The billion-dollar Rosetta, in development for 10 years, will spend another 10 years on a roundabout voyage that will include three velocity-boosting flybys of Earth and one of Mars. Its objective in 2014 is to place itself in low orbit around the comet Churyumov- Gerasimenko, a 4-kilometer-diameter body which at that point will be 675 million kilometres from the sun.

It will spend six months examining the comet before selecting a landing spot for the small 100-kilogram Philae lander. Then both Rosetta and Philae will travel with Churyumov-Gerasimenko as it moves toward the center of the solar system. The two spacecraft will have front-row seats as the comet, heated by the sun, sheds its icy outer layer and produces the "tail" for which comets have been held in awe by humans for centuries.

Churyumov-Gerasimenko was chosen as an emergency replacement target after Rosetta missed its January 2003 launch date following the December 2002 failure of its Ariane 5 carrier rocket. The failure forced a broad review of the vehicle's reliability.

After two additional delays Feb. 26 and Feb. 27, the rocket performed as expected today, putting the 3,065-kilogram Rosetta into an escape orbit following a 105-minute coast phase before the rocket's second stage was ignited for final orbital injection. It was the first time the Ariane 5 had performed this ballistic-coast maneuver.

Scientists speculate that the particles that comets shed as they approach the sun may have been responsible for the water on Earth, and perhaps even the building blocks of life.

"The only phrase I can use for this mission is mind-blowing," said John Ellwood, Rosetta project manager at the European Space Agency (ESA), during a press briefing at the Guiana Space Center spaceport. "So many things we are doing with this mission have never been done before."

Other European Rosetta scientists said Rosetta, like the ESA Mars Express satellite now orbiting Mars alongside two NASA spacecraft, will further help Europe shed its eternal second-fiddle status when compared to the better-financed U.S. space program.

As is the case with Mars, ESA and NASA's interest in comets has dovetailed to witness several spacecraft in orbit at the same time. NASA's Stardust collected dust samples from the comet Wild 2 in January and is scheduled to return the samples to Earth in 2006. In December, NASA's Deep Impact is scheduled to be launched on a mission to hurl a projectile at the comet P/Tempel to study the ice and other material that is thrown off from the impact.

Rosetta has 11 observing instruments, and Philae, 10. But ESA officials said the mission was selected solely on the value of the mission as a comet orbiter. The lander was added subsequently. "When Rosetta was first approved in 1993, we had to demonstrate that the orbiter alone would justify it," said Gerhard Schwehm, lead Rosetta project scientist at ESA.

NASA contributed three of the 21 instruments being carried by Rosetta and Philae. The German, French and Italian space agencies are the biggest contributors to the science payload.