Two months after returning to Earth, Scott Kelly is still nursing stiff legs, sore feet and fatigue.

The former NASA astronaut spent 340 days in space — the longest American spaceflight so far. When Kelly landed in Kazakhstan, he complained of joint pains, muscle soreness and skin problems far worse than what he has previously experienced.

In his first major address to employees of NASA on May 25, Kelly shared that his appearance upon landing failed to express how he was feeling at that time. Kelly continued to say that even when he went home in Houston, he would still experience flu-like symptoms and skin issues that could have prompted him to go the emergency room if he had not just gotten back from space. Although he would prefer not having to go through it, he knows it was not without a purpose.

"That's why we do this," Kelly said. "We need to learn these things if we're going to go to Mars."

Part of NASA's One Year Crew goal is to recognize the effect of microgravity on a human body — its coping mechanisms — as a preparation for longer spaceflights, especially now that the agency is gearing up for a 2030 Mars journey. Spaceflight to Mars is expected to last for about a year.

Upon his return, Kelly underwent several tests that would look at his overall health, such as physical, mental and psychological conditions to address his concerns and also understand the biophysical changes his body was subjected to.

Kelly's earlier space missions lasted only about 5 months, from which he was able to recover after six months. For his present recovery, Kelly said it would probably take some time, even longer.

"I suspect it's probably going to be much longer, especially considering how sore my feet still are after 2 and a half months," Kelly said. "But the good news is I do feel better all the time."

Kelly retired from NASA after returning from space and is now busy doing talks and writing a book that would detail his experience in space.

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