It's the most exclusive fraternity on Earth.
And it was 40 years ago today that the first of those men blasted off into space on the missions of their lifetimes.
Apollo 11 launched from Cape Kennedy on July 16, 1969, carrying Neil Armstrong, Michael Collins and Edwin "Buzz" Aldrin.
Four days later, the world watched as Armstrong stepped onto the moon and made his "giant leap for mankind."
Between 1969 and 1972, five more NASA missions landed on the moon, giving a total of 12 people the chance to walk, run and, even, golf on the lunar surface.
"For the astronauts, going to the moon was professional first and a personal experience second," said Andrew Chaikin, a science journalist who has spent more than 100 hours interviewing the lunar astronauts for his books "Voices From the Moon" and "Man on the Moon."
"Imagine getting to do the most important thing that you could possibly do in your profession, doing it for national prestige and doing it with the whole world watching and knowing that it was something only a handful of people could accomplish," he continued. "That's what it was."
These men were as prepared as they could be for their out of world experiences.
"But when they came back from the moon, we gave them the mission they never trained for," Chaikin said.
Fame, adulation, the constant stream of questions. These were the challenges they wrestled with in the aftermath.
"But they came through. And they came through in spectacular fashion," he said.
Some struggled more than others -- and some changed more than others – but they were all, in some way, moved by having seen Earth shrink before their eyes.
Here are their stories.
His were the first human feet to touch the extraterrestrial.
As the world watched on July 20, 1969, Neil Armstrong stepped onto the moon and gave American history one of its most memorable quotes.
But, though he's the most famous moon walker, he's also one of the most quiet, very selective about offers to speak and be interviewed.
He resigned from NASA in 1971 and accepted a teaching position at the University of Cincinnati's Department of Aerospace Engineering, which he held until 1979.
For the next decade or so, he was chairman of an aviation software company, Computing Technologies for Aviation, Inc. in Charlottesville, Va.
Along the way, he's collected honors from 17 countries, including the Presidential Medal of Freedom, the Explorers Club Medal and the Congressional Space Medal of Honor.
He had two sons with his first wife Janet, who divorced him in 1994. He later re-married.
In 2005, he stepped back into the public spotlight to endorse a biography, titled "First Man" by James Hansen. The book was as much an exploration of "American hero worship" as it was an exploration of the man himself.
"Friends and colleagues all of a sudden looked at us, treated us slightly differently than they had months or years before when we were working together. I never quite understood that," Armstrong said.
In an interview with CBS News' 60 Minutes soon after the book's release, he commented on his discomfort with his celebrity.
"I guess we all like to be recognized not for one piece of fireworks, but for the ledger of our daily work," Armstrong said. "I wasn't chosen to be first. I was just chosen to command that flight. Circumstance put me in that particular role. That wasn't planned by anyone."
As the second man to walk on the moon, Edwin "Buzz" Aldrin has cemented a spot in history, but the now 79-year-old has spent much of his career looking to the future.
In the decades since the moon landing, Aldrin has lectured and written extensively about the country's future in space. He retired from NASA in 1971 but continued to work on space exploration, proposing a plan to reach Mars and receiving three patents for a modular space station.
He has also penned several books, including an autobiography, "Return to Earth."
In an interview with Chaikin for "Voices From the Moon," he shared the difficulty in articulating his experience.
"I'm not really sure how a layperson reader is going to grasp whatever words are going to try and describe this. I've felt totally inadequate in ever trying to do it with spoken words," he said.
In a rare show of violence, he punched filmmaker and skeptic Bart Sibrel in 2002 when Sibrel challenged him to prove that he actually went to the moon.
Pete Conrad wasn't the first to walk on the moon, but some remember him as the first to dance on it.
In "Rocketman," a book about the astronaut's "incredible ride to the moon and beyond," his wife Nancy Conrad and Howard A. Klausner, describe the third man to walk on the moon as an adventurous, free-spirited space cowboy.
