Outer space, or simply space, is the expanse that exists beyond the
Earth and between celestial bodies.
Outer space is not completely
empty—it is a hard vacuum containing a low density of particles,
predominantly a plasma of hydrogen and helium, as well as
electromagnetic radiation, magnetic fields, neutrinos, dust, and
cosmic rays. The baseline temperature of outer space, as set by the
background radiation from the Big Bang, is 2.7 kelvins
(−270.45 °C; −454.81 °F). The plasma
between galaxies accounts for about half of the baryonic (ordinary)
matter in the universe; it has a number density of less than one
hydrogen atom per cubic metre and a temperature of millions of
kelvins; local concentrations of this plasma have condensed
into stars and galaxies. Studies indicate that 90% of the mass in most
galaxies is in an unknown form, called dark matter, which interacts
with other matter through gravitational but not electromagnetic
forces. Observations suggest that the majority
of the mass-energy in the observable universe is dark energy, a type
of vacuum energy that is poorly understood.
Intergalactic space takes up most of the volume of the universe, but
even galaxies and star systems consist almost entirely of empty space.
Outer space does not begin at a definite altitude above the Earth's
surface. However, the Kármán line, an altitude of 100 km
(62 mi) above sea level, is conventionally
used as the start of outer space in space treaties and for aerospace
records keeping. The framework for international space law was
established by the Outer Space Treaty, which entered into force on 10
October 1967. This treaty precludes any claims of national sovereignty
and permits all states to freely explore outer space. Despite the
drafting of UN resolutions for the peaceful uses of outer space,
anti-satellite weapons have been tested in
Humans began the physical exploration of space during the 20th century
with the advent of high-altitude balloon flights, followed by manned
Earth orbit was first achieved by
Yuri Gagarin of the
Soviet Union in 1961, and unmanned spacecraft have since reached all
of the known planets in the Solar System. Due to the high cost of
getting into space, manned spaceflight has been limited to low Earth
orbit and the Moon.
Outer space represents a challenging environment for human exploration
because of the hazards of vacuum and radiation.
Microgravity also has
a negative effect on human physiology that causes both muscle atrophy
and bone loss. In addition to these health and environmental issues,
the economic cost of putting objects, including humans, into space is
2 Formation and state
3.1 Effect on human bodies
5 Legal status
7.1.1 Cislunar space
7.2 Interplanetary space
7.3 Interstellar space
7.4 Intergalactic space
8 Exploration and applications
9 See also
11 External links
In 350 BCE, Greek philosopher
Aristotle suggested that nature abhors a
vacuum, a principle that became known as the horror vacui. This
concept built upon a 5th-century BCE ontological argument by the Greek
philosopher Parmenides, who denied the possible existence of a void in
space. Based on this idea that a vacuum could not exist, in
the West it was widely held for many centuries that space could not be
empty. As late as the 17th century, the French philosopher
René Descartes argued that the entirety of space must be
In ancient China, the 2nd-century astronomer
Zhang Heng became
convinced that space must be infinite, extending well beyond the
mechanism that supported the
Sun and the stars. The surviving books of
the Hsüan Yeh school said that the heavens were boundless, "empty and
void of substance". Likewise, the "sun, moon, and the company of stars
float in the empty space, moving or standing still".
The Italian scientist
Galileo Galilei knew that air had mass and so
was subject to gravity. In 1640, he demonstrated that an established
force resisted the formation of a vacuum. However, it would remain for
Evangelista Torricelli to create an apparatus that would
produce a partial vacuum in 1643. This experiment resulted in the
first mercury barometer and created a scientific sensation in Europe.
The French mathematician
Blaise Pascal reasoned that if the column of
mercury was supported by air, then the column ought to be shorter at
higher altitude where the air pressure is lower. In 1648,
his brother-in-law, Florin Périer, repeated the experiment on the Puy
de Dôme mountain in central France and found that the column was
shorter by three inches. This decrease in pressure was further
demonstrated by carrying a half-full balloon up a mountain and
watching it gradually expand, then contract upon descent.
Magdeburg hemispheres (lower left) used to demonstrate
Otto von Guericke's vacuum pump (right)
In 1650, German scientist
Otto von Guericke
Otto von Guericke constructed the first
vacuum pump: a device that would further refute the principle of
horror vacui. He correctly noted that the atmosphere of the Earth
surrounds the planet like a shell, with the density gradually
declining with altitude. He concluded that there must be a vacuum
Earth and the Moon.
