Scott Kelly, a former shuttle pilot and space station commander, is heading back to the orbital outpost, this time for an experimental, year-long stint.

The 48-year-old Navy pilot and identical twin brother of former astronaut Mark Kelly, will be joined by veteran cosmonaut Mikhail Kornienko, for a planned 12-month mission beginning Spring 2015, NASA announced Monday.

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NASA and its partners in the International Space Station are interested in learning more about how the human body fares during long-duration stays in space. Typically, crews spend four- to six months living aboard the station, which orbits about 250 miles above Earth.

The U.S. space agency is beginning to work on a spaceship and rocket to carry astronauts farther into space, eventually working up to a human mission to Mars in the 2030s.

NASA wants the data from a 12-month station mission to assess crew performance and health and to test what countermeasures best cut the risks for future long-duration stays in space. Currently, a mission to Mars would take about 18 months of travel time, plus whatever time the astronauts spend orbiting the planet or on its surface.

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The current U.S. record-holder for the longest stay in space is former astronaut and space station commander Michael Lopez-Alegria, who spent 215 days in orbit between September 2006 and April 2007.

The Russians, who previously operated their own space station called Mir, are the world champs when it comes to long-duration spaceflight. The endurance record is held by Valery Polyakov, a doctor, who lived off-planet for 438 days.

Kelly and Kornienko, 52, have already met. Kelly served as a backup crew member for the station's Expedition 23 and 24 crews, which included Kornienko, a flight engineer.

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Astronaut, Cosmonaut to Spend Year in Space