By Helen Briggs
BBC News Online science reporter
A shadow hangs over the rocket that will blast next month's flagship European comet mission into space.
Rosetta will orbit the comet and put a lander on its surface
British scientist André Balogh has told the BBC he fears the Rosetta probe could miss its flight due to technical problems on the launch vehicle.
But the European Space Agency says the rocket's faults are not major and can be sorted out before the launch date.
The £600m Rosetta mission aims to put a lander on Comet Churyumov-Gerasimenko to study primordial ices and gases.
Rosetta is currently scheduled to leave Europe's Kourou spaceport, in French Guiana, on 26 February, atop an Ariane 5 G+ rocket.
The probe should have launched a year ago but was grounded after another Ariane 5 vehicle exploded four minutes into a flight from Kourou.
The delay that resulted from the accident investigation led to Rosetta's original quarry, Comet Wirtanen, being abandoned and the mission re-designed.
Scientists have now selected a new target comet - a ball of ice, rock and dust that has the full name of 67P/Churyumov-Gerasimenko.
But they are worried that technical issues related to Rosetta's new launch vehicle, raised in reviews of its flight readiness, may delay the mission yet again.
André Balogh, professor of space physics at Imperial College London, UK, has an instrument on Rosetta.
"If Rosetta misses this launch window, for whatever reason, it will be very difficult to find another target comet for it using Ariane," he told BBC News Online.
"Therefore people have suggested turning to the Russians and using a Proton launcher. However, that would mean extra expense and very significant expense but it would be the only possible future for a successful launch of Rosetta to a comet."
The final decision on whether to launch will be made by the European Space Agency (Esa) in consultation with the rocket's operators, Arianespace.
The investigation that followed the failure of the rocket bumped the Rosetta mission
Professor David Southwood, head of science at Esa, said two "open technical items" relating to "the mechanical behaviour of the system as it takes off and the mechanical structure of the boosters" had yet to be resolved but they were not a particular cause for concern.
"I have no indication that they won't be resolved," he said. "I will be astonished if we don't go ahead."
Arianespace confirmed on Tuesday that the launch campaign was continuing in Kourou.
"Today, the launch vehicle is undergoing its preparation in Kourou and we are all getting ready to launch Rosetta on 26 February," said a spokesperson.
If all goes to plan, Rosetta will reach the comet in 2014 and drop a small lander, the size of a washing machine, on to the comet's surface.
The lander will send close-up pictures of the comet's nucleus back to Earth, and drill into the heart of the "dirty snowball" to sample its primordial ices and gases.
"It's a cornerstone not just of our [scientific] programme but I think also of the scientific approach to understanding the Solar System," said Professor Southwood.