Spacecraft will attempt to chase down a comet

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European scientists are "crossing fingers" that a spacecraft will finally lift off next week on a mission to chase and land on a comet.

European scientists are "crossing fingers" that a spacecraft will finally lift off next week on a mission to chase and land on a comet.

The £600m Rosetta mission, which has been delayed for more than a year by questions over the safety of its Ariane 5 launch rocket, is due to take off from French Guiana next Thursday. Professor David Southwood, director of science for the European Space Agency, said yesterday: "We want all our fingers crossed for this one," although he added: "I will be astonished if we don't take off."

The mission is unique in the exploration of space, with its aim to land on and examine the "nucleus" of the comet named 67P/Churyumov-Gerasimenko, or "CG". The comet is one of the thousands of billions of such icy bodies which follow huge elliptical orbits around the Sun. When they near the Sun they throw off a tail of heated dust and gas that in some cases is visible during the day.

Spacecraft have already landed on asteroids, the rocky leftovers from the formation of the solar system. Comets are different; little is known about their origins. They might be formed in other parts of the galaxy and travel between stars. Some scientists think comets provided the water for the Earth's oceans when the planet was forming. Others suggest that they could even carry life or the building blocks of life.

Though the Rosetta will not test for signs of life - because that was not considered when it was being designed more than a decade ago - it will analyse the chemicals in the nucleus of the comet. But first it will have to find the nucleus. The gases and dust thrown off comets as they near the Sun occupy huge volumes of space, but the nucleus - composed, scientists believe, principally of ice - is comparatively tiny.

Dr Ian Wright, of the Open University, who has worked on the lander system for the project, put the challenge in perspective: if a comet was the size of London, including the M25, then finding the nucleus was like "landing on a 12-inch ruler perched on Nelson's head in Trafalgar Square".

The nucleus of Comet CG is about 1.3 miles (two kilometres) in radius, roughly the size of Heathrow airport, while the lander is about the size of a large washing machine.

Even if the Rosetta locates the nucleus, the mission scientists are still unsure how solid it will be. "It could be anything from the hardness of concrete to candyfloss," Dr Wright said.

Dr Chandra Wickramasinghe, of Cardiff University, suggested the nucleus might be hard to find because it could be covered in a layer of tar-like material from decomposed organic compounds. He said: "That would make it black, which would make it very difficult to pick out."

If its landing is successful, the Rosetta will shootharpoons to anchor it to the comet's surface because the gravity would be 100,000 times less than Earth's.

But the Rosetta has a long and lonely mission to the comet; the meeting will not happen until April 2014, when the comet is far beyond the asteroid belt between Mars and Saturn.