At the time, for a public buoyed by the success of the Apollo missions and nourished on the vision of "2001: A Space Odyssey," Man seemed to have a dazzling, unlimited future in space.

Next stop: Mars. Then the outer planets, starting perhaps with the satellites of Jupiter. And after that -- who knows?

The air fizzed with talk of human colonies dotted across the Solar System, of "terraforming" lifeless planets into lush gardens, and the possibilities for interstellar travel were given a serious airing.

Three decades on, the reality is sadly very different.

No one has even set a date for a manned trip to Mars. Indeed, Man no longer even visits the Moon.

Instead, his trips are restricted to the International Space Stationand to taxi trips by the veteran US shuttle and the Soviet-designed Soyuz -- the equivalent in low Earth orbit of walking around one's backyard.

The cosmic explorers these days are not made of flesh and blood, they are robots made of alloys and silicon.

What caused the retreat?

Problem No. 1 is money, say experts.

"Amongst the space community there's disappointment that the achievements of Apollo have not been followed up with equivalent achievements," says Doug Millard, curator of space technology at Britain's Science Museum.

"But the situation is no longer the same. The catalyst that sent us to the Moon was geopolitical, it was the Cold War fight which provided the funding for it."

Without the threat factor or national prestige to foot the bill, the cost of manned missions is intimidatingly high.

NASA's budget has been crippled by the spiralling cost of the ISS, whose total bill is estimated at anywhere between 60 billion and 100 billion dollars. Russia's space programme is so cash-strapped that it is contemplating taking up pop stars or quiz-show winners to help make ends meet.

Among the emerging space powers, only China, again for reasons of national glory, has chosen to develop an independent manned programme. But it is treading almost exactly the same path taken by the Cold War rivals more than 40 years ago -- indeed, experts say its hardware is mainly of the same vintage.

But the other big hurdle is technological: the awareness has dawned that today's chemical rockets are just too slow and our spaceships too fragile and tiny to cover the vast distances between planets.

"Going to the Moon is one thing, you can take them there in one or two days, but going to Mars is quite a different story," Hans Rickman, general secretary of the Paris-based International Astronomical Union (IAU), said in an interview.

Apollo 17 made a there-and-back mission to the Moon from December 7-19 1972. But a round trip to the Red Planet, depending on the relative orbital positions of Earth and Mars, could take a couple of years.

A spaceship to Mars would have to be roomy, shielded from cosmic radiation and asteroid collision, and supplied with tonnes of food, water and life-support systems for its crew.

These kind of challenges call for an incremental approach, says Dick Taylor, secretary of the British Interplanetary Society.

As the cost of putting big payloads into space from the Earth is so expensive, the best thing would be to exploit the low gravity of the Moon, he says.

Remote-controlled robots could build a lunar factory, using minerals and gases extracted from moon rocks, that would turn out the propulsion systems and accommodation modules for long-term missions. Big corporations should be encouraged to take part, perhaps earning money from lunar tourism.

"You've got to open up space to commercial use if you want to unleash creative energies and reduce costs," Taylor argues.

Many space scientists are furious about the ISS, dismissing it as a cash-gobbler that has drained funds away from probes that deliver more data at less cost and at no risk to human life.

That may be so. But, says Millard, there will always be a place for manned flight, with its images of courageous men and women battling the cold, airless, boundless cosmos.

"There are lots of intangibles about human space flight which echo voyages of exploration and discovery over the centuries," he believes.

"It is essentially an inquisitiveness, a nosiness, and if we lose that spirit of adventure that could be quite worrying. It's something that helps to define us as human beings."

SPACE.WIRE