"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.