The Cida collector

If confirmed, this discovery could mean that molecules drifting in space before our Earth was formed may have helped start life on our planet and by implication elsewhere in the Cosmos.

"When they got in contact with liquid water on the young Earth, they could have triggered the type of chemical reactions which are prerequisite for the origin of life," said Jochen Kissel, of the Max Planck Institut fur Extraterrestrische Physik, Germany.

So far, five interstellar dust particles have been captured by the German-built Cometary and Interstellar Dust Analyser (Cida), onboard the spacecraft.

Seven year voyage

Nasa's Stardust spacecraft was launched in February 1999. It will visit comet Wild-2 in 2004 and return samples from the comet, as well as interstellar dust, to Earth two years later.

To reach the comet, Stardust will circle the Sun three times. During its seven-year voyage through the Solar System, the spacecraft will encounter streams of interstellar dust several times.

The particles have an impact speed of about 30 km per second (18 miles per second) and on impact are broken up into molecular fragments. However, their original mass can still be estimated.

"It is the size of these molecular fragments, with nuclear masses of up to 2000 units (water has 18 units), which surprised us as much as the seeming absence of any mineral constituents," explains Dr Kissel.

Life in space

The apparent discovery that cosmic particles consist mostly of large and complex molecules could have far-reaching implications.

Only organic (carbon-based) molecules can reach these sizes, which are far larger than anything that has been found in space before.

The biggest molecules previously found in space are the polycyclic aromatic hydrocarbons, which reach a few hundred mass units.

It has been suggested that the molecular building blocks of life could be common in space and eventually land on a planet that by chance offers the right conditions for molecular evolution to take place.

The emergence of the simplest lifeforms may follow.