Mission to Pluto will take about 9 years

CAPE CANAVERAL, Fla. — It will be the fastest spacecraft ever launched, zooming past the moon in nine hours and reaching Jupiter in just over a year at a speed nearly 100 times that of a jetliner.

Its target is Pluto — the solar system's last unexplored planet, 3 billion miles from Earth. And the New Horizons spacecraft, set for liftoff on Tuesday, 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, Colo., 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 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."