The ISS year-long mission was an 11-month-long scientific research project aboard the International Space Station, which studied the health effects of long-term spaceflight. Astronaut Scott Kelly and cosmonaut Mikhail Kornienko spent 340 days in space, with scientists performing medical experiments. Kelly and Kornienko launched on 27 March 2015 on Soyuz TMA-16M along with Gennady Padalka. The mission encompassed Expeditions 43, 44, 45 and 46. The pair safely landed in Kazakhstan on March 2, 2016, returning aboard Soyuz TMA-18M with Sergey Volkov. The mission supported the NASA Twins study, which helps shed light on the health effects of long-duration spaceflight, which is of interest for Mars missions especially.

The effects of spaceflight on the human body are complex and largely harmful over both short and long term. Significant adverse effects of long-term weightlessness include muscle atrophy and deterioration of the skeleton. Other significant effects include a slowing of cardiovascular system functions, decreased production of red blood cells, balance disorders, eyesight disorders and changes in the immune system. Additional symptoms include fluid redistribution, loss of body mass, nasal congestion, sleep disturbance, and excess flatulence. Overall, NASA refers to the various deleterious effects of spaceflight on the human body by the acronym RIDGE.