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 harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's sur ...
and
its atmosphere and between
celestial bodies
An astronomical object, celestial object, stellar object or heavenly body is a naturally occurring physical entity, association, or structure that exists in the observable universe. In astronomy, the terms ''object'' and ''body'' are often us ...
. Outer space is not completely empty—it is a
near-perfect vacuum[ containing a low density of particles, predominantly a plasma of ]hydrogen
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-to ...
and helium
Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic ta ...
, as well as electromagnetic radiation
In physics, electromagnetic radiation (EMR) consists of waves of the electromagnetic (EM) field, which propagate through space and carry momentum and electromagnetic radiant energy. It includes radio waves, microwaves, infrared, (visib ...
, magnetic field
A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...
s, neutrinos, dust
Dust is made of fine particles of solid matter. On Earth, it generally consists of particles in the atmosphere that come from various sources such as soil lifted by wind (an aeolian process), volcanic eruptions, and pollution. Dust in ...
, and cosmic ray
Cosmic rays are high-energy particles or clusters of particles (primarily represented by protons or atomic nuclei) that move through space at nearly the speed of light. They originate from the Sun, from outside of the Solar System in our own ...
s. The baseline temperature
Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer.
Thermometers are calibrated in various temperature scales that historically have relied o ...
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 source ...
from the Big Bang
The Big Bang event is a physical theory that describes how the universe expanded from an initial state of high density and temperature. Various cosmological models of the Big Bang explain the evolution of the observable universe from the ...
, is .
The plasma between galaxies is thought to account for about half of the baryonic (ordinary) matter in the universe, having a number density
The number density (symbol: ''n'' or ''ρ''N) is an intensive quantity used to describe the degree of concentration of countable objects (particles, molecules, phonons, cells, galaxies, etc.) in physical space: three-dimensional volumetric num ...
of less than one hydrogen atom
A hydrogen atom is an atom of the chemical element hydrogen. The electrically neutral atom contains a single positively charged proton and a single negatively charged electron bound to the nucleus by the Coulomb force. Atomic hydrogen cons ...
per cubic metre
The cubic metre (in Commonwealth English and international spelling as used by the International Bureau of Weights and Measures) or cubic meter (in American English) is the unit of volume in the International System of Units (SI). Its symbol is m ...
and a kinetic temperature
Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer.
Thermometers are calibrated in various temperature scales that historically have relied on ...
of millions of kelvins.[ Local concentrations of matter have condensed into ]star
A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...
s and galaxies
A galaxy is a system of stars, stellar remnants, interstellar gas, dust, dark matter, bound together by gravity. The word is derived from the Greek ' (), literally 'milky', a reference to the Milky Way galaxy that contains the Solar System ...
. 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 thought to account for approximately 85% of the matter in the universe. Dark matter is called "dark" because it does not appear to interact with the electromagnetic field, which means it does not ...
, which interacts with other matter through gravitation
In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the stron ...
al but not electromagnetic force
In physics, electromagnetism is an interaction that occurs between particles with electric charge. It is the second-strongest of the four fundamental interactions, after the strong force, and it is the dominant force in the interactions o ...
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 comprising all matter that can be observed from Earth or its space-based telescopes and exploratory probes at the present time, because the electromagnetic radiation from these ob ...
is ''dark energy
In physical cosmology and astronomy, dark energy is an unknown form of energy that affects the universe on the largest scales. The first observational evidence for its existence came from measurements of supernovas, which showed that the univ ...
'', a type of vacuum energy
Vacuum energy is an underlying background energy that exists in space throughout the entire Universe. The vacuum energy is a special case of zero-point energy that relates to the quantum vacuum.
The effects of vacuum energy can be experimental ...
that is poorly understood. Intergalactic space takes up most of the volume of the universe
The universe is all of space and time and their contents, including planets, stars, galaxies, and all other forms of matter and energy. The Big Bang theory is the prevailing cosmological description of the development of the univers ...
, 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'' or ''galaxy'', although, broadly speaking ...
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 (or von Kármán line ) is an attempt to define a boundary between Earth's atmosphere and outer space, and offers a specific definition set by the Fédération aéronautique internationale (FAI), an international record-keeping ...
, an altitude of above 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
The stratosphere () is the second layer of the atmosphere of the Earth, located above the troposphere and below the mesosphere. The stratosphere is an atmospheric layer composed of stratified temperature layers, with the warm layers of air h ...
and the mesosphere
The mesosphere (; ) is the third layer of the atmosphere, directly above the stratosphere and directly below the thermosphere. In the mesosphere, temperature decreases as altitude increases. This characteristic is used to define its limits: it ...
are also sometimes referred to as "near space". 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 effort ...
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 Moo ...
, 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 that each state has exclusive sovereignty over its territory. The principle underlies the modern international system of sovereign states and is enshrined in the Un ...
and permits all states to freely explore outer space. Despite the drafting of UN resolution
A United Nations resolution (UN resolution) is a formal text adopted by a United Nations (UN) body. Although any UN body can issue resolutions, in practice most resolutions are issued by the Security Council or the General Assembly.
Legal stat ...
s for the peaceful uses of outer space, anti-satellite weapon
Anti-satellite weapons (ASAT) are space weapons designed to incapacitate or destroy satellites for strategic or tactical
purposes. Several nations possess operational ASAT systems. Although no ASAT system has been utilised in warfare, a few ...
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
Earth orbits the Sun at an average distance of 149.60 million km (92.96 million mi) in a counterclockwise direction as viewed from above the Northern Hemisphere. One complete orbit takes days (1 sidereal year), during which time E ...
, 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 pilot and cosmonaut who became the first human to journey into outer space. T ...
of the Soviet Union
The Soviet Union,. officially the Union of Soviet Socialist Republics. (USSR),. was a transcontinental country that spanned much of Eurasia from 1922 to 1991. A flagship communist state, it was nominally a federal union of fifteen nationa ...
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 with or without humans on board. Most spaceflight is uncrewed and conducted mainly with spacecraft such as satellites in ...
has been limited to low Earth orbit and the Moon
The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...
. On the other hand, uncrewed spacecraft
Unmanned spacecraft or uncrewed spacecraft are spacecraft without people on board, used for robotic spaceflight. Uncrewed spacecraft may have varying levels of autonomy from human input; they may be remote controlled, remote guided or ev ...
have reached all of the known planet
A planet is a large, rounded astronomical body that is neither a star nor its remnant. The best available theory of planet formation is the nebular hypothesis, which posits that an interstellar cloud collapses out of a nebula to create a you ...
s in the Solar System
The Solar System Capitalization 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 "Solar ...
. Outer space represents a challenging environment for human exploration because of the hazards of vacuum and radiation
In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or through a material medium. This includes:
* ''electromagnetic radiation'', such as radio waves, microwaves, infrared, visi ...
. Microgravity
The term micro-g environment (also μg, often referred to by the term microgravity) is more or less synonymous with the terms '' weightlessness'' and ''zero-g'', but emphasising that g-forces are never exactly zero—just very small (on the ...
also has a negative effect on human physiology
Physiology (; ) is the scientific study of functions and mechanisms in a living system. As a sub-discipline of biology, physiology focuses on how organisms, organ systems, individual organs, cells, and biomolecules carry out the chemic ...
that causes both muscle atrophy
Muscle atrophy is the loss of skeletal muscle mass. It can be caused by immobility, aging, malnutrition, medications, or a wide range of injuries or diseases that impact the musculoskeletal or nervous system. Muscle atrophy leads to muscle weakness ...
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 comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...
s, galaxies
A galaxy is a system of stars, stellar remnants, interstellar gas, dust, dark matter, bound together by gravity. The word is derived from the Greek ' (), literally 'milky', a reference to the Milky Way galaxy that contains the Solar System ...
and other astronomical object
An astronomical object, celestial object, stellar object or heavenly body is a naturally occurring physical entity, association, or structure that exists in the observable universe. In astronomy, the terms ''object'' and ''body'' are often u ...
s, leaving behind a deep vacuum
A vacuum is a space devoid of matter. The word is derived from the Latin adjective ''vacuus'' for "vacant" or " void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists often ...
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, is the local and global geometry of the universe. The local features of the geometry of the universe are primarily described by its curvature, whereas the topology of the universe describes ge ...
has been determined from measurements of the cosmic microwave background
In Big Bang cosmology the cosmic microwave background (CMB, CMBR) is electromagnetic radiation that is a remnant from an early stage of the universe, also known as "relic radiation". The CMB is faint cosmic background radiation filling all spac ...
using satellites like the Wilkinson Microwave Anisotropy Probe
The Wilkinson Microwave Anisotropy Probe (WMAP), originally known as the Microwave Anisotropy Probe (MAP and Explorer 80), was a NASA spacecraft operating from 2001 to 2010 which measured temperature differences across the sky in the cosmic mic ...
. These observations indicate that the spatial geometry
Three-dimensional space (also: 3D space, 3-space or, rarely, tri-dimensional space) is a geometric setting in which three values (called ''parameters'') are required to determine the position of an element (i.e., point). This is the informal ...
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 not ex ...
, indicates that space has a non-zero vacuum energy
Vacuum energy is an underlying background energy that exists in space throughout the entire Universe. The vacuum energy is a special case of zero-point energy that relates to the quantum vacuum.
The effects of vacuum energy can be experimental ...
, which is called dark energy
In physical cosmology and astronomy, dark energy is an unknown form of energy that affects the universe on the largest scales. The first observational evidence for its existence came from measurements of supernovas, which showed that the univ ...
.
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 thought to account for approximately 85% of the matter in the universe. Dark matter is called "dark" because it does not appear to interact with the electromagnetic field, which means it does not ...
, and baryonic matter
In particle physics, a baryon is a type of composite subatomic particle which contains an odd number of valence quarks (at least 3). Baryons belong to the hadron family of particles; hadrons are composed of quarks. Baryons are also classifie ...
