After flying its last mission into space last year, the shuttle Discovery arrived Tuesday at Dulles International Airport outside Washington, D.C., atop a NASA 747 to enter the National Air and Space Museum at its giant Udvar-Hazy facility in suburban Virginia.

Sister ships Endeavour, Atlantis and early prototype shuttle Enterprise will be similarly displayed in Los Angeles, Cape Canaveral and New York respectively.

"It's hard to escape the idea that we are going backwards," Chiao - a veteran of three shuttle missions and a trip to the International Space Station on a Russian Soyuz rocket and now a private consultant and adviser to industry group the Space Foundation - told Reuters.

"I struggle to even think of it (the shuttle) in the past tense. It was a great vehicle."

With Russia's commitment to human spaceflight also seen wavering and some observers questioning whether even emerging powerhouse China will stick to its brashly self-confident plans, some begin to suspect the world is simply giving up.

While it hopes to resume manned spaceflight around 2017 once it has re-engineered some of its unmanned rockets, in the short term the United States has been left totally dependent on Russia for transport to and from the International Space Station.

NASA retains the longer term plans to return to the Moon, land people on a near-Earth asteroid and ultimately reach Mars. But veterans of the space program say there is a growing mismatch between resources and ambition.

"If you ask people whether space exploration is important, then eight or nine times out of 10 they will tell you that it is," said the Smithsonian Institution's Roger Launius, senior curator for space.

"The only problem comes when you tie that to funding. No president or congressman is ever going to say that he is putting an end to the human spaceflight program. But they may make decisions that stop you getting the resources to do anything,"

SIMPLY LOSING INTEREST?

After a series of accidents and technical glitches, Russia itself says it is no longer prioritizing manned spaceflight. Some of its experts say they fear the focus on the space station has left its program falling behind on more commercially lucrative ventures such as putting satellites in orbit.

China, which first put a man in space in 2003 and intends to do so again later this year, happily talks of long-term plans leading to Mars. Its space strategy includes its own manned space station by 2020 and a lunar landing by 2030.

That would be almost half a century since the last Apollo Moon landing in 1972. But with China facing its own very particular challenges of a rapidly aging population, some believe appetite there too may wane.

"People have lost interest in spaceflight," says James Lewis, a former U.S. Foreign Service officer and now senior fellow for technology and policy at the Center for Strategic and International Studies, a Washington think tank. "You hear complaints even in China that the money would be better spent on the ground on healthcare and social services."

Some experts believe the decisions to be made on spaceflight in the coming decades could determine the fate of the entire human race. Some scientists, such as British physicist Stephen Hawking, have long warned that unless mankind moves beyond a single planet it risks extinction from either nuclear war or a natural catastrophe such as a meteorite impact.

"I think it's essentially a binary choice," says John Logsdon, veteran space historian and now a professor at the Space Policy Institute at George Washington University. "In 50 years time, we will either have Antarctica-style research stations elsewhere in the solar system or the era of government-funded manned space exploration will be over for good."

A VERY DIFFERENT FUTURE?

At the National Air and Space Museum, its curator says he doubts many visitors realize that humanity might be quietly turning away from the kind of spacefaring future portrayed in "Star Trek" and countless other science fiction works.

"People very rarely think about that, but if you were to ask them they would say that probably is what they would still expect," says Launius. "Will it really happen that way? Probably not ... it's certainly a lot more complicated."

Some wonder whether the entire focus on national space programs and manned spaceflight may be somewhat misplaced. Like so much else in the aerospace and defense sector, the future might have much more to do with robot drones and an ever increasing role for private companies.

Improvements in information technology, potentially including artificial intelligence, could revolutionize the use of robot probes. Some scientists even suggest intelligent machines, not organic lifeforms like humans, might be most likely to colonize space.

Private companies too are rapidly expanding their activities, from bidding for contracts to supply the International Space Station to British entrepreneur Richard Branson's Virgin Galactic orbital tourism venture.

For now, many doubt private firms will ever find an economic argument for true deep space travel. But whatever the method, astronaut Chiao remains broadly optimistic.

"At the time of the moon landings, you would have thought we would already be on Mars by now," he says. "I still think we will get there, although it is taking a lot longer. If we did not, it would be a real tragedy."

(Additional reporting by Sally Huang and Ben Blanchard in Beijing; Editing by Jackie Frank)

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