Where does Earth's atmosphere end and outer space begins?

When climbers climb Mount Everest, they usually have to carry oxygen cylinders, which are necessary equipment to allow them to breathe freely at high altitudes. The closer we get to the edge of Earth's atmosphere, the less oxygen is available, compared to the abundance of oxygen at sea level.

This is just one example of how Earth's atmosphere changes with altitude. From the troposphere near sea level to the outermost exosphere, the layers of Earth's atmosphere have different basic compositions. According to the National Weather Service, each layer of the atmosphere begins and ends with four key characteristics, namely temperature changes, chemical composition, density, and the movement of the gases inside.

So, given these 4 features, where exactly does Earth's atmosphere end? Where does outer space begin? Each layer of Earth's atmosphere performs its own role, including blocking cancer-causing cosmic radiation and providing the pressure needed to form liquid water, ensuring the blue planet can host a wide variety of life forms.

The farther from the Earth's surface, the less dense the atmosphere becomes, and its composition changes, with lighter atoms and molecules starting to dominate while heavier molecules remain near the Earth's surface. As you ascend in the atmosphere, the pressure or weight of the atmosphere above you rapidly diminishes. Although commercial aircraft have pressurized cabins, rapid changes in altitude can affect the slender Eustachian tubes that connect the ears, nose, and throat. That's why our ears may experience tinnitus when a plane takes off.

Eventually, the air becomes too thin for conventional planes to generate enough lift to fly. Here is the area delineated by scientists, marking the end of the atmosphere and the beginning of outer space. This boundary is called the Kármán line, named after the Hungarian-American engineer and physicist Theodore von Kármán. Carmen in 1957 became the first person to attempt to define the boundary between Earth and outer space.

Schematic diagram of the vertical structure of the earth's atmosphere, which can be roughly divided into troposphere, stratosphere, mesosphere, thermosphere, and exosphere

The Karman line marks the boundary between Earth and space, not only indicating the altitude limit of aviation flight but also critical to the successful orbit of spacecraft and satellites around the Earth. The Karman line is an approximate area above which a satellite will be able to orbit the Earth at least once without burning up or deorbiting.

This line is usually defined as about 100 kilometers above the Earth's surface as the line between the atmosphere and space (however, this definition is not accepted by all organizations, such as the United States Air Force and NASA that define the line between the atmosphere and space is 80 kilometers, and there is currently no provision for this boundary in international law). It is possible for an object to orbit the Earth at altitudes below the Karman line, but it would require extremely high orbital speeds, which would be difficult to maintain due to friction. Many people may feel this way about the Carmen line as a hypothetical, yet real, threshold between air travel and space travel.

For a satellite, a variety of factors, including its size and shape, will determine how much air resistance it encounters, and thus its ability to successfully orbit the Earth. Low-Earth orbit satellites gradually slow down due to drag in Earth's outermost atmosphere and usually deorbit after a few years of operation. According to the European Space Agency, the altitude of satellites in low-Earth orbit is usually below 1,000 kilometers, but sometimes as low as 160 kilometers.

However, this does not mean that Earth's atmosphere cannot be detected beyond 1,000 kilometers. In the region where artificial satellites operate, the atmosphere has not disappeared. It would take another tens of thousands of kilometers before the evidence of Earth's atmosphere would disappear. The outermost atoms of Earth's atmosphere, the hydrogen atoms that make up the geocorona (the luminous part of the exosphere), can extend even beyond the orbit of the moon (in other words, if we treat the exosphere as part of the atmosphere rather than outer space, That atmosphere can extend into the sky at an altitude of about 10,000 kilometers).

So, if we were able to go through the Karman line, would we observe anything? Will we realize that we are crossing the boundary between Earth and space? The answer is no, we will not observe any particular changes. In general, the stratification of the atmosphere is basically based on the physical properties of the atmosphere itself, but the Karman line is not a physical boundary in essence, but a relatively abrupt boundary defined based on some factors in the aerospace field. ; therefore, one does not notice the process of crossing this line, nor does it have any thickness. However, because the gases in the atmosphere scatter blue visible light more than other colors of visible light, a blue halo appears at the edge of the atmosphere; as altitude increases, the atmosphere becomes thinner and the blue halo also increases will fade away, leaving only the black space as the background.

As the plane flies higher, the air becomes thinner, and less and less lift is provided. In order to keep flying in the air, planes need higher and higher speeds. According to this trend, the speed required to ensure that the aircraft can fly in the air will reach the orbital speed of that altitude at a certain altitude. Above this altitude, aerodynamics fail and space comes into play. In fact, the calculated result of the height of the Karmen line is not exactly 100 kilometers, but Carmen still recommends 100 kilometers above sea level as the boundary between outer space and the earth's atmosphere. On the other hand, the atmosphere is also changing all the time, and various parameters also vary from time to time, and the calculation results are not constant. Therefore, although the value is 100 kilometers, the Karmen line is not a strict definition with absolute precision. It was only after it was adopted by the International Aviation Federation that the Karmen line became a widely accepted boundary in various related fields.

If someone is able to get to the Carmen line, even for a short period of time, is it possible to survive? What if you're near the Carmen line but don't have a custom spacesuit or oxygen tank for mountaineering? Can we breathe at such a high altitude? Can birds reach such heights?

Scientists say it is possible in principle for birds to fly all the way to the Carmen line, but in reality, animals cannot survive above the "Armstrong limit". The Armstrong limit is at an altitude of about 20 kilometers above the ground, where the pressure is so low that it causes the fluid in the lungs to boil.

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