SEATTLE — NASA’s New Horizons probe marked six years of spaceflight Jan. 19, beginning the last leg of its journey to the small, faraway world of Pluto.

New Horizons 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.”

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 said in an interview. “We’re really in the final stages.”

The flight control center for the $650 million NASA Discovery-class mission is located at the Johns Hopkins University Applied Physics Laboratory, the Laurel, Md.-based lab that partnered with the Southwest Research Institute to build the plutonium-powered spacecraft.

Late cruise is expected to 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 closest approach will occur that July, when New Horizons comes within about 9,600 kilometers of Pluto.

During the flyby, New Horizons will study Pluto and its moons Charon, Nix, Hydra and S/2011 P1 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.

Pluto is found in the Kuiper Belt, the ring of icy objects beyond Neptune’s orbit. As of Jan. 19, New Horizons had traveled about 3.45 billion kilometers, with roughly another 1.6 billion kilometers left to go before the close encounter.

The probe’s work will not 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 55,404 kilometers per hour relative to the sun.

While New Horizons spends most of its cruise time hibernating, it is 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.The excitement of the New Horizons team has been tempered with some sadness as the scientists mourn the Jan. 12 death of Patsy Tombaugh, 99, the widow of Pluto’s discoverer.

Pluto was discovered in 1930 by American astronomer Clyde Tombaugh, who died in 1997.

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

“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 will 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.”