Russian Mikhail Korniyenko and American Scott Kelly will ride a Russian Soyuz spacecraft to the station in the spring of 2015, Russian space agency Roskosmos said on Monday.

It would be the longest space flight by an American. Russian cosmonaut Valery Polyakov holds the record for the longest spell in orbit, a 438-day mission aboard Russia's Mir space station in 1994 and 1995.

The record for an American is held by Michael Lopez-Alegria, who completed a 215-day mission aboard the International Space Station in 2006-2007.

Most stints on the station, a $100 billion, permanently staffed laboratory that orbits about 250 miles above Earth, have lasted no more than six months.

Doctors are particularly concerned about the effect of long spells in space on bones, vision and the cardiovascular system.

"The goal of the year-long expedition aboard the orbital laboratory is testing human body reactions to the harsh conditions of space and the ability to adapt to them," Roskosmos said in a statement on its website.

It would help reduce risks on future missions to the Moon's orbit, the asteroids and eventually Mars, it said.

"The choice of participants in the year-long flight was hard because there were many worthy candidates, but we chose the most responsible," said Roskosmos head Vladimir Popovkin.

He said Kelly and Korniyenko, who were in separate ISS crews in 2010-2011, were "selflessly loyal to the business of space".

Since the United States ended its space shuttle program last year, it has relied solely on Russia to fly astronauts to the ISS at a cost of more than $60 million per seat.

But Russia's pioneering space industry has suffered embarrassing and costly failures in the past two years. Prime Minister Dmitry Medvedev met Popovkin and other senior officials on Monday to discuss ways to improve the performance of Roskosmos.

(Editing by Steve Gutterman and Tom Pfeiffer)

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