Since, in the long run, every planetary society will be endangered by impacts from space, every surviving civilization is obliged to become spacefaring — not because of exploratory or romantic zeal, but for the most practical reason imaginable: staying alive.

Virtually every major technological advance in the history of the human species - back to the invention of stone tools and the domestication of fire - has been ethically ambiguous.

We have entered, almost without noticing, an age of exploration and discovery unparalleled since the Renaissance.

Religions contradict one another-on small matters, such as whether we should put on a hat or take one off on entering a house of worship, or whether we should eat beef and eschew pork or the other way around, all the way to the most central issues, such as whether there are no gods, one God, or many gods.

After the earth dies, some 5 billion years from now, after it's burned to a crisp, or even swallowed by the Sun, there will be other worlds and stars and galaxies coming into being - and they will know nothing of a place once called Earth.

Across the sea of space, the stars are other suns.

We began as wanderers, and we are wanderers still.