It may not be the real thing, but they want it to seem as realistic as possible.
In Moscow, six people have made their farewells before embarking on a simulated expedition to Mars.
But while the journey will not be long, the mission will be in real time.
That means the team will be sealed inside what is known as the Mars-500 module for almost a year and a half, to test all aspects of what would be involved in a trip to the red planet.
This includes the 250 day voyage to Mars, 30-days of exploration on the surface, plus the return trip.
The conditions will be as realistic as possible, except there will be no radiation risks and weightlessness.
Among the all-male team are three Russians, two Europeans and a Chinese man. Each is being paid the equivalent of 82,000 euros for the mission.
The crew will communicate with ground-control via e-mails and the module is equipped with a laboratory, a living area, a warehouse and a small garden.
One of the major aims is to see how the team gets on together, living in cramped conditions