Six would-be cosmonauts have entered a sealed facility where they will spend 18 months with no windows and only e-mail contact with the outside world. The men are taking part in the Mars500 project, which aims to simulate a mission to Mars. They entered the craft, located at a medical institute in Moscow, just before 1100 BST on Thursday.
Scientists say the study will help them understand how humans would cope on a long journey to another world.
During a press conference on Thursday morning, the six men - three Russians, two Europeans and a Chinese man - all described what motivated them to take part in the experiment.
Twenty-six year old Wang Yue from China, the youngest of the volunteers, said he was excited to be involved in a project that he felt would be "excellent for science and for all of humankind".
French volunteer Romain Charles acknowledged that it would be a "difficult" mission and said that he would miss his family and "the Sun and fresh air".
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