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Outer space, commonly shortened to space, is the expanse that exists beyond
Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remaining 70.8% is Water distribution on Earth, covered wit ...

Earth
and its atmosphere and between
celestial bodies In astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that studies astronomical object, celestial objects and celestial event, phenomena. It uses ...
. Outer space is not completely empty—it is a
hard vacuum A vacuum is space Space is the boundless three-dimensional extent in which objects and events have relative position and direction. Physical space is often conceived in three linear dimensions, although modern physicists usually consi ...
containing a low density of particles, predominantly a
plasma Plasma or plasm may refer to: Science * Plasma (physics), one of the four fundamental states of matter * Plasma (mineral) or heliotrope, a mineral aggregate * Quark–gluon plasma, a state of matter in quantum chromodynamics Biology * Blood plasma ...
of
hydrogen Hydrogen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same ...

hydrogen
and
helium Helium (from el, ἥλιος, helios Helios; Homeric Greek: ), Latinized as Helius; Hyperion and Phaethon are also the names of his father and son respectively. often given the epithets Hyperion ("the one above") and Phaethon ("the shining") ...

helium
, as well as
electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy and force. ...

electromagnetic radiation
,
magnetic field A magnetic field is a vector field In vector calculus and physics, a vector field is an assignment of a vector to each point in a subset of space. For instance, a vector field in the plane can be visualised as a collection of arrows with ...

magnetic field
s,
neutrinos A neutrino ( or ) (denoted by the Greek letter ) is a fermion In particle physics, a fermion is a particle that follows Fermi–Dirac statistics and generally has half odd integer spin: spin 1/2, Spin (physics)#Higher spins, spin 3/2, etc. T ...
,
dust Dust is made of s of solid . On Earth, it generally consists of particles in the that come from various sources such as lifted by wind (an ), , and . Dust in homes is composed of about 20–50% dead . The rest, and in offices, and other ...
, and
cosmic ray Cosmic rays are high-energy proton A proton is a subatomic particle, symbol or , with a positive electric charge of +1''e'' elementary charge and a mass slightly less than that of a neutron. Protons and neutrons, each with masses of approx ...
s. The baseline
temperature Temperature ( ) is a physical quantity that expresses hot and cold. It is the manifestation of thermal energy Thermal radiation in visible light can be seen on this hot metalwork. Thermal energy refers to several distinct physical concept ...

temperature
of outer space, as set by the
background radiation Background radiation is a measure of the level of ionizing radiation present in the environment at a particular location which is not due to deliberate introduction of radiation sources. Background radiation originates from a variety of sources, ...
from the
Big Bang The Big Bang is the prevailing of the from the through its subsequent large-scale evolution. The model describes how the from an initial state of high and , and offers a comprehensive explanation for a broad range of observed phenomen ...

Big Bang
, is . The plasma between galaxies is thought to account for about half of the baryonic (ordinary) matter in the universe, having a
number densityThe number density (symbol: ''n'' or ''ρ''N) is an intensive quantity used to describe the degree of concentration In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds ...
of less than one
hydrogen atom #REDIRECT Hydrogen atom A hydrogen atom is an atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday o ...

hydrogen atom
per
cubic metre The cubic metre (in Commonwealth English The use of the English language English is a of the , originally spoken by the inhabitants of . It is named after the , one of the ancient that migrated from , a peninsula on the (not to b ...
and a temperature of millions of kelvins. Local concentrations of matter have condensed into
star A star is an astronomical object consisting of a luminous spheroid of plasma Plasma or plasm may refer to: Science * Plasma (physics), one of the four fundamental states of matter * Plasma (mineral) or heliotrope, a mineral aggregate * Quark ...

star
s and
galaxies A galaxy is a gravitationally bound system of star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest star to ...

galaxies
. Studies indicate that 90% of the mass in most galaxies is in an unknown form, called
dark matter Dark matter is a hypothetical form of matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of atoms, ...

dark matter
, which interacts with other matter through
gravitation Gravity (), or gravitation, is a natural phenomenon Types of natural phenomena include: Weather, fog, thunder, tornadoes; biological processes, decomposition, germination seedlings, three days after germination. Germination is t ...

gravitation
al but not
electromagnetic force Electromagnetism is a branch of physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related ...
s. Observations suggest that the majority of the mass-energy in the
observable universe The observable universe is a ball-shaped region of the universe The universe ( la, universus) is all of space and time and their contents, including planets, stars, galaxy, galaxies, and all other forms of matter and energy. The Big Bang th ...
is ''
dark energy In physical cosmology Physical cosmology is a branch of cosmology concerned with the study of cosmological models. A cosmological model, or simply cosmology, provides a description of the largest-scale structures and dynamics of the unive ...

dark energy
'', a type of
vacuum energy Vacuum energy is an underlying background energy In physics, energy is the physical quantity, quantitative physical property, property that must be #Energy transfer, transferred to a physical body, body or physical system to perform Work ...
that is poorly understood. Intergalactic space takes up most of the volume of the
universe The universe ( la, universus) is all of space and time and their contents, including planets, stars, galaxy, galaxies, and all other forms of matter and energy. The Big Bang theory is the prevailing cosmological description of the development ...

universe
, but even galaxies and
star system A star system or stellar system is a small number of stars that orbit each other, bound by gravitational attraction. A large group of stars bound by gravitation is generally called a ''star cluster Star clusters are large groups of star ...

star system
s consist almost entirely of empty space. Outer space does not begin at a definite altitude above the Earth's surface. The
Kármán line The Kármán line is an attempt to define a boundary between Earth's atmosphere File:Atmosphere gas proportions.svg, Composition of Earth's atmosphere by volume, excluding water vapor. Lower pie represents trace gases that together compose ab ...
, an altitude of above sea level, is conventionally used as the start of outer space in space treaties and for aerospace records keeping. The framework for international
space law Space law is the body of law governing space-related activities, encompassing both international and domestic agreements, rules, and principles. Parameters of space law include space exploration, liability for damage, weapons use, rescue efforts, ...
was established by the
Outer Space Treaty russian: link=yes, Договор о космосе es, link=yes, Tratado sobre el espacio ultraterrestre , long_name =Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, including the Moon ...

