After arriving in lunar orbit and later separating from the Command Module to begin their landing sequence, Armstrong and Aldrin had little idea that their moon landing plans had already been modified by an overlooked effect of Newtonian physics.

A couple of hours earlier, as the spidery Lunar Module “Eagle” undocked from the Command Module “Columbia,” residual pressure inside the tunnel that connected the two spacecraft before undocking wasn’t sufficiently vented, causing Eagle to get an additional boost as it separated.

It was slight, but at around nine minutes before touchdown, Armstrong realized they were going to overshoot their landing site, estimating they’d miss by approximately three miles (which was a close educated guess, they actually missed by four). As the moon is littered with boulders and craters, the planned landing site was chosen as it was comparatively smooth. So with the modified flight plan, the duo had to find another suitable place to safely touch down.

As if that wasn’t enough drama, the Eagle’s computer had been distracting them with program alarms throughout their descent. Radio communications with Mission Control were also patchy. The recurring alarm was being triggered by the onboard landing computer that was warning of an overload. Fortunately, as the alarm was intermittent, Mission Control deemed the risk of computer overload low and green-lit the landing.

As the minutes ticked down, and the pair watched the lunar surface getting closer by the second, another problem became clear: they were burning more fuel than calculated. Due to their overshot landing, they were nearly running on empty so there was even more urgency to find a landing spot.

“You never [want to] go under the ‘Dead Man’s Curve,’” Flight Controller Steve Bales later recalled in an interview. “It was an altitude [where] you just don't have enough time to do an abort before you had crashed … Essentially, you're a dead man.”

With only 30 seconds of fuel left in the tank, Armstrong guided the Eagle softly down onto its impromptu landing site that, moments later, would become “Tranquility Base”—the first (temporary) human outpost on the moon.

As the adrenaline ebbed and the astronauts carried out their post-landing tasks, another problem was brewing. Although it had been shut down, sensors were detecting a pressure build-up in the landing engine fuel line. This could mean only one thing: ice had accumulated in the line, plugging it, and the backed-up fuel vapor was getting heated by the hot engine.

Discussions between NASA and Grumman Aircraft Engineering Corporation, the company that oversaw the Lunar Module’s development, deemed this increase in pressure a hazard that may trigger a deadly explosion if not remedied. So they drew up plans to vent the system.

“We all felt that the consequences of an explosion, even of the relatively small amount of fuel remaining in that short section of line, was unpredictable and unacceptable,” wrote aerospace engineer and “Father of the Lunar Module” Thomas J. Kelly in his book 2001 book, Moon Lander.

Before the instructions could be relayed to Armstrong and Aldrin, however, the ice plug thawed, the gas was released, and the problem remedied itself.

Even though the ground beneath Tranquility Base had the appearance of being free from any boulders that may have damaged the Lunar Module as it touched down, before Apollo 11, scientists couldn’t be absolutely sure that Armstrong and Aldrin would land on stable ground. What if the stuff acted like quicksand? There was also the possibility that the fluffy accumulations of moon dust covered jagged shards of rock that could cause injuries to moonwalkers or to the lander itself.

Although previous robotic missions, such as the Surveyor landers, were designed to study the lunar surface as a prelude to planning later Apollo missions, it wasn’t until Armstrong’s “one small step” crunched into the grey powder that NASA was sure the surface was safe for extravehicular activity (EVA).

While this may be a minor point in the annals of the Apollo Program, lunar dust is no joke. Created over billions of years by meteorite impacts, the moon lacks processes that would erode these minuscule particles into smoother shapes. Apollo astronauts found the abrasive dust to be more than a nuisance.

Later missions after Apollo 11 featured longer EVAs, and there are reports about these tiny shards of rock permeating Lunar Module interiors, coating helmet visors, jamming zippers and even penetrating layers of protective spacesuit material.

“All of the astronauts complained of the problems with dust,” said Brian O’Brien, a Rice University professor from 1963 to 1968 who built radiation and dust experiments for the Apollo missions. “The very access to the moon stirs up dust. And the walking of an astronaut or the movement of a rover stirs up dust. The dust will travel ballistically, because there’s no atmosphere, and it will stick to anything and everything.”

Though scientists are now acutely aware of the impacts of space radiation and dust on astronaut health, in those pioneering days of 1969, there was some degree of trial and error.

By 1969, only a handful of robotic landers had touched down on the lunar surface. And while these landers confirmed that the moon’s surface was rocky, dusty, covered in craters and devoid of complex life forms, some precautions for possible infection by alien microbes had to be taken—but only after the Apollo astronauts became infected by these hypothetical space germs.

After risking their lives for the advancement of humanity, Armstrong, Aldrin and Collins had the dubious pleasure of being stuck in planetary protection quarantine on their return, just in case a deadly space-borne plague had hitched a ride with them.

As soon as their re-entry capsule splashed down in the Pacific Ocean on July 24, the trio was transferred to a mobile quarantine facility inside which they were transported to NASA Lunar Receiving Laboratory at Johnson Space Center where they had access to a larger quarantine facility until their release on August 10, 1969.

Probably the most terrifying moment of the Apollo 11 landing, however, didn’t actually happen.

A speech, prepared for then-President Richard Nixon in the event of mission failure, was released to the public 30 years later, detailing the White House’s response should the unthinkable have happened. Any number of things could have gone wrong during that pioneering mission, so, to prepare, the president was ready to address the nation when it became obvious the mission was lost.

The text ends on a poignant note: “For every human being who looks up at the moon in the nights to come will know that there is some corner of another world that is forever mankind.”

The “In Event of Moon Disaster” speech was never read but was instead filed away as a reminder that space exploration is a dangerous endeavor that has claimed the lives of many brave explorers since the beginning of the space age. Meanwhile, the men of Apollo 11 became the first humans to set foot—and survive—on an alien world.