It's not easy living in space. But then, it's not easy coming back to Earth either.

Picking up where the PBS documentary A Year In Space left off, the sequel, aptly entitled Beyond A Year In Space, chronicles what it's been like for U.S. astronaut Scott Kelly since returning from his 340 days in space. It also introduces the next generation of astronauts who could spend even longer stretches in space on future missions.

Kelly spent a year living aboard the International Space Station, along with Russian cosmonaut Mikhail Kornienko, so that scientists can study the long-term effects of space on the body. (Warning: It's not all pretty.) By luck, Scott Kelly has an identical twin brother, Mark Kelly, astronaut and husband of former congresswoman Gabby Giffords, who remained on Earth as a sort of experimental control for researchers.

Scott Kelly takes a selfie inside the Cupola, a special module which gives a 360-degree viewing of the Earth.

The documentary opens with Kelly's last day in space. It follows him as the Soyuz capsule brings him safely back to Earth -- landing on the broad steppes of Kazakhstan. Upon landing researchers put Kelly through a battery of tests to gauge his strength and coordination, tests that continue into the weeks and months that follow. A year without gravity has left his muscles and joints stiff and sore, despite a rigorous exercise schedule aboard the ISS.

"Gravity definitely gives you a beat-down when you get back," he says.

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Perhaps no one has held a job that is so simultaneously glamorous and unglamorous.

Scott Kelly takes a selfie during a spacewalk on Oct. 28, 2015.

On one hand, he holds the record for a U.S. person in space. On the other hand, he returns to suffer achy, flu-like symptoms, "to the point where," he says, "if I hadn't just gotten back from space I would have gone to the emergency room."

His skin suffers something like an allergic reaction -- breaking out in hives anywhere pressure is put on it. It has grown accustomed to being essentially untouched. (Astronauts don't so much wear clothes in space, as float around inside them.) The reactions last about a week.

He offers up countless blood and biological samples, and grew accustomed to drawing his own blood on the ISS. (Note to any young aspiring astronauts: the squeamish do not make it to space.)

[contextly_sidebar id="UgAVf3EURc6iTGy1po7SPnQd6dSVhqO3"]Kelly's health may have suffered in deeper, less obvious ways too. Aboard the ISS he and his crew mates were without the protection Earth's atmosphere provides in shielding us from radiation. High-energy particles, from the Sun and other cosmic bodies, rip through DNA strands, clearing the way for potential mutations. Our bodies are able to repair minor radiation damage, however high exposure to these particles increases risk for cancer.

Studying how Kelly's body responds to a year in space will be invaluable for the upcoming crop of astronauts, who are hoping to spend roughly twice as long as Kelly in space, on missions to Mars.

While Kelly will probably never fly again, his contributions to space flight will last forever.

"I think I'd be lying to say I didn't wish I'd be the guy going to Mars someday or be the guy that got to walk on the Moon, work on the Moon, visit the Moon," Kelly says. "But there's only one space program in this country and I just feel really privileged to have the experience I had. So it's not like I dwell on it."

Beyond A Year In Space premieres tonight at 9 p.m. on KQED. It will be preceded by an encore broadcast of A Year in Space at 8 p.m.

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Living on Space Station Takes Toll on Astronaut’s Health: Watch Full Documentary Online