When are you in space

When are you in space

With the flight of Richard Branson onboard the VSS Unity in July 2021 the discussion on what is space has been more intense than ever. This blog post aims not to finish the discussion but to provide insight into what the various altitude means.

FAI and the Karman line

The FAI or Fédération Aéronautique Internationale follows the Karman Line. Theodore von Karman. Set out to find an altitude where a vehicle’s orbital velocity is more important than lift. For instance, during the X-2 flight of Iven Carl Kincheloe Jr, the vehicle flew at 3200 km/h at 38.5 km. At this altitude, about 2 per cent of the plane lift is carried by centrifugal force and 98% of its lift from the wings. However, at 91.440 km this relation is reversed and 98% of the velocity is centrifugal or Keppler force. The calculated value by Karman was then rounded up to 100 km and became the FAI’s definition of space.


The United States air force defines space at an altitude of 50 miles or 80 kilometres (80.4672 km to be exact). The border was set as 80km is considered the top of the mesopause, a high-altitude layer in the atmosphere. NASA adopted the 80 km limit in 2005 to remove any discrepancies between USAF and NASA astronauts. In 2018 Jonathan McDowell and Thomas Gangale dove into the 50-mile limit and stated that a satellite can have a stable orbit with a periapsis of 100 km, but not a periapsis of 80 km. Basically, below 80 km the atmosphere becomes so dominant when flying at orbital velocities that one simply re-enters the atmosphere.

Stable circular orbit

The USAF and NASA look at elliptical orbits that are stable and look at circular stable orbits. It is quite hard to define an exact altitude at which a stable circular orbit is possible one can look at missions such as GOCE. The ESA Gravity Field and Steady-State Ocean Circulation Explorer flew in a 254.9 km orbit and required an ion thruster to stay in orbit. In May 2013, the satellite was lowered to 229 km and on October 18th the propellants ran out and the satellite re-entered on the 11th of November. Orbits below 450 km are recently classified as Very Low Earth orbits and are orbits where no long-term stable orbits are possible.

Armstrong limit

The third limit one can follow is the limit of the human physique. When a human is exposed to a pressure of less than 6.25 kPa, the liquids inside the body start to boil. This roughly translates to 19 km altitude. Above this limit, a pilot has seconds to dive when losing air pressure. In modern regulations, a pressure suit is required above 15 km. This limit of 6.25 kPa is called the Armstrong limit after Harry George Armstrong.

As a reference, any altitude above roughly 2.5 km requires a pressure cabin or oxygen as altitude sickness might start to occur. Although this altitude varies per person and is dependent on the pressure drop gradient. The current record for high altitude life stands at two years at an altitude of 5950 m. Above 8000 is defined as the “death zone”, and at this altitude, humans cannot work for long durations of time.

Sphere of influence

When zooming out even further one can wonder when a spacecraft is released from earth’s pull. A satellite in earth orbit can only remain in orbit due to its forward velocity. However, when you leave the SOI of Earth (Sphere Of Influence), you are no longer affected by the earth’s gravity. Or in more correct terms, the earth is no longer the dominant gravitational pull. The SOI of the earth is about 929000 km after which the dominant gravity is that of the sun and the spacecraft is in heliocentric orbit.


It should be clear now that the definition of space is fairly arbitrary and not defined by a hard law of physics. Depending on what one takes as a reference.

  • Human oxygen needs: 8 km
  • The boiling point of blood: 15km
  • Lowest possible periapsis of an elliptical orbit: 80 km
  • Orbital velocity: 100 km (rounded up)
  • Lowest possible long-term stable orbit: ~450km
  • Sphere of Influence of Earth: 92900 km

SpaceShipTwo shortly after release

X15 in flight

An X-15 in flight. The X-15 flew to space several times. Flights 91 and 90 passed the Karman line, and 13 flights passed the USAF definition of space.

Render of GOCE in flight. The Ion thruster together with solar panel wings can clearly be seen