Steve Connor: A year and a half in isolation may be hard, but it's not the real thing

Spending long periods in a confined space is a trusted method of testing potential astronauts for the "right stuff", but nothing can really prepare them for the extraordinary reality of a genuine space mission.

The most obvious difference for the six men who will spend 520 days in five inter-connected modules on the ground in Moscow – apart from the lack of real danger – is that they will not be exposed to the weightlessness of space, a state that takes its toll on the human body.

Astronauts in space are effectively in free-fall (they are not in fact in a gravity-free environment) and their bodies begin to deteriorate slowly unless they take appropriate counter-measures, such as regular excercising to keep their muscles and bones from wasting away.

In the weightlessness of space, fluids within the body get redistributed, leading to a build-up in the chest and head. The heart rate falls, fewer red blood cells are produced, the immune system weakens and astronauts begin to experience sleep problems, nasal congestion, puffiness in the face and excessive flatulence.

Ten years ago, the European Space Agency tried to mimic the effects of long-term weightlessness – or microgravity as some experts prefer to call it – by recruiting a couple of dozen able-bodied young men to spend three months in bed.

The beds were set at an angle of 6 degrees so that their feet were slightly higher than their heads; this filled their upper bodies with fluids, as happens in space. But lying horizontally for such long periods can only simulate some of the medical effects of long-term space flight.

And the psychological pressures of spending long periods in the Mars500 "spaceship" with five other men cannot match the pressures of a real mission to Mars. For one thing, the researchers are free to leave if they can't cope any more.

Claustrophobia may not always be associated with space travel but it can become a problem. I realised this when I entered a mock-up of the Russian Mir Space Station in Moscow. The claustrophobia was overwhelming, and I couldn't get out fast enough.

Yet for trained astronauts, claustrophobia may be the least of their problems. The Russian cosmonaut Sergei Krikalyov was on board Mir when the Soviet Union collapsed in 1991. Nothing could have prepared him for his role as "the last citizen of the USSR". That truly is the right stuff.