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Edge of space)
The boundary to space or edge of space, according to definitions by the FAI, lies at a height of 100 km (about 62 miles) above Earth's surface (ie. in technical terms 100 km above mean sea level). This height is also known as the Karman line, after Theodore von Karman. It is deemed to be roughly equivalent to being "just outside of Earth's atmosphere".
Overview
Strictly speaking, there is no such thing as an end to Earth's atmosphere: An atmosphere doesn't technically end at any given height, but becomes progressively thinner with altitude. Also, depending on how the various layers that make up the space around the Earth are defined (and depending on whether these layers are considered as part of the actual atmosphere), the definition of the edge of space could vary considerably: If one were to consider the thermosphere and exosphere only part of the atmosphere and not of space, one might have to place the boundary to space as high as about 10,000 km (6200 miles) up. It is quite reasonable to state that attaining a height of 100 km is escaping the Earth's atmosphere, as spacecraft at this altitude cannot rely on it anymore for weight support or propulsion. Perhaps more importantly, the FAI's definitions regarding the boundary to space are internationally accepted.
The FAI apparently doesn't itself use the precise words "boundary to space" or "edge of space"; the FAI uses the term "Karman line" or speaks of a "100 Km. altitude boundary for astronautics", as also reflected in their following two definitions (quoted verbatim from their website):
- Aeronautics -- For FAI purposes, aerial activity, including all air sports, within 100 kilometres of Earth's surface.
- Astronautics -- For FAI purposes, activity more than 100 kilometres above Earth's surface.
Also see the article on the Karman line under External links below, which has an excellent explanation on how this boundary was determined.
A diluted definition
Some people (including, unfortunately, the FAI in some of their publications) also use the expression "edge of space" to refer to a very vaguely defined (essentially undefined) region below the actual 100 km boundary to space, which is often meant to include substantially lower regions as well. Thus, certain balloon or airplane flights might be described as "reaching the edge of space", when they really don't even go half as high as 100 km up. In such statements, "reaching the edge of space" merely refers to going somewhat higher than average aeronautical vehicles would commonly go.
The U.S. definition
U.S. authorities define the boundary to space to lie at a height of 50 miles (about 80 km) above mean sea level. This definition is thought by some to be outdated, and is not commonly accepted internationally. Some cynics argue that the U.S. definition exists solely for historical reasons, because after the Yuri Gagarin PR disaster the U.S. needed an astronaut, and fast, so they might have decided to lower the bar a bit. If that were true however, it would have been an unnecessary step: Alan Shepard, the first U.S. astronaut, travelled 185 km (115 miles) up.
Gagarin's flight and FAI spaceflight guidelines
According to strict FAI rules, Gagarin's flight did not constitute a spaceflight. The FAI's Sporting Code, states that a pilot and his or her craft must take off together and land together, a condition that Gagarin's space flight did not meet (and other Vostok flights as well). He parachuted from the craft and landed separately. The Soviet Union covered up this fact for many years after the flight.
Space does not equal orbit
A common misunderstanding about the boundary to space is that people wrongly assume this to also be the height where satellites and other spacecraft would orbit Earth. While a satellite could theoretically orbit Earth at any altitude, atmospheric drag practically precludes an orbit that is too low. Minimal practical altitudes for a stable orbit begin at around 350 km (220 miles) above mean sea level, so to actually perform an orbital spaceflight, a spacecraft would need to go higher and (more importantly) faster than what would be required for a sub-orbital spaceflight.
Reaching orbit requires tremendous speed. A craft has not reached orbit until it is circling Earth so quickly that the upward centrifugal "force" cancels the downward gravitational force on the craft. Having climbed up out of the atmosphere, a craft entering orbit must then turn sideways and continue firing its rockets to reach the necessary speed; for low Earth orbit, the speed is about 7.9km per second (18,000 mph). Thus, achieving the necessary altitude is only the first step in reaching orbit.
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