On 19 December 1972, a final sonic boom above the south Pacific signalled the end of the Apollo programme, as a tiny space capsule burst back through the blue sky. On board were the last three astronauts to visit the moon on Apollo 17. Riding home with them was the precious negative of a photograph that would go on to become the most reproduced image in human history.
Frame number 22725 in magazine NN was a single shot of the whole Earth – later branded "the Blue Marble". Snapped 12 days earlier by astronaut-geologist Harrison Schmitt as the spacecraft accelerated away from the Earth, the picture was immediately captivating.
Journeying southwards, towards the moon, Schmitt had seen his home planet upside down, with the continent of Antarctica sprawling over the top. Below it the entire African land mass arced downwards towards the cradle of civilisation in the Middle East, with the edge of southern Europe right at the bottom. On a rare, relatively cloudless day, so many human histories, causes and stories were on show in one view.
Subsequently, this single image was embraced by everyone from NGOs working in the developing world to the environmental movements seeking to protect our planet. For 40 years it has been used to change minds, behaviours and political policies.
Just four years separated Blue Marble from another profound Apollo picture – Earthrise, captured by Bill Anders on Christmas Eve 1968. Anders's Apollo 8 portrait of our vibrant-blue planet, juxtaposed against the barren, brown-grey horizon of the moon, drew attention to Earth's apparent fragility.
Such images led one commentator to conclude that "on the way to the moon we'd discovered the Earth". They prompted many into thinking differently about our home planet. One such person was Stewart Brand, who self-published his ecologically themed Whole Earth Catalogue the same year, with a colour image of the entire Earth seen from space on the cover.
Brand's vision was for his new quarterly magazine to create a "self-sustaining, critical information service", and he soon nurtured it into a forum for the exchange of ideas suggested by the readers themselves. The Whole Earth Catalogue ran into the mid-80s, when Brand's concept for a "self-sustaining, critical information service" would find a new platform in Usenet newsgroups on the internet, and eventually on the world wide web.
American poet Archibald MacLeish, also influenced by these visions of the whole Earth from space, penned an essay in the New York Times, as Apollo 8 was heading home in December 1968, pointing out the eternal loveliness of such pictures of Earth from space. For MacLeish these images suddenly revealed us all as "brothers who know now they are truly brothers… riders on the Earth together".
Bill Anders's 1968 Earthrise image also captured the attention of peace activist John McConnell, who printed it on flags and handed them out in Central Park, New York, the following summer as Apollo 11 became the first mission to land on the moon. His actions would later lead to the founding of Earth Day – an annual celebration of awareness and appreciation of Earth's natural environment that is still held today in more than 175 countries. Shortly afterwards Friends of the Earth was formed by David Brower and other campaigners who felt that if there was one thing the Earth needed it was friends.
Around this time the Lindisfarne Association invited Apollo 9 astronaut Rusty Schweickart to their annual conference, to speak to them about his Earth orbital flight. Schweickart just stood up and spoke from the heart, recounting the story of his space walk, when he had nothing to do but look down on the Earth from 160 miles above it for five precious minutes after a camera jammed.
Schweickart's mind-expanding view and the epiphany that it triggered led him to vividly appreciate the insanity of humans fighting over borders that were invisible to him from up there. "Hundreds of people in the Middle East killing each other over some imaginary line that you're not even aware of, that you can't see," he recounted. "And from where you see it, the thing is a whole, and it's so beautiful," he remembered of his view of Earth. "You wish you could take one in each hand, one from each side in the various conflicts, and say, 'Look. Look at it from this perspective. Look at that. What's important?'"
Schweickart's speech, later turned into an essay entitled "No Frames, No Boundaries", was embraced by those at the conference, including Carl Sagan, who borrowed from it to pen his uplifting poem Pale Blue Dot, published in his 1994 book of the same name.
The sentiment of Schweickart's sermon was eventually taken as the founding principle of his Association of Space Explorers. The ASE was established in 1985 and today numbers 375 astronauts from 35 nations, who work to foster environmental awareness and planetary stewardship.
Forty years after Apollo, the ASE still has its work cut out. Today, thanks to the human impact on the environment over the past four decades, many of these national borders that the early astronauts struggled to resolve are now clearly visible from space.
And the "brothers" that Archibald MacLeish saw in those whole-Earth images are still killing each other around the world. The messages of peace and better environmental stewardship from those space flights of the 1960s and 70s sometimes feel forgotten.
But, thanks to another gift of Apollo, human unity is stronger now than it's ever been. The technological boost that the space race provided has changed the course of human history in far more profound ways than anyone could have predicted.
In 1961, when a new president, just a few months into his term in the White House, challenged America to "land a man on the moon and return him safely to the Earth", no one in America knew how to make it happen. But that wasn't going to stop them rising to Kennedy's dare.
