The Astronaut’s Dilemma: What Happens to Human Bodies in Space

There’s been a lot of stuff in the news lately about space and its affect on the human body. Just recently, with the return of the last ISS crew to Earth, one astronaut had to be hospitalized overnight (for reasons that, due to NASA’s very legitimate ferocious protection of its astronauts privacy particularly concerning health matters, are currently unknown). Meanwhile astronaut Suni Williams, currently on the ISS, is fending off tabloid reports stating that she is losing a lot of body mass in space (she isn’t, but since when does that stop a tabloid?).

We’re sending people into space for longer and longer periods as we take the first steps needed to get humans to Mars. That trip will take years, so knowing what happens to the human body in space is critical for the survival of future astronauts. Or, well, knowing how to counteract what happens to the human body is space is critical, at least if you ever want to be able to come back to Earth and, you know, move. Because let me tell ya, space does a number on human biology. 

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Getting a Little Green Around the Gills…

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Right from the beginning there were concerns about how space would affect the human body, even on the extremely brief flights that the first astronauts and cosmonauts took. Even Alan Shepard’s 15 minute suborbital flight in May 1961 caused worry. That’s one reason why the first astronaut that the US sent into space, in January 1961, was not a human but a chimp named Ham—NASA wanted to make certain a living creature could function in the space environment. 

Once we got around to launching people, even on the early, short flights there were some immediately obvious affects of the microgravity of Earth orbit on the human body. For one thing, even extremely experienced test pilots can get space sick. That happens when the contents of your stomach suddenly start to float. Fortunately most space travelers get over this stage after some time to acclimate. 

But your stomach contents aren’t the only thing floating. The fluids in your head are as well. This affects your orientation as the inner ear freaks out a little (which also contributes to the whole space sickness thing—every flight to space includes vomit bags as key equipment) and also your eyesight as the fluid in your eyeball starts to float, reshaping your eye (eyeglasses are also standard equipment on the ISS). Given enough time in space, your eyesight may change permanently.

Things that would normally drain downward out of your head just don’t, and astronauts wind up with puffy faces and a state of semi-congestion that has nothing to do with having a cold. Perhaps related to this, astronauts also frequently report changes in how they taste things.

These are the sorts of effects that show up fast and that were dealt with as far back as Project Gemini. But once we entered the space station era, we started staying in space for much longer periods of time.

The longest flight duration aboard ISS currently stands at 374 days, by cosmonauts Oleg Kononenko and Nikolai Chub, followed closely by astronaut Frank Rubio’s 371 days. But the record for longest flight ever belongs to cosmonaut Valeri Polyakov, who spent 437 days and 18 hours aboard the Russian space station Mir in a single go in the mid-90s (and since Mir was a grungy death trap, that’s an even bigger achievement than it sounds). This is where we start to see more effects of microgravity starting to come into play.

The Hip Bone’s Connected to the Thigh Bone…

So what do humans risk when they spend a lot of time in a this environment? The one that often gets the most attention is bone and muscle deterioration. Your musculoskeletal system is used to hauling your mass around in Earth’s gravity. The bones and muscles in your legs in particular are used to doing a whole bunch of work every time you stand on your feet. 

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That doesn’t happen in orbit. Not only are you not carrying a bunch of weight around, you’re not even standing on your feet. Bones actually need some work to in order to keep laying down healthy new tissue, and without the work needed to move around in normal Earth gravity, your bones will start to weaken. This progresses over time and eventually you’ll essentially have space-related osteoporosis. This is fine while you’re still in orbit, but if you want to come back to Earth again and, you know, put weight on your bones, it’s bad news.

Then there’s the muscles. Muscles, you probably know, get stronger when we use them. Again, the work we do moving our bodies around in normal Earth conditions just doesn’t happen in orbit. Muscle breakdown starts quickly without intervention. If you just let this go on, by the time you get back to Earth after a space station tour your muscles won’t be able to support your weight.