(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 ]black hole
A black hole is a region of spacetime where gravity is so strong that nothing, including light or other electromagnetic waves, has enough energy to escape it. The theory of general relativity predicts that a sufficiently compact mass can def ...
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 of a value relative to some contextually understood reference value, usually 10, interpreted as the base of the logarithm and the representative of values of magnitude one. Logarithmic dis ...
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
A vacuum is a space devoid of matter. The word is derived from the Latin adjective ''vacuus'' for "vacant" or "void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists often dis ...
. It has effectively no friction
Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. There are several types of friction:
*Dry friction is a force that opposes the relative lateral motion of ...
, allowing stars, planets
A planet is a large, rounded astronomical body that is neither a star nor its remnant. The best available theory of planet formation is the nebular hypothesis, which posits that an interstellar cloud collapses out of a nebula to create a youn ...
, and moons
A natural satellite is, in the most common usage, an astronomical body that orbits a planet, dwarf planet, or small Solar System body (or sometimes another natural satellite). Natural satellites are often colloquially referred to as ''moons'' ...
to move freely along their ideal orbit
In celestial mechanics, an orbit is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an object or position in space such as ...
s, following the initial formation stage. The deep vacuum of intergalactic space
Outer space, commonly shortened to space, is the expanse that exists beyond Earth and its atmosphere and between celestial bodies. Outer space is not completely empty—it is a near-perfect vacuum containing a low density of particles, predo ...
is not devoid 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, which are made up of interacting subatomic part ...
, as it contains a few hydrogen atoms
A hydrogen atom is an atom of the chemical element hydrogen. The electrically neutral atom contains a single positively charged proton and a single negatively charged electron bound to the nucleus by the Coulomb force. Atomic hydrogen constit ...
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, electromagnetic radiation (EMR) consists of waves of the electromagnetic (EM) field, which propagate through space and carry momentum and electromagnetic radiant energy. It includes radio waves, microwaves, infrared, (visib ...
can travel great distances without being scattered: the mean free path
In physics, mean free path is the average distance over which a moving particle (such as an atom, a molecule, or a photon) travels before substantially changing its direction or energy (or, in a specific context, other properties), typically as ...
of a photon
A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they alwa ...
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 or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the Endling, last individual of the species, although the Functional ext ...
, which is the absorption
Absorption may refer to:
Chemistry and biology
*Absorption (biology), digestion
**Absorption (small intestine)
*Absorption (chemistry), diffusion of particles of gas or liquid into liquid or solid materials
*Absorption (skin), a route by which s ...
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 or sound, are forced to deviate from a straight trajectory by localized non-uniformities (including ...
of photons by dust and gas, is an important factor in galactic and intergalactic astronomy
Astronomy () is a natural science that studies celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and evolution. Objects of interest include planets, moons, stars, nebulae, g ...
.[
Stars, planets, and moons retain their ]atmosphere
An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A ...
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 applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country a ...
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 working for the advancement of the chemical sciences, especially by developing nomenclature and terminology. It is ...
(IUPAC) definition of standard pressure
Standard temperature and pressure (STP) 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 ...
. 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 that is a ...
from the Sun
The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
and the dynamic pressure
In fluid dynamics, dynamic pressure (denoted by or and sometimes called velocity pressure) is the quantity defined by:Clancy, L.J., ''Aerodynamics'', Section 3.5
:q = \frac\rho\, u^2
where (in SI units):
* is the dynamic pressure in pascals ( ...
of the solar wind
The solar wind is a stream of charged particles released from the upper atmosphere of the Sun, called the corona. This plasma mostly consists of electrons, protons and alpha particles with kinetic energy between . The composition of the sol ...
. 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 ...
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 ...
.[
The ]temperature
Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer.
Thermometers are calibrated in various temperature scales that historically have relied o ...
of outer space is measured in terms of the kinetic
Kinetic (Ancient Greek: κίνησις “kinesis”, movement or to move) may refer to:
* Kinetic theory, describing a gas as particles in random motion
* Kinetic energy, the energy of an object that it possesses due to its motion
Art and ent ...
activity of the gas,[ as it is on Earth. The radiation of outer space has a different temperature than the ]kinetic temperature
Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer.
Thermometers are calibrated in various temperature scales that historically have relied on ...
of the gas, meaning that the gas and radiation are not in thermodynamic equilibrium
Thermodynamic equilibrium is an axiomatic concept of thermodynamics. It is an internal state of a single thermodynamic system, or a relation between several thermodynamic systems connected by more or less permeable or impermeable walls. In the ...
. All of the observable universe is filled with photons that were created during the Big Bang
The Big Bang event is a physical theory that describes how the universe expanded from an initial state of high density and temperature. Various cosmological models of the Big Bang explain the evolution of the observable universe from the ...
, which is known as the cosmic microwave background radiation
In Big Bang cosmology the cosmic microwave background (CMB, CMBR) is electromagnetic radiation that is a remnant from an early stage of the universe, also known as "relic radiation". The CMB is faint cosmic background radiation filling all space ...
(CMB). (There is quite likely a correspondingly large number of neutrino
A neutrino ( ; denoted by the Greek letter ) is a fermion (an elementary particle with spin of ) that interacts only via the weak interaction and gravity. The neutrino is so named because it is electrically neutral and because its rest mass ...
s called the cosmic neutrino background
The cosmic neutrino background (CNB or CB) is the universe's background particle radiation composed of neutrinos. They are sometimes known as relic neutrinos.
The CB is a relic of the Big Bang; while the cosmic microwave background radiation ( ...
.) The current black body
A black body or blackbody is an idealized physical body that absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence. The name "black body" is given because it absorbs all colors of light. A 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 away from Earth in the constellation Centaurus. It is also known as the Bow Tie Nebula and catalogued as LEDA 3074547. The nebula's temperature is measured at making i ...
is 1 K,[ while the ]solar corona
A corona ( coronas or coronae) is the outermost layer of a star's atmosphere. It consists of plasma.
The Sun's corona lies above the chromosphere and extends millions of kilometres into outer space. It is most easily seen during a total solar ...
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
"Dynamo Electric Machine" (end view, partly section, )
A dynamo is an electrical generator that creates direct current using a commutator. Dynamos were the first electrical generators capable of delivering power for industry, and the foundati ...
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. This has been used to show ordered magnetic fields exist in several nearby galaxies. Magneto-hydrodynamic
Magnetohydrodynamics (MHD; also called magneto-fluid dynamics or hydromagnetics) is the study of the magnetic properties and behaviour of electrically conducting fluids. Examples of such magnetofluids include plasmas, liquid metals, ...
processes in active
Active may refer to:
Music
* ''Active'' (album), a 1992 album by Casiopea
* Active Records, a record label
Ships
* ''Active'' (ship), several commercial ships by that name
* HMS ''Active'', the name of various ships of the British Royal ...
elliptical galaxies
An elliptical galaxy is a type of galaxy with an approximately ellipsoidal shape and a smooth, nearly featureless image. They are one of the four main classes of galaxy described by Edwin Hubble in his Hubble sequence and 1936 work ''The Real ...
produce their characteristic jets and radio lobe
A radio galaxy is a galaxy with giant regions of radio emission extending well beyond its visible structure. These energetic radio lobes are powered by jets from its active galactic nucleus. They have luminosities up to 1039 W at radio wav ...
s. Non-thermal radio sources
An astronomical radio source is an object in outer space that emits strong radio waves. Radio emission comes from a wide variety of sources. Such objects are among the most extreme and energetic physical processes in the universe.
History
In 193 ...
have been detected even among the most distant, 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 particle
In physical sciences, a subatomic particle is a particle that composes an atom. According to the Standard Model of particle physics, a subatomic particle can be either a composite particle, which is composed of other particles (for example, a p ...
s known as cosmic rays. These particles have energies ranging from about 106 eV up to an extreme 1020 eV of ultra-high-energy cosmic ray
In astroparticle physics, an ultra-high-energy cosmic ray (UHECR) is a cosmic ray with an energy greater than 1 EeV (1018 electronvolts, approximately 0.16 joules), far beyond both the rest mass and energies typical of other cosmic ray pa ...
s.[ 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 ]electron
The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family,
and are generally thought to be elementary particles because they have n ...
s is only about 1% of that of protons. Cosmic rays can damage electronic components and pose a health threat to space travelers. According to astronauts, like Don Pettit
Donald Roy Pettit (born April 20, 1955) is an American astronaut and chemical engineer. He is a veteran of two long-duration stays aboard the International Space Station, one Space Shuttle mission and a six-week expedition to find meteorites in An ...
, space has a burned/metallic odor that clings to their suits and equipment, similar to the scent of an arc welding
Arc welding is a welding process that is used to join metal to metal by using electricity to create enough heat to melt metal, and the melted metals, when cool, result in a binding of the metals. It is a type of welding that uses a welding powe ...
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
''Arabidopsis thaliana'', the thale cress, mouse-ear cress or arabidopsis, is a small flowering plant native to Eurasia and Africa. ''A. thaliana'' is considered a weed; it is found along the shoulders of roads and in disturbed land.
A winter ...
'' and ''Nicotiana tabacum
''Nicotiana tabacum'', or cultivated tobacco, is an annually grown herbaceous plant of the ''Nicotiana'' genus. The plant is tropical in origin, is commonly grown throughout the world, and is often found in cultivation. It grows to heights be ...
'' germinated after being exposed to space for 1.5 years. A strain of ''Bacillus subtilis
''Bacillus subtilis'', known also as the hay bacillus or grass bacillus, is a Gram-positive, catalase-positive bacterium, found in soil and the gastrointestinal tract of ruminants, humans and marine sponges. As a member of the genus ''Bacillus ...
'' has survived 559 days when exposed to low Earth orbit or a simulated martian environment. The lithopanspermia
Panspermia () is the hypothesis, first proposed in the 5th century BCE by the Greek philosopher Anaxagoras, that life exists throughout the Universe, distributed by space dust, meteoroids, asteroids, comets, and planetoids, as well as by spacecr ...
