KEY POINTS:
Nasa, the United States space agency, has given the first details of its plans to set up a permanent settlement on the moon by 2024, as the first part of a plan for manned exploration of Mars and beyond.
The project, first outlined by President George W. Bush almost three years ago, would return a man to the moon for the first time since the Apollo 17 mission of December 1972. That stay lasted just three days.
Under the new programme, beginning in 2020 astronauts would make stays of a week. These would be gradually lengthened, so that within four years the base would be permanently manned, with astronauts living there for six-month stretches at a time.
The chosen site will be at or near one of the moon's poles, probably the south pole, because of the long periods of sunlight these regions enjoy.
This will permit solar power generation, and the production of electricity - in keeping with Nasa's aim of "living off the land". The south pole has a special attraction: The suspected nearby presence of key elements, notably helium-3, a lighter form of the gas that can be used for nuclear power. There have also been signs that deep craters may contain ice, which would provide water and fuel.
The moon base, said Scott Horowitz, Nasa's director of lunar exploration, "will be a central theme in our plan for going back to the moon, in preparation togo to Mars and beyond."
He added that Nasa scientists so far knew less about the lunar poles than they did about Mars, even though the moon was only 400,000km from Earth.
The rockets and landing capsules - the Ares I and Orion programmes respectively - which will ferry astronauts back and forth would be exclusively American, Nasa said. But the agency wants to bring other countries, including Britain, India, Russia and China into the project, as well as the European Space Agency.
The key question, unanswered by Nasa this week, is how much the base may cost. Unofficial estimates put the price tag at around US$100 billion ($145 billion), compared with Nasa's current annual budget of US$17-18 billion. But costs will be spread over two decades, and the agency hopes to offset part of it by attracting private sector investment. The total cost of sending a man to Mars will be far higher, however - at least US$600 billion.
The US also appears determined to avoid the problems which have plagued the still uncompleted international space station by bringing other countries into the process at an early stage.
Contacts with key partners have already taken place, and a conference is scheduled for early next year.
Nasa claims the end of the shuttle programme in 2010, and the winding down of the space station, mean that the lunar base can be paid for within existing budget ceilings.
But critics dispute this, arguing that less high-profile but highly valuable scientific programmes will have to be scaled back if the agency is to avoid going cap in hand to Congress for extra money.
Did you know?
* In 1835, a hoax published in the New York Sun fooled some into thinking there were exotic animals on the moon, from blue unicorns to fire-wielding beavers.
* In 1971, Apollo 14 astronaut Stuart Roosa took seeds with him on the command module. Later, they were germinated on Earth, and came to be called the moon trees.
* Because of the moon's weak gravity, it is much easier to reach lunar escape velocity, making it an ideal staging post for missions to Mars.
* On the lunar near side, Earth appears 60 times brighter than the moon does on Earth.
* The moon has no light of its own and shines by reflecting sunlight.
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