A four-person crew will land in 2020 to start building the lunar base, shown here in an artist's rendering. ((John Frassanito and Associates/NASA/Associated Press))

At one time the moonwas a nice place to visit. Now NASA has decided it wants people to live there.

"We're going for a base on the moon," said Scott (Doc) Horowitz, NASA's associate administrator for exploration, in a teleconference from the Johnson Space Center in Houston on Monday.

Thespace agency announced plans to build a base on the moon and permanently staff it by 2024, with astronauts staying for up to six months at a time.

The habitat will most likely be built on the moon's south pole and will serve as a science outpost and test centre for technologies needed for future expeditions to Mars.

The ambitious plan is considered the next phase of space exploration after space shuttles are retired in 2010. It will also take some time to get going. Construction of the base is not set to begin until the first series of flights to the moon scheduled for 2020.

The location of the base is a departure from the Apollo missions of the 1960s, when astronauts landedin the middle area of the moon.

The south pole was chosen because it is sunlit three-quarters of the time — providing greaterexposurefor solar power — and also has possible resources to mine, said associate deputy administrator Doug Cooke.

"This is not your father's Apollo," said John Logsdon, director of the Space Policy Institute at George Washington University. "I think it's the only way to sustain something like this over decades. This is not flag-and-footprints. This is the idea of starting an outward movement that includes long stays on the moon."

NASA will use two vehicles to get to the moon — the Orion exploration vehicle to reach it and the all-purpose lunar lander to touch down and provide the beginnings of base camp.

An estimated timeline has NASA testing its first spaceships in 2009, making the first manned flight of Orion in 2014 and landing the first four-person crew in 2020.

Earlier this year NASA announced plans to launch a lunar probe in 2009 to search for water and mineral resources.

For the first four years of the base, astronauts would only stay a week at a time. After that NASA envisions astronauts staying for six-month stints.

In 2004, a year after the space shuttle Columbia accident that killed seven astronauts, U.S. President George W. Bush announced a plan to return astronauts to the moon by 2020, followed by a mission to Mars.

Last year NASA said it would cost $104 billion just to get back to the moon for its first trip, but on Monday NASA officials declined to estimate the costs of a permanent base.