[♪ INTRO] Stories about human spaceflight are usually full of heroism, math, and the triumph of human ingenuity over incredible odds.

The story of how an astronaut got the farts on Apollo 16 is no exception. It all started on the Apollo 15 mission, which landed on the Moon in 1971.

The mission was successful, but while they were in space, the astronauts experienced some arrhythmia, or irregular beating of the heart. Arrhythmia can signal all kinds of serious cardiovascular problems, like heart attack and stroke. So even though the astronauts made it back just fine, NASA very much wanted to make sure that astronauts on future missions avoided arrhythmia while they were in space, where humans have yet to establish any solid healthcare infrastructure.

So NASA physicians did some bloodwork on the Apollo 15 crew when they returned to Earth, and found that they had too little potassium in their systems, a condition called hypokalemia. Potassium is super important for your heart because it’s a major part of nerve cell function. And your nervous system is what controls your heartbeat.

Specifically, potassium is used in the sodium-potassium pump, which is found in all kinds of cells, including those in every part of the nervous system. An enzyme separates out the sodium and potassium on either side of a nerve cell’s membrane, building up voltage across the cell surface. For carefully timed bursts of cellular activity, you usually want the cell’s membrane to reach a particular voltage before firing, called the action potential.

Once you hit that voltage, the cell fires; otherwise, it just waits for the voltage to build up again. A network of nerve cells throughout the heart rely on the sodium-potassium pump, among other cell-signalling pathways, to time your heartbeat. If components of that system fall out of whack, like if you aren’t getting enough potassium, your heartbeat can get off-kilter.

So be sure to eat your bananas, friends. But back to our astronauts. To keep the Apollo 16 astronauts safe from the arrhythmia experienced by the Apollo 15 crew, NASA prescribed a ton of potassium.

Not enough to mess up the sodium-potassium pump in the other direction, but still a whole bunch. Potassium is typically very bitter, so it needs to be dissolved into something with a lot of flavor. NASA decided to use lots of citrus drinks.

The crew had various citrus-flavored rehydratable drinks that they had to drink every day, multiple times a day. As you can probably imagine, the astronauts got really tired of this. And the diet had some physical downsides, which we know because NASA records all its communication with astronauts and makes the transcripts publicly available.

At one point, the mission commander, John Young, was unaware that he had a hot mic, and well, I’ll just let you listen to what he said to fellow astronaut Charles Duke. “I’ve got the farts, again. I got ‘em again, Charlie. I don't know what gives ‘em to me.

I think it's acid stomach, I really do. I haven't eaten this much citrus fruit in 20 years! I’ll tell you one thing, in another 12 days, I ain’t never eatin’ any more.” So of course the press reported this, and because farts are always funny, everyone had a good laugh about it.

That is, everyone except for Florida Citrus Mutual, the organization representing Florida’s citrus industry. They issued some official statements saying that the astronauts were using artificial citrus drinks, and not wholesome, Floridian OJ, part of this complete, balanced breakfast. So citrus is not to blame, and they would like you to please continue to buy their products.

And that was true. The flatulence wasn’t caused by fruit. It was the potassium.

See, potassium is used in another enzyme pump, in the digestive system. It’s the hydrogen-potassium pump. Your stomach acid is kept nice and acidic by an enzyme that pumps potassium in and hydronium ions out.

The hydronium quickly dissociates to regular water and free hydrogen ions. Having free hydrogen ions in a solution is what makes it acidic. In fact, pH stands for “power of hydrogen.” So all the extra potassium the astronauts were taking in caused the hydrogen-potassium pump to overwork, which gave them “the farts.” But their heartbeats were good and steady, so at least they solved that problem!

There has actually been a great deal of research into space farts by NASA, because they pose a safety hazard. Methane is a flammable gas, so having a container of people emitting flammable gas on top of a rocket could conceivably pose some problems. But the amount of methane the Apollo 16 astronauts produced didn’t cause a fire, and the astronauts didn’t experience any arrhythmia.

So unfortunately for the Apollo 17 crew, NASA physicians required the potassium diet for them, too. Thanks for watching this episode of SciShow Space. For more amazing stories from the history of space travel, plus all kinds of other fascinating space science, you can go to youtube.com/scishowspace and subscribe.

Not all of our episodes are about farts, I promise. [♪ OUTRO]