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Caitlin: On May 25, 1961, U.S. President John F. Kennedy challenged the nation to land someone on the Moon before the end of the decade.

America had barely just started sending people to space at all. But just eight years later -- and 47 years ago, today -- Neil Armstrong and Edwin “Buzz” Aldrin walked on the Moon, on the Apollo 11 mission. They were the first humans to ever set foot on another world.

Apollo 11 launched on July 16, 1969, at 9:32 AM local time from -- appropriately enough -- Cape Kennedy, Florida. Armstrong, Aldrin, and a third astronaut, Michael Collins, were on board. To get all the way to the Moon, they were perched in a small capsule atop the Saturn V, the most powerful rocket in history.

Fully loaded with fuel, the Saturn V was over a hundred meters tall and weighed almost three million kilograms -- that’s 15 times as much as a blue whale. The rocket worked in three stages: The first stage burned eight hundred thousand liters of a fuel called RP-1, a type of kerosene that’s used in rockets. It ran out of the RP-1 in the upper atmosphere, where the first stage detached and fell into the Atlantic Ocean below.

This made way for a second stage that burned through a million liters of liquid hydrogen. The second stage ran out of fuel at the edge of space, which is when the third and final stage took over, burning even more liquid hydrogen. It kept burning until the ship was high enough and going fast enough to orbit the Earth.

Then, the engines were turned off -- but they still had some fuel left to burn. After a couple of hours, the crew aimed at the Moon and turned the rockets back on. Two days later, they became the third crew in history to orbit the Moon, and their mission was still just beginning.

Armstrong and Aldrin crawled into the Lunar Module and started drifting down towards the surface, while Michael Collins stayed in lunar orbit. Until now, things had gone pretty smoothly. The rockets fired like they were supposed to, and Apollo 11 seemed to be following the path that NASA scientists had mapped out.

But no one had tried to land on the lunar surface before, and that’s when they started to run into some problems. For one thing, the Lunar Module’s computers almost fried themselves. But the computer was also aiming the ship at rocks.

They were supposed to land in the middle of the Sea of Tranquility, which is a big, flat basin on the Moon’s surface. But early in the descent, Armstrong and Aldrin noticed that they were passing landmarks about three seconds earlier than they were supposed to. Three seconds might not sound like that big of a deal, but it meant that they were going to land a few kilometers away from where they’d originally planned.

And as they got closer, Armstrong noticed that they were headed right for a field of boulders. If they hit one of those boulders, that would be the end of the mission -- and maybe even their lives. With only about 30 seconds’ worth of fuel left to spare, Armstrong switched control of the craft over to a semi-manual mode and piloted it to safety.

And that was it: The first humans in history had landed on the Moon. According to the official schedule, they were supposed to follow the landing with a few hours of rest before getting ready to open the door. But they were a little too excited to sleep.

I mean, who wouldn’t be? They were on the MOON! So the schedule was switched around a bit and they immediately started preparing to go outside. And so, on July 20, 1969, at 10:56 PM on the east coast of the United States, an estimated six hundred million people worldwide -- a fifth of the world’s population at the time -- watched Neil Armstrong become the first person to set foot on the Moon. With Buzz Aldrin following closely behind him.

Little known to most of that immense audience, though, NASA wasn’t positive that the astronauts would get back off the Moon. If it took more fuel than planned, or if something went wrong or broke on the way up, they would be stranded there. And there was no rescue plan.

This was a likely enough possibility that speechwriter William Safire was asked to write a speech for then-president Richard Nixon to read in case the worst happened. And something did break while they were on the Moon’s surface. When he was getting back into the cramped lunar module, Aldrin accidentally broke a knob off of a circuit breaker that was responsible for arming the main engine.

Luckily, he and Armstrong realized that they could fix the problem by sticking a felt-tipped pen where the knob went. They safely rejoined Michael Collins in orbit and headed home. Eight days, three hours, eighteen minutes, and thirty-five seconds after leaving Earth, Apollo 11 splashed down in the Pacific Ocean.

At the time, scientists didn’t actually know if there was any life on the Moon, so the entire crew was quarantined for almost a month, just in case they brought back anything dangerous. Then, finally, they were released, and went back to living life like the rest of us -- if the rest of us were huge international celebrities.

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