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Outer space, often simply called space, refers to the relatively empty regions of the universe outside the atmospheres of celestial bodies. Outer space is used to distinguish it from airspace (and terrestrial locations). Contrary to popular understanding, outer space is not completely empty (i.e. a perfect vacuum) but contains a low density of particles, predominantly hydrogen plasma, as well as electromagnetic radiation. Hypothetically, it also contains dark matter and dark energy.
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There is no clear boundary between Earth\'s atmosphere and space as the density of the atmosphere gradually decreases as the altitude increases. Nevertheless, the Fédération Aéronautique Internationale has established the Kármán line at an altitude of 100 km (62 miles) as a working definition for the boundary between aeronautics and astronautics. This is used because above an altitude of roughly 100 km, as Theodore von Kármán calculated, a vehicle would have to travel faster than orbital velocity in order to derive sufficient aerodynamic lift from the atmosphere to support itself. The United States designates people who travel above an altitude of 80 km (50 statute miles) as astronauts. During re-entry, roughly 120 km (75 miles) marks the boundary where atmospheric drag becomes noticeable, depending on the ballistic coefficient of the vehicle.
Outer space within the solar system is called interplanetary space, which passes over into interstellar space at the heliopause. The vacuum of outer space is not really empty; it is sparsely filled with cosmic rays, which include ionized atomic nuclei and various subatomic particles. There is also gas, plasma and dust, small meteors, and several dozen types of organic molecules discovered to date by microwave spectroscopy. According to the Big bang theory and confirmed by observations, outer space also contains 2.7 K blackbody radiation left over from the \'big bang\' and the origin of the universe. Close to Earth, outer space contains material left over from previous manned and unmanned launches that are a potential hazard to spacecraft. Some of this debris re-enters Earth\'s atmosphere periodically.
The absence of air makes outer space (and the surface of the Moon) ideal locations for astronomy at all wavelengths of the electromagnetic spectrum, as evidenced by the spectacular pictures sent back by the Hubble Space Telescope, allowing light from about 13.7 billion years ago — almost to the time of the Big Bang — to be observed.
Study of space via human spaceflight has been ongoing since 1961.
While not being an actual perfect vacuum, outer space contains such sparse matter that it can be effectively thought of as one. The pressure of interstellar space is on the order of 10 pPa (1×10-11 Pa)Zheng, MiMi (2002). Pressure in Outer Space. The Physics Factbook.. For comparison, the pressure at sea level (as defined in the unit of atmospheric pressure) is about 101 kPa (1×105 Pa), or about 10 quadrillion times greater.
Contrary to popular belief,NASA Human Body in a Vacuum a person suddenly exposed to the vacuum would not explode, freeze to death (space may be cold, but it\'s mostly vacuum and can hardly transfer heat, so the main temperature worry for space suits is how to get rid of naturally generated body heat), or die from boiling blood, but would take a short while to die by asphyxiation (suffocation). Air would immediately leave the lungs due to the enormous pressure gradient. Any oxygen dissolved in the blood would empty into the lungs to try to equalize the partial pressure gradient. Once the deoxygenated blood arrived at the brain, death would quickly follow. Water vapor would also rapidly evaporate off from exposed areas such as the lungs, cornea of the eye and mouth, cooling the body.
There are many artificial satellites orbiting Earth, including geosynchronous communication satellites 35,786 km (22,241 miles) above mean sea level at the Equator. There is also increasing reliance, for both military and civilian uses, on satellites which enable the Global Positioning System (GPS). A common misconception is that people in orbit are outside Earth\'s gravity because they are "floating". They are floating because they are in "free fall": they are accelerating toward Earth, along with their spacecraft, but are simultaneously moving sideways fast enough that the "fall" away from a straight-line path merely keeps them in orbit at a constant distance above Earth\'s surface. Earth\'s gravity reaches out far past the Van Allen belt and keeps the Moon in orbit at an average distance of 384,403 km (238,857 miles).
To perform an orbital spaceflight, a spacecraft must travel away from Earth faster than it must for a sub-orbital spaceflight. A spacecraft has not entered orbit until it is traveling with a sufficiently great horizontal velocity such that the acceleration due to gravity on the spacecraft is less than or equal to the centripetal acceleration being caused by its horizontal velocity (see circular motion). So to enter orbit, a spacecraft must not only reach space, but must also achieve a sufficient orbital speed (angular velocity). For a low-Earth orbit, this is about 7.9 km/s (18,000 mph); by contrast, the fastest airplane speed ever achieved (excluding speeds achieved by deorbiting spacecraft) was 2.02 km/s (4,520 mph) in 1967 by the North American X-15Linda Shiner. X-15 Walkaround: A short guide to the fastest airplane ever.. Air & Space Magazine. Retrieved on 2007-01-19.. Konstantin Tsiolkovsky was the first to realize that, given the energy available from any available chemical fuel, a several-stage rocket would be required. The escape velocity to pull free of Earth\'s gravitational field altogether and move into interplanetary space is about 40,000 km/h (25,000 mph or 11,000 m/s). The energy required to reach velocity for low Earth orbit (32 MJ/kg) is about twenty times the energy required simply to climb to the corresponding altitude (10 kJ/(km·kg)).
There is a major difference between sub-orbital and orbital spaceflights. The minimum altitude for a stable orbit around Earth (that is, one without significant atmospheric drag) begins at around 350 km (220 miles) above mean sea level. A common misunderstanding about the boundary to space is that orbit occurs simply by reaching this altitude. Achieving orbital speed can theoretically occur at any altitude, although atmospheric drag precludes an orbit that is too low. At sufficient speed, an airplane would need a way to keep it from flying off into space, but at present, this speed is several times greater than anything within reasonable technology.
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