When American astronaut Neil Armstrong took that first small step on the Moon in 1969, it seemed to many that mankind was poised to seize the high frontier. Three decades on, the dream of a human destiny in space has faded amid escalating costs. The jewel in the crown of the space programme - the International Space Station - has failed to capture public imagination. It rotates without achieving much of value, scientific or otherwise.

So why go into space? Nations entered the space race for military and political reasons. With the end of the Cold War in 1989, the emphasis shifted from aspects such as the arms race to loftier motives, including understanding the Earth’s climate and resources. There was much talk of profitable by-products, a host of which can be seen in the timeline below.

Undoubtedly, there is money to be made in space, and many companies are already out there making it. For example, telecommunications satellites and global positioning systems are used the world over. But the space programme will never pay its way overall. It will always need public funds.

Governments are happy to come up with cash for national priorities, such as spying, weather forecasting and crop monitoring - all of which are best done from orbit. Astronomers are also big users of satellites because the Earth’s atmosphere spoils the view of the universe from the ground. Data sent back by the Wilkinson Microwave Anisotropy Probe recently enabled cosmologists to measure the age of the universe and determine its mix of constituents with astonishing accuracy. The Hubble Space Telescope has also proved an enormous boon, while space probes sent to the planets have taught scientists a vast amount about how the solar system formed and evolved.

Some of these things are surely worth paying for: even in relatively poor countries people support astronomy.

The real bone of contention lies with manned space flight. From the scientific point of view, the return on investment from employing astronauts is meagre. So long as the scope of the manned space programme is restricted to low Earth orbit, sending astronauts aloft will fail to inspire new generations of taxpayers.

Taking a more ambitious view, however, there is one glittering prize that beckons - the planet Mars. Mars is the most Earth-like of our sister planets and therefore the safest to explore. It has the necessary raw materials to support a permanent human colony. Such a colony is the best guarantee we have for the long-term survival of our species, because it could act as a lifeboat in the event of a truly major calamity on Earth, such as the impact of a large comet or the outbreak of a killer plague.

Mars is also compelling for human exploration because it may well hold the key to life on Earth. Though a freeze-dried desert today, billions of years ago the red planet was warm and wet, with rivers, lakes and active volcanoes, making it favourable for biology. Because Mars suffered less damage from intense early bombardment by asteroids and comets, it was a better place than Earth for life to start.

Life’s origin remains one of the greatest outstanding scientific puzzles.

Scientists can’t even agree if it was a freak event, unique in the universe, or a more or less automatic process to be expected whenever there are Earth-like conditions. Nothing could better help solve this enigma than a sample of biology which showed that life started from scratch independently of terrestrial life, on another planet.

But if life occurs, or occured, on Mars, finding traces of it won’t be easy. Only trained scientists on the ground are likely to identify the right clues. Any residual life would be in the form of microbes dwelling deep underground; getting at them would demand drilling rigs and other complex equipment needing human operation.

But the payoff could be sensational. With an alien life-form for comparison, unravelling the mystery of life on Earth would be much easier, enormously boosting biotechnology and even paving the way for the creation of artificial life in a test tube. More significantly, knowing that life had happened twice in the solar system would imply that the universe is teeming with it - a discovery certain to transform our view of ourselves and our place in the great cosmic scheme of things.

Professor Paul Davies, winner of the 2002 Royal Society Michael Faraday Prize, is in the Australian Centre for Astrobiology at Macquarie University, Sydney. His latest book is The Origin of Life, (Penguin)