LOS ANGELES - After traveling 240,000 miles in just 76 stressful hours, Apollo 11 had entered lunar orbit on July 19, 1969 and the next day, July 20, humankind had achieved the unthinkable.
A feat of human ingenuity that seemed so far out of reach at the time finally was made possible and the sky was no longer the limit, but so much could have gone wrong with the lunar expedition.
Putting a human on the moon was such a pipe dream at the time that President Richard Nixon even had his speechwriter prepare an obituary that would be read to the public just in case the astronauts never made it home.
"They were moving really fast," said Rod Pyle, who has written 15 books on space exploration and worked at the Jet Propulsion Laboratory in California.
"The general confidence of the hardware was pretty high, but they were moving so quickly back in those days that they would only test these things a few times before they actually committed them to a mission," said Pyle.
From the moment the Saturn V rocket carried the three astronauts out of Earth's atmosphere to Armstrong's famous first words on the moon declaring "one small step for a man, one giant leap for mankind," so much could have gone wrong. From equipment failure to hostile alien pathogens, the mission was brimming with potential dangers.
Here are just a few of the potential horrors NASA anticipated:
Aliens - Alien germs to be exact. While the chances of actually discovering any life outside our planet during the mission was slim, scientists weren't taking any chances.
A quarantine protocol had been established for the space explorers upon their return to Earth in order to defend against nefarious space microbes.
Unfortunately, there was one fatal flaw with the whole plan:
"So when these guys landed in the water, the Navy divers swam over, opened the hatch, threw in three biological contamination garments, and they had to be quarantined for three weeks, but of course when you open the hatch to throw the garments in, then anything could have come out of there. We could have had tentacle monsters running around," said Pyle.
Pyle says very few scientists from NASA and the Centers for Disease Control and Prevention felt there was a substantial risk of any dangerous space born microbes, but they wanted to make sure.
The astronauts were put in a three-week quarantine, and so far there has been no documentation of any serious illnesses affecting anyone who has gone to space and returned.
Lost in space - Had they gotten stranded, Pyle said NASA would've cut off communication with the astronauts, leaving it up to the three men how they wanted to spend their last days on the moon.
Pyle said the prevailing view was that the astronauts should stay as long as possible in the name of science because that's what the mission was about.
"There were no cyanide pills, but you don't need one because all you have to do is open a valve on the lunar module hull or crack a door open and the air leaves and you're dead in 30 seconds," said Pyle.
While the team had food rations, weight was such a crucial component for the success of the mission that they really didn't have any extra consumables had a catastrophe occurred causing them to be castaways.
Pyle said NASA was literally shaving grams in order to conserve weight for the trip, even going so far as to offer bounties as much as $7,000 to anyone able to shave a gram off the lunar module.
Equipment failures - Probably the biggest risk facing the success of the mission and safety of the astronauts had to do with the equipment.
During the mission, Buzz Aldrin accidentally broke the circuit breaker switch on the lunar module that was meant to ignite the engine so they could return to the command module, which easily could have left them stranded.
Aldrin was able to improvise using a felt-tip pen as the switch instead. But Armstrong said in a 2011 interview that the module's lunar launch probably could have worked even without a pen.
Galactic cosmic radiation - NASA new that flying beyond the Earth's magnetic field, which protects us from harmful sun flares and radiation, would've put the astronauts in the dangerous situation of being exposed to a lot more radiation than you would normally encounter on Earth.
Radiation from the sun, but also the outer stretches of the solar system, called galactic cosmic radiation (GCR) comes from within the Milky Way galaxy and is composed of harmful particles traveling at nearly the speed of light.
"During the Apollo missions, there was never a major solar flare or coronal mass ejection, but it was really just a matter of rolling the dice and playing the odds," said Pyle.
Pyle said that had there been a major flare, there would have been nowhere to hide.
Psychological effects - As for the Apollo astronauts themselves, there was initially concern that hurtling through space for long periods of time would have serious negative effects on their minds.
According to a 1975 government report detailing the medical aspects of the program, none of the astronauts experienced any issues. "It is perhaps remarkable that there was virtually no difficulty from a psychological and psychodynamic viewpoint among highly competitive, driving, and forceful individuals," the report said.
What weighed heavily on the astronauts, however, was what to do once they returned.
Pyle cited the "overview effect," which is a shift in cognitive awareness reported by some astronauts when they view the tiny blue planet from the moon as a fragile ball of life hanging in the void of space.
Pyle said a number of astronauts came back from lunar missions and became preachers, getting seriously into religion and metaphysics.
"Now that we have a space station and we've had people up there for a year at a time, we know that your eyeballs kind of change shape, and your vision can go out, your bones get brittle, your body changes," said Pyle.
While many psychological effects are not as much of a concern for a short trip like the Apollo 11 mission, or anyone on the space station who can come back to Earth, the effects do matter for a longer journey.