Where Does Outer Space Actually Begin?

On July 11, 2021, high over the New Mexico desert, Virgin Galactic’s SpaceShipTwo detached from its mothership and ignited its engines, rocketing billionaire Richard Branson and five crewmates past the edge of space. The 15-minute suborbital hop made Branson the first person to fly into space aboard his own privately-built spacecraft and officially ushered in the age of commercial space tourism. Yet despite its historic nature, many were quick to criticize the flight and the very concept of space tourism, dismissing it as a battle of egos between billionaires and an irresponsible waste of resources in the midst of a global pandemic and climate crisis. But another, more technical accusation was that Branson hadn’t actually gone into space at all. According to critics, including rival billionaire Jeff Bezos, Branson’s flight, which topped out at 53 miles or 88 kilometres in altitude, fell well short of the 100 kilometres internationally recognized as the boundary of outer space. By contrast, Bezos’s own spaceflight, scheduled for July 20th aboard his company Blue Origin’s New Shepard rocket, is expected to reach around 103 kilometres in altitude, though at the time of the writing of the making of this video, has not yet occurred. In true feuding fashion, Blue Origin made a point of underscoring this difference by releasing a snarky infographic, which in addition to reminding Virgin Galactic that:

“…for 96% of the world’s population, space begins at 100km up.”

… also helpfully listed New Shepard’s superior safety record, environmental impact, and window size. But feuding billionaires aside, who actually decides where outer space begins, and how did they arrive at that number? Well, as with so many topics covered on this channel, the answer is: it’s complicated. 

The problem of defining the boundary of outer space is a tricky one, since the earth’s atmosphere doesn’t simply end at some hard boundary. Rather, the atmosphere gets progressively thinner with altitude, eventually dissipating into the hard vacuum of interplanetary space. The boundary between the atmosphere and outer space would thus seem to be completely arbitrary. However, over the years numerous attempts have been made to find an unambiguous technical basis for defining this boundary. In the early days of American ballistic missile development, engineers used two different definitions for the edge of outer space. The first was based on atmospheric frictional heating, and defined outer space as the altitude above which the air was thin enough that a vehicle travelling at orbital speeds would not burn up. The second was based on propulsion and defined outer space as the altitude above which there is insufficient altitude for air-breathing engines like jet engines or ramjets to operate. Both definitions placed the edge of space at around 50 miles or 80 kilometres in altitude.

However, these early definitions were based on limitations in contemporary materials science and propulsion technology, meaning that the boundary of outer space would change as technology improved. Therefore the most widely-accepted definition is based on the work of Hungarian-born physicist Theodore von Kármán, one of the founders of NASA’s Jet Propulsion Laboratory. In 1956, von Kármán proposed that the boundary of outer space be defined as the altitude at which aerodynamic lift becomes irrelevant in keeping a vehicle airborne. As an aircraft climbs, the air gets progressively thinner, and in order to stay airborne the aircraft must either fly faster – pushing more air over its wings – or be fitted with larger wings like the high-flying Lockheed U-2 spy plane. At a certain altitude, however, even an aircraft with wings of infinite size would have to fly so fast to generate sufficient lift that it would be travelling at orbital velocity, where the force of gravity acting on the aircraft is balanced out by the centrifugal force generated by its motion around the earth. At this point the aircraft will have become an orbiting spacecraft, rendering its wings and the lift they generate totally redundant. von Kármán summarized this principle in his autobiography, published posthumously in 1967:

“Where space begins can actually be determined by the speed of the space vehicle and its altitude above the earth. Consider, for instance, the record flight of Captain Iven Kincheloe in an X-2 rocket plane. Kincheloe flew 2000 miles per hour at 126,000 feet, or 24 miles up. At this altitude and speed aerody- namic lift still carries 98 per cent of the weight of the plane, and only two per cent is carried by centrifugal force, or Kepler Force, as the space scientists call it. But at 300,000 feet or 57 miles up this relationship is reversed because there is no longer any air to contribute lift. Only centrifugal force prevails. This is certainly a physical boundary, where aerodynamics stops and astronautics begins.”

