Pluto orbits the sun in an elliptical orbit tilted 17 degrees to the plane of the other planets. The orbit comes as close as 2.8 billion miles to the sun and as far as 4.6 billion miles. Between 1979 and 1999, Pluto was closer to the sun than Neptune. It takes the planet 247 years to complete a single orbit, or plutonian year, and it currently is moving away from the sun.

The ice dwarf is tipped over on its side and has a diameter of about 1,460 miles. Gravity is about 6 percent of Earth's and surface temperatures are as low as 387 degrees below zero Fahrenheit. Astronomers have detected spectroscopic signs of nitrogen, carbon monoxide, methane and ice on the surface. The planet has an extremely thin atmosphere composed of nitrogen with small amounts of methane, carbon monoxide and hydrocarbons.

Pluto and its large moon Charon whirl about each other every 6.4 days. Charon is 12,200 miles from Pluto and both worlds turn in gravitational lockstep, always keeping the same face toward each other. Pluto's two other, much smaller known moons orbit two to three times farther out than Charon.

Little more is known about the Pluto system, not surprising given the planet is 50,000 times fainter than Mars when viewed from Earth.

To carry out its mission, New Horizons must traverse some 3 billion miles and then hit a keyhole in space just 186 miles across, a target point at the far end of the launch trajectory that will allow the spacecraft to pass midway between Pluto and Charon.

Detailed observations will begin about five months before the flyby, collecting data that will help flight planners fine-tune the spacecraft's course. Starting about three months out, at a distance of about 62 million miles, New Horizons will begin mapping Pluto and Charon. A few weeks later, the spacecraft's images will become sharper than those taken by the Hubble Space Telescope. Daily observations will commence one month before the encounter.

By that point, scientists hope to know whether two recently discovered moons - and others that may be discovered between now and then - have contributed to any as-yet-unseen rings of debris around Pluto that could pose a threat to the spacecraft as it zips through the system. Stern said New Horizons should still have half a tank of hydrazine rocket fuel left by then, more than enough to change course if necessary to avoid any threats. "Our plan, however, is to fly about halfway between Pluto and Charon, splitting that difference between them," he said. "But it's contingent upon some work we need to do with the Hubble Space Telescope To ensure we avoid any potential debris field ... associated with the orbits of the small satellites, which might generate rings in the system.

"We would want to make sure we would not fly through rings or ringlets. We want to do deeper searches for additional satellite and we want to pin down their orbits so as we fly through that bullŐs-eye, we fly between satellite orbits with good separation and not at the distances that ring system debris may be present."

While trying to avoid danger, the new moons offer an unexpected scientific opportunity.

"It looks because of the geometry we might actually be able to fly pretty close to both of those satellites, maybe one of them before we encounter Pluto and one after," Weaver said. "My own personal feeling right now is we almost have a family here. We have the mother and the father, Pluto and Charon, and these objects seem to be rotating around Pluto all in the same plane. It looks like the same event that produced Pluto and Charon, which was probably catastrophic collision that caused all heck to break loose, a whole bunch of stuff formed a debris disc and you had Pluto and Charon forming. In addition to that in the little disc of material that was left over probably had enough clumpiness in it and maybe these little moons formed."

But Pluto and Charon are the main reason for launching New Horizons, along with the opportunity to study a second Kuiper Belt object in the years after the main encounter.

"At the time of the Voyager missions in the early 70s, the importance of Pluto and, of course, the Kuiper belt, because it wasn't discovered (yet), wasn't appreciated at all," Stern said. "It wasn't very high on the priority list because we didn't realize how much it had to teach us about the origin of the solar system and some of the kinds of physics, particularly atmospheric physics, that we want to learn about. Today, with our better knowledge of the geography of our solar system, we see how central it is to modern planetary science."

New Horizons' close encounter with Pluto will last a full day, 12 hours before and after. The spacecraft cannot enter orbit around the planet because no current rocket can launch a probe carrying enough fuel to arrest the velocity needed to get it there in a reasonable amount of time.

