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After 340 days in space, astronaut Scott Kelly back on Earth

Astronaut Scott Kelly is safely back on Earth, and he's already thinking about when he can leave again.

Kelly, who landed on the steppes of Kazakhstan late Tuesday, has spent more consecutive time in space — 340 days — than any other American.

His most recent stint was aboard the International Space Station, where he's lived since March 27. In all, the astronaut has now spent a total of 520 days in orbit across four missions.

Also leaving the station Tuesday were Russian cosmonauts Mikhail Kornienko and Sergey Volkov.

"Physically, I feel pretty good," Kelly said at a news conference days before his return. "The hardest part is being isolated in a physical sense from people on the ground that are important to you."

Those aren't idle words. Kelly is helping NASA develop protocols and techniques the agency will use for a crewed mission to Mars. His twin brother, Mark, a retired astronaut, remained on the ground while Scott circled the Earth.

The two have been subject to extensive and ongoing testing, including DNA testing, to determine how long term exposure to space radiation affects humans. NASA is also looking at mental health to help understand how long periods of near-isolation affects space travelers.

Speaking with reporters before leaving the station, Kelly crossed his arms and bobbed in zero gravity. Among the experiments astronauts were conducting: how to prevent muscle loss without the force of gravity.

Kelly said he wasn't "climbing the walls" as the end of his journey neared, although he did have a bit of fun in the waning days: He donned a gorilla costume and chased a fellow crew member around the station in a widely shared NASA video. He also reshared many of his favorite photographs taken from the station as it orbited the Earth at nearly five miles per second.

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Kelly will fly to Houston on Wednesday for examination by NASA doctors. And then?

"And then I'm going to go home and jump in my pool," he said.