By Helen Briggs BBC News Online science reporter

Rosetta is set to take off from South America on the night of 12 January for a nine-year-voyage past the planets.

Rosetta will join the nucleus of the comet as it travels in towards the Sun from the cold parts of the Solar System Dr Geraint Jones, Imperial College

Launch preparations are continuing amid an independent investigation into a failed rocket launch in December.

The maiden flight of the new heavy-lift Ariane 5 ended in disaster when it exploded over the Atlantic Ocean.

The first aim of the inquiry is to find out whether it has any implications for Rosetta, which is due to be launched on the basic version of the rocket.

Rosetta must be launched by the end of January if it is to reach its destination, Comet Wirtanen.

"If it doesn't launch we are in deep trouble," Dr Rita Schulz of the European Space Agency (Esa), deputy project scientist for the Rosetta mission, said. "There is no back-up target."

Building blocks

The Esa probe must skirt the Earth twice and Mars once to gather the power to chase the comet.

It will catch up with the large, "dirty snowball" in the dark depths of the Solar System where it will try to land a robot on a comet for the first time.

Wirtanen: One of family of comets near Jupiter

The spacecraft will then follow the comet as it hurtles towards the Sun at up to 135,000 kilometres per hour, studying the dust and gas particles that trail behind it.

Professor David Southwood, director of science at Esa, said the idea of a spacecraft being able to orbit and land on a comet was remarkable.

He told BBC News Online: "Comets, we believe, are the building blocks of the Solar System - so to go out and 'scratch and sniff'' a comet is going to take us back to the beginning where we all came from."

Long odyssey

The journey to Wirtanen is the furthest journey for a spacecraft powered only by the Sun.

Its destination comet was discovered in 1948 by Carl Wirtanen of the Lick Observatory in California.

Wirtanen is one of a group of comets whose paths are influenced by Jupiter, the largest planet in the Solar System.

We don't know what it's like to stand on the nucleus of a comet Dr Geraint Jones, Imperial College London

The mission is named after the famous Rosetta stone - carved with inscriptions in Egyptian and Greek - found by a French soldier in Egypt in 1799.

The discovery of the basalt stone slab, written at the beginning of the 2nd century BC, unlocked the secrets of ancient Egyptian writing.

Scientists hope that Rosetta will provide a similar insight into the ancient history of space and the planets.

Fleeting glimpse

Comets are the leftovers of the stuff that formed the Earth, Moon and planets 4.5 billion years ago.

Previous spacecraft have encountered the space bodies now and again but have been able to get only a fleeting glimpse of their make-up.

Comets

Balls of ice and dust that did not get incorporated into planets when the Solar System was formed. Analysis could reveal more about the Solar System and perhaps life on Earth

"Until now, missions to comets have flown past very quickly in a matter of hours," said Dr Geraint Jones of the space and atmospheric physics department at Imperial College London.

"The big difference with Rosetta is that it will actually join the nucleus of the comet as it travels in towards the Sun from the cold parts of the Solar System."

Rosetta will reach the comet at its farthest point from the Sun, around November 2011.

The spacecraft will drop a lander on to its nucleus and will chase Wirtanen as it speeds towards the Sun.

Rosetta will then orbit the comet, scanning its surface for an entire year, as it moves closer to the star.

Instruments on board the spacecraft will study the dust and gas particles that surround the comet and trail behind it as streaming tails.

They will measure the chemical composition of gases given off as ices evaporate and capture images of the centre of the comet as it melts.

Many of the molecules that are given off by the nucleus cannot be seen using telescopes.

"There's a good chance that Rosetta will be able to detect molecules that haven't been detected before because they are so unstable," said Dr Jones.

Perilous landing

The most exciting part of the mission will be the attempt to land a robotic probe on the icy heart of the comet.

It is hoped that the main orbiter will be able to get to about one kilometre from Wirtanen before releasing the lander.

The landing site may be snow or ice

If all goes well, the robot will take close-up images of the comet and a microscope will look in detail at its surface.

"Because the nucleus is so small, the pull of gravity will be extremely weak," explained Dr Jones. "It will be more like a docking than a landing."

This sort of manoeuvre has only been tried once before, when the Nasa spacecraft Near landed on the asteroid Eros.

Eros is much bigger, though, than Wirtanen, so exerts a bigger pull. Furthermore, nobody knows whether the heart of a comet is soft, slushy stuff or hard ice.

"We don't know what it's like to stand on the nucleus of a comet - what the landscape would look like," he said. "Just that is exciting in itself.

"For years people have discussed if the nucleus is a hard, icy surface or if it is covered in fluffy snow.

"Finally, we will learn what the truth is."