Astronaut Scott Kelly and Russian cosmonaut Mikhail Kornienko will stay in space for a year. Both of them will fly off in Soyuz rocket from Kazakhstan on Saturday.

Both of them have received training of more than two years and after that they are being sent to space, where they will live at the International Space Station. For NASA, it will be their longest space mission ever.

For the Russian space agency, it will be their longest one in almost two decades. Space industry has advanced significantly and now, it has become vital for the world's space agencies to know how the body adapts to a year- long weightlessness before the agencies become fully commit to even longer missions like on Mars.

It would not be the last mission, which will be a year-long, as more such missions have been planned. NASA said that their aim of 12 test subjects.

NASA's space station program scientist, Julie Robinson was of the view that they are aware of what happens with the body in the six-month stay. But they are completely unaware of about what happens between six and 12 months in space.

Weak bones and muscles, impaired vision and immune system are considered to be common space side-effects. Experts said that there is a psychological effect as well. Russian cosmonaut Gennady Padalka, who will be accompanying Kelly and Kornienko into orbit, thinks that it will be psychological effects that will be tougher to deal with.

Kelly and Kornienko are not worried about themselves. They are concerned about their family and friends, whom they will be leaving behind for a complete year, until next March. Kelly said that if something happens, the only thing that is sure is they are not coming back.

Both veteran space fliers are well aware of the risks. Kelly has gone in space for three times for a total of 180 days and Kornienko has gone once for 176 days.