Ten years in the making, with a 10-year trip in front of her and a billion euros (1.25 billion dollars) invested in her, the unmanned craft Rosetta lay atop an Ariane 5 launcher ahead of the 0736 GMT Thursday launch at Europe's space base.

If all goes well, Rosetta will rendezvous with Comet Churyumov-Gerasimenko 675 million kilometers (421 million miles) from the Sun.

After firing its thrusters to close on the target, in November 2014 Rosetta will send down a tiny robot lab, Philae, that will gently land on the surface of "C-G" and carry out an ambitious programme to assess the comet's geology.

"Rosetta should bring us entirely new knowledge about comet nuclei," the European Space Agency (ESA) said on Monday. "For the first time in history a comet travelling sunwards will be investigated from close quarters."

The hope is find what comets are made of -- and the answer, say astrophysicists, could shed light on how life began on Earth.

Comets have always been associated in the human mind with good or ill omens: the birth and death of kings, of failed harvests, floods, wars and earthquakes.

These enigmatic rocks are cursed to encircle the Sun and leave a fiery "tail" in their wake: this reflective glow in the solar rays is caused by the stripping away of their coating of ice and dust as they near the star.

For space scientists, comets are the most primitive material left over from the making of the Solar System some 4.6 billion years ago.

Barely touched by gravity and heat, strangely black in spite of their icy surface, extraordinarily light in density, comets may contain volatile and complex carbon molecules, some experts believe.

These molecules may have been the chemical kick to start life on Earth, according to the so-called "panspermia" theory. It postulates that the Earth, in its infancy, was bombarded with comets and asteroids, whose elements reacted with the oceans to provide the building blocks for DNA.

"This mission has the potential to make spectacular discoveries about the origin of Earth and, perhaps, about the origin of life itself," says Jean-Pierre Bibring, of France's National Centre for Space Studies (CNES).

To get Rosetta and Philae to the rendezvous point in deep space will require what ESA engineers dub a "billiard-ball journey."

The plucky pair will need three flybys of Earth and one from Mars, using the gravitational pull each time as a slingshot to reach more than 100,000 kph (60,000 mph) to match C-G's speed.

The minutest error in navigation will send the mission hurtling out of the Solar System, for the spacecraft has to meet up with an object just four kilometers (2.5 miles across).

Thursday's launch is ESA's second stab to get Rosetta into space. A launch a year ago was scrubbed because of reliability fears about the Ariane 5, and that target, Comet Wirtanen, was substituted for "C-G".

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

SPACE.WIRE