Rosetta is due to be launched on February 26 from ESA's space base in Kourou, French Guiana, on a 10-year mission to chase the comet Churyumov-Gerasimenko and land a laboratory probe on its surface.

"This mission is almost science fiction," Marcello Coradini, responsible for Solar Systems programmes at ESA, told a press conference.

The launch window closes on March 17, after which the only other possibility would be in January 2005, possibly using a Russian Proton rocket, he added.

The mission is the longest and most ambitious deep-space mission ever attempted by Europe.

Scientists believe comets are virtually pristine material left over the creation of the Universe.

Some suspect that comets harbour complex carbon molecules that may have seeded Earth with the chemical building blocks for making life when the planet was bombarded by space rocks in its infancy.

"The Rosetta mission has a potential for making spectacular discoveries about the origin of the world and, perhaps, about the origin of life itself," French astrophysicist Jean-Pierre Bibring said.

Rosetta, an orbiter stuffed with instruments, will rendezvous with the comet about half a billion kilometers (350 million miles) from Earth in August

After scanning the comet for a good landing site, it will send down a 100-kilo (220-pound) probe, which will anchor itself to the icy crust and run chemistry tests.

Rosetta was initially scheduled to have been launched in early 2003, but liftoff was postponed to let engineers check the Ariane-5 launcher after a heavy version of this rocket failed disastrously on its maiden flight.

That caused a change of target. The comet Wirtanen was swapped for Churyumov-Gerasimenko.

Comets, famously described as "dirty snowballs", originate from a belt of enigmatic bodies that girdle the Sun beyond the orbit of Neptune.

As they head towards the inner Solar System on their long trek around the Sun, some of the ice is melted by the solar heat, ejecting gassy jets and dusty debris that typically show in the sky as the comet's "tail."

Rosetta is named after the stone that helped archaeologists decrypt Egyptian hieroglyphics.

Its lander has been dubbed Philae, after an obelisk which now stands in Dorset, southern England, that also provided one of the keys for figuring out Rosetta.

Philae was the name put forward by a 15-year-old Italian girl, winner of a competition to name the probe among young Europeans.

SPACE.WIRE