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How dangerous is a space walk like the one you performed?
It’s always scary. The biggest risk is getting a tear in your space suit. You’ve got 11 layers of Dacron and Gore-Tex; 4.3 pounds of oxygen per square inch inside, a vacuum on the outside. Even a small cut may not be survivable.

Or you could get hit by a piece of space debris. The junk is flying by at seven to ten kilometers [four to six miles] a second. Hubble’s had lots of debris hits over the years. But the odds on any given space walk of an astronaut getting hit are only one in 20,000. On the other hand, NASA has calculated that the chances of what they call a “probability of loss of crew” on any given space mission are one in 70. I worry about that before every shuttle launch we make.

On a space walk, another big concern is that you lose things. You let go of a wrench and it’s gone for good. The worst thing, of course, would be if it’s you that’s gone. There are a lot of operations where you have to switch to a new tether. You think you’re tethered to something but you’re not, and when you let go, you’re gone. We sometimes have a backpack with jets on it so that if you became separated, you have one chance to fire these little rockets and get yourself back—but not on this mission.

Does adventure alone drive you?
I’m an incurable romantic. But I’m not an adrenaline junkie. I don’t particularly want to jump out of an airplane with a parachute if I don’t have to. I don’t want to go bungee jumping. I like adventure with a real purpose that I can buy into. To help enable the kind of science Hubble is performing makes my life worthwhile.

What’s the future of space exploration?
I think we should clearly declare Mars as the next big goal. We need to do it. We need to move off the planet. And Mars is the next best place.

Manned or unmanned?
I was talking with Steve Squyres [principal investigator for the Mars Exploration Rovers mission]. I said I thought that when you broke it down to cost per unit of science done, it came out about even between robots and humans. I said, “I’ll bet a 90-day mission by robot could be accomplished in two hours by a human.” Steve thought a minute, then said, “Nah, probably more like two minutes.”

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