Where does outer space begin

     In the margins of Earth's atmosphere, a new era of commercial and military space exploration is unfolding, as private companies like Virgin Galactic and Blue Origin prepare to offer space tourism experiences, while military forces experiment with hypersonic flight in the upper reaches, and hucksters advertise high-altitude stratospheric balloon flights, blurring the lines between the Earth's atmosphere and the vast expanse of space and raising the long-standing question of where exactly outer space begins and how high one must travel to be considered a true space traveler, a question that is not easily answered due to the gradual, exponential decrease in atmospheric density, leading to ongoing debates and discussions about the precise definition of the boundary between the Earth's gaseous envelope and the vacuum of space, a quest that has become increasingly important as the race to the final frontier intensifies, with implications for the legal and regulatory frameworks governing space activities and the recognition of those who venture into the upper atmosphere.

    The definition of outer space is the most widely accepted. In 1963, it was released by the lawyer for space Andrew Haley. Modern commentators often assume that the Kármán altitude line defines its order of magnitude estimate of 100 km.

    The original point of Kármán was that there was an altitude where it was impossible to generate a wing lift. The drag dominates gravity forces on an orbiting satellite at a similar height, showed by essentially identical calculations.

    The drag is proportional to the density of the atmosphere and the velocity of the satellite. The high density causes a rapid slow-down below this height and causes the satellite to fail. The solid braking also generates high heating and the satellite quickly splits off and melts in the absence of a heat shield. 

     Practical evidence suggests a reasonable boundary is the 80 km line. Perigee satellites in the range of 80-90 km fall below 100 km every few hours. The boundary of 80 km corresponds fairly well with the typical mesopausal altitude. The boundary used since 1961 to award "astronaut wings" to US military pilots, including several who flew the suborbital space plane X-15, is closely equal to 50 statute miles.

    All vehicles that need aerodynamic lift to remain aloft are restricted to the stratosphere. Above the stratopause is the mesosphere region between 50 km and 80 km. That is a forbidden zone, where neither aircraft nor satellites can fly. Russia and other spacefaring countries have repeatedly suggested adopting the 100 km boundary, or something near it. The US government, though, has long resisted any official legal definition of space. 

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