You've said we need to become a space-faring species because humans face so many survival threats on Earth. What worries you most?
Probably an asteroid impact. That's a very low probability, but it could wipe out everyone. Although if you have a full-blown nuclear exchange between big powers, the people who survive would probably die out.
In your book Mission to Mars, you predict it won't be long before we land there, and recently said the date might be 2040. Could we have gone to Mars much earlier?
The information I got about a year ago is that when President Kennedy approached NASA in the early 1960s, he said, "I want to go to Mars." The NASA people spent a full weekend studying that, went to the president and said they had done the calculations and we were in no position to get to Mars. They told him they thought they could get to the moon.
How do you see a U.S. commitment to sending people to Mars unfolding?
The next president will use the 50th anniversaries of the Apollo missions to say the U.S. will lead international efforts to land on Mars within two decades.
The mission will only land on Mars?
The president will commit to a permanent presence on Mars from the first landing on, thereby gaining a legacy that is unmatched.
How old is too old to go to Mars?
People who are 10 years old now will be the right age to go. They’re in the target audience of my new book, Welcome to Mars. In 15 years they'll be old enough to start training. The life expectancy of those who choose to stay on Mars as permanent settlers might be reduced to what it would have been in 1900 or 1850.
Reduced life expectancy because of radiation?
Yes, though nobody seems interested yet in my idea of wearing a lead helmet and a lead jockstrap to stop radiation.
Will radiation be the greatest danger to humans who try to colonize Mars?
No, it will be mental status. It is the growing isolation, the irritation, the realization that this is the way it's going to be.
What makes you so aware of this problem?
I inherited depression. My grandfather committed suicide. My mother did. I had my concerns with it after getting an assignment that I didn't really want in the Air Force, after NASA. I experienced depression, alcoholism.
I've got 36 years of sobriety, but I still see a shrink every couple of weeks. When you're depressed, you're convinced it will never end. But when you're on top of things, you're convinced that will never end.
What is the one thing you wish you could have done in space that you didn't?
For the Gemini 12 mission, I allowed NASA to cancel the experiment with a backpack-maneuvering unit. It would have been nice to fly around in that unit like George Clooney in the movie Gravity. I would have been the first. I wasn't assertive enough.
Did you change when you saw Earth as a small blue marble from the moon?
Actually being on the surface of the moon and leaning back and looking up at Earth? No big deal. Really. I had been thinking about this and planning this for six months. I knew the moon would get bigger and Earth would get smaller.
So, what were your emotions?
There was a magnificence of achievement being there. But that was in contrast to a total loneliness, a lifelessness, desolation. You couldn't find a place anywhere on Earth like that desolation.
You were 39 then. How do you feel today?
I lead a life that is far more active than ever — or that anybody 20 years younger could match.
Then do you want to go to Mars?
I won't be around, but I at least hope to get the ball rolling.
Stephen Petranek is the author of the new book How We'll Live on Mars and former editor-in-chief of Discover magazine.