The American Space Agency, Nasa, has been explaining its research programme to look for extraterrestrial life. Its Astrobiology Institute - a "virtual" institute linking 11 different labs via the Internet - has come up with the "roadmap" it hopes will lead it to its goal. The map begins on Earth.

David Morrison, Director of Space at Nasa's Ames Research Centre, laid out the map for the American Association for the Advancement of Science annual expo in Anaheim, California.

It shows a convoluted course that wends its way through super-hot undersea vents, travels through the frozen Antarctic and the oceans, before shooting off toward Mars and Europa - one of Jupiter's moons - and beyond.

"It's an effort to answer some fundamental questions that have been with us for a very long time," he said. "What was the origin and evolution of life? Is there life on other worlds, and what's the future of life on Earth and in space?"

Future funding

The Astrobiology Institute is carefully separated from the non-governmental Search for Extraterrestrial Intelligence Institute, known as SETI. Nasa had a similar project that lost if funding shortly after being set up.

Now the new institute has secured $9 million for this year and $20 million for 2000 to try to determine under what conditions life can survive and thrive and whether these conditions do or ever did exist elsewhere in the solar system.

Much of the evidence that will prove the existence of extraterrestrial life, Nasa believes, is to be found on Earth. We already know that life can exist on our own planet under the most extreme conditions. The recent discovery of hydrothermal vents on the ocean floor which support whole ecosystems even in the absence of light has demonstrated how versatile life can be.

Some of the work will inevitably be done in space. Nasa will look for the conditions that could support life, or could have supported life millions of years ago. Venus for example, although completely inhospitable now, could have supported some form of microbial life earlier in its evolution.

Pluto's moon

The most likely places to look are beneath the surface of planets such as Mars, and Jupiter's icy satellite Europa. Pluto's moon Charon has also been added to the list.

The first step is to decide where to go to take samples, and the recent Pathfinder mission to Mars has provided valuable guidance.

"We now have a new strategy that will allow us to select the right places to go and hopefully bring back the right samples to answer this very fundamental question," explained Professor Jack Farmer of Arizona State University.

All this information will be useful if we eventually decide to visit other planets. Understanding how plants and animals can adapt to extreme conditions is vital if men and women are to survive in space for long periods.

"For a long-term habitation of Mars, you'll have to grow crops, deal with microbes and so forth," David Morrison said.