In the 24 Big Questions Science Still Needs To Answer by Kelly Oakes in BuzzFeed
http://www.buzzfeed.com/kellyoakes/big-questions-science-still-needs-to-answer
Kevin Fong (space medicine expert) addressed this interesting question and added:
At the moment, astronauts that go up to the International Space Station are limited
to spending six months in space at a time because of the effects of microgravity on
the human body. But from March 2015 astronaut Scott Kelly is going to spend a year
in orbit. And his twin, astronaut Mark Kelly, will be staying on Earth.
Nasa hopes this unique twin study will give us some insight into how we can
counteract the damaging effects of space on our bodies.