An unmanned NASA spacecraft the size and shape of a concert piano hurtled yesterday toward Pluto on a three-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 just taking their first physics class.

The New Horizons spacecraft blasted off aboard an Atlas V rocket in a spectacular start to the 700-million-dollar mission. Though it is the fastest spacecraft ever launched, capable of reaching 36,000 mph, it will take nine years to reach Pluto and the frozen, sunless reaches of the solar system.

The probe, powered by 24 pounds of plutonium, will not land on Pluto but will photograph it, analyse 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 lift-off two days in a row because of wind gusts at the launch pad and a power outage at the spacecraft’s control centre in Maryland.

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.

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 analyse 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.