NASA astronauts Sunita Williams and Butch Wilmore recently returned home after an unexpected nine-month stay on the International Space Station. Now, they’re in recovery mode.

“When you’re in this environment, your body doesn’t have to work as hard, so your muscles weaken,” Kelly Bishop of the Sky and Telescope Magazine told Boston Public Radio. “Your bone density goes down. And the fluid in your body migrates to your head; you feel like you have a cold all the time because your head is kind of flush. Your eyeballs actually change shape a little bit.”

A lot of that corrects itself upon returning to Earth’s gravity. But the process isn’t easy.

“Your skin — because all your clothing [was] floating — gets super sensitive. So when you get back down on the ground, it’s like sandpaper on your arms,” Bishop explained.

Astronauts also have to adjust to the sensations of water and weight in their limbs, which can limit mobility right after touchdown. Former NASA astronaut Jack Fischer explained the sensation to NPR: “You come back and all of a sudden your leg weighs 30 pounds again.”

Williams and Wilmore boarded Boeing’s Starliner spacecraft for what was supposed to be an eight-day test flight last summer. Their stay in space was extended to 286 days after the Starliner experienced a technical malfunction, and determined to be incapable of safely returning the astronauts to Earth. Williams and Wilmore, along with NASA’s Nick Hague and Russia’s Aleksandr Gorbunov, who joined the two in September, finally returned home last week.

NASA now has the astronauts on a 45-day recovery program that includes medical testing and daily sessions with trainers.

But not all of the effects are reversible. Time in space will weaken a person’s bones, for example, and expose them to higher-than-normal levels of radiation.

“Those two on this trip probably got as much radiation as a resident near the Fukushima plant in Japan would have ... it’s not dangerous, but there are limits to this,” Bishop said.

He said Williams and Wilmore may never fly again.

“Trust me, Butch and Suni would not have traded a moment of it,” Bishop said.

Hague, Williams and Wilmore will be answering questions about their experience during a news conference on March 31.