CAPE CANAVERAL, Fla. (AP) – Pluto is considered a planetary oddball – a tiny, frozen world so unlike the other planets that some astronomers doubt whether it even deserves to be called a planet.

Scientists came a step closer Thursday to understanding Pluto, launching an unmanned spacecraft on a 3 billion-mile journey to the solar system’s last unexplored planet.

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 mph, it will take 9½ years to reach Pluto.

“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 anti-nuclear 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.

“We’re realizing just how much there is to the deep, outer solar system,” said Alan Stern, the mission’s principal investigator. “I think it’s exciting that textbooks have to be rewritten, over and over.”

Pluto is the only planet discovered by a U.S. citizen, Clyde Tombaugh in 1930, though some astronomers dispute its right to be called a planet. It is an icy dwarf unlike the rocky planets of Mercury, Venus, Earth and Mars and the gaseous planets of Jupiter, Saturn, Uranus and Neptune.

New Horizons contained some of Tombaugh’s ashes. His 93-year-old widow, Patricia, was in tears as she watched the liftoff from about four miles away, said daughter Annette Tombaugh-Sitze, who arrived with her family from New Mexico.

“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 TV’s “The Honeymooners.”

The probe will rely on the natural decay of the plutonium to generate electricity for its instruments. NASA and the Energy Department had put the chances of a launch accident that could release radiation at 1 in 350. As a precaution, the agencies brought in 16 mobile field teams that can detect radiation and 33 air samplers and monitors.

“Certainly there are feelings of relief that we didn’t have to actually execute any of our contingency plans,” said Bob Lay, emergency management director for surrounding Brevard County.

Griffin said he had an answer for those who may question spending $700 million on a mission to study Pluto and the Kuiper (pronounced KY-per) Belt, which is too far away to observe in any detail from Earth.

“Of what value do you think it might be to be able to study the primordial constituents from which the solar system and all the planets and we, ourselves, were formed?” Griffin said.

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On the Net:

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

Nuclear protesters: http://www.space4peace.org

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