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
A microorganism, or microbe,, ''mikros'', "small") and ''organism'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usually written as a single word but is sometimes hyphenated (''micro-organism''), especially in old ...
-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
The vapour pressure of water is the pressure exerted by molecules of water vapor in gaseous form (whether pure or in a mixture with other gases such as air). The saturation vapour pressure is the pressure at which water vapour is in thermodynamic ...
at the temperature of the human body is called the Armstrong line
The Armstrong limit or Armstrong's line is a measure of altitude above which atmospheric pressure is sufficiently low that water boils at the normal temperature of the human body. Exposure to pressure below this limit results in a rapid loss of ...
, named after American physician Harry G. Armstrong
Harry George Armstrong (February 17, 1899 – February 5, 1983) was a major general in the United States Air Force, a physician, and an airman. He is widely recognized as a pioneer in the field of aviation medicine. The "Armstrong limit", the alti ...
. 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 pressure
Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country a ...
, 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.][
As a consequence of rapid decompression, ]oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements ...
dissolved in the blood empties into the lungs to try to equalize the partial pressure
In a mixture of gases, each constituent gas has a partial pressure which is the notional pressure of that constituent gas as if it alone occupied the entire volume of the original mixture at the same temperature. The total pressure of an ideal g ...
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
Ebullism is the formation of gas bubbles in bodily fluids due to reduced environmental pressure, for example at high altitude. It occurs because a system of liquid and gas at equilibrium will see a net conversion of liquid to gas as pressure lo ...
.[ 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
Ebullism is the formation of gas bubbles in bodily fluids due to reduced environmental pressure, for example at high altitude. It occurs because a system of liquid and gas at equilibrium will see a net conversion of liquid to gas as pressure lo ...
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
A pressure suit is a protective suit worn by high-altitude pilots who may fly at altitudes where the air pressure is too low for an unprotected person to survive, even breathing pure oxygen at positive pressure. Such suits may be either full-pr ...
. 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
Decompression sickness (abbreviated DCS; also called divers' disease, the bends, aerobullosis, and caisson disease) is a medical condition caused by dissolved gases emerging from solution as bubbles inside the body tissues during decompressio ...
and gas embolisms if not managed.
Humans evolved for life in Earth gravity
In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the stro ...
, and exposure to weightlessness has been shown to have deleterious effects on human health. Initially, more than 50% of astronauts experience space motion sickness
Space adaptation syndrome (SAS) or space sickness is a condition experienced by as many as half of all space travelers during their adaptation to weightlessness once in orbit. It is the opposite of terrestrial motion sickness since it occurs whe ...
. This can cause nausea
Nausea is a diffuse sensation of unease and discomfort, sometimes perceived as an urge to vomit. While not painful, it can be a debilitating symptom if prolonged and has been described as placing discomfort on the chest, abdomen, or back of th ...
and vomiting
Vomiting (also known as emesis and throwing up) is the involuntary, forceful expulsion of the contents of one's stomach through the mouth and sometimes the nose.
Vomiting can be the result of ailments like food poisoning, gastroenter ...
, vertigo
Vertigo is a condition where a person has the sensation of movement or of surrounding objects moving when they are not. Often it feels like a spinning or swaying movement. This may be associated with nausea, vomiting, sweating, or difficulties w ...
, headaches, lethargy
Lethargy is a state of tiredness, sleepiness, weariness, fatigue, sluggishness or lack of energy. It can be accompanied by depression, decreased motivation, or apathy. Lethargy can be a normal response to inadequate sleep, overexertion, overwo ...
, 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 mass. It can be caused by immobility, aging, malnutrition, medications, or a wide range of injuries or diseases that impact the musculoskeletal or nervous system. Muscle atrophy leads to muscle weakness ...
and deterioration of the skeleton
A skeleton is the structural frame that supports the body of an animal. There are several types of skeletons, including the exoskeleton, which is the stable outer shell of an organism, the endoskeleton, which forms the support structure inside ...
, or spaceflight osteopenia. These effects can be minimized through a regimen of exercise. Other effects include fluid redistribution, slowing of the cardiovascular system
The blood circulatory system is a system of organs that includes the heart, blood vessels, and blood which is circulated throughout the entire body of a human or other vertebrate. It includes the cardiovascular system, or vascular system, tha ...
, decreased production of red blood cell
Red blood cells (RBCs), also referred to as red cells, red blood corpuscles (in humans or other animals not having nucleus in red blood cells), haematids, erythroid cells or erythrocytes (from Greek ''erythros'' for "red" and ''kytos'' for "hol ...
s, balance disorders, and a weakening of the immune system
The immune system is a network of biological processes that protects an organism from diseases. It detects and responds to a wide variety of pathogens, from viruses to parasitic worms, as well as cancer cells and objects such as wood splinte ...
. 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
In epidemiology, a risk factor or determinant is a variable associated with an increased risk of disease or infection.
Due to a lack of harmonization across disciplines, determinant, in its more widely accepted scientific meaning, is often us ...
. Exposure to high-energy, ionizing cosmic rays
Cosmic rays are high-energy particles or clusters of particles (primarily represented by protons or atomic nuclei) that move through space at nearly the speed of light. They originate from the Sun, from outside of the Solar System in our ...
can result in fatigue, nausea, vomiting, as well as damage to the immune system and changes to the white blood cell
White blood cells, also called leukocytes or leucocytes, are the cells of the immune system that are involved in protecting the body against both infectious disease and foreign invaders. All white blood cells are produced and derived from mult ...
count. Over longer durations, symptoms include an increased risk of cancer
Cancer is a group of diseases involving abnormal cell growth with the potential to invade or spread to other parts of the body. These contrast with benign tumors, which do not spread. Possible signs and symptoms include a lump, abnormal b ...
, plus damage to the eyes
Eyes are organs of the visual system. They provide living organisms with vision, the ability to receive and process visual detail, as well as enabling several photo response functions that are independent of vision. Eyes detect light and con ...
, nervous system
In biology, the nervous system is the highly complex part of an animal that coordinates its actions and sensory information by transmitting signals to and from different parts of its body. The nervous system detects environmental changes ...
, lungs
The lungs are the primary organs of the respiratory system in humans and most other animals, including some snails and a small number of fish. In mammals and most other vertebrates, two lungs are located near the backbone on either si ...
and the gastrointestinal tract
The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus. The GI tract contains all the major organs of the digestive system, in humans and ...
.[ On a round-trip ]Mars
Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury. In the English language, Mars is named for the Roman god of war. Mars is a terrestrial planet with a thin at ...
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.][
]
Boundary
There is no clear boundary between Earth's atmosphere
The atmosphere of Earth is the layer of gases, known collectively as air, retained by Earth's gravity that surrounds the planet and forms its planetary atmosphere. The atmosphere of Earth protects life on Earth by creating pressure allowing fo ...
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
The (; FAI; en, World Air Sports Federation) is the world governing body for air sports, and also stewards definitions regarding human spaceflight. It was founded on 14 October 1905, and is headquartered in Lausanne, Switzerland. It maintain ...
has established the Kármán line
The Kármán line (or von Kármán line ) is an attempt to define a boundary between Earth's atmosphere and outer space, and offers a specific definition set by the Fédération aéronautique internationale (FAI), an international record-keeping ...
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
Theodore von Kármán ( hu, ( szőllőskislaki) Kármán Tódor ; born Tivadar Mihály Kármán; 11 May 18816 May 1963) was a Hungarian-American mathematician, aerospace engineer, and physicist who was active primarily in the fields of aeronaut ...
calculated, a vehicle would have to travel faster than orbital velocity to derive sufficient aerodynamic lift from the atmosphere to support itself.[
* Up until 2021, the United States designated people who travel above an altitude of as ]astronaut
An astronaut (from the Ancient Greek (), meaning 'star', and (), meaning 'sailor') is a person trained, equipped, and deployed by a human spaceflight program to serve as a commander or crew member aboard a spacecraft. Although generally r ...
s. Astronaut wings
The United States Astronaut Badge is a badge of the United States, awarded to military and civilian personnel who have completed training and performed a successful spaceflight. A variation of the astronaut badge is also issued to civilians who ...
are now only awarded to spacecraft crew members that "demonstrated activities during flight that were essential to public safety, or contributed to human space flight safety".[
* ]NASA
The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research.
NASA was established in 1958, succeedin ...
's Space Shuttle used , or , as its re-entry
Atmospheric entry is the movement of an object from outer space into and through the gases of an atmosphere of a planet, dwarf planet, or natural satellite. There are two main types of atmospheric entry: ''uncontrolled entry'', such as the entr ...
altitude (termed the Entry Interface), which roughly marks the boundary where atmospheric drag
In fluid dynamics, drag (sometimes called air resistance, a type of friction, or fluid resistance, another type of friction or fluid friction) is a force acting opposite to the relative motion of any object moving with respect to a surrounding flu ...
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 Moo ...
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
Sovereignty is the defining authority within individual consciousness, social construct, or territory. Sovereignty entails hierarchy within the state, as well as external autonomy for states. In any state, sovereignty is assigned to the perso ...
, 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 weapon
A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission (fission bomb) or a combination of fission and fusion reactions ( thermonuclear bomb), producing a nuclear explosion. Both bomb ...
s in outer space. The treaty was passed by the United Nations General Assembly
The United Nations General Assembly (UNGA or GA; french: link=no, Assemblée générale, AG) is one of the six principal organs of the United Nations (UN), serving as the main deliberative, policymaking, and representative organ of the UN. Curr ...
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 effort ...
treaties have been negotiated and drafted by the UN's Committee on the Peaceful Uses of Outer Space
The United Nations Committee on the Peaceful Uses of Outer Space (COPUOS) is a United Nations committee whose main task is to review and foster international cooperation in the peaceful uses of outer space, as well as to consider legal issues ar ...
