The concept goes back to Apollo

Nasa wants to send a crew of four to the Moon in 2018 for a one-week stay.

The Crew Exploration Vehicle, the spacecraft that will take them there, should be ready by 2012, officials say.

Astronauts on the 2018 mission might return to the Moon in a capsule similar to those used in the Apollo programme but using space shuttle technology.

Humans have not set foot on the Moon since the Apollo 17 mission in 1972

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.

Expensive plans

The American space agency briefed congressional committees on Friday on its plans to spend a reported $100bn developing the spacecraft and associated technology for the lunar programme.

Research aboard the International Space Station is also expected to be cut.

Missions to the Moon will use a landing capsule

The space shuttle is to be retired by 2010 in order to pay for its replacement, known as the Crew Exploration Vehicle (CEV).

Nasa would begin the lunar expedition by lifting a lunar landing capsule and a propulsion stage into space.

It would then launch the CEV atop a new launcher using existing shuttle rocket technology.

The CEV would dock with the lunar lander and the propulsion stage and start its journey to the Moon.

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.

Even before the official announcement, there has been recognition that the proposals will be tough to see through, given the US government's commitments to the Iraq war and recovery from Hurricane Katrina.

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

"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."