In space, your body is like a soda can

The absence of air pressure in space exposes the fragility of human bodies.
An astronaut in a white spacesuit with Earth in the background.
An astronaut floats in space in a spacesuit. NASA

Excerpted from A City on Mars: Can We Settle Space, Should We Settle Space, and Have We Really Thought This Through? by Kelly and Zach Weinersmith. Copyright © 2023. Available from Penguin Press, an imprint of Penguin Publishing Group, a division of Penguin Random House.

On Earth, air pushes on your skin from every direction with a consistent pressure of about 14 pounds per square inch, or using ridiculous non-American units, 1 atmosphere. That’s about the weight of 1 liter of water on every square centimeter of your skin. You don’t notice this for the same reason a seabottom shrimp doesn’t notice that the surrounding liquid could implode a submarine—your body is adapted to the pressure near Earth’s surface. It counterbalances the typical push of your surroundings, and you only rarely experience sudden pressure changes. 

But consider a soda. When you buy a sealed bottle of Diet Pepsi, you know it’s full of gas, but you don’t see a lot of bubbles. That’s because the bottle is held at about four times the surface air pressure of Earth, keeping carbon dioxide suspended sedately inside. When you open the top, you expose its contents to Earth’s relatively gentle atmosphere. All that dissolved gas rushes out in the familiar bubbling foam. If you want to avoid the sudden burst of gas, you can always open your bottle forty meters under the sea, where the pressure will keep the gas in place, and the seawater will make the Diet Pepsi taste no worse. 

Your body is like the soda, except that the gas suspended in your fluids is nitrogen, absorbed from the atmosphere. If you were teleported to outer space, where the air pressure level is “none,” your bodily fluids would react like the Diet Pepsi when opened, only instead of a burst of foam, you’d get nitrogen bubbles blocking your veins and arteries, preventing the normal flow of blood, oxygen, and nutrients. This danger is familiar to divers going from low depths back to the surface. If you switch from high to low pressure too quickly, you get “decompression sickness,” colloquially known as “the bends” because it often affects joints, causing the sufferer to bend in agony. If it’s in your lungs, that’s “the chokes.” If it’s in your brain, you’ve got “the staggers.” 

If you’re exposed to space, most likely you’ll just have the death. In fact, the only people who’ve ever died in space were killed by sudden loss of pressure. It was June 30, 1971, and cosmonauts Georgy Dobrovolsky, Vik tor Patsayev, and Vladislav Volkov were returning from Salyut-1. The three cosmonauts spent weeks performing zero gravity acrobatics, televised for the adoring Soviet public. They entered the capsule, and after some brief issues getting the hatch to seal, undocked and began their descent. When the ground crew arrived and the capsule was opened, the men were found, still seated, serene in death. Attempts to revive them proved useless—each had suffered massive brain hemorrhaging. Subsequent investigation determined that when they undocked from their space station, a valve on the return craft had unexpectedly popped open, exposing them to a near-perfect vacuum. 

Decompression sickness isn’t just a danger during accidents; it’s an issue any time you use a pressure suit. You may imagine a space suit as something like bulky clothing, but normal clothing doesn’t have to provide a sealed habitat inside itself. It’d be more accurate to imagine a leather balloon that happens to be shaped like a human. And like a balloon, the higher the internal pressure, the harder it is to bend. In a human-shaped balloon, high pressure means difficulty bending at the joints. Like, a lot of difficulty. A phenomenon called “fingernail delamination” is well documented, and we encourage you not to learn what it is. Thus, although the International Space Station is kept at Earth pressure, both American and Russian space suits only have around one third of that. 

So, why don’t astronauts get bendy, choky, staggery, and deathy when they don space suits? Because they prebreathe pure oxygen before spacewalks, removing most of the nitrogen from their blood. No nitrogen, no nitrogen bubbles. Movies may have led you to believe heroic astronauts can slip on a space suit and leap to the rescue, but under current designs this would result in Brad Pitt clutching his joints and shambling to a very painful (if handsome) death. 

The astute nerd will ask why not just keep the ISS at the same low pressure as the suit. The short answer is that although humans can survive in low pressure as long as there’s enough oxygen floating around, engineers would have to design all equipment to operate in a low-pressure, pure-oxygen environment. 

But pure oxygen is dangerous. In 1967, during prep for the Apollo 1 flight, a spark went off in the crew’s capsule, causing an intense fire in the pure oxygen environment. The three astronauts—Edward White II, Roger Chaffee, and Gus Grissom—could not be rescued, because the sudden increase in temperature and pressure made it impossible to use the inward-opening hatch, while the intense heat prevented rescuers from saving them. 

Less well known is a similar and earlier incident from the Soviet Union. In early 1961, Valentin Bondarenko was training to be a cosmonaut, and one of the training exercises was to spend ten days in a high-oxygen pressurized chamber. Near the end of confinement, he removed a medical sensor from his body and wiped the sticky glue from the sensor off with an alcohol swab. He absent-mindedly threw it aside, where it landed on an electric hot plate. The resulting fire quickly got out of control, consuming his suit. Oxygen had to be bled out of the chamber before rescuers could reach him, and he died of shock soon after. This happened just a month before Gagarin became the first human to reach outer space. The Soviets preferred to keep their failures a secret, and so when the Apollo 15 astronauts left a plaque on the Moon with the names of astronauts and cosmonauts who lost their lives in the race for the Moon, Bondarenko was not included. His story was only finally shared a quarter century after his passing. 

Buy A City on Mars by Kelly and Zach Weinersmith here.