CAPE CANAVERAL, Fla. — A former space shuttle commander whose twin brother is married to former Rep. Gabrielle Giffords will attempt the longest spaceflight ever by an American.

NASA astronaut Scott Kelly and Russian cosmonaut Mikhail Kornienko will spend an entire year aboard the International Space Station beginning in 2015.

Both countries' space agencies announced the names of the two veteran spacefliers on Monday. The extended mission was approved almost two months ago to provide a medical foundation for future missions around the moon, as well as far-flung trips to asteroids and Mars.

Both men already have lived aboard the space station for six months. NASA wanted experienced space station astronauts to streamline the amount of training necessary for a one-year stint. Officials had said the list of candidates was very short. They will begin training next year.

"Their skills and previous experience aboard the space station align with the mission's requirements," Bill Gerstenmaier, head of human exploration for NASA, said in a statement. "The one-year increment will expand the bounds of how we live and work in space and will increase our knowledge regarding the effects of microgravity on humans as we prepare for future missions beyond low-Earth orbit."

Kelly's identical twin brother, Mark Kelly, retired from the astronaut corps last year and moved to Tucson, Ariz., his wife's hometown. The former congresswoman was critically wounded in an assassination attempt in January 2011, while Scott Kelly was living aboard the space station.

NASA said neither crew member was available Monday to comment and that news conferences would be held next week to outline the mission.

Astronauts normally spend about four to six months aboard the space station. The longest an American lived there was seven months, several years back.

Russia, though, will continue to hold the world space endurance record.

Four cosmonauts spent at least a year aboard the old Mir space station. A Russian physician, Valery Polyakov, logged nearly 15 continuous months there in the mid-1990s.

Boris Morukov, head of the Moscow-based Institute for Medical and Biological Problems, Russia's main space medicine research center, told the Interfax news agency that communications and food rations for Kelly and Kornienko may be limited during their yearlong mission to better simulate interplanetary travel.

Kelly and Kornienko will launch aboard a Russian rocket from Kazakhstan. Americans must buy seats on Russian spacecraft now that NASA's shuttles have retired to museums, until private U.S. companies have vessels capable of carrying human passengers. That's still four or five years off.

Kelly is a 48-year-old, divorced Navy captain with two daughters. Kornienko, 52, a rocket engineer, is married with a daughter.

"We have chosen the most responsible, skilled and enthusiastic crew members to expand space exploration, and we have full confidence in them," Russian Space Agency chief Vladimir Popovkin said in the announcement.