Outer Space Treaty
, which entered into force on 10 October 1967. This treaty precludes any claims of
national sovereignty Westphalian sovereignty, or state sovereignty, is a principle in international law International law, also known as public international law and law of nations, is the set of rules, norms, and standards generally accepted in relations between nati ...
and permits all states to freely explore outer space. Despite the drafting of UN resolutions for the peaceful uses of outer space,
anti-satellite weapon Anti-satellite weapons (ASAT) are space weaponSpace weapons are weapon A weapon, arm or armament is any implement or device that can be used with the intent to inflict damage or harm. Weapons are used to increase the efficacy and efficiency of ...
s 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. This was followed by crewed rocket flights and, then, crewed
Earth orbit A geocentric orbit or Earth orbit involves any object orbit In celestial mechanics, an orbit is the curved trajectory of an physical body, object such as the trajectory of a planet around a star, or of a natural satellite around a planet, ...
, first achieved by
Yuri Gagarin Yuri Alekseyevich Gagarin; Gagarin's first name is sometimes transliterated as ''Yuriy'', ''Youri'', or ''Yury''. (9 March 1934 – 27 March 1968) was a Soviet The Soviet Union,. officially the Union of Soviet Socialist Republics. (U ...
of the
Soviet Union The Soviet Union,. officially the Union of Soviet Socialist Republics. (USSR),. was a that spanned during its existence from 1922 to 1991. It was nominally a of multiple national ; in practice and were highly until its final years. The ...
in 1961. Due to the high cost of getting into space, human
spaceflight Spaceflight (or space flight) is an application of astronautics to fly spacecraft into or through outer space, either human spaceflight, with or uncrewed spaceflight, without humans on board. Most spaceflight is uncrewed and conducted mainly wit ...
has been limited to low Earth orbit and the
Moon The Moon is Earth's only natural satellite. At about one-quarter the diameter of Earth (comparable to the width of Australia (continent), Australia), it is the largest natural satellite in the Solar System relative to the size of its plane ...

Moon
. On the other hand,
uncrewed spacecraft Uncrewed spacecraft or unmanned spacecraft are spacecraft A spacecraft is a vehicle or machine designed to fly in outer space. A type of artificial satellite alt=, A full-size model of the Earth observation satellite ERS 2 "> ...
have reached all of the known
planet A planet is an astronomical body orbiting a star or Stellar evolution#Stellar remnants, stellar remnant that is massive enough to be Hydrostatic equilibrium, rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and ...

planet
s in the
Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Sola ...

Solar System
. Outer space represents a challenging environment for human exploration because of the hazards of vacuum and
radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy and f ...

radiation
.
Microgravity The term micro-g environment (also μg, often referred to by the term microgravity) is more or less synonymous with the terms ''weightlessness Weightlessness is the complete or near-complete absence of the sensation of weight In science ...
also has a negative effect on human
physiology Physiology (; ) is the scientific Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity or awareness, of someone or something, such as facts A fact is an occurrence in the real world. ...
that causes both
muscle atrophy Muscle atrophy is the loss of skeletal muscle Skeletal muscles (commonly referred to as muscles) are Organ (biology), organs of the vertebrate muscular system that are mostly attached by tendons to bones of the skeleton. The muscle cells of ske ...
and bone loss. In addition to these health and environmental issues, the economic cost of putting objects, including humans, into space is very high.


Formation and state

The size of the whole universe is unknown, and it might be infinite in extent. According to the Big Bang theory, the very early Universe was an extremely hot and dense state about 13.8 billion years ago which rapidly expanded. About 380,000 years later the Universe had cooled sufficiently to allow protons and electrons to combine and form hydrogen—the so-called recombination epoch. When this happened, matter and energy became decoupled, allowing photons to travel freely through the continually expanding space. Matter that remained following the initial expansion has since undergone gravitational collapse to create
star A star is an astronomical object consisting of a luminous spheroid of plasma Plasma or plasm may refer to: Science * Plasma (physics), one of the four fundamental states of matter * Plasma (mineral) or heliotrope, a mineral aggregate * Quark ...

star
s,
galaxies A galaxy is a gravitationally bound system of star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest star to ...

galaxies
and other
astronomical object In astronomy, an astronomical object or celestial object is a naturally occurring physical object, physical entity, association, or structure that exists in the observable universe. In astronomy, the terms ''object'' and ''body'' are often us ...
s, leaving behind a deep vacuum that forms what is now called outer space. As light has a finite velocity, this theory also constrains the size of the directly observable universe. The present day
shape of the universe The shape of the universe, in physical cosmology Physical cosmology is a branch of cosmology concerned with the study of cosmological models. A cosmological model, or simply cosmology, provides a description of the largest-scale structures ...
has been determined from measurements of the
cosmic microwave background The cosmic microwave background (CMB, CMBR), in Big Bang The Big Bang Scientific theory, theory is the prevailing cosmological model explaining the existence of the observable universe from the Planck units#Cosmology, earliest known perio ...
using satellites like the
Wilkinson Microwave Anisotropy Probe The Wilkinson Microwave Anisotropy Probe (WMAP), originally known as the Microwave Anisotropy Probe (MAP), is an inactive uncrewed spacecraft operating from 2001 to 2010 which measured temperature differences across the sky in the cosmic micr ...
. These observations indicate that the spatial geometry of the observable universe is " flat", meaning that photons on parallel paths at one point remain parallel as they travel through space to the limit of the observable universe, except for local gravity. The flat Universe, combined with the measured mass density of the Universe and the accelerating
expansion of the Universe The expansion of the universe is the increase in distance between any two given gravitationally unbound parts of the observable universe with time. It is an intrinsic expansion whereby ''the scale of space itself changes''. The universe does n ...
, indicates that space has a non-zero
vacuum energy Vacuum energy is an underlying background energy In physics, energy is the physical quantity, quantitative physical property, property that must be #Energy transfer, transferred to a physical body, body or physical system to perform Work ...
, which is called
dark energy In physical cosmology Physical cosmology is a branch of cosmology concerned with the study of cosmological models. A cosmological model, or simply cosmology, provides a description of the largest-scale structures and dynamics of the unive ...

dark energy
. Estimates put the average energy density of the present day Universe at the equivalent of 5.9 protons per cubic meter, including dark energy,
dark matter Dark matter is a hypothetical form of matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of atoms, ...

dark matter
, and
baryonic matter In particle physics Particle physics (also known as high energy physics) is a branch of physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is th ...
(ordinary matter composed of atoms). The atoms account for only 4.6% of the total energy density, or a density of one proton per four cubic meters. The density of the Universe is clearly not uniform; it ranges from relatively high density in galaxies—including very high density in structures within galaxies, such as planets, stars, and s—to conditions in vast
voids Void may refer to: Science, engineering, and technology * Void (astronomy), the spaces between galaxy filaments that contain no galaxies * Void (composites), a pore that remains unoccupied in a composite material * Void, synonym for vacuum, a s ...
that have much lower density, at least in terms of visible matter. Unlike matter and dark matter, dark energy seems not to be concentrated in galaxies: although dark energy may account for a majority of the mass-energy in the Universe, dark energy's influence is 5
orders of magnitude An order of magnitude is an approximation of the logarithm In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geome ...
smaller than the influence of gravity from matter and dark matter within the Milky Way.


