It's been one of the greatest challenges that ever came along in my life; it was one of the more difficult things to do.

To move forward, what's required is a unified space agenda based on exploration, science, development, commerce, and security.

Once you've been first, it cannot be done again. Not by you, not by anyone else.

By refocusing our space program on Mars for America's future, we can restore the sense of wonder and adventure in space exploration that we knew in the summer of 1969. We won the moon race; now it's time for us to live and work on Mars, first on its moons and then on its surface.

I was saddened to learn of the passing of Leonard Nimoy, a fellow space traveler because he helped make the journey into the final frontier accessible to us all.... Indeed, there are strange new worlds to explore -- to seek out new life and start new civilizations. It is time to boldly go where no man -- or woman -- has gone before. Thanks to Leonard Nimoy and his beloved Mr. Spock, the bar has been set high for us to continue humanity's quest to probe outward in the universe.

Absolutely the United States should lead in space, for the survival of the United States. It's inspiring for the next generation. If we lose leadership, then we'll be using Chinese capability to inspire Americans.

The first footfalls on Mars will mark a historic milestone, an enterprise that requires human tenacity matched with technology to anchor ourselves on another world.