NASA, Russia eye yearlong space station assignments

* Mission would help prepare for flights beyond Earth

* Cosmonaut Valery Polyakov spent 438 days in orbit

* Longest flight by U.S. astronaut is 215 days

By Irene Klotz

CAPE CANAVERAL, Fla., Oct 4 (Reuters) - NASA is considering doubling the amount of time an astronaut spends at the International Space Station to a year, laying the groundwork for future missions deeper into space, officials said Thursday.

If approved, a mission likely would begin in 2015, said NASA spokesman Rob Navias.

Russian news agency RIA Novosti reported this week that the experimental yearlong endurance mission would include a Russian cosmonaut and a NASA astronaut.

"If the mission proves to be effective, we will discuss sending yearlong missions ... on a permanent basis," Alexei Krasnov, head of human spaceflight with the Russian space agency Roscosmos, said in the RIA Novostia report.

Navias said an agreement had not yet been signed.

"NASA has been exploring the idea of a one-year increment on the space station," Navias said. "That would be a natural progression as part of preparations for missions beyond low-Earth orbit." The agency had previously planned missions at no more than six months.

Medical and biological studies are key areas of research aboard the station, a $100 billion, permanently staffed laboratory a partnership of 15 countries, that flies about 250 miles (400 km) above Earth.

Doctors are particularly concerned about the effect of long stays in the weightless environment of space has on bone loss, vision changes and impacts to an astronaut's cardiovascular system.

Russian cosmonaut Valery Polyakov holds the record for the longest spaceflight, a 438-day mission aboard Russia's Mir space station in 1994 and 1995.

"Only four people - all Russians - have ever spent a year or more in orbit," Navias said. "Getting contemporary data with modern equipment would be helpful."

The longest flight by an American is a 215-day mission by astronaut Michael Lopez-Alegria aboard the International Space Station in 2006-2007.

"I think a year is do-able," Lopez-Alegria told Reuters. "The countermeasures, particularly for bone-density loss, are so much better now."

Lopez-Alegria said it's not being in space that takes its toll on the body, it's coming back.

"You don't really notice it until you come back and begin recovery. Then you decide whether you've pushed it too far or not," he said.

Yearlong flights also could open seats on Russian Soyuz capsules, now the only vehicles flying people to the space station, for paying tourists or researchers.

The U.S. firm Space Adventures, which has brokered trips to the space station for seven people so far, plans an announcement next week in Russia with opera singer Sarah Brightman, a space enthusiast who already has booked a ride on Virgin Galactic's planned suborbital SpaceShipTwo vehicle.

The company, an offshoot of Richard Branson's Virgin Group, is selling tickets for $200,000. Commercial service is expected to begin in late 2013 or 2014.

Since the United States retired its fleet of space shuttles, Soyuz capsules have been fully booked ferrying crews to and from the space station. The last paying passenger to visit the orbital outpost was Cirque du Soleil founder Guy Laliberte, who flew in October 2009.

(Editing by Kevin Gray and Philip Barbara)