Where Does Space Begin?

The edge of space does not have a clear boundary. Rather, space begins in the same gradual way that the atmosphere ends. Though, the generally accepted defense of the start of space is the karman line, which is located 62 miles above the Earth's surface. This is where the effects of the atmosphere no longer are the dominant natural force guiding the path of an aircraft (e.g. the effect of “lift” generated by wings is effectively absent). Atmospheric drag is also minimal to nonexistent.

Instead, astrodynamics prevail as the dominant natural force guiding flight. This is how objects orbit the Earth. As an object “falls”, the curve of the Earth prevents it from reaching the ground. This keeps it perpetually falling, and thus, it is perpetually in motion at a high velocity.

Thus, a balance is maintained between the objects's fall and the curve of the Earth. However, in order to orbit the Earth, it must reach a velocity of 17,500mph when it is launched.

However, a stable orbit is not possible at an altitude of 62 miles. Atmospheric drag is still present, and this causes the orbit to rapidly degrade. Thus, a complete circle around the Earth would not occur. A complete orbit is possible at altitudes of around 80 miles. Though, satellites typically orbit significantly higher at altitudes of 160-1,200 miles. 

However, the effects of the atmosphere lessen dramatically long before the karman line. Approximately 50% of the atmosphere is located below an altitude of about 18,000 feet (about 3.5 miles). The daytime sky is black at around 60,000 feet (about 12 miles), and the saliva would boil right off of your tongue at 63,000 feet (about 12.5 miles). 

Approximately 99% of the atmosphere is located below 100,000 feet (about 20 miles). The ozone layer ends at around 150,000 feet (about 30 miles). Altitudes above this are exposed to sterilizing uvc radiation, which doesn't reach the ground.

Where are the Most Distant Air Molecules?

Incredibly, there are exceedingly rare particles originating from Earth's atmosphere at the distance of the moon. However, they are far too sparse to interact with one another or have any influence whatsoever on materials, spacecraft or life forms. Rather, it is simply that a minute concentration of hydrogen atoms are perpetually in orbit around the Earth at these vast distances.

Due to rare collisions between atmospheric atoms in orbit around the Earth and the effects of the solar wind, atoms from the atmosphere can be occasionally launched into higher orbits. While it is exceedingly rare, they can reach the distance of the moon's orbit and beyond.

What About Earth's Magnetic Field? 

The magnetic field of the Earth shields it from hazardous radiation and plasma from the sun. This extends to a distance of about 40,000 miles on the daytime side of the planet, but it forms a large “tail” on the nighttime side. It can reach hundreds of thousands of miles from Earth on this side. If you travel beyond the Earth's magnetic field, you are fully exposed to the solar wind.

Where Does Interstellar Space Begin?

Interstellar space begins at the heliopause, which is where the effects of the solar wind end. The solar wind protects the planets from the effects of interstellar radiation, which originates from magnetars, neutron stars and distant supernovas.  This is located at about 120 astronomical units. Three space probes have traveled beyond this distance. 

However, the sun's gravity reaches much further. Objects can be in a stable orbit around the sun at distances of approximately 1.5 light years. Though, this is far beyond the edge of the solar wind.