ISS astronauts back on earth

"The air out here feels great. I've no idea why you guys are so bundled up," Commander Scott Kelly said on Wednesday after he and fellow crew members Mikhail Kornienko and Sergey Volkov returned to Earth.

The space agency also posted a video of the Russian spacecraft Soyuz reentering the earth's atmosphere.

The "one-year crew" mission began in March 2015 and underwent several tests as part of a NASA study to see how long duration space flights affected the human body.

The absence of gravity in space means that muscle mass and bone density could reduce and eyesight could diminish because of an increase in the cerebrospinal fluid around the optical nerve.

Scientists in Kazakhstan were planning to observe how Kelly's body responded to different positions and movements.

Scott Kelly, 52, is also an interesting case for scientists because he is a twin. Experts compared his development and changes to his brother Mark, also a former astronaut. NASA said it was the first experiment conducted on two people with the same genetics.

Kelly's 340-day-mission was the longest single stay in space by a US astronaut. He also holds a US record for spending the most cumulative time in space. Russian cosmonaut Valeri Polyakov holds the record of 437 days, the longest cumulative time in space.

The US space traveler tweeted this message towards the end of his trip.

mg/kms (dpa, AFP)