When American astronaut Scott Kelly arrived back on Earth after spending almost a year on the International Space Station, there was one thing he was itching to do: jump in a pool.

And so, as he relates in his new book, “Endurance,” when he got home, he walked through the front door, out the back door and, without even stopping to remove his clothes, let himself fall into the water, just so that he could remember what it was like to be submerged by water.

(He even had someone take a video of the moment)

“And then I came back inside and had a beer and an apple pie that had been sent from the White House,” Kelly said, remembering the moment on CTV’s Your Morning Friday.

Those first few days after returning from 340 days in zero gravity brought on a lot of the usual symptoms Kelly remembered from three other space missions: nausea, dizziness, feeling like he weighed 1,000 pounds after weighing nothing for a year.

“That part wasn’t too bad,” he said. “But for me, what was most alarming and different that I didn’t experience the other times was that when I stood up, I could feel the blood rush through my legs. I would get rashes because my skin hadn’t touched anything for a year. All these symptoms were new things for me.”

Before the mission began in March, 2015, Kelly decided to keep a journal of his experience, knowing he would be setting a record for NASA's longest single space flight ever. He took notes about how the space station looked, how it sounded and smelled, just so he could relay the experience to others.

Now back on Earth and retired for the last year and a half, Kelly no longer feels the physical effects of the long space flight. But, he says, there are lasting effects. For example, the fluid shifts that occur during space flight cause increased pressure in the brain, which pushes on the back of the eyes, causing them to change shape. Though some astronauts experience vision changes, Kelly says his vision is fine.

And then there’s the solar radiation Kelly absorbed, which could lead to health problems down the road.

Because Kelly has an identical twin brother – Mark, also an astronaut -- NASA was able to conduct extensive testing on Kelly, and then compare his results to those of his brother on Earth.

One of these tests involved measuring telomeres, which are the tips at the ends of our chromosomes that are an indication of our physical age.

“The hypothesis was that mine would get shorter and more frayed; in other words, in the direction of aging because of this harsh environment. But what they saw was the exact opposite,” Kelly said. “So yeah, I got younger than my brother.”

NASA scientists have said that Kelly’s longer telomeres could be linked to increased exercise and reduced caloric intake during the mission. Interestingly, his telomere soon began to shorten again after his return to Earth.

As hard as the mission was on his body, Kelly says the hardest part at the time was knowing that no matter what happened to his family on Earth, he couldn’t come back to help them.

“I experienced that when I was in space for six months when my sister-in-law, Congresswoman Gabriel Giffords was shot in Tuscon, Ariz. and I still had a few months of front of me,” Kelly said.

“So I was always more concerned about what if something happens to my kids, my family members – that was always more concerning to me than my own personal safety.

“…You’re signing up for this idea that if something bad happens on Earth to your family members, you are not coming home.“