By Paul Rincon
Science reporter, BBC News
An impossible dream? A manned mission to Mars would be very costly.
In the summer of 1969, two weeks after Neil Armstrong became the first man to walk on the Moon, rocket scientist Wernher von Braun delivered to Nasa a detailed, fully costed plan for landing humans on Mars.
Dr von Braun, who had masterminded America's lunar programme, intended to send astronauts on an initial excursion to the Red Planet lasting two years, with a fly-by of Venus on the journey home.
The mission was to reach Mars by 1982 and would be accomplished using a nuclear-powered rocket. This would be parked in Earth orbit and used to ferry 800-tonne spaceships to Mars and back.
The annual cost, von Braun estimated, would peak at $7bn in 1974, running at $6bn thereafter.
But by the end of the 1960s, politicians and the media had had their fill of space; the timing was also wrong economically. As a result, the proposal fell on deaf ears.
Fast forward to this millennium: In 2001, the European Space Agency (Esa) approved an exploration programme, called Aurora, with an ultimate goal of staging a human mission to Mars by 2030.
Then, in January 2004, President George W Bush announced a programme for returning humans to the Moon by 2020 and - at an undetermined date - to Mars.
In the years since von Braun unveiled his masterplan, interest in a manned Mars mission has remained high. But no other proposal submitted in the intervening decades had gained any momentum.
Why? In short, the challenges are immense. A manned mission to Mars would be both more difficult than sending humans to the Moon and more costly. Today, estimates of the cost range enormously from $20bn to $450bn.
President Bush outlined a new vision for Nasa in 2004
"Whenever you ask someone the question: 'Do you think it would be cool to send a man to Mars?' They all say: 'Yeah, that'd be neat, I'd like that,'" says Dr Roger Launius chair of space history at the National Air and Space Museum in Washington DC.
"Then when you give them a price tag - however many billions it is - they say: 'Well maybe we can spend that money better elsewhere.' That's one of the big challenges that anyone who seeks to explore space has."
Nasa chief Mike Griffin recently said he did not foresee humans landing on Mars for another 20 years or more.
Scott Hovland, head of human systems at the European Space Agency (Esa) in Nordwijk, the Netherlands, says: "I have to admit that, right now, a manned Mars mission is far away in the future.
"There are still a lot of things that we really have to look into and better understand before we can commence a trip like that."
Spreading the cost
According to Mr Hovland, it is likely that a manned mission to Mars could only be achieved through broad international co-operation.
The US space agency has been concentrating its efforts on returning humans to the Moon. But it will this year begin a fresh assessment of potential approaches for sending humans to Mars: its Mars Architecture Study.
Europe is carrying out a variety of studies on long duration missions as part of its Aurora exploration programme.
Esa is collaborating with Russia on a unique experiment due to begin next year at Moscow's Institute for Biomedical Problems. The Mars 500 study aims to simulate the tough conditions of a trip to the Red Planet for six volunteers.
They will be locked up for 17 months in an airtight capsule, eating sterile food and breathing processed air. Scientists will study the behaviour of the "crew" as they try to live together in a space measuring 550 cubic metres (19,250 cubic feet).
"Boredom may be their biggest problem," Bruno Gardini, Aurora project manager at the European Space Agency, told BBC News.
"Over 500 days, their activity is fairly low. So what do they do? They get on each other's nerves."
It sounds a bit like the basis for a reality TV show. But the simulated mission, due to begin in late 2008, will help scientists better understand the demands of long-haul space travel as well as informing astronaut selection.
Mr Gardini adds: "You can do psychological profiling, but when you put them under these conditions, you may discover aspects of their personalities of which they are not aware - which are sometimes good and sometimes bad."
Solar flares could prove deadly for astronauts en route to Mars
But an interplanetary journey will present other hazards not addressed in this experiment.
Once crews venture outside Earth's protective magnetic field, they will be exposed to higher levels of cosmic radiation. Deep space is filled with protons from solar flares, gamma rays from newborn black holes and galactic cosmic rays (GCRs) from distant exploding stars.
Astronauts would be exposed to a continuous stream of GCRs. These travel at close to the speed of light and can penetrate both the skins of spacecraft and of people like tiny bullets. As they pass through the body, they break strands of the DNA molecule, damage genes and kill cells.
Short but intense fluxes of particles from solar flares could contribute to a large - even lethal - radiation dose over a short period of time.
"It's going to be a problem. We have to find ways to shield the crew properly," says Mr Hovland.
Bruno Gardini adds: "With this kind of radiation, you cannot simply shield the craft with heavy metal. It can actually make it worse, because when the cosmic rays hit the metal atoms, they can produce secondary particles."
Experiments on the International Space Station have shown that plastics such as polyethylene provide good shielding. Plastics contain lots of hydrogen, which is good at absorbing cosmic rays.
Polyethylene bricks could shield astronauts against radiation
Even if the whole spacecraft is not built from plastic, it could be used to shield key areas such as living quarters. However, the best form of shielding appears to be water. The ship could be designed so that water tanks were placed on the outside, and radiation would hit them first.
Astronaut selection might also be influenced by observations that some people are more susceptible to harm from radiation than others.
Men are generally able to withstand higher doses than women. And older people, with their slower metabolisms, tend to be more resistant than the young. But other considerations are likely to prevent an elderly, all-male crew becoming humanity's first deputation to Mars.
Weightlessness also poses a challenge on such a long journey, because of the muscle atrophy and bone mineral loss experienced by astronauts during extended periods in zero-gravity.
The gravity on Mars is about 30% that of Earth. But after months spent in weightlessness, just walking on the Martian surface could place enormous physical stresses on the human body.
These problems mean mission planners must minimise the journey time. Propulsion will therefore be critical to the success of a manned mission. Cryogenic, storable and nuclear methods of propulsion are all sufficiently well understood to be used on a human mission to Mars.
But a shorter transfer requires more fuel to speed up the rocket. This in turn hikes up the mass of the spacecraft and, accordingly, the launch cost.
Robot explorers have transformed our view of Mars in recent years
A propulsion system big enough for the trip would probably need to be assembled in low-Earth orbit, says Scott Hovland.
That rules out cryogenic fuels, such as liquid oxygen and liquid hydrogen, because they have to be kept at very low temperatures.
"We don't have the technology to keep them in the liquid phase [in orbit]. So we would get boil-off, which means we would lose fuel," Mr Hovland explains.
So-called storable rocket fuels do not suffer from this problem, but are less efficient. Nuclear propulsion must therefore be considered as a serious contender. But additional shielding would be required to protect the crew from radioactive fuel.
A manned mission to Mars would probably use a so-called split-mission architecture, for which cargo is sent first and astronauts are sent later on a faster spacecraft.
This would reduce fuel costs and the journey time.
Onboard systems that recycle air and water could cut down on "storables" that would need to be taken on the journey. Scientists are also looking at whether a Mars crew could grow some of their own food.
Nasa has carried out studies on nuclear propulsion systems
CO2 could even be collected from the Martian atmosphere and broken down to make methane (CH4) - a potential rocket propellant for the return journey.
But much more work needs to be done before the dream of humans setting foot on another planet can be realised.
And, perhaps, the strong incentive required for governments to commit resources is still lacking.
"I've been inspired by the Apollo missions since I was a child. So for me, the very idea of a person going to Mars - the exploration part of this - is enough," says Scott Hovland.
"But then, I'm not the person paying for it."