The US space agency (Nasa) has unveiled its proposals for returning humans to the Moon in 2020.
But what's behind the drive to go back, and how will astronauts get there?
Why does the US want to go back to the Moon after all these years?
The Moon will serve as a staging post for a Mars mission
Nasa's Apollo programme, which was designed to land humans on the Moon and return them safely to Earth, ran from 1963 through to 1972.
In total, 12 astronauts set foot on the lunar surface over six missions - beginning with Neil Armstrong on Apollo 11, which flew in 1969.
The new proposals to return humans there are part of a US vision for space exploration, announced by President Bush in January 2004.
Within this wider context, the Moon is intended primarily as a testing ground for a manned mission to Mars - a longer-term goal of this vision.
The proposed Moon missions will establish whether it is possible to set up a lunar base, where astronauts can practise for long-term stays on other worlds such as the Red Planet.
What are the astronauts going to do there?
Astronauts will be able to practise "living off the land" on another world before making the long trek to Mars.
Nasa would learn how to turn water-ice stores at the Moon's south pole to produce potable water and fuel. And they would learn to use the sun's rays for power.
But this idea has come in for criticism. Some observers argue that new practices and technologies could be tested for a fraction of the price at remote and inhospitable places on Earth, such as Antarctica.
Nasa has said it is open to international partnership in its exploration of the Moon and that the exact nature of the activities carried out there would largely depend on which countries chose to participate.
So will we finally have a base on the Moon?
With a minimum of two lunar missions per year, Nasa says, momentum will build quickly toward a permanent outpost. But the agency has stopped short of committing itself to building a permanent lunar base.
Critics say that by failing to incorporate this, and a mission to Mars, into its exploration architecture, Nasa is missing an opportunity.
Henry Vanderbilt, head of the Space Access Society advocacy group, said that "like Apollo, Nasa's new plan has built into it the seeds of its shutdown by some future Congress once the warm glow of the first few daring missions has once again faded."
Isn't this just a re-tread of the Apollo missions?
Nasa says it evaluated a large number of possible architectures when formulating its proposals. But, as Nasa chief Mike Griffin says, spaceflight technology has not changed significantly since Apollo.
"Maybe we would like for it to have and maybe if we spent more money on it, it would have," he explained, "but none of those things came true."
In addition, he said, "the physics of atmospheric entry haven't changed recently" and the architecture study carried out by Nasa this summer, only served to prove "how much all those Apollo guys got right".
However, this does not apply to the areas of electronics, avionics and software, where there has been a small revolution. And it is in this area that there are expected to be major advances over the Apollo missions.
The mission is similar in concept to Apollo
The new Crew Exploration Vehicle (CEV) will have a base three times wider than the Apollo command and service modules and will be able to carry six crewmembers - double the number Apollo missions could support.
They will also be able to stay there for longer. The first mission will be able to stay there for seven days. If a lunar outpost is established, crews could stay up to six months.
In addition, while Apollo was limited to landings along the Moon's equator, the CEV carries enough propellant to land anywhere on the Moon's surface.
It will also employ launch technology used by the space shuttle to slash development costs and make use of existing workforce expertise.
What about Mars?
The US space agency believes that going back to the Moon will give it a head start in getting to Mars. In fulfilling its Moon plans, Nasa will develop the heavy-lift rocket, the manned vehicle and propulsion system it requires to mount a Mars mission.
The new lunar lander will also use methane fuel, something Nasa thinks its astronauts can produce from chemical components in the Martian atmosphere.
Will Nasa continue to service the International Space Station?
Announcing its exploration architecture on Monday, Nasa reiterated its commitment to complete the International Space Station (ISS). The grounding of shuttle flights following the Columbia disaster in 2003 put the ISS severely behind schedule and all international partners are keen to proceed apace with construction.
It was hoped that this could be done using the remaining scheduled shuttle flights before the vehicle is retired in 2010. But the grounding of Nasa's three shuttle orbiters this summer has served only to fuel doubts over the viability of continuing the programme.
The CEV could be used to ferry crew and cargo to the ISS
The CEV would be able to carry crew and up to several thousand kilos of pressurised cargo to "rationalise" the space station.
But Nasa chief Mike Griffin also gave a clear indication on Monday that the agency sees a future role for commercial operators in servicing the ISS.
"As commercial operators come into the mix, we can stand down some or all of our own operations to accomodate commercial operators," Dr Griffin explained.