By Paul Rincon
Science reporter, BBC News
Nasa has released details of its strategy for sending a human crew to Mars within the next few decades.
Landing humans on Mars would pose significant challenges
The US space agency envisages despatching a "minimal" crew on a 30-month round trip to the Red Planet in a 400,000kg (880,000lb) spacecraft.
Details of the concept were outlined at a meeting in Houston, Texas.
In January 2004, President George W Bush launched a programme for returning humans to the Moon by 2020 and - at an undetermined date - to Mars.
The "Mars ship" would be assembled in low-Earth orbit using three to four Ares V rockets - the new heavy-lift launch vehicle that Nasa has been developing.
Notionally despatched in February 2031, the mission's journey from Earth to Mars would take six to seven months in a spacecraft powered by an advanced cryogenic fuel propulsion system.
Estimates of the cost of mounting a manned Mars mission vary enormously, from $20bn to $450bn.
The details are subject to change, and may not represent the way Nasa eventually chooses to go to the Red Planet.
However, the document says this is the agency's current "best strategy" for landing humans on the Martian surface.
Grow your own
The cargo lander and surface habitat would be sent to Mars separately, launched before the crew in December 2028 and January 2029.
According to the Nasa presentation seen by BBC News, astronauts could grow their own fruit and vegetables on the way.
Once there, astronauts could spend up to 16 months on the Martian surface, and would use nuclear energy to power their habitat.
The "Mars-ship" would be assembled in Earth orbit
But the document points out that options for aborting the mission or furnishing the crew with new supplies would be extremely limited.
The difficulties of re-supply mean the astronauts would have to be remarkably self-sufficient.
They would need to be well-versed in the maintenance and repair of equipment and perhaps even able to manufacture new parts.
The spacecraft itself would be equipped with so-called "closed-loop" life support systems, in which air and water would be recycled.
Plants would be grown onboard to feed the crew and contribute to the "psychological health" of the astronauts.
But the report, authored by Nasa official Bret Drake, who sits on the agency's Robotic and Human Lunar Expeditions Strategic Roadmap Committee, says that many challenges remain for ensuring safe passage for the crew.
Nasa needs to come up with solutions for effectively protecting the astronauts from the high levels of cosmic radiation they will be exposed to in deep space and on the surface of Mars.
They will also need medical equipment for the diagnosis and treatment of illnesses or injuries.
Nasa proposes using the Moon as a testing ground for many of these new systems.
Details of the plan, which comes under Nasa's new Constellation programme, were presented at a meeting of Nasa's Lunar Exploration and Analysis Group.