Given that even a six month stint at the International Space Station (ISS) causes astronauts to lose bone density and, in some cases, results in visual impairment, researchers have wondered if the human body can withstand a mission to Mars, which could take up to three years. Now, a groundbreaking study involving American twin astronauts Mark and Scott Kelly has found that while the body undergoes drastic changes when exposed to the weightless environment and space radiation for long durations, it mostly reverts to normal upon returning to Earth. This has led the experts to conclude that astronaut health can be "mostly sustained" for a year in space.