The question of how far up space begins is not answered by a simple, universally agreed-upon line. Unlike borders on Earth, the transition from the atmosphere to outer space is gradual, stretching across a zone where the air steadily thins. Defining this boundary requires establishing a scientifically informed marker to separate the domain of aeronautics from astronautics. This marker is necessary for international record-keeping, regulatory purposes, and space treaties. The altitude where the atmosphere ceases to have any meaningful effect on flight provides the answer to “how many feet up is space.”
The International Standard for Outer Space
The internationally recognized boundary, known as the Kármán Line, is set at an altitude of 100 kilometers above mean sea level. This measure is the standard accepted by the Fédération Aéronautique Internationale (FAI), the global organization that maintains world air and space records. In imperial units, the Kármán Line sits at approximately 328,084 feet.
This altitude was named in honor of Hungarian-American engineer Theodore von Kármán. Kármán calculated the altitude where the atmosphere becomes too thin for conventional aircraft to fly, proposing a boundary later formalized by the FAI. The adoption of 100 km was a practical choice, representing a convenient, rounded metric number close to his theoretical calculation.
The Kármán Line serves as the international demarcation for space, despite not being codified in binding international law. It provides a standardized measure for defining the beginning of spaceflight and is referenced in international space regulations. Crossing this 100-kilometer threshold is widely regarded as the point at which an object enters outer space.
The Aerodynamics That Define the Kármán Line
The scientific justification for the 100-kilometer boundary lies in the physics of atmospheric density and lift generation. As an aircraft climbs, the air becomes progressively thinner, reducing the molecules available to create aerodynamic lift. To maintain altitude, a traditional aircraft must fly at increasingly greater speeds to compensate for this decrease in lift.
The Kármán Line represents the altitude where the air is so rarefied that the speed required to generate sufficient lift would exceed orbital velocity. At this point, the vehicle relies on centrifugal force to counteract Earth’s gravity. Any vehicle operating above this altitude functions as a spacecraft, using propulsion and trajectory adjustments instead of aerodynamic control surfaces.
The transition from aerodynamic support to orbital mechanics defines the significance of the Kármán Line. Above 100 kilometers, the atmosphere’s effect on flight is negligible, and a vehicle is essentially in a state of free fall. This boundary demonstrates the shift in physics from aviation, which uses air, to astronautics, which relies on rocketry and orbital dynamics.
Government and Military Boundaries
While 100 kilometers is the international standard, some government and military organizations use a lower altitude for defining space. The United States military and the Federal Aviation Administration (FAA) have historically recognized the boundary of space at 50 miles. This altitude is equivalent to approximately 80 kilometers, or 264,000 feet.
This 50-mile boundary is primarily used for operational and ceremonial purposes within the U.S. It has been the threshold for awarding astronaut wings to military pilots, such as those who flew the X-15 rocket plane. The FAA also used this mark to define eligibility for its Commercial Space Astronaut Wings program, recognizing the crew of suborbital commercial flights.
The 50-mile definition is not based on the aerodynamic principles of the Kármán Line, but serves as a domestic, practical marker for recognizing high-altitude achievements. This distinction highlights the difference between a physics-based boundary and an administrative boundary used for military and regulatory recognition. The answer to “how many feet up is space” depends on whether one is adhering to the international standard or a domestic operational measure.