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.