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What defines the boundary of Earth's atmosphere?
Executive summary
There is no single, physically sharp top to Earth’s atmosphere; it thins gradually from the surface out to the exosphere and beyond, so scientists and agencies use practical conventions to mark “the boundary” — most commonly the Kármán line at 100 km (62 mi), though other choices (80 km used by some U.S. agencies, and much higher extents of the exosphere) are also cited [1] [2] [3]. Different definitions serve different purposes: engineering (where aerodynamic lift fails), legal/regulatory (where air law ends), or atmospheric science (where measurable gas becomes vanishingly rare) [4] [5] [6].
1. The physics: a gradual fade, not a cliff
Earth’s air does not end abruptly; atmospheric density decreases continuously with altitude so that there is no single physical altitude where “air stops.” Encyclopaedia Britannica and Wikipedia both state the atmosphere becomes progressively thinner with no definite boundary, and the exosphere gradually blends into interplanetary space [2] [1]. UCAR’s Center for Science Education likewise emphasizes that the exosphere “fades away” and that measurable traces of gas extend for hundreds or thousands of kilometers [6].
2. The Kármán line: an engineering and convention anchor
The most widely cited conventional boundary is the Kármán line at 100 km above mean sea level, adopted by bodies such as the Fédération Aéronautique Internationale (FAI) and commonly used in public discussion; it was motivated by von Kármán’s estimate of where aerodynamic lift becomes ineffective and spacecraft operations, not traditional aeronautics, become necessary [2] [4] [7]. Astronomy.com and NASA summaries explain that above this altitude aerodynamic flight gives way to orbital/propulsive flight — a practical engineering distinction [5] [3].
3. Competing conventions: 80 km, orbital limits, and political choices
Not all organizations accept 100 km. NASA, the U.S. FAA and the U.S. Air Force have used an 80 km (50-mile) figure for awarding astronaut wings, showing a lower operational boundary for some purposes [2]. Other proposals tie the lower boundary of space to the lowest perigee at which sustained orbit is possible (around ~150 km for a circular orbit without propulsion, per some analyses), or even to where the thermosphere/exosphere definitions are placed — which can extend thousands of kilometres [4] [6].
4. The exosphere and the “top” of the atmosphere: very far out
Atmospheric models and educational resources put the exosphere as the outermost layer and describe it as the region where particles can escape to space; its top is poorly defined and has been placed anywhere from tens of thousands to hundreds of thousands of kilometres in different descriptions (UCAR gives 100,000–190,000 km as some estimates) [6]. NOAA and NASA note that the exosphere extends into the domain where satellites orbit and where molecules slowly leak into space, reinforcing that the practical “edge” depends on what you’re measuring [8] [3].
5. Why a definition matters: law, trophies, and engineering
Choosing a boundary is not purely scientific. The Kármán line carries legal and jurisdictional implications (the notion of a Kármán “jurisdictional line” has been suggested) and matters for awards (who counts as an astronaut), air/space law, and regulatory authority; Wikipedia and EBSCO point out that different choices have consequences and that international agreement is incomplete [4] [7]. Astronomy.com highlights the mixed motives: physical reasoning plus human needs to draw a line for governance and recognition [5].
6. Where experts disagree and what reporting leaves out
Experts disagree on the precise altitude where space begins: proposals in reporting range from ~30 km up to extremely large distances, depending on criteria [5]. Available sources do not mention a single definitive international legal altitude accepted by all states; rather, they document the practical adoptions (FAI’s 100 km, some U.S. agencies’ 80 km) and continuing debate [2] [4] [7]. Detailed atmospheric measurements or the latest diplomatic negotiations beyond those citations are not covered in these sources.
7. Bottom line for readers
If you need a quick answer for popular usage, the Kármán line at 100 km is the commonly cited boundary [7] [2]. If the question is scientific — “where does the atmosphere physically end?” — the correct answer is that it doesn’t end at one altitude: it fades into space, with the exosphere extending far beyond the Kármán line and with useful alternate definitions (80 km, orbital stability limits, or exospheric extents) chosen for specific legal, operational, or scientific reasons [6] [3] [4].