Indeed, up until this point many had considered Captain Kincheloe’s September 7, 1956 flight to have been the first space flight in history. However, according to von Kármán’s definition, the boundary of outer space lies at an altitude of 275,000 feet – around 52 miles or 84 kilometres – more than twice that achieved by Kincheloe. According to this definition, the first manmade object to reach space was a German V2 ballistic missile fired on June 20, 1944, while the first human to reach space was Soviet Major Yuri Gagarin on April 12, 1961. It is also worth pointing out that the “Kepler Force” von Kármán refers to is not an actual scientific term; the centrifugal force that keeps an orbiting spacecraft aloft is derived from the theories of Sir Isaac Newton, not Johannes Kepler. Still, the basic physical principle holds.

While von Kármán never formally published his definition, in 1957 American lawyer Andrew G. Haley, widely considered to be the world’s first “space lawyer,” adapted von Kármán’s theories to create a formal legal definition for the boundary of outer space, which he dubbed the “Kármán Line” in the physicist’s honour. In his 1963 book Space Law and Government, Haley states:

“[The boundary is] a critical jurisdictional line, marking the theoretical limit of air flight, which I term the Kármán Line. It must be noted with care that the exact location of this line of primary jurisdiction is not presented as an apodictic solution of the problem. The Kármán primary jurisdictional line may eventually remain, or, after due consideration of such developments as improved techniques of cooling and the discovery of more heat resistant materials, this line may be changed significantly. But, while these changes will be in the exact location of the Kármán Line, the existence of the line is certain and wherever the line is finally drawn will be the place where “airspace” terminates.

While this early definition still integrated older ideas about aerothermal heating, these factors were later dropped in favour of von Kármán’s aerodynamic lift approach. In 1960, the Fédération Aéronautique Internationale or FAI, the international body which administers aviation records, officially adopted the Kármán Line as the formal boundary for outer space, setting it at an even 100 kilometres in altitude. Coincidentally, this altitude corresponds with the approximate location of the turbopause, a layer of the upper atmosphere where nitrogen and oxygen molecules are no longer evenly mixed but rather stratify according to molecular weight. This abrupt change in the physical properties of the atmosphere adds further legitimacy to the 100km definition for the boundary of outer space.

However, not everyone agrees with the FAI’s definition. Instead of rounding up from von Kármán’s figure of 52 miles, NASA and the United States Armed Forces rounded down and set the boundary of outer space at 50 miles or 80 kilometres. Thus, any American Air Force, Navy, Marine Corps, Coast Guard, or civilian pilot flying above 50 miles in altitude is considered to have completed a spaceflight and qualifies to receive a United States Astronaut Badge – commonly referred to as “Astronaut Wings.” Thus, while according to the FAI Richard Branson’s July 11, 2021 flight did not reach outer space, according to United States rules, it did.

While most astronaut wings have been earned by serving military officers participating in NASA’s Mercury, Gemini, Apollo, and Space Shuttle programs, they were also awarded to seven USAF and NASA pilots of the North American X-15 rocket-powered research plane, which on 13 occasions in the 1960s reached altitudes in excess of 50 miles. For this reason, the United States has been hesitant to adopt the FAI’s definition of 100 kilometres, as this would mean stripping all but one of these pioneering pilots of their astronaut wings. The sole exception would be Joseph A. Walker, who flew the X-15 above the Kármán Line on July 19 and August 22, 1963. Further complicating the issue is the fact that the official Air Force criteria for awarding Astronaut Wings do not specify whether altitude is to be measured in statute miles of 5,280 feet or nautical miles of 6,076 feet – the traditional unit of measurement in aviation. Of the thirteen X-15 flights that exceeded 50 statute miles in altitude, only four exceeded 50 nautical miles. Yet for the remaining nine flights, astronaut wings were awarded to all the Air Force but not the civilian pilots – a snub that was not corrected until 2005.

