Even if it seems a simple question, the height at which space begins isn’t clear, and the definitions of the frontier between the atmosphere and space can variate depending on the source. Indeed, there isn’t a specific point where the atmosphere finishes, but the latter gets thinner and thinner, until arriving at a point where it disappears.
The most accepted definition
The most accepted definition for the “beginning of space” is the one that situates it at 100 kilometres (62 miles) over sea level. This height is known as the Kármán line, in honour of the Hungarian physicist and engineer Theodore von Kármán.
This physicist determined that aeronautics would no longer work at this altitude. In other words, the Kármán line is the height at which an aircraft can no longer fly, as the air density is too low for enough lift, necessary to fly, to be generated by a plane.
The Fédération Aéronautique Internationale (FAI), the international body that sets standards and keeps records in the fields of aeronautics and astronautics, situates the beginning of space at a high of 100 kilometres, keeping loyal to the Kármán line.
Some alternative proposals
Despite all this, the 100 kilometres of the Kármán line are, in a certain way, arbitrary, and are not accepted by everybody. For example, the FFA (Federal Aviation Authority), from the United States, affirms that space starts at 80 kilometres above sea level, and not at the Kármán line. In this way, it awards the “astronaut” status to those this height of 80 kilometres (or 50 miles). This astronaut status applies to both NASA’s employees and the military personnel that gets to this altitude.
There are even other criteria regarding the beginning of space, including the speed at which ions in the atmosphere move. Other people consider that space begins when a spacecraft is in orbit around the Earth…
In conclusion, there isn’t a fixed point at which we consider that space begins. Every definition for the start of space depends on the processes in the terrestrial atmosphere that we judge important: the point where a plane can no longer fly, arrive at the Earth’s orbit, etc.