The young minds (the average age of Apollo 11's mission control team was 28) who were put to work on Apollo were all recent graduates who had benefited from former president Eisenhower's National Defence Education Act, a massive capital investment in the US education system, started in the late 50s in response to Sputnik.
As progress in human space flight accelerated through the 60s, PhD intake at American universities, particularly in the field of physics, increased almost threefold. Apollo was making America cleverer.
Nasa knew that its entire moonshot challenge would rely on one thing above all others – navigation. So, within weeks of Kennedy's speech to Congress, they had appointed some of these bright new PhDs at the Massachusetts Institute of Technology (MIT) to work out how to make a soft landing on a moving target hurtling through space a quarter of a million miles from Earth.
The prototype inertial guidance system they came up with could not be relied on completely and would need to be manually realigned during the flight. To assist the astronauts in this task, and to help them control the fly-by-wire systems in their new Apollo spacecraft, a small, lightweight computer was proposed by MIT. In the early 60s computers still tended to take up entire rooms. If the boffins in Boston were going to miniaturise it enough to pack it into a modestly sized craft, they'd need some new technology, and they turned to an emerging invention called the integrated circuit. Fairchild Semiconductor was one of the few companies experimenting with these new micro-electronic components at the time; keen to help them perfect the performance of these novel miniature circuits, Nasa ordered one million of them.
The agency really needed only a few hundred for its Apollo programme, but, aware that they would be betting the lives of their astronauts on them, they were keen to make sure the manufacturers could make them has reliable as possible.
Such a financial kickstart to a fledgling industry, coupled with the third great gift of Apollo – inspiration – would prove to be a powerful driver for social change in the decades that followed. In 1969 two employees from Fairchild would go on to found a new company called Intel.
Those graduating across the world in the 70s and 80s had watched Apollo's engineers dream the impossible and then build it. As an act of human ingenuity, Apollo made them giddy, intoxicated on admiration and inspiration.
As William Bainbridge put it, in his book The Spaceflight Revolution, Apollo was "a grand attempt to reach beyond the world of mundane life and transcend the ordinary limits of human existence through accomplishment of the miraculous – a story of engineers who tried to reach the heavens".
And the generation that followed them took this philosophy and ran with it, harnessing the new Apollo-driven technologies of micro-electronics to wire up the modern world and reinvent society.
Canadian-born space entrepreneur Bob Richards points out: "It was a great example of what can happen when human beings come focused on a big, bold goal and inspire not only themselves but the generation that comes after them." Richards, who describes himself as "an orphan of Apollo", was one of that new generation, founding the global movement Students for the Exploration and Development of Space, or Seds, in 1980. Caught up in the wonder of it all, Jeff Bezos served for a year as president of this student group. He would eventually go on to change the world in his own Apollo-inspired way, creating the giant e-commerce website Amazon.
Bezos is not alone. Many hi-tech entrepreneurs who have built the new tools of the internet and the computing and communications infrastructure that underpins it also cite Apollo as their inspiration.
Prof Sir Martin Sweeting founded the world-renowned small satellite company SSTL, which revolutionised the industry. "Apollo started me on this whole pathway of getting involved in space," says Sweeting. "The idea of being able to participate in something as exciting as a lunar landing, it stimulated an ambition, the dream of building my own satellite with my friends." The idea of a private individual launching a satellite was considered pretty crazy at the time, he points out. "After building the first one, I had a lot of advice to go out and get a proper job. I'm sure that without Apollo I would have followed a more conventional career."
In the hands of people like Sweeting, the new micro-electronics technologies made affordable by Apollo led to the pocket calculators of the 70s, the simple home computers and the burgeoning internet of the 80s, the emerging world wide web of the 90s, and the video streams and social networks of the 21st century.
Former Nasa flight director Glynn Lunney witnessed this technological trickle-down straight from Apollo to the rest of us. "Apollo really did drive our industry. We were asking people to do things that were probably 10 or 20 years faster than they otherwise would have done. And they knew it. They stepped up to it and succeeded. Today's cell phones, wireless equipment, iPads and so on are a result of the fact that the country did this hi-tech thing and created this large portfolio of available technologies."
Today's population, over half of whom weren't born when Apollo 17 returned from the moon, use these inventions to connect and communicate with each other freely and without a thought for geographical and cultural differences.
Thanks to this final legacy of the lunar landings, the Earth of today has at last become that borderless world the astronauts looked back on during those heady days of the space race. The gifts of Apollo continue to ripple down the decades, and they still have the power to unite and inspire us.
Dr Christopher Riley is visiting professor of science and media at the University of Lincoln. His documentary, For All Mankind, airs on BBC Radio 4 at 9pm on 19 December, and his biopic of Neil Armstrong is on BBC2 television at 9pm on 30 December
