10-year Journey to Probe the Comet Mystery

A spacecraft the size of a delivery van is about to begin one of the most ambitious journeys ever begun: a 4.4bn-mile trip to deliver a lander the size of a washing machine to a comet beyond the orbit of Jupiter.

The European probe Rosetta will begin its 10-year journey on Thursday from Kourou in French Guiana, on an Ariane 5 launcher. It will spend 10 years in orbit, gathering speed as it flies three times past Earth and once past Mars, gaining momentum each time from a series of "gravitational slingshot" manoeuvres.

It will reach 75,000mph to meet up with a comet called Churyumov-Gerasimenko and go into orbit around it in May 2014.

In November that year it will drop a package of instruments called Philae on to a heavenly body with a diameter not much bigger than the runways at Heathrow airport.

The £600m mission was first proposed in 1985 and will end in 2015.

"From start to finish it will be a big fraction of a scientific life," said one of the investigators Ian Wright, of the Open University.

Rosetta will be the first spacecraft to go into orbit around a comet's nucleus. It will be the first to ride alongside a comet as it loops around the sun. It will be the first to see how a modest lump of ice and dust from the outer suburbs of the solar system transforms into a spectral vision with a glowing tail.

It will make the first soft landing on a comet, and it will provide the first chance to directly analyse the stuff of a comet.

It will be the first spacecraft to get as far as Jupiter using solar panels to generate power. And it will have the first close encounter with one of the mysterious lumps of rock in the asteroid belt between Mars and Jupiter.

Rosetta gets its name from the Rosetta stone, now on display in the British Museum, which offered the first key to deciphering Egyptian hieroglyphics. Philae is named after an obelisk found in the upper Nile that provided another powerful clue: it carried the cartouches of Cleopatra and Ptolemy.

To continue the theme, the name Ptolemy has been given to a package of instruments the size of a shoebox intended to analyse the isotopic structure of the ice and dust of the comet.

Comets are surviving fragments of the original building material of the solar system. There may be billions of them parked far beyond the orbit of Pluto.

Scientists have flown spacecraft past three comets in the past 19 years, but Rosetta will provide the first intimate study. The accuracy required is phenomenal. To observers on Earth, comets look big. Imagine that the coma of a comet occupied a space the size of Greater London within the M25, said Dr Wright.

"The size then of the nucleus would be equivalent to a 12-inch ruler balanced on the top of Nelson's column. Beyond the comet, you have a long way to go to get to the nucleus."

The technical challenges have been huge. More than half the mass of the three-tonne spacecraft will be fuel to allow it to complete its mission.

It needs instruments and communication systems able to survive 12 years in a climate that falls to minus 150 C (minus 235F) beyond Jupiter but rises to the heat of a Provençal summer nearer the sun.

When the encounter begins, light - and therefore radio commands - will take almost an hour to reach the spaceship, so it has had to be designed to be able to think for itself as well as respond to its controllers.

The lander will have to touch down on a comet with a texture that could be as hard as concrete or as yielding as candyfloss, with a gravitational tug 10,000 times weaker than Earth's, so it will have to be attached with a harpoon to stop itself bouncing off again.