If not for Apollo–and, therefore, if not for the political purpose it served—I doubt whether the historic American expeditions of exploration and discovery throughout the Solar System would have occurred, The Mariners, Vikings, Pioneers, Voyagers ,and Galileo are among the gifts of Apollo. Magellan and Cassini are more distant descendants. Something similar is true for the pioneering Soviet efforts in Solar System exploration, including the first soft landings of robot spacecraft–Luna 9, Mars 3, Venera 8-on other worlds.

Apollo conveyed a confidence, energy, and breadth of vision that did capture the imagination of the world. That too was part of its purpose. It inspired an optimism about technology, an enthusiasm for the future. If we could fly to the Moon, as so many have asked, what else were we capable of? Even those who opposed the policies and actions of the United States—even those who thought the worst of us—acknowledged the genius and heroism of the Apollo program. With Apollo, the United States touched greatness.

When you pack your bags for a big trip, you never know what’s in store for you. The Apollo astronauts on their way to and from the Moon photographed their home planet. It was a natural thing to do, but it had consequences that few foresaw. For the first time, the inhabitants of Earth could see their world from above—the whole Earth, the Earth in color, the Earth as an exquisite spinning white and blue ball set against the vast darkness of space. Those images helped awaken our slumbering planetary consciousness. They provide incontestable evidence that we all share the same vulnerable planet. They remind us of what is important and what is not. They were the harbingers of Voyager’s pale blue dot.

We may have found that perspective just in time, just as our technology threatens the habitability of our world. Whatever the reason we first mustered the Apollo program, however mired it was in Cold War nationalism and the instruments of death, the inescapable recognition of the unity and fragility of the Earth is its clear and luminous dividend, the unexpected final gift of Apollo. What began in deadly competition has helped us to see that global cooperation is the essential precondition for of survival.

Travel is broadening.

It’s time to hit the road again.

When you look down at the Earth from orbital altitudes, you see a lovely, fragile world embedded in black vacuum. But peering at a piece of the Earth through a spacecraft porthole is nothing like the joy of seeing it entire against the backdrop of black, or—better—sweeping across your field of view as you float in space unencumbered by a spacecraft. The first human to have this experience was Alexei Leonov, who on March 18, 1965, left Voskhod 2 in the original space “walk”: “I looked down at the Earth,” he recalls, “and the first thought that crossed my mind was `The world is round, after all.’ In one glance I could see from Gibraltar to the Caspian Sea… I felt like a bird—with wings, and able to fly.”

When you view the Earth from farther away, as the Apollo astronauts did, it shrinks in apparent size, until nothing but a little geography remains. You’re struck by how self-contained it is. An occasional hydrogen atom leaves; a pitter-patter of cometary dust arrives. Sunlight, generated in the immense, silent thermonuclear engine deep in the solar interior, pours out of the Sun in all directions, and the Earth intercepts enough of it to provide a little illumination and enough heat for our modest purposes. Apart from that, this small world is on its own.

From the surface of the Moon you can see it, perhaps as a crescent, even its continents now indistinct. And from the vantage point of the outermost planet it is a mere point of pale light.

From Earth orbit, you are struck by the tender blue arc of the horizon—the Earth’s thin atmosphere seen tangentially. You can understand why there is no longer such a thing as a local environmental problem. Molecules are stupid. Industrial poisons, greenhouse gases, and substances that attack the protective ozone layer, because of their abysmal ignorance, do not respect borders. They are oblivious of the notion of national sovereignty. And so, due to the almost mythic powers of our technology (and the prevalence of short-term thinking), we are beginning—on Continental and on planetary scales—to pose a danger to ourselves. Plainly, if these problems are to be solved, it will require many nations acting in concert over many years.

I’m struck again by the irony that spaceflight—conceived in tile cauldron of nationalist rivalries and hatreds—brings with it a stunning transnational vision. You spend even a little time contemplating the Earth from orbit and the most deeply engrained nationalisms begin to erode. They seem the squabbles of mites on a plum.

If we’re stuck on one world, we’re limited to a single case; we don’t know what else is possible. Then—like an art fancier familiar only with Fayoum tomb paintings, a dentist who knows only molars, a philosopher trained merely in NeoPlatonism, a linguist who has studied only Chinese, or a physicist whose knowledge of gravity is restricted to falling bodies on Earth—our perspective is foreshortened, our insights narrow, our predictive abilities circumscribed. By contrast, when we explore other worlds, what once seemed the only way a planet could be turns out to be somewhere in the middle range of a vast spectrum of possibilities. When we look at those other worlds, we begin to understand what happens when we have too much of one thing or too little of another. We learn how a planet can go wrong. We gain a new understanding, foreseen by the spaceflight pioneer Robert Goddard, called comparative planetology.

The exploration of other worlds has opened our eyes in the study of volcanos, earthquakes, and weather. It may one day have profound implications for biology, because all life on Earth is built on a common biochemical master plan. The discovery of a single extraterrestrial organism—even something as humble as a bacterium—would revolutionize our understanding of living things. But the connection between exploring other worlds and protecting this one is most evident in the study of Earth’s climate and the burgeoning threat to that climate that our technology poses. Other worlds provide vital insights about what dumb things not to do on Earth.

Three potential environmental catastrophes—all operating on a global scale—have recently been uncovered: ozone layer depletion, greenhouse warming, and nuclear winter. All three discoveries, it turns out, have strong ties to the exploration of the planets.