The three-tonne probe Rosetta powered into space at 0717 GMT atop an Ariane 5 from the European Space Agency's pad in Kourou, French Guiana, ending a 13-month delay and two launch postponements.

"The launch went absolutely perfectly," Jean-Yves Le Gall, chief executive of launch operators Arianespace, said from Kourou.

"Rosetta has been placed in a perfect orbit... this achievement marks the renewal of Europe's mission in space."

Flight engineers and scientists in Kourou applauded and clapped each other on the back, while at ESA headquarters here relieved employees broke into cheers and toasted the success in champagne as France's science minister, Claudie Haignere, phoned her congratulations.

"Another mission is on its way to explore the Universe," ESA Director General Jean-Jacques Dordain said, as he ticked off the current European probes sent to explore the Moon, Mars and Saturn, with Venus to follow suit next year.

"The launch of Rosetta ... shows what Europe is capable of in space," declared EU Research Commissioner Philippe Busquin. "We take another step towards becoming a globally responsible space power."

Rosetta is one of the most ambitious space projects ever conceived, an extraordinary mix of technical and mathematical prowess, cross-border collaboration and financial stamina.

Its five-billion-kilometer (three-billion-mile) trek will require four planetary flybys of Earth and Mars to build velocity to meet Comet Churyumov-Gerasimenko 675 million kilometers (420 million miles) from the Sun in early 2014.

Crammed with remote sensors, Rosetta will dog the four-kilometer (2.5-mile) -wide comet as it races around the Sun and then drop a fridge-sized mini-laboratory onto its surface to carry out chemical and geological analysis.

Data will be relayed home by the orbiter.

Underpinning the scheme is the belief that comets contain clues as to how the Solar System was formed and even about how life itself began on Earth.

Comets are believed to be orbiting clusters of frozen gas and dust -- the primitive material from which the planets accumulated, more than four-and-a-half billion years ago.

According to the so-called panspermia theory, these wanderers of the Solar System may be replete in complex, volatile molecules.

By bombarding Earth in its infancy, comets could have seeded the planet with the ingredients for water and DNA, this theory goes.

Rosetta's initial launch in early 2003 was scrapped because of reliability fears about the Ariane 5 and its original target, Comet Wirtanen, replaced.

Its launch last Thursday was cancelled by high-altitude winds, and an attempt on Friday was nixed after an insulation tile detached from the fuel tank.

The spacecraft gets its name from the Rosetta Stone, which explained Egyptian hieroglyphics, thus laying bare the culture of the Pharoahs.

Its lander, Philae, was named in a competition won by Serena Olga Vismara, a 15-year-old from Milan.

"I looked on the Internet and found that Philae was an obelisk which provided clues to understanding Rosetta," she told AFP during a visit to Kourou last week. "Philae and Rosetta belong together."

Also invited last week were two Ukrainian astronomers who co-discovered Churyumov-Gerasimenko back in September 1969 and had no inkling that their finding would become so celebrated.

"It's an incredible moment for me," said Svetlana Gerasimenko, 58.

"Now I have to wait 10 years" before Rosetta teams up with the comet, she said. "I am already nervous."

"Comets are as many as fish in the ocean, and to think that they have chosen mine is amazing," co-discoverer Klim Churyumov said. "I am 67, but Rosetta encourages me to live for another decade."

SPACE.WIRE