When a spacecraft blasts off the surface of Earth, it eventually exits our planet’s airspace and enters outer space. Where, precisely, that boundary lies is up for some debate.
Many experts cite what’s called the Kármán line as that edge of space, which establishes the boundary as the altitude above which conventional aircraft can’t fly. That line is often placed at approximately 62 miles above our planet’s surface.
That figure has many practical uses, but its scientific accuracy has been questioned. Assigning a precise value to the edge of space is anything but straightforward.
“In science, the boundaries we draw don’t exist in nature exactly,” says Jonathan McDowell, an astrophysicist at the Center for Astrophysics | Harvard & Smithsonian. “Where a boundary exists is where some quantity changes very rapidly over a short distance… And that is true at this edge of the atmosphere. But what you choose to call space and what you choose to call Earth—that’s a human decision that’s not forced on us by physics.”
The implications of deciding where Earth ends and space begins go beyond whether or not travelers earn their astronaut wings. Air traffic is typically regulated on the national level, with countries controlling the airspace over their land. Flying too low, for example, has the potential to inadvertently start an international conflict.
But “space is intrinsically global,” McDowell says. Different international treaties apply to space. As more nations launch satellites, and private spaceflight companies build a suborbital space tourism industry, defining the distinction between Earth’s airspace and outer space is becoming increasingly important.
Physics behind the Kármán line
The Kármán line is based on physics, in that it describes how the characteristics of Earth’s atmosphere at different altitudes affect a craft’s ability to fly. Planes stay airborne largely from lift generated by their wings against the thickness of Earth’s atmosphere. But as our atmosphere rises in altitude, it thins. At a certain point, the air is too thin for traditional aircraft, and any craft above that altitude require a propulsion system, such as a rocket, to remain aloft. That distinction is the Kármán line.
The line is named for Theodore von Kármán, an engineer and physicist who was born in Hungary in 1881. He became a prominent expert in rockets during World War II, and co-founded the United States’ Jet Propulsion Laboratory. He is credited as being the first to calculate the altitude above which a craft would need to use a propulsion system to fly.
Von Kármán originally calculated the boundary to be roughly 50 miles above sea level. But, today, the Kármán line is commonly defined as an altitude of around 62 miles, or 100 kilometers. In fact, the agency that keeps track of standards and records in air and space, the Fédération Aéronautique Internationale, also uses this figure to define where space begins.
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The thinking behind that round number of 100 kilometers, McDowell says, is that the boundary can’t be defined precisely because of the variability of the atmosphere.
But McDowell wasn’t so sure that was the case. So he re-examined the history and calculations of the Kármán line in a paper published in the journal Acta Astronautica in 2018. He found that von Kármán’s original calculation was more accurate than previously thought, and with decades of advancement on atmospheric models, the variability is probably only within a few miles of the original 52 mile calculation.
Is the Kármán line the only possible edge of space?
Some scientists have proposed other characteristics to define the boundary between Earth and space, such as the region in our planet’s orbit where a satellite breaks up upon reentry, McDowell says. “That, again, turns out to be in the 80s to 90s kilometers,” he says, which, in miles, is in the 50s.
Many US agencies, including the Federal Aviation Administration, typically use 50 miles as their boundary, too. The FAA and Air Force actually bestow astronaut wings on those who fly above an altitude of 50 miles. (However, not all passengers on a commercial flight will earn their wings, as in 2021 the FAA added criteria regarding a traveler’s contributions to a commercial space mission.)
But NASA Mission Control takes a different approach. Instead of focusing on aerodynamic lift, the space agency defines the point of reentry into Earth’s airspace from outer space as the place at which atmospheric drag becomes noticeable, at about 76 miles.
There are other boundaries that some might consider for the edge of space, suggests McDowell. One is the Armstrong Limit, named for Harry G. Armstrong, an early American aerospace medicine physician, which is the altitude at which a human’s blood boils if they’re not protected from the low atmospheric pressure by a spacesuit, approximately 11 to 12 miles up.
“You can play all kinds of games about what the criterion should be,” McDowell says.
Another is more of a joke, McDowell says: The Ripley Line, which would be where nobody could hear you scream in space. “A very rough” calculation of the altitude of that boundary came out to a few hundred miles, he says, “but that could easily be totally wrong.”
Where is the edge of space on other planets?
This question of where a planet ends and space begins can be extrapolated to other worlds, McDowell says. A sort of Kármán line might exist on a world like Mars, because it also has an atmosphere (albeit a thinner one than Earth’s), he suggests. But the moon, for example, has no atmosphere. So does that mean it is entirely in space? Or is there a different kind of boundary, perhaps a gravitational one, that should be considered?
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“In the future, when we have, you know, Lunar City, and you’re taking off from Lunar City into orbit around the moon, at what point do you get handed over from local air traffic control to deep space traffic control?” he says. Or, “when do you have to consider the difference between a local flight or a local athlete jumping very high into lunar gravity, versus something that counts as space traffic?”
While there are practical, logistical reasons to define such a boundary for places that are current targets for human spaceflight, McDowell says there’s another reason to create clear definitions.
“Definitions help us understand how to think about the objects we study,” he says. “They help us to then frame our questions in a different way. These concepts evolve. As we understand more about a class of thing, we get new questions to ask about it.”
