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 does 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 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 prize, which was claimed when Burt Rutan’s
SpaceShipOne became the first privately-built spacecraft to carry a crew back in 2004.
But... just a moment. Kármán... who was he? 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.
Lift is largely generated by an airplane’s wings as it flies through the air, creating a
force that opposes the plane’s weight, keeping it airborne. But this concept doesn’t work in space.
Without enough air, there’s no lift, which is why spaceships don't usually resemble aircraft.
The Space Shuttle and Virgin Galactic's SpaceShipTwo, in fact, look a bit like
planes because they were designed to glide back to a runway on Earth after venturing to space.
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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”. How come people can’t agree?
“It’s very political, it turns out” - McDowell says.
There is no easy distinction between “space” and “not space,” in part because Earth’s atmosphere
doesn’t simply vanish; rather, it gradually becomes thinner and thinner over about 1000 km.
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.
Furthermore, “there’s no single altitude above which a satellite can stably remain in orbit;
that depends on the type of satellite and its orbital trajectory” - McDowell says.
A prodigious maker of lists, McDowell was compiling records for rockets, astronauts,
and other space objects, and he went looking for an accepted international boundary that would help
him decide which records to include. When he realized none existed,
he decided to find one by revisiting the types of calculations von Kármán did.
He pulled publicly available orbital paths for 43,000 satellites and sorted them based on the
lowest points in their orbits (called perigee) during decommissioning and atmospheric re-entry.
From there, he realized that satellites could orbit the planet numerous times below an altitude
of 100 km, but those dipping beneath 80 km met a quick and flaming end more often than not.
After that, he redid the von Kármán math and found that atmospheric contributions on orbiting
spacecraft become negligible at around 80 km up. “What you don’t see is satellites dipping down to
70 km and coming back out,” he says. “There is a fairly sharp boundary, a decently sharp boundary,
between how low the perigee can be and where you just won’t make it back out again.”
With suborbital spaceflight companies edging closer to the edge of space,
could 2021 be the year we formally define it? McDowell thinks that’s unlikely, although he’s
sure the conversation will pick up speed as commercial spaceflight ventures begin
spending more time in the region between 80 and 400 km, where the space station orbits.
“I think that as space activity moves more into this regime, the pressure to agree on
a boundary will be greater,” he says. 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. Ok, but will paying suborbital spaceflight
passengers be called “astronauts? “As of now, yes, at least if they
make the trip from a U.S. launch site. The FAA and the U.S. Air Force both agree that flying
higher than 80 km above our planet qualifies a person for the title” - McDowell says.
What do NASA astronauts think about that? Some people might argue that getting into
orbit is what defines an astronaut. However, “I think Alan Shepard and Gus Grissom would
disagree,” says Terry Virts, a former commander of the International Space Station who has spent
more than 213 days in orbit. “They’re the first two U.S. astronauts who didn’t get into orbit.”
Virts says there’s a big difference between riding along on a five-minute suborbital flight
and performing a six-month orbital mission, but when it comes down to it, folks on both types
of trips have earned the “astronaut” title. “If you’re strapping your butt to a rocket,
I think that’s worth something,” Virts says. “When I was an F-16 pilot, I didn’t feel jealous about
Cessna pilots being called pilots. I think everybody’s going to know if you paid to be
a passenger on a five-minute suborbital flight or if you’re the commander of an interplanetary space
vehicle. Those are two different things.” Massimino agrees that there’s an important
distinction between being selected as a NASA astronaut—“the training, the struggle,
the rejections, all of that”—and being a paying customer. But he’s also completely on board
with space tourists earning the title. “I think if you get above that line,
you certainly qualify as an astronaut, absolutely,” he says. “The more the merrier!”