As the world listened to Conrad's first moments on the moon, he hummed. And then he shouted.
"Whoopie! That may have been one small step for Neil, but it's a long one for me!" he roared.
Conrad was selected as an astronaut by NASA in 1962 and traveled to the moon in November 1969 as the commander of Apollo 12, the second lunar landing.
Years later, in an interview with Chaikin, he said, "I remember thinking I got to go to the moon. And I can also remember thinking, it's going to change a lot of people, to do that. … I thought very hard about that I didn't want it to change me… [And now] I don't think it changed me."
After 20 years of service, including 11 as an astronaut in the space program, Conrad retired from the U.S. Navy to begin a career in business. He accepted an executive position with the American Television and Communications Corp., a cable television company in Denver, and later became vice president for the Douglas Aircraft Company.
In July 1999, at age 69, Conrad died after a motorcycle crash near Ojaj, Calif. He was survived by his wife, three sons and several grandchildren.
In his book, "Carrying the Fire," fellow astronaut Michael Collins describes his courageous and notoriously puckish colleague as "One of the few who lives up to the image."
In 1969, Alan Bean touched down on the Moon. But through his paintings, the astronaut cum artist says he tries to extend the moment.
"Our time on the Moon ended much too quickly and, in the years since then, I have created paintings to try to capture the feeling of our Apollo 12 mission, as well as all the other the Apollo missions, too," he says in an introduction on his Web site.
After 18 years with NASA, in 1981, the Apollo 12 moonwalker decided to pursue painting full-time to share the sights no other artist had ever seen. But he approached it the way he might a moon mission.
"So I took some time off and painted full-time to see if I'd like it. I simulated it, which is always good. I learned that at NASA; and the more I simulated being an artist, the more I realized it's much more difficult than I'd thought," he said. "But at the same time I liked it. I cared about it! I had many nice job offers for a lot of money, but I didn't care about them. I care about these paintings. I care about them every day."
After landing on the moon's Ocean of Storms with Pete Conrad in 1969, Bean stayed with NASA, painting on the side.
But after retiring at age 49, he devoted his time to painting in his Houston studio. In 1984, he publicly displayed his work for the first time in Houston. His exhibit, "First Artist on Another Planet," is currently at the Smithsonian National Air and Space Museum.
Each of his paintings includes small pieces of moon memorabilia – from parts of his name tag to Apollo mission patches and the U.S. flag. (His Web site points out that "each of the fragments is embedded with small amounts of lunar dust.")
"I remember thinking in lunar orbit, that if I got back from this, I was going to live my life differently, in that I was going to try to live it… like I want to live it," he said in an interview with Chaikin. "Mostly it made me have a lot of courage to do what I wanted to do and be happy about it… that's one thing that really allowed me to be an artist. I probably wouldn't have had the courage to be an artist."
Ten years after becoming the first American to journey into space, Alan Shepard logged another extraterrestrial milestone – he became the fifth person to walk on the moon.
In February 1971, at age 47, Shepard reached the moon with the Apollo 14 mission. He hadn't flown anything since he rocketed 116 miles above Florida in 1961.
At the end of the second moonwalk, the avid golfer carried out a different kind of scientific experiment: he pulled out a makeshift golf club and whacked two golf balls. One landed in a nearby crater. The other, he said, traveled "miles and miles and miles."
"All of us wanted to think of something which would demonstrate – especially to young people – the lack of atmosphere and the difference of gravity," he said in a 1991 interview with the American Academy of Achievement. Shepard wanted to show that with only one-sixth the gravitational pull of Earth, the ball would travel six times as far.
But he didn't see the moon as just a giant golf course.
"The first time really seeing it in the black sky, the blue planet all by itself up there. That was an emotional moment. Some of the emotion was a result of having successfully arrived, a little sense of relief, but I think all of us, in our own ways, have expressed the same kind of feeling," Shepard said in 1991.
"Maybe if people had a chance to see this, they wouldn't be so parochial, they wouldn't be so interested in their own particular territories," he said. "To me and, I think, to all of us, it was a realization that our world is finite, it is small, it is fragile, and we need to start thinking about how to take care of it."
After Apollo 14, he reprised his role as chief of the Astronaut Office (a position he held before the mission) and remained in that administrative role until retiring to a corporate position in Houston in 1974.
In 1984, he joined other astronauts in founding the Mercury Seven Foundation to raise money for scholarships for science and engineering college students. In 1995, it was renamed the Astronaut Scholarship Foundation and he was president and chairman until 1997.
After a long illness, said to be leukemia, he died in July 1998.
In the 1979 book "The Right Stuff," author Tom Wolfe said he had two sides, "the Icy Commander and Smilin' Al,'' but also that he but he ''set a standard of coolness and competence that would be hard to top.''
One of the more controversial moonwalkers, in the years after his 1971 journey to the moon, Edgar Mitchell has made headlines for arguing that alien visits to Earth have been covered up by governments for more than 60 years.
"I happen to be privileged enough to be in on the fact that we have been visited on this planet and the UFO phenomenon is real," Mitchell said on Britain's Kerrang Radio in July 2008.
"It has been covered up by governments for quite some time now," added Mitchell, who grew up in Roswell, N.M., the location of the controversial 1947 incident (or perhaps non-incident) in which some believe the U.S. military covered up the crash scene of an alien spacecraft.
The Apollo 14 astronaut was the sixth man to walk on the moon but retired from NASA the following year.
In 1973, he founded the Institute of Noetic Sciences to sponsor research into the nature of consciousness. He published "Psychic Exploration" in 1974.
He traces his interest in consciousness back to his moments on the moon.
"There was a vague feeling that something was different. That my life had gotten very disturbing, very distressing at a subconscious level," he said in an interview with Chaikin. "What I do remember is the awesome experience [on the trip back from the moon] of recognizing the universe was not simply random happenstance … That there was something more operating than just chance… I've assiduously spent the last fifteen years figuring out what was true."
In April, the 78-year-old spoke at the National Press Club in Washington after the X-Conference, a convention of UFO researchers and activists.
"We are being visited," he said, according to the U.K.'s Guardian. "It is now time to put away this embargo of truth about the alien presence. I call upon our government to open up ... and become a part of this planetary community that is now trying to take our proper role as a spacefaring civilization."
Reaching the moon was a spiritual experience for astronaut James Irwin.
"I felt the power of God as I'd never felt it before," he said about the July 1971 Apollo 15 mission. He was the lunar module pilot for the flight and explored the moon's surface for three days.
One year after the mission, Irwin resigned from NASA and the Air Force to form the religious organization High Flight Foundation in Colorado Springs, Colo.
According to High Flight's Web site, the astronaut started the organization to encourage others to experience "the Highest Flight possible with God."
"Jesus walking on the earth is more important than man walking on the moon," it quotes Irwin as saying.
The group organizes religious retreats and trips to the Holy Land. Irwin even led expeditions to Turkey's Mount Ararat in search of evidence of Noah's Ark.
In 1991, at age 61, he died of a heart attack.
"The Earth reminded us of a Christmas tree ornament hanging in the blackness of space. As we got farther and farther away, it [the Earth] diminished in size. Finally it shrank to the size of a marble, the most beautiful you can imagine," Irwin said. "That beautiful, warm, living object looked so fragile, so delicate, that if you touched it with a finger it would crumble and fall apart. Seeing this has to change a man."
Not even 40 years old, David Scott reached the pinnacle of his career and then wondered, "what's next?"
"When I landed on the moon and came back from the moon, I was 39 years old. My career had been finished. I'd finished my career. That's it. Now go find a new career," the Apollo 15 astronaut said in an interview with Chaikin.
But after his voyage to the moon in 1971, Scott stayed with NASA for about six more years, as deputy director and then director of the NASA Dryden Flight Research Center at Edwards Air Force Base.
In 1977, he left the space agency to found Scott Science and Technology, a specialized space project management and technical services company.
In 2004, he published "Two Sides of the Moon: Our Story of the Cold War Space Race" with Russian astronaut Alexei Leonov, the first man to walk in space.
Of the 12 men to have walked on the moon, John Young was the last to retire from NASA.
After 42 years with NASA in various administrative and leadership positions, the Apollo 16 astronaut retired in 2004.
In a 2004 interview with the Houston Chronicle, he described his experience on the moon.
"One-sixth gravity on the surface of the moon is just delightful. It's not like being in zero gravity, you know. You can drop a pencil in zero gravity and look for it for three days. In one-sixth gravity, you just look down and there it is," he said.
He also advocated for a return to the moon and beyond.
"The moon has a lot of resources that we'll learn how to use in this century and that will be great. ... The technologies we need to live and work on the moon will save us right here on this planet," he said. "Bad things are inevitably going to happen to us, like comet or asteroid impacts or super volcanoes. Flying in space is risky business, but just staying on this planet is risky business too."
At 36 years old, Charles Duke was the youngest man to walk on the moon.
In 1972, he flew on Apollo 16 and spent more than 71 hours on the moon. But he has said that it was his hero's welcome was overwhelming.
"When I came back, I got so many questions about, "How did it feel? Did it change your life? How do you view Earth? You almost have to manufacture a response to that question," he said.
Her retired from NASA and the space program in 1975 to enter private business.
He started a beverage company, Orbit Corp., and has been active in real estate development and public speaking. He is also president of Duke Ministry for Christ.
"My father was born shortly after the Wright brothers. He could barely believe that I went to the moon," he said in the documentary "In the Shadow of the Moon." "But my son Tom was 5 -- and he didn't think it was any big deal."
The commander of the last scheduled manned U.S. mission to the moon, Cernan spent more than 73 hours on the lunar surface in 1972.
The experience left him believing in a greater power.
"I felt that the world was just too beautiful to have happened by accident. There has to be something bigger than you and bigger than me," Cernan said "In the Shadow of the Moon." "And I mean this in a spiritual sense, not a religious sense. There has to be a creator of the universe who stands above the religions that we ourselves create to govern our lives."
He moved to an administrative position with NASA after his return and then retired in 1976, after about 20 years with the Navy and 13 years with NASA.
After leaving NASA, he joined Coral Petroleum, Inc. of Houston and later started his own company, a space-related technology and marketing firm. He also has been a special consultant to ABC News, covering space programming.
He currently lives in Houston and is on the Board of Directors of the Young Astronaut Council and the US Space Foundation.
After 10 years with NASA, Harrison Schmitt left science for politics.
The Apollo 17 astronaut was one of the last to walk on the moon. Three years after his 1972 lunar mission, he left NASA to run for the U.S. Senate in his home state of New Mexico.
He spent one six-year term as a Republican, serving on the Commerce, Science and Transportation Committee, the Science, Technology and Space Subcommittee of Commerce and others.
He later worked as consultant, freelance writer and speaker on space, technology and public policy issues. In a 2006 interview with Astrobiology Magazine, he said that his experience on the moon was almost indescribable.
"Being there is an essential ingredient. It's the same as trying to describe to someone what it's like to stand on the rim of the Grand Canyon. Or to have your first child. Any meaningful event that you've had in your life is probably that kind of experience. It has a personal meaning, and it will be different for every individual," he said.
"But sometimes people just want a description of what it was like," he continued, "The black sky, the brilliantly illuminated slopes of the mountains, the bright sun, and then our Earth as a big blue marble hanging over one of the mountains. The physical feeling of walking on the moon is like walking on a giant trampoline, to some degree."