In the 1970s, a young Nasa scientist named Jim Garvin made his superiors a proposal. The success of the 1976 Viking landings on Mars, he said, had convinced him that it was worth sending a manned mission to the Red Planet. The technical challenges, he admitted, were vast, and there was little or no chance of sending astronauts there and expecting to bring them back to Earth alive. But the value of the science a lone explorer could do there was such that it was worth the risk. Garvin volunteered to make the trip himself - on a one-way ticket. He would spend weeks, months, even years conducting the scientific experiments that could prove whether Mars had ever bloomed with life, and send his results back to Earth. Then, when his supplies of oxygen, food and water were exhausted, he would die a lonely death, consoled by his contribution to human history.
Nasa never took Garvin up on his offer - though it is said they considered his proposal seriously before turning it down. And Garvin has gone on to become one of the US space agency's senior officials - he is now chief scientist for Mars exploration. His dream of sending astronauts to Mars, however, has now entered centre stage as never before.
On January 14, a new plan for US space exploration was unveiled by President Bush, promising to send humans back to the Moon for the first time since the end of the Apollo programme in 1972, as a prelude to a manned mission to Mars. Over the next decade, Nasa will retire the space shuttle, complete the International Space Station, and then turn its attention further afield. Moon landings can be expected to start again by 2014, with a permanent lunar base to follow in the latter half of the decade.
Then will come the real challenge - sending men and women to Mars, probably by about 2030. The timescale means that the astronaut who will eventually emulate Neil Armstrong, becoming the first human to set foot on the Red Planet, has already been born and is probably already at school.
The plan is certainly grandiose and ambitious, and it has plenty of critics. While President Bush promised an extra $1bn (pound;541 million) a year for Nasa's budget over each of the next five years, this is a drop in the ocean when set against the estimated $400 billion to $500 billion (pound;216 billion to pound;276 billion) cost of sending astronauts to Mars. It is probably not a coincidence that the President is up for re-election in November: the tragic loss of the space shuttle Columbia last February and the landing of the Spirit and Opportunity Mars rovers have reminded the American public of the excitement of space flight.
Douglas Osheroff, a Nobel prize-winning physicist at Stanford University, who helped investigate the Columbia disaster, is one of many experts who would prefer to see any extra money invested in unmanned robotic probes which offer much better value than manned spaceflight. The Spirit rover, for example, cost $400 million - about the same as a single shuttle launch and a fraction of the estimated $100 billion final cost of the International Space Station.
"The cost of a manned enclave on the Moon, I think, is going to make the space station look cheap," he said. "That's the only good thing about it. I think we're still 30 years from going to Mars, and if there's any reason to do that, I don't know."
This is a question asked by many - why go? With environmental and humanitarian issues reaching crisis point on this planet, the amount of money spent on getting to another one can seem obscene. Nasa tries to justify it thus: Human evolution: Mars is the next logical step in the expansion of the human race into the stars; Comparative planetology: by understanding Mars and its evolution, a better understanding of Earth will be achieved; International co-operation: an international effort has the potential to bring about a sense of global unity as never seen before; Technological advancement: the development of new and improved technologies for the Mars mission will enhance the lives of those on Earth while encouraging high-tech industry; Inspiration: the human Mars exploration mission will test our technological abilities to their maximum. Our accomplishments will serve to inspire future generations; Investment: the cost is reasonable when compared with the costs of other current societal expenditures.
There is another reason. The plan has given Nasa what it has lacked ever since Eugene Cernan and Harrison Schmitt became the last humans to walk on the Moon in 1972 - a sense of vision. With the Moon and Mars to work towards, the US space programme now has an ultimate goal that will inform all its individual projects.
"The human space programme has been bogged down in Earth orbit for many years with no exploration, no pushing the frontiers of knowledge," says Louis Friedman, executive director of the Planetary Society. "Mars exploration would be a tremendous boost, far better than aimlessly drifting in space."
What, though, of the technical challenges? How is it going to be possible to put humans on Mars, when two-thirds of all attempts to send robots there have ended in failure? Britain's Beagle 2 lander, which never phoned home after entering the Martian atmosphere in the early hours of Christmas Day, is only the latest probe to fall victim to the "Mars jinx". Nasa might have succeeded with Spirit and Opportunity, but it suffered a double embarrassment in 1999, with the loss of both the Mars Polar Lander and the Mars Climate Orbiter spacecraft. What must be done to prepare the way for a Martian Armstrong moment?
First, there will be a new generation of robotic probes and landers. Every two years, when Mars's orbit brings it relatively close to Earth, a flotilla of craft will set off for the planet, continuing the mapping and scientific investigations that are essential if a manned landing is to be plausible and worthwhile.
