Where does the sky end?
Look up. There - the sky. Astronomically speaking, the cosmos begins immediately after a thin layer of the atmosphere. It's easy to see, looking at the Earth from the orbital altitude. But how far is it to space? Where does what we call the sky end?
The point at which the Earth’s atmosphere ends and the cosmic vacuum begins determines cosmos. As you can easily guess, such a subjective definition leads to the fact that people are actively arguing about where exactly the cosmos begins.
The National Aeronautics Advisory Committee (NASA’s predecessor) derived the first official definition of space: space is where atmospheric pressure is less than one pound per square foot. 424 grams per 0.3 square meters.
At this altitude, planes stopped working - about 81 kilometers from the ground. Any test pilot or astronaut NASA, crossing this height, was rewarded with "wings" of the astronaut.
Soon after the appearance of this definition, the aerospace engineer Theodor von Karman estimated that at an altitude of more than 100 km the atmosphere should be so thin that the aircraft would have to move with the first space velocity to transport cargo. And this is from the field of cosmonautics. Later the World Air Sports Federation accepted this height as the «Karman line».
When Felix Baumgartner broke the record of the highest free fall in 2012, he jumped from a height of 39 kilometers, that is, not reaching even half way to space (as defined by NASA).
But the atmosphere of the Earth goes far into space.
Even during rotation at an altitude of more than 400 kilometers or more, the International Space Station requires constant acceleration - friction against the atmosphere slows down. Satellites in a higher orbit can fly for tens or hundreds of years without slowing down.
The outer atmosphere of the Earth, also known as the exosphere, extends to a height of 10,000 kilometers above the planet. It is quite difficult to call it an atmosphere, but there are many more gas particles than in interplanetary space.