Next Stop, Pluto

A space probe is set to take off for what astronomers used to think was the last unexplored planet

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When NASA first considered a mission to Pluto more than 15 years ago, the idea was to visit the last and most distant of the nine planets, an oddball whose icy composition, tilted orbit and tiny size made it unlike anything else in the solar system. But when the New Horizons probe finally takes off from Cape Canaveral—as early as next week, if all goes well—it will be heading for something else entirely. "This little misfit is now central to our understanding of the origin of our solar system," says Alan Stern of the Southwest Research Institute in Boulder, Colo., and lead scientist for New Horizons.

Reason: Pluto, astronomers have learned, is no oddball. It's one of thousands of icy bodies in a diskshape swarm known as the Kuiper Belt that orbits the sun in the dark, frigid realm beyond Neptune. Since the discovery last summer of an object called 2003 UB313, Pluto is not even the biggest. And because those little worlds have been in deep freeze since the solar system was formed more than 4 billion years ago, they represent a frozen record of what conditions were like back then.

Those primordial conditions are what Stern and his colleagues will be trying to understand when New Horizons reaches Pluto and its three moons (two were found just this past fall) in 2015. As the probe zips by, cameras will snap pictures of surface features about the size of a football field, analyze Pluto's thin atmosphere and measure its temperature.

All of that will add enormously to our knowledge. But it won't help scientists decide whether Pluto should keep its status as a planet, a debate that only intensified when 2003 UB313 was discovered; if Pluto is a planet, then its bigger cousin must be as well. The International Astronomical Union promises a decision, but Stern doesn't know when it will come. For now, he's not thinking much about that. He has a spacecraft to launch. [The following text appears as part of a complex diagram]

THE MISSION Using Jupiter's gravity to speed it on its way, New Horizons will be the first probe to take close-up images of Pluto and analyze its atmosphere, thus enabling astronomers to understand how the icy bodies of the Kuiper Belt came to be

• SUN • EARTH LAUNCH - Between Jan. 17 and Feb. 14 • SATURN • JUPITER Jupiter gravity assist - February to March 2007 • URANUS • NEPTUNE • PLUTO PLUTO-CHARON ENCOUNTER July 2015. During flyby, the probe will pass within a mere 6,000 miles (10,000 km) of Pluto—40 times as close as the Earth is to our own moon • KUIPER BELT Voyage into Kuiper Belt 2016-2020 What is the Kuiper Belt? Named for Gerard Kuiper, who predicted its existence in the 1950s, it is a vast, disk-shaped cloud of thousands of icy bodies that starts near Neptune and reaches to about 4.5 billion miles (7.5 billion km)from the sun

ORBIT OF PLUTO • From Pluto, the sun appears about 1,000 times as dim as it does from Earth • Pluto and its largest moon, Charon, are locked in synchronous orbit, always keeping the same face toward each other

THE SPACECRAFT New Horizons is about the size of a grand piano, packed with highly sensitive instruments

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