A team of Nasa recruits has begun living in a dome near a barren volcano in Hawaii to simulate what life would be like on Mars.
The isolation experience, which will last a year starting on Friday, will be the longest of its type attempted.
Experts estimate that a human mission to the Red Planet could take between one and three years.
The six-strong team will live in close quarters under the dome, without fresh air, fresh food or privacy.
They closed themselves away at 15:00 local time on Friday (01:00 GMT Saturday).
A journey outside the dome – which measures only 36ft (11m) in diameter and is 20ft (6m) tall – will require a spacesuit.
A French astrobiologist, a German physicist and four Americans – a pilot, an architect, a journalist and a soil scientist – make up the Nasa team.
The men and women will each have a small sleeping cot and a desk inside their rooms. Provisions include powdered cheese and canned tuna.
Missions to the International Space Station last six months. The US space agency has recently conducted four-month and eight-month-long isolation experiments.
While others focus on the technical and scientific challenges of the journey, the isolation experiments address the human element of exploration and problems that arise living in tight quarters.
“I think one of the lessons is that you really can’t prevent interpersonal conflicts. It is going to happen over these long-duration missions, even with the very best people,” said Kim Binsted, a Nasa investigator.