Space station crew back on Earth after record US spaceflight

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.