--- title: "Outer space" id: "outer-space-1" project: "Deep Space Exploration Project Publication2" url: "https://hedronlabs.clickhelp.co/articles/deep-space-exploration-project-publication2/outer-space-1" updated: 2023-08-24T16:48:35Z --- # Outer space # What is outer space? Outer space, commonly shortened to space, is an infinite expanse that exists beyond [Earth](https://en.wikipedia.org/wiki/Earth) and [its atmosphere](https://en.wikipedia.org/wiki/Atmosphere_of_Earth) and between [celestial bodies](https://en.wikipedia.org/wiki/Astronomical_object). Outer space is not completely empty; it is a [near-perfect vacuum](https://en.wikipedia.org/wiki/Ultra-high_vacuum) containing a low density of particles, predominantly a [plasma](https://en.wikipedia.org/wiki/Plasma_%28physics%29) of [hydrogen](https://en.wikipedia.org/wiki/Hydrogen) and [helium](https://en.wikipedia.org/wiki/Helium), as well as [electromagnetic radiation](https://en.wikipedia.org/wiki/Electromagnetic_radiation), [magnetic fields](https://en.wikipedia.org/wiki/Magnetic_field), [neutrinos](https://en.wikipedia.org/wiki/Cosmic_neutrino_background), [dust](https://en.wikipedia.org/wiki/Cosmic_dust), and [cosmic rays](https://en.wikipedia.org/wiki/Cosmic_ray). The baseline [temperature](https://en.wikipedia.org/wiki/Temperature) of outer space, as set by the [background radiation](https://en.wikipedia.org/wiki/Cosmic_background_radiation) from the [Big Bang](https://en.wikipedia.org/wiki/Big_Bang), is 2.7 [kelvins](https://en.wikipedia.org/wiki/Kelvin) (−270 [°C](https://en.wikipedia.org/wiki/Celsius); −455 [°F](https://en.wikipedia.org/wiki/Fahrenheit)). The [plasma between galaxies](https://en.wikipedia.org/wiki/Warm%E2%80%93hot_intergalactic_medium) is thought to account for about half of the [baryonic (ordinary) matter](https://en.wikipedia.org/wiki/Baryonic_matter) in the universe, having a [number density](https://en.wikipedia.org/wiki/Number_density) of less than one [hydrogen atom](https://en.wikipedia.org/wiki/Hydrogen_atom) per [cubic metre](https://en.wikipedia.org/wiki/Cubic_metre) and a [kinetic temperature](https://en.wikipedia.org/wiki/Kinetic_temperature) of millions of kelvins. Local concentrations of matter have condensed into [stars](https://en.wikipedia.org/wiki/Star) and [galaxies](https://en.wikipedia.org/wiki/Galaxy). Intergalactic space takes up most of the volume of the [universe](https://en.wikipedia.org/wiki/Universe), but even galaxies and [star systems](https://en.wikipedia.org/wiki/Star_system) consist almost entirely of empty space. Most of the remaining [mass-energy](https://en.wikipedia.org/wiki/Mass%E2%80%93energy_equivalence) in the [observable universe](https://en.wikipedia.org/wiki/Observable_universe) is made up of an unknown form, dubbed [dark matter](https://en.wikipedia.org/wiki/Dark_matter) and [dark energy](https://en.wikipedia.org/wiki/Dark_energy). Outer space does not begin at a definite altitude above Earth's surface. The [Kármán line](https://en.wikipedia.org/wiki/K%C3%A1rm%C3%A1n_line), an altitude of 100 km (62 mi) above [sea level](https://en.wikipedia.org/wiki/Sea_level), is conventionally used as the start of outer space in space treaties and for aerospace records keeping. Certain portions of the upper [stratosphere](https://en.wikipedia.org/wiki/Stratosphere) and the [mesosphere](https://en.wikipedia.org/wiki/Mesosphere) are sometimes referred to as "near space". The framework for international [space law](https://en.wikipedia.org/wiki/Space_law) was established by the [Outer Space Treaty](https://en.wikipedia.org/wiki/Outer_Space_Treaty), which entered into force on 10 October 1967. This treaty precludes any claims of [national sovereignty](https://en.wikipedia.org/wiki/National_sovereignty) and permits all states to freely [explore outer space](https://en.wikipedia.org/wiki/Space_exploration). Despite the drafting of [UN resolutions](https://en.wikipedia.org/wiki/UN_resolution) for the peaceful uses of outer space, [anti-satellite weapons](https://en.wikipedia.org/wiki/Anti-satellite_weapon) have been tested in Earth orbit. Humans began the physical exploration of space during the 20th century with the advent of high-altitude [balloon flights](https://en.wikipedia.org/wiki/Balloon_%28aircraft%29). This was followed by crewed [rocket flights](https://en.wikipedia.org/wiki/Rocket_launch) and, then, crewed [Earth orbit](https://en.wikipedia.org/wiki/Earth_orbit), first achieved by [Yuri Gagarin](https://en.wikipedia.org/wiki/Yuri_Gagarin) of the [Soviet Union](https://en.wikipedia.org/wiki/Soviet_Union) in 1961. The economic cost of putting objects, including humans, into space is very high, limiting human [spaceflight](https://en.wikipedia.org/wiki/Spaceflight) to low Earth orbit and the [Moon](https://en.wikipedia.org/wiki/Moon). On the other hand, [uncrewed spacecraft](https://en.wikipedia.org/wiki/Uncrewed_spacecraft) have reached all of the known [planets](https://en.wikipedia.org/wiki/Planet) in the [Solar System](https://en.wikipedia.org/wiki/Solar_System). Outer space represents a challenging environment for human exploration because of the hazards of vacuum and [radiation](https://en.wikipedia.org/wiki/Radiation). [Microgravity](https://en.wikipedia.org/wiki/Microgravity) has a negative effect on human [physiology](https://en.wikipedia.org/wiki/Physiology) that causes both [muscle atrophy](https://en.wikipedia.org/wiki/Muscle_atrophy) and [bone loss](https://en.wikipedia.org/wiki/Spaceflight_osteopenia). Outer space does not begin at a definite altitude above Earth's surface. The [Kármán line](https://en.wikipedia.org/wiki/K%C3%A1rm%C3%A1n_line), an altitude of 100 km (62 mi) above [sea level](https://en.wikipedia.org/wiki/Sea_level), is conventionally used as the start of outer space in space treaties and for aerospace records keeping. Certain portions of the upper [stratosphere](https://en.wikipedia.org/wiki/Stratosphere) and the [mesosphere](https://en.wikipedia.org/wiki/Mesosphere) are sometimes referred to as "near space". The framework for international [space law](https://en.wikipedia.org/wiki/Space_law) was established by the [Outer Space Treaty](https://en.wikipedia.org/wiki/Outer_Space_Treaty), which entered into force on 10 October 1967. This treaty precludes any claims of [national sovereignty](https://en.wikipedia.org/wiki/National_sovereignty) and permits all states to freely [explore outer space](https://en.wikipedia.org/wiki/Space_exploration). Despite the drafting of [UN resolutions](https://en.wikipedia.org/wiki/UN_resolution) for the peaceful uses of outer space, [anti-satellite weapons](https://en.wikipedia.org/wiki/Anti-satellite_weapon) have been tested in Earth orbit.