The decision to postpone the �600m Rosetta mission is a major blow to the scientists and engineers who have spent more than a decade working on the project.

The plan to chase down Comet Wirtanen and put a lander on its icy surface in 2012 was a bold one that had generated huge excitement.

It's very frustrating, but I can understand why Esa took the decision Prof Colin Pillinger, Open University

But the European Space Agency (Esa) knew it had no choice but to suspend Rosetta preparations when concerns surfaced about the reliability of its launch rocket.

The enforced delay means the spacecraft will now overshoot the narrow window it had to get into position to catch the comet.

The project's researchers will have to redesign the mission and find a new comet to target. It could be a year before Rosetta finally leaves Earth and the delay will undoubtedly add substantially to the already large budget cost.

Alternative comets

Dr Gerhard Schwehm, the lead scientist on Rosetta, told the BBC: "It's a disappointment now that we have to look for another opportunity. We have invested so much effort but it's quite clear Rosetta is not lost - we just have to wait a little bit longer."

Professor Colin Pillinger, from the UK's Open University, who helped develop one of the lander instruments designed to analyse comet core samples, said: "Planning for this mission has gone on for 10 years and a few more months won't make any difference.

"Prudence is such in the business that you can't risk sending a billion dollars up in smoke. Better to be safe than sorry. It's very frustrating, but I can understand why Esa took the decision. They did it for us."

It is known that the agency has been discussing back-up plans for some time.

When it became apparent in late December that Rosetta might not fly on time, Esa began the process of drawing up a shortlist of possible alternative comet targets.

Low risk and cost

It is likely a report detailing these alternatives will now be submitted to the agency's scientific policy committee, which meets next in February.

There will be three or four comets to consider and "a collective decision will be made about which one to go for", said Peter Barratt, a spokesman for PParc, the UK's space funding body.

There's no way we would want to jeopardise the time invested already Peter Barratt, PParc

"They will be looking for the best return for the science with minimal risk and cost implications.

"The critical thing will be what are these other targets and what are their orbits. It might mean waiting 12 months until the orbit requirements are met, but there's no way we would want to jeopardise the time invested already."

When it finally does fly, Rosetta should add substantially to our understanding of comets.

Studying the materials trapped in these icy bodies could reveal details about the origins of the Solar System and how water - and possibly even life - arrived on Earth.

Rosetta carries 11 different scientific instruments. It will go into orbit around the comet to map it before sending down a lander on to the surface.

The lander will take pictures of the surface and sample its chemistry.