Environment

Outer space is the closest known approximation to a perfect vacuum. It has effectively no
friction Friction is the force In physics, a force is an influence that can change the motion (physics), motion of an Physical object, object. A force can cause an object with mass to change its velocity (e.g. moving from a Newton's first law, st ...

friction
, allowing stars,
planets A planet is an astronomical body orbit In physics, an orbit is the gravitationally curved trajectory of an physical body, object, such as the trajectory of a planet around a star or a natural satellite around a planet. Normally, orbit r ...

planets
, and
moons A natural satellite, or moon, is, in the most common usage, an astronomical body that orbit In physics, an orbit is the gravitationally curved trajectory of an physical body, object, such as the trajectory of a planet around a star or ...

moons
to move freely along their ideal
orbit In celestial mechanics, an orbit is the curved trajectory of an physical body, object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an satellite, artificial satellite around an object or po ...

orbit
s, following the initial formation stage. The deep vacuum of
intergalactic space Outer space is the expanse that exists beyond Earth and between celestial bodies. Outer space is not completely empty—it is a hard vacuum containing a low density of particles, predominantly a plasma of hydrogen Hydrogen is the c ...

intergalactic space
is not devoid of
matter In classical physics Classical physics is a group of physics theories that predate modern, more complete, or more widely applicable theories. If a currently accepted theory is considered to be modern, and its introduction represented a major ...
, as it contains a few hydrogen atoms per cubic meter. By comparison, the air humans breathe contains about 1025 molecules per cubic meter. The low density of matter in outer space means that
electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy and force. ...

electromagnetic radiation
can travel great distances without being scattered: the
mean free path In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regular succ ...

mean free path
of a
photon The photon ( el, φῶς, phōs, light) is a type of elementary particle In , an elementary particle or fundamental particle is a that is not composed of other particles. Particles currently thought to be elementary include the fundamental s ...

photon
in intergalactic space is about 1023 km, or 10 billion light years. In spite of this,
extinction Extinction is the termination of a kind of organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biol ...
, which is the
absorption Absorption may refer to: Chemistry and biology *Absorption (chemistry), diffusion of particles of gas or liquid into liquid or solid materials *Absorption (skin), a route by which substances enter the body through the skin *Absorption (pharmacolo ...
and
scattering Scattering is a term used in physics to describe a wide range of physical processes where moving particles or radiation of some form, such as light Light or visible light is electromagnetic radiation within the portion of the electromagnetic ...

scattering
of photons by dust and gas, is an important factor in galactic and intergalactic
astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that studies astronomical object, celestial objects and celestial event, phenomena. It uses mathematics, phys ...
. Stars, planets, and moons retain their
atmosphere An atmosphere (from the greek words ἀτμός ''(atmos)'', meaning 'vapour', and σφαῖρα ''(sphaira)'', meaning 'ball' or 'sphere') is a layer or a set of layers of gases surrounding a planet or other material body, that is held in ...

atmosphere
s by gravitational attraction. Atmospheres have no clearly delineated upper boundary: the density of atmospheric gas gradually decreases with distance from the object until it becomes indistinguishable from outer space. The Earth's atmospheric
pressure Pressure (symbol: ''p'' or ''P'') is the force In physics, a force is an influence that can change the motion (physics), motion of an Physical object, object. A force can cause an object with mass to change its velocity (e.g. moving fr ...

pressure
drops to about Pa at of altitude, compared to 100,000 Pa for the
International Union of Pure and Applied Chemistry The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations that represents chemists in individual countries. It is a member of the International Science Council (ISC). IUPAC ...
(IUPAC) definition of
standard pressure Standard temperature and pressure are standard sets of conditions for experimental measurements to be established to allow comparisons to be made between different sets of data. The most used standards are those of the International Union of Pure ...
. Above this altitude, isotropic gas pressure rapidly becomes insignificant when compared to
radiation pressure Radiation pressure is the mechanical pressure exerted upon any surface due to the exchange of momentum between the object and the electromagnetic field. This includes the momentum of light or electromagnetic radiation of any wavelength which is Ab ...

radiation pressure
from the
Sun The Sun is the star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many othe ...

Sun
and the
dynamic pressure In incompressible fluid dynamics In physics and engineering, fluid dynamics is a subdiscipline of fluid mechanics that describes the flow of fluids—liquids and gases. It has several subdisciplines, including aerodynamics (the study of air a ...
of the
solar wind The solar wind is a stream of charged particle In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies matter, i ...

solar wind
. The
thermosphere The thermosphere is the layer in the Earth's atmosphere directly above the mesosphere and below the exosphere. Within this layer of the atmosphere, ultraviolet radiation causes photoionization/photodissociation of molecules, creating ions; the th ...
in this range has large gradients of pressure, temperature and composition, and varies greatly due to
space weather Space weather is a branch of space physics and aeronomy, or heliophysics, concerned with the time varying conditions within the Solar System, including the solar wind, emphasizing the space surrounding the Earth, including conditions in the magn ...
. The temperature of outer space is measured in terms of the activity of the gas, as it is on Earth. The radiation of outer space has a different temperature than the kinetic temperature of the gas, meaning that the gas and radiation are not in
thermodynamic equilibrium Thermodynamic equilibrium is an axiomatic An axiom, postulate or assumption is a statement that is taken to be true True most commonly refers to truth Truth is the property of being in accord with fact or reality.Merriam-Webster's Online ...
. All of the observable universe is filled with photons that were created during the
Big Bang The Big Bang is the prevailing of the from the through its subsequent large-scale evolution. The model describes how the from an initial state of high and , and offers a comprehensive explanation for a broad range of observed phenomen ...

