Where does the space station's oxygen come from?

If there is one thing in this world that is essential for survival but doesn't cost money, it must be oxygen.

Oxygen is everywhere on the surface of the earth, and it is colorless and odorless, so we often ignore its existence. However, this oxygen that we do not value is extremely difficult to find in space. Space is almost a vacuum, where there is not only no oxygen for humans to breathe, but also no other gases. In this case, how can humans who cannot stop breathing for a moment settle in space? The space station has been running for decades, why hasn't the oxygen in it been exhausted? A vacuum environment is very dangerous for humans, not only because it is impossible to breathe in a vacuum environment, but also because in a vacuum environment, a person's blood may boil and evaporate. Why? Because the boiling point of a liquid is closely related to the surrounding air pressure.

In an absolute vacuum environment, the boiling point of water is close to 0°C. Although space is not an absolute vacuum, the boiling point of water will not exceed 10°C, so it is obvious what will happen when the human body is exposed to a vacuum environment.

So if humans want to go to space, the first problem they have to solve is air. To go to space, you must use chemical rockets, and the payload of chemical rockets is very limited. Any increase in weight will lead to a significant increase in launch costs, so the earliest space probes carried pure oxygen. The air on the surface of the earth is composed of a variety of gases, including nitrogen, argon, carbon dioxide, and water vapor, etc., and people only need oxygen to breathe, so in order to save transportation space and reduce transportation weight, spacecraft have become a low-pressure pure oxygen environment.

A pure oxygen environment is extremely dangerous, so accidents are inevitable. In the 1960s, both the United States and the Soviet Union had experienced pure oxygen test chamber fires, and several astronauts died as a result, so the proportion of oxygen had to be reduced afterwards.

In the later International Space Station, the composition of the air is almost the same as that on the surface of the earth. So the question is, after the proportion of oxygen is reduced, the space and weight occupied will increase significantly. Is the air in the International Space Station transported from the earth? Of course not, if so, it would be too expensive. On average, it costs about $50,000 to transport one kilogram of materials into space, and a person needs about 600 liters of oxygen per day. Astronauts live in the space station for several months, and it is too expensive to rely on transportation to deliver oxygen.

In fact, the oxygen in the space station is mainly provided by electrolysis of water.

Electrolysis of water is a very simple chemical reaction that only requires electricity and water, and electricity can be provided by solar panels on the space station. The efficiency of electrolysis of water to produce oxygen is very high. After electrolysis, 1 liter of water can produce more than 600 liters of oxygen, which means that 1 liter of water can meet a person's daily oxygen needs. So the question is, where does the water needed for electrolysis come from? Water can be recycled. In addition to generating oxygen, electrolysis of water can also generate hydrogen, and human breathing produces carbon dioxide. Combining hydrogen with carbon dioxide can be reduced to water and methane again.

In the space station, water consumption is very small, because not only the process of electrolysis of water, but also other water uses, even drinking water can be recycled.

Can the water we drink be recycled? Yes. An American astronaut once said a famous saying that is still widely circulated among astronauts: "Today's coffee is tomorrow's coffee." In the space station, water resources are extremely precious, so even urine will be restored to drinkable water through a powerful filtration system. However, although the air circulation system and water circulation system in the space station are extremely powerful, they cannot be recycled 100%, so occasionally a supply ship is needed to transport some oxygen and water, and of course the food that the space station cannot be self-sufficient.

Astronauts will not stay in the space station forever. Sometimes they need to go out of the vehicle to perform tasks, and space travel is different from diving.

When diving, you only need an oxygen cylinder to provide the oxygen needed for breathing, but when going out of space, you not only need oxygen for breathing, but also need to fill oxygen to pressurize the space suit, because the human body cannot be exposed to a vacuum. So why is the space suit filled with oxygen instead of air? Because the low-pressure pure oxygen environment is currently the most ideal environment in the space suit. Astronauts go out of the cabin to perform tasks, and space is almost a vacuum. If the pressure in the space suit is too high, it will cause the space suit to over-expand, and the astronaut will become a balloon man and unable to move freely. Moreover, the environment inside the space suit is not as complicated as that in the space station, so there will be no danger caused by the pure oxygen environment.

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