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[b]Where does 'space' begin?[/b]
By Louise Thorn, Astronomy Education Assistant, National Air and Space Museum
When humankind first began going into space, there was no worldwide consensus as to what "space" was, and surprisingly, there still isn't. The need for an official definition became apparent with the launch of Earth's first artificial satellite, Sputnik, on October 4, 1957, which traveled around Earth at altitudes between 227 kilometers (142 miles) at the closest point and 945 kilometers (591 miles) at the farthest.
In the 1960s, the X-15 space plane missions gave us the first glimpses of human space travel. The pioneering X-15 pilots were awarded "astronaut wings" whenever they exceeded the threshold of 80 kilometers (50 miles) altitude, the definition of "space" as deemed by the U.S. Air Force.
However, the Federation Aeronautique Internationale (FAI), the recognized world authority for aeronautical records, declared that aircraft would have to exceed 100 kilometers (62.5 miles) in order for the pilot to qualify for astronaut status. To win the X Prize, competitors must reach the 100 kilometer altitude.
Why 100 kilometers? Because for an aircraft, flying above this altitude is impossible, since lift is no longer effective in the thinning atmosphere. And a spacecraft moving below this altitude would encounter increasing drag, thus reducing its circular orbital speed and causing it to spiral toward Earth. Tests with a Soviet low-altitude satellite in the 1960s confirmed this theory by completing a few orbits above 100 kilometers before rapidly tumbling to earth as it dipped below the 100 kilometer threshold.
This 100 kilometer boundary is known as the Karman Line, having been proposed by Hungarian scientist Theodore Von Karman. Simply put, it denotes where aeronautics becomes astronautics, aeronautics being flight that requires atmosphere, and astronautics being flight that does not require atmosphere. Although 100 kilometers sounds suspiciously convenient, the Karman line can actually be scientifically measured as a definite change.
As the Space Race between the United States and the Soviet Union intensified, the United Nations decided to create an Office for Outer Space Affairs to add legal definition to space activities. Intriguingly, the 1967 Outer Space Treaty itself does not include a definition of where outer space begins, so we can find no clues here.
The only legal document containing a definition of space is the Australian Space Activities Act of 1998 which requires all objects launched from the continent to an altitude greater than 100 kilometers be licensed. Although major space agencies such as the National Aeronautics and Space Administration and the European Space Agency use the general consensus of 100 kilometers, there is still no legally binding stipulation to accept this value. Other definitions of space include: in medicine, the point at which humans must have life-support systems to survive is 24 kilometers/15 miles; in propulsion, the distance where a propulsion system needs to provide its own oxygen as well as fuel is 45 kilometers/28 miles; and the 'atmospheric interface' encountered by the Space Shuttle orbiter on re-entry is at a distance of 122 kilometers/76 miles.
So, to answer our question, "Where does space begin?" we can only reply, "Depends on who you ask!"
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