It is a bit cliche, but remains a Shakespearean truism: What is past is prologue.

Replanting human footprints on the Moon in the 21st century not only draws upon a rekindling of skill and daring, it also revisits a future interrupted. The Apollo Moon landing effort was curtailed in the early 1970s. Scheduled missions were not flown. The historic "one small step" was followed by retreat.

Several writers have recently focused on the Apollo years--not only recalling the epoch-making steps by humanity outward to another world, but also glimpsing at a future yet to unfold.

History book memories

That first landing of humans on the Moon back in July 1969 was an "unbelievable, extremely extraordinary period of time in our history," said James Hansen, author of First Man: The Life of Neil A. Armstrong (Simon & Schuster, 2005). Hansen's book is the first authorized biography of Armstrong.

Speaking to an attentive audience at Books & Company in Dayton, Ohio last November, and broadcast on CSPAN2's Book TV show, Hansen served up several reflective considerations.

"Roughly two-thirds of the Earth's population today was not alive in 1969," Hansen said. For them, "it's just in the history books...they had no first-hand experience" compared to many that watched the unfolding event through television.

Of the 12 human beings that trod across the surface of another heavenly body, Hansen added, only nine are still living today. "Only six commanders have piloted a spacecraft down to a lunar landing," he said, with only four of those people now alive.

Next voices from the Moon

Presently, the age of moonwalkers ranges from 71 to 75 years old, Hansen pointed out. Furthermore, in the year 2019--a moment in time that celebrates the 50th anniversary of Apollo 11's landing--the surviving moonwalkers will be in their mid-to-late 80s.

"Hopefully they will all still be with us," Hansen noted, "to tell us and share their histories."

But Hansen added: "Realistically, it's quite possible that we won't have them all...and it's a sad thought to think."

Even given NASA's plans to dispatch humans back to the Moon, Hansen related, "it is in the realm of possibility that they will part our company, all of them, before we do return. I think it's also possible that the next voices that we hear from the surface of the Moon will be speaking Chinese. The Chinese have a very aggressive program and it's very possible that they might be the next ones there."

Rudest of technology

"I was just 11 years old when Apollo 11 landed on the Moon, said Rod Pyle, author of Destination Moon: The Apollo Missions in the Astronauts' Own Words (Smithsonian Books/Collins, 2005)

"But even as a kid I knew that Apollo was a transcendent, ephemeral, spectacular moment--the apex of our civilization," he told SPACE.com. There might never be another time like this in his life, "where men from Earth reached out with the rudest of technology and explored another world."

In researching the book, Pyle said some delightful details of the Moon landings rose to the surface. For one, just after Armstrong and Aldrin landed, climbing fuel pressure in the Eagle's decent stage threatened an abort of the mission, he recalled.

Image Gallery: NASA's New Spaceship

VIDEO: A New Era of Space Exploration
Video animation of how NASA plans to get back to the Moon by 2018.

Then there's Apollo16's John Young complaining via open microphone to all in listening range back on Earth, in no uncertain terms, about his flatulence inside the tiny lunar module due to his dietary restrictions, Pyle said.

"One thing that became resoundingly clear as I was writing this book was what a great time the astronauts had on the Moon's surface," Pyle said. "These men were enjoying their time up there...almost romping as they churned through their intense schedule of activities."

Brute force

Pyle also spotlighted the now-primitive cold-war technology of the 1960's utilized for the Apollo project.

"The scale of the enterprise is such that it will still be a challenge today, almost 40 years later," Pyle said. "The Apollo missions were achieved by a brute-force application of technology. While the computer programming in the tiny memory units onboard was some of the most elegant and compact code ever written, the rockets and flight hardware were amazingly simple and basic by today's standards."

Perhaps it shouldn't be surprising that today, Pyle said, "as we purport a desire to return to the Moon, NASA is showing a distinct preference for Apollo-era style hardware."

"While obviously updated with new composite materials and vastly improved computing power, the new moonships specified by the likely contractors are, as new NASA chief Mike Griffin recently stated: like 'Apollo on steroids'. And this is a good thing. For as we approach this still highly risky endeavor, we can and should resort to established and proven technology to get there... as well we might have used had we chosen to continue our brilliant course moonward and beyond in the 1970's," Pyle stated.

Non-hardware matters

There are key questions that need to be answered, according to Pyle: can we return to the Moon? Can we do it safely and achieve our one-time goal of extended exploration? Can we beat the Chinese and Japanese, both of whom have expressed a strong interest in a moon landing?

"It is hard to say. What the nation needs now is not the technology, for we possess that," Pyle said. "We have the machines, the people and the know-how to make this journey, to establish a permanent lunar base...and to reach out to Mars."

Still, there are non-hardware matters that need resolution.

"What we may lack is the national will, the idealism, and the raw nerve to go," Pyle concluded. "And of those distinctly American characteristics, of our ability to find them within and apply them to the great endeavor, the exploration of space...only time will tell."

Financial, environmental, and national security carrot

One moonwalker has recently shared his visionary view of the role the Moon could play in the 21st century.

Harrison Schmitt was the last human to step onto the Moon, doing so as an Apollo 17 astronaut in December 1972. His three days of on-the-spot lunar exploration has spurred years of thinking about the Moon, its uses, and humankind's destiny in space.

Schmitt has authored Return to the Moon: Exploration, Enterprise, and Energy in the Human Settlement of Space (Copernicus Books/Praxis Publishing, Ltd., 2005).

Within its pages, the former moonwalker--as well as U.S. Senator--draws upon the Apollo legacy, but also plots out a sweeping, rejuvenated government and entrepreneurial space enterprise. It is a venture that includes returning to the Moon for its helium-3 fusion resource--a "financial, environmental, and national security carrot,"--to fuel an energy-hungry Earth, Schmitt suggests.

In a foreword to the book, fellow moonwalker, Neil Armstrong calls Schmitt's plan "thought provoking and exciting...a proposal worthy of careful examination."

NASA: overhaul needed

Schmitt adds in his new book, however, that NASA "lacks the critical mass of youthful energy and imagination required for work in deep space."

While some steps to remedy these issues are being taken, the agency needs a major overhaul. It has become too bureaucratic, too risk-adverse to efficiently carry out U.S. President Bush's Moon, Mars and beyond mandate, Schmitt reports.

"Whenever and however a return to the Moon occurs," Schmitt writes, "one thing is certain: that return will be historically comparable to the movement of our species out of Africa about 150,000 years ago. Further, if led by an entity representing the democracies of the Earth, a return to the Moon to stay will be politically comparable to the first permanent settlement of North America by European immigrants."

"Apollo bent our evolutionary path into the future," Schmitt feels. "We owe the future of humankind another walk on the Moon."