After returning from the longest U.S. space mission on record, NASA astronaut Scott Kelly says he feels confident human beings are capable of someday making the long journey to Mars.

Kelly, 52, made the comments Friday during a news conference at NASA's Johnson Space Center in Houston, where he answered questions about his 340-day mission aboard the International Space Station.

"Going to Mars, if it takes 2½ years, yeah, that's doable," the astronaut said. "There's still things we have to learn, but we can learn them."

He and Russian cosmonaut, Mikhail Kornienko, served about twice as long as previous ISS crew members as part of a pilot program to help NASA and its partners prepare for eventual human missions to Mars that will last at least two years.

"I could have gone longer on this flight if there was good reason," Kelly said.

The world record holder for longest single spaceflight in human history is Russian cosmonaut Valeri Polyakov, who spent 438 days in orbit aboard the Mir space station in 1994-95.

Kelly left the space station on Tuesday in a Russian Soyuz capsule that landed in Kazakhstan, then headed home to Houston.

"I'm used to going 17,500 miles per hour, but this airplane doesn't quite do that," Kelly quipped after a belated 2:30 a.m. ET touchdown on Thursday at Ellington Field near the Johnson Space Center.

He was welcomed by a crowd that included Jill Biden, wife of U.S. Vice-President Joe Biden, who gave him beer and apple pie.

"Welcome back to Earth," U.S. President Barack Obama wrote to Kelly on Twitter. "Your year in space is vital to the future of American space travel. Hope gravity isn't a drag!"

But amid the celebration, Kelly is busy undergoing medical checks at the space centre as he starts recovering from his time in the weightless and high-radiation environment of space. Scientists are studying how the human body fares during long stays in space.

Before landing, Kelly told reporters he had experienced some changes in his vision, an affliction that affects about half of astronauts on long-duration space missions.

On Friday, he discussed some of the other physical tolls the space mission has taken on his body, including muscle soreness and fatigue.

"I think coming back to gravity is harder than leaving gravity," he said.  "I also have an issue with my skin because it hasn't touched anything in that long. ... It's very, very sensitive, almost like a burning sensation."

Kelly's participation in the mission also gave doctors an opportunity to look at possible genetic changes from spaceflight.

Researchers have been studying whether he and his identical twin brother, former astronaut Mark Kelly, are as genetically similar now as they were before Scott Kelly's launch in March 2015.

"This kind of genetic-based research is new for us, so that's exciting," Scott Kelly said. 

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With files from Associated Press