International Space Station's 'one year crew' returns to Earth

The 340-day mission saw Scott Kelly claim the record for the longest single stay in space by an US astronaut

US astronaut Scott Kelly and Russian cosmonaut Mikhail Kornienko returned to Earth today after spending almost a year in space in a ground-breaking experiment foreshadowing a potential manned mission to Mars.

The 340-day mission saw Kelly claim the record for the longest single stay in space by an US astronaut, while Kornienko is now fifth on the list for lengthiest mission by a Russian cosmonaut.

"We have landing," Russian Mission Control confirmed after the trio touched down southeast of the settlement of Dzhezkazgan in central Kazakhstan at around 0430 GMT.

After returning from his lengthy stint in space Kelly was clearly in high spirits as he was lowered to the ground by burly Russian rescue workers at the landing site.

"The air out here feels great. I've no idea why you guys are so bundled up," NASA TV reported him as saying as he sat upright in a chair on the steppe in temperatures just below zero degrees Celsius (32 degrees Fahrenheit).

Kelly and Kornienko returned with Russia's Sergei Volkov, who was stationed at the ISS for over five months and was met upon landing by his father, retired cosmonaut Aleksandr Volkov.

The "one-year crew" mission -- which began on March 27 last year -- was the longest by any astronauts aboard the ISS and seen as a vital chance to measure the effects of a prolonged period in space on the human body.

"Scott Kelly's one-year mission aboard the International Space Station has helped to advance deep space exploration and America's Journey to Mars," NASA administrator Charles Bolden in a statement.

"Scott has become the first American astronaut to spend a year in space, and in so doing, helped us take one giant leap toward putting boots on Mars."

The pair were subjected to a battery of tests before and after their ascent towards the ISS and underwent more tests soon after landing.

Weightlessness reduces muscle mass and bone density and is believed to diminish eyesight by increasing cerebrospinal fluid around the optic nerve.

Kelly, 52, was also part of an experiment comparing his development and changes in space with his identical twin brother -- Mark -- back on Earth. He will now arrive by chartered flight in Houston for a NASA medical examination.

In his year aboard the space station Kelly has been an avid Internet poster, capturing stunning views on his Instagram page and tweeting regularly to nearly a million followers while travelling some 230 million kilometres.

In one particularly eye-catching stunt, Kelly posted a short video of himself dressed up in a gorilla suit and floating through the ISS in pursuit of a colleague.

"Needed a little humour to lighten up a year in space," he wrote on Twitter on February 23, when he posted the video.

One image the bald-headed astronaut tweeted captured the economic divide between North and South Korea as visible from space, with the South aglow with electric lights and the North cast in a blanket of darkness.

Another impressive shot was one of the Milky Way which Kelly described as "old, dusty, gassy and warped. But beautiful.