Big Bang
, which is known as the
cosmic microwave background radiation The cosmic microwave background (CMB, CMBR), in Big Bang The Big Bang Scientific theory, theory is the prevailing cosmological model explaining the existence of the observable universe from the Planck units#Cosmology, earliest known perio ...
(CMB). (There is quite likely a correspondingly large number of
neutrino A neutrino ( or ) (denoted by the Greek letter ) is a fermion In particle physics, a fermion is a particle that follows Fermi–Dirac statistics and generally has half odd integer spin: spin 1/2, Spin (physics)#Higher spins, spin 3/2, etc. T ...

neutrino
s called the
cosmic neutrino background The cosmic neutrino background (CNB or CνB) is the universe's background particle radiation composed of neutrino A neutrino ( or ) (denoted by the Greek letter ) is a fermion (an elementary particle In particle physics Particle physics ...
.) The current
black body A black body or blackbody is an idealized physical object, physical body that absorption (electromagnetic radiation), absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence (optics), angle of incidence. The ...

black body
temperature of the background radiation is about . The gas temperatures in outer space can vary widely. For example, the temperature in the
Boomerang Nebula The Boomerang Nebula is a protoplanetary nebula located 5,000 light-years A light-year, alternatively spelt lightyear, is a unit of length A unit of length refers to any arbitrarily chosen and accepted reference standard for measurement o ...

Boomerang Nebula
is 1 K, while the
solar corona A corona (, in turn derived from Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the f ...
reaches temperatures over 1.2–2.6 million K. Magnetic fields have been detected in the space around just about every class of celestial object. Star formation in spiral galaxies can generate small-scale
dynamo A dynamo is an that creates using a . Dynamos were the first electrical generators capable of delivering power for industry, and the foundation upon which many other later devices were based, including the , the , and the . Today, the simple ...

dynamo
s, creating turbulent magnetic field strengths of around 5–10 μ G. The Davis–Greenstein effect causes elongated dust grains to align themselves with a galaxy's magnetic field, resulting in weak optical
polarization Polarization or polarisation may refer to: In the physical sciences *Polarization (waves), the ability of waves to oscillate in more than one direction, in particular polarization of light, responsible for example for the glare-reducing effect of ...
. This has been used to show ordered magnetic fields exist in several nearby galaxies. Magneto-hydrodynamic processes in
active Active may refer to: Music * Active (album), ''Active'' (album), a 1992 album by Casiopea * Active Records, a record label Ships * Active (ship), ''Active'' (ship), several commercial ships by that name * HMS Active, HMS ''Active'', the nam ...
elliptical galaxies The giant elliptical galaxy ESO 325-G004 An elliptical galaxy is a type of galaxy A galaxy is a gravitation Gravity (), or gravitation, is a natural phenomenon by which all things with mass Mass is both a property Proper ...
produce their characteristic jets and radio lobes. Non-thermal Astronomical radio source, radio sources have been detected even among the most distant, redshift, high-z sources, indicating the presence of magnetic fields. Outside a protective atmosphere and magnetic field, there are few obstacles to the passage through space of energetic subatomic particles known as cosmic rays. These particles have energies ranging from about 106 Electronvolt, eV up to an extreme 1020 eV of ultra-high-energy cosmic rays. The peak flux of cosmic rays occurs at energies of about 109 eV, with approximately 87% protons, 12% helium nuclei and 1% heavier nuclei. In the high energy range, the flux of electrons is only about 1% of that of protons. Cosmic rays can damage electronic components and pose a Health threat from cosmic rays, health threat to space travelers. According to astronauts, like Don Pettit, space has a burned/metallic odor that clings to their suits and equipment, similar to the scent of an arc welding torch.


Effect on biology and human bodies

Despite the harsh environment, several life forms have been found that can withstand extreme space conditions for extended periods. Species of lichen carried on the ESA BIOPAN facility survived exposure for ten days in 2007. Seeds of ''Arabidopsis thaliana'' and ''Nicotiana tabacum'' germinated after being exposed to space for 1.5 years. A strain of ''bacillus subtilis'' has survived 559 days when exposed to low-Earth orbit or a simulated martian environment. The Panspermia, lithopanspermia hypothesis suggests that rocks ejected into outer space from life-harboring planets may successfully transport life forms to another habitable world. A conjecture is that just such a scenario occurred early in the history of the Solar System, with potentially microorganism-bearing rocks being exchanged between Venus, Earth, and Mars. Even at relatively low altitudes in the Earth's atmosphere, conditions are hostile to the human body. The altitude where atmospheric pressure matches the vapor pressure of water at the Human body temperature, temperature of the human body is called the Armstrong line, named after American physician Harry G. Armstrong. It is located at an altitude of around . At or above the Armstrong line, fluids in the throat and lungs boil away. More specifically, exposed bodily liquids such as saliva, tears, and liquids in the lungs boil away. Hence, at this altitude, human survival requires a pressure suit, or a pressurized capsule. Out in space, sudden exposure of an unprotected human to very low Atmospheric pressure, pressure, such as during a rapid decompression, can cause pulmonary barotrauma—a rupture of the lungs, due to the large pressure differential between inside and outside the chest. Even if the subject's airway is fully open, the flow of air through the windpipe may be too slow to prevent the rupture. Rapid decompression can rupture eardrums and sinuses, bruising and blood seep can occur in soft tissues, and shock can cause an increase in oxygen consumption that leads to Hypoxia (medical), hypoxia. As a consequence of rapid decompression, oxygen dissolved in the blood empties into the lungs to try to equalize the partial pressure gradient. Once the deoxygenated blood arrives at the brain, humans lose consciousness after a few seconds and die of hypoxia within minutes. Blood and other body fluids boil when the pressure drops below 6.3 kPa, and this condition is called ebullism. The steam may bloat the body to twice its normal size and slow circulation, but tissues are elastic and porous enough to prevent rupture. Ebullism is slowed by the pressure containment of blood vessels, so some blood remains liquid. Swelling and ebullism can be reduced by containment in a pressure suit. The Crew Altitude Protection Suit (CAPS), a fitted elastic garment designed in the 1960s for astronauts, prevents ebullism at pressures as low as 2 kPa. Supplemental oxygen is needed at to provide enough oxygen for breathing and to prevent water loss, while above pressure suits are essential to prevent ebullism. Most space suits use around 30–39 kPa of pure oxygen, about the same as on the Earth's surface. This pressure is high enough to prevent ebullism, but evaporation of nitrogen dissolved in the blood could still cause decompression sickness and air embolism, gas embolisms if not managed. Human evolution, Humans evolved for life in Earth Gravitation, gravity, and exposure to weightlessness has been shown to have deleterious effects on human health. Initially, more than 50% of astronauts experience space motion sickness. This can cause nausea and vomiting, Vertigo (medical), vertigo, headaches, lethargy, and overall malaise. The duration of space sickness varies, but it typically lasts for 1–3 days, after which the body adjusts to the new environment. Longer-term exposure to weightlessness results in
muscle atrophy Muscle atrophy is the loss of skeletal muscle Skeletal muscles (commonly referred to as muscles) are Organ (biology), organs of the vertebrate muscular system that are mostly attached by tendons to bones of the skeleton. The muscle cells of ske ...
and deterioration of the Human skeleton, skeleton, or spaceflight osteopenia. These effects can be minimized through a regimen of exercise. Other effects include fluid redistribution, slowing of the cardiovascular system, decreased production of red blood cells, balance disorders, and a weakening of the immune system. Lesser symptoms include loss of body mass, nasal congestion, sleep disturbance, and puffiness of the face. During long-duration space travel, radiation can pose an acute health hazard. Exposure to high-energy, ionizing cosmic rays can result in fatigue, nausea, vomiting, as well as damage to the immune system and changes to the white blood cell count. Over longer durations, symptoms include an increased risk of cancer, plus damage to the Human eye, eyes, nervous system, Human lung, lungs and the Human gastrointestinal tract, gastrointestinal tract. On a round-trip Mars mission lasting three years, a large fraction of the cells in an astronaut's body would be traversed and potentially damaged by high energy nuclei. The energy of such particles is significantly diminished by the shielding provided by the walls of a spacecraft and can be further diminished by water containers and other barriers. The impact of the cosmic rays upon the shielding produces additional radiation that can affect the crew. Further research is needed to assess the radiation hazards and determine suitable countermeasures.


