At what height does Outer Space begin?

 

How high do you have to go to earn Astronaut wings?

 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 100 kilometers above Earth's surface, and it's generally accepted as the place where Earth ends and outer space begins. From a cosmic point of view, 100 km is a stone’s throw and is also a limit that falls abundantly within the domain of the gravitational attraction of the Earth and its 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.

Where, exactly, is the edge of space? It depends on who you ask. With more countries and commercial companies heading into the stratosphere,  the debate about how to define outer space is heating up. Ask someone where outer space is, and they’ll probably point at the sky. It’s up, right? Simple. Except, no one really knows where “air space” ends and “outer space” begins. That might sound trivial, but defining that boundary could matter for a variety of reasons - including,  but not limited to, which high-flying humans get to be designated as astronauts. Now, with Virgin Galactic seemingly on the cusp of launching paying passengers onto suborbital trajectories, many people are wondering whether those lucky space tourists will earn their astronaut wings. As of right now, they will, according to U.S. practices. Is that a problem? “No, I think it’s great!” says NASA astronaut Mike Massimino, who in 2002  with the mission STS -109 Columbia contributed to the repair of the Hubble Space Telescope.

Here, we take a look at the ways space is currently defined, the confusion surrounding the demarcation, and what the future might bring. International treaties define “space” as being free for exploration and use by all, but the same is not true of the sovereign airspace above nations. The laws governing air space and outer space are different.  flying a satellite 88 km above China is just fine if space begins at 80 km up, but define the edge at 96 km, and you might find your satellite being treated as an act of military aggression.  “Where do a country’s air space stop and space begin?” asks Jonathan McDowell of the  Harvard-Smithsonian Center for Astrophysics. “Once you agree on a boundary of space,  you agree on a boundary where space law applies.” However, the United States and some other countries have resisted a formal, international delimitation of space, stating that it’s not necessary and that “no legal or practical problems have arisen in the absence of such a definition. Others argue that maintaining a distinct boundary will be crucial,  given an increase in the number of national space programs and in private spaceflight endeavors that are boosting the amount of suborbital traffic.

So, how is “space” currently defined? Experts have suggested the actual boundary between Earth and space lies anywhere from a mere 30 km above the surface to more than 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 the 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 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 prizes, which was claimed when Burt Rutan’s  SpaceShipOne became the first privately-built spacecraft to carry a crew back in 2004. Von Kármán suggested that the most reasonable edge of space would be near where orbital forces exceed aerodynamic ones. And, opting for a nice, round altitude, he decided that  100 kilometers was a good boundary. Still, despite now having his name attached to the boundary of space, von Kármán himself never actually published this idea.  As von Kármán himself wrote in his posthumously published autobiography, The Wind and Beyond:  

This is certainly a physical boundary, where aerodynamics stops and astronautics begins,  and so I thought why should it not also be a jurisdictional boundary? ... Below this line,  space belongs to each country. Above this level, there would be free space

The Federation Aeronautique Internationale (FAI), which keeps track of standards and records in astronautics and aeronautics, also defines space as beginning a hundred kilometers up.  It is, after all, a nice round number. But the Federal Aviation Administration,  the U.S. Air Force, NOAA, and NASA generally use 80 kilometers as the boundary, with the Air Force granting astronaut wings to flyers who go higher than this mark.  At the same time, NASA Mission Control places the line at 122 kilometers, because that is “the point at which atmospheric drag becomes noticeable”. Technically, the International Space Station - which orbits at an average height of 400  km - would not be in space if we defined “space” as the absolute absence of an atmosphere, The question is, could 2021 be the year we formally define it?  In fact, the FAI says that because of  “compelling” recent analyses suggesting that space ought to begin around 80 km up, it will propose a  meeting this coming year to evaluate the idea.