The Christmas spirit didn't bypass the six-man crew confined to an isolation chamber for 520 days to simulate a grueling round-trip flight to Mars. Nope. They built themselves a green Christmas tree out of cardboard boxes, and decorated it with paper ornaments printed on their onboard computer.

After the tree was put together and decorated with a garland, French crew member Romain Charles said, "Now it is really Christmas."

The all-male crew — including Charles as well as three Russians, an Italian-Colombian and a Chinese space trainer — are part of an experiment to find out how a crew would handle the stress, claustrophobia and fatigue that real astronauts would face in long-term interplanetary travel.

A little holiday spirit, it appears, is helping them get along fine.

Their 20,000-cubic-foot "spaceship" is safely parked inside the Moscow-based Institute for Medical and Biological Problems. The crew's only contact with the outside world is through an Internet connection — which experiences delays and spotty service by design, to imitate the effects of space travel.

The crew is a bit over six months into their virtual journey, currently high in Mars orbit spiraling slowly down to the point where their lander can be dispatched to the simulated Martian surface in February. They'll return to Earth — that is, get out of confinement — next November.

You can learn more about the mission on the ESA's Mars500 website, or read this story in our archives.

John Roach is a contributing writer for msnbc.com. Connect with the Cosmic Log community by hitting the "like" button on the Cosmic Log Facebook page or following msnbc.com's science editor, Alan Boyle, on Twitter (@b0yle).