Regions

Space is a partial vacuum: its different regions are defined by the various atmospheres and "winds" that dominate within them, and extend to the point at which those winds give way to those beyond. Geospace extends from Earth's atmosphere to the outer reaches of Earth's magnetic field, whereupon it gives way to the solar wind of interplanetary space. Interplanetary space extends to the heliopause, whereupon the solar wind gives way to the winds of the interstellar medium. Interstellar space then continues to the edges of the galaxy, where it fades into the intergalactic void.


Geospace

wikt:geospace, Geospace is the region of outer space near Earth, including the upper atmosphere and magnetosphere. The Van Allen radiation belts lie within the geospace. The outer boundary of geospace is the magnetopause, which forms an interface between the Earth's magnetosphere and the solar wind. The inner boundary is the ionosphere. The variable space-weather conditions of geospace are affected by the behavior of the Sun and the solar wind; the subject of geospace is interlinked with heliophysics—the study of the Sun and its impact on the planets of the Solar System. The day-side magnetopause is compressed by solar-wind pressure—the subsolar distance from the center of the Earth is typically 10 Earth radii. On the night side, the solar wind stretches the magnetosphere to form a magnetotail that sometimes extends out to more than 100–200 Earth radii. For roughly four days of each month, the lunar surface is shielded from the solar wind as the Moon passes through the magnetotail. Geospace is populated by electrically charged particles at very low densities, the motions of which are controlled by the Earth's magnetic field. These plasmas form a medium from which storm-like disturbances powered by the solar wind can drive electrical currents into the Earth's upper atmosphere. Geomagnetic storms can disturb two regions of geospace, the radiation belts and the ionosphere. These storms increase fluxes of energetic electrons that can permanently damage satellite electronics, interfering with shortwave radio communication and Global Positioning System, GPS location and timing. Magnetic storms can also be a hazard to astronauts, even in low Earth orbit. They also create aurora (astronomy), aurorae seen at high latitudes in an oval surrounding the geomagnetic poles. Although it meets the definition of outer space, the atmospheric density within the first few hundred kilometers above the Kármán line is still sufficient to produce significant Drag (physics), drag on satellites. This region contains material left over from previous crewed and uncrewed launches that are a potential hazard to spacecraft. Some of this space debris, debris re-enters Earth's atmosphere periodically.


Cislunar space

gravity of Earth, Earth's gravity keeps the
Moon The Moon is Earth's only natural satellite. At about one-quarter the diameter of Earth (comparable to the width of Australia (continent), Australia), it is the largest natural satellite in the Solar System relative to the size of its plane ...

Moon
in orbit at an lunar distance (astronomy), average distance of . The region outside atmosphere of Earth, Earth's atmosphere and extending out to just beyond the orbit of the Moon, Moon's orbit, including the Lagrange points, is sometimes referred to as cislunar space. Deep space is defined by the United States government and others as any region beyond cislunar space. The ITU-R, International Telecommunication Union responsible for radio communication (including satellites) defines the beginning of deep space at about 5 times that distance (). The region where Earth's gravity remains dominant against gravitational perturbation (astronomy), perturbations from the Sun is called the Hill sphere. This extends into translunar space to a distance of roughly 1% of the mean distance from Earth to the Sun, or .


Interplanetary space

Interplanetary space is defined by the solar wind, a continuous stream of charged particles emanating from the Sun that creates a very tenuous atmosphere (the heliosphere) for billions of kilometers into space. This wind has a particle density of 5–10 protons/cm3 and is moving at a velocity of . Interplanetary space extends out to the Astropause, heliopause where the influence of the galactic environment starts to dominate over the magnetic field and particle flux from the Sun. The distance and strength of the heliopause varies depending on the activity level of the solar wind. The heliopause in turn deflects away low-energy galactic cosmic rays, with this modulation effect peaking during solar maximum. The volume of interplanetary space is a nearly total vacuum, with a mean free path of about one astronomical unit at the orbital distance of the Earth. This space is not completely empty, and is sparsely filled with cosmic rays, which include ionized atomic nucleus, atomic nuclei and various subatomic particles. There is also gas, Plasma (physics), plasma and dust, small meteors, and several dozen types of organic chemistry, organic molecules discovered to date by rotational spectroscopy, microwave spectroscopy. A cloud of interplanetary dust is visible at night as a faint band called the zodiacal light. Interplanetary space contains the magnetic field generated by the Sun. There are also magnetospheres generated by planets such as Jupiter, Saturn, Mercury and the Earth that have their own magnetic fields. These are shaped by the influence of the solar wind into the approximation of a teardrop shape, with the long tail extending outward behind the planet. These magnetic fields can trap particles from the solar wind and other sources, creating belts of charged particles such as the Van Allen radiation belts. Planets without magnetic fields, such as Mars, have their atmospheres gradually eroded by the solar wind.