Back in the 15th century, German theologian Nicolaus Cusanus
speculated that the
Universe lacked a center and a circumference. He
believed that the Universe, while not infinite, could not be held as
finite as it lacked any bounds within which it could be
contained. These ideas led to speculations as to the
infinite dimension of space by the Italian philosopher Giordano Bruno
in the 16th century. He extended the Copernican heliocentric cosmology
to the concept of an infinite
Universe filled with a substance he
called aether, 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 philosophers,
including Aristotle, who conceived of it as the medium through which
the heavenly bodies move.
The concept of a
Universe filled with a luminiferous aether retained
support among some scientists until the early 20th century. This form
of aether was viewed as the medium through which light could
propagate. In 1887, the Michelson–Morley experiment
tried to detect the Earth's motion through this medium by looking for
changes in the speed of light depending on the direction of the
planet's motion. However, the null result indicated something was
wrong with the concept. The idea of the luminiferous aether was then
abandoned. It was replaced by Albert Einstein's theory of special
relativity, which holds that the speed of light in a vacuum is a fixed
constant, independent of the observer's motion or frame of
The first professional astronomer to support the concept of an
Universe was the Englishman
Thomas Digges in
1576. But the scale of the
Universe remained unknown until
the first successful measurement of the distance to a nearby star in
1838 by the German astronomer Friedrich Bessel. He showed that the
61 Cygni had a parallax of just 0.31 arcseconds (compared to
the modern value of 0.287″). This corresponds to a distance of over
10 light years. In 1917, Heber Curtis noted that novae in
spiral nebulae were, on average, 10 magnitudes fainter than galactic
novae, suggesting that the former are 100 times further
away. The distance to the Andromeda
Galaxy was determined
in 1923 by American astronomer
Edwin Hubble by measuring the
brightness of cepheid variables in that galaxy, a new technique
discovered by Henrietta Leavitt. This established that the
Andromeda galaxy, and by extension all galaxies, lay well outside the
The modern concept of outer space is based on the "Big Bang"
cosmology, first proposed in 1931 by the Belgian physicist Georges
Lemaître. This theory holds that the universe originated
from a very dense form that has since undergone continuous expansion.
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 made a similar calculation to derive a temperature of
3.18 K in 1926. German physicist
Erich Regener used the total
measured energy of cosmic rays to estimate an intergalactic
temperature of 2.8 K in 1933. American physicists
Ralph Alpher and
Robert Herman predicted 5 K for the temperature
of space in 1948, based on the gradual decrease in background energy
following the then-new
Big Bang theory. The modern
measurement of the cosmic microwave background is about 2.7K.
The term outward space was used in 1842 by the English poet Lady
Emmeline Stuart-Wortley in her poem "The Maiden of
Moscow". The expression outer space was used as an
astronomical term by
Alexander von Humboldt
Alexander von Humboldt in 1845. It
was later popularized in the writings of
H. G. Wells
H. G. Wells in
1901. The shorter term space is older, first used to mean
the region beyond Earth's sky in John Milton's
Paradise Lost in
Formation and state
This is an artist's concept of the metric expansion of space, where
a volume of the
Universe is represented at each time interval by the
circular sections. At left is depicted the rapid inflation from the
initial state, followed thereafter by steady expansion to the present
day, shown at right.
Main article: Big Bang
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 stars, galaxies and other
astronomical objects, leaving behind a deep vacuum that forms what is
now called outer space. As light has a finite velocity,
this theory also constrains the size of the directly observable
universe. This leaves open the question as to whether the
Universe is finite or infinite.
The present day shape of the universe has been determined from
measurements of the cosmic microwave background using satellites like
the Wilkinson Microwave Anisotropy Probe. These observations indicate
that the spatial geometry of the observable universe is "flat",
meaning that photons on parallel paths at one point remain parallel as
they travel through space to the limit of the observable universe,
except for local gravity. The flat Universe, combined with
the measured mass density of the
Universe and the accelerating
expansion of the Universe, indicates that space has a non-zero vacuum
energy, which is called dark energy.
Estimates put the average energy density of the present day Universe
at the equivalent of 5.9 protons per cubic meter, including dark
energy, dark matter, and baryonic matter (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, however, 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
holes—to conditions in vast voids 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 smaller
than the influence of gravity from matter and dark matter within the
Part of the
Hubble Ultra-Deep Field
Hubble Ultra-Deep Field image showing a typical section
of space containing galaxies interspersed by deep vacuum. Given the
finite speed of light, this view covers the past 13 billion years
of the history of outer space.
