To the layman, the idea that the Earth has an edge is easy to accept: there have even been pictures from space when viewed just right, one can see the edge of the Earth’s atmosphere against the black emptiness of the cosmic void.
Most people just assume that there is a border: air below, vacuum above; cross the border, and you are in space.
The fact is, as is often the case, more complex. The Earth’s atmosphere gradually thins out, and even then, outer space is not a perfect vacuum. So, how do you define the border between a shell of gas that gradually thins out to a vacuum that is imperfect, and itself has molecules and particles in it.
Different scientists and governments define things differently. For example, as far as aeronautical record keeping is concerned, and human traveling higher than 50 miles is considered as having traveled into space. This definition is important politically, as you can’t stop a space satellite from crossing your border, but you are free to shoot down any aircraft which isn’t in space and crosses your border (hence, the United States government has never really defined where space starts and ends to avoid problems with spy satellites).
To make matters more difficult, the altitude where scientists generally put the border is in that region where balloons and aircraft really can’t travel because it is too high, but it is generally too low for satellites to take measurements. The only way to make measurements is to fire a rocket that arcs through the region and takes a few minutes of measurements while it happens to be moving through.
The Canadian Space Agency (yes, they have one!) recently launched a rocket with an instrument payload designed by the University of Calgary.
The instruments were looking for the altitude at which the pressure of the Earth’s atmosphere is balanced by the pressure of incoming particles from space. This essentially defines the edge of the atmosphere. Ironically, this is almost the exact same place which defines the edge of the solar system (where the outward pressure of the solar wind matches the inward pressure of particles from the interstellar void).
While the measurements were made about two years ago, the results were just recently published, and it seems that the answer is: 118 km above the Earth is where space begins (or about 73-and-a-third miles).
Why spend all of that money on finding the edge of the atmosphere? One of the most important areas of stud today is understanding the relationship between solar energy entering our planet and its effect on the atmosphere in everything from understanding global warming to hurricane formation and the changes in weather. Part of that understanding is knowing exactly how much energy penetrates the Earth’s atmosphere, and knowing that requires a knowledge of the size of the atmosphere.
I forget who said it, but someone once said, that space isn’t distant at all ….. about an hour’s drive, if you could drive straight up. If you could drive 75 miles-per-hour, you could actually make it in under an hour!