. Still, there remains no legal prohibition against deploying conventional weapons in space, and anti-satellite weapon
Anti-satellite weapons (ASAT) are space weapons designed to incapacitate or destroy satellites for strategic or tactical
purposes. Several nations possess operational ASAT systems. Although no ASAT system has been utilised in warfare, a few ...
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
Ecuador ( ; ; Quechua: ''Ikwayur''; Shuar: ''Ecuador'' or ''Ekuatur''), officially the Republic of Ecuador ( es, República del Ecuador, which literally translates as "Republic of the Equator"; Quechua: ''Ikwadur Ripuwlika''; Shuar: ' ...
, Colombia
Colombia (, ; ), officially the Republic of Colombia, is a country in South America with insular regions in North America—near Nicaragua's Caribbean coast—as well as in the Pacific Ocean. The Colombian mainland is bordered by the ...
, Brazil
Brazil ( pt, Brasil; ), officially the Federative Republic of Brazil (Portuguese: ), is the largest country in both South America and Latin America. At and with over 217 million people, Brazil is the world's fifth-largest country by area ...
, Congo, Zaire
Zaire (, ), officially the Republic of Zaire (french: République du Zaïre, link=no, ), was a Congolese state from 1971 to 1997 in Central Africa that was previously and is now again known as the Democratic Republic of the Congo. Zaire was, ...
, Uganda
}), is a landlocked country in East Africa. The country is bordered to the east by Kenya, to the north by South Sudan, to the west by the Democratic Republic of the Congo, to the south-west by Rwanda, and to the south by Tanzania. The ...
, Kenya
)
, national_anthem = " Ee Mungu Nguvu Yetu"()
, image_map =
, map_caption =
, image_map2 =
, capital = Nairobi
, coordinates =
, largest_city = Nairobi
, ...
, and Indonesia
Indonesia, officially the Republic of Indonesia, is a country in Southeast Asia and Oceania between the Indian and Pacific oceans. It consists of over 17,000 islands, including Sumatra, Java, Sulawesi, and parts of Borneo and New Gui ...
) met in Bogotá
Bogotá (, also , , ), officially Bogotá, Distrito Capital, abbreviated Bogotá, D.C., and formerly known as Santa Fe de Bogotá (; ) during the Spanish period and between 1991 and 2000, is the capital city of Colombia, and one of the larges ...
, Colombia. With their " 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.][
]
Earth orbit
A spacecraft enters orbit when its centripetal acceleration
In mechanics, acceleration is the rate of change of the velocity of an object with respect to time. Accelerations are vector quantities (in that they have magnitude and direction). The orientation of an object's acceleration is given by ...
due to gravity
In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the stro ...
is less than or equal to the centrifugal
Centrifugal (a key concept in rotating systems) may refer to:
*Centrifugal casting (industrial), Centrifugal casting (silversmithing), and Spin casting (centrifugal rubber mold casting), forms of centrifigual casting
*Centrifugal clutch
*Centrifug ...
acceleration due to the horizontal component of its velocity. For a low Earth orbit
A low Earth orbit (LEO) is an orbit around Earth with a period of 128 minutes or less (making at least 11.25 orbits per day) and an eccentricity less than 0.25. Most of the artificial objects in outer space are in LEO, with an altitude never m ...
, 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
The North American X-15 is a hypersonic rocket-powered aircraft. It was operated by the United States Air Force and the National Aeronautics and Space Administration as part of the X-plane series of experimental aircraft. The X-15 set spee ...
.[
To achieve an orbit, a ]spacecraft
A spacecraft is a vehicle or machine designed to fly in outer space. A type of artificial satellite, spacecraft are used for a variety of purposes, including communications, Earth observation, meteorology, navigation, space colonization, ...
must travel faster than a sub-orbital spaceflight
A sub-orbital spaceflight is a spaceflight in which the spacecraft reaches outer space, but its trajectory intersects the atmosphere or surface of the gravitating body from which it was launched, so that it will not complete one orbital re ...
. The energy required to reach Earth orbital velocity at an altitude of is about 36 MJ/kg, which is six times the energy needed merely to climb to the corresponding altitude.[ Spacecraft with a ]perigee
An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. For example, the apsides of the Earth are called the aphelion and perihelion.
General description
There are two apsides in any el ...
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
In celestial mechanics, escape velocity or escape speed is the minimum speed needed for a free, non- propelled object to escape from the gravitational influence of a primary body, thus reaching an infinite distance from it. It is typically ...
required to pull free of Earth's gravitational field altogether and move into interplanetary space is about .[
]
Regions
Space is a partial vacuum: its different regions are defined by the various magnetic fields and "winds" that dominate within them, and extend to the point at which those fields 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 magnetic fields of the interstellar medium
In astronomy, the interstellar medium is the matter and radiation that exist in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, as well as dust and cosmic rays. It fills interstella ...
. Interstellar space then continues to the outer fringes of the galaxy, where it fades into the intergalactic void.
Regions near the Earth
Near-Earth space is the region of outer space above the Kármán line, from low Earth orbit
A low Earth orbit (LEO) is an orbit around Earth with a period of 128 minutes or less (making at least 11.25 orbits per day) and an eccentricity less than 0.25. Most of the artificial objects in outer space are in LEO, with an altitude never m ...
s out to geostationary orbit
A geostationary orbit, also referred to as a geosynchronous equatorial orbit''Geostationary orbit'' and ''Geosynchronous (equatorial) orbit'' are used somewhat interchangeably in sources. (GEO), is a circular geosynchronous orbit in altitu ...
s. This region includes the major orbits for artificial satellite
A satellite or artificial satellite is an object intentionally placed into orbit in outer space. Except for passive satellites, most satellites have an electricity generation system for equipment on board, such as solar panels or radioisoto ...
s and is the site of most of humanity's space activity. The region has seen high levels of space pollution, mainly in the form of space debris, threatening any space activity in this region.
Geospace
Outer space, commonly shortened to space, is the expanse that exists beyond Earth and its atmosphere and between celestial bodies. Outer space is not completely empty—it is a near-perfect vacuum containing a low density of particles, pred ...
is a region of outer space near Earth that includes the upper atmosphere Upper atmosphere is a collective term that refers to various layers of the atmosphere of the Earth above the troposphere and corresponding regions of the atmospheres of other planets, and includes:
* The mesosphere, which on Earth lies between th ...
and magnetosphere
In astronomy and planetary science, a magnetosphere is a region of space surrounding an astronomical object in which charged particles are affected by that object's magnetic field. It is created by a celestial body with an active interior d ...
. The Van Allen radiation belts lie within the geospace. The outer boundary of geospace is the magnetopause
The magnetopause is the abrupt boundary between a magnetosphere and the surrounding plasma. For planetary science, the magnetopause is the boundary between the planet's magnetic field and the solar wind. The location of the magnetopause is det ...
, which forms an interface between the Earth's magnetosphere and the solar wind. The inner boundary is the ionosphere
The ionosphere () is the ionized part of the upper atmosphere of Earth, from about to above sea level, a region that includes the thermosphere and parts of the mesosphere and exosphere. The ionosphere is ionized by solar radiation. It plays ...
.[ 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
Heliophysics (from the prefix " helio", from Attic Greek ''hḗlios'', meaning Sun, and the noun "physics": the science of matter and energy and their interactions) is the physics of the Sun and its connection with the Solar System. NASA define ...
—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
In astronomy and planetary science, a magnetosphere is a region of space surrounding an astronomical object in which charged particles are affected by that object's magnetic field. It is created by a celestial body
An astronomical object, c ...
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
Earth's magnetic field, also known as the geomagnetic field, is the magnetic field that extends from Earth's interior out into space, where it interacts with the solar wind, a stream of charged particles emanating from the Sun. The magneti ...
. 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 storm
A geomagnetic storm, also known as a magnetic storm, is a temporary disturbance of the Earth's magnetosphere caused by a solar wind shock wave and/or cloud of magnetic field that interacts with the Earth's magnetic field.
The disturbance that d ...
s 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 GPS
The Global Positioning System (GPS), originally Navstar GPS, is a satellite-based radionavigation system owned by the United States government and operated by the United States Space Force. It is one of the global navigation satellite sy ...
location and timing. Magnetic storms can also be a hazard to astronauts, even in low Earth orbit. They also create aurorae
An aurora (plural: auroras or aurorae), also commonly known as the polar lights, is a natural light display in Earth's sky, predominantly seen in high-latitude regions (around the Arctic and Antarctic). Auroras display dynamic patterns of bri ...
seen at high latitudes in an oval surrounding the geomagnetic pole
The geomagnetic poles are antipodal points where the axis of a best-fitting dipole intersects the surface of Earth. This ''theoretical'' dipole is equivalent to a powerful bar magnet at the center of Earth, and comes closer than any other po ...
s.[
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 on ]satellite
A satellite or artificial satellite is an object intentionally placed into orbit in outer space. Except for passive satellites, most satellites have an electricity generation system for equipment on board, such as solar panels or radioiso ...
s.[ This region contains material left over from previous crewed and uncrewed launches that are a potential hazard to spacecraft. Some of this ]debris
Debris (, ) is rubble, wreckage, ruins, litter and discarded garbage/refuse/trash, scattered remains of something destroyed, or, as in geology, large rock fragments left by a melting glacier, etc. Depending on context, ''debris'' can refer to ...
re-enters Earth's atmosphere periodically.[
Translunar space is the region of ]lunar
Lunar most commonly means "of or relating to the Moon".
Lunar may also refer to:
Arts and entertainment
* ''Lunar'' (series), a series of video games
* "Lunar" (song), by David Guetta
* "Lunar", a song by Priestess from the 2009 album ''Prior t ...
transfer orbit
{{Astrodynamics
Orbits
Astrodynamics
In orbital mechanics, a transfer orbit is an intermediate elliptical orbit that is used to move a spacecraft in an orbital maneuver from one circular, or largely circular, orbit to another.
There are several ...
s, between the Moon and Earth.
Cislunar space is a region outside of Earth that includes lunar orbit
In astronomy, lunar orbit (also known as a selenocentric orbit) is the orbit of an object around the Moon.
