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Its target is Pluto — the solar system's last unexplored planet, 3 billion miles from Earth.

And the New Horizons spacecraft could reach it within nine years.

Pluto, a tiny, icy misfit of a planet — some say it's not a planet at all — neither resembles the rocky bodies of Mercury, Venus, Earth and Mars, nor the giant gaseous planets of Jupiter, Saturn, Uranus and Neptune. For years after its discovery 75 years ago, it was considered a planetary oddball.

But in recent years, astronomers have come to realize that Pluto's class of planetary bodies, ice dwarfs, isn't so odd after all. In fact, ice dwarfs are the most populous group in the solar system. Now, scientists have a chance to learn more about them and the origins of the planetary system.

"Just as a Chihuahua is still a dog, these ice dwarfs are still planetary bodies," said Alan Stern of the Southwest Research Institute in Boulder, Colorado, the mission's principal investigator. "The misfit becomes the average. The Pluto-like objects are more typical in our solar system than the nearby planets we first knew."

When the 7-foot-tall (2.1-meter-tall) New Horizons spacecraft reaches Pluto as early as 2015, the spacecraft will study the ninth planet's large moon, Charon, as well as two other moons just discovered last year. The $700 million mission should provide scientists with a better understanding of the Kuiper Belt, a mysterious region that lies beyond Neptune at the outer limits of the planetary system.

Besides being home to Pluto, the Kuiper Belt is believed to hold thousands of comets and icy planetary objects that make up a third zone of the solar system, the rocky and gaseous planets making up the other two. Scientists believe they can learn about the evolution of the solar system by studying the Kuiper Belt since it possesses debris left over from the formation of the outer solar system. Depending on its fitness after arriving at Pluto, New Horizons will attempt to identify one or two objects in the Kuiper Belt.

"It provides for us a window 4 1/2 billion years back in time to observe the formation conditions of giant planets," Stern said. "This is a little bit about rewriting the textbooks about the outer planets."

A successful journey to Pluto will complete a survey of the planets that NASA began in the early 1960s with the Mariner program's exploration of Mars, Mercury and Venus by unmanned spacecraft. The best images of Pluto currently come from the Hubble Space Telescope, but they suffer from low-resolution fuzziness, making it difficult for scientists to interpret what they're seeing.

The 1,054-pound piano-sized spacecraft will be launched on an Atlas V. The rocket's makers, Lockheed Martin, experienced problems on another Atlas propellant tank similar to the one being flown to Pluto, forcing a delay of New Horizons' launch by several days to give the contractor extra time for inspection.

"Because we have such a long way to go, we put this small spacecraft on one of the largest rockets the U.S. has in its inventory," said project manager Glen Fountain of the Johns Hopkins University Applied Physics Laboratory.

When New Horizons reaches Jupiter in 13 months, it will use that giant planet's gravity as a slingshot, shaving five years off the trip to Pluto. During the trip between Jupiter and Pluto, the probe will go into hibernation, closing down most systems to conserve power. It will send weekly "beeps" back to Earth, providing updates on the vehicle's condition.

If the spacecraft is unable to launch during its monthlong window that closes Feb. 14, the next opportunity is in February 2007, but that would push back an arrival at Pluto to 2020 since New Horizons wouldn't be able to get the gravity assist from Jupiter then.

Powered by nuclear fuel that will produce less energy than is used by two 100-watt lightbulbs, New Horizons is loaded with seven instruments that will be able to photograph the surfaces of Pluto and Charon and examine Pluto's atmospheric composition and structure. Two of the cameras, Alice and Ralph, are named for the bickering couple from television's "The Honeymooners."

The spacecraft has a thermos-bottle design that will allow it to stay at room temperature. Tucked inside the probe will be a U.S. flag and a CD containing about a half million names of ordinary citizens who signed up on a NASA Web site.

Pluto and the Kuiper Belt have been full of surprises in recent years.

Scientists discovered in 2001 that binary objects — pairs like Pluto and Charon — litter the Kuiper Belt, and a year later they learned that Pluto's atmosphere undergoes rapid and dramatic global change. Last summer, scientists discovered Pluto's two extra moons.

Scientists expect more unexpected discoveries from the New Horizons mission.

Said Stern, "You can see why we think it's going to be like kids in a candy shop."

Spacecraft instruments are selected to meet a mission's science goals. On New Horizons, for example, NASA set out a list of things it (and the planetary science community) wanted to know about Pluto: What is its atmosphere made of, and how does it behave? What does the surface of Pluto look like? Are there big geological structures? How do particles ejected from the sun (known as the solar wind) interact with Pluto's atmosphere?

The New Horizons team selected instruments that not only would directly measure NASA's items of interest, but also provide backup to other instruments on the spacecraft should one fail during the mission.

The science payload includes seven instruments:

RALPH: Visible and infrared imager/spectrometer; provides color, composition and thermal maps.

ALICE: Ultraviolet imaging spectrometer; analyzes composition and structure of Pluto's atmosphere and looks for atmospheres around Charon and Kuiper Belt Objects (KBOs).

REX: (Radio Science EXperiment) Measures atmospheric composition and temperature; passive radiometer.

LORRI: (Long Range Reconnaissance Imager) telescopic camera; obtains encounter data at long distances, maps Pluto's farside and provides high resolution geologic data.

SWAP: (Solar Wind Around Pluto) Solar wind and plasma spectrometer; measures atmospheric "escape rate" and observes Pluto's interaction with solar wind.

PEPSSI: (Pluto Energetic Particle Spectrometer Science Investigation) Energetic particle spectrometer; measures the composition and density of plasma (ions) escaping from Pluto's atmosphere.

SDC: (Student Dust Counter) Built and operated by students; measures the space dust peppering New Horizons during its voyage across the solar system.

After launch aboard a Lockheed-Martin Atlas V rocket, the New Horizons spacecraft set out on a journey to the edge of the solar system. Liftoff occurred Jan. 19, 2006 at 2:00:00 p.m. EST from Launch Complex 41 at Cape Canaveral Air Force Station in Florida. New Horizons is headed for a distant rendezvous with the mysterious planet Pluto almost a decade from now.

As the first spacecraft to visit Pluto and its moon Charon, New Horizons looks to unlock one of the solar system's last, great planetary secrets. The New Horizons spacecraft will cross the entire span of the solar system and conduct flyby studies of Pluto and Charon in 2015. The seven science instruments on the piano-sized probe will shed light on the bodies' surface properties, geology, interior makeup and atmospheres.

New Horizons Mission at http://pluto.jhuapl.edu/