Climate has been changing for billions of years.

I'm convinced that sending people to Mars is so expensive that if you go once and bring the people back and then go again and bring the people back, we're eventually going to run out of money. But what if we send people the first time and they don't come back? What if they stay there?

Astronauts are not superhuman. They lead ordinary lives and have varied personalities.

Unfortunately, pioneers will always pave the way with sacrifices.

A mind concerned about danger is a clouded mind. It's paralyzing.

When I am getting ready to cross a street, I look both ways before crossing. My bones, my muscles, are not what they used to be, so I am careful when I go up and down stairs, because I've heard stories of older people falling and having very disabling injuries. I have enough things that begin to go a little bit wrong as I get a little bit older.

Computers allow us to squeeze the most out of everything, whether it's Google looking up things, so I guess that tends to make us a little lazy about reading books and doing things the hard way to understand how those things work.