(CNN) Apollo 11 astronaut Michael Collins remembers looking up a the sky as a child, seeing "the most marvelous things up there" and wanting to know more about them, he told CNN Chief Medical Correspondent Dr. Sanjay Gupta. That's when he knew that he wanted to fly.

Collins recently sat down with Gupta to talk about his memories of the Apollo 11 mission as its 50th anniversary approaches.

Becoming an astronaut

Collins came from a military family. His father and brother were Army generals, and his uncle was the Army chief of staff. He decided to "sneak off" to the US Air Force instead.

In 1961, Collins was a student at the Test Pilot School at Edwards Air Force Base in California. That year, President John F. Kennedy said that the United States would put a man on the moon by the end of the decade and return him safely to Earth, Collins remembers vividly.

Collins and about 80% of his peers were "gung-ho," he recalled. NASA and the idea of the Mercury and Gemini programs, which set up for the Apollo program, were attractive, and the space program seemed like a promotion. The other 20% would rather fly and test new airplanes for the Air Force rather than getting "locked up in a capsule and shot off like a round of ammunition," Collins said.

Collins, a fighter pilot for four years, graduated flight school at age 22. He "flunked out" the first time he applied to the space program. He says there are 15 or 20 reasons why he might have flunked, but he likes to tell the story of the famed Rorschach inkblots mishap during his psychiatric exam.

"I leafed through a whole series of them, and then the last one was a blank sheet of paper, pure white, 8 by 10," he said. " 'Here, so what do you see?' they asked. I say, 'well, of course that's eleven polar bears fornicating in a snowbank.' And I could see the examiner's eyes kind of tighten. He didn't think that was funny. He didn't like people making light of his card set. Anyway, for whatever reason, I flunked. The next year, [in the inkblot] I saw my mother and my father, and my father was slightly larger and more authoritarian but not too much more than my mother, and I passed."

Collins was selected as part of the third class of astronauts in 1963. His first mission was Gemini 10. His second was Apollo 11.

The six years between 1963 and 1969 flew by. Collins and his fellow astronauts worked hard, rising early and neglecting breaks on the weekends. They rarely saw their families and flew from coast to coast, visiting facilities where parts of the spacecraft were being manufactured.

They attended classes to learn everything about the spacecraft they would be flying and spent countless hours in simulators that replicated their missions to conquer every possible error.

Physical fitness was not one of the NASA requirements, Collins said. The astronauts had an initial thorough exam before they were accepted into the program, testing their senses and capabilities. But after that, physical fitness was up to the individual to maintain.

"We had an annual physical exam that we had to pass, and it was an extremely rigorous exam. They would assign two flight surgeons to one of us, and one would look in this ear, and one would look in that ear. If they didn't see each other, you passed," he joked. "That was the physical exam that NASA offered us, and they required us to really do whatever we felt like we should be doing in terms of our own conditioning."

At the time, one of the requirements to become an astronaut was graduation from an accredited test pilot school. Test pilots were used to mental stress and physical danger, so Collins believes that NASA was more focused on other aspects. The agency's priority was making sure that the astronauts could operate a complex machine that would be going a quarter of a million miles from Earth for the first time.

Apollo 11

Collins learned that he would join Buzz Aldrin and Neil Armstrong on Apollo 11 during a call from Deke Slayton, whose résumé included World War II pilot, test pilot, one of the original Mercury Seven astronauts and NASA's first chief of the Astronaut Office and director of Flight Crew Operations.

He put the crews together and was "sort of one of those unsung behind-the-scenes heroes," Collins said. "He was a wonderful, wonderful boss."

Slayton called Collins and asked, "hey, you still want to do this thing?"

"Oh, absolutely!" Collins replied. "You better believe!"

Kennedy's wish loomed large in Collins' mind. Then 39, he felt the astronauts carried the weight of the world.

He didn't talk about the dangers of spaceflight with his wife, Pat.

"We talked about superficialities and maybe alluded to those serious difficulties that a space flight entails," Collins recalled. "We'd nibble all around the edges of the danger involved."

Launch day on July 16, 1969, arrived quickly.

The three astronauts got out of their vehicle at the base of a tower that went 365 feet into the air. An elevator took them up to their command module, Columbia. Everything had to be "all neat and apple pie" before they could board. Collins looked to his left and saw a clear ocean. On his right was "the most gigantic pile of complex machinery you've ever seen."

"And I can remember thinking 'ooh, I think I'd rather look at the simple one rather than that complicated one. Maybe that's too complicated for me over there.' "

The Apollo 11 crew, from left: Neil Armstrong, commander; Michael Collins, command module pilot; and Edwin "Buzz" Aldrin, lunar module pilot.

The men knew that the chances of it failing somewhere along the line were relatively high, but they were optimistic about surviving, Collins said.

After that, the mission unfolded in a series of imperative events.

