NASA astronaut Scott Kelly and Russian cosmonaut Mikhail Kornienko have returned to earth after nearly a year on the International Space Station.

A Soyuz capsule carrying Kelly, Kornienko and Sergey Volkov, another Russian cosmonaut, made a parachute landing on the steppe near the Kazakh city of Zhezkazgan at 10.26am (3.26pm AEDT) on Wednesday, about three-and-a-half hours after departing the station.

Kelly and Kornienko have been aboard the space station for 340 days, about twice as long as previous crews. Their flight sets a record for the space station and for the longest US space mission, intended to pave the way for human travel to Mars.

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Volkov, who has been in space for five-and-a-half months, was the first to emerge from the capsule, to be greeted by his father Alexander Volkov, also a cosmonaut.

Kelly, emerging next, waved his hand energetically and smiled before beginning a satellite telephone conversation.

In their nearly year-long stay in space, Kelly, 52, and Kornienko, 55, have been the subjects of dozens of medical experiments and science studies trying to learn more about how the human body adjusts to weightlessness and the high-radiation environment of space.

Scott Kelly grew flowers during his year in space. Photo: Scott Kelly/Twitter

The research aims to help the US space agency and its partners develop plans for eventual human missions to Mars that will last at least two years.

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 US, 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 $US100 billion ($A139.30 billion) station flies about 400km above earth.

After 340 days in space, @StationCDRKelly is back on Earth from his #YearInSpace mission. https://t.co/KX5g7yYnYG https://t.co/dhCfP1M6tZ

— NASA (@NASA) March 2, 2016

New views of the reentry of the #YearInSpace crew earlier returning to Earth from @Space_Station: https://t.co/96LV6YYR3r — NASA (@NASA) March 2, 2016

-AAP