Where is the edge of space?

Where is the edge of space and what is the Karman line?

credit: National Geographic

Definition where the space starts is not completely clear. 

Satellite in space collides with billions of air molecules from the atmosphere. The frequency and intensity of these collisions increases with increasing velocity. At orbital velocity, the energy from those collisions with air molecules produces heat that would melt even metal. It is known as the friction heating effect. 

All orbiting satellites must operate in space in order to avoid the frictional heating effect. It is difficult to estimate. Traces of the gases which we are breathing extends beyond 161 km above the Earth's surface. Above this altitude it is safe for satellite to move at orbital speed without the danger of incineration from the frictional heating. 

Many other definitions exist where the space really begins. 

Generally, it is told that space starts at the point where orbital dynamic forces become more important than aerodynamic forces, or where the atmosphere alone is not enough to support a flying vessel at suborbital speeds.

Karman line is a proposed boundary separating Earth's atmosphere and outer space. The line is not sharp boundary, rather encircle Earth at an altitude between 80 to 100 km above mean sea level. 

The Karman line is named after Theodore von Kármán (1881–1963), who was a Hungarian-American engineer and physicist active in aeronautics and astronautics. In 1957, he was the first person to attempt to calculate a theoretical limit of altitude for airplane flight. His rough calculations came close to 84 km. 

In early 1960s Federation Aeronautique Internationale (FAI) adopted a 100 km as a boundary. Another often accepted limit is 80 km, preferred for example by NASA. 

Common estimates of Karman line's altitude place it between layers of the atmosphere. The atmosphere gradually thins out with the distance from the surface. Atmosphere is divided into six major layers. Those layers fade into each other, therefore it is difficult to say where exactly is the Karman line. As air density fluctuates over time, so does the Karman line. 

Karman line ranges from lower thermosphere to the transition between the thermosphere and the upper mesosphere underneath it. 

While the Karman line is defined only for Earth, if it was calculated for Mars and Venus, it would be about 80 km and 250 km high, respectively.

You can imagine the line as a boundary of 80 ±10 km, where a craft above that will continue orbiting the Earth, whereas below that will descent to the Earth's surface. 

The lack of air prevents airplanes to fly to space. Planes use a phenomenon called lift. Another reason is that both jet and propeller plane engines require oxygen from the atmosphere to burn the fuel. 

Flight in the space is possible due to the orbits. Orbits allow a satellite generally to circle the Earth without continuously needing the fuel. Once the satellite achieves the orbital speed, gravity keeps it moving around the Earth.