Interstellar space

Interstellar space is the physical space within a galaxy beyond the influence each star has upon the encompassed plasma. The contents of interstellar space are called the interstellar medium. Approximately 70% of the mass of the interstellar medium consists of lone hydrogen atoms; most of the remainder consists of helium atoms. This is enriched with trace amounts of heavier atoms formed through stellar nucleosynthesis. These atoms are ejected into the interstellar medium by stellar winds or when evolved stars begin to shed their outer envelopes such as during the formation of a planetary nebula. The cataclysmic explosion of a supernova generates an expanding shock wave consisting of ejected materials that further enrich the medium. The density of matter in the interstellar medium can vary considerably: the average is around 106 particles per m3, but cold molecular clouds can hold 108–1012 per m3. A List of interstellar and circumstellar molecules, number of molecules exist in interstellar space, as can tiny 0.1 Micrometre, μm dust particles. The tally of molecules discovered through radio astronomy is steadily increasing at the rate of about four new species per year. Large regions of higher density matter known as molecular clouds allow chemical reactions to occur, including the formation of organic polyatomic species. Much of this chemistry is driven by collisions. Energetic cosmic rays penetrate the cold, dense clouds and ionize hydrogen and helium, resulting, for example, in the trihydrogen cation. An ionized helium atom can then split relatively abundant carbon monoxide to produce ionized carbon, which in turn can lead to organic chemical reactions. The local interstellar medium is a region of space within 100 parsecs (pc) of the Sun, which is of interest both for its proximity and for its interaction with the Solar System. This volume nearly coincides with a region of space known as the Local Bubble, which is characterized by a lack of dense, cold clouds. It forms a cavity in the Orion Arm of the Milky Way galaxy, with dense molecular clouds lying along the borders, such as those in the constellations of Ophiuchus and Taurus (constellation), Taurus. (The actual distance to the border of this cavity varies from 60 to 250 pc or more.) This volume contains about 104–105 stars and the local interstellar gas counterbalances the Stellar-wind bubble, astrospheres that surround these stars, with the volume of each sphere varying depending on the local density of the interstellar medium. The Local Bubble contains dozens of warm interstellar clouds with temperatures of up to 7,000 K and radii of 0.5–5 pc. When stars are moving at sufficiently high peculiar velocities, their astrospheres can generate bow shocks as they collide with the interstellar medium. For decades it was assumed that the Sun had a bow shock. In 2012, data from Interstellar Boundary Explorer, Interstellar Boundary Explorer (IBEX) and NASA's Voyager program, Voyager probes showed that the Sun's bow shock does not exist. Instead, these authors argue that a Mach number, subsonic bow wave defines the transition from the solar wind flow to the interstellar medium. A bow shock is the third boundary of an astrosphere after the Heliosphere#Termination shock, termination shock and the astropause (called the heliopause in the Solar System).


Intergalactic space

Intergalactic space is the physical space between galaxies. Studies of the large scale distribution of galaxies show that the Universe has a foam-like structure, with Galaxy groups and clusters, groups and clusters of galaxies lying along filaments that occupy about a tenth of the total space. The remainder forms huge voids that are mostly empty of galaxies. Typically, a Void (astronomy), void spans a distance of (10–40) ''h''−1 Mpc, where ''h'' is the Hubble constant in units of , or the dimensionless Hubble constant. Surrounding and stretching between galaxies, there is a rarefaction, rarefied plasma that is organized in a galaxy filament, galactic filamentary structure. This material is called the intergalactic medium (IGM). The density of the IGM is 5–200 times the average density of the Universe. It consists mostly of ionized hydrogen; i.e. a plasma consisting of equal numbers of electrons and protons. As gas falls into the intergalactic medium from the voids, it heats up to temperatures of 105 K to 107 K, which is high enough so that collisions between atoms have enough energy to cause the bound electrons to escape from the hydrogen nuclei; this is why the IGM is ionized. At these temperatures, it is called the warm–hot intergalactic medium (WHIM). (Although the plasma is very hot by terrestrial standards, 105 K is often called "warm" in astrophysics.) Computer simulations and observations indicate that up to half of the atomic matter in the Universe might exist in this warm–hot, rarefied state. When gas falls from the filamentary structures of the WHIM into the galaxy clusters at the intersections of the cosmic filaments, it can heat up even more, reaching temperatures of 108 K and above in the so-called intracluster medium (ICM).


Earth orbit

A spacecraft enters orbit when its Centripetal force, centripetal acceleration due to Gravitation, gravity is less than or equal to the centrifugal force, centrifugal acceleration due to the horizontal component of its velocity. For a low Earth orbit, this velocity is about ; by contrast, the fastest piloted airplane speed ever achieved (excluding speeds achieved by deorbiting spacecraft) was in 1967 by the North American X-15. To achieve an orbit, a spacecraft must travel faster than a sub-orbital spaceflight. The energy required to reach Earth orbital velocity at an altitude of is about 36 Megajoule, MJ/kg, which is six times the energy needed merely to climb to the corresponding altitude. Spacecraft with a Apsis, perigee below about are subject to drag from the Earth's atmosphere, which decreases the orbital altitude. The rate of orbital decay depends on the satellite's cross-sectional area and mass, as well as variations in the air density of the upper atmosphere. Below about , decay becomes more rapid with lifetimes measured in days. Once a satellite descends to , it has only hours before it vaporizes in the atmosphere. The escape velocity required to pull free of Earth's gravitational field altogether and move into interplanetary space is about .


Boundary

There is no clear boundary between Earth's atmosphere and space, as the density of the atmosphere gradually decreases as the altitude increases. There are several standard boundary designations, namely: * The Fédération Aéronautique Internationale has established the
Kármán line The Kármán line is an attempt to define a boundary between Earth's atmosphere File:Atmosphere gas proportions.svg, Composition of Earth's atmosphere by volume, excluding water vapor. Lower pie represents trace gases that together compose ab ...
at an altitude of as a working definition for the boundary between aeronautics and astronautics. This is used because at an altitude of about , as Theodore von Kármán calculated, a vehicle would have to travel faster than Orbital speed, orbital velocity to derive sufficient aerodynamic lift from the atmosphere to support itself. * The United States designates people who travel above an altitude of as astronauts. * NASA's Space Shuttle used 400,000 feet (, ) as its atmospheric reentry, re-entry altitude (termed the Entry Interface), which roughly marks the boundary where atmospheric drag becomes noticeable, thus beginning the process of switching from steering with thrusters to maneuvering with aerodynamic control surfaces. In 2009, scientists reported detailed measurements with a Supra-Thermal Ion Imager (an instrument that measures the direction and speed of ions), which allowed them to establish a boundary at above Earth. The boundary represents the midpoint of a gradual transition over tens of kilometers from the relatively gentle winds of the Earth's atmosphere to the more violent flows of charged particles in space, which can reach speeds well over .


