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( Outer space)
Outer space (often called space) comprises 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. There is no distinct boundary between airspace and outer space. Definitions vary, but the most common is that space consists of everything beyond the Kármán line, which is 100 kilometers (62 mi) above the surface of the Earth. At this height, there is still detectable atmosphere, but not enough to sustain practical aerodynamic lift (The United States sometimes defines outer space as everything beyond 50 miles (80 km) in altitude.) 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. The term outer space was first recorded by H. G. Wells in 1901.[1] The shorter term space is actually older, first used to mean the region beyond Earth's sky in John Milton's Paradise Lost in 1667.[2] Outer space is the closest natural approximation of a perfect vacuum. It has effectively no friction, allowing stars, planets and moons to move freely along ideal gravitational trajectories. But no vacuum is truly perfect, not even in intergalactic space where there are still a few hydrogen atoms per cubic centimeter. (For comparison, the air we breathe contains about 1019 molecules per cubic centimeter.) The deep vacuum of space could make it an attractive environment for certain industrial processes, for instance those that require ultraclean surfaces; however, it is currently much less costly to create an equivalent vacuum on Earth than to leave the Earth's gravity well.
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