As used in the space program, this refers not to the orbit of the Moon about the Earth, but to orbits by spacecraft around the Moon. Th ...
, the Moon's orbital space around Earth and the Lagrange point
In celestial mechanics, the Lagrange points (; also Lagrangian points or libration points) are points of equilibrium for small-mass objects under the influence of two massive orbiting bodies. Mathematically, this involves the solution of t ...
s.[ xGeo space is a concept used by the US to refer to space of ]High Earth Orbit
A high Earth orbit is a geocentric orbit with an altitude entirely above that of a geosynchronous orbit (). The orbital periods of such orbits are greater than 24 hours, therefore satellites in such orbits have an apparent retrograde motion ...
s, ranging from beyond geosynchronous orbit
A geosynchronous orbit (sometimes abbreviated GSO) is an Earth-centered orbit with an orbital period that matches Earth's rotation on its axis, 23 hours, 56 minutes, and 4 seconds (one sidereal day). The synchronization of rotation and orbita ...
(GEO) at approximately ,[ out to the L2 Earth-Moon Lagrange point at . This is located beyond the orbit of the Moon and therefore includes cislunar space.]
The region where Earth's gravity well remains dominant against gravitational perturbations from the Sun is the planet's Hill sphere
The Hill sphere of an astronomical body is the region in which it dominates the attraction of satellites. To be retained by a planet, a moon must have an orbit that lies within the planet's Hill sphere. That moon would, in turn, have a Hill sph ...
.[ This includes all space from the Earth to a distance of roughly 1% of the mean distance from Earth to the Sun, or . Beyond Earth's Hill sphere extends along ]Earth's orbit
Earth orbits the Sun at an average distance of 149.60 million km (92.96 million mi) in a counterclockwise direction as viewed from above the Northern Hemisphere. One complete orbit takes days (1 sidereal year), during which time Earth ...
al path its orbital and co-orbital space. This space is co-populated by groups of co-orbital Near-Earth Object
A near-Earth object (NEO) is any small Solar System body whose orbit brings it into proximity with Earth. By convention, a Solar System body is a NEO if its closest approach to the Sun (Apsis, perihelion) is less than 1.3 astronomical unit ...
s (NEOs), such as horseshoe librators and Earth trojan
An Earth trojan is an asteroid that orbits the Sun in the vicinity of the Earth–Sun Lagrangian points (leading 60°) or (trailing 60°), thus having an orbit similar to Earth's. Only two Earth trojans have so far been discovered. The name ...
s, with some NEOs at times becoming temporary satellite A temporary satellite is an object which has been captured by the gravitational field of a planet and thus has become the planet's natural satellite, but, unlike irregular moons of the larger outer planets of the Solar System, will eventually either ...
s and quasi-moons to Earth.
Deep space is defined by the United States government and others as any region beyond cislunar space.[ The International Telecommunication Union responsible for radio communication, including with satellites, defines the beginning of deep space at ,][ which is about five times the Moon's orbital distance.
]
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
The heliosphere is the magnetosphere, astrosphere and outermost atmospheric layer of the Sun. It takes the shape of a vast, bubble-like region of space. In plasma physics terms, it is the cavity formed by the Sun in the surrounding interstell ...
) for billions of kilometers into space. This wind has a particle density of 5–10 proton
A proton is a stable subatomic particle, symbol , H+, or 1H+ with a positive electric charge of +1 ''e'' elementary charge. Its mass is slightly less than that of a neutron and 1,836 times the mass of an electron (the proton–electron mass ...
s/cm3 and is moving at a velocity of . Interplanetary space extends out to the 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
The astronomical unit (symbol: au, or or AU) is a unit of length, roughly the distance from Earth to the Sun and approximately equal to or 8.3 light-minutes. The actual distance from Earth to the Sun varies by about 3% as Earth orbits ...
at the orbital distance of the Earth. This space is not completely empty, and is sparsely filled with cosmic rays, which include ionized atomic nuclei
The atomic nucleus is the small, dense region consisting of protons and neutrons at the center of an atom, discovered in 1911 by Ernest Rutherford based on the 1909 Geiger–Marsden gold foil experiment. After the discovery of the neutron ...
and various subatomic particles. There is also gas, plasma and dust, small meteor
A meteoroid () is a small rocky or metallic body in outer space.
Meteoroids are defined as objects significantly smaller than asteroids, ranging in size from grains to objects up to a meter wide. Objects smaller than this are classified as mi ...
s, and several dozen types of organic
Organic may refer to:
* Organic, of or relating to an organism, a living entity
* Organic, of or relating to an anatomical organ
Chemistry
* Organic matter, matter that has come from a once-living organism, is capable of decay or is the product ...
molecule
A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and b ...
s discovered to date by microwave spectroscopy
Microwave spectroscopy is the spectroscopy method that employs microwaves, i.e. electromagnetic radiation at GHz frequencies, for the study of matter.
History
The ammonia molecule NH3 is shaped like a pyramid 0.38 Å in height, with an equilatera ...
.[ A cloud of interplanetary dust is visible at night as a faint band called the ]zodiacal light
The zodiacal light (also called false dawn when seen before sunrise) is a faint glow of diffuse sunlight scattered by interplanetary dust. Brighter around the Sun, it appears in a particularly dark night sky to extend from the Sun's direction ...
.
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
Stellar nucleosynthesis is the creation (nucleosynthesis) of chemical elements by nuclear fusion reactions within stars. Stellar nucleosynthesis has occurred since the original creation of hydrogen, helium and lithium during the Big Bang. A ...
. These atoms are ejected into the interstellar medium by stellar wind
A stellar wind is a flow of gas ejected from the upper atmosphere of a star. It is distinguished from the bipolar outflows characteristic of young stars by being less collimated, although stellar winds are not generally spherically symmetric.
...
s or when evolved stars begin to shed their outer envelopes such as during the formation of a planetary nebula
A planetary nebula (PN, plural PNe) is a type of emission nebula consisting of an expanding, glowing shell of ionized gas ejected from red giant stars late in their lives.
The term "planetary nebula" is a misnomer because they are unrelate ...
. The cataclysmic explosion of a supernova
A supernova is a powerful and luminous explosion of a star. It has the plural form supernovae or supernovas, and is abbreviated SN or SNe. This transient astronomical event occurs during the last evolutionary stages of a massive star or whe ...
generates an expanding shock wave
In physics, a shock wave (also spelled shockwave), or shock, is a type of propagating disturbance that moves faster than the local speed of sound in the medium. Like an ordinary wave, a shock wave carries energy and can propagate through a me ...
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
A molecular cloud, sometimes called a stellar nursery (if star formation is occurring within), is a type of interstellar cloud, the density and size of which permit absorption nebulae, the formation of molecules (most commonly molecular hydroge ...
can hold 108–1012 per m3.
A number of molecules exist in interstellar space, as can tiny 0.1 μm
The micrometre ( international spelling as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer ( American spelling), also commonly known as a micron, is a unit of length in the International System of Uni ...
dust particles. The tally of molecules discovered through radio astronomy
Radio astronomy is a subfield of astronomy that studies celestial objects at radio frequencies. The first detection of radio waves from an astronomical object was in 1933, when Karl Jansky at Bell Telephone Laboratories reported radiation comin ...
is steadily increasing at the rate of about four new species per year. Large regions of higher density matter known as molecular cloud
A molecular cloud, sometimes called a stellar nursery (if star formation is occurring within), is a type of interstellar cloud, the density and size of which permit absorption nebulae, the formation of molecules (most commonly molecular hydroge ...
s 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
The trihydrogen cation or protonated molecular hydrogen is a cation (positive ion) with formula , consisting of three hydrogen nuclei (protons) sharing two electrons.
The trihydrogen cation is one of the most abundant ions in the universe. It is ...
. An ionized helium atom can then split relatively abundant carbon monoxide
Carbon monoxide ( chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the simpl ...
to produce ionized carbon, which in turn can lead to organic chemical reactions.
The local interstellar medium is a region of space within 100 parsec
The parsec (symbol: pc) is a unit of length used to measure the large distances to astronomical objects outside the Solar System, approximately equal to or (au), i.e. . The parsec unit is obtained by the use of parallax and trigonometry, an ...
s (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
The Local Bubble, or Local Cavity, is a relative cavity in the interstellar medium (ISM) of the Orion Arm in the Milky Way. It contains the closest of celestial neighbours and among others, the Local Interstellar Cloud (which contains the Sol ...
, which is characterized by a lack of dense, cold clouds. It forms a cavity in the Orion Arm
The Orion Arm is a minor spiral arm of the Milky Way Galaxy that is across and approximately in length, containing the Solar System, including Earth. It is also referred to by its full name, the Orion–Cygnus Arm, as well as Local Arm, Orion ...
of the Milky Way galaxy, with dense molecular clouds lying along the borders, such as those in the constellation
A constellation is an area on the celestial sphere in which a group of visible stars forms a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object.
The origins of the earliest constellation ...
s of Ophiuchus
Ophiuchus () is a large constellation straddling the celestial equator. Its name comes from the Ancient Greek (), meaning "serpent-bearer", and it is commonly represented as a man grasping a snake. The serpent is represented by the constella ...
and 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 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 shock
In astrophysics, a bow shock occurs when the magnetosphere of an astrophysical object interacts with the nearby flowing ambient plasma such as the solar wind. For Earth and other magnetized planets, it is the boundary at which the speed of th ...
s 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 (IBEX) and NASA's Voyager
Voyager may refer to:
Computing and communications
* LG Voyager, a mobile phone model manufactured by LG Electronics
* NCR Voyager, a computer platform produced by NCR Corporation
* Voyager (computer worm), a computer worm affecting Oracle ...
probes showed that the Sun's bow shock does not exist. Instead, these authors argue that a 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 ]termination shock
The heliosphere is the magnetosphere, astrosphere and outermost atmospheric layer of the Sun. It takes the shape of a vast, bubble-like region of space. In plasma physics terms, it is the cavity formed by the Sun in the surrounding interstell ...
