The Ariane 5 rocket was scheduled to blast off at 0736 GMT, although the crucial final separation between the launcher's upper stage and its payload was due to take place a couple of hours after liftoff.

The spacecraft Rosetta will travel five billion kilometers (three billion miles) before rendezvousing in deep space with Comet Churyumov-Gerasimenko in 2014 and hound it during its path around the Sun.

The orbiter will scan the comet with its remote sensors and then send down a miniature laboratory, Philae, to assess the surface chemistry and geology.

Ten years of scientific and technical prowess have been thrown into the project, at a cost of a billion euros (1.25 billion dollars).

The reason for the mission: comets may contain vital clues as to how the Solar System was formed and, maybe, how life itself began on Earth.

"Where do we come from? Who are we? And where do we go? These are questions this mission may address," Gerhard Schwehm, a comet specialist at the European Space Agency (ESA), said on Wednesday.

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.

As comets near the Sun, the solar heat warms up their frigid outer layer, which strips away in gassy outgushes and a wake of dust that the sunlight spectacularly reflects as a long "tail."

Some astrophysicists contend that comets are rich in complex, volatile moleculates which seeded Earth with the chemical means for life when the planet was in its infancy and under bombardment by space rubble.

To get Rosetta and Philae to the meeting point will need what ESA engineers dub a "billiard-ball journey."

The mission will use three flybys of Earth and one from Mars, exploiting the gravitational pull each time as a slingshot to reach more than 100,000 kilometers (60,000 miles) per hour to match the comet's speed.

The minutest error in navigation will send the three-tonne craft hurtling out of the Solar System, for Rosetta has to meet up with an object just four kilometers (2.5 miles across), 675 million kilometers (420 million miles) from the Sun.

The spacecraft will have to survive long periods in hibernation, cope with lengthy gaps in radio contact with Earth and survive a 220 Cdifference between the utter chill of deep space and the warmth of perihelion, when the comet will be at its closest point to the Sun.

Engineers have had to give the craft wide powers of decision-making and have equipped it with a revolutionary generation of solar panels, designed to suck energy out of dim sunlight.

"It's absolutely mindblowing when you think about what we are trying to do," said John Ellwood, in overall charge of the Rosetta project at ESA.

"It's an amazing challenge. We've got to press technology to the edges to do something like that."

The launch is ESA's second stab to get Rosetta into space. A planned launch a year ago was scrubbed because of reliability fears about the Ariane 5, and that target, Comet Wirtanen, was substituted by Churyumov-Gerasimenko.

Rosetta is named after the stone that explained Egyptian hieroglyphics, thus laying bare the culture of the Pharoahs to modern eyes. Philae is the name of an obelisk that itself provided a key to understanding Rosetta.

The comet was named after two Ukrainian astronomers, Klim Churyumov and Svetlana Gerasimenko, who discovered it in a series of observations in Kazakhstan in September 1969.

SPACE.WIRE