Liftoff had been scheduled for January 2003, but was postponed to let ESA carry out reliability checks on the Ariane 5 after a 10-tonne version of this rocket failed disastrously on its maiden flight a month earlier.

Rosetta will take until 2014 to reach the comet Churyumov-Gerasimenko as it races through the Solar System on a long loop around the Sun.

Comets are believed to consist of virgin material left over by the creation of the Solar System billions of years ago.

Understanding their chemical composition may shed light on how Earth and the other planets were created and evolved.

Rosetta's initial target was the comet Wirtanen, but the programme had to be rescheduled because of the technical review of the Ariane 5 range.

The probe, carrying a small lander, will be taken aloft from Kourou, French Guiana, aboard a standard Ariane 5.

Meanwhile, the 10-tonne version of this rocket, the Ariane 5-ECA, will be put back into service in mid-2004 after a programme to fix the problems that led to the December 2002 failure, Arianespace said.

It said it expected to carry out up to half a dozen Ariane launches this year.

SPACE.WIRE