The greatest thrill for me in reliving this adventure has been not just that we have completed the preliminary reconnaissance with spacecraft of the entire solar system, and not just that we’ve discovered astonishing structures in the realm of the galaxies, but especially that some of Cosmos’ boldest dreams about this world are coming closer to reality.

Since this series’ maiden voyage, the impossible has come to pass: Mighty walls that maintained insuperable ideological differences have come tumbling down; deadly enemies have embraced and begun to work together. The imperative to cherish the Earth and to protect the global environment that sustains all of us has become widely accepted, and we’ve begun, finally, the process of reducing the obscene number of weapons of mass destruction. Perhaps we have, after all, decided to choose life.

But we still have light years to go to ensure that choice. Even after the summits and the ceremonies and the treaties, there are still some 50,000 nuclear weapons in the world—and it would require the detonation of only a tiny fraction of them to produce a nuclear winter, the predicted global climatic catastrophe that would result from the smoke and the dust lifted into the atmosphere by burning cities and petroleum facilities.

The world scientific community has begun to sound the alarm about the grave dangers posed by depleting the protective ozone shield and by greenhouse warming, and again we’re taking some mitigating steps, but again those steps are too small and too slow.

The discovery that such a thing as nuclear winter was really possible evolved out of studies of Martian dust storms. The surface of Mars, fried by ultraviolet light, is also a reminder of why it’s important to keep our ozone layer intact. The runaway greenhouse effect on Venus is a valuable reminder that we must take the increasing greenhouse effect on Earth seriously.

Important lessons about our environment have come from spacecraft missions to the planets. By exploring other worlds, we safeguard this one. By itself, I think this fact more than justifies the money our species has spent in sending ships to other worlds.

It is our fate to live during one of the most perilous and, at the same time, one of the most hopeful chapters in human history. Our science and our technology have posed us a profound question. Will we learn to use these tools with wisdom and foresight before it’s too late? Will we see our species safely through this difficult passage so that our children and grandchildren will continue the great journey of discovery still deeper into the mysteries of the cosmos?

That same rocket and nuclear and computer technology that sends our ships past the farthest known planet can also be used to destroy our global civilization. Exactly the same technology can be used for good and for evil. It is as if there were a God who said to us, “I set before you two ways: You can use your technology to destroy yourselves or to carry you to the planets and the stars. It’s up to you.”