Two veteran spacefarers have been chosen to spend a year aboard the International Space Station - many times longer than the usual tour of duty - as part of research into the physiological effects of long stays in space. The work will feed into American and Russian plans to send people to Mars, which would require astronauts to be in harsh conditions for several years at a time. US astronaut Scott Kelly and Russian cosmonaut Mikhail Kornienko will fly to the ISS in 2015 and come back to earth, in Kazakhstan, in 2016. Both have been in space for around six months each, over different missions. The pair's health, performance and mental well-being will be monitored on the year-long trip. Scientists will measure the long-term effects of microgravity on everything from the pair's muscle mass and vision to their strength and bone density. William Gerstenmaier, associate administrator for human exploration and operations at Nasa, said that the duo's skills and experience aboard the space station were important. "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." Vladimir Popovkin, head of the Russian Federal Space Agency, said the selection process for the mission had been rigorous and that they had chosen "the most responsible, skilled and enthusiastic crew members to expand space exploration, and we have full confidence in them". Kelly is a 48-year-old retired US Navy captain who first went into space as a pilot aboard the space shuttle mission STS-103 in 1999. He was commander on a subsequent shuttle flight and has been on two expeditions to the space station. Kornienko, 52, is a specialist in airborne systems and a former paratrooper. He went to the ISS in 2010 and has spent at least 176 days in space. They will have two years of training in the US and Russia.