US astronaut Scott Kelly and Russian cosmonaut Mikhail Kornienko returned to Earth Wednesday after 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 a 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 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.

- Eyeing 'boots on Mars' -

"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 under went more tests soon after landing.

Kelly and Kornienko "fulfilled a program of 11 tests" following their three-and-a-half hour descent from the ISS, Russian space agency Roscosmos said in a statement.

These included "getting up from a chair without using their hands", "standing vertically for three minutes" and "walking in tandem with eyes closed" among others.

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 those of his identical twin brother Mark back on Earth, who is a former astronaut.

"My brother... is back home on our planet! Talk about aliens. He's been off the planet for a year," tweeted Mark Kelly, who retired as an astronaut in 2011.

Kelly will now be flown to Houston for a NASA medical examination.

- Gorilla in space -

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 143 million miles (230 million kilometres).

In one particularly eye-catching stunt, the bald-headed astronaut 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 he tweeted captured the economic divide between North and South Korea, with the South aglow with electric lights and the North covered 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."

"Spaceflight is the biggest team sport there is, and it's incredibly important that we all work together to make what is seemingly impossible possible," Kelly said when handing over command of the ISS to fellow NASA astronaut Tim Kopra on Monday.

The ISS trio leave behind Kopra, Russian cosmonaut Yuri Malenchenko and the European Space Agency's British astronaut Tim Peake.

NASA's Jeff Williams and Roscosmos' Oleg Skriprochka and Alexey Ovchinin, will join them following a launch from Baikonur later this month.

The American record for consecutive days in space was previously held by Michael Lopez-Alegr�a, who logged 215 straight days in 2007.

The world record for longest single stay in space is held by Russian Valery Polyakov, who spent some 438 days on the Mir space station between 1994 and 1995.

The ISS space laboratory has been orbiting Earth at about 28,000 kilometres per hour (17,000 miles per hour) since 1998.

Space travel has been one of the few areas of international cooperation between Russia and the West that has not been wrecked by the Ukraine crisis.

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