Even before Neil Armstrong set foot on the Moon, it seemed obvious that Mars would be next.
Wernher von Braun, the great architect of the American space program, had envisioned it as the next logical step after landing on the Moon. Less than two months after Apollo 11, a presidential task force convened to chart the future of human spaceflight came to the same conclusion.
When that idea died on Richard Nixon's desk a year later, it was not for lack of American know-how, but for lack of money. Later this week, when President Bush is expected to announce his plan to build a lunar base and send an astronaut to Mars, it will again be as much an issue of dollars as of science.
Like the scientists of 30 years ago, many today look a few decades into the future and see a human mission to Mars as a real - though difficult - possibility. What's different now is that NASA can hope to achieve that goal without devouring a budget that could fund a third-world nation.
The notion of going into space on a budget has never been more controversial than now, a year after a space-shuttle disaster blamed in part on NASA's attempt to stretch money too far. But that accident gave policymakers new impetus to look at the direction of the space program. Moreover, advances in robotic missions, like the Spirit rover now on Mars, suggest that - with appropriate vigilance - NASA can indeed do more with less.
The result could be the most significant shift in the US space program since the 1960s, as well as a new sense of purpose for an agency that has struggled to recapture the public's imagination.
"The challenge of the next decade will be to repeat that [Apollo] performance, but do it at one-third the price," says Howard McCurdy a NASA historian at American University in Washington. "The challenge will be going to the Moon, but in a very different way."
The White House has provided little information since the first leaks of the plan last week. But several reports have traced some generalities. They suggest that the shuttle will only be used until the International Space Station is finished, and will then be retired. Russian, European, and Japanese rockets would be used to service the space station, and the $3 billion-plus needed to operate the shuttles could be put into a new program.
Goals would include the design of a new spacecraft to ferry astronauts to the space station, the moon, and perhaps even Mars, and the establishment of a moon base as a way to test technologies and procedures for a Mars trip. No time frame has been confirmed, but the administration would reportedly ask for $800 million more than normal for NASA this year, with that number increasing 5 percent annually.
The increase represents less than 6 percent of NASA's current budget, and in a time of massive deficits, keeping that number low will be crucial to the plan's long-term support on Capitol Hill and across America.
The model for how to do that will not be the shuttle program, but programs like the Spirit rover poised to nose around the Martian landscape later this week. Over the years, operating the shuttle has only become costlier, while offering minimal scientific benefit. Robotic missions, by contrast, have become far more scientifically advanced, while dramatically cutting costs.
The 1976 Viking lander, for instance, was basically an immovable weather station that cost $4 billion in today's dollars. The two Mars rovers - Spirit and Opportunity, the latter scheduled to arrive Jan. 24 - are mobile mini-geologists that can sample dust and grind rocks, for a combined cost of $820 million. Scientists imagine robots as forerunners of a human presence both on the moon and on Mars, recording crucial data, carrying out experiments, and even building structures.
If you send humans first, "They simply get there, and then you have to bring them back," says Story Musgrave, a former astronaut and veteran of six shuttle flights. "Robots let you go assuredly because you already have a place."
On the moon, the ideas are not far beyond our scientific grasp - an outpost in the highlands near the south pole, perhaps, where mountains the height of the Rockies offer year-round sunlight, and research indicates the possibility of water-ice deposits. Or a permanent base that could also be an observatory for astronomers.
Its most valuable function, however, would probably be as a step to Mars. The technological challenges for a Mars trip are so great that few scientists can imagine it in less than 20 years. The obstacles "are significant, and we should not commit humans until they are resolved," says John Logsdon, a scientist who was on the Columbia accident investigation board.
The likely three-year round trip means that engineers must design a ship that can last that long without support from Earth. It means that they must understand how such a trip - perhaps in zero gravity - would affect human physiology. And it means that scientists must find a way to deflect solar radiation, which could build to fatal levels.
The International Space Station will play a part. But getting father from Earth is key. "Being on the moon has a number of engineering and science things to teach us," says James Oberg, a former member of NASA mission control. "It would give us a better understanding of space reliability and a wider use of robotic systems."
More than that, he and others add, it would allow NASA to be essentially reborn. As the shuttle and space station projects have drifted along, overbudget and aimless, NASA has gradually lost the diligence and technological know-how that defined its early days, critics say. A trip to Mars, they argue, will focus engineers and scientists on a clear goal for the first time since Apollo, and take human spaceflight back to the ideal that captured the American imagination: exploration.
Says Dr. McCurdy: "One hope is that by creating a long-term vision, it will create the discipline needed to make this work."