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 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 spans a distance of 7–30 megaparsecs.
Surrounding and stretching between galaxies, there is a rarefied plasma[ that is organized in a 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
In astronomy, the intracluster medium (ICM) is the superheated plasma that permeates a galaxy cluster. The gas consists mainly of ionized hydrogen and helium and accounts for most of the baryonic material in galaxy clusters. The ICM is heated to t ...
(ICM).
History of discovery
In 350 BCE, Greek philosopher Aristotle
Aristotle (; grc-gre, Ἀριστοτέλης ''Aristotélēs'', ; 384–322 BC) was a Greek philosopher and polymath during the Classical period in Ancient Greece. Taught by Plato, he was the founder of the Peripatetic school of ...
suggested that ''nature abhors a vacuum'', a principle that became known as the '' horror vacui''. This concept built upon a 5th-century BCE ontological
In metaphysics, ontology is the philosophical study of being, as well as related concepts such as existence, becoming, and reality.
Ontology addresses questions like how entities are grouped into categories and which of these entities exi ...
argument by the Greek philosopher Parmenides
Parmenides of Elea (; grc-gre, Παρμενίδης ὁ Ἐλεάτης; ) was a pre-Socratic Greek philosopher from Elea in Magna Graecia.
Parmenides was born in the Greek colony of Elea, from a wealthy and illustrious family. His date ...
, who denied the possible existence of a void in space. Based on this idea that a vacuum could not exist, in the West
West or Occident is one of the four cardinal directions or points of the compass. It is the opposite direction from east and is the direction in which the Sun sets on the Earth.
Etymology
The word "west" is a Germanic word passed into some ...
it was widely held for many centuries that space could not be empty. As late as the 17th century, the French philosopher René Descartes
René Descartes ( or ; ; Latinized: Renatus Cartesius; 31 March 1596 – 11 February 1650) was a French philosopher, scientist, and mathematician, widely considered a seminal figure in the emergence of modern philosophy and science. Ma ...
argued that the entirety of space must be filled.
In ancient China
The earliest known written records of the history of China date from as early as 1250 BC, from the Shang dynasty (c. 1600–1046 BC), during the reign of king Wu Ding. Ancient historical texts such as the '' Book of Documents'' (early chapt ...
, the 2nd-century astronomer Zhang Heng
Zhang Heng (; AD 78–139), formerly romanized as Chang Heng, was a Chinese polymathic scientist and statesman who lived during the Han dynasty. Educated in the capital cities of Luoyang and Chang'an, he achieved success as an astronomer, mat ...
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
Galileo di Vincenzo Bonaiuti de' Galilei (15 February 1564 – 8 January 1642) was an Italian astronomer, physicist and engineer, sometimes described as a polymath. Commonly referred to as Galileo, his name was pronounced (, ). He ...
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
Evangelista Torricelli ( , also , ; 15 October 160825 October 1647) was an Italian physicist and mathematician, and a student of Galileo. He is best known for his invention of the barometer, but is also known for his advances in optics and wo ...
to create an apparatus that would produce a partial vacuum in 1643. This experiment resulted in the first mercury barometer
A barometer is a scientific instrument that is used to measure air pressure in a certain environment. Pressure tendency can forecast short term changes in the weather. Many measurements of air pressure are used within surface weather analysis ...
and created a scientific sensation in Europe. The French mathematician Blaise Pascal
Blaise Pascal ( , , ; ; 19 June 1623 – 19 August 1662) was a French mathematician, physicist, inventor, philosopher, and Catholic writer.
He was a child prodigy who was educated by his father, a tax collector in Rouen. Pascal's earliest ...
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
Atmospheric pressure, also known as barometric pressure (after the barometer), is the pressure within the atmosphere of Earth. The standard atmosphere (symbol: atm) is a unit of pressure defined as , which is equivalent to 1013.25 millibars ...
is lower. In 1648, his brother-in-law, Florin Périer, repeated the experiment on the Puy de Dôme
Puy de Dôme (, ; oc, label=Auvergnat, Puèi Domat or ) is a lava dome and one of the youngest volcanoes in the region of Massif Central in central France. This chain of volcanoes including numerous cinder cones, lava domes and maars is ...
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
Otto von Guericke ( , , ; spelled Gericke until 1666; November 20, 1602 – May 11, 1686 ; November 30, 1602 – May 21, 1686 ) was a German scientist, inventor, and politician. His pioneering scientific work, the development of experimental me ...
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
Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematicall ...
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
Nicholas of Cusa (1401 – 11 August 1464), also referred to as Nicholas of Kues and Nicolaus Cusanus (), was a German Catholic cardinal, philosopher, theologian, jurist, mathematician, and astronomer. One of the first German proponents of Renai ...
speculated that the Universe
The universe is all of space and time and their contents, including planets, stars, galaxies, and all other forms of matter and energy. The Big Bang theory is the prevailing cosmological description of the development of the univers ...
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
Giordano Bruno (; ; la, Iordanus Brunus Nolanus; born Filippo Bruno, January or February 1548 – 17 February 1600) was an Italian philosopher, mathematician, poet, cosmological theorist, and Hermetic occultist. He is known for his cosmolog ...
in the 16th century. He extended the Copernican heliocentric
Heliocentrism (also known as the Heliocentric model) is the astronomical model in which the Earth and planets revolve around the Sun at the center of the universe. Historically, heliocentrism was opposed to geocentrism, which placed the Earth ...
cosmology
Cosmology () is a branch of physics and metaphysics dealing with the nature of the universe. The term ''cosmology'' was first used in English in 1656 in Thomas Blount's ''Glossographia'', and in 1731 taken up in Latin by German philosopher ...
to the concept of an infinite Universe filled with a substance he called aether, which did not resist the motion of heavenly bodies. English philosopher 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 Greek
Ancient Greek includes the forms of the Greek language used in ancient Greece and the ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Dark Ages (), the Archaic pe ...
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
Luminiferous aether or ether ("luminiferous", meaning "light-bearing") was the postulated medium for the propagation of light. It was invoked to explain the ability of the apparently wave-based light to propagate through empty space (a vacuum), so ...
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
The Michelson–Morley experiment was an attempt to detect the existence of the luminiferous aether, a supposed medium permeating space that was thought to be the carrier of light waves. The experiment was performed between April and July 188 ...
tried to detect the Earth's motion through this medium by looking for changes in the speed of light
The speed of light in vacuum, commonly denoted , is a universal physical constant that is important in many areas of physics. The speed of light is exactly equal to ). According to the special theory of relativity, is the upper limit fo ...
depending on the direction of the planet's motion. The null result In science, a null result is a result without the expected content: that is, the proposed result is absent. It is an experimental outcome which does not show an otherwise expected effect. This does not imply a result of zero or nothing, simply a res ...
indicated something was wrong with the concept. The idea of the luminiferous aether was then abandoned. It was replaced by Albert Einstein
Albert Einstein ( ; ; 14 March 1879 – 18 April 1955) was a German-born theoretical physicist, widely acknowledged to be one of the greatest and most influential physicists of all time. Einstein is best known for developing the theor ...
's theory of special relativity
In physics, the special theory of relativity, or special relativity for short, is a scientific theory regarding the relationship between space and time. In Albert Einstein's original treatment, the theory is based on two postulates:
# The law ...
, which holds that the speed of light in a vacuum is a fixed constant, independent of the observer's motion or frame of reference
In physics and astronomy, a frame of reference (or reference frame) is an abstract coordinate system whose origin, orientation, and scale are specified by a set of reference points― geometric points whose position is identified both math ...
.
The first professional astronomer
An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of Earth. They observe astronomical objects such as stars, planets, moons, comets and galaxies – in either ...
to support the concept of an infinite Universe was the Englishman Thomas Digges
Thomas Digges (; c. 1546 – 24 August 1595) was an English mathematician and astronomer. He was the first to expound the Copernican system in English but discarded the notion of a fixed shell of immoveable stars to postulate infinitely many s ...
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
Friedrich Wilhelm Bessel (; 22 July 1784 – 17 March 1846) was a German astronomer, mathematician, physicist, and geodesist. He was the first astronomer who determined reliable values for the distance from the sun to another star by the method ...
. He showed that the star system 61 Cygni
61 Cygni is a binary star system in the constellation Cygnus, consisting of a pair of K-type dwarf stars that orbit each other in a period of about 659 years. Of apparent magnitude 5.20 and 6.05, respectively, they can be seen ...
had a parallax
Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight and is measured by the angle or semi-angle of inclination between those two lines. Due to foreshortening, nearby object ...
of just 0.31 arcsecond
A minute of arc, arcminute (arcmin), arc minute, or minute arc, denoted by the symbol , is a unit of angular measurement equal to of one degree. Since one degree is of a turn (or complete rotation), one minute of arc is of a turn. The n ...
s (compared to the modern value of 0.287″). This corresponds to a distance of over 10 light year
A light-year, alternatively spelled light year, is a large unit of length used to express astronomical distances and is equivalent to about 9.46 trillion kilometers (), or 5.88 trillion miles ().One trillion here is taken to be 1012 ...
s. In 1917, Heber Curtis
Heber Doust Curtis (June 27, 1872 – January 9, 1942) was an American astronomer. He participated in 11 expeditions for the study of solar eclipses, and, as an advocate and theorist that additional galaxies existed outside of the Milky Way, wa ...
noted that nova
A nova (plural novae or novas) is a transient astronomical event that causes the sudden appearance of a bright, apparently "new" star (hence the name "nova", which is Latin for "new") that slowly fades over weeks or months. Causes of the dramat ...
e 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
The Andromeda Galaxy (IPA: ), also known as Messier 31, M31, or NGC 224 and originally the Andromeda Nebula, is a barred spiral galaxy with the diameter of about approximately from Earth and the nearest large galaxy to the Milky Way. The gal ...
was determined in 1923 by American astronomer Edwin Hubble
Edwin Powell Hubble (November 20, 1889 – September 28, 1953) was an American astronomer. He played a crucial role in establishing the fields of extragalactic astronomy and observational cosmology.
