Outer 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 gas, as well as electromagnetic radiation.
There is no clear boundary between the Earth's atmosphere and space as the density of the atmosphere gradually decreases as the altitude increases. Nevertheless, the Federation Aeronautique Internationale has established the Kármán line at an altitude of 100 km (62 miles) as a working definition for the boundary between atmosphere and space. This is used because, as Karman calculated, above an altitude of roughly 100 km, 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 50 statute miles (80 km) as Astronauts. During re-entry, 120 km (75 miles) marks the boundary where atmospheric drag becomes noticeable.
Solar System outer space.
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 several dozen organic molecules discovered to date by Microwave spectroscopy. According to the Big bang theory,2.7 K blackbody radiation was left over from the 'big bang' and the origin of the universe, and cosmic rays, which include ionized atomic nuclei and various subatomic particles. There is also gas, plasma and dust, and small meteors and material left over from previous manned and unmanned launches that are a potential hazard to Spacecraft. Some of this debris re-enters the 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 14 billion years ago, back almost to the time of the Big Bang to be observed. Pictures and other data from unmanned space vehicles have provided invaluable information about the planets, asteroids and comets in our solar system.
The "vacuum of space".
While not being an actual perfect vacuum, outer space contains such sparse matter that it can be effectively thought of as one. The pressure gradient of a vessel kept at sea-level Atmospheric pressure and the surrounding area is equal to roughly 101 kPa, roughly equal to a vessel at an underwater depth of about 10 metres (34 ft).
Contrary to popular belief, a person suddenly exposed to the vacuum would not explode, freeze to death, 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, and so any dissolved oxygen in the blood would empty into the lungs to try to equalise 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 and cornea of the eye, cooling the body. Any exposed skin would immediately succumb to sunburn.
Satellites in outer space.
There are many artificial satellites orbiting the Earth, including geosynchronous communication satellites 35,786 km (22,241 miles) above mean sea level at the Equator. Their orbits never "decay" because there is almost no matter there to exert frictional drag. There is also increasing reliance, for both military and civilian uses, of satellites which enable the Global Positioning System (GPS). A common misconception is that people in orbit are outside Earth's gravity because they are obviously "floating". They are floating because they are in "free fall": the force of gravity and their linear velocity is creating an inward centripetal force which is stopping them from flying out into space. 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). The gravity of all celestial bodies drops off toward zero with the inverse square of the distance.
Milestones on the way to outer space.
Regions of outer space.
Space does not equal orbit.
To perform an Orbital spaceflight, a spacecraft must travel faster than for a sub-orbital spaceflight. A spacecraft has not entered orbit until it is travelling 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 caused being 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). Konstantin Tsiolkovsky was the first to realize that, given the energy available from any available chemical fuel, a several-stage rockets 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)).
So there is a major difference between sub-orbital and orbital spaceflights. The minimum altitude for a stable orbit around the 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.