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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.
Another hurdle to strictly defining the boundary to space is the dynamic nature of Earth's atmosphere. For example; at an altitude of 1000 km, the atmosphere's density may vary by a factor of five, depending on the time of day, time of year, AP magnetic index, and recent solar flux.
The FAI apparently doesn't itself use the precise words "boundary to space" or "edge of space"; the FAI uses the term "Kármán line" or speaks of a "100 km. altitude boundary for astronautics", as also reflected in their following two definitions (quoted verbatim from their website):
Also see the article on the Kármán line under External links below, which has an excellent explanation on how this boundary was determined.
Some people (including 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.
Reentry from orbit begins at 122 km.
A common misunderstanding about the boundary to space is that orbit occurs by reaching this altitude. Orbit, however, requires orbital speed and can theoretically occur at any altitude. Atmospheric drag precludes an orbit that is too low.
Minimal altitudes for a stable orbit begin at around 350 km (220 miles) above mean sea level, so to actually perform an orbital spaceflightAn orbital spaceflight (or orbital flight) in the general sense is a spaceflight where the spacecraft reaches the height of, and through having an appropriate velocity enters into, orbit around an astronomical body. Most commonly the term is applied speci, a spacecraft would need to go higher and (more importantly) faster than what would be required for a sub-orbital spaceflightA sub-orbital spaceflight (or sub-orbital flight) is a spaceflight that does not involve putting a vehicle into orbit. Sub-orbital flights have been undertaken to test spacecraft and launch vehicles intended for later orbital flight, but some vehicles, su.
Reaching orbit requires tremendous speed. A craft has not reached orbit until it is circling Earth so quickly that the upward centrifugal "force"In classical mechanics, centrifugal force is the experience of the inertia of an object moving in circular motion, causing it to move away from the center. It is often designated a " fictitious force". We know from Newton's first law of motion that a body cancels the downward gravitational forceThis article covers the physics of gravitation. See also gravity (disambiguation). Gravitation is the tendency of masses to move toward each other. The first mathematical formulation of the theory of gravitation was made by Sir Isaac Newton and proved ast 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 orbitA Low Earth Orbit (LEO) is an orbit in which objects such as satellites are below intermediate circular orbit (ICO) and far below geostationary orbit, but typically around 350 1400 km above the Earth's surface. Orbits lower than this are not stable, and w, the speed is about 7.9km per second (18,000 mph). Thus, achieving the necessary altitude is only the first step in reaching orbit.
The energy required to reach velocity for low earth orbit (32 MJ/kg) is about twenty times the energy to reach the corresponding altitude (10 kJ/km/kg).
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