But ours was a nation unafraid of risk, unfrightened by the possibility of failure. Unlike the Soviet space program, which was buried in layers of secrecy and propaganda, the Apollo 11 mission was televised around the world, so that millions of people watched the culmination in "one small step for man, one giant leap for mankind."

In all, twelve men — all Americans — walked on the moon between 1969 and 1972. No one has gone back since then; instead, we've done all our exploring with robots (like the rovers presently on Mars, or the Cassini probe now photographing Saturn), while humans have been confined to Earth orbit.

That's all changing. Since President Bush's announcement in January of his vision to "gain a new foothold on the moon and to prepare for new journeys to the worlds beyond our own," NASA has been transforming itself into an agency capable once again of going places — of exploring space instead of just floating around our own planet. According to the president's timetable, a new series of robotic missions to the moon will start by 2008, and human exploration of the moon will resume some time between 2015 and 2020.

In other words, American astronauts may be back on the lunar surface in time for the 50th anniversary of the first moon landing. This brings to mind three related questions. First, what caused the long delay — that is, what happened to keep us from the moon since 1972? Second, why is it going to take so long to get back to the moon this time — perhaps twice as long as the eight years it took last time? And finally, why should we bother going at all?

As the Apollo 11 astronauts were hurtling from the moon back toward home, NASA officials expressed the optimism of many Americans. "It seems quite clear," said George Mueller, NASA associate administrator, "that the planets of the solar system are well within our ability to explore, both manned and unmanned, at the present time." Even allowing for the enormous exaggeration in the last four words of Mueller's statement, it would have been inconceivable to most people watching the live coverage of Apollo 11 on TV that we wouldn't reach Mars before the end of the century, that we would retreat from the moon after a few short years, and that our great adventure in space would shrivel into just a fragile space shuttle and a half-built space station.

There are many explanations for NASA's decline. Some have said NASA lost its "can-do" spirit and became just another bureaucracy, losing its ability to achieve great things. Others have argued that we only succeeded in the 1960s because of the spur of competition; after we beat the Soviets in the race to the moon, we lost the impetus for further exploration; NASA was thus a victim of its own success. Some critics have blamed NASA's apparent mediocrity on a lack of public attention; others have blamed a lack of political will.

Each of those explanations is partially true: the public and its leaders lost interest, NASA lost sight of big goals, and the agency got tired and sloppy with age. But the chief problem has been much more fundamental: a failure to understand and articulate why we should go into space. The board that investigated the accident that destroyed the shuttle Columbia last year summed up the problem nicely, finding "a lack, over the past three decades, of any national mandate providing NASA a compelling mission requiring human presence in space." The board recommended in its final report "that the White House, Congress, and NASA should honor the memory of Columbia's crew by reflecting on the nation's future in space and the role of new space transportation capabilities in enabling whatever space goals the nation chooses to pursue."

Now President Bush has given NASA a new mission, and work is already well underway to reorganize the agency to focus on that mission. A number of major changes to the structure of NASA will take effect on August 1, 2004, including clearer lines of accountability and a great deal of streamlining in NASA's headquarters and bureaucracy.

Congress has not yet had much to say about the president's plan or NASA's reorganization, but since this is an election year, some opposition is to be expected. The chief gripe of the plan's opponents has been that the president's goal of exploring space is too extravagant, too expensive, too wasteful — but it is space activity done without a goal that results in waste. And even if opponents criticize the cost or quibble over the details of the president's plan, a larger point is undeniable: NASA cannot be allowed to revert to its old ways, to the ethos of aimlessness. There can be no going back to the days before President Bush's speech, or before the Columbia accident.

Nor, though, can we go back to the days of Apollo — which explains why it may take as long as 16 years to get to the moon this time, when it only took eight years to get to the moon after President Kennedy's 1961 announcement. Inside the space community, the Apollo lunar landings, for all their wondrous achievement, are sometimes derided as mere "flags-and-footprints" missions, just for show. Now, however, the president has set us on a path toward long-term exploration and permanent outposts. This time, we're going to stay. Surely the new space program could have a tighter timeline, and we could return to the moon and move on to Mars more quickly than the president's plan presently calls for — but if we're seriously committed to a future in space, we have to develop a careful plan and reliable hardware that will not just get us there, but will let us stay.

All that remains is the final question: Why should we go at all? There are valid scientific and economic reasons for space exploration, but the chief justification is difficult to express — at least, that is, without sounding corny. It has to do with American creativity — with our vast technical expertise and our most innovative minds bent upon solving difficult problems. It has to do with American character — the call of the frontier and the courage of the pioneer. And it has to do with American greatness — the daring of the hero, the risk of the adventure, and the glory of a future worthy of the people of this nation.

— Adam Keiper is managing editor of The New Atlantis: A Journal of Technology and Society.