Then, there is the matter of designing and building a spacecraft that can get astronauts safely to the Red Planet. And, as Garvin's one-way mission is no longer an option, to get them back. At present, Mars is a little more than 100 million miles (160 million km) from Earth, and, as the two planets' orbits take different paths, the flying distance is never less than 250 million miles (402 million km). Beagle 2 took seven months to reach Mars, and a manned mission would probably take longer - 2003 marked Mars's closest approach to Earth for 60,000 years.
A manned Mars spacecraft, too, would have to be many times heavier than unmanned probes and it would have to be capable of taking off again from the Red Planet, escaping its gravity, and bringing its crew back to Earth.
This means that today's chemical propulsion systems will be inadequate.
Last year, President Bush announced Project Prometheus, which aims to develop a nuclear- powered rocket to take humans to Mars. Its success is probably going to be critical if the trip is ever to be made.
Another possibility is assembling a Mars spacecraft in orbit - or at Nasa's Moon base, if it is ever built - to take advantage of the much lower escape velocity needed to send a vessel to Mars from these starting points. At present, the main cost of spaceflight is launching from Earth orbit, which is estimated at $1m (pound;541,000) for every kilogram. No gravity equals much cheaper launches.
One Nasa plan suggests that a manned Mars mission could use as many as 12 separate spacecraft, working in relays. During a first "launch window" when the Earth and Mars were close, three unmanned spacecraft would set off for Mars. One would take a "crew return vehicle", to be parked in Martian orbit. Another would carry an "ascent vehicle" to the surface, which astronauts would later use to leave the planet and rendezvous with the return vehicle. A third would ferry a rover, oxygen units, nuclear plants, spares and a laboratory to the surface.
When the next launch window occurred two years later, three more craft would launch, taking back-up vehicles and equipment, along with a crew of six or seven people. The whole process would be repeated in the next two launch windows, allowing a fresh crew to replace the initial ones. After spending a year or more on Mars and waiting for the planets to converge, the astronauts would climb into the ascent vehicle, transfer to the return vehicle, and then make the seven- or eight-month journey home.
One of the proposed functions for a permanent base on the Moon, under the Nasa plan, would be to test much of the technology needed for this sort of relay system. Prototype crew return and ascent vehicles could be tested in a relatively controlled environment, just three days from Earth should anything go wrong.
All this would require humans to spend unprecedented lengths of time in space. At present, the record for a continuous flight in space is held by the cosmonaut Valery Polyakov, who spent 438 days in orbit; a crew flying to Mars could expect to spend at least two years away from Earth. Such a long flight has the potential to endanger human health in several ways, and these remain perhaps the most difficult of all the challenges facing a manned mission to Mars. They are also what makes the prospect of permanent colonisation of the Red Planet such a distant piece of science fiction.
Leaving aside the hazards of launch, entry to the Martian atmosphere, descent and landing, the astronauts are going to have to live in microgravity conditions for months on end during the voyage there and back, and under Martian gravity that is about 40 per cent of that on Earth. This can cause all sorts of health difficulties, such as bone loss, muscle wastage, kidney stones and immune system deficiencies. These cause enough problems, such as an inability to walk, when astronauts return to Earth, and have immediate access to specialist medical care. When landing on Mars, they could prove disastrous. A crew member whose weakened heart muscles gave out, or whose thinned leg bones snapped under Martian gravity, would stand little chance of survival. Nasa needs to find ways of reducing these problems before a manned attempt on Mars can even be contemplated. Research on the International Space Station will now concentrate on manned spaceflight: many of the experiments will investigate the long-term effects of microgravity on its crew, who will spend months and even years in orbit.
Any eventual Moon base will have a similar brief.
Potentially greater still are the dangers that radiation would pose. The Earth's magnetic field protects people from the fiercest effects of the Sun's radiation, and even the space station is in a low enough orbit to benefit from the shielding of the magnetosphere. Missions to the Moon and Mars, however, would have no such protection and they would be exposed to radiation levels many times higher than those on Earth. If not killed outright by its effects, they might develop cancer on their return. Living on Mars would present the same problems, as the planet has no protective magnetic field of its own. If microgravity doesn't get them, there is every chance that radiation will.
This means that heavy radiation shielding is going to be needed for every phase of the mission. All this means extra weight and extra expense unless light but effective shields can be developed. That will be another major focus of Nasa's research.
Another challenge is the Martian environment. Temperatures can reach as low as - 133C, and the planet is regularly swept by destructive dust storms. A life support mechanism much more sophisticated than that which sustained the Apollo astronauts on the Moon is going to be required.
