During his year in space, NASA astronaut Scott Kelly said he could do another year if he had to. But now that Kelly has returned to Earth and retired from NASA, he says the experience took an emotional and physical toll.

The down side of long-term stints on the International Space Station came up today when Alfred A. Knopf announced it would be publishing Kelly’s memoir, titled “Endurance: My Year in Space and Our Journey to Mars.”

The announcement included a telling quote from the 52-year-old spaceflier:

“During my time in orbit, I lost bone mass, my muscles atrophied, and my blood redistributed itself in my body, which strained my heart. Every day, I was exposed to 10 times the radiation of a person on Earth, which will increase my risk of a fatal cancer for the rest of my life. Not to mention the psychological stress, which is harder to quantify and perhaps as damaging.”

Kelly has previously referred to the rigors of life in space, though not quite as starkly. In addition to the problems he listed in today’s statement, he’s mentioned vision impairment and skin sensitivity. All these are well-known side effects of long-term spaceflight.

“The hardest part is being isolated in a physical sense from people on the ground that are important to you,” Kelly said just before he came back to Earth in March.

Despite the hardships, Kelly’s book will make the case for continuing space exploration and moving onward to Mars. Here’s what he had to say on that topic today:

“There are few aspects of everyday life that aren’t touched by the technologies developed for space travel … but these innovations aren’t the only benefits of spaceflight. … The superhuman accomplishment of innovation, perseverance, and cooperation carried out by thousands of Americans working towards one audacious goal speaks for itself.”

Kelly’s retirement from NASA took effect on April 1, and that’s freed him up to take on a variety of outside gigs – including the book deal. Knopf said Kelly would write “Endurance” with Margaret Lazarus Dean as co-author, with publication set for late 2017.

Kelly is also due to publish several books about his time in space for young readers, plus a book of photographs taken aboard the space station. Terms of the deal were not disclosed.

In addition to the book announcement, Kelly put in an appearance at the Microsoft Envision conference in New Orleans this week. He’s also taking on a role as an ambassador for Breitling, a Swiss watch manufacturer that made two of the timepieces he wore in orbit.

Although the list of Kelly’s commercial ventures is quickly growing, the veteran Navy pilot and spaceflier is still keeping some connections to NASA for the good of the order.

He and his twin brother, retired astronaut Mark Kelly, have been taking part in a comparative study on the longer-term health effects of long-term spaceflight, and Scott has said he’ll continue to participate in the study “for as long as is necessary.”