This study aims to determine whether astronauts who have not flown in space can provide an unbiased comparison to astronauts who have flown in space when analyzing long-term health outcomes such as incidence of chronic disease and mortality. Various propensity score methods failed to achieve good balance between groups, demonstrating that even with sophisticated rebalancing methods the group of non-flight astronauts cannot be demonstrated to be an unbiased comparison group for examining the effect of the hazards of spaceflight on incidence and mortality from chronic diseases.