Where does the Earth end and space begin?

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Many people want to travel in space one day. But actually, when can we say we are in space? Why are we not in space when we travel by plane? The answers are simple: you just need to know where the Earth ends and space begins.

A boundary that lies between the layers of the Earth’s atmosphere

When you look up at the sky, it feels like space is infinite, and it probably is. On the other hand, one must know that there is a clear distinction between the sky in our atmosphere and the infinite space. So the question is, where does the Earth end and where does space begin? To answer this question, one must first understand the structure of the atmosphere. First, the atmosphere is a stratified structure that provides our planet with sufficient temperature, density, and exposure to sunlight for life on Earth to exist.

The atmosphere is divided into five layers: the troposphere, the stratosphere, the mesosphere, the thermosphere, and the exosphere. Each of these layers plays a role in making the Earth a habitable planet. There are boundaries between each of these layers and they are defined by four key parameters: temperature change, chemical composition, density, and gas motion. And between the mesosphere and the thermosphere is a line that officially serves as the boundary between Earth and space.

This boundary, which marks the beginning of space, is called the Karman line. Named after the Hungarian physicist Theodore von Karman, the Karman line is a crucial element in aviation and astronomy. In fact, the Karman line is mandatory when considering the altitude at which an aircraft can fly. Engineers and scientists are also considering the Karman line when deciding how to keep satellites and spacecraft in orbit around the Earth.

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A more or less well-defined boundary between space and Earth

The Karman line is defined as the approximate height at which orbital dynamic forces are more critical than aerodynamic forces. In other words, it is the altitude from which the air tightness becomes so low that the atmosphere alone can no longer support spacecraft at suborbital speeds. Theodore von Karman estimated that this famous line is about 80 kilometers above sea level. As science progressed, scientists finally agreed that this limit is higher, about 100 kilometers above sea level.

In any case, the border used by space and aviation agencies is generally 80 kilometers, although this is the source of many disagreements. If there is so much debate on the subject, it is because the atmosphere does not disappear but thins out with height. This means that part of the atmosphere is in space. In addition, the Earth’s atmosphere is estimated to extend about 1,000 kilometers, which is well beyond the Karman line. Confusion is thus allowed to the extent that space is defined as the absence of atmosphere.

This confusion is problematic because international laws and treaties define space as a region that is free for anyone to explore and use. On the other hand, this is not at all the case with regard to the airspace over the various nations. However, international law does not provide the exact limit of the altitude at which this airspace ends. This lack of universal definition of the boundary between space and Earth can thus lead to the intrusion into sovereign airspace; an action that can be misunderstood as aggression.

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