Even before then, if we take a cooperative path, there will come a time—perhaps in the first decades of the new century and the new millennium—when an interplanetary spacecraft is assembled in Earth orbit, the progress in full view on the evening news. Astronauts and cosmonauts, hovering like gnats, guide and mate the prefabricated parts. Eventually the ship, tested and ready, is boarded by its international crew, and boosted to escape velocity. For the whole of the voyage to Mars and back, the lives of the crew members depend on one another, a microcosm of our actual circumstances down here on Earth. Perhaps the first joint interplanetary mission with human crews will be only a flyby or orbit of Mars. Earlier, robot vehicles, with aerobraking, parachutes, and retrorockets, will have set gently down on the Martian surface to collect samples and return them to Earth, and to emplace supplies for future explorers. But whether or not we have compelling, coherent reasons, I am sure—unless we destroy ourselves first—that the day will come when we humans set foot on Mars. It is only a matter of when.

According to solemn treaty, signed in Washington and Moscow on January 27, 1967, no nation may lay claim to part or all of another planet. Nevertheless—for historical reasons that Columbus would have understood well—some people are concerned about who first sets foot on Mars. If this really worries us, we can arrange for the ankles of the crew members to be tied together as they alight in the gentle Martian gravity.

The crews would acquire new and previously sequestered samples, in part to search for life, in part to understand the past and future of Mars and Earth. They would experiment, for later expeditions, on extracting water, oxygen, and hydrogen from the rocks and the air and from the underground permafrost—to drink, to breathe, to power their machines and, as rocket fuel and oxidizer, to propel the return voyage. They would test Martian materials for eventual fabrication of bases and settlements on Mars.

And they would go exploring. When I imagine the early human exploration of Mars, it’s always a roving vehicle, a little like a jeep, wandering down one of the valley networks, the crew with geological hammers, cameras, and analytic instruments at the ready. They’re looking for rocks from ages past, signs of ancient cataclysms, clues to climate change, strange chemistries, fossils, or—most exciting and most unlikely—something alive. Their discoveries are televised back to Earth at the speed of light. Snuggled up in bed with the kids, you explore the ancient riverbeds of Mars.

What?, I sometimes ask myself in amazement: Our ancestors walked from East Africa to Novaya Zemlya and Ayers Rock and Patagonia, hunted elephants with stone spearpoints, traversed the polar seas in open boats 7,000 years ago, circumnavigated the Earth propelled by nothing but wind, walked the Moon a decade after entering space—and we’re daunted by a voyage to Mars? But then I remind myself of the avoidable human suffering on Earth, how a few dollars can save the life of a child dying of dehydration, how many children we could save for the cost of a trip to Mars—and for the moment I change my mind. Is it unworthy to stay home or unworthy to go? Or have I posed a false dichotomy? Isn’t it possible to make a better life for everyone on Earth and to reach for the planets and the stars?

We had an expansive run in the ‘60s and ‘70s. You might have thought, as I did then, that our species would be on Mars before the century was over. But instead, we’ve pulled inward. Robots aside, we’ve backed off from the planets and the stars. 1 keep asking myself Is it a failure of nerve or a sign of maturity?

Maybe it’s the most we could reasonably have expected. In a way it’s amazing that it was possible at all: We sent a dozen humans on week-long excursions to the Moon. And we were given the resources to make a preliminary reconnaissance of the whole Solar System, out to Neptune anyway—missions that returned a wealth of data, but nothing of short-term, everyday, bread-on-the-table practical value. They lifted the human spirit, though. They enlightened us about our place in the Universe. It’s easy to imagine skeins of historical causality in which there were no race to the Moon and no planetary program.

But it’s also possible to imagine a much more serious devotion to exploration, because of which we would today have robot vehicles probing the atmospheres of all the Jovian planets and dozens of moons, comets, and asteroids; a network of automatic scientific stations emplaced on Mars would daily be reporting their findings; and samples from many worlds would be under examination in the laboratories of Earth—revealing their geology, chemistry, and perhaps even their biology. Human outposts might be already established on the near-Earth asteroids, the Moon, and Mars.

There were many possible historical paths. Our particular causality skein has brought us to a modest and rudimentary, although in many respects heroic, series of explorations. But it is far interior to what might have been—and what may one day be.

To carry the green Promethean spark of Life with us into the sterile void and ignite there a firestorm of animate matter is the very destiny of our race,” reads the brochure of something called the First Millennial Foundation. It promises, for $120 a year, “citizenship” in “space colonies—when the time comes.” “Benefactors” who contribute more also receive “the undying gratitude of a star-flung civilization, and their name carved on the monolith to be erected on the Moon.” This represents one extreme in the continuum of enthusiasm for a human presence in space. The other extreme—better represented in Congress—questions why we should be in space at all, especially people rather than robots. The Apollo program was a “moondoggle,” the social critic Amitai Etzioni once called it; with the Cold War over, there is no justification whatever, proponents of this orientation hold, for a manned space program. Where in this spectrum of policy options should we be?

Ever since the United States beat the Soviet Union to the Moon, a coherent, widely understood justification for humans in space seems to have vanished. Presidents and Congressional committees puzzle over what to do with the manned space program. What is it for? Why do we need it? But the exploits of the astronauts and the moon landings had elicited—and for good reason—the admiration of the world. It would be a rejection of that stunning American achievement, the political leaders tell themselves, to back off from manned spaceflight. Which President, which Congress wishes to be responsible for the end of the American space program? And in the former Soviet Union a similar argument is heard: Shall we abandon, they ask themselves, the one remaining high technology in which we are still world leaders? Shall we be faithless heirs of Konstantin Tsiolkovsky, Sergei Korolev, and Yuri Gagarin?

The first law of bureaucracy is to guarantee its own continuance. Left to its own devices, without clear instructions from above, NASA gradually devolved into a program that would maintain profits, jobs, and perquisites. Pork-barrel politics, with Congress playing a leading role, became an increasingly powerful force in the design and execution of missions and long-term goals. The bureaucracy ossified. NASA lost its way.

On July 20, 1989, the twentieth anniversary of the Apollo 11 landing on the Moon, President George Bush announced a long-term direction for the U.S. space program. Called the Space Exploration Initiative (SEI), it proposed a sequence of goals including a U.S. space station, a return of humans to the Moon, and the first landing of humans on Mars. In a later statement, Mr. Bush set 2019 as the target date for the first footfall on that planet.