spaceSpace and Physics

The Strangest Things That Happen To Your Body While In Space

So you want to go to space? Enjoy your fingernails dropping off and radiation shooting through your eyeballs.

James Felton

James Felton

James Felton

James Felton

Senior Staff Writer

James is a published author with four pop-history and science books to his name. He specializes in history, strange science, and anything out of the ordinary.

Senior Staff Writer

Astronauts on a space walk outside the ISS.

Astronauts on a space walk outside the ISS.

Image credit: NASA

Going to space is, let's face it, one of the coolest things most of us will never get to do. But while you're in space, your body is undergoing some other strange effects, which might make you feel a little better about not being able to go there.

Your fingernails can fall off

In the vacuum of space, the pressure around you is so low that the boiling point of the fluids inside your body decreases to below that of your body temperature, and gas bubbles will begin to form inside you. 


"Some degree of consciousness will probably be retained for 9 to 11 seconds. In rapid sequence thereafter, paralysis will be followed by generalized convulsions and paralysis once again," NASA's Bioastronautics Data Book explains.

"During this time, water vapor will form rapidly in the soft tissues and somewhat less rapidly in the venous blood. This evolution of water vapor will cause marked swelling of the body to perhaps twice its normal volume unless it is restrained by a pressure suit."

Your blood would stop circulating, while gas and water vapor would flow slowly out of your airways, and the resulting moisture evaporation would cause your mouth and nose to freeze slightly before the rest of you. 

As such, the International Space Station (ISS), other space stations, and spacecraft are all artificially pressurized for advantages such as "allowing you to breathe" and so that you don't "die in the vacuum of space". When astronauts go on spacewalks, their suits are also pressurized to keep them safe and healthy. But there are disadvantages, one being that astronauts tend to develop injuries including onycholysis, where the fingernail separates from the nail bed.


"Injuries to the hands are common among astronauts who train for extravehicular activity," a NASA study explains. "When the gloves are pressurized, they restrict movement and create pressure points during tasks, sometimes resulting in pain, muscle fatigue, abrasions, and occasionally more severe injuries such as onycholysis. Glove injuries, both anecdotal and recorded, have been reported during EVA training and flight persistently through NASA's history regardless of mission or glove model."

Cosmic ray visual phenomena

Dating back to the Apollo program, many astronauts have reported mysterious bright flashes of light in their eyes. One survey of 59 astronauts found that 47 had seen these flashes while on spaceflights, which were most often noticed before sleep, sometimes disturbing it.

"The light flashes predominantly appear white, have elongated shapes, and most interestingly, often come with a sense of motion," the study explains. "The motion is described as sideways, diagonal, or in-out, but never in the vertical direction."

The flashes have long been believed to be caused by cosmic rays passing through astronauts' eyeballs, though the exact mechanism is still unclear, even after NASA conducted tests asking astronauts to wear an Apollo light flash moving emulsion detector (ALFMED) during the Apollo 16 and 17 missions.


The ALFMED can be seen in footage from Apollo 16.

"Flashes could be seen with the eyes open or closed when the spacecraft was dark. They discovered that it was not necessary to be dark adapted to see the flashes," NASA wrote following the experiment. "This indicates that Cerenkov radiation from energetic cosmic rays traversing the eyeball, which had been the most widely accepted explanation for the light flashes, probably did not cause all or most of the flashes because light from this source is quite faint. Some of the flashes observed in space may be caused by direct ionization interactions of cosmic rays with the retina."

You get baby feet

Without the constant hassle of having to walk around, your feet get a well-earned break, but the downsides include dodging callouses floating around in your living quarters.


"The calluses on your feet in space will eventually fall off," astronaut Scott Kelly explained in a Reddit AMA. "So, the bottoms of your feet become very soft like newborn baby feet. But the top of my feet develop rough alligator skin because I use the top of my feet to get around here on space station when using foot rails."

Your face gets puffy as your fluids go everywhere

We have evolved to live in gravity, and leaving it has a number of effects on your body.

"While on Earth, gravity causes most of the body’s fluids to be distributed below the heart. In contrast, living in space with less gravity allows fluids in the body to spread equally throughout the body," NASA explains. "When astronauts first travel into space, they feel as if they have a cold and their faces look puffy." Fortunately, this only lasts a few days.

Your bones reabsorb into your body and your muscle mass reduces

Going to space and experiencing weightlessness from the freefall of the ISS for prolonged periods of time can leave your body with reduced muscle mass and bone tissue upon your return. The reason is pretty simple – your bones and muscles don't have to support your weight in weightless environments. 


"Weightlessness is cool," astronaut Chris Hadfield explained in a video for the Canadian Space Agency. "But it doesn't come for free. Without constant load on your body, you can get incredibly lazy. Your muscles will start dissolving. Your bones will start getting reabsorbed back into your body."

To try and minimize this and remain strong enough to walk when they return to Earth's gravity, astronauts have to do space exercises, including being strapped to a space treadmill.

The effects don't last too long. Returning to Earth, astronauts just need to hit the gym for some time in order to regain their lost muscle mass and get back to their previous physical condition.

You need to pee a lot

As the fluids shift around, many astronauts report not feeling thirsty.

"The body records this shift as an increase in blood volume," NASA explains. "The body takes care of this fluid shift by eliminating what it thinks are extra fluids as it would normally – that’s right – through the kidneys – resulting in visits to the restroom. Once this 'extra fluid' is flushed from the body, astronauts adjust to space and usually feel fine."


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