Hubble proved that many objects previousl ...
by measuring the brightness of cepheid variable
A Cepheid variable () is a type of star that pulsates radially, varying in both diameter and temperature and producing changes in brightness with a well-defined stable period and amplitude.
A strong direct relationship between a Cepheid vari ...
s in that galaxy, a new technique discovered by Henrietta Leavitt
Henrietta Swan Leavitt (; July 4, 1868 – December 12, 1921) was an American astronomer. A graduate of Radcliffe College, she worked at the Harvard College Observatory as a "computer", tasked with examining photographic plates in order to measu ...
.[ This established that the Andromeda galaxy, and by extension all galaxies, lay well outside the ]Milky Way
The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye. ...
.
The modern concept of outer space is based on the "Big Bang" cosmology, first proposed in 1931 by the Belgian physicist Georges Lemaître
Georges Henri Joseph Édouard Lemaître ( ; ; 17 July 1894 – 20 June 1966) was a Belgian Catholic priest, theoretical physicist, mathematician, astronomer, and professor of physics at the Catholic University of Louvain. He was the first to t ...
.[ This theory holds that the universe originated from a very dense form that has since undergone continuous ]expansion
Expansion may refer to:
Arts, entertainment and media
* ''L'Expansion'', a French monthly business magazine
* ''Expansion'' (album), by American jazz pianist Dave Burrell, released in 2004
* ''Expansions'' (McCoy Tyner album), 1970
* ''Expansio ...
.
The earliest known estimate of the temperature of outer space was by the Swiss physicist 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
Sir Arthur Stanley Eddington (28 December 1882 – 22 November 1944) was an English astronomer, physicist, and mathematician. He was also a philosopher of science and a populariser of science. The Eddington limit, the natural limit to the lumi ...
made a similar calculation to derive a temperature of 3.18 K in 1926. German physicist Erich Regener
Erich Rudolf Alexander Regener (12 November 1881 – 27 February 1955) was a German physicist known primarily for the design and construction of instruments to measure cosmic ray intensity at various altitudes. He is also known for predicting ...
used the total measured energy of cosmic ray
Cosmic rays are high-energy particles or clusters of particles (primarily represented by protons or atomic nuclei) that move through space at nearly the speed of light. They originate from the Sun, from outside of the Solar System in our own ...
s to estimate an intergalactic temperature of 2.8 K in 1933. American physicists Ralph Alpher
Ralph Asher Alpher (February 3, 1921 – August 12, 2007) was an American cosmologist, who carried out pioneering work in the early 1950s on the Big Bang model, including Big Bang nucleosynthesis and predictions of the cosmic microwave backgroun ...
and Robert Herman
Robert Herman (August 29, 1914 – February 13, 1997) was an American scientist, best known for his work with Ralph Alpher in 1948–50, on estimating the temperature of cosmic microwave background radiation from the Big Bang explosion.
Biograp ...
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 event is a physical theory that describes how the universe expanded from an initial state of high density and temperature. Various cosmological models of the Big Bang explain the evolution of the observable universe from the ...
theory. The modern measurement of the cosmic microwave background
In Big Bang cosmology the cosmic microwave background (CMB, CMBR) is electromagnetic radiation that is a remnant from an early stage of the universe, also known as "relic radiation". The CMB is faint cosmic background radiation filling all spac ...
is about 2.7K.
The term ''outward space'' was used in 1842 by the English poet Lady Emmeline Stuart-Wortley
Lady Emmeline Charlotte Elizabeth Stuart-Wortley (née Manners; 1806 – 20 October 1855) was an English poet and writer, best known for her ''Travels in the United States, etc. During 1849 and 1850''. She was editor of '' The Keepsake'' volumes ...
in her poem "The Maiden of Moscow". The expression ''outer space'' was used as an astronomical term by Alexander von Humboldt
Friedrich Wilhelm Heinrich Alexander von Humboldt (14 September 17696 May 1859) was a German polymath, geographer, naturalist, explorer, and proponent of Romantic philosophy and science. He was the younger brother of the Prussian minister ...
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
John Milton (9 December 1608 – 8 November 1674) was an English poet and intellectual. His 1667 epic poem ''Paradise Lost'', written in blank verse and including over ten chapters, was written in a time of immense religious flux and politica ...
's ''Paradise Lost
''Paradise Lost'' is an epic poem in blank verse by the 17th-century English poet John Milton (1608–1674). The first version, published in 1667, consists of ten books with over ten thousand lines of verse. A second edition followed in 16 ...
'' in 1667.[ " Spaceborne" denotes existing in outer space, especially if carried by a ]spacecraft
A spacecraft is a vehicle or machine designed to fly in outer space. A type of artificial satellite, spacecraft are used for a variety of purposes, including communications, Earth observation, meteorology, navigation, space colonization, ...
; similarly, " space-based" means based in outer space or using space technology
Space technology is technology for use in outer space, in travel (''astronautics'') or other activities beyond Earth's atmosphere, for purposes such as spaceflight, space exploration, and Earth observation. Space technology includes space vehicles ...
.
Exploration
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
''Explorer II'' was a manned U.S. high-altitude balloon that was launched on November 11, 1935, and reached a record altitude of . Launched at 8:00 am from the Stratobowl in South Dakota, the helium balloon carried a two-man crew consisting of ...
crewed balloon flight reached an altitude of .[ This was greatly exceeded in 1942 when the third launch of the German A-4 rocket climbed to an altitude of about . In 1957, the uncrewed satellite ]Sputnik 1
Sputnik 1 (; see § Etymology) was the first artificial Earth satellite. It was launched into an elliptical low Earth orbit by the Soviet Union on 4 October 1957 as part of the Soviet space program. It sent a radio signal back to Earth for ...
was launched by a Russian R-7 rocket
The R-7 family of rockets (russian: Р-7) is a series of rockets, derived from the Soviet R-7 Semyorka, the world's first ICBM. More R-7 rockets have been launched than any other family of large rockets.
When Soviet nuclear warheads became ligh ...
, 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 pilot and cosmonaut who became the first human to journey into outer space. T ...
was sent into orbit on Vostok 1
Vostok 1 (russian: link=no, Восток, ''East'' or ''Orient'' 1) was the first spaceflight of the Vostok programme and the first human orbital spaceflight in history. The Vostok 3KA space capsule was launched from Baikonur Cosmodrome on Apr ...
. The first humans to escape low Earth orbit were Frank Borman
Frank Frederick Borman II (born March 14, 1928) is a retired United States Air Force (USAF) colonel (United States), colonel, aeronautical engineer, test pilot, businessman, and NASA astronaut. He was the commander of Apollo 8, the first missio ...
, Jim Lovell
James Arthur Lovell Jr. (; born March 25, 1928) is an American retired astronaut, naval aviator, test pilot and mechanical engineer. In 1968, as command module pilot of Apollo 8, he became, with Frank Borman and William Anders, one of th ...
and William Anders
William Alison Anders (born 17 October 1933) is a retired United States Air Force (USAF) major general, former electrical engineer, nuclear engineer, NASA astronaut, and businessman. In December 1968, he was a member of the crew of Apollo 8, t ...
in 1968 on board the U.S. Apollo 8
Apollo 8 (December 21–27, 1968) was the first crewed spacecraft to leave low Earth orbit and the first human spaceflight to reach the Moon. The crew orbited the Moon ten times without landing, and then departed safely back to Earth. The ...
, 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
''Luna 1'', also known as ''Mechta'' (russian: Мечта , '' lit.'': ''Dream''), ''E-1 No.4'' and ''First Lunar Rover'', was the first spacecraft to reach the vicinity of Earth's Moon, and the first spacecraft to be placed in heliocentric or ...
'', which performed a fly-by of the Moon in 1959. In 1961, ''Venera 1
''Venera 1'' (russian: Венера-1 meaning ''Venus 1''), also known as Venera-1VA No.2 and occasionally in the West as ''Sputnik 8'' was the first spacecraft to fly past Venus, as part of the Soviet Union's Venera programme. Launched in Febr ...
'' became the first planetary probe. It revealed the presence of the solar wind and performed the first fly-by of Venus
Venus is the second planet from the Sun. It is sometimes called Earth's "sister" or "twin" planet as it is almost as large and has a similar composition. As an interior planet to Earth, Venus (like Mercury) appears in Earth's sky never f ...
, although contact was lost before reaching Venus. The first successful planetary mission was the 1962 fly-by of Venus by Mariner 2
Mariner 2 (Mariner-Venus 1962), an American space probe to Venus, was the first robotic space probe to conduct a successful planetary encounter. The first successful spacecraft in the NASA Mariner program, it was a simplified version of the B ...
. The first fly-by of Mars was by Mariner 4
Mariner 4 (together with Mariner 3 known as Mariner-Mars 1964) was the fourth in a series of spacecraft intended for planetary exploration in a flyby mode. It was designed to conduct closeup scientific observations of Mars and to transmit the ...
in 1964. Since that time, uncrewed spacecraft have successfully examined each of the Solar System's planets, as well their moons and many minor planet
According to the International Astronomical Union (IAU), a minor planet is an astronomical object in direct orbit around the Sun that is exclusively classified as neither a planet nor a comet. Before 2006, the IAU officially used the term ''minor ...
s 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
''Voyager 1'' is a space probe launched by NASA on September 5, 1977, as part of the Voyager program to study the outer Solar System and interstellar space beyond the Sun's heliosphere. Launched 16 days after its twin '' Voyager 2'', ''V ...
'' became the first man-made object to leave the Solar System and enter interstellar space
Outer space, commonly shortened to space, is the expanse that exists beyond Earth and its atmosphere and between celestial bodies. Outer space is not completely empty—it is a near-perfect vacuum containing a low density of particles, predo ...
.
