The effects of spaceflight, and to be precise — of microgravity — on the human body are fairly well-known. Since the beginning of manned space flight astronauts and cosmonauts alike have been used as guinea pigs to study the long- and short-term consequences of living in an environment where up and down doesn’t really apply anymore.
In microgravity, such as on the International Space Station, the usual downward pull is barely measurable, and our bodies react quickly: bones don’t need to be as strong, so bone density diminishes, with most of the calcium concentrated in urine. Our muscles dwindle, because you can literally propel yourself across the room with the gentle push of a finger pad. And our heart no longer has to struggle against the length of the blood vessels all the way to our toes, so it, too, shrinks and powers down.
Our spine is affected, our vision, digestion — the list is endless.
If we ever want to venture beyond the moon, we need to know how microgravity affects the body over very long time periods, and how to counteract those effects, because unless a space traveler chooses to stay in a microgravity environment for the rest of his or her life, they will experience major issues coming back to Earth.
While several astronauts and cosmonauts have stayed in orbit for months (cosmonauts even longer) there needs to be more focused attention on the issue. So both NASA and the Russian space agency have chosen a candidate to be the research subject for such a mission. Russia picked Mikhail Kornienko, a physician (and it makes perfect sense that one of them should be a doctor!), and NASA picked space flight veteran Scott Kelly. Kelly has the awesome advantage of having a twin brother, Mark Kelly, who also used to be an astronaut.