Former U.S. space shuttle pilot and station commander Scott Kelly, 48, who last flew in 2011, will be paired with Russian cosmonaut Mikhail Kornienko, 52, for a 12-month space station assignment beginning in early 2015. Kornienko served as a flight engineer aboard the station in 2010.

"The one-year increment will expand the bounds of how we live and work in space and will increase our knowledge ... as we prepare for future missions," NASA associate administrator William Gerstenmaier said in a statement.

Only four people have lived off-planet for a year or longer, all Russians who served aboard the now-defunct Mir space station. The single longest stay in space was a 438-day mission in 1994-1995 by cosmonaut Valery Polyakov, a physician.

The current U.S. record for a long-duration flight is held by former International Space Station commander Michael Lopez-Alegria, who spent 215 days in orbit between September 2006 and April 2007.

"You don't really notice it until you come back and begin recovery," Lopez-Alegria told Reuters. "When you come back, you decide whether you've pushed it too far or not."

With the retirement of the space shuttles last year and the completion of the U.S. construction of the $100 billion station, NASA is working on a new space transportation system that can fly astronauts to the moon, asteroids and other destinations in deep space. The goal is to send a crew to Mars in the mid-2030s.

The year-long station missions are intended to collect medical data and to test protocols for countering some of the adverse impacts of long-duration spaceflight, including bone and muscle loss, risks to eyesight and reproductive systems and changes in the immune and cardiovascular systems.

Kelly and Kornienko are scheduled to begin a two-year training program early next year.

(Editing by Jim Loney)

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