The all-male crew of three Russians, a Chinese, a Frenchman and an Italian-Colombian has been inside windowless capsules at a Moscow research center since June. Their mission aims to help real space crews in the future cope with the confinement and stress of interplanetary travel.
The researchers communicate with the outside world via e-mails and video messages - occasionally delayed to give them the feel of being farther than a few yards (meters) away from mission control. The crew members eat canned food similar to that eaten on the International Space Station and shower only once a week.
None of the men has considered abandoning the mission, although they are free to walk out at any time, mission director and former cosmonaut Boris Morukov told reporters on Friday.
"They are still motivated, but there is a certain fatigue, which is natural," he said.
The six men are due to "land" on Mars on Feb. 12 and spend two days researching the planet. They then begin the months-long return flight to Earth, expected to be the most challenging part of the mission.
"It will be very tough on the boys because of the monotony," Morukov said. "The fatigue and the thought that the mission is over can be fraught with negative consequences."
The Mars-500 experiment is being conducted by the Moscow-based Institute for Medical and Biological Problems, the European Space Agency and China's space training center.
In an effort to reproduce the conditions of space travel, with exception of weightlessness, the crew has living quarters the size of a bus connected with several other modules for experiments and exercise. A separate built-in imitator of the Red Planet's surface is attached for the mock landing.
A real mission to Mars is decades away because of its huge costs and major technological challenges, particularly the task of creating a compact shield that would protect the crew from deadly space radiation.
Mission control will create mock emergencies for the crew to keep them busy on their way back to Earth, Morukov said Friday.
The six men had to spend a day in the dark in December, after mission control simulated an emergency blackout, leaving them with only a few back-up generators so they had to prioritize their needs.
The length of interplanetary missions is why the conditions of a Mars flight cannot be tested on astronauts at the International Space Station. The space station is in Earth orbit and in case of emergency the astronauts can always return to Earth in two hours' time. On interplanetary missions, there is no quick way back.
Although the crew is all-male, Morukov stressed that the organizers did not intentionally leave out women but just couldn't find the right candidate.
A similar experiment in 1999-2000 at the same Moscow research center went awry when a Canadian woman complained of being forcibly kissed by the Russian team captain. She also said two Russian crew members had a fist fight that left blood splattered on the walls. Russian officials downplayed the incidents, attributing them to cultural gaps and stress.
The organizers earlier said each crew member will be paid about $97,000 for taking part in the experiment.