MOSCOW (AP) — Pale but smiling, an international crew of researchers on Friday walked out of a set of windowless modules after a grueling 520-day simulation of a flight to Mars.

The all-male crew of three Russians, a Frenchman, an Italian-Colombian and a Chinese slowly emerged from the western Moscow facility, which simulated the confinement, stress and fatigue of interplanetary travel — minus the weightlessness. Dressed in blue track suits emblazoned with the mission emblem, they carefully walked down a metal ladder to a greeting crowd of officials and journalists.

“The crew has completed the experiment,” team leader Alexey Sitev reported to Russian space officials. “The mission is accomplished, the crew is in good health and is ready for new missions.”

Psychologists said long confinement without daylight and fresh air put the team members under stress as they grow increasingly tired of each other’s company. They said that psychological conditions can even be more challenging on a mock mission than a real flight because the crew won’t experience any of the euphoria or dangers of actual space travel.

Despite that, the crew showed no sign of stress as they walked to microphones to speak before cameras. “We hope that we can help in designing the future missions to Mars,” Frenchman Romain Charles said with a smile.