Outer space, sometimes 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, dark matter and dark energy.
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 several dozen types of 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.
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": the force of gravity is creating an inward centripetal force which is stopping them from flying out into space, balanced by the (reactive) centrifugal force induced by their linear velocity. 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 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. Pictures and other data from unmanned space vehicles have provided invaluable information about the planets, asteroids and comets in our solar system.
NASA has had many successful space missions and programs, including over 100 manned missions. Many of the notable manned missions were from the Apollo program, a sequence of missions to the Moon which included the achievement of the first man to walk on the Moon, during Apollo 11. The Space Shuttle Program has also been a huge success, despite the loss of two of the Space Shuttles, Challenger and Columbia which resulted in the deaths of their entire crews. The Space Shuttles were able to dock with the space station Mir while it was operational, and are now able to dock with the International Space Station - a joint project of many space agencies. NASA's future plans for space exploration are with the Project Constellation.