Approaching from the planet's southern hemisphere, the spacecraft will pass within about 5,500 miles of Pluto's surface at 31,300 mph, streaking well inside the orbit of Charon. Fourteen minutes later, the probe will fly past Charon at a distance of 16,800 miles. Fifty-one minutes after Pluto closest approach, New Horizons will disappear behind the planet as viewed from Earth. Two hours and 15 minutes past Pluto closest approach, the spacecraft will pass behind Charon as viewed from Earth, giving the radio science team a chance to look for signs of an atmosphere.

At Pluto's distance from Earth, it will take several hours to transmit a single image back to Earth. High-priority observations will be sent back right away, but it will take nine months to transmit the complete data set.

Weaver said the Pluto-Charon system is unique in the known solar system.

"If you think about this mission in the context of comparative planetology, we have the terrestrial planet region in the inner solar system comprised of Mercury, Venus, Earth and Mars," he said. "Pluto and Charon and the Kuiper Belt objects are nothing like those rocky objects. Then you have the gas giants, the ones that are dominated by thick atmospheres of molecular hydrogen and helium - Jupiter, Saturn, Uranus and Neptune. And then we have the final new frontier in the solar system, the realm of the icy dwarfs and that's what Pluto, Charon and the KB objects belong to. And that has not yet been explored.

"Another interesting thing about Pluto and Charon is they're the only example in the solar system of a binary planet," Weaver continued. "They're more like brothers or sisters than a planet and a moon. They're sort of at one end of the spectrum. Most of the planet-moon systems have much bigger discrepancies between the mass and the size of the planet and its moon and Pluto and Charon are at the other end of the spectrum. By understanding how all of these things formed, that's going to tell us something about planetary dynamics and the inner solar system from the time of its formation.

"Another interesting aspect of Pluto is that it does have an atmosphere and in fact, that's one of the reasons why it's being called a planet. But PlutoŐs atmosphere is weird. It's sort of a transitional case between a traditional planetary atmosphere and a cometary atmosphere, which is not connected to the object at all, it just flies off into space. It's in between. Pluto does have a bound atmosphere but it also has some escaping gases. In fact, it's the only current planet in the solar system whose atmosphere is in the process right now of hydrodynamic escape.

"And not only does the gas escape, as Pluto moves father away from the sun and gets colder and colder, what eventually will happen is the atmosphere will freeze out onto the surface," Weaver said. "We want to get to Pluto with New Horizons before that atmosphere freezes out so we can learn something about Pluto's atmosphere, which in turn will tell us something more generally about the formation and evolution of planetary atmospheres."

Before New Horizons gets to Pluto, astronomers plan to select a second Kuiper Belt object for close-up observations. Plenty of fuel is expected to be available for any necessary course change and the RTG power source should provide enough electricity to operate the instrument package for a full 10 years beyond the Pluto flyby.

"Pluto, of course, is embedded in the Kuiper Belt, the largest structure in our planetary system, dotted with almost a half million worlds and worldlets that are four billion years old," Stern said. "In a real sense, this is not just a journey four billion miles away but also four billion years back in time. It's really no wonder, because of the value of this ancient structure to our understanding of the origin of the solar system, that the National Academy (of Sciences) ranked this mission number one on the runway for a new start in this decade among medium-sized planetary missions like New Horizons."

But Pluto is the star of the show.

"It's a very unusual pair of worlds, the first binary planet system we've ever explored and the first ice dwarf, a whole new class of planets," Stern said. "Whereas we know of four terrestrial planets and four gas giants, we expect there are hundreds, if not thousands, of these ice dwarfs. So this class of planets, which we have not yet reconnoitered, is in reality the most populous class of planetary body in our solar system. That's what this mission is about.

"Some of the other attributes of the Pluto system that make it interesting is it's a scientific wonderland for atmospheric scientists. Its atmosphere is escaping like a comet, but on a planetary scale. It's surface, which is only 40 degrees above absolute zero, is covered in exotic ices, it has strong seasonal and global change effects that we already know are taking place."

"You can see why we think it's going to be like kids in a candy shop when we arrive in a system like this," Stern said. "It just keeps getting better."