NASA unveiled its US$104 billion plan on Monday (local time) to return Americans to the moon by 2018, aboard a capsule-like vehicle the space agency's chief described as like "Apollo on steroids."

Like the Apollo programme that carried the first humans to the moon in 1969, the new system would put crew members into a capsule sitting atop a rocket, and would have a separate heavy-lift vehicle to take only cargo into orbit.

"It is very Apollo-like ... but bigger," NASA chief Michael Griffin said at a briefing. "Think Apollo on steroids."

The capsule's base would be considerably larger than Apollo's, 5.5 metres compared with 3.9 metres, and it would weigh about 50 per cent more, Griffin said. It would carry six people, instead of Apollo's three, and be able to stay in lunar orbit for six months.

The first human mission to the moon since 1972 would likely take place in 2018, Griffin said, carrying four people for a four- to seven-day stay.

He defended the programme's cost, which is expected to spark criticism in light of US commitments in Iraq and areas devastated by Hurricane Katrina. He said the programme will cost 55 per cent of what Apollo cost, in constant dollars spread over 13 years.

"There will be a lot more hurricanes and a lot more other natural disasters to befall the United States and the world" before the launch in 2018, Griffin said. "... We must deal with our short-term problems while not sacrificing our long-term investments in our future. When we have a hurricane, we don't cancel the Air Force ... and we're not going to cancel NASA."

The new launch system is part of US President George W. Bush's 2004 Vision for Space Exploration, which called for a human mission to the moon by 2020 and an eventual trip to Mars and other planets in our solar system.

The new space system is meant to replace the aging and now-grounded shuttle fleet, but would use some shuttle components, including its solid rocket boosters, main engine and massive external tank, Griffin said.

In the meantime, the United States is committed to completing the International Space Station, using the hobbled shuttle fleet.

The shuttles are slated to retire in 2010, but Griffin said the new Crew Exploration Vehicle, as it is known, will not be up and running until 2012, leaving the United States with no way to get people into space on its own.