The BBC's Pallab Ghosh: "The mission is to catch a comet"

A model of the space craft has been shown off at the Royal Society in London. Scheduled for launch in January, 2003, Rosetta will be the first space probe to land on a comet. Its quarry is Comet Wirtanen, a ball of rock and ice just 600 metres across.

The £650m mission to Wirtanen will last eight years and cover a distance of 5.3 billion kilometres. The space craft will use the gravity of Earth and Mars to shoot itself out into the outer Solar System until it is travelling at the same velocity as the comet, around 130,000 kilometres per hour.

Comets could hold secrets to life on Earth

Once in orbit around the Wirtanen, the probe will release a lander that will touch down on the comet's surface. The lander and probe will monitor the changes taking place on the comet as it hurtles in towards the Sun.

As the comet nucleus evaporates, 12 experiments will map its surface and study the dust and gas particles it ejects. Scientists are very keen to see the results. Comets are among the most primitive objects in the Solar System.

Virtually unchanged after 4.6 billion years in the deep freeze of space, they contain unaltered dust and ice left over from the time the planets were formed. Some scientists even think comets contain simple organic compounds that could have been responsible for seeding life on Earth.

Clues to life on Earth

Esa simulation of Rosetta lander docking with comet nucleus

At a news conference at the Royal Society, Rosetta Project scientist Dr Gerhard Schwehm said: "The primary scientific objective of this mission is to study the origins of the solar system, the clues to its evolution.

"The really key question is what role did comets play in the evolution of life."

The lander will probe and analyse the comet's interior

Dr Schwehm used the conference to show off a quarter-size model of Rosetta and its lander. The real spacecraft, with its two giant solar panels, measures 32 metres across.

The UK has instruments on both the orbiter and lander. It is also contributing to a study of the physical properties of the comet's surface, inner structure and tail.