Passion is what gets you through the hardest times that might otherwise make strong men weak, or make you give up.

The history of exploration across nations and across time is not one where nations said, 'Let's explore because it's fun.' It was, 'Let's explore so that we can claim lands for our country, so that we can open up new trade routes; let's explore so we can become more powerful.'

Imagination alone is not enough, because the reality of nature is far more wondrous than anything we can imagine

We already know the limits of Einstein's theories. From the centers of black holes at the very beginning of the universe - we call these singularities - Einstein's equations fail. In fact, people have joked that's where God is dividing by zero.

People say, oh we just need charismatic leaders to continue on to Mars. Now we've gone to the moon, of course Mars is next. No. Mars was never, of course, next. It is next if you think we went to the moon because we're explorers, but if you know we went to the moon because we were at war then we're never going to Mars. There's no military reason to do it, to justify the expenditure.

People generally don't recognize how long it takes to conceive, publish, and write a book.

When I needed to overcome the low expectations of others or the bias that would be expressed in one circumstance or another, I'd keep on keeping on. And I climb over the obstacle, go around it, dig under it, fly over it. That's what kept me going. Otherwise I would have never been an astrophysicist.