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 on Monday. "Think Apollo on steroids."
The capsule's base would be considerably larger than Apollo's - 18 feet (5.5 metres) compared with 12.8 feet (3.9 metres) - and it would weigh about 50% more, Griffin said. It would be able to 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.
They would get there in several stages, with a cargo vehicle launching to Earth orbit, where it would dock with a later launch of the crew capsule. It would then be propelled to lunar orbit, with a landing craft, whose bottom half is meant to stay on the moon as a long-term base.
Moon voyagers would return to the capsule in the top half of the lander and travel back to Earth, floating down safely with the help of parachutes and airbags to the projected landing site at Edwards Air Force Base in California.
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, its main engine and its massive external tank, Griffin said.
Griffin defended the programme's cost, which is expected to spark criticism in light of current US commitments in Iraq and in the areas devastated by Hurricane Katrina. He noted that this programme will cost 55% 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 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.