Legal status

The
Outer Space Treaty russian: link=yes, Договор о космосе es, link=yes, Tratado sobre el espacio ultraterrestre , long_name =Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, including the Moon ...

Outer Space Treaty
provides the basic framework for international space law. It covers the legal use of outer space by nation states, and includes in its definition of ''outer space'', the Moon, and other celestial bodies. The treaty states that outer space is free for all nation states to explore and is not subject to claims of national sovereignty, calling outer space the "province of all mankind". This status as a common heritage of mankind has been used, though not without opposition, to enforce the right to access and shared use of outer space for all nations equally, particularly non-spacefaring nations. It also prohibits the development of nuclear weapons in outer space. The treaty was passed by the United Nations General Assembly in 1963 and signed in 1967 by the USSR, the United States of America and the United Kingdom. As of 2017, 105 state parties have either ratified or acceded to the treaty. An additional 25 states signed the treaty, without ratifying it. Since 1958, outer space has been the subject of multiple United Nations resolutions. Of these, more than 50 have been concerning the international co-operation in the peaceful uses of outer space and preventing an arms race in space. Four additional
space law Space law is the body of law governing space-related activities, encompassing both international and domestic agreements, rules, and principles. Parameters of space law include space exploration, liability for damage, weapons use, rescue efforts, ...
treaties have been negotiated and drafted by the UN's United Nations Committee on the Peaceful Uses of Outer Space, Committee on the Peaceful Uses of Outer Space. Still, there remains no legal prohibition against deploying conventional weapons in space, and
anti-satellite weapon Anti-satellite weapons (ASAT) are space weaponSpace weapons are weapon A weapon, arm or armament is any implement or device that can be used with the intent to inflict damage or harm. Weapons are used to increase the efficacy and efficiency of ...
s have been successfully tested by the US, USSR, China, and in 2019, India. The 1979 Moon Treaty turned the jurisdiction of all heavenly bodies (including the orbits around such bodies) over to the international community. The treaty has not been ratified by any nation that currently practices human spaceflight. In 1976, eight equatorial states (Ecuador, Colombia, Brazil, Republic of the Congo, Congo, Democratic Republic of the Congo, Zaire, Uganda, Kenya, and Indonesia) met in Bogotá, Colombia. With their "Bogota Declaration, Declaration of the First Meeting of Equatorial Countries", or "the Bogotá Declaration", they claimed control of the segment of the geosynchronous orbital path corresponding to each country. These claims are not internationally accepted.


Discovery, exploration and applications


Discovery

In 350 BCE, Greek philosopher Aristotle suggested that ''nature abhors a vacuum'', a principle that became known as the ''Horror vacui (physics), horror vacui''. This concept built upon a 5th-century BCE Ontology, ontological argument by the Greek philosopher Parmenides, who denied the possible existence of a void in space. Based on this idea that a vacuum could not exist, in the Western culture, West it was widely held for many centuries that space could not be empty. As late as the 17th century, the French philosopher René Descartes argued that the entirety of space must be filled. In ancient China, the 2nd-century astronomer Zhang Heng became convinced that space must be infinite, extending well beyond the mechanism that supported the Sun and the stars. The surviving books of the Hsüan Yeh school said that the heavens were boundless, "empty and void of substance". Likewise, the "sun, moon, and the company of stars float in the empty space, moving or standing still". The Italian scientist Galileo Galilei knew that air had mass and so was subject to gravity. In 1640, he demonstrated that an established force resisted the formation of a vacuum. It would remain for his pupil Evangelista Torricelli to create an apparatus that would produce a partial vacuum in 1643. This experiment resulted in the first mercury barometer and created a scientific sensation in Europe. The French mathematician Blaise Pascal reasoned that if the column of mercury was supported by air, then the column ought to be shorter at higher altitude where the air pressure is lower. In 1648, his brother-in-law, Florin Périer, repeated the experiment on the Puy de Dôme mountain in central France and found that the column was shorter by three inches. This decrease in pressure was further demonstrated by carrying a half-full balloon up a mountain and watching it gradually expand, then contract upon descent. In 1650, German scientist Otto von Guericke constructed the first vacuum pump: a device that would further refute the principle of ''horror vacui''. He correctly noted that the atmosphere of the Earth surrounds the planet like a shell, with the density gradually declining with altitude. He concluded that there must be a vacuum between the Earth and the Moon. Back in the 15th century, German theologian Nicolaus Cusanus speculated that the Universe lacked a center and a circumference. He believed that the Universe, while not infinite, could not be held as finite as it lacked any bounds within which it could be contained. These ideas led to speculations as to the infinite dimension of space by the Italian philosopher Giordano Bruno in the 16th century. He extended the Copernican heliocentric cosmology to the concept of an infinite Universe filled with a substance he called Aether (classical element), aether, which did not resist the motion of heavenly bodies. English philosopher William Gilbert (astronomer), William Gilbert arrived at a similar conclusion, arguing that the stars are visible to us only because they are surrounded by a thin aether or a void. This concept of an aether originated with ancient Greece, ancient Greek philosophers, including Aristotle, who conceived of it as the medium through which the heavenly bodies move. The concept of a Universe filled with a luminiferous aether retained support among some scientists until the early 20th century. This form of aether was viewed as the medium through which light could propagate. In 1887, the Michelson–Morley experiment tried to detect the Earth's motion through this medium by looking for changes in the speed of light depending on the direction of the planet's motion. The null result indicated something was wrong with the concept. The idea of the luminiferous aether was then abandoned. It was replaced by Albert Einstein's theory of special relativity, which holds that the speed of light in a vacuum is a fixed constant, independent of the observer's motion or frame of reference. The first professional astronomer to support the concept of an infinite Universe was the Englishman Thomas Digges in 1576. But the scale of the Universe remained unknown until the first successful measurement of the distance to a nearby star in 1838 by the German astronomer Friedrich Bessel. He showed that the star system 61 Cygni had a stellar parallax, parallax of just 0.31 arcseconds (compared to the modern value of 0.287″). This corresponds to a distance of over 10 light years. In 1917, Heber Doust Curtis, Heber Curtis noted that novae in spiral nebulae were, on average, 10 magnitudes fainter than galactic novae, suggesting that the former are 100 times further away. The distance to the Andromeda Galaxy was determined in 1923 by American astronomer Edwin Hubble by measuring the brightness of cepheid variables in that galaxy, a new technique discovered by Henrietta Leavitt. This established that the Andromeda galaxy, and by extension all galaxies, lay well outside the Milky Way. The modern concept of outer space is based on the Big Bang cosmology, "Big Bang" cosmology, first proposed in 1931 by the Belgian physicist Georges Lemaître. This theory holds that the universe originated from a very dense form that has since undergone continuous Hubble's law, expansion. The earliest known estimate of the temperature of outer space was by the Swiss physicist Charles Édouard Guillaume, Charles É. Guillaume in 1896. Using the estimated radiation of the background stars, he concluded that space must be heated to a temperature of 5–6 K. British physicist Arthur Eddington made a similar calculation to derive a temperature of 3.18 K in 1926. German physicist Erich Regener used the total measured energy of
cosmic ray Cosmic rays are high-energy proton A proton is a subatomic particle, symbol or , with a positive electric charge of +1''e'' elementary charge and a mass slightly less than that of a neutron. Protons and neutrons, each with masses of approx ...
s to estimate an intergalactic temperature of 2.8 K in 1933. American physicists Ralph Alpher and Robert Herman predicted 5 K for the temperature of space in 1948, based on the gradual decrease in background energy following the then-new
Big Bang The Big Bang is the prevailing of the from the through its subsequent large-scale evolution. The model describes how the from an initial state of high and , and offers a comprehensive explanation for a broad range of observed phenomen ...

