Where does the atmosphere end? Where is the edge of space?

Category: Earth Science      Published: June 6, 2024

By: Christopher S. Baird, author of The Top 50 Science Questions with Surprising Answers and Associate Professor of Physics at West Texas A&M University

Public Domain Image, source: NASA.

Earth's atmosphere does not have a well-defined edge. The air that makes up the atmosphere generally gets thinner the higher that you go, until it blends into the interplanetary medium. As a result, there is no clearly defined edge of space. The air molecules that make up the atmosphere are held near the earth by earth's gravity. That's why heavier molecules are more abundant in the lower regions of the atmosphere. Gravity also makes the lower layers of the atmosphere contain more molecules and atoms per cubic meter than the higher layers.

Why don't all of the molecules that make up the atmosphere simply fall down to earth's surface? If the molecules of the atmosphere were cold enough (and therefore slow enough), they would indeed fall to earth's surface and stay there, after having condensed to a liquid or frozen to a solid. However, having a high enough temperature to be a gas means having high enough speeds to bounce off the ground and bounce of each other, sending these molecules upwards. As part of the normal state of affairs when molecules are in a gas state, the molecules are constantly colliding into each other. During some of these collisions, one molecule gains a lot of speed and the other loses a lot of speed. As a result, some of the molecules in a gas always have much higher speeds than the average. In the atmosphere, these molecules have enough speed to bounce high above earth's surface, despite earth's downward gravitational pull. Also, some molecules experience a series of collisions with other molecules that are just right that they keep bouncing these molecules higher.

The upper layers of the atmosphere therefore reach high above the surface of the earth, despite gravity's downward pull, because of this collision process. In other words, the upper atmospheric layers consist of the lightest molecules that have gained the most speed and been bounced the highest during collisions. In general, molecular collisions in a gas tend to push molecules from regions of high density out toward regions of lower density. This process is called diffusion. It is the same reason that a gas tends to expand to fill its container. For the air in the atmosphere, the container is earth's gravity well. Diffusion tends to push molecules up away from earth's surface while gravity tends to pull these molecules down toward earth's surface. The result is that the atmosphere is not collapsed to earth's surface but also is not infinitely spread out. Rather, it is spread out over a certain range of heights.

Another interesting complicating factor is that earth's gravity weakens gradually as you get farther away from the earth, i.e. higher up in the atmosphere. As a result, the upper region of the atmosphere is able to spread upwards even more. The very highest region of the atmosphere—the upper exosphere—very gradually gets less and less dense as you go higher up away from the earth, gradually blending in with the interplanetary medium. Additionally, some of the hydrogen atoms that are in the upper exosphere gain enough speed from a collision to permanently escape earth's gravitational well. This means that the uppermost portion of earth's atmosphere is continuously, slowly leaking atoms into space.

For all of these reasons, it is not meaningful to say that the atmosphere has a physical edge at a particular location. For practical purposes, we can arbitrarily define an effective edge of the atmosphere based on what property of the atmosphere that we are interested in. We just have to be careful to not think that an effective edge of the atmosphere is a literal edge beyond which there exists zero atmosphere. The table below lists various useful choices for the effective edge of the atmosphere. All of the altitude values mentioned below are the height above mean sea level.

Maximum Service Ceiling

From the viewpoint of human activity, one of the most important things that we do in the atmosphere is travel in airplanes. For this reason, we can define an effective edge of the atmosphere as the point above which we can't fly airplanes because the air is too thin to provide enough lift and to provide sufficient air to burn the fuel in jet engines. However, even with this definition in mind, there is not a clearly defined edge because high-speed airplanes can generate sufficient lift from very low density air and therefore can fly higher up in the atmosphere than regular airplanes.

Most commercial aircraft are certified to fly no higher than 13 km. For a particular airplane, the highest that it can fly while still operating normally is called the "service ceiling". Therefore, the altitude of 13 km can be thought of as the edge of the atmosphere in the sense of being the maximum service ceiling of commercial airplanes (with a few exceptions). This effective edge of the atmosphere is within the upper part of the troposphere or the lower part of the stratosphere, depending on the situation. With that said, there are many other interesting things happening in the atmosphere above 13 km, so this would be a poor definition for the edge of the atmosphere for most other applications.

Armstrong Limit

Another way to define an effective edge of the atmosphere is the altitude above which humans cannot survive while being openly exposed to the air, even with an oxygen supply. This is called the "Armstrong Limit" and it is at an altitude of 19 km. At this altitude, the air pressure of the atmosphere becomes so low that water boils at the normal temperature of a human body. This means that humans openly exposed to the atmosphere at this altitude and higher will experience their bodily fluids boiling away and will die within a minute, even if they have an oxygen mask on. Therefore, from the perspective of the limitations of the human body, we can think of the effective edge of the atmosphere as existing at an altitude of 19 km. This effective edge of the atmosphere is in the lower stratosphere. However, if you pressurize the aircraft cabin or have the crew where pressure suits, you can safely fly manned airplanes higher than this.

Jet Airplane Limit

We can instead consider all airplanes (not including rocket planes) instead of just commercial airplanes and unpressurized airplanes. The highest altitude ever achieved by an air-breathing jet-propelled aircraft is 38 km. Therefore, from the perspective of all airplanes except for rocket planes, the altitude of about 38 km can be thought of as the effective edge of the atmosphere. This effective edge of the atmosphere is in the middle of the stratosphere.

