CAPE CANAVERAL, Fla. -- An unmanned NASA spacecraft the size and shape of a concert piano hurtled toward Pluto yesterday on a 3-billion-mile journey to the solar system's last unexplored planet -- a voyage so long that some of the scientists who will be celebrating its arrival are still in junior high.
The New Horizons spacecraft blasted off aboard an Atlas V rocket in a spectacular start to the $700 million mission. Though it is the fastest spacecraft ever launched, capable of reaching 36,000 miles per hour, it will take 9 1/2 years to reach Pluto and the frozen, sunless reaches of the solar system.
''God has laid out the solar system in a way that requires a certain amount of patience on the part of those who choose to explore it," NASA administrator Michael Griffin said.
The probe, powered by 24 pounds of plutonium, will not land on Pluto but will photograph it, analyze its atmosphere and send data back across the solar system to Earth.
The launch went off without incident, to the relief of antinuclear activists who had feared an accident could scatter lethal radioactive material.
NASA had postponed the liftoff two days in a row because of wind gusts at the launch pad and a power outage at the spacecraft's control center in Maryland.
''It looked beautiful," said Ralph McNutt Jr. of the Johns Hopkins University of Applied Physics Laboratory, one of the mission's scientists. ''I was getting a little bit antsy."
Pluto is the solar system's most distant planet and the brightest body in a zone known as the Kuiper Belt, made up of thousands of icy, rocky objects, including tiny planets whose development was stunted for unknown reasons. Scientists believe studying those ''planetary embryos" can help them understand how planets were formed.
Pluto is the only planet discovered by a US citizen, Clyde Tombaugh in 1930, though some astronomers dispute its right to be called a planet.
It is a celestial oddball -- an icy dwarf unlike the rocky planets of Mercury, Venus, Earth, and Mars, and the gaseous planets of Jupiter, Saturn, Uranus, and Neptune.
Tombaugh's 93-year-old widow, Patricia, was in tears as she watched the liftoff from about 4 miles away, said her daughter, Annette Tombaugh-Sitze, who arrived with her family from New Mexico. New Horizons contained some of Tombaugh's ashes.
''It was so awe-inspiring to watch something like this," Tombaugh-Sitze said. ''It's something you can't put into words. You just feel it."
The spacecraft will use Jupiter's gravity as a sling to shave five years off the trip, allowing it to arrive as early as July 2015.
The 1,054-pound spacecraft was loaded with seven instruments that will photograph the surfaces of Pluto and its large moon, Charon, as well as analyze Pluto's atmosphere. Two of the cameras, Alice and Ralph, are named for the bickering couple from the television show ''The Honeymooners."