While the likes of Jeff Bezos and Elon Musk are more than happy to stick to the 100 kilometre definition and keep Richard Branson out of the astronaut club, the FAI may soon be rethinking its long-held rules. Indeed, it is unknown just how the FAI arrived at the 100 kilometres figure, given that von Kármán’s original calculations gave a figure much closer to NASA and the U.S. Military’s 80-kilometre definition. According to space law expert Thomas Gangale, the likeliest explanation is that lawyers and government officials from Andrew Haley onward simply kept rounding up until they arrived at the round, pleasing number of 100 kilometres. Indeed, the attribution of the 100 kilometre boundary to von Kármán may in itself be a mistake. As early as 1951, lawyer John Cobb Cooper and Antonio Ambrosini, the first chairman of the Ad Hoc Committee on the Peaceful Uses of Outer Space, suggested 100 kilometres as a legal definition for the edge of space. According to Gangale, given that his calculations were never formally published, von Kármán’s name likely became attached to the boundary due to his greater fame.

Unsurprisingly, numerous alternate definitions for the boundary of space have since been proposed. In 2018, Harvard astrophysicist Jonathan McDowell revisited von Kármán’s calculations and came up with a refined figure of 80 kilometres, plus or minus 10 kilometres. According to the McDowell, this is the minimum altitude at which a spacecraft can complete an orbit of the earth without atmospheric friction causing it to deorbit or burn up. The margin for error accounts for the fact that solar heating causes the atmosphere to expand and contract, meaning the exact boundary of space is constantly fluctuating. A more extreme proposal places the boundary at an altitude of 600 miles or 960 kilometres, the point at which space becomes more densely populated with solar particles than atmospheric gas molecules. However, under this definition a large number of orbiting vehicles, including many communications satellites and the International Space Station, would no longer qualify as spacecraft. Another definition, used during the Space Shuttle program, set the edge of space at 76 miles or 122 kilometres. This was the altitude at which atmospheric drag became significant for the reentering shuttle, allowing the crew to switch from orbital maneuvering thrusters to aircraft-style control surfaces. Finally, in 2009, researchers from the University of Calgary used an instrument called a Supra-Thermal Ion Imager to determine the precise boundary between more gentle winds of the upper atmosphere and the more violent ion flows of the solar wind. The resulting data places the boundary of space at an altitude of 73 miles or 118 kilometres. In the face of such analyses, on November 30, 2018, the FAI announced it would investigate the possibility of changing its formal definition of outer space from 100 to 80 kilometres.

While all this might seem like mere semantics, the legal definition of the edge of space has profound consequences for national sovereignty and international relations. According to international law, nations have jurisdiction over the airspace above their land-based territory and territorial waters, typically defined as extending 12 miles or 19 kilometres beyond the shoreline. How far up this sovereign airspace extends, however, is a matter of considerable debate, with some definitions setting the vertical sovereignty boundary at 30 kilometres – the highest most aircraft and balloons can fly – and others extending it all the way to the edge of the atmosphere. Beyond this, according to the 1967 Outer Space Treaty, outer space is governed similarly to international waters, with all nations being free to conduct peaceful exploration regardless of the territory being overflown. Precisely defining the boundary of outer space is therefore crucial to determining whether a particular space activity violates a nation’s sovereign airspace. This issue was of particular interest to the United States military early in the space race, as it prepared to conduct surveillance overflights of the Soviet Union using spy satellites – and for more on this please check out our video Kaputnik: America’s Largely Forgotten Disastrous First Attempt to Launch a Satellite. In more recent years, the Space Shuttle regularly passed as low as 30 miles or 48 kilometres over foreign territory while reentering the atmosphere, though NASA always sought formal permission to conduct such overflights.

Such issues further contribute to the United States’ reluctance to change its 50 mile definition of outer space – or even to formally define this boundary at all. At a United Nations meeting in Vienna in 2001, the United States Delegation stated:

“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.”

According to Thomas Gangale, the U.S. Military has good reasons for wanting to keep the boundary of outer space ambiguous:

“They want to keep it undefined, primarily because the US military feels it gives them the flexibility to do things at any altitude. You can’t break a law that doesn’t exist.”

So, petty and arbitrary as it may sound, the question of where the atmosphere ends and outer space begins goes far beyond which billionaire gets to call himself an astronaut.

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Expand for References


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