Balloon Limit

Although the atmosphere above 38 km is too thin to supply sufficient air to a jet, we can overcome this problem by using a balloon, which simply floats up to high altitudes because of its low density. The highest altitude ever achieved by a balloon is about 54 km. The balloons that reach high altitudes are specially designed scientific research balloons. Therefore, from the perspective of balloons, we can think of the effective edge of the atmosphere as existing at an altitude of about 54 km. This effective edge of the atmosphere is in the lower mesosphere.

Effective Karman Line

To send a manmade object higher than 54 km, you have to use rockets or cannons. Once you use rocket propulsion, there is theoretically no limit to how far away you can travel from the earth. This is because rockets do not use the ambient air to generate lift or burn fuel. Using rockets enables you to send spacecraft to other planets and beyond. With that said, often the purpose of sending a rocket up through the atmosphere is not to travel to other planets, but to place a satellite in orbit around the earth. We can therefore define effective edges of the atmosphere based on satellite orbits.

The lowest that most satellites can reach and still experience stable orbits is at an altitude of about 80 km. Satellites can only reach this low and still successfully stay in orbit if they have elliptical orbits. Traveling along an elliptical orbit enables a satellite to experience major atmospheric drag for only a short fraction of its orbit, when it is closest to the earth. This part of the orbit is called "perigee". Below about 80 km, the air is so dense that the air resistance on the satellite is too strong to allow it to stay in orbit (although, in some special circumstances, this threshold can drop down to 70 km). From this perspective, we can say that the effective edge of the atmosphere is at an altitude of 80 km, which we can call the "lowest sustainable perigee" or the "Effective Karman Line". You can also think of this as the point at which the velocity required by a rocket plane to sustain flight using lift from wings is higher than orbital velocity. In other words, it is the dividing line between rocket-plane winged flight and spaceflight. With that said, depending on the state of the atmosphere, latitude, and aerodynamics, the effective Karman Line can vary between 70 km and 90 km. This effective edge of the atmosphere is at the mesopause, which is the line dividing the mesosphere and the thermosphere. The U.S. military, NASA, and FAA require a person to travel above 80 km in order to be awarded an astronaut badge. In the context of human flight, the United States legally considers the altitude of 80 km to be the edge of space.

Karman Line

International law defines the edge of the atmosphere, for legal purposes, to be at an altitude of 100 km. This means that, internationally, people become astronauts when they fly above an altitude of 100 km. The altitude of 100 km is called the Karman Line and is in the lower thermosphere. The reasons for choosing this altitude as the effective edge of space are historically convoluted. Theodore von Karman himself apparently never chose exactly 100 km as a good choice for the effective edge of the atmosphere. It was originally thought by some that the altitude of 100 km was the physically meaningful dividing line between rocket-plane winged flight and spaceflight, and the point below which satellites in elliptical orbits cannot maintain stable orbits. However, more recent research found that this physically meaningful dividing line is actually at an altitude of about 80 km. Therefore, in view of our modern scientific understanding, the internationally accepted Karman Line at 100 km is arbitrary and not tied to any meaningful physical parameter. The Effective Karman Line at 80 km is the physically meaningful dividing line between rocket-plane winged flight and spaceflight. It would be more accurate to call 100 km the "Historical Karman Line" and 80 km the "Physical Karman Line".

Circular Orbit Limit

If we instead focus on satellites in stable circular orbits rather than in elliptical orbits, we get a more broadly useful choice for the edge of space. The lowest that a satellite can be and still maintain a stable circular orbit around the earth without propulsion is at an altitude of about 125 km. Below this altitude, the atmosphere is thick enough to slow down satellites so much that they are unable to maintain a circular orbit without propulsion. Therefore, from the perspective of satellites in stable circular orbits, we can think of the edge of the atmosphere as being at an altitude of 125 km. This effective edge of the atmosphere is in the lower thermosphere.

Thermopause

We can define the edge of the atmosphere as the point at which the atmosphere is so thin that barometric conditions no longer apply and the atoms no longer act like a gas. Above this altitude, in the exosphere, the hydrogen atoms are so widely dispersed that they effectively never collide with each other. This demarcation line is called the thermopause. It is the dividing line between the thermosphere and the exosphere. Above the thermopause, the hydrogen atoms are so spread out that they move along independent ballistic trajectories rather than acting collectively. Depending on solar activity, the thermopause is at an altitude of between 500 and 1000 km. Note that the International Space Station and many satellites orbit below this altitude.

Interplanetary Medium Begins

Lastly, we can define an effective edge of the atmosphere as the point where the force from sunlight striking a hydrogen atom is stronger than earth's gravitational force on that atom. As a result, hydrogen atoms above this point become permanently knocked free of earth's gravity by sunlight. From this perspective, the effective edge of the atmosphere is at an altitude of about 200,000 km, which is about halfway to the moon! This altitude is the upper edge of the exosphere, which is the uppermost layer of the atmosphere. Therefore, this altitude is the highest level that we can meaningfully call the edge of the atmosphere.

If you forced me to pick what I think is the most meaningful choice of altitude for the edge of space, I would say that the effective edge of the atmosphere for human flight should be the Effective Karman Line at 80 km and the effective edge of the atmosphere for purposes of scientific research should be where the interplanetary medium begins, at an altitude of 200,000 km.

For more information, you can read this excellent research article by Jonathan C. McDowell.

Topics: air, atmosphere, gas, molecular diffusion, space