Outer space, often referred to simply as space, refers to the empty areas within the universe that are not occupied by the atmospheres of celestial bodies. Although the earliest use of the term "outer space" is attributed to Alexander von Humboldt in 1845, it was popularized by English science fiction writer H. G. Wells, who used the term in his 1901 book The First Men in the Moon. Outer space is not a perfect vacuum as some might imagine, but rather it contains electromagnetic radiation and hydrogen. Outer space is measured in relation to Earth’s atmosphere, which consists of five layers: stratosphere, thermosphere, troposphere, exosphere, and mesosphere. Based on general boundaries of Earth's surface, outer space begins 62 miles above Earth’s surface.
Theories of Where Outer Space Begins
Although it is clear that outer space does not begin immediately at the end of Earth’s atmosphere, its exact location is unknown despite significant scientific inquiry. However, it is generally accepted that outer space begins approximately 62 miles above the surface of the Earth. This point is popularly known as the Karman Line, named after Hungarian-American engineer and physicist Theodore von Karman. According to Karman, the Karman Line defines the altitude at which aeronautics can no longer work. The line is recognized by the Fédération Aéronautique Internationale (FAI), which is the world governing body of air sports.
In 2009, researchers from the University of Calgary claimed that outer space begins 73.3 miles above Earth’s atmosphere. Drawing on data from the Supra-Thermal Ion Imager, the study represents another theory that attempts to explain the start of outer space. However, critics argue that this point is very far away from the end of Earth’s atmosphere, and therefore could not possibly be the Karman Line.
Differences Between Outer Space and the Atmosphere
A significant difference the atmosphere and outer space is that gases below the Karman Line (within the atmosphere) are well mixed with nitrogen making up 78% of the atmosphere, oxygen 21%, and rare gases 1%. In contrast, gases above the Karman Line (in outer space) diffusively separate due to gravitational force. Additionally, temperature in space is near absolute zero, whereas an atmosphere has a higher temperature depending on the object that it covers, such as stars and planets. Furthermore, the atmosphere can support life, while outer space cannot. The Armstrong Line determines the altitude beyond which nobody can survive.
Challenges of Identifying Exactly Where Spaces Begins
A major challenge of identifying the exact location of outer space is the fact that stars, moons, planets, and asteroids maintain their atmospheres through gravitational attraction. As such, it is almost impossible to clearly mark the boundary between these atmospheres. The popular method of assessing shifting in atmospheres is usually the density of gases. The further a gas is from an object in space, the lower its density.