In general the six became more sedentary by the end of the mission - a problem if it had been an actual long-term space trip, where exercise is essential to maintain bone and muscle strength.
Those findings were reported by the Penn-led team on Monday, in Proceedings of the National Academy of Sciences. Still to come are evaluations of the crew members' psychological health.
The crew never left the ground while living in the modules, from June 2010 until November 2011, so they did not experience the sensation of weightlessness or the intense radiation of outer space.
But, otherwise, conditions resembled those for interplanetary travel as much as organizers could manage, said Jeffrey P. Sutton, director of the National Space Biomedical Research Institute, the primary funder of the Penn-led team.
"It's more spacious than a submarine, but kind of along those lines," said Sutton, who also directs the Center for Space Medicine at the Baylor College of Medicine.
Most of the time was spent in three cylindrical buildings, the largest of which measured about 13 by 80 feet, containing a greenhouse, refrigerator, and gym.
The simulation was developed by the Institute of Biomedical Problems of the Russian Academy of Sciences, with assistance from the European Space Agency.
The Penn-led study of sleep patterns and psychological health was the lone American research effort out of more than 100.
Sutton, whose nonprofit institute was established by NASA in 1997, coordinated the participation of the U.S. team. The lead scientists were David F. Dinges, a professor at Penn's Perelman School of Medicine, and Mathias Basner, an assistant professor at the school.
The scientists measured whether the crew members were awake, resting, or asleep by means of special electronic wristwatches that detected movement.
The findings serve as a reminder of the need to maintain a regular day schedule not just for astronauts but for people on Earth, said Dinges, a prominent sleep researcher. A night-owl regimen can wreak havoc with sleep, digestion, cognitive performance, and mood.
On a space mission, where exposure to natural light is erratic, measures to enforce a 24-hour day are essential, Dinges said.
"We'd better make sure the habitat interior and activity levels are structured to reinforce Earth's normal circadian cycle in a regular way, not just through light, but through when exercise is done and everything else," Dinges said.
Indeed, Thomas Jefferson University researcher George Brainard already is working with NASA to test special lights that are enriched in the blue portion of the spectrum, designed to boost astronaut alertness during the day.
The 520-day mission in Moscow was the longest simulation of space travel to date, more than double the length of a previous effort. As for actual space travel, Russian cosmonaut Valeri Polyakov spent 437 consecutive days aboard the International Space Station, but he did not undergo the kind of continuous analysis performed during the ground simulation.
Using conventional rocket propulsion, 520 days is probably the minimum amount of time it would take to get to and from the red planet, allowing for a month-long visit on Martian soil, Sutton said. In reality, a crew would want to stay much longer; missions with a total duration of three years are commonly discussed, he said.
No U.S. timetable for a trip to Mars exists, though scientists have tossed around a rough date of 2030.
Scientists not involved with the Penn study said that, inevitably, the simulation could not perfectly replicate conditions in space.
Jack Stuster, a behavioral scientist at California-based Anacapa Sciences Inc. who has studied astronauts on the International Space Station from afar, said one reason the members of the simulated mission became more sedentary may be that they were not given enough work to do.
Nick Kanas, a professor of psychiatry at the University of California, San Francisco, said another element missing from the simulation was the danger inherent in a real space mission.
Yet both said the study was a valuable contribution to the field.
As for the yet-to-be published psychological evaluation, Dinges acknowledged that the 17-month stay in a confined space was not without moments of conflict. But, overall, crew members have said they got along pretty well, celebrating holidays such as New Year's and Halloween together, and spending free time on video games and movies. Surgeon Sukhrob Kamolov, one of the Russian crew members, told Reuters that cooperation was essential.
"We know that flies can turn into elephants in space so whatever conflict arose, we tried to nip it in the bud," he said.
Contact Tom Avril at 215-854-2430 or firstname.lastname@example.org .