The US space agency Nasa has announced plans to return to the Moon by 2020.

Nasa administrator Michael Griffin said four astronauts would be sent in a new space vehicle, in a project that would cost $104bn.

"We will return to the Moon no later than 2020 and extend human presence across the solar system and beyond," Mr Griffin said on Monday.

Nasa sent several manned missions to the Moon between 1969 and 1972. A total of 12 astronauts walked on the Moon.

Different modules could be launched separately into space then joined together for the journey to lunar orbit.

The new missions would use rocket technology used for the space shuttle to cut the costs of development.

'Apollo on steroids'

Dr Griffin said the new rocket would be "very Apollo-like, with updated technology. Think of it as Apollo on steroids."

Nasa is charged with implementing the vision for space exploration, laid out in January 2004 by President George W Bush.

This vision aims to return humans to the Moon, to use it as a staging point for a manned mission to Mars.

"We believe this architecture...achieves those goals in the most cost-effective, efficient manner that we could do it," said Dr Griffin in a news briefing at Nasa headquarters in Washington.

The space shuttle is to be retired by 2010 in order to pay for its replacement, the Crew Exploration Vehicle (CEV) - to be ready by 2012. This vehicle would be shaped like an Apollo capsule, but three times larger, and able to take four astronauts to the Moon at a time.

Fly me to the Moon

Nasa would begin the first lunar expedition by launching a lunar landing capsule and a propulsion stage atop a new heavy-lift rocket.

This will consist of a lengthened shuttle external tank and a pair of five-segment solid rocket boosters capable of putting up to 125 metric tonnes in orbit - about one and a half times the weight of a shuttle orbiter.

The cargo it carries could wait for up to 30 days in orbit for the astronauts to launch aboard their capsule.

Carrying a crew of four, the CEV would blast off atop a single solid rocket booster consisting of four segments - exactly like those flown with the shuttle.

Once in orbit, the manned orbiter would dock with the lunar lander and the propulsion stage and begin the journey to the Moon.

Apollo again?

After a three-day journey, the four astronauts would climb into the lander craft, leaving the capsule to wait for them in orbit.

After landing and exploring the surface for seven days, the crew will blast off in a portion of the lander, docks with the capsule and return to Earth, parachuting through the atmosphere to dry land.

Nasa says it will be able to recover the capsule, replace the heat shield and re-launch the craft up to 10 times.

Dr Griffin attempted to head off criticism that the proposals looked like a re-tread of the Apollo missions, but did acknowledge the similarities: "It's a significant advancement over Apollo," he said.

"Much of it looks the same, but that's because the physics of atmospheric entry haven't changed recently...We really proved once again how much of it all the Apollo guys got right."

Reconstruction costs

The Nasa chief also dismissed suggestions that reconstruction of the Gulf Coast in the wake of Hurricane Katrina could derail the programme.

"We must deal with our short-term problems while not sacrificing our long-term investments in our future," said the Nasa chief.

"When we have a hurricane, we don't cancel the Air Force. We don't cancel the Navy. And we're not going to cancel Nasa."

But Representative Bart Gordon, a Tennessee Democrat on the US House Science Committee, said in a statement: "This plan is coming out at a time when the nation is facing significant budgetary challenges.

"Getting agreement to move forward on it is going to be heavy lifting in the current environment, and it's clear that strong presidential leadership will be needed."

Nasa also envisions the possibility of building a semi-permanent lunar base, where astronauts would make use of the Moon's natural resources for water and fuel