Big Bang
theory. The modern measurement of the cosmic microwave background is about 2.7K. The term ''outward space'' was used in 1842 by the English poet Lady Emmeline Stuart-Wortley in her poem "The Maiden of Moscow". The expression ''outer space'' was used as an astronomical term by Alexander von Humboldt in 1845. It was later popularized in the writings of H. G. Wells in 1901. The shorter term ''space'' is older, first used to mean the region beyond Earth's sky in John Milton's ''Paradise Lost'' in 1667.


Exploration and application

For most of human history, space was explored by observations made from the Earth's surface—initially with the unaided eye and then with the telescope. Before reliable rocket technology, the closest that humans had come to reaching outer space was through balloon flights. In 1935, the U.S. Explorer II crewed balloon flight reached an altitude of . This was greatly exceeded in 1942 when the third launch of the German V-2 rocket, A-4 rocket climbed to an altitude of about . In 1957, the uncrewed satellite Sputnik 1 was launched by a Russian R-7 Semyorka, R-7 rocket, achieving Earth orbit at an altitude of . This was followed by the first human spaceflight in 1961, when
Yuri Gagarin Yuri Alekseyevich Gagarin; Gagarin's first name is sometimes transliterated as ''Yuriy'', ''Youri'', or ''Yury''. (9 March 1934 – 27 March 1968) was a Soviet The Soviet Union,. officially the Union of Soviet Socialist Republics. (U ...
was sent into orbit on Vostok 1. The first humans to escape low-Earth orbit were Frank Borman, Jim Lovell and William Anders in 1968 on board the U.S. Apollo 8, which achieved lunar orbit and reached a maximum distance of from the Earth. The first spacecraft to reach escape velocity was the Soviet ''Luna 1'', which performed a fly-by of the Moon in 1959. In 1961, ''Venera 1'' became the first planetary probe. It revealed the presence of the solar wind and performed the first fly-by of Venus, although contact was lost before reaching Venus. The first successful planetary mission was the 1962 fly-by of Venus by Mariner 2. The first fly-by of Mars was by Mariner 4 in 1964. Since that time, uncrewed spacecraft have successfully examined each of the Solar System's planets, as well their moons and many minor planets and comets. They remain a fundamental tool for the exploration of outer space, as well as for observation of the Earth. In August 2012, ''Voyager 1'' became the first man-made object to leave the Solar System and enter interstellar space. The absence of air makes outer space an ideal location for astronomy at all wavelengths of the electromagnetic spectrum. This is evidenced by the spectacular pictures sent back by the Hubble Space Telescope, allowing light from more than 13 billion years ago—almost to the time of the Big Bang—to be observed. Not every location in space is ideal for a telescope. The Interplanetary dust cloud, interplanetary zodiacal dust emits a diffuse near-infrared radiation that can mask the emission of faint sources such as extrasolar planets. Moving an infrared telescope out past the dust increases its effectiveness. Likewise, a site like the Daedalus (crater), Daedalus crater on the far side of the Moon could shield a radio telescope from the Electromagnetic interference, radio frequency interference that hampers Earth-based observations. Uncrewed spacecraft in Earth orbit are an essential technology of modern civilization. They allow direct monitoring of Weather satellite, weather conditions, relay Communications satellite, long-range communications like television, provide a means of Satellite navigation, precise navigation, and allow remote sensing of the Earth. The latter role serves a wide variety of purposes, including tracking soil moisture for agriculture, prediction of water outflow from seasonal snow packs, detection of diseases in plants and trees, and Spy satellite, surveillance of military activities. The deep vacuum of space could make it an attractive environment for certain industrial processes, such as those requiring ultraclean surfaces. Like asteroid mining, space manufacturing would require a large financial investment with little prospect of immediate return. An important factor in the total expense is the high cost of placing mass into Earth orbit: $–$ per kg, according to a 2006 estimate (allowing for inflation since then). The cost of access to space has declined since 2013. Partially reusable rockets such as the Falcon 9 have lowered access to space below 3500 dollars per kilogram. With these new rockets the cost to send materials into space remains prohibitively high for many industries. Proposed concepts for addressing this issue include, fully reusable launch systems, non-rocket spacelaunch, momentum exchange tethers, and space elevators. Interstellar travel for a human crew remains at present only a theoretical possibility. The distances to the nearest stars mean it would require new technological developments and the ability to safely sustain crews for journeys lasting several decades. For example, the Project Daedalus, Daedalus Project study, which proposed a spacecraft powered by the nuclear fusion, fusion of deuterium and helium-3, would require 36 years to reach the "nearby" Alpha Centauri system. Other proposed interstellar propulsion systems include light sails, Bussard ramjet, ramjets, and beam-powered propulsion. More advanced propulsion systems could use antimatter as a fuel, potentially reaching relativistic speed, relativistic velocities.


See also

* Animals in space * Earth's location, Earth's location in the Universe * List of government space agencies * List of topics in space * Outline of space science * Panspermia * Space and survival * Space environment * Space race * Space station * Space technology * Space weather * Space weathering * Timeline of knowledge about the interstellar and intergalactic medium * Timeline of Solar System exploration * Timeline of spaceflight


References


Citations


Sources

* * * * Note: this source gives a value of molecules per cubic meter. * * * * Note: a light year is about 1013 km. * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
* * * * *


External links


Newscientist Space

Space.com
{{DEFAULTSORT:Outer Space Outer space, Space Environments Vacuum