Mission to catch a comet

December 20, 2002

The countdown to one of the longest shots in the history of space exploration has begun, writes Steve Farrar.

The Rosetta probe will have to avoid destruction on the launch pad and being drowned in cosmic snow in its bid to catch a comet during an 11.5-year journey set to begin on January 13 next year.

Leading Rosetta scientists, who gathered in London for a Royal Astronomical Society discussion meeting, acknowledged that the European Space Agency mission was risky. They were dismayed when a few days before the conference a modified version of the Ariane 5 rocket due to send Rosetta into space exploded three minutes after blast-off.

Rita Schulz, deputy project scientist, said Rosetta would be powered by a tried-and-tested version of the rocket but admitted that if the launch was delayed beyond January 29, it would miss its chance. "We would be in deep trouble; there is no back-up target," she said.

Another major concern will come in a decade's time when a lander reaches Comet Wirtanen.

In the reduced gravity of Wirtanen's 1km-wide nucleus, the fridge-size lander, which weighs 100kg on earth, will have the weight of a terrestrial matchstick.

Jens Biele, a researcher at the German Aerospace Centre, said the lander would deploy a tripod of legs, fire rockets and shoot an anchor into the surface in a bid to avoid bouncing off or sinking. "In just five seconds during touchdown, the fate of this ten-year mission will be decided," he said.

Rosetta will reach Wirtanen at its furthest point from the sun, then follow it on its plunge into the inner solar system, as it becomes active and grows a tail.

The main probe will orbit the nucleus during this time, subjecting the comet to a variety of tests.

UK involvement in Rosetta is a quarter of what had originally been hoped for, but there will still be work on several experiments including mapping Wirtanen's interior, studying its interaction with the solar wind and analysing its chemical make-up. Ian Wright of the Open University said scientists would be looking at material from the birth of the solar system.

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