By Jackie Wattles, CNN

(CNN) -- Astronauts have for decades described their trips to space as "breathtaking" and humbling, a reminder of the Earth's fragility and humanity's need to serve as stewards of our home planet.

Actor William Shatner, who joined a suborbital space tourism flight last year, experienced the same phenomenon, but he had a very distinct observation when he turned his gaze from the Earth to black expanse of the cosmos: "All I saw was death," he wrote in a new book.

Shatner's biography, called "Boldly Go," which he co-wrote with TV and film writer Joshua Brandon, is filled with similarly grim anecdotes about Shatner's experience bolting above the Earth's atmosphere aboard a real-life rocket after his memorable stint playing a spaceship captain on the 1960s TV show "Star Trek" and several franchise movies in the following decades.

"I saw a cold, dark, black emptiness. It was unlike any blackness you can see or feel on Earth. It was deep, enveloping, all-encompassing. I turned back toward the light of home. I could see the curvature of Earth, the beige of the desert, the white of the clouds and the blue of the sky. It was life. Nurturing, sustaining, life. Mother Earth. Gaia. And I was leaving her," reads an excerpt from "Boldly Go" that was first published by Variety.

"Everything I had thought was wrong," it reads. "Everything I had expected to see was wrong."

While he had expected to be awed at the vision of the cosmos, seen without the filter of the Earth's atmosphere, he instead became overwhelmed by the idea that humans are slowly destroying our home planet. He felt one of the strongest feelings of grief he's ever encountered, Shatner wrote.

Shatner's book was released October 4 by publishing house Simon & Schuster. CNN interviewed him in June about the book, his trip to space with the Jeff Bezos-backed space tourism company Blue Origin, and what's next for the 91-year-old. A transcript of the interview, edited for length and clarity, is below.

CNN: We all saw how emotional you were when you stepped out of the Blue Origin spacecraft after landing. How did that experience change you?

William Shatner: Fifty-five or 60 years ago I read a book called "Silent Spring" by Rachel Carson. She wrote about the environmental issues that are still happening today. I've been a verbal ecologist since then. I've been aware of the changing Earth and my apprehension for all of us.

It's like somebody owing money on a mortgage, and they don't have the payments. And they think, "Oh, well, let's go to dinner and not think about it."

But it's so omnipresent! The possibilities of an apocalypse are so real. It's hard to convince people — and especially certain political people — that this is not on our doorstep any longer. It's in the house.

When I got up to space, I wanted to get to the window to see what it was that was out there. I looked at the blackness of space. There were no dazzling lights. It was just palpable blackness. I believed I saw death.

And then I looked back at the Earth. Given my background and having read a lot of things about the evolution of Earth over 5 billion years and how all the beauty of nature has evolved, I thought about how we're killing everything.

I felt this overwhelming sadness for the Earth.

I didn't realize it until I got down. When I stepped out of the spacecraft, I started crying. I didn't know why. It took me hours to understand why I was weeping. I realized I was in grief for the Earth.

I don't want to ever forget, nor have I forgotten, the momentousness of that occasion.

CNN: What else have you realized about the experience in the months since you took your spaceflight?

Shatner: I had an awareness that human beings may be the only species alive on this planet that is aware of the enormity and the majesty of the universe.

Think about what we've discovered in just the last 100 years given the 200,000 years that humans have existed. We've discovered how mountains have formed, the Big Bang. And I kept thinking about how mankind is evolving rapidly into a knowledgeable creature at the same time it's killing itself.

It's a race.

CNN: Space tourism companies such as Blue Origin have also received a lot of criticism from people who view those efforts as more of a vanity project for wealthy individuals rather than something that can be truly transformational. How do you respond to that criticism?

Shatner: The whole idea here is to get people accustomed to going to space, as if it's like going to the Riviera. It's not only a vanity -- it's a business.

But what Jeff Bezos wants to do and what is slowly accruing because of our familiarity with space is get those polluting industries up into orbit and get the earth back to what it was. (Editor's note: Bezos has routinely talked about moving heavy industries into orbit to help preserve the Earth, and that idea also has its skeptics and critics.)

CNN: What do you think about the 'astronaut' title. Are people who pay for brief, suborbital flights to space astronauts?

Shatner: I call them half astronauts.

CNN: What should we be doing in space next?

Shatner: The ability to go to Mars which is lurking in the background, which I think that should take a backseat to going to the moon, setting up the moon as a base and mining whatever the moon has to offer, rather than mining it here.

Those are just my own opinions. What's-his-name would not agree. He wants to go to Mars. (Editor's note: SpaceX CEO Elon Musk founded his company with the goal of setting up a colony on Mars.)

CNN: Are you are you anxious to go back to space?

Shatner: If you had a great love affair, could you go back? Or would that demean it?

CNN: You mentioned you got a chance to speak with famed astrophysicist Stephen Hawking before he died. What was that experience like?

Shatner: I was never able to ask him about String Theory, which I wanted to. We had to get him all the questions in advance. And he had said when we made the arrangement, 'I want to ask Shatner a question.'

Finally, I'm leaning in, you know, we're sitting side by side looking at the cameras.

So he laboriously typed out, 'What is your favorite Star Trek episode?' which is the question every fan asks, and I started laughing. He didn't have the ability to laugh (because of his degenerative disease, Amyotrophic Lateral Sclerosis, or ALS).

But his laughter showed in the redness of his face and he got so red. He then invited me to dinner. I had a beautiful moment with him.

CNN: What are you doing next?

Shatner: I should take the opportunity to say I have an album out there called "Bill." And I kept making songs with my collaborators. The song "So Fragile, So Blue," is very much about my experience in space. I recently performed with (musician) Ben Folds at the Kennedy Center. That could be a TV show or an album.

I also have a really wonderful show called "The UnXplained" on the on the History Channel.

And then I have my book, called "Boldly Go," coming out in the fall.