Paris - The United States has pledged to colonize the Moon by 2020 and send astronauts to Mars, but many scientists say dangerous and costly manned space missions should be a thing of the past, not the future.

Intelligent robots and satellites such as those already exploring the Red Planet, they say, do a good job and are a lot less fragile than human organisms too easily stranded millions of miles from home.



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The scientific tug of war over the merits of sending humans into deep space is at least as old as Sputnik, the 83.5 kilo (184 pound) sphere of aluminum crammed with two radio transmitters -- the world's first satellite -- that Russia lobbed into orbit 50 years ago on October 4.

Russian leaders did not foresee the frenzied response Sputnik would provoke, especially in the United States.

But when Washington declared victory in the ensuing space race a dozen years later with the far more impressive feat of putting two men on the moon, no one expected that Apollo 11 would remain the fulcrum of human space exploration for nearly four decades and counting.

"Apollo gave us a false sense of security, it showed us what could be done," commented Doug Millard, space curator of the Science Museum in London. "But all we have managed to do since then -- no matter how magnificent it might be -- is to send humans round and round in orbit around Earth."

Delving deeper into the final frontier, however, is coming back into vogue.

NASA's renewed commitment to lunar and Mars missions, entrepreneurial ventures ranging from space tourism to moon mining, and emerging "space powers" China and India are all promising to launch a new generation of flesh-and-blood explorers beyond the reassuring grip of near-Earth gravity.

"We are looking at the moon and Mars to build a civilisation for tomorrow and after that," the head of NASA, Michael Griffin, told an international astronautical congress in India last week.

NASA has targeted the Moon's south pole for a lunar base as a jumping off point for further exploration of our solar system. Mars is the primary target, with the Phoenix spacecraft set to land on its northern plains next year to see if the Red Planet can support human life.

The US space agency's successful Mars rovers Opportunity and Spirit have been collecting useful information from the surface for years already, but NASA scientists say that is not enough.

"We are many decades from robots that can match humans, even in the lab, and laboratory robotics is about 20 years ahead of space robotics," Steve Squyres, an astronomy professor at Cornell University and principal investigator of the Mars Exploration Rover Mission, recently told a science forum.

Griffin predicts that human footprints will grace Mars by 2037.

But many space experts are skeptical. "I would be surprised if we do it this century," said Millard.

"Going to the Moon was almost like going out for a little swim with a snorkel. Going to Mars is a totally different order of magnitude," he added, citing at least three serious constraints.

One is how to protect human beings from the hazards, many of them poorly understood, of long term space travel.

Cosmic radiation, weightlessness, psychological stress -- no one knows what it will feel like to watch one's home planet dwindle into invisibility -- all pose serious challenges to any future Mars mission.

Another vexing question beside safety is whether Washington, with or without help from other nations, will get enough bang for its new Buck Rogers.

Adding a human being to an exploratory space mission boosts the cost roughly a hundred fold, said Millard.

"You can now put together a pretty decent unmanned mission for a few hundred million euros (dollars), but you are usually talking about many billions for a manned mission," especially if it is something new, Millard said.

The US Congress has already shown a reluctance to fund such ambitious projects.

But there is an even more fundamental problem. "We really don't know how to land anything big on Mars. The Moon has no atmosphere. Mars does, and it is very different to ours," he said.

Beyond questions of feasibility, Millard -- who is not opposed to maintaining a human presence in space -- and other experts critical of long-distance manned space exploration are concerned about priorities.

Rather than preparing for some distant future when human beings might have to flee our planet, we might be better off trying to maintain the one we have, they suggest.

"We need to do as much as we can to use space to look back on Earth, especially given the concerns about climate change," he said.

Investing more in satellites such as Envisat -- the largest Earth observation spacecraft ever built -- will help us monitor Earth's atmosphere, oceans and ice caps.