The first people to make the perilous journey to Mars will have to cope with long periods of boredom, the constant worry of returning home safely and the joy and pain of each other's company.
According to the latest research into long-duration space travel, they will also endure the sort of radiation exposure that few people of Earth have experienced.
A study has found that astronauts will receive more than half a lifetime's radiation dose during the return journey of a future manned mission to Mars - a calculation that does not take into account the time spent on the surface of the Red Planet.
Measurements of cosmic rays within the Mars Science Laboratory, the unmanned spacecraft that delivered the Curiosity rover to the planet last year, found that radiation exposure would be higher than some experts had predicted for a human mission to Mars and back.
It is the first time that scientists have made radiation measurements on a Mars mission from within a space probe that has similar radiation shielding to a manned spacecraft. Other space probes to Mars had little or no shielding.
The researchers found that during the 360-day round trip to Mars a person on board would receive more than 60 per cent of the maximum lifetime dose allowed for an astronaut. Any Martian explorer would be exposed to further radiation during the 500 days or so they spent on the surface of the planet.
Nasa currently stipulates that its astronauts should not receive more than 1,000 milli-Sieverts of radiation over their lifetime - which equates to a 5 per cent increase in the risk of cancer. The radiation device on the Mars Science Laboratory measured about 660mSv - equivalent to getting a whole body CT scan once every five or six days.
Cary Zeitlin of Southwest Research Institute in Boulder, Colorado, who led the study published in Science, said that the relatively high radiation exposure would pose serious problems for a future manned mission to Mars, which Nasa is tentatively planning for some time beyond 2020.
“Radiation exposure at the level we measured is right at the edge, or possibly over the edge of what is considered acceptable in terms of career exposure limits defined by Nasa and other space agencies,” Dr Zeitlin said.
“Those limits depend on our understanding of the health risks associated with exposure to cosmic radiation, and at present that understanding is quite limited,” he said.
However, other experts believe that the risk is manageable. New materials could improve the shielding against cosmic and solar radiation and astronauts could be selected on the basis of their genetic resistance to radiation damage, which increases the risk of cancer by damaging DNA.
“These results show that cosmic rays are not a showstopper. This confirms what you might expect: the radiation risk is quite acceptable. Frankly, it's a modest portion of the risks on a Mars mission,” Robert Zubrin, president of the Mars Society in Colorado, told the journal Science.
The other risks associated with a manned mission to Mars include
The Mars500 project, where six men spent 520 days and nights confined to a terrestrial “spacecraft” in a research institute near Moscow, found that one of the biggest problems was getting enough of the right sort of sleep.
There were wide variations in their circadian rhythms - the 24-hour sleep-wake cycle - of the six-crew. One man shifted to a 25-hour period, meaning that every 12th day his body was telling him it was midnight when it was in fact midday.
Lack of natural light, fresh air and contact with the outside world disturbed the sleeping patterns of the six volunteers. Only one of the crew retained a near-normal sleep pattern, when he was active during the day and rested at night.
Selecting astronauts and the basis of being regular sleepers could help to overcome this difficulty.
It is well established that long period in low or zero gravity causes major changes to the physiology of the body. The lack of gravity causes muscles to waste away and bones to become frail, while the heart becomes weaker at pumping blood around the body.
Regular exercise using a cycle-like device, a zero-gravity treadmill and a set of resistance “weights” not only boost muscles and bone, it maintains levels of red blood cells, which are crucial of overall fitness.
Being cooped up for a year or more in the claustrophobic confines of a spacecraft can severely test “interpersonal relationships”. One suggestion is to send a pair of astronauts who are already in a proven personal relationship, such as a married couple. This could lead to the first interplanetary marriage guidance if strains develop.
Space food tends to be an acquired taste. Spicing up the menu with something fresh and tasty would be good for morale as well as health and psychological wellbeing.
Growing your own food in some kind of cosmic greenhouse attached to a space ship would be one solution - a cosmic allotment would also give astronauts something to do.
Astronauts on board the International Space Station also said that the cuisine of the European Space Agency was favoured over the American or Russian equivalent. So, hiring a French or Italian chef to prepare the space menu may help to maintain the mood of any Martian explorers.
Getting back home
One of the biggest technical challenges is working out how to get home again. If a manned mission to Mars manages to explore the surface of the planet it may be necessary to find a source of water and fuel that could be used for both drinking and the return journey home.
Mars is thought to have deposits of water, which with the help of solar panels could be converted into oxygen and hydrogen, the raw material of rocket fuel. Any return mission to Mars will need a lot of pre-planning to ensure that astronauts are able to get back home - although some very dedicated or foolhardy people may be prepared to volunteer for a one-way ticket.