Application
The absence of air makes outer space an ideal location for astronomy at all wavelengths of the electromagnetic spectrum
The electromagnetic spectrum is the range of frequencies (the spectrum) of electromagnetic radiation and their respective wavelengths and photon energies.
The electromagnetic spectrum covers electromagnetic waves with frequencies ranging fro ...
. This is evidenced by the spectacular pictures sent back by the Hubble Space Telescope
The Hubble Space Telescope (often referred to as HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the first space telescope, but it is one of the largest and most vers ...
, 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 zodiacal dust emits a diffuse near-infrared radiation that can mask the emission of faint sources such as extrasolar planets. Moving an infrared telescope
An infrared telescope is a telescope that uses infrared light to detect celestial bodies. Infrared light is one of several types of radiation present in the electromagnetic spectrum.
All celestial objects with a temperature above absolute ze ...
out past the dust increases its effectiveness.[ Likewise, a site like the Daedalus crater on the ]far side of the Moon
The far side of the Moon is the lunar hemisphere that always faces away from Earth, opposite to the near side, because of synchronous rotation in the Moon's orbit. Compared to the near side, the far side's terrain is rugged, with a multitu ...
could shield a radio telescope
A radio telescope is a specialized antenna and radio receiver used to detect radio waves from astronomical radio sources in the sky. Radio telescopes are the main observing instrument used in radio astronomy, which studies the radio frequency ...
from the radio frequency interference
Electromagnetic interference (EMI), also called radio-frequency interference (RFI) when in the radio frequency spectrum, is a disturbance generated by an external source that affects an electrical circuit by electromagnetic induction, electrost ...
that hampers Earth-based observations.[
Uncrewed spacecraft in Earth orbit are an essential technology of modern civilization. They allow direct monitoring of ]weather conditions
Weather is the state of the atmosphere, describing for example the degree to which it is hot or cold, wet or dry, calm or stormy, clear or cloudy. On Earth, most weather phenomena occur in the lowest layer of the planet's atmosphere, the t ...
, relay long-range communications like television, provide a means of precise navigation, and allow remote sensing
Remote sensing is the acquisition of information about an object or phenomenon without making physical contact with the object, in contrast to in situ or on-site observation. The term is applied especially to acquiring information about Ear ...
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 surveillance
Surveillance is the monitoring of behavior, many activities, or information for the purpose of information gathering, influencing, managing or directing. This can include observation from a distance by means of electronic equipment, such as ...
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
Asteroid mining is the hypothetical exploitation of materials from asteroids and other minor planets, including near-Earth objects.
Notable asteroid mining challenges include the high cost of spaceflight, unreliable identification of asteroids ...
, 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
Falcon 9 is a partially reusable medium lift launch vehicle that can carry cargo and crew into Earth orbit, produced by American aerospace company SpaceX.
The rocket has two stages. The first (booster) stage carries the second stage and pay ...
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 system
A reusable launch vehicle have parts that can be recovered and reflown, while carrying payloads from the surface to outer space. Rocket stages are the most common launch vehicle parts aimed for reuse. Smaller parts such as rocket engines and boos ...
s, non-rocket spacelaunch
Non-rocket spacelaunch refers to theoretical concepts for launch into space where much of the speed and altitude needed to achieve orbit is provided by a propulsion technique that is not subject to the limits of the rocket equation. Although al ...
, momentum exchange tethers, and space elevators
A space elevator, also referred to as a space bridge, star ladder, and orbital lift, is a proposed type of planet-to-space transportation system, often depicted in science fiction. The main component would be a cable (also called a tether) anc ...
.
Interstellar travel
Interstellar travel is the hypothetical travel of spacecraft from one star system, solitary star, or planetary system to another. Interstellar travel is expected to prove much more difficult than interplanetary spaceflight due to the vast diffe ...
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 Daedalus Project study, which proposed a spacecraft powered by the fusion
Fusion, or synthesis, is the process of combining two or more distinct entities into a new whole.
Fusion may also refer to:
Science and technology Physics
*Nuclear fusion, multiple atomic nuclei combining to form one or more different atomic nucl ...
of deuterium
Deuterium (or hydrogen-2, symbol or deuterium, also known as heavy hydrogen) is one of two stable isotopes of hydrogen (the other being protium, or hydrogen-1). The nucleus of a deuterium atom, called a deuteron, contains one proton and one ...
and helium-3
Helium-3 (3He see also helion) is a light, stable isotope of helium with two protons and one neutron (the most common isotope, helium-4, having two protons and two neutrons in contrast). Other than protium (ordinary hydrogen), helium-3 is the ...
, would require 36 years to reach the "nearby" Alpha Centauri
Alpha Centauri ( Latinized from α Centauri and often abbreviated Alpha Cen or α Cen) is a triple star system in the constellation of Centaurus. It consists of 3 stars: Alpha Centauri A (officially Rigil Kentaurus), Alpha Centa ...
system. Other proposed interstellar propulsion systems include light sails, ramjets
A ramjet, or athodyd (aero thermodynamic duct), is a form of airbreathing jet engine that uses the forward motion of the engine to produce thrust. Since it produces no thrust when stationary (no ram air) ramjet-powered vehicles require an ass ...
, and beam-powered propulsion
Beam-powered propulsion, also known as directed energy propulsion, is a class of aircraft or spacecraft propulsion that uses energy beamed to the spacecraft from a remote power plant to provide energy. The beam is typically either a microwave or a ...
. More advanced propulsion systems could use antimatter
In modern physics, antimatter is defined as matter composed of the antiparticles (or "partners") of the corresponding particles in "ordinary" matter. Antimatter occurs in natural processes like cosmic ray collisions and some types of radioac ...
as a fuel, potentially reaching relativistic velocities.[
In addition to astronomy and space travel, the ultracold temperature of outer space can be used as a renewable ]cooling
Cooling is removal of heat, usually resulting in a lower temperature and/or phase change. Temperature lowering achieved by any other means may also be called cooling.ASHRAE Terminology, https://www.ashrae.org/technical-resources/free-resources/as ...
technology for various applications on Earth through passive daytime radiative cooling
Passive daytime radiative cooling (PDRC) is a renewable cooling method proposed as a solution to global warming of enhancing terrestrial heat flow to outer space through the installation of thermally-emissive surfaces on Earth that require zer ...
,[ which enhances longwave infrared (LWIR) ]thermal radiation
Thermal radiation is electromagnetic radiation generated by the thermal motion of particles in matter. Thermal radiation is generated when heat from the movement of charges in the material (electrons and protons in common forms of matter) i ...
heat transfer
Heat transfer is a discipline of thermal engineering that concerns the generation, use, conversion, and exchange of thermal energy ( heat) between physical systems. Heat transfer is classified into various mechanisms, such as thermal conducti ...
on the Earth's surface
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surface ...
through the infrared window
The infrared atmospheric window refers to a region of the Infrared spectrum where there is relatively little absorption of terrestrial thermal radiation by atmospheric gases. The window plays an important role in the atmospheric greenhouse effe ...
into outer space to lower ambient temperatures. It became possible with the discovery to suppress solar heating with photonic metamaterials.
See also
* Animals in space
Animals in space originally served to test the survivability of spaceflight, before human spaceflights were attempted. Later, other non-human animals were flown to investigate various biological processes and the effects microgravity and space ...
* Earth's location in the Universe
Knowledge of the location of Earth has been shaped by 400 years of telescopic observations, and has expanded radically since the start of the 20th century. Initially, Earth was believed to be the center of the Universe,
which consisted only of ...
* List of government space agencies
This is a list of government agencies engaged in activities related to outer space and space exploration.
As of 2022, 77 different government space agencies are in existence, 16 of which have launch capabilities. Six government space agencie ...
* List of topics in space
* Olbers' paradox
Olbers's paradox, also known as the dark night sky paradox, is an argument in astrophysics and physical cosmology that says that the darkness of the night sky conflicts with the assumption of an infinite and eternal static universe. In the hy ...
* Outline of space science
The following outline is provided as an overview and topical guide to space science:
Space science encompasses all of the scientific disciplines that involve space exploration and study natural phenomena and physical bodies occurring in outer ...
* Panspermia
Panspermia () is the hypothesis, first proposed in the 5th century BCE by the Greek philosopher Anaxagoras, that life exists throughout the Universe, distributed by space dust, meteoroids, asteroids, comets, and planetoids, as well as by spacecraf ...
* Space and survival
Space and survival is the idea that the long-term survival of the human species and technological civilization requires the building of a spacefaring civilization that utilizes the resources of outer space, and that not doing this will lead to hu ...
* Space environment
Space environment is a branch of astronautics, aerospace engineering and space physics that seeks to understand and address conditions existing in space that affect the design and operation of spacecraft. A related subject, space weather, deals ...
* Space race
The Space Race was a 20th-century competition between two Cold War rivals, the United States and the Soviet Union, to achieve superior spaceflight capability. It had its origins in the ballistic missile-based nuclear arms race between the tw ...
* Space station
A space station is a spacecraft capable of supporting a human crew in orbit for an extended period of time, and is therefore a type of space habitat. It lacks major propulsion or landing systems. An orbital station or an orbital space station i ...
* Space technology
Space technology is technology for use in outer space, in travel (''astronautics'') or other activities beyond Earth's atmosphere, for purposes such as spaceflight, space exploration, and Earth observation. Space technology includes space vehicles ...
* 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 ma ...
* Space weathering
Space weathering is the type of weathering that occurs to any object exposed to the harsh environment of outer space. Bodies without atmospheres (including the Moon, Mercury, the asteroids, comets, and most of the moons of other planets) take on ...
*
* Timeline of Solar System exploration
This is a timeline of Solar System exploration ordered by date of spacecraft launch. It includes:
*All spacecraft that have left Earth orbit for the purposes of Solar System exploration (or were launched with that intention but failed), includ ...
* Timeline of spaceflight
References
Citations
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External links
{{DEFAULTSORT:Outer Space
Environments
Vacuum