Although there is no definite boundary where space starts, there is a limit, and if you crossed it, you would he entered space… at least, officially. This boundary is known as the Karman line (also spelled Kármán line), which is often referred to as the ‘edge of space’.
The Karman Line lies near the bottom of the Thermosphere (Photo Credit: Wikipedia.org) The Karman Line is an imaginary boundary that lies 100 kilometers above Earth and is widely considered the boundary between the outer space and Earth’s atmosphere. The line owes its name to Theodore von Karman – a Hungarian American physicist who was the first to determine that around this altitude, the atmosphere became too thin to support aeronautical flights. In simple terms, this means that at this altitude, an aircraft cannot operate using regular flight tech to generate aerodynamic lift and expect to stay airborne above this altitude (it would instead require rockets). This definition is accepted by the Fédération Aéronautique Internationale (FAI), an international body that sets standards for astronautics and aeronautics.
Note that the atmosphere still exists beyond the Karman line; in fact, tiny particles of the exosphere (the outermost layer of the atmosphere) can be found 10,000 kilometers above the surface of Earth! From that perspective, the International Space Station, which orbits at only 330-435 kilometers above the Earth, isn’t even worthy of its name.
So, it’s not that the atmosphere ends at the Karman line, it’s simply that at that altitude, it becomes too insignificant to support flights that rely on ‘air quality’. Here’s a fun fact: you don’t he to fly beyond the Karman line to officially be considered an ‘astronaut’; according to the US Air Force’s definition of an ‘astronaut’, all you he to do is fly more than 50 miles (80 km) above mean sea level to earn that title!