Paris

Rosetta, the US$700-million European Space Agency (ESA) mission to land a probe on the comet Wirtanen in 2011, is set to miss its January take-off window and could be postponed for a year or more, Nature has learned.

The mission's postponement was set to be agreed on 14 January, after the failure of the Ariane 5 ECA 'heavy lifter' rocket on its maiden flight in December (see Nature 420, 723; 200210.1038/420723a) cast doubt on the safety of the standard Ariane 5 model, on which Rosetta was scheduled to fly.

Hopes that the launch would go ahead before the take-off window closes on 31 January were temporarily boosted last week, when an inquiry traced the cause of the accident to a cooling fault on the new engine — suggesting that the standard rocket is not at risk. And scientists at a pre-launch press briefing in London on 13 January said that they were still hoping for a January launch.

David Southwood, ESA's director of science, was scheduled to hold another briefing in Paris on 15 January “to take stock of the status of this mission”. But Nature has learned that Arianespace, which operates the Ariane rockets, has advised against a launch — and ESA has agreed with its assessment. Southwood has previously said: “If there was the slightest risk in my mind, we would not launch; there is no way I'm not going to gamble a billion euros of other people's money.”

The quickest alternative launch options would use gravitational assistance from Venus instead of Mars to swing the craft towards Wirtanen — but this would raise heat problems. Others would use Mars, as in the original plan, but this would mean finding a different comet to land on instead of Wirtanen. Sources close to ESA say that no plan will be finalized before a thorough analysis has been undertaken of the cost, scientific merit and technical risk of all options.