"I liken it to a daisy chain, long and very fragile daisy chain," Collins said. "It emanates from Cape Canaveral, and then it goes out into space and around the moon and circles it back in. And it's got all these links in it, and if one link fails, well, all the rest downstream are useless. So for eight days, to and from, there was always one thing coming up, the next big event which could ruin you, be the end of you. That was how it worked."

While Aldrin and Armstrong separated from Columbia in the lunar lander, the Eagle, to land on the moon, Collins kept circling the moon. Once Armstrong and Aldrin were finished, he would rendezvous and dock with the Eagle after it left the lunar surface. That maneuver was the one they had prepared for most during training on Earth. Collins had a 8-by-10 notebook with 18 scenarios around his neck. But it went perfectly.

Photos: Reaching for the moon At 9:32 a.m. on July 16, 1969, NASA's Apollo 11 spacecraft was launched by a Saturn V rocket from the Kennedy Space Center in Florida. On board were astronauts Neil Armstrong, Buzz Aldrin and Michael Collins. Four days later, on July 20, they would become the first men to land on the moon. Hide Caption 1 of 10 Photos: Reaching for the moon Former U.S. President Lyndon B. Johnson and then-Vice President Spiro Agnew were among the spectators at the historic launch. Hide Caption 2 of 10 Photos: Reaching for the moon A photo of the full moon taken from Apollo 11. At the time, the spacecraft was 10,000 nautical miles from Earth. Hide Caption 3 of 10 Photos: Reaching for the moon A spectacular view of home, taken halfway between Earth and the moon. Hide Caption 4 of 10 Photos: Reaching for the moon Aldrin is photographed by Armstrong inside Apollo 11's lunar module, just prior to the moon landing. In orbit, Aldrin later took what would become known as the world's first space selfie Hide Caption 5 of 10 Photos: Reaching for the moon Aldrin is photographed on the moon after Armstrong went first and called it "one small step for man, one giant leap for mankind." Armstrong and Aldrin explored the Sea of Tranquility region of the moon. Meanwhile, Collins remained inside the command module for the duration of the mission. Hide Caption 6 of 10 Photos: Reaching for the moon A close-up view of Aldrin's boot print in the lunar soil. Hide Caption 7 of 10 Photos: Reaching for the moon Back on Earth, NASA officials celebrate the successful conclusion of Apollo 11's eight-day mission. Hide Caption 8 of 10 Photos: Reaching for the moon The U.S. Navy assists the returning astronauts after their re-entry vehicle landed safely in the Pacific Ocean on July 24, 1969. Hide Caption 9 of 10 Photos: Reaching for the moon President Richard Nixon welcomes the astronauts back to Earth: from left, Armstrong, Collins and Aldrin. The astronauts were received by the President from their mobile quarantine unit, which was thought to help prevent the spread of contagions caught on the moon. The quarantine practice was discontinued a couple years later after Apollo 14's mission. Hide Caption 10 of 10

Collins was often called "the loneliest man" once he returned to Earth, but he didn't feel that way -- even when he lost contact with Mission Control during his flybys on the far side of the moon. While Armstrong and Aldrin were busy landing, setting up experiments and collecting samples from the lunar surface, Collins had to keep all of the subsystems running on Columbia by himself.

"It was a happy home. I liked Columbia," he said. "It reminded me, in a way, of almost like a church or a cathedral. It had the apse, the three couches, and then you went down into where the altar was. That was the guidance and navigation system. And it was laid out almost like a cathedral. And I had hot coffee. I had music I could play if I wanted to. I had people to talk to on the radio, sometimes too many people talking too much on the radio. So I enjoyed that interlude. Being by myself in a machine up in the air somewhere was not unknown to me, and so everything was working well within Columbia, and I enjoyed it."

The crew. Found this at the bottom of a box. Don't think it was ever used by @NASA. #TBT @TheRealBuzz pic.twitter.com/ZXINsWPcix — Michael Collins (@AstroMCollins) June 13, 2019

When the three men were reunited after the docking, Collins wanted to celebrate with Aldrin and Armstrong. But they had mission items to tend to. The daisy chain wouldn't be complete until they landed safely on Earth.

"I remember I was going to grab Buzz by the shoulders and kiss him on the forehead, and then I decided, 'No, that's just not right.' So I don't know. I shook his hand or patted him or something. And Neil, I didn't even bother touching Neil when he came through. That was it. We didn't say 'oh, you have landed on another planet' or anything like that."

Although Collins did look for the little bit of cognac he thought they had stowed away, he never found it.

Apollo 11 astronauts Neil Armstrong, Buzz Aldrin and Michael Collins waving to crowds at a parade celebrating their return from the moon.

Memories of Armstrong and Aldrin

Collins remembers thinking that Armstrong was a wonderful choice for Apollo 11. He couldn't imagine anyone finer.

"He was probably the best test pilot amongst us in that he had been flying the X-15 rocket plane, which today still holds some speed and altitude records, and so he was the most experienced test pilot," Collins said.