Outer space is the closest known approximation to a perfect vacuum. It
has effectively no friction, allowing stars, planets, and moons to
move freely along their ideal orbits, following the initial formation
stage. However, even the deep vacuum of intergalactic space is not
devoid of matter, as it contains a few hydrogen atoms per cubic
meter. By comparison, the air humans breathe contains
about 1025 molecules per cubic meter. The low
density of matter in outer space means that electromagnetic radiation
can travel great distances without being scattered: the mean free path
of a photon in intergalactic space is about 1023 km, or
10 billion light years. In spite of this, extinction,
which is the absorption and scattering of photons by dust and gas, is
an important factor in galactic and intergalactic
Stars, planets, and moons retain their atmospheres 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 drops to about
0.032 Pa at 100 kilometres (62 miles) of altitude,
compared to 100,000 Pa for the International Union of Pure and Applied
Chemistry (IUPAC) definition of standard pressure. Above this
altitude, isotropic gas pressure rapidly becomes insignificant when
compared to radiation pressure from the
Sun and the dynamic pressure
of the solar wind. The thermosphere in this range has large gradients
of pressure, temperature and composition, and varies greatly due to
The temperature of outer space is measured in terms of the kinetic
activity of the gas, as it is on Earth. However, the radiation of
outer space has a different temperature than the kinetic temperature
of the gas, meaning that the gas and radiation are not in
thermodynamic equilibrium. All of the
observable universe is filled with photons that were created during
the Big Bang, which is known as the cosmic microwave background
radiation (CMB). (There is quite likely a correspondingly large number
of neutrinos called the cosmic neutrino background.) The
current black body temperature of the background radiation is about
3 K (−270 °C; −454 °F). The gas
temperatures in outer space are always at least the temperature of the
CMB but can be much higher. For example, the corona of the
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 dynamos, creating turbulent magnetic field
strengths of around 5–10 μG. 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 processes in active elliptical galaxies produce
their characteristic jets and radio lobes. Non-thermal radio sources
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
particles known as cosmic rays. These particles have energies ranging
from about 106 eV up to an extreme 1020 eV of
ultra-high-energy cosmic rays. The peak flux of cosmic
rays occurs at energies of about 109 eV, with approximately 87%
protons, 12% helium nuclei and 1% heavier nuclei. In the high energy
range, the flux of electrons is only about 1% of that of
Cosmic rays can damage electronic components and
pose a health threat to space travelers. According to
astronauts, like Don Pettit, space has a burned/metallic odor that
clings to their suits and equipment, similar to the scent of an arc
Despite the harsh environment, several life forms have been found that
can withstand extreme space conditions for extended periods. Species
of lichen carried on the ESA
BIOPAN facility survived exposure for ten
days in 2007. Seeds of
Arabidopsis thaliana and Nicotiana
tabacum germinated after being exposed to space for 1.5
years. A strain of bacillus subtilis has survived 559 days
when exposed to low-
Earth orbit or a simulated martian
environment. The lithopanspermia hypothesis suggests that
rocks ejected into outer space from life-harboring planets may
successfully transport life forms to another habitable world. A
conjecture is that just such a scenario occurred early in the history
of the Solar System, with potentially microorganism-bearing rocks
being exchanged between Venus, Earth, and Mars.
Effect on human bodies
Space exposure and Weightlessness
Because of the hazards of a vacuum, astronauts must wear a
pressurized space suit while off-
Earth and outside their spacecraft.
Even at relatively low altitudes in the Earth's atmosphere, conditions
are hostile to the human body. The altitude where atmospheric pressure
matches the vapor pressure of water at the temperature of the human
body is called the Armstrong line, named after American physician
Harry G. Armstrong. It is located at an altitude of around
19.14 km (11.89 mi). 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
Once in space, sudden exposure of unprotected humans to very low
pressure, such as during a rapid decompression, can cause pulmonary
barotrauma—a rupture of the lungs, due to the large pressure
differential between inside and outside the chest. Even if
the subject's airway is fully open, the flow of air through the
windpipe may be too slow to prevent the rupture. Rapid
decompression can rupture eardrums and sinuses, bruising and blood
seep can occur in soft tissues, and shock can cause an increase in
oxygen consumption that leads to hypoxia.
As a consequence of rapid decompression, oxygen dissolved in the blood
empties into the lungs to try to equalize the partial pressure
gradient. Once the deoxygenated blood arrives at the brain, humans
lose consciousness after a few seconds and die of hypoxia within
minutes. Blood and other body fluids boil when the
pressure drops below 6.3 kPa, and this condition is called
ebullism. The steam may bloat the body to twice its normal
size and slow circulation, but tissues are elastic and porous enough
to prevent rupture.
Ebullism is slowed by the pressure containment of
blood vessels, so some blood remains liquid.
