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A majority of modern astronauts experience any one of a suite of symptoms scientists collectively call Space Motion Sickness, or SMS. But despite knowing about it for nearly as long as humans have gone into space, we still don't know exactly what causes it, or how to predict which astronauts might lose their lunch.

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[♪ INTRO] In 1961, cosmonaut Gherman Titov became the first person to throw up in space.

Hey, whatever gets you into the history books, right? And these days, between 60 to 90 percent of astronauts report experiencing some degree of Space Motion Sickness, or SMS.

While it doesn’t always lead to losing their astronaut lunches, the crummy symptoms like nausea and dizziness aren’t great when there are spacewalks to complete and research to run. Or dangerous accidents to fix. But despite how long scientists have known that space trips can cause motion sickness, they’ve yet to actually pin down the why.

And if we’re going to keep sending humans to space, we need to know what’s causing SMS to help prevent it. Any hypothesis looking to explain SMS has to account for a few key pieces of data. For one thing, astronauts don’t just get it when they first go into space.

Many feel motion sick for a bit immediately after returning to Earth. And in each case, it only lasts for two or three days. The hypothesis also needs to help predict who is likely to get sick.

Because right now, an astronaut having a stomach of steel all through training doesn’t guarantee they’ll have a smooth, sick-free trip to space and back. The first hypothesis that scientists came up with was the Fluid Shift Hypothesis. It’s based on the fact that in microgravity, the fluids inside an astronaut’s body aren’t all pulled in one downward direction, so they wind up getting redistributed.

You can see the effect in photos of astronauts’ faces. They tend to appear puffier, at least until they pee out enough of that liquid. But this redistribution can also increase the pressure of fluids within the inner ear, which could impact the vestibular system.

It’s the vestibular system that tells your brain where your head is in three-dimensional space. Three fluid-filled loops in each ear keep track of any rotating that your head does, and the accelerations that come along with that. Meanwhile, two other chambers on each side monitor your head’s position relative to the direction that Earth’s gravity is pulling on it.

You know, whether it’s tilted or not. The Fluid Shift Hypothesis proposes that microgravity basically messes with the base settings of your vestibular system. Your brain gets disoriented, and boom.

SMS. But not so fast! There was one piece of evidence that led most researchers to dismiss it.

Or, more accurately, a lack of evidence. The crews of early space missions, especially the Mercury and Gemini missions, rarely reported SMS symptoms. Their missions may have been more limited than a six-month stay on the International Space Station, or even a week or two aboard the Space Shuttle, but their bodies still would have experienced these fluid shifts.

But, it’s possible that the threat of being known for getting spacesick made them reluctant to admit they did get SMS. While that fluid shift may play some role in disorienting astronauts, we’ve got to continue our search for the true culprit of SMS. The Sensory Conflict Hypothesis is most similar to how we think motion sickness works down here on Earth.

Like, if you’re riding in a car along a winding or bumpy road, vestibular signals tell the brain you’re moving. But if you're focused on reading a book inside that car, visual signals report everything’s holding still. So you’ve got two different systems in your body sending mismatched messages, your brain gets confused, and you feel sick to your stomach.

Now, how might this translate to space? Remember, the vestibular system tells you how tilted your head is compared to the direction of gravity. And it can do this thanks to gumdrop-like otoliths, literally ear-stones, that sit inside the chambers in your ear.

When your head tilts, the stones tip, too. But their job kinda goes out the window when your body and the spaceship or space station around you are continuously falling around the planet, and from your perspective it seems like gravity doesn’t exist anymore. Any signal sent from the vestibular system that “You’re tilting!” wouldn’t match any of the typical visual signals you’d be getting.

And that’s the sensory conflict which may lead to SMS. Unlike the Fluid Shift Hypothesis, Sensory Conflict might explain why early astronauts reported fewer spacesick symptoms. They were pretty much strapped into tin cans for most of their flights, and weren’t moving around the cabin much.

As for why this sensory conflict occurs only when arriving in space and coming home, it may be that astronaut brains adapt specifically by reinterpreting any tilt message to simply mean “You’re moving”. The few days that process takes is the window when folks are most likely to experience motion sickness. Despite being such a popular hypothesis to explain SMS and how an astronaut gets over it, Sensory Conflict can’t paint the whole picture.

It can’t explain why only some astronauts suffer from SMS, and can’t predict who will experience it. So scientists have another idea: the Otolith Asymmetry Hypothesis. Those ear-stones are made up of tiny calcium carbonate crystals, and it’s possible that your left ear might have more, or simply larger, stones than the right ear.

Or vice versa. This asymmetry would make one ear slightly more sensitive to gravity than the other, but you’ve spent your whole life naturally figuring out how to compensate for that in Earth’s standard gravity. Once again, switching to the microgravity of outer space messes all of that up.

Some researchers believe that the larger that asymmetry is to begin with, the more motion sickness an astronaut is likely to feel while the brain retrains itself. In fact, research published back in 2008 demonstrated that people with greater otolith asymmetry tend to experience more motion sickness after spinning around in a centrifuge. Even though the centrifuge could replicate a shift in gravity, it was still a ground-based experiment.

But we’re now in the era of private spaceflight. Billionaire-turned-space tourist Jared Issacman is funding a series of crewed SpaceX flights under the mission name Polaris. The first of those missions, Polaris Dawn, hopes to launch as early as March 2023.

And over five days, the four-person crew will conduct a bunch of experiments, including one that tests the Otolith Asymmetry Hypothesis. So we might find out more about how the vestibular system responds to space travel very soon! In the meantime, astronauts will just have to deal with the symptoms of SMS, or make due with medications that come with their own negative side effects, like drowsiness.

Which may be better than nausea, but being sleepy on the job in a dangerous environment far from home isn’t great, either. But all things considered, astronauts still seem to think that spending time on space exploration is worth it. And so do we!

For the last nine years here at SciShow Space, we’ve brought the wonders of the universe home to you. Going forward, we’ve decided to move that content—along with the usual hosts Reid and Savannah—over to our main channel, SciShow. So we’ll keep making cool space videos, just in a new place!

All of the videos already on this channel will continue to be available on SciShow Space forever, for you to watch any time. Thank you so much for your support over the years. We’ll see you next time on the main SciShow channel! [♪ OUTRO]