An American astronaut and two Russian cosmonauts are to blast off early Saturday to begin a year away from Earth.

Of the three-man team, American Scott Kelly and Russian Mikhail Kornienko are to stay at the International Space Station until March 2016. Their trip is NASA's first stab at a one-year spaceflight, anticipating Mars expeditions that would last two-to-three years.

Kelly, Kornienko and another Russian, Gennady Padalka, will begin their marathon mission with a Soyuz rocket launch from Kazakhstan and are to arrive at the orbiting outpost six hours later.

Kelly's identical twin Mark, a retired astronaut, agreed to take part in many of the same medical experiments as his orbiting sibling to help scientists see how a body in space compares with its genetic double on Earth.