NASA / Johns Hopkins University Applied Physics Laboratory / Southwest Research Institute
An artist's concept of the New Horizons spacecraft as it visits Pluto in 2015. Instruments will map Pluto and its moons, providing detail not only on the surface of the dwarf planet, but also about its shape, which could reveal whether or not an ocean lies beneath the ice.
updated 1/19/2012 5:42:23 PM ET 2012-01-19T22:42:23

A super-speedy NASA probe marks six years of spaceflight Thursday, beginning the last leg of its journey to the small, faraway world of Pluto.

The New Horizons spacecraft launched Jan. 19, 2006, on a mission to become the first probe to visit the dwarf planet Pluto and its moons. That unprecedented encounter is slated to begin in January 2015, so New Horizons has now entered the home stretch of its nine-year trip, researchers said.

"It's really around the corner," said New Horizons principal investigator Alan Stern of the Southwest Research Institute in Boulder, Colo. "We're just more and more excited."

Entering 'late cruise' phase
The New Horizons team breaks the spacecraft's flight to Pluto into three three-year segments, Stern said — early cruise, mid-cruise and late cruise.

"We are now turning the corner from mid-cruise to late cruise," Stern told "We're really in the final stages."

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Late cruise should be a busy time for mission scientists and engineers, as they check out the spacecraft's systems and prepare for the flyby of Pluto and its four known moons. That encounter technically begins in January 2015, Stern said, though the closest approach will occur that July, when New Horizons comes within about 6,000 miles (9,600 kilometers) of Pluto.

During the flyby, New Horizons will study Pluto and its moons with seven different instruments, performing the first in-depth reconnaissance of these frigid, far-flung objects.

In fact, researchers have said, the mission will give scientists their first good look at any dwarf planet — a class of bodies suspected to be far more numerous in our solar system than terrestrial and giant planets combined.

1 billion miles to go
Pluto is found in the Kuiper Belt, the ring of icy objects beyond Neptune's orbit. As of today, New Horizons has put about 2.14 billion miles (3.45 billion km) on its odometer, with roughly another 1 billion miles (1.6 billion km) left to go before the close encounter.

The probe's work won't be done after it flies by the Pluto system in 2015. The mission team wants New Horizons to study one or two other Kuiper Belt objects as well.

NASA has billed New Horizons as the fastest spacecraft ever launched from Earth. According to the mission team, the probe is now speeding through space at 34,426 mph (55,404 kph) relative to the sun.

While New Horizons spends most of its cruise time hibernating, it's awake for now. Scientists and engineers are performing various tests on the spacecraft throughout January, Stern said, adding that the spacecraft is in good health.

'An American story'
The excitement of the New Horizons team has been tempered with some sadness this month, as the scientists mourn the death of Patsy Tombaugh, the widow of Pluto's discoverer.

Pluto was discovered in 1930 by American astronomer Clyde Tombaugh, who died in 1997. Patsy passed away on Jan. 12 at the age of 99.

Patsy Tombaugh was very enthusiastic about the New Horizons mission, and the team will miss her a great deal, Stern said.

ESO / L. Calçada
An artist’s impression of how the surface of Pluto might look. The image shows patches of pure methane on the surface.

"She was such a wonderful woman," Stern said. "It was sad to see her pass without getting to see what her husband's planet really looked like."

But the Tombaughs' two children, Annette and Alden, should get to see what New Horizons discovers. They'll be the mission team's guests of honor when the probe makes its closest approach to Pluto in July 2015, Stern said.

On top of its technical and scientific ambitions, New Horizons is also part of "a very personal story, an American story," Stern said. "I think that just makes it nice."

You can follow senior writer Mike Wall on Twitter:@michaeldwall. Follow for the latest in space science and exploration news on Twitter @Spacedotcom and on Facebook.

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