After 257 days of 'flight' they 'landed' and two of the team - Russian physiologist Alexander Smoleevskiy and Italian engineer Diego Urbina - donned space suits and conducted a 40-minute walk on a mock-up of the surface of Mars.
The simulator comprises a landing module, an research module, a residential segment, a storage, a greenhouse, along with a separate module imitating the surface of Mars with a volume of 1,200 cubic meters.
The Mars500 project is an attempt to simulate the experience of a manned trip to Mars, with an international team of researchers locked in a windowless capsule for about a year and a half -- time required for a round trip to the next planet out in the solar system.
Organizers at the European Space Agency and Russia's Institute of Biomedical Problems hope the project will shed light on the physical and psychological effects of the long isolation that future Mars astronauts will experience.
The earth-bound cosmonauts communicate with Mission Control via the internet, with occasional disruptions and a 20-minute delay to imitate the effects of space travel. They are performing tasks similar to astronauts at the international space station, such as maintenance and scientific experiments, but for a longer period of time.
They follow a seven-day week with two days off, except when special and emergency situations are simulated. The crew will also drive a rover on the simulated Mars surface and use its robotic arm to pick up rocks and sensors,
Speaking at a press conference at Mission Control on Monday before the walk, Vitaly Davydov, deputy head of the Russian Space Agency, said that the "main task of the project is to determine the list of problems" that real space crews flying to Mars might be encountering.
He said such a manned flight to the planet could take place in two decades. "Twenty years is a good time to prepare for such an expedition. I think it is quite realistic."
A human mission to Mars with current-day propulsion requires only 180 days in transit each way. The Mars500 crew will have endured isolation for a period significantly longer than that.
When the Mars500 mission is complete, it will be the longest high-fidelity spaceflight simulation in history.