Spending a year in space takes such a toll on the human body that astronauts literally have to learn how to walk again once they’re back on Earth. At least, that’s what seems to have happened to Scott Kelly — the American astronaut who spent 340 days on the International Space Station (ISS) between 2015 and 2016.

In an exclusive video given to The Verge by PBS, Kelly is seen trying to walk on a straight line right after landing in the steppes of Kazakhstan. He slowly gets up and stumbles. Putting one foot in front of the other looks like a gargantuan task, as if his legs are made of jelly. Six hours after landing, his steps are a bit quicker, but still uncertain. And after 22 hours, he’s much more stable, but still wobbly. It’s as if Kelly is a one-year-old just learning how to walk.

On Earth, we know where is up and where is down.

That’s because zero gravity messes with our sense of orientation. On Earth, we know where is up and where is down. In space, not so much. Sensors inside our ears, which are part of the vestibular system that controls balance, are thrown off — often causing astronauts to feel dizzy or queasy the first few days in space. Once they get back to Earth, it takes a while for their bodies to readjust. Hence, the walking problems.

And it’s not just the messed-up balance system, either. The first time I interviewed Kelly — two months after he’d come back from his year in space — he told me his feet still hurt. Two months after being thrust back into Earth’s gravity. This is just one way long periods in space affect the human body. And that’s exactly why Kelly spent a year on the ISS to begin in: by understanding how zero gravity changes us, the next generation of astronauts will be better prepared for deep-space travel.

The clip showing Kelly wobbling after his trip to space is part of a new PBS / Time Inc. special about Kelly's homecoming and adjustment to life on Earth. The show premieres on November 15th at 9PM ET on PBS. You can take a look at the trailer below: