In the history of humanity, only 24 men have shared the experience.
Forty years ago today, the first of the two dozen astronauts to fly to or around the moon rocketed away from Earth to make history. Twelve had the chance to walk on the moon's surface, though only nine of those are still alive today.
When they returned to Earth, they were scientists and explorers with no peers, at the pinnacles of their careers.
But for some the adventure was so epic it changed the course of their lives. Inspired and transformed by seeing Earth shrink to the size of their thumbs, many let new philosophical and spiritual sensibilities guide them. Others chose entirely new career paths.
Those who have interviewed the lunar astronauts at length, or are familiar with the space program, say that each individual responded to the experience differently.
But though some changed more drastically than others, and some have been more public about their choices than others, every one of the astronauts was in some way moved by the intense, unparalleled experience.
"There is this sort of general view [and] urban myth that they went to the moon and came back and went crazy. I've met, personally, the people who went the moon and you couldn't hope to meet saner people," said David Sington, a documentary filmmaker who directed the 2007 film "In the Shadow of the Moon," about the lunar astronauts.
What the moon missions gave the astronauts, he said, was "the ultimate perspective."
"To see your own hometown, you have to leave and go and come back," he said. "They did the ultimate trip. They left Earth and leaving Earth and coming back allowed them to see what it really is."
For Edgar Mitchell, the Apollo 16 astronaut who walked on the moon in 1971, that perspective led him to the study of consciousness and the belief in extraterrestrial life.
"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 1986 interview with Andrew Chaikin, author of "Man on the Moon" and "Voices from the Moon."
On the trip back from the moon, "what I do remember is the awesome experience of recognizing the universe was not simply random happenstance. … That there was something more operating than just chance," Mitchell said, adding that in the years since the moonwalk he has "assiduously" tried to "figure out what was true."
In 1973, he founded the Institute of Noetic Sciences to sponsor research into the nature of consciousness. He published "Psychic Exploration" in 1974.
In recent years, Mitchell has grabbed 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," he said on Britain's Kerrang Radio in July 2008.
Mitchell grew up in Roswell, N.M., the location of the controversial 1947 incident (or perhaps nonincident) in which some believe the U.S. military covered up the crash scene of an alien spacecraft.
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 space-faring civilization."
Buzz Aldrin, who made the first moon landing with Neil Armstrong, apparently claimed that he saw a UFO while on the Apollo 11 mission, according to a 2005 documentary for the Science Channel. But later, Aldrin would say his comments were taken out of context and the object he saw from the moon's surface was not unidentified at all.
In January, Skeptical Inquirer magazine published a story in which Aldrin repeated his denials and said the UFO was nothing more than a small panel that had connected parts of the spacecraft and was meant to detach itself as the spacecraft approached the moon.
Aldrins UFO allegations and denials only helped fuel conspiracy theorists on all sides, from those who believe in UFO's to those who think the whole moon landing program was a hoax.
Aldrin punched a leading Apollo skeptic in the face in 2002 after he confronted Aldrin and demanded he admit he never set foot on the moon.
But after returning to Earth, Aldrin did have a transformation of his own. He suffered from severe depression and alcoholism, both of which he wrote about extensively in his memoir, "Return to Earth." Those who know him say it wasn't so much the journey to the moon but the fame and glory in the aftermath that led to his troubles.
Apollo 15 astronaut James Irwin was so spiritually moved by traveling to the moon that he left NASA one year after his mission to form a religious organization, High Flight Foundation in Colorado Springs, Colo.
"I felt the power of God as I'd never felt it before," he said about the July 1971 experience. He was the lunar module pilot for the flight and explored the moon's surface for three days.
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. Before his death in 1991, Irwin led expeditions to Turkey's Mount Ararat in search of evidence of Noah's Ark.
"The Earth reminded us of a Christmas tree ornament hanging in the blackness of space. As we got farther and farther away, it 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."
Other moonwalkers have expressed similar sentiments.
Speaking in Sington's documentary "In the Shadow of the Moon," astronaut Gene Cernan, who made the last moon landing in 1972, said he became a believer in the idea of a greater power after traveling to outer space.
"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," said Cernan in the documentary.
"And I mean this in a spiritual sense, not a religious sense," said Cernan. "There has to be a creator of the universe who stands above the religions that we ourselves create to govern our lives."
Charles Duke, the youngest man to walk on the moon, found religious affirmation after flying on Apollo 16 in 1972. He left NASA in 1975 to enter private business and formed the Duke Ministry for Christ.
But though moon travel appears to have changed these astronauts, space experts say they were already interested in the pursuits they chose after their moon missions.
"There is some evidence that astronauts are changed by going into space," said Mike Neufeld, the chairman of the space history division at the National Air and Space Museum in Washington, D.C.
But while Neufeld noted that Mitchell's thoughts in extraterrestrial life have certainly come to light over the years, the former moonwalker had always been interested in the paranormal.
"He was interested in ESP before he was ever launched -- he conducted an ESP experiment on Apollo 14," said Neufeld.
Alan Bean, the Apollo 12 moonwalker who later became a full-time painter, said the moon missions gave the astronauts the courage to live their lives the way they'd always wanted to live them.
"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."
"It doesn't change you, it reveals who you are," he said.
ABC News' Emily Friedman contributed to this report.