Also the combo of fluid shifting and muscular atrophy has effects on the cardiovascular system. The heart doesn’t have to work as hard to pump blood which, unchecked, will lead to a weakening of the heart muscle. Your blood volume will decrease in orbit, which means when you come back to Earth there just isn’t as much blood to pump through your veins and also suddenly it’s all being pulled downward. Beware of passing out when you get out of your spacecraft.

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Fortunately there are ways to slow some of these processes by forcing the body to work in ways that resemble what it does on Earth. ISS residents have very strict fitness routines—two hours a day in general. They have weightlifting machines that pull downwards on the weights to create resistance, and a pair of treadmills that astronauts can strap themselves to in order to simulate a downward, gravity-like pull. Suni Williams once ran the Boston marathon on one of those treadmills, because she’s awesome that way. 

(Fun fact, one of the treadmills is called the Combined Operational Load-Bearing External Resistance Treadmill, aka COLBERT for comedian Stephen Colbert. NASA held a contest to name a new ISS node and Colbert got all of his fans to write in his name. He won, but NASA had reservations about naming their new station node after a person. They named the module Tranquility and compensated Colbert by naming the treadmill after him.)

So Many Other Things Because Biology is Weird

There’s a whole raft of other random issues that pop up in the human body when it spends a long time in space. For one thing, without the downward pull of gravity compressing the spine, astronauts grow an inch or two when spending a while in space. This reverses itself once gravity is back, which can be uncomfortable.

The fact that folks in microgravity don’t spend a lot of time putting pressure on the bottoms of their feet can lead to the loss of the rough skin we all get down there as a result of just walking around. This is no issue in space, but can mean the act of walking is painful for the feet for a while after the return home, until those callouses reappear (conversely, because astronauts often hold themselves in place by hooking their feet into loops on the walls of the ISS, they can develop callouses on top of their feet. These also go away once there’s a normal up and down again).

Sleep can be an issue for some astronauts. You can’t lie down in a bed in space—you kind of just strap yourself to a wall instead. Usually in designated sleep cubbies, but on rare occasions when things have been particularly crowded on the ISS space travelers have used any stretch of wall that’s out of the way. The inability to lie down, coupled with the fact that the inside of the ISS is never silent nor dark, has led to some astronauts having major issues with sleep. Adding to this, circadian rhythms get blown all to heck. Humans evolve these in response to one sunrise and one sunset every day. But someone in Earth orbit is seeing a sunrise and a sunset every 90 minutes. 

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Then there’s the big long-term concern: radiation. Folks on the ISS are not completely outside of the protection of Earth’s magnetic field (heck, folks on the Moon won’t be completely outside the protection of Earth’s magnetic field), but they are outside the layers of protection provided by Earth’s atmosphere. That generally doesn’t have effects while crew are still in space, but can have major longer-term implications, such as cataracts, increased cancer risk, and even the possibility of long-term effects on brain and immune systems functions.

That’s just from the normal increased radiation levels encountered in orbit. During a major solar flare, radiation goes up dramatically. At times, ISS crews have had to shelter in more heavily shielded parts of the station (mostly those crew cubbies and the galley) to keep themselves safe.

Going to Mars

There’s no way around it—the human body evolved to exist in Earth-level gravity with Earth-level temperatures and Earth-level pressures and Earth-level radiation. What I’ve outlined above is only a fraction of the issues facing human biology that decides spending long periods of time in space is a good idea. I didn’t even touch on things like the psychological challenges of long-duration spaceflight, which is a whole ‘nother barrel of monkeys.

But if we’re going to put folks in a tin can and spend a few years getting them to Mars and back, these are some of the obstacles that need to be solved. Mars is an even more hostile radiation environment than Earth orbit and you can’t just get shipments of fresh food from home.

That’s why places like the ISS are so vital to humanity’s continued presence in space. Where else can we figure stuff like this out? You can’t learn to run if you don’t first learn to walk, and walking starts with those first baby steps. That’s where we are now. Hopefully we’ll learn how to run eventually, and we’ll leave some red Martian dust in our footprints behind when we do.