By Helen Briggs BBC News Online science reporter

An upgraded Ariane 5 rocket carrying two French satellites exploded over the Atlantic on its maiden flight, casting doubt on the Rosetta mission.

To have it fail in such a spectacularly bad way is the worst thing we can imagine at the moment for Rosetta Dr Chris Carr, Imperial College London

The voyage to orbit and land on a comet is one of the European Space Agency (Esa's) most ambitious, daring and costly ventures.

Space officials now face the dilemma of risking another failed launch or going back to the drawing board after 10 years of planning.

One of the two leading British scientists on the project said the mission may be in jeopardy.

Comet odyssey

Dr Chris Carr of Imperial College London said Wednesday's launch was critical to the Rosetta mission.

He told BBC News Online: "We were all waiting and waiting for this launch to be successful.

Wirtanen: One of family of comets near Jupiter

"To have it fail in such a spectacularly bad way is the worst thing we can imagine at the moment for Rosetta."

The probe is due to launched on the night of January 12 on a rocket similar to the one that exploded on take-off from the Kourou spaceport in French Guiana.

It cannot be launched on another vehicle and it must take-off by the beginning of February.

The spacecraft is destined to skirt the Earth twice and Mars once in its journey to comet Wirtanen.

If it is unable to leave Earth within the narrow launch window, the planets will be in the wrong position for it to reach the comet.

Astronomers have been observing Wirtanen for many years, because they knew it was the target of the Esa mission.

'Grand mission'

Dr Carr believes there will be a delay of at least six months if the launch has to be scrubbed.

"The space craft Rosetta was designed solely for the mission it is flying," he said. "It is extremely difficult to target another comet even if we can find one that is scientifically-interesting."

The prospect is a nightmare scenario for Esa. On Monday, Prof David Southwood, director of science at Esa, told BBC News Online: "It's just about a month until we launch and it's been a long time coming.

"It is the grand mission so it's going to be an incredible feeling when it goes up, for me and indeed for colleagues and scientists across Europe."

Nervous wait

Esa has set up a major investigation into the Ariane 5 disaster and data analysts are working to identify the cause.

Officials at Arianespace said on Thursday they were confident the Rosetta launch would go ahead as planned.

An independent commission will report as soon as possible on whether the accident has any repercussions for the Rosetta launch.

Meanwhile, scientists on the project face an anxious wait. Dr Christopher Lee, operations manager of the Rosetta plasma consortium, said they were continuing preparations for a January launch.

"We are saddened by the loss of the latest Ariane flight and its payload and are deeply concerned about its impact on the Rosetta mission," he said.

"However, we have to be patient and wait for news from Esa and Arianespace to see what the effects will be."