50 Years Later: Astronaut Michael Collins on Apollo 11 mission

Three men went to the moon, but only two walked on the lunar surface.

Astronaut Michael Collins, the man who manned the command module during the moonwalk, reflected on the historic Apollo 11 mission as it celebrates its 50th anniversary on Saturday, July 20.

"The thing that amazes me about Apollo 11 is everything worked as advertised," Collins told ABC News' Senior Transportation Correspondent David Kerley.

A vivid image of the moon is still in Collins' mind to this day.

"My God that moon is immense and it's so three-dimensional," Collins said. "You have the sunlight illuminating the ring of the surface of the moon...but somehow beyond its size and its gloss it projects a feeling of fragility. It hit me that this is a fragile little tiny thing."

Collins is referred to by some as the loneliest man in the universe. He circled the moon alone for more than a day without witnessing the actual landing. He could occasionally hear audio from fellow Apollo 11 astronauts Neil Armstrong and Buzz Aldrin walking on the moon, but the signal would disappear when the command module went behind the moon. When the signal was down, he could not talk to the crew or be reached by mission control in Houston.

"I liked being in the command module by myself," Collins said. "I had my own way of doing things, I had hot coffee. I took the center seat out and it was almost like being in a little church."

Referring to himself as Armstrong and Aldrin's "ride home," he worried about a scenario where they would be stuck on the moon.

"They only had one little motor," Collins said. "That was it. That motor had to work perfectly, if it didn't and they were stuck on the moon I was going to come home and I would not have been a happy returnee. I'd be a marked man for the rest of my life."

Even though he didn't walk on the moon, Collins said that he did not feel left out.

"They were wonderful crew mates," Collins said, "each in his own way. Buzz was from a technical background. Neil was not only a highly-experienced test pilot. He knew the whys and wherefores of the design of those spacecrafts. I felt very much an equal partner with them, I felt very much part of the trio."

When it came time to say goodbye to the command module, Collins left a message inside: "the finest ship to come down the line. God bless her."

He scribbled it first with a pencil, then came back and made it darker with a ballpoint pen.

"I didn't really want to say goodbye to Columbia without saying 'goodbye'," Collins said. "And that was my way of saying goodbye and thank you. Columbia had been such a wonderful machine and taken care of us."

For the next space venture Collins has his eyes set on Mars.

"I think in going to Mars we should go not primarily as Americans, but as human inhabitants of Earth," Collins said.

ABC News' Nate Luna and Christine Theodorou contributed to this report.