By Paul Rincon
BBC News Online science staff
European space scientists are counting down to the launch of Rosetta, the mission to put a lander on a comet.
The lander will approach the surface at less than one metre per second
The £600m, 12-year space expedition is scheduled to launch from French Guiana's Kourou spaceport on 26 February aboard an Ariane-5 G+ rocket.
But the high-risk mission will need to overcome major technical challenges.
"Rosetta will be the first ever spacecraft to perform a soft landing on a comet's nucleus," UK science minister Lord Sainsbury told a news conference.
"This will allow Rosetta to carry out more in-depth study (of a comet) than has ever been done before."
The Rosetta spacecraft will despatch a lander, named Philae, to touch down on the icy surface of the comet Churyumov-Gerasimenko.
The mission continues a long association between the European Space Agency (Esa) and comet exploration that was cemented when the Giotto probe obtained close-up images of Comet Halley's nucleus in 1986.
"Rosetta is psychologically very important for us in Europe, because we did get closest to Halley (with Giotto). So comets are ours," Professor David Southwood, director of science at Esa, told BBC News Online.
"The best part of 20 years ago, we could have chosen to go to Mars, but it was interesting that we staked our claim with comets.
"I don't think I regret that because in the last three years there has been a flurry of comet missions. I think that's a sign of how important Rosetta is."
The probe will study the comet's materials, which are thought to have remained relatively unchanged since the formation of the Solar System.
Rosetta will slip into orbit around the comet in 2014
"We will (effectively) go back 4.6bn years, to when the Solar System was in its infant stages and the planets were forming out of a cloud of dust and gas," said project scientist Dr Gerhard Schwehm.
By summer 2014, Rosetta will enter orbit around the comet and begin edging towards its nucleus. The orbiter's cameras will then map the nucleus in detail to help scientists choose a suitable landing site.
Once this has been chosen, the lander Philae will be released from a distance of one kilometre. Approaching the comet nucleus at about one metre per second, Philae will fire two harpoons to anchor itself to the surface.
Mission scientists are under no illusions about the task they face.
"The main merit of Churyumov-Gerasimenko is that it's something you can land on. That may seem like a funny thing to say but the last thing you want to do is bounce off or break up," Professor Southwood explained.
"So we are really flying blind; it's tricky."
Scientists need to land the washing machine-sized Philae on a surface roughly the area of Heathrow airport.
"Rosetta will have been 10 years and several hundreds of millions of miles and then have to land on something like that - it's an audacious mission," said mission scientist Dr Ian Wright of the Open University.
The launch will go ahead despite the last minute detection of mechanical problems on Rosetta's Ariane vehicle in January.
The probe should have launched a year ago but was grounded after another Ariane 5 vehicle exploded four minutes into a flight from Kourou.
Rosetta's original quarry, Comet Wirtanen, had to be abandoned and the mission re-designed following the delay caused by the investigation.