Why does the FAA uses 50 miles for defining outer space?

Why does the FAA uses 50 miles for defining outer space?” is a question some of us might have asked ourselves, especially when looking at the question of the delimitation of outer space, the different approaches – spatialist or functionalist – to space activities. For this new space law article on Space Legal Issues, let’s have a look at the choice of the Federal Aviation Administration (FAA) to use 50 miles (roughly eighty kilometers) as the boundary between the atmosphere and outer space.

Outer space, beyond being the final frontier, is different things to different people. For pilots, outer space is beyond the atmosphere, where they no longer have aerodynamic control and vehicles must be controlled in their position and altitude by thrusters. For a meteorologist, outer space is where there is insufficient atmosphere to cause a measurable barometric pressure. For a planetary scientist, outer space is that edge of the Earth’s influence called the magnetopause, the last vestiges of Earth’s magnetic field in wispy remnants of ionized particles marking the presence of our planet. For cosmologists, outer space is beyond that, beyond the very fringes of our Solar System, past even the distant orbiting, icy rocks of the Kuiper Belt and the Oort Cloud, extending billions of miles and out to the very limits of where the pressure of sunlight is bounced against the interstellar gas position known as the heliopause. However, when we use human beings as a measure of outer space, the distance above our home planet is dramatically less.

The argument about where the atmosphere ends and space begins predates the launch of the first Sputnik. The most widely – but not universally – accepted boundary, is the so-called Kármán line, nowadays usually set to be one hundred kilometers, but boundaries ranging from thirty kilometers to one and a half million kilometers have been suggested. Although the subject has not been much addressed in the physics literature, there is an extensive law/policy literature on the subject.

The Armstrong limit

The Armstrong limit or Armstrong’s line is a measure of altitude above which atmospheric pressure is sufficiently low that water boils at the normal temperature of the human body. Exposure to pressure below this limit results in a rapid loss of consciousness, followed by a series of changes to cardiovascular and neurological functions, and eventually death, unless pressure is restored within sixty to ninety seconds.

On Earth, the limit is around eighteen to nineteen kilometers above sea level. The term is named after United States Air Force General Harry George Armstrong, who was the first to recognize this phenomenon. At or above the Armstrong limit, exposed body fluids such as saliva, tears, urine, and the liquids wetting the alveoli within the lungs (but not vascular blood) will boil away without a full-body pressure suit, and no amount of breathable oxygen delivered by any means will sustain life for more than a few minutes. The NASA technical report Rapid Decompression Emergencies in Pressure-Suited Subjects, which discusses the brief accidental exposure of a human to near vacuum, notes that “The subject later reported that his last conscious memory was of the saliva on his tongue beginning to boil”.

Well below the Armstrong limit, humans typically require supplemental oxygen in order to avoid hypoxia (a condition in which the body or a region of the body is deprived of adequate oxygen supply at the tissue level).

The Kármán line

The Kármán line is an attempt to define a boundary between Earth’s atmosphere and outer space. This is important for legal and regulatory measures: aircraft and spacecraft fall under different jurisdictions and are subject to different treaties. The Fédération Aéronautique Internationale (or World Air Sports Federation), an international standard-setting and record-keeping body for aeronautics and astronautics, defines the Kármán line as the altitude of one hundred kilometers (sixty-two miles) above Earth’s mean sea level. Other organizations do not use this definition.

The line is named after Theodore von Kármán, a Hungarian American engineer and physicist, who was active primarily in aeronautics and astronautics. He was the first person to calculate the altitude at which the atmosphere becomes too thin to support aeronautical flight; the reason is that a vehicle at this altitude would have to travel faster than orbital velocity to derive sufficient aerodynamic lift to support itself. The line is approximately at the turbopause, above which atmospheric gases are not well-mixed.

The 50 miles line

In the late 1950s the USAF decided to award astronaut wings to pilots flying above 50 statute miles. This boundary was chosen as a nice round figure, but I want to argue that it is also the right choice from a physical point of view. It seems natural to choose the outermost (physical atmospheric) boundary, the mesopause, as the physical boundary which marks the edge of space. It turns out that the traditional value for the height of the mesopause, eighty kilometers, is also within five hundred meters of the 50 mile astronaut wings boundary historically used by the USAF. I therefore suggest that we adopt as the formal boundary of space an altitude of exactly eighty kilometers, representing the typical location of the mesopause”.

After combing through numerous sets of orbital statistics for spacecraft over the years, McDowell came up with an estimate that he says is more precise than the one currently used by the FAI: eighty kilometers, plus or minus ten kilometers. In easy-to-understand terms, this is the lowest altitude a satellite can go and still complete orbits around the Earth. To stay in orbit, and also reach such a low altitude, the vehicle has to be in an elliptical orbit. That’s one where the spacecraft swings out far away from Earth most of the time and comes in close to eighty kilometers for just a brief part of the trip. In this configuration, a spacecraft can stay in orbit for days or weeks, according to McDowell. McDowell says that 50 miles (eighty kilometers) is the point at which gravity becomes more important than the atmosphere. “You’re in space if you can basically ignore the atmosphere. And that doesn’t mean it has no effect, but gravity is the dominant thing you have to worry about”.

Even above 50 miles, Earth’s atmosphere still exists – it’s just super thin. Satellites that orbit much higher than 50 miles are still interacting with the particles from our atmosphere. The air is just so thin that it’s not detrimental to a spacecraft’s orbit. “So then the question is, where do you draw a boundary where you’re no longer in space? It’s when you can’t even dip through the atmosphere briefly at orbital speed and keep on going” says McDowell.

So if this is the most technical answer, how did the FAI’s formal definition end up set at one hundred kilometers? Theodore von Kármán himself set his own limit at eighty-three kilometers in 1956; however he wasn’t even trying to find the boundary of outer space. He was mostly trying to define how high a plane could fly and still achieve lift. Ultimately, this limit was misinterpreted as the boundary of outer space: “Around 1960, the FAI decided to set the limit at one hundred kilometers, just for the purpose of record setting flights – that any flight above that would be considered to be a spaceflight”.

However, not everyone adheres to the FAI’s definition of outer space. The US Air Force, for instance, already sets the limit at 50 miles, or roughly eighty kilometers, and will give badges to any of its personnel that fly above this height. NASA does the same. And while the Federal Aviation Administration (FAA) does not have an official definition, it usually gives out astronaut badges to those who have gone above 50 miles. It’s something that may become more defined as more commercial actors go to space. While different organizations have their own definitions, there is no universal agreement. In fact, the U.S.A. maintains that defining space through international law just isn’t necessary: “With respect to the question of the definition and delimitation of outer space, we have examined this issue carefully and have listened to the various statements delivered at this session. Our position continues to be that defining or delimiting outer space is not necessary. No legal or practical problems have arisen in the absence of such a definition. On the contrary, the differing legal regimes applicable in respect of airspace and outer space have operated well in their respective spheres. The lack of a definition or delimitation of outer space has not impeded the development of activities in either sphere”.