethren are "planetary embryos," says Dr. Stern, offering a view of planets "frozen at the mid-state of gestation."
If all goes well, scientists will get their first close-up look into this planetary kindergarten in the summer of 2015, when the golden bullet of New Horizons skips by Pluto on its way to other as-yet undetermined Kuiper Belt objects. If New Horizons fails to launch before Feb. 4, however, the probe will miss a rendezvous with Jupiter designed to give it a push, adding as many as five years to the mission.
In either case, the flyby will pass in a blink. Though scientists will observe the planet for five months, the richest data will come from the 12-hour window just before New Horizons skims the planet. At one point, just 6,800 miles above the planet, New Horizons will be close enough to see objects the size of a house.
For scientists who have pored over photos that show Pluto as only an indistinct blob, those 12 hours yield more than a lifetime of work has. Indeed, the most apt planetary symbol for Pluto could be a question mark. Some 76 years after the discovery of Pluto, what scientists know about it would fill only a few 3-by-5 notecards, Stern says.
What they do know about Pluto, though, portrays it as a curious place. The planet is tilted so that it spins on its side. When New Horizons arrives, everything below about 40 degrees south latitude will be in shadow.
Elsewhere on the surface, the sun rises in the west, casting little more than a feeble twilight over the landscape, with the moon Charon looming massive overhead. Despite being half the size of tiny Mercury, Pluto has at least three moons, and Charon is so large that it doesn't revolve around Pluto. Both actually revolve around a point in space between the two, making them a double planet.
Yet these peculiarities - and Pluto's off-kilter orbit - have long cast its planet status in doubt. When discovered in 1930, Pluto was a unique object, so it became a planet almost by default. Now, with scientists finding dozens of Pluto-like worlds in the thicket of the Kuiper Belt - including one almost certainly larger than Pluto - the very notion of what a planet is might change.
Either Pluto is not a planet, or it is an "ice dwarf" planet - dividing the solar system into the four inner rocky planets, the four middle gas giants, and perhaps hundreds of outer ice dwarfs, throwing grade-school science fairs into chaos. The International Astronomical Union is reviewing its definition to give the term some certainty.
Whatever the decision, Pluto certainly appears to be more than just an inert ball of ice. While Charon is a uniform dull gray, likely pitted by eons of meteor impacts, Pluto has wide swaths of light and dark terrain overlaid with an orange patina - most likely methane frost. Indeed, some scientists suggest that Pluto could have wind, weather, and even snow.
That Pluto has an atmosphere at all is among its greatest puzzles. Scientists know that it creates a sort of reverse greenhouse effect on Pluto, making the surface colder than it should be. Yet data suggest that it should simply disappear into space. Perhaps it is maintained by geysers spewing nitrogen, as is the case on Neptune's moon Triton. Perhaps the light areas of Pluto are evidence of volcanism that is spewing chemicals into the atmosphere, as is the case on Saturn's moon Enceladus.
"There's all this stuff going on there," says Dr. Marc Buie of Lowell Observatory in Flagstaff, Ariz. "It's that exploration of the unknown that is so fascinating."