His muscles are sore, his joints ache, he’s fatigued and his skin is super-sensitive, the first U.S. astronaut to spend almost a year in space told reporters Friday.

“I’m kind of surprised how I do feel different physically than the last time, with regards to muscle soreness and joint pain,” Scott Kelly, 52, said during his first press conference since returning to Earth after 340 days in orbit.

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Then, there’s what Kelly called “the skin issue.”

“Because I hadn’t touched anything for so long, (had) any significant contact, it’s very, very sensitive, almost like a burning feeling wherever I sit or lie or walk,” Kelly said.

Kelly and two Russian cosmonauts, including Mikhail Korneinko who was Kelly's crewmate through the year in space, landed in Kazakhstan late Tuesday.

Kelly, a veteran of three previous spaceflights, flew back to the Johnson Space Center in Houston the next day and immediately began a series of post-landing medical tests that will last about a year.

“Initially this time coming out of the capsule, I felt better than I did last time, but at some point those two lines have crossed and my level of muscle soreness and fatigue is a lot higher than it was last time,” Kelly said. “That was kind of unexpected.”

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NASA and its space station partners flew Kelly and Kornienko for a year as a pilot program intended to pave the way for missions to Mars lasting more than two years. Typically, crews serve aboard the space station for six months.

Kelly told reporters the year passed slowly in space, but he could have stayed longer if it was for “a good reason,” such as a mission to Mars.

“It seemed like I lived there forever,” Kelly said.

The daily, 250-mile-high views of the planet left Kelly with a deeper appreciation of the environment.

“You can see a lot of pollution over parts of Asia that is almost continuous, constantly there. You can’t really see the ground very well. And those fires in California over the summer, that smoke was pretty extensive, covering large parts of the U.S.” Kelly said.

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“But the predominant thing is you just notice how thin the atmosphere is, how fragile it looks. So that, combined with these large swabs of pollution, is somewhat alarming.

“People often say … that we need to save the planet. The planet will be just fine. It’s us that is going to have a problem … I think the planet will eventually recover (but) it probably will be without people,” Kelly said.

“For us to take care of the air we breathe and the water we drink is critical. I do believe we have an impact on that and we have the ability to change it if we make the decision to,” he added.