The Mars500 experiment, at the Institute for Medical and Biological Problems in the Russian capital, is being carried out with the European Space Agency and China's space training center. It was on a hot day last June when the six-man international crew entered the simulated spacecraft at the research centre. Ever since, they have been crammed inside under conditions that replicate the monotony and claustrophobia of a journey to Mars in every way except the weightlessness. The crew has small living compartments and separate modules for exercise and experiments.