A Belgian, two Frenchmen and a Colombian-Italian have agreed to be locked away in steel containers for 18 months to simulate a mission to Mars.
Their self-imposed exile will test the physical and mental requirements of ultra-long duration spaceflight.
The Europeans will join a predominantly Russian crew for the Mars500 project, which is due to start in May.
All the food and water needed for the "journey" will have to be loaded into the "spacecraft" before "departure".
There will even be a simulated landing.
After about 250 days, the crew will be split in two, and three "cosmonauts" will move into a separate container to walk on the "surface of the Red Planet" wearing modified Russian Orlan spacesuits.
"I think you've got to be a little bit crazy to undertake this venture, but it's a healthy craziness," said Diego Urbina, a 26-year-old electronic engineer from Italy.
"I will definitely miss my family and friends, and Nature itself - all the things we take for granted here on Earth, such as the internet. And girls - that's something I'll definitely miss," he told BBC News.
Urbina joins the Belgian Jerome Clevers (28) and the Frenchmen Arc'hanmael Gaillard (34) and Romain Charles (30) as the European Space Agency (Esa) candidates on the project. Their number will shortly be reduced to two.
The selected pair will then join three Russians and a Chinese individual in the simulation spacecraft.
The experiment is being run by Russia's Institute of Biomedical Problems (IBMP) with the key participation of Esa.
The Mars500 facility, which is located on the IBMP site in Moscow, comprises four sealed interconnected modules. The total interior volume is about 550 cubic metres. There are no windows.
The walls in the living quarters have been covered with a wooden panelling to make them feel slightly less austere.
Another module holds the Red Planet landscape.
Looking after the participants' needs will be a mission control-room sited just outside the containers.
But the experiment's designers are determined to make the training exercise as realistic as possible, so they will introduce a time delay in communications after two months.
Because it can take about 20 minutes for a message to travel from Mars to Earth, it will take this amount of time in the simulation, also.
The crew and their ground controllers will send text and voice messages to each other and then have to wait for the replies.
It means there can be no real-time conversations, not even with friends and family - and, in moments of crisis, it will mean the crew will have to make crucial decisions themselves.
"Daily life, I hope, will be quite busy," said auto-industry engineer Romain Charles. "First, when we wake up, we will do some medical checks; and from there we'll all go to different tasks from cooking to cleaning and experiments. And we'll have some spare time to maybe read or watch videos or write to our relatives."
Martin Zell heads up the International Space Station (ISS) utilisation department at Esa. He believes the Mars500 experience will be extremely taxing.
"It is literally isolated compared to ISS where you have a lot of arriving and departing vehicles, and at least three of the crew being exchanged every two or four months at least.
"[On the ISS,] you have basically unlimited contact with the ground and you always have new experiments. Whereas on the Mars simulation, you will depart with a certain set of experiments, consumables and equipment - and the door never opens."
Scientific investigations during the "voyage" will asses the effect that isolation has on various psychological and physiological aspects such as stress, hormone levels, sleep quality, mood and the impact of dietary supplements.
The organisers say that if a crew-member decides halfway through the simulation that he really cannot cope with the separation anymore and wants to leave, he will be allowed to do so; although every effort will be made to try to persuade the individual to stay.
Another group of experimental cosmonauts completed a shorter mission of 105 days in the same facility last year and came through unscathed. Scientists have every reason to hope the new crew will fare just as well.
The Mars500 is so called because it simulates the duration of a possible human Mars mission in the future using conventional propulsion: 250 days for the trip to the Red Planet, 30 days on the martian surface and 240 days for the return journey, totalling 520 days (in reality it would probably take a lot longer than this).
"Mars is the ultimate destination where we will one day live and work," said Simonetta Di Pippo, the director of human spaceflight at Esa.
"What we're doing right now is working with 13 other space agencies. We have put together a framework for future global endeavour because it's clear this challenge cannot be undertaken by one nation alone. It is, by definition, a global challenge."
It will not happen for many decades, however. The technology does not currently exist to send people to Mars and bring them home safely.
In recent months, the Americans have talked about a "flexible path" out into the Solar System. This would involve sending humans to targets that are progressively further away and more challenging. These targets might include a return to the Moon and visits to asteroids.