These days, spacecraft are venturing into the final frontier at a record pace. And a deluge of
paying space tourists should soon follow. But to earn their astronaut wings, high-flying civilians will have to make it past the so-called Kármán line. This boundary sits some 62 miles (100 kilometers) above Earth's surface, and it's generally accepted as the place where Earth ends and outer space begins.
From a cosmic perspective, 100 km is a stone's throw; it's only one-sixth the driving distance between San Francisco and Los Angelas. It’s also well within the clutches of Earth's overpowering gravitational pull and expansive atmosphere. So, how did humans come to accept this relatively nearby location as the defining line between Earth and space?
The answer is partly based on physical reality and partly based on an arbitrary human construct. That's why the exact altitude where space begins is something scientists have been debating since before we even sent the first spacecraft into orbit.
What is the Kármán Line?
Experts have suggested the actual boundary between Earth and space lies anywhere from a mere 18.5 miles (30km) above the surface to more than a million miles (1.6 million km) away. However, for well over half a century, most — including regulatory bodies — have accepted something close to our current definition of the Kármán Line.
The Kármán line is based on physical reality in the sense that it roughly marks the altitude where traditional aircraft can no longer effectively fly. Anything traveling above the Kármán line needs a propulsion system that doesn’t rely on lift generated by Earth’s atmosphere — the air is simply too thin that high up. In other words, the Kármán line is where the physical laws governing a craft's ability to fly shift.
However, the Kármán line is also where the human laws governing aircraft and spacecraft diverge. There are no national borders that extend to outer space; it’s governed more like international waters. So, settling on a boundary for space is about much more than the semantics of who gets to be called an astronaut.
The United Nations has historically accepted the Kármán line as the boundary of space. And while the U.S. government has been reticent to agree to a specific height, people who fly above an altitude of 60 miles (100 km) typically earn astronaut wings from the Federal Aviation Administration. Even the Ansari X-prize chose the Kármán line as the benchmark height required to win its $10 million prize, which was claimed when Burt Rutan’s SpaceShipOne became the first privately-built spacecraft to carry a crew back in 2004.
Origins: Theodore von Kármán
The Kármán line gets its name from Hungarian-born aerospace pioneer Theodore von Kármán. In the years around World War I, the engineer and physicist worked on early designs for helicopters, among other things.
Then, in 1930, von Kármán moved to the United States and became a go-to expert in rockets and supersonic flight around World War II. Eventually, in 1944, Kármán and his colleagues founded the Jet Propulsion Laboratory, now a preeminent NASA lab.
In addition to the boundary line of space, von Kármán’s name is attached to a number of engineering equations, laws, constants, and aerospace designs, as well as a handful of awards in the field. But the Kármán line is by far his most famous claim to fame, which he earned by being among the first to calculate the altitude above which aerodynamic lift could no longer keep an aircraft aloft.