Collins remembers Armstrong as incredibly intelligent, with an interest in the history of science.

After the successful Apollo 11 flight, Collins saw another side of Armstrong as the three astronauts embarked on a trip around the world to talk about their experiences. Armstrong was their spokesman.

"He was just amazing," Collins recalled. "He'd delve into the background of this thing and that, and wherever we went, he'd pick out details that appealed to the local population, and you could see by the time he got through his little introductory speech, they almost felt like they were crawling on board with us.

"It was just an amazing feat, and I think he's often overlooked in a way. First Man -- he's not overlooked. But what people maybe don't know about First Man was that First Man was one marvelous proponent of the virtues of the United States and spread those all over the globe."

Aldrin was also highly qualified, a West Point graduate who was third in his class of 500. He was a fighter pilot during the Korean War and went to the Massachusetts Institute of Technology afterward, pursuing a doctorate in orbital mechanics.

"At that time, NASA was considering bringing two spacecraft together in space as the most ticklish part perhaps of the lunar profile, and here was this guy who had a Ph.D. in exactly that," Collins said. "So he was very, very well qualified."

But, Collins joked, sometimes that's all Aldrin wanted to talk about.

"Now, we tried sometimes not to sit next to Buzz at a party because he was working 24 hours, around the clock, with 'now, if you get an ellipse and its epicenter is incongruent with the other, and then of course this comes through and the perigee is down there,' " Collins said. They would respond with, "OK, Buzz, OK."

The three men didn't remain in close touch afterward, mainly because Collins lived in Washington, Armstrong lived in Ohio and Aldrin moved around. It wasn't easy to get together. But they had shared in something wonderful and fulfilled Kennedy's mandate.

From left, Neil Armstrong, Michael Collins and Edwin "Buzz" Aldrin are subjected to a period of quarantine upon their return to Earth on July 24, 1969.

Life after Apollo

Apollo 11 was the proudest moment of Collins' life. He may not have had the best seat on Apollo 11, but he was happy with the seat he had, he said. He felt privileged to be there.

His biggest regret was for those who couldn't be there -- pilots who died in training accidents along the way, the Apollo 1 astronauts and his friend Charlie Bassett, who died in an airplane crash. "I thought 'boy, he'd be right up there at the tippy-top of the list of who goes first on the moon.' So I regret that aspect of it."

The Apollo 1 crew.

After his return, Collins retired from NASA, wanting to spend more time with his family. "Pat and I took on a totally different kind of life," he said.

"I figured I'd done the space thing and at the highest level," he said. "I made a speech at a joint session of Congress and the secretary of state, Bill Rogers, liked it, and he got President Nixon involved. And so the next thing I know, I was offered a job as assistant secretary of state. A little less than two years when up popped one that I was better qualified for, which was the director of the Air and Space Museum of the Smithsonian, and that was just getting started. I started with a vacant lot and a hole in the ground and then a building and so forth."

Collins will turn 89 in October. He began running with astronaut Ed White during the Gemini program and ran 50 miles when he turned 50, completing triathlons along the way.

He still believes that exercise is important and walks using trekking poles due to balance issues caused by peripheral neuropathy, nerve damage that can cause weakness and pain in the feet. He follows a simplified version of the Mediterranean diet and reads to keep his mind sharp. "2001: A Space Odyssey" is his favorite movie, but otherwise, he doesn't watch much TV or movies or engage on social media. And he's got his eye on living to 100.

If there's one question he's tired of hearing after all of these years, it's "what was it like up there?" It's part of why he wrote his 1974 book, "Carrying the Fire," which has been rereleased for the Apollo anniversary.

From left, Michael Collins, Neil Armstrong and Buzz Aldrin are honored during the 40th anniversary of their Apollo 11 mission.

Throughout the Apollo mission, Kennedy's wish was in the back of Collins' mind. He still reflects on that aspect of the mission 50 years later.

"Apollo 11 was the culmination," he said. "We were finally able to do what Kennedy had asked us to do, and so I think Neil and Buzz and I, all three, we felt that this was a culmination of a long, successful series. And we tried our best to fulfill it."

Seeing the moon up close was spectacular, but he recalls that the view of Earth kept snaring the astronauts' attention.

"I said 'hey, Houston, I've got the world in my window,' " Collins said. "And the world is about the size of your thumbnail if you hold it out arm's length in front of you. The whole focus of your attention goes into this little thing out there. It's in a black void, which makes its colors even more impressive. Primarily, you get the blue of the oceans, the white of the clouds, you get a little streak of tan that we call continents, but they're not that noticeable. It just looks glorious."

But Collins noticed something unique about his perspective of our home planet.

"Strangely enough, it looks fragile somehow," he said. "You want to take care of it. You want to nurture it. You want to be good to it. All the beauty, it was wonderful, it was tiny, it's our home, everything I knew, but fragile, strange."