Space starts at either 100 kilometers (62 miles) or 80 kilometers (50 miles) above Earth’s surface, depending on which authority you ask. There is no single, universally agreed-upon boundary, and the debate over where the atmosphere ends and space begins has real consequences for who gets to call themselves an astronaut and which laws govern a vehicle’s flight.
The Two Main Boundaries
The most widely cited line is the Kármán line at 100 kilometers (62 miles), recognized by the Fédération Aéronautique Internationale (FAI), the international body that keeps records for aeronautics and astronautics. The concept is named after physicist Theodore von Kármán, who calculated the approximate altitude where the atmosphere becomes too thin for conventional aerodynamic flight. Above this point, a vehicle would need to travel faster than orbital velocity for wings to generate any meaningful lift, so aeronautics effectively gives way to astronautics.
The United States uses a lower threshold. The FAA, NOAA, NASA, and the U.S. Air Force generally place the boundary at 50 miles (80 kilometers). The Air Force has awarded astronaut wings to pilots who crossed this mark since the X-15 program in the 1960s, and the FAA follows the same standard for commercial spaceflight crews today.
Adding another layer, NASA Mission Control uses a separate operational boundary at 76 miles (122 kilometers), the point where atmospheric drag becomes noticeable enough to affect a returning spacecraft’s trajectory. So even within a single agency, “where space starts” can shift depending on the purpose of the measurement.
Why There’s No Single Answer
The atmosphere doesn’t end with a sharp cutoff. It thins gradually over hundreds of kilometers. The thermosphere, starting around 80 kilometers up, is cloud-free and nearly vacuum-like, yet still technically contains trace molecules. Above roughly 700 kilometers begins the exosphere, where air particles are so sparse they don’t behave like a gas at all. The exosphere stretches out to about 10,000 kilometers before merging with the solar wind. By that measure, you could argue Earth’s atmosphere extends more than halfway to the Moon.
Because the transition is gradual rather than abrupt, any boundary is somewhat arbitrary. It reflects a practical choice: the point where one set of physics (aerodynamics, air pressure, weather) stops mattering and another set (orbital mechanics, vacuum, radiation) takes over.
International law hasn’t resolved the question either. The United Nations Committee on the Peaceful Uses of Outer Space has acknowledged that the lack of a formal definition and delimitation of outer space remains a significant gap in international law. Countries govern their own airspace but share outer space as a commons under the 1967 Outer Space Treaty. Where one ends and the other begins is still an open legal question.
The Scientific Case for 80 Kilometers
In 2018, astrophysicist Jonathan McDowell at the Harvard-Smithsonian Center for Astrophysics published a detailed reexamination of the Kármán line. He argued that von Kármán’s original 100-kilometer figure rested on outdated assumptions about the speed a vehicle would need to fly suborbitally, essentially pegging it to a technology-specific threshold rather than a fundamental physical one.
McDowell’s analysis found that 80 kilometers is the altitude where atmospheric density drops low enough that a vehicle must travel at roughly 1 kilometer per second to maintain even a brief ballistic trajectory. Below that altitude, aerodynamic forces still play a meaningful role. Above it, you’re in a regime where only orbital mechanics matter. His conclusion lined up neatly with the boundary the U.S. had already been using for decades.
What Your Body Would Experience
Long before reaching either proposed boundary, the environment becomes lethal. At about 19,200 meters (63,000 feet), roughly one-fifth of the way to the 100-kilometer line, you cross what’s known as the Armstrong limit. At this altitude, air pressure drops low enough that bodily fluids begin to boil at normal body temperature. Animal studies at these altitudes have documented severe oxygen deprivation, a doubling of body volume from gas expansion, loss of consciousness within about 10 seconds, and complete circulatory arrest within 30 seconds. Without a pressurized suit or cabin, survival is measured in seconds, not minutes.
This is worth knowing because it highlights how “space-like” conditions begin far below any official space boundary. The protective blanket of atmosphere that keeps you alive thins dramatically in the first 20 kilometers. The remaining 60 to 80 kilometers to the recognized edge of space are increasingly empty.
How High Commercial Flights Actually Go
The 20-kilometer gap between the two boundaries is exactly where commercial spaceflight companies have staked competing claims. Blue Origin’s New Shepard capsule reaches apogees around 107 kilometers (66 miles), comfortably above both the 80-kilometer U.S. line and the 100-kilometer Kármán line. Virgin Galactic’s SpaceShipTwo, by contrast, has flown passengers above 80 kilometers but below 100 kilometers, placing them in a zone that counts as space by U.S. standards but falls short of the international FAI threshold.
This isn’t just trivia. Whether your flight “counts” as a spaceflight determines whether you receive astronaut wings, how the flight is regulated, and how the experience is marketed. Both companies have strong commercial incentives to define space in whichever way includes their vehicle.
Where Satellites Actually Orbit
Regardless of where you draw the line on paper, practical spaceflight demands much higher altitudes. The lowest stable orbits sit around 150 kilometers above Earth’s surface. Below that, residual atmospheric drag slows a spacecraft enough that it will lose altitude and eventually reenter. At 150 kilometers, the atmosphere is thin enough that a vehicle traveling around 30,000 kilometers per hour (19,000 mph) can circle the planet without continuous propulsion, needing only occasional small adjustments to maintain its orbit.
The International Space Station orbits between roughly 370 and 460 kilometers, well into the thermosphere. Even at that altitude, it experiences enough faint drag that it needs periodic reboosts. True freedom from Earth’s atmosphere, in operational terms, starts considerably higher than any proposed boundary line.