A common definition of space is known as the Kármán line, an imaginary limit 100 kilometers (62 miles) above the mean sea level. In theory, once this 100 km line is crossed, the atmosphere becomes too thin to provide enough lift for conventional aircraft to maintain their flight. In the 1950s, United States Air Force pilots received a special set of astronaut wings for flying their planes over 50 miles, which is considered the outermost edge of the atmosphere. This number also fits in perfectly with several other cultural and atmospheric factors.
To stay in the air and escape Earth's gravity, a plane must travel at a speed of over 17,000 miles per hour. Most satellites orbit much higher than the proposed Karman line and are well within reach of orbital space. There are no national boundaries that extend to outer space; it is governed rather like international waters. So what does it mean if the boundary between Earth and space is 20 percent lower than what's generally accepted? It won't change the way rockets are launched or any other physical interaction with space, but it could pose some important political and territorial problems.
NASA uses 76 miles as a re-entry altitude, the point at which atmospheric resistance is noticeable, and the space shuttle can go from driving with thrusters to maneuvering like a conventional airplane. The government has been reluctant to accept a specific height; people who fly above 60 miles (100 km) usually gain astronaut wings from the Federal Aviation Administration. When McDowell used a mathematical model to find the exact point at which several satellites were finally released from their orbits and returned to the atmosphere, he discovered that this could occur between 41 and 55 miles (66 and 88 km). The International Space Station maintains an orbit of approximately 250 miles (400 kilometers) above sea level, while the Hubble Space Telescope operates at an altitude of approximately 340 miles (550 kilometers).
So how did humans come to accept this relatively close location as the defining line between Earth and space? From these cases, it seemed clear that the physics of space still prevailed far below the Karman line. In addition to the space limit line, the name von Kármán is associated with a series of engineering equations, laws, constants and aerospace designs, as well as with a handful of awards in this field. The Air Force in the 1950s gave away astronaut wings to test pilots who flew more than 50 miles (80 km) high.