By Dr David Whitehouse
BBC News Online science editor
Scientists are divided about the use of the Moon as a base to develop ways to travel to Mars, according to reports given to the US government.
What would a lunar base look like?
Some have said the possibility of water-ice existing at the lunar poles would allow a moonbase to use the ice as rocket fuel for a Mars mission.
Others contend that it would be too difficult to extract.
And there is disagreement about whether the Moon is a good alternative to space as a base for advanced telescopes.
In January, President Bush redirected the US space effort, sending astronauts back to the Moon and then on to Mars.
Dirt and gravity
As the US prepares for its new direction in space, a series of testimonies presented to it at a hearing entitled Lunar Science And Resources, show how opinion is divided when it comes to using water-ice discovered at the lunar poles as rocket fuel, and even in the value of the Moon as a base for scientific research.
"The discovery of accessible deposits of water on the Moon would profoundly affect the economics and viability of a lunar base," said Cornell University astronomer Donald Campbell.
But he added that recovering water deposits would not be an easy task, since they were likely to exist in the bottoms of very cold, permanently dark craters at the Moon's poles.
Meanwhile, Daniel Lester of the McDonald Observatory, University of Texas, was not keen on using the Moon as a base for advanced astronomical telescopes. He believes space telescopes are better.
"In comparison to zero-g sites in free space, the Moon, as a telescope platform, offers mainly dirt and gravity," he said.
"While dirt has been viewed by some as providing harvestable resources, it also translates into serious performance liabilities."
Telescopes on the moon could be useful
Dr Lester maintains that surface dust kicked up by both meteorites and activity near the telescope (whether blast waves from rockets or footsteps of astronauts) will degrade optical surfaces and reduce performance of loaded mechanical bearings on which such lunar telescopes would critically depend for precision motions.
"In short, we should ask whether dirt and gravity offer any general value to astronomy. The answer, I believe, is no."
He is also unimpressed by claims that the far side of the Moon is scientifically important as a radio-quiet site.
Because the far hemisphere is never in a line of sight to the Earth and its strong human radio traffic and natural radio emission, astronomical observations should be largely free from interference.
"While this is potentially enabling, the scientific need for such a radio-quiet site has never been entirely persuasive," he said.
Paul Spudis of the Lunar and Planetary Institute is more optimistic.
He believes the polar ice can be mined to support human life on the Moon and in space, and to make rocket propellant (by splitting it into liquid hydrogen and oxygen).
Is a lunar base a rehearsal for Mars?
He also believes the Moon is a testing ground - a small, nearby body where we can learn the strategies and operations we need to explore the Solar System.
Telescopes erected on the lunar surface will possess many advantages over both Earth-based and space-based instruments, he says.
"Even during the lunar day, brighter sky objects are visible through the reflected surface glare. The far side of the Moon is permanently shielded from the din of electromagnetic noise produced by our industrial civilisation," he said.
And Dr Spudis does not think lunar dust is a problem. "Recent suggestions that lunar dust poses unsolvable problems and difficulties for telescopes on the Moon are incorrect; lunar dust does not "coat" surfaces if left undisturbed."
John Lewis of Arizona University did not think lunar ice would be easy to mine.
"I regard this suggestion with deep scepticism because of the immense technical difficulty of mining steel-hard and highly abrasive permafrost under conditions of permanent darkness, at the bottom of steep and rugged craters, at temperatures so low that most metals in mining equipment are as brittle as glass.
Astronauts back on the Moon in a decade
"Further, the location of the hydrogen-bearing deposits (almost certainly dominated by water-ice) at the poles is the most remote from sensible locations for a lunar base of any place on the Moon.
"The lunar ice deposits are of great scientific interest for the stories they can tell about comet and asteroid bombardment of the lunar surface. Scientific investigation of these deposits need not, and arguably should not, involve human presence."
He said the use of lunar-derived propellants to support expeditions to Mars made no logistic sense.
"The Moon is not 'between' Earth and Mars; it is a different destination, poorly suited to function as a support base for travel to Mars," he said.