# Where Does Space Begin? The Kármán Line

Where does the Earth’s atmosphere end and space begin? The answer depends on who you ask because there is no firm boundary marking the end of the atmosphere. A popular answer is the value used for international treaties. Under this definition, the Kármán line marks the beginning of space at 100 kilometers or 62 miles above sea level. Here’s a look at different answers about where space begins.

If you fly higher than 80 km (50 miles), you’re an astronaut. However, many governments define the start of space as 100 km (62 miles).

### Why It’s Difficult to Find the Start of Space

There are two main reasons why it’s hard to define the beginning of space.

1. The Earth’s atmosphere slowly fades into a vacuum. It isn’t a shell surrounding the planet.
2. The atmosphere changes its shape and size. Pressure from the solar wind pushes on the side of the atmosphere facing the Sun. Meanwhile, the atmosphere trails behind the Earth on the dark side of the planet. Even averaging these values poses a challenge, because the intensity of the solar wind changes.

### What Is the Kármán Line?

The Kármán line takes its name from engineer and physicist Theodore von Kármán. In the mid-20th century, Kármán calculated the altitude where the atmosphere becomes too thin to support aeronautical flight as 83.6 kilometers or 51.9 miles. His value approximately coincides with the turbopause. Above the turbopause, atmospheric gases don’t mix uniformly. But, the turbopause varies in altitude, sometimes extending up to 100 kilometers. It occurs near the bottom edge of the thermosphere.

Given the variable nature of the turbopause, Kármán decided to round the air-space boundary up to 100 kilometers. The Fédération Aéronautique Internationale (FAI) adopted the Kármán line, and many governing bodies followed suit.

### How NASA and the U.S. Air Force Define the Edge of Space

NASA and the U.S. Air Force use a different value for the start of space. They award astronaut status to people reaching 80 kilometers or 50 miles above sea level. The Federal Aviation Administration (FAA) similarly uses the 80 km or 50 mile boundary. For practical purposes, NASA Mission Control uses 122 km or 76 miles, because atmospheric drag affects objects below this altitude.

### Interplanetary, Interstellar, and Intergalactic Space

Space is space, right? Actually, no. Although all of space is a partial vacuum, there are different regions of space.

• Geospace: When we talk about where space begins, we’re discussing the start of geospace. Geospace is the region of space that surrounds the Earth. It includes the ionosphere and thermosphere and ends at the magnetopause. Beyond the magnetopause, the Earth’s atmosphere isn’t affected by the solar wind. In other words, it’s where solar weather ceases to be a concern. The magnetopause is compressed toward the Earth to about 10 Earth radii on the side facing the sun, while it extends out to 100-200 Earth radii on the night side of the planet.
• Cislunar space: Cislunar space describes the region of space surrounding the Earth out to the edge of the Moon’s orbit.
• Deep space: Deep space is space beyond cislunar space. How far beyond the Moon it starts depends on who you ask. The International Telecommunication Union managing satellites defines deep space as starting 5 times the distance to the moon or 2×106 km.
• Interplanetary space: Interplanetary space is the region of deep space encompassing the solar system. The edge of interplanetary space is where the galaxy exerts more influence than the solar wind. Since solar activity varies, there is no firm edge to interplanetary space. Interplanetary space contains some ionized atomic nuclei, gases, dust, organic molecules, and small meteors. The dust appears from Earth in the form of the zodiacal light.
• Interstellar space: Interstellar space is the region within a galaxy between the influence of stars. Interstellar space is a near-perfect vacuum, but about 70% of the matter it contains consists of hydrogen atoms, with the remainder consisting of helium atom and a few traces of heavier elements.
• Intergalactic space: Intergalactic space is the space between galaxies. Rarefied plasma (mostly ionized hydrogen) forms filaments between galaxies.

### Why It’s Important to Define the Edge of Space

There are two reasons why it’s important to define the boundary between the atmosphere and outer space.

1. The airspace above nations is governed by the nations, while space is free to everyone. So, where you can place a satellite depends on the definition of space.
2. While it’s not as important as the first reason, defining the start of space determines who gets to be considered an astronaut.

### Space Starts at a Different Place on Other Planets

The Kármán line only applies to the Earth. The comparable beginning of space for Mars is about 80 km (50 miles). On Venus, space starts around 250 km (160 miles) from the surface.

### References

• Gangale, Thomas (2017). “The Non Karman Line: An Urban Legend of the Space Age”. Journal of Space Law. 41 (2).
• Liddle, Andrew (2015). An Introduction to Modern Cosmology. John Wiley.
• McDowell, Jonathan C. (2018). “The edge of space: Revisiting the Karman Line”. Acta Astronautica. 151: 668–677. doi:10.1016/j.actaastro.2018.07.003
• Voosen, Paul (2018). “Outer space may have just gotten a bit closer”. Science. doi:10.1126/science.aau8822