Where does space begin?

...and where does the atmosphere end?

We have the impression that space begins beyond the terrestrial atmosphere but that is not a good definition because the more the altitude increases and the less the atmosphere is dense, without really disappearing, it gradually becomes scarce.
The atmosphere begins at the Earth's surface (troposphere 0 km to 20 km altitude) and continues at extremely high altitudes (exosphere 1000 km to 50,000 km altitude).
From data collected by the Soho mission, a Franco-Russian team calculated that the atmosphere extended up to 630,000 km beyond the Earth (1.5 times farther than the Moon).
The atmosphere is diluted in space until it forms a scattered cloud extremely held in hydrogen atoms (10 to 70 atoms per cm³) called « geocrown ».
At the level of the Moon, there are only 200 atoms per cubic decimetre, in other words almost empty.

Image: a Franco-Russian team calculated that the atmosphere extended up to 630,000 km beyond the Earth (almost twice as far as the Moon).

Kármán Line

On July 11, 2021, space tourism began!
Richard Branson and his SpaceShipTwo (SS2) VSS Unity rocket, has more than 600 customers willing to pay $ 250,000 to visit space.
On July 20, 2021, a place at 28 million dollars was auctioned to accompany Jeff Bezos in space (1st space tourism flight of the company Blue Origin).
But where does space begin?
Space is located at the limit between aeronautical flight (planes) and astronautical flight (satellites). This boundary is called the Kármán line (Theodore von Kármán, Hungarian engineer and physicist 1881-1963).
This imaginary boundary was calculated 100 km above the surface of the Earth. It corresponds to the altitude from which the atmosphere becomes too thin for aeronautics. An airplane can only stay in flight by being carried by the surrounding air, its airfoil providing lift. The higher a plane flies, the less lift the thin air provides, which requires increasing speed to maintain lift and compensate for the decrease in air density as it rises in altitude.

The question which then arises is: at what speed must one move in the atmosphere to be supported by the aerodynamic thrust force?
The plane must reach orbital speed.
The Kármán line marks the altitude where the required flight speed equals the orbital speed.
To place a satellite in orbit, it is necessary to give it a minimum speed so that it can circle it completely. This speed should approach 7.9 km / s (28,440 km / h). So that the satellite is not slowed down in its course by atmospheric friction, it must be positioned above the atmosphere, ie at least 100 km above the Earth.
The orbital speed varies according to the height of the Earth's orbit:
- For the International Space Station, the orbital speed is around 7.5 km / s.
- In geostationary orbit around the Earth (35  786 km above the Earth's geoid) the orbital speed is 3 km / s.
- The Moon orbits at 1.052 km / s.
In 2021, space tourism takes passengers slightly below 100 km, so these are suborbital flights.

Image: The altitude of 120 km marks the border where the atmospheric effects become noticeable for an object when it re-enters the atmosphere.