Put these problems together, and it is easy to see why so many are sceptical that President Bush's approach will bear fruit. But in the longer term, there is no reason why these problems should not be overcome. Eugene Cernan, the last man to walk on the Moon, puts it succinctly. "The first people to walk on Mars are already alive. It's our job to figure out how to get them there."
Mark Henderson is science correspondent for The Times
Diameter: 4,246 miles (6,800km)
Day: 24 hours, 37 minutes, 22 seconds
Year: 687 Earth days (669 Martian days)
Average:55 LESS THAN C (-67 LESS THAN F)
Min: - 133 LESS THAN C (-207.4 LESS THAN F), at the poles in winter
Max: 27 LESS THAN C (80.6 LESS THAN F) in summer on equator
* Mars is currently little more than 100 million miles (160 million km) from Earth. On August 27 last year, it was just 35 million miles (56 million km) away - the closest for 60,000 years.
* Olympus Mons, at 14 miles (22.5km) high and 341 miles (549km) across, is the largest volcano in the Solar System. It has two moons, Phoibos and Deimos, meaning: "fear" and "dread" in Greek.
* Mars was a key presence in Babylonian astrology, in which it was known as Nergal, the Star of Death.
* The ancient Greeks knew it as Pyroeis, the Fiery Star. It later became Ares, after the God of war.
* The Romans named it after their war god. It can be seen easily from Britain as the brightest object in the southern night sky.
Plotting the dream
Though most scientists think a manned mission to Mars remains unlikely before 2030 at the earliest, a programme of exploration for the Red Planet has already been mapped out for the next decade. Both Nasa and the European Space Agency (ESA) are planning an array of missions to be sent in the two-yearly "launch windows" during which the orbits of the Earth and Mars bring the planets closer together.
Already in orbit around Mars: Mars Global Surveyor; Mars Odyssey: Nasa probes that are imaging the surface and collecting information on the planet's geology.
2003-4: Mars Express: ESA orbiter, launched June 2003, arrived safely in orbit around Mars on Christmas Day 2003. It carries the best high resolution stereo camera yet sent to the planet, ground-penetrating radar that will search for buried ice, and instruments to measure atmospheric composition, the geological make-up of the surface, and the way water is evaporating from the atmosphere under the influence of the solar wind.
Beagle 2: British, ESA-supported lander, carried by Mars Express arrived on Mars on Christmas Day 2003. Nothing was heard from it after its scheduled landing and it is now presumed lost, though attempts to contact it will continue into this month. Its mission was to conduct experiments on the ground and atmosphere to determine whether the planet ever supported life.
Spirit and Opportunity: Twin Nasa rovers. Spirit landed at Gusev Crater on January 4 this year. Opportunity landed at Meridiani Planum on January 25.
Golf cart-sized, six-wheeled robots searched for signs that Mars was once habitable.
Nozomi: Japanese orbiter was abandoned after software problems.
2005: Mars Reconnaissance Orbiter: Nasa orbiter, which will make the most detailed map yet of the Martian surface to select future landing sites.
2007: Phoenix Scout: Nasa lander, which will touch down close to the planet's north pole to look for water ice and determine the geology and atmospheric composition at this latitude. It will use much of the technology that originally flew on the lost 1999 Mars Polar Lander.
ESA plans to start testing technology for a "sample return" mission, bringing soil and rocks back to Earth for analysis. A second version of Beagle 2 could also fly again at this time.
2009: Mars Science Laboratory: Nuclear-powered Nasa rover, the size of a jeep, which would land in 2010 and remain active for a Martian year - 687 Earth days. It will be more than twice as long (2.4m) and five times as heavy (one ton) as the Spirit and Opportunity rovers. It will be able to gather rocks and soil and crush and analyse them to look for traces of life.
Mars Telecommunications Orbiter: Dedicated Nasa communications satellite, designed to relay signals from future Mars missions, both robotic and unmanned, back to Earth.
ExoMars: ESA rover, designed to search for traces of life.
2011 or 2013: Possible launch dates for first "sample return" missions.
Both Nasa and ESA are planning these, as Earth-based laboratories are much better equipped to analyse Martian rocks and soil than anything that could be sent to the planet.
2014: Nasa plans to start Moon landings, and will eventually set up a permanent Moon base to test technologies for Mars exploration.
Late 2020s: Possible manned mission to orbit Mars, but not to land.
c.2030: Likely date for first manned mission to land on Mars.
Of the nine planets in the Solar System, only four are made up of rock - Mercury, Venus, Earth and Mars. These are the closest to the Sun and form the inner planetary system. The next four - Jupiter, Saturn, Uranus and Neptune - are gas giants, while the final planet, Pluto, is a small chunk of ice. All the planets orbit the Sun in an anti-clockwise direction and the entire system orbits the centre of our galaxy, the Milky Way.