Scott Kelly and Mikhael Komienko will be heading to the International Space Station in the spring of 2015 for a year-long stay. (NASA)

An American and a Russian astronaut will spend a rare year-long stay on board the International Space Station, according to NASA and Roscosmos, the Russian Federal Space Agency.

Scott Kelly and Mikhael Komienko will head off into space in the spring of 2015, according to Monday's announcement.

Their goal will be to gather more data about how weightlessness affects humans over long periods in space.

"We have chosen the most responsible, skilled and enthusiastic crew members to expand space exploration, and we have full confidence in them," said the head of Roscosmos, Vladimir Popovkin.

Ultimately, by becoming guinea pigs, the pair will help reduce the health risks for future long-term missions to the moon and also, perhaps, to Mars.

Data on the effects of gravity on muscle mass, strength, vision and bone density will be collected.

Until now, crews have only spent up to six months at the space station. But there have been previous studies on the physiological and psychological effects of long-term human spaceflight.

Russian cosmonaut Valery Polyakov spent nearly 438 consecutive days — from January 1994 to March 1995 — in space aboard the Mir Space Station. Also, in 2011, six volunteers from Europe, Russia and China spent 520 days confined in a capsule at a research institute in Moscow.

Kelly, 48, is a U.S. Navy captain who first served as a pilot on a space shuttle mission in1999 and was a commander in 2007.

In 2010, he was the flight engineer on the International Space Station Expedition 25 and served as commander of Expedition 26 in 2011. He has logged more than 180 days in space.

Kornienko, 52, is a former paratroop officer and graduated from the Moscow Aviation Institute as a specialist in airborne systems. He has worked in the space industry since 1986 and was trained as an International Space Station Expedition 8 backup crew member.

As as a flight engineer on the station's Expedition 23/24 crews in 2010, he has logged more than 176 days in space.

The pair will begin a two-year training program starting early next year.