Those of us who are now middle-aged can remember the murky live TV pictures of Neil Armstrong's 'one small step' We imagined follow-up projects: a permanent 'lunar base', rather like the one at the South Pole; or even huge 'space hotels' orbiting the Earth. Manned expeditions to Mars seemed a natural next step. But none of these has happened. The year 2001 didn't resemble Arthur C. Clarke's depiction, any more than 1984 fortunately) resembled Orwell's.

The Apollo program was an isolated episode, motivated primarily by the urge to 'beat the Russians'. It's 30 years since the last lunar landing. To young people, the Apollo programme is a remote historical episode: they know the Americans landed men on the Moon, just as they know the Egyptians built the pyramids; but the motivations seem almost as bizarre in the one case as in the other. The 1995 (?) film Apollo 13 -- a 'docudrama', starring Tom Hanks, of the near-disaster that befell three astronauts on a voyage round the Moon --- was for me (and I suspect for many others of similar vintage) an evocative reminder of an episode we followed anxiously at the time. But to a young audience, the outdated gadgetry and the 'right stuff' values seemed almost as antiquated as a traditional 'Western'.

The practical case for manned spaceflight gets ever-weaker with each advance in robotics and miniaturisation. But the use of space for communications meteorology and navigation has forged ahead. Space exploration for scientific purposes can be better (and far more cheaply) carried out by fleets of unmanned probes, exploiting the advances that have given us , using the same technical advances that have given us mobile phones and high-power laptop computers here on Earth.

But what is the future for manned spaceflight? The International Space Station is neither practical nor inspiring. Thirty years after men walked on the Moon, a new generation of astronauts is going round and round the Earth, in more comfort than MIR (its Soviet predecessor) offered, but much more expensively. The reduction in the number of astronauts to three makes it even less likely that they will be able to pursue any serious or interesting projects: they will be preoccupied simply in 'housekeeping' tasks.

There is maybe just one argument for the space station: if one believes that in the long run space travel will become routine, it ensures that the 40 years experience of the US and Russia isn't dissipated.

Does our future lie in space? There have been two 'space tourists so far, Dennis Tito, an elderly American financier, and Mark Shuttleworth, an South African internet entrepreneur. Each of them paid 20 million dollars. There is a queue of others who wish to follow them, even at that price. But there would be far more if the tickets were cheaper.

Present launching techniques are as extravagant as air travel would be if the plane had to be rebuilt after every flight. Spaceflight will only be affordable when its technology comes closer to that of supersonic aircraft. Tourist trips into orbit may then become routine. And when circling round and round the Earth seems so tame and routine that it palls, some will yearn to go further-- to the Moon, or even to Mars.

If a private consortium were ever to fund a high-risk expedition to Mars, it might well follow the cut-price 'Mars direct' strategy advocated by the maverick American engineer Robert Zubrin. An unmanned rocket-load of supplies, together with a factory to make fuel for the return trop, would be sent in advance. The crew would then follow -- heading straight for Mars, rather than using the huge and expensive International Space Station as a staging-post.

Would anyone want to go? There may be a parallel here with terrestrial exploration. This was driven by a variety of motives. The explorers who set out from Europe in the 15th and 16th century were mainly bankrolled by monarchs, in the hope of recouping exotic merchandise or colonising new territory. Some, for instance Captain Cook's three 18-th century expeditions to the South Seas, were publicly funded, at least in part as a scientific enterprise. And for some -- generally the most foolhardy of all -- the enterprise was primarily a challenge and adventure: the same motive that drives test pilots, mountaineers, round-the-world sailors and the like.

Any of these motives could drive the first travellers to Mars, or the first long-term denizens of a lunar base. The level of risk would be no higher than it was for the 'classic' explorers. Travellers into space would never be venturing into the unknown to the extent that the great terrestrial navigators were. There would admittedly be a 30 minute turnaround for messages to and from Mars.. But traditional explorers took months to send messages home -- or had no opportunity whatever, as in the case of Captain Scott and other polar pioneers.

Space travel is difficult and extravagant primarily because it takes several tonnes of chemical fuel to propel one tonne of payload away from the grip of the Earth's gravity. But if there were (say) ten time more power, or ten times more thrust for each kilogram of fuel, then mid-course adjustments could be made whenever necessary, just as we do when driving along a winding road. Keeping a car on the road would be a high-precision enterprise if the journey had to be programmed beforehand, with no chance of adjustments on the way.

We don't yet know what technology might provide more efficient propulsion systems -- nuclear or solar energy are two obvious options. It would greatly help if the propulsion system and the fuel needed to escape from Earth's gravity could be on the ground rather than having to be part of the cargo. One possibility is the space elevator -- a rope made of carbon fibre extending 25000 kilometres up into space, and held vertical by a geostationary satellite. This would allow payloads and passengers to be hoisted to geostationary orbit by power supplied from the ground. The rest of the voyage could be powered by a low-thrust (perhaps nuclear) rocket.

If there were plenty of power, space travel would be an almost unskilled exercise. The target (the moon, Mars or an asteroid) is always in view. One just has to steer towards it, and use retrojets to brake by the right amount at journey's end.

Before humans venture into deep space, the entire solar system will have ben explored and mapped by flotillas of tiny robotic craft, controlled by the ever more powerful and miniaturised 'processors' that nanotechnology will make possible. Before any human explorers head for Mars, they will have sent not just the mechanical provisions envisaged by Zubrin, but perhaps also 'seeds' for plants genetically-engineered to grow and reproduce on the Red Planet.

Within a century there could be a permanent presence. Moreover, once the infrastructure was there, the journey could be at a modest cost.

