NASA astronauts Scott Kelly (R) and Tim Kopra shake hands as Kelly turns over command of the International Space Station to Kopra, February 29, 2016.[Photo/Agencies]

Kelly and his identical twin brother Mark, a former NASA astronaut, are also participating in genetic studies, the first to assess if genetic changes occur during long spaceflights.

Kelly's 340-day mission eclipses the previous US record-long spaceflight of 215 days, set by former astronaut Michael Lopez-Alegria aboard the space station in 2007.

The world's longest missions were carried out by four Soviet-era cosmonauts aboard the now-defunct Mir space station, including a flight from January 1994 to March 1995, spanning nearly 438 days by record holder Valeri Polyakov, a physician.

The International Space Station, a joint project of the United States, Russia, Europe, Japan and Canada, followed Mir and has been permanently staffed by rotating crews since 2000.

About the size of a five-bedroom house, the $100-billion station flies about 250 miles (400 km) above Earth.