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

Introduction

The question of where Earth’s atmosphere ends and the edge of space begins is more complex than it might first appear. The atmosphere gradually thins out with altitude, lacking a definitive boundary. This article will explore various perspectives on the “edge” of the atmosphere, defining it based on different criteria and applications.

Understanding the Atmosphere

Earth’s atmosphere is held close to the surface by gravitational forces. As altitude increases, the density of air decreases due to the gravitational pull, which is stronger closer to the Earth’s surface. This results in heavier molecules being more concentrated at lower altitudes. The behavior of atmospheric molecules is dictated by their temperature and the principles of gas dynamics, including diffusion.

Why Doesn’t the Atmosphere Simply Fall to Earth?

In theory, if the atmospheric molecules were cold enough, they would condense into liquid or solid forms and remain on the Earth’s surface. However, the gas state of these molecules allows them to collide and exchange energy, which often enables some molecules to achieve sufficient velocity to escape gravitational pull. This process of molecular collisions and diffusion allows the atmosphere to extend high above the Earth despite gravity’s influence.

Defining the Edge of the Atmosphere

Since the atmosphere doesn’t have a clear-cut boundary, we can define various effective edges based on different criteria. Below is a table summarizing several useful definitions of the edge of the atmosphere:

Altitude (km)	Name	Description
$13$	Maximum Service Ceiling	The highest altitude for most commercial airplanes.
$19$	Armstrong Limit	The altitude above which humans cannot survive without pressurized suits.
$38$	Jet Airplane Limit	The maximum altitude for air-breathing jet airplanes.
$54$	Balloon Limit	The maximum altitude balloons can float.
$80$	Effective Karman Line	U.S. legal edge of space; dividing line between winged flight and spaceflight.
$100$	Karman Line	International legal edge of space.
$125$	Circular Orbit Limit	The lowest altitude for stable circular orbits of satellites.
$500-1000$	Thermopause	The point where atoms behave independently of gas laws.
$200,000$	Interplanetary Medium Begins	The altitude above which atoms escape Earth’s gravity permanently.

Key Definitions

Maximum Service Ceiling

The maximum service ceiling of commercial aircraft is approximately $13 \text{ km}$. Above this altitude, the air is too thin for sufficient lift and combustion.

Armstrong Limit

The Armstrong Limit, located at $19 \text{ km}$, is critical for human survival as the air pressure drops to values where bodily fluids can boil.

Jet Airplane Limit

At approximately $38 \text{ km}$, air-breathing jet airplanes reach their operational limits.

Balloon Limit

Scientific research balloons can ascend to about $54 \text{ km}$, marking an effective edge for balloon-based exploration.

Effective Karman Line

The Effective Karman Line is set at $80 \text{ km}$, distinguishing between atmospheric flight and spaceflight. It is where satellites can maintain elliptical orbits.

Karman Line

Internationally recognized at $100 \text{ km}$, the Karman Line serves as the legal boundary for space.

Circular Orbit Limit

Satellites require an altitude of at least $125 \text{ km}$ to maintain stable circular orbits without propulsion.

Thermopause

The thermopause lies between $500$ and $1000 \text{ km}$, separating the thermosphere from the exosphere.

Interplanetary Medium Begins

At approximately $200,000 \text{ km}$, the gravitational influence of Earth weakens sufficiently for hydrogen atoms to escape into space.

Conclusion

In summary, while there is no definitive edge to the atmosphere, various altitudes serve as practical benchmarks for different applications, ranging from human flight to satellite operations. For human activity, the Effective Karman Line at $80 \text{ km}$ is a meaningful threshold, while for scientific research, the upper edge of the exosphere at $200,000 \text{ km}$ provides a significant point of demarcation. Understanding these different perspectives helps clarify the complex nature of our atmosphere and its transition into space.