There are of course issues of environmental ethics. Would it be appropriate to exploit Mars, as happened when the pioneers advanced westward across the United States? Or should it be preserved as a natural wilderness, like the Antarctic? The answer would I think depend on what the pristine state of mars actually is. If there were any life already on Mars -- especially if it had different DNA, testifying to quite separate origin from any life on Earth --- then there would be widely voiced insistence that it should be preserved as unpolluted as possible. What would actually happen would depend partly on the character of the first expeditions. If they were governmental (or international), Antarctic- style restraint might be feasible. On the other hand, if the explorers were privately-funded adventurers of a free-enterprise (even 'anarchic') disposition, the Wild West model would be more likely to prevail.

The focus will not forever be exclusively on Mars. As the visionary American scientist Freeman Dyson reminds us, the main habitable surface in the Solar System is not on the planets, but on the vast numbers of smaller bodies: he envisages life spreading and diversifying among asteroids and comets -- even in the cold outer reaches of the Solar System.

One scenario for the second half of the century -- which I think is technically and sociologically realistic -- would involve a permanently manned Lunar Base, some pioneers on Mars, and perhaps small artificial habitats cruising the Solar system, attaching themselves to asteroids or comets. Space will also be pervaded by robots and 'fabricators', allowing large construction projects, using raw materials that need not come from Earth,

In the centuries ahead, some groups could develop into communities quite independent of Earth, unconstrained by any restrictions. Some would surely exploit the full range of genetic techniques and diverge into a new species. There will be a new impetus to evolution and diversity - -- driven by genetics, perhaps by intelligent machines.

The creatures that could, within a few hundred years, occupy sites in our Solar System, would all be recognisably humanoid. However, interstellar travel, if it ever happened, would be a task for post-humans -- voyages lasting many generations, or in suspended animation, or via transmission of 'encoded' information (a kind of 'space travel' that could happen at the speed of light). leading to the assembly of artefacts or the 'seeding' of living organisms in propitious locations. This would be as epochal an evolutionary transition as that which led to land-based life on Earth. But it could still be the beginning of cosmic evolution.

A hackneyed anecdote among astronomy lecturers describes a worried questioner asking: "how long did you say it would be before the sun burnt the earth to a crisp?" On receiving the answer, "five billion years," the questioner responds with relief: "thank God for that, I thought you said five million." What happens in far future aeons may seem blazingly irrelevant to the practicalities of our lives. But I don't think the cosmic context is entirely irrelevant to the way we perceive our Earth and its future.

An iconic image from the 1960s was the first photograph from space, showing our home planet of land, oceans and clouds: The beauty and vulnerability of 'spaceship Earth' contrasts with the stark and sterile moonscape on which the Apollo astronauts left their footprints. This image added no new facts -- just a new perspective that many hoped would make us more mindful of the collective ties that bind us to our environment.

Likewise, our perspective is changed if we realise the huge expanses of time that lie ahead. Our Sun is less than half way through its life, and our Universe has a future that could be infinite. We are not the culmination of evolution: indeed we are still near the cosmic beginning.

Even if life is now unique to Earth it could eventually 'take over' the cosmos. There is plenty of time for life to spread through the entire Galaxy, and even beyond. The entire Galaxy, extending for a hundred thousand lightyears, could be 'greened' in less time than it took for us to evolve from the first primates.

We cannot conceive what might happen still further ahead. Intelligently-controlled modifications could lead to faster and more dramatic changes than Darwinian natural selection allows. The future may lie in artefacts created by us and in some way descended from us, that develop via their own directed intelligence. Our universe has the potential to harbour a teeming complexity of life far beyond what we can even conceive. In the long term the universe may, in a sense, become alive.

But how important is the Earth in this cosmic perspective? This depends on whether life is already pervasive in the universe, or if (as is equally conceivable) we on Earth are unique within the entire Galaxy. We have no idea -- the SETI Institute in Mountain View, California is listening for signals that could come from intelligent aliens, or perhaps from intelligent machines that have been created by some long-dead civilisation. perhaps the cosmos is already teeming with life; perhaps, on the other hand, the odds are stacked so heavily against life's emergence that we on earth are unique.

It would in some ways be disappointing if searches for alien intelligence were doomed to fail. On the other hand, it would boost our cosmic self-esteem: if our tiny Earth were a unique abode of intelligence, we could view it in a less humble perspective than it would merit if the Galaxy already teemed with complex life.

If the cosmos is already teeming with life, the Earth's fate would be of 'merely' terrestrial significance. Life could 'take over' the cosmos whatever happens here.

Thomas Wright of Durham expressed this thought 250 years ago: "In this great Celestial Creation, the Catastrophy of a World, such as ours, or even the total Dissolution of a System of Worlds, may possibly be no more to the great Author of Nature , than the most common Accident in Life with us, and in all Probability such final and general DoomsDays may be as frequent there, as even Birth- Days or Mortality with us upon this Earth."

But suppose Earth is the unique abode of intelligence in the Galaxy. The fate of humanity could then have an importance that is truly cosmic -- reverberating through the whole of Thomas Wright's 'Celestial Creation': what happens here might conceivably make the difference between a near eternity filled with ever more complex and subtle forms of life and one filled with nothing but base matter.

The first aquatic creatures that crawled onto dry land more than 300 million years ago, may have been unprepossessing brutes. But if they had been clobbered, the potential of land-based life would have been destroyed. Likewise even the most misanthropic among us should be mindful of the post-human potentialities that would be foreclosed were humans to be snuffed out. We should fervently hope that we avoid irreversible catastrophe at least until self- sustaining communities had started to establish themselves beyond the Earth. This thought should give us even stronger motives to cherish our Earth, this 'pale blue dot' in the cosmos, and not foreclose life's future-- a future that could be even more prolonged than the time span over which simple life has evolved into humans. Perhaps also it offers an extra motive for being interested in the long-term role that humans may have in space.