Oleg Kononenko
Russia
Russian cosmonaut Oleg Kononenko has accumulated 878 days in space across five missions -- the longest total time any human has spent off Earth. Long-duration spaceflight carries severe health risks including muscle atrophy, bone-density loss, and radiation exposure due to microgravity. Kononenko has also performed multiple extravehicular activities (EVAs), and his cumulative data is invaluable for planning future crewed missions to Mars.