The project, titled Mars500, will see two European candidates chosen to join four Russian crew members for the study, which will require them to live, eat, sleep and work inside a sealed laboratory in Moscow that will simulate a mission to Mars, ESA said.

The study, which is a joint venture between the ESA's Directorate of Human Spaceflight and the Russian Institute for Biomedical Problems (IBMP), aims to research the physical and psychological effects of long-term confinement.

The eight shortlisted European candidates for the test are all male, between 28 and 39, and come from Denmark, Sweden, Germany, France and Belgium, the ESA said in a press release.

The selection process started in June 2007 and attracted around 5,600 applicants. The final selection of the ESA's two crew members plus two others who will train as their back-ups will be announced in mid-December.

"Surprisingly, there is no competitive mood among the remaining candidates," Mars500 Program Manager Jennifer Ngo-Anh told Deutsche Welle. "They are just getting really excited and are highly motivated to participate."

Tough medical tests

The candidates were subjected to various medical tests at the Central Clinical Hospital of the Russian Academy of Sciences. They underwent extensive examinations, as well as screenings by a neurologist, a dentist, a psychologist and an ophthalmologist.

"The tests were very invasive. They pushed the men to their limits," said Jennifer Ngo-Anh, adding that examinations such as the colonoscopy caused real discomfort.

This testing was necessary to ensure that the final crew consists of healthy individuals who are able to cope with isolated and unusual living conditions.

A new venture for ESA

This is the first time ESA has embarked on an experiment of this kind. The initial 105-day study will be followed by a longer follow-up later in 2009, in which another six-member crew will spend 520 days together.

The distance between Earth and Mars varies between 55 million and more than 400 million kilometers (250 million and 34 million miles), which means that a round trip to the planet would take at least 18 months.

ESA and NASA have separately sketched dates around three decades from now for a historic manned flight to Mars.