Swelling and ebullism can be reduced by containment in a pressure
suit. The Crew Altitude Protection Suit (CAPS), a fitted elastic
garment designed in the 1960s for astronauts, prevents ebullism at
pressures as low as 2 kPa. Supplemental oxygen is needed
at 8 km (5.0 mi) to provide enough oxygen for breathing and
to prevent water loss, while above 20 km (12 mi) pressure
suits are essential to prevent ebullism. Most space suits
use around 30–39 kPa of pure oxygen, about the same as on the
Earth's surface. This pressure is high enough to prevent ebullism, but
evaporation of nitrogen dissolved in the blood could still cause
decompression sickness and gas embolisms if not managed.
Humans evolved for life in
Earth gravity, and exposure to
weightlessness has been shown to have deleterious effects on human
health. Initially, more than 50% of astronauts experience space motion
sickness. This can cause nausea and vomiting, vertigo, headaches,
lethargy, and overall malaise. The duration of space sickness varies,
but it typically lasts for 1–3 days, after which the body adjusts to
the new environment. Longer-term exposure to weightlessness results in
muscle atrophy and deterioration of the skeleton, or spaceflight
osteopenia. These effects can be minimized through a regimen of
exercise. Other effects include fluid redistribution,
slowing of the cardiovascular system, decreased production of red
blood cells, balance disorders, and a weakening of the immune system.
Lesser symptoms include loss of body mass, nasal congestion, sleep
disturbance, and puffiness of the face.
For long-duration space travel, radiation can pose an acute health
Exposure to high-energy, ionizing cosmic rays can result in fatigue,
nausea, vomiting, as well as damage to the immune system and changes
to the white blood cell count. Over longer durations, symptoms include
an increased risk of cancer, plus damage to the eyes, nervous system,
lungs and the gastrointestinal tract. On a round-trip Mars
mission lasting three years, a large fraction of the cells in an
astronaut's body would be traversed and potentially damaged by high
energy nuclei. The energy of such particles is
significantly diminished by the shielding provided by the walls of a
spacecraft and can be further diminished by water containers and other
barriers. However, 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
SpaceShipOne completed the first manned private spaceflight in 2004,
reaching an altitude of 100.12 km (62.21 mi).
There is no clear boundary between
Earth's atmosphere and space, as
the density of the atmosphere gradually decreases as the altitude
increases. There are several standard boundary designations, namely:
Fédération Aéronautique Internationale
Fédération Aéronautique Internationale has established the
Kármán line at an altitude of 100 km (62 mi) as a working
definition for the boundary between aeronautics and astronautics. This
is used because at an altitude of about 100 km (62 mi), as
Theodore von Kármán
Theodore von Kármán calculated, a vehicle would have to travel
faster than orbital velocity to derive sufficient aerodynamic lift
from the atmosphere to support itself.
The United States designates people who travel above an altitude of 50
miles (80 km) as astronauts.
Space Shuttle used 400,000 feet (76 mi, 122 km) as
its re-entry altitude (termed the Entry Interface), which roughly
marks the boundary where atmospheric drag becomes noticeable, thus
beginning the process of switching from steering with thrusters to
maneuvering with aerodynamic control surfaces.
In 2009, scientists reported detailed measurements with a
Ion Imager (an instrument that measures the direction
and speed of ions), which allowed them to establish a boundary at
118 km (73 mi) 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
268 m/s (600 mph).
Main article: Space law
2008 launch of the SM-3 missile used to destroy American
reconnaissance satellite USA-193
Outer Space Treaty
Outer Space Treaty provides the basic framework for international
space law. It covers the legal use of outer space by nation states,
and includes in its definition of outer space the
Moon and other
celestial bodies. The treaty states that outer space is free for all
nation states to explore and is not subject to claims of national
sovereignty. It also prohibits the deployment of nuclear weapons in
outer space. The treaty was passed by the United Nations General
Assembly in 1963 and signed in 1967 by the USSR, the United States of
America and the United Kingdom. As of 2017, 105 state parties have
either ratified or acceded to the treaty. An additional 25 states
signed the treaty, without ratifying it.
Since 1958, outer space has been the subject of multiple United
Nations resolutions. Of these, more than 50 have been concerning the
international co-operation in the peaceful uses of outer space and
preventing an arms race in space. Four additional space
law treaties have been negotiated and drafted by the UN's Committee on
the Peaceful Uses of Outer Space. Still, there remains no legal
prohibition against deploying conventional weapons in space, and
anti-satellite weapons 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.
However, this treaty has not been ratified by any nation that
currently practices manned spaceflight.
