updated 12/16/2010 3:48:16 PM ET 2010-12-16T20:48:16

A NASA Pluto probe may be slumbering at the moment, but it's still tearing through space at a blistering pace, closing in on the orbit of Uranus.

The New Horizons probe is the fastest spacecraft ever launched from Earth, having sped from its home planet in 2006 at about 36,000 mph (nearly 58,000 kph). It had covered half the distance of its nearly 3 billion-mile (4.8 billion-kilometers) voyage by last February, and the spacecraft should reach Pluto in July 2015.

Currently, New Horizons is about 18.5 times farther from the sun than the Earth is, and it should pass the orbit of Uranus in March 2011, NASA officials said.

Guided by recent Pluto revelations

The spacecraft is flying to study Pluto and its three known moons Nix, Hydra and Charon. In recent years, a number of revelations have come out regarding Pluto from the Hubble Space Telescope, such as the discovery of Nix and Hydra, as well as apparent geyser eruptions and seasonal color changes on the dwarf planet.

"These discoveries have helped develop our encounter with Pluto, which is now fully planned," said New Horizons principal investigator Alan Stern of the Southwest Research Institute in Boulder, Colo. "We have a list of things of do, which has been converted into a timeline of events, which has been converted into spacecraft software with all the commands to run the spacecraft and instruments."

New Horizons "is very different from most missions in the solar system today," Stern told "It's like we're back at the early days of planetary exploration."

Dozens of missions have been sent to Mars and the moon, and multiple spacecraft have checked out Venus, Jupiter, Mercury and Saturn, as well as various comets and asteroids. But faraway Pluto has escaped such attention thus far, Stern said, as has its neighborhood, the Kuiper Belt the ring of icy bodies beyond Neptune's orbit.

Shedding light on dark, cold worlds

"With New Horizons, this is our first reconnaissance of Pluto, of this kind of world we've never sent a mission to the dwarf planets before, never sent a mission to the Kuiper Belt," Stern said. "This is the first time we're going to see a new type of planet since the '70s, when we had our first mission to a giant planet, Jupiter."

Pluto and the Kuiper Belt remain mysterious in many ways, and New Horizons should help fill in some major gaps, Stern said.

"So we don't have a narrow scope here we're going to write the book on Pluto and the dwarf planets," he said. "We're here to map Pluto, map its surface composition, measure its atmospheric composition, pressure and temperature and assay the same kinds of measurements for all of its satellites."

Pluto was demoted from full-fledged planet to a newly created category, "dwarf planet," in 2006. Stern doesn't disagree with the "dwarf" designation, but he has argued that stripping Pluto of its planethood was wrong and unscientific. [ Fighting for Pluto's Planet Title: Q & A with Alan Stern ]

Probe sleeping, for now

New Horizons is hibernating now, as it does for most of the year. It wakes up for about two months each summer to test its systems, calibrate its instruments and gather tracking data needed to make course corrections as necessary, Stern said.

The spacecraft also rouses itself for about 10 days in both November and January to do more tracking and maintenance activities. So far, everything has checked out fine.

"New Horizons is healthy," Stern said. "All systems and instruments are working well, and we have never had a case where we've had to use a backup system owing to a problem. We have good fuel reserves, too, and we're bang on course to Pluto."

After it reaches Pluto, New Horizons will not stop its flight to orbit the dwarf planet. Instead, after its flyby, it will dash out into the Kuiper Belt to investigate the icy bodies lurking in that mysterious realm. The New Horizons team has not yet chosen which Kuiper Belt object they might visit after Pluto.

The search for candidates will start next year, Stern said. Scientists will use giant Earth-based telescopes to pick out flyby targets beyond Pluto, to be reached in the late 2010s and early 2020s.

"I want to know as much about what candidates are out there and as much about the candidates as possible before making the best possible decision," Stern said. "Making a decision too early would be like picking fruit that's not ripe yet."

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