On Dec. 21, 1968, Apollo 8 was launched on one of the greatest journeys in the history of human exploration.

Imagine If Columbus took only the Santa María, sans lifeboats, 3,000 miles across the Atlantic to the Caribbean island of Hispaniola. Unable to go ashore without landing craft, he circled it and recorded his observations in log books. Returning later with his three-ship flotilla to plant the flag would still be dramatic, but also a tad anticlimactic.

It's hard to believe that Apollo 8's voyage around the moon had originally been scheduled as a less audacious Earth-orbit mission to test the whole moonship "flotilla": the monstrous, still problem-prone Saturn 5 booster, along with the recently redesigned, and only once flown by astronauts Apollo command ship fashioned to carry a three-man crew round-trip from Earth to moon orbit in tandem with the lunar lander, which ferries two astronauts to and from the moon's surface.

In 1968 the command ship was ready, but the lander was behind schedule, and if NASA were to wait for it to test the whole system in Earth orbit before heading moonward, then the goal set by Pres. Kennedy of a landing there by the end of 1969 would be near impossible. Not only was the clock ticking, but also the CIA had informed the agency that it believed the Soviet Union was on the verge of launching cosmonauts on a moon mission.

In August 1968, NASA's Apollo Spacecraft Program Office manager, George Low, proposed making Apollo 8 a mission to circumnavigate the moon some 234,000 miles (376,585 kilometers) away. Working out the details in secret, the Apollo team realized they could do it and, while there, go into  orbit.

Orbiting made the stakes even higher: If a failure of the spacecraft's rocket engine left astronauts Frank Borman, Jim Lovell and Bill Anders stranded, NASA Administrator Jim Webb feared we would have "ruined the moon" for poets, lovers, and everyone who would look at the orb and know there were three dead astronauts circling there. And, if the new navigational, communication or reentry components had failed, the crew could have been lost in space or have burned up in Earth's atmosphere on returning home.

That year, public enthusiasm for manned spaceflight was as high as it ever would be, but a failure might have dampened enthusiasm, thereby slowing or even stopping the U.S. from fulfilling its attempt to fulfill Kennedy's goal.

Apollo 8 realized many firsts, including the first time human's had set their eyes on the moon's far side as well as the fastest astronauts (or, for that matter, anybody) had ever traveled—25,000, or 40,230 kilometers, per hour—through space and into Earth's atmosphere during reentry.

But what stands out for many is that it was also the first time people back on Earth had been able to see their world as a sphere floating in the pitch-black void of space. The iconic Apollo 8 "Earthrise" photo taken over the lunar horizon is credited with inspiring global environmental consciousness and the cultural viewpoint of our world as a unique and extremely fragile planet that must be preserved.

What started as a gamble became one of humanity's greatest moments in exploration—and a public relations coup for NASA. Not only that, but people needed a moment like this in a year like 1968—one shattered by antiwar and race riots; the assassinations of Martin Luther King and Robert F. Kennedy; as well as war, both hot and cold.

So, there it was, just what the doctor ordered: On Christmas Eve, pajama-clad kids who normally would be looking skyward for eight tiny reindeer were glued to their TV sets, along with an estimated half a billion people around the world, gaping at the moon's surface hurtling by just 69 miles (111 kilometers) away and listening to awe-inspired astronauts who read Genesis and sent yuletide greetings to "all of you on the good Earth"—the same good Earth they had seen on their black-and-white TVs as a fuzzy, cloud-veiled, gibbous globe, like they had never seen it before. Small wonder then that when Frank Borman got back, someone had sent him a message: "Thanks for saving 1968."

For more on Apollo 8, see our In-Depth Report. NASA's Christmas Eve "Genesis" transmission video is here.