New Delhi, July 5 -- Ross Elder, Ellen Ellis, Matthew Montgomery and James Spicer have not left a 1,700-square-foot printed habitat at Johnson Space Center since October 19. They have about three months left. On July 1, while that crew was still inside, NASA opened applications for volunteers willing to do something harder.

confined transit first, surface operations second, with the psychological and physiological toll of each phase measured separately and then together.

NASA says the combined structure exists because the two phases stress a crew differently, and the agency has never measured what happens when the same four people absorb both stresses back to back without a reset in between. A real lunar or Mars mission does not give as...