In 1976, eight equatorial states (Ecuador, Colombia, Brazil, Congo,
Zaire, Uganda, Kenya, and Indonesia) met in Bogotá, Colombia. With
their "Declaration of the First Meeting of Equatorial Countries", or
Bogotá Declaration", they claimed control of the segment of the
geosynchronous orbital path corresponding to each country.
These claims are not internationally accepted.
Main article: Geocentric orbit
A spacecraft enters orbit when its centripetal acceleration due to
gravity is less than or equal to the centrifugal acceleration due to
the horizontal component of its velocity. For a low
Earth orbit, this
velocity is about 7,800 m/s (28,100 km/h;
17,400 mph); by contrast, the fastest manned airplane
speed ever achieved (excluding speeds achieved by deorbiting
spacecraft) was 2,200 m/s (7,900 km/h; 4,900 mph) in
1967 by the North American X-15.
To achieve an orbit, a spacecraft must travel faster than a
sub-orbital spaceflight. The energy required to reach
velocity at an altitude of 600 km (370 mi) is about
36 MJ/kg, which is six times the energy needed merely to climb to
the corresponding altitude.
Spacecraft with a perigee
below about 2,000 km (1,200 mi) 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 300 km (190 mi), decay becomes more
rapid with lifetimes measured in days. Once a satellite descends to
180 km (110 mi), it has only hours before it vaporizes in
the atmosphere. The escape velocity required to pull free
of Earth's gravitational field altogether and move into interplanetary
space is about 11,200 m/s (40,300 km/h;
Space is a partial vacuum: its different regions are defined by the
various atmospheres and "winds" that dominate within them, and extend
to the point at which those winds give way to those beyond. Geospace
Earth's atmosphere to the outer reaches of Earth's
magnetic field, whereupon it gives way to the solar wind of
interplanetary space. Interplanetary space extends to the
heliopause, whereupon the solar wind gives way to the winds of the
Interstellar space then continues to
the edges of the galaxy, where it fades into the intergalactic
Aurora australis observed from the
Space Shuttle Discovery, on
STS-39, May 1991 (orbital altitude: 260 km)
Geospace is the region of outer space near Earth, including the upper
atmosphere and magnetosphere. The Van Allen radiation
belts lie within the geospace. The outer boundary of geospace is the
magnetopause, which forms an interface between the Earth's
magnetosphere and the solar wind. The inner boundary is the
ionosphere. The variable space-weather conditions of
geospace are affected by the behavior of the
Sun and the solar wind;
the subject of geospace is interlinked with heliophysics—the study
Sun and its impact on the planets of the Solar
The day-side magnetopause is compressed by solar-wind pressure—the
subsolar distance from the center of the
Earth is typically 10 Earth
radii. On the night side, the solar wind stretches the magnetosphere
to form a magnetotail that sometimes extends out to more than
Earth radii. For roughly four days
of each month, the lunar surface is shielded from the solar wind as
Moon passes through the magnetotail.
Geospace is populated by electrically charged particles at very low
densities, the motions of which are controlled by the Earth's magnetic
field. These plasmas form a medium from which storm-like disturbances
powered by the solar wind can drive electrical currents into the
Earth's upper atmosphere. Geomagnetic storms can disturb two regions
of geospace, the radiation belts and the ionosphere. These storms
increase fluxes of energetic electrons that can permanently damage
satellite electronics, interfering with shortwave radio communication
and GPS location and timing. Magnetic storms can also be
a hazard to astronauts, even in low
Earth orbit. They also create
aurorae seen at high latitudes in an oval surrounding the geomagnetic
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
satellites. This region contains material left over from
previous manned and unmanned launches that are a potential hazard to
spacecraft. Some of this debris re-enters Earth's atmosphere
Lunar Orbital Station, one of the proposed space stations for manned
cislunar travel in the 2030s
Earth's gravity keeps the
Moon in orbit at an average distance of
384,403 km (238,857 mi). The region outside Earth's
atmosphere and extending out to just beyond the Moon's orbit,
including the Lagrangian points, is sometimes referred to as cislunar
The region of space where Earth's gravity remains dominant against
gravitational perturbations from the
Sun is called the Hill
sphere. This extends well out into translunar space to a
distance of roughly 1% of the mean distance from
Earth to the
Sun, or 1.5 million km
(0.93 million mi).
Deep space has different definitions as to where it starts. It has
been defined by the United States government and others as any region
International Telecommunication Union responsible for radio
communication (including satellites) defines the beginning of deep
space at about 5 times that distance (2×106 km).
Main article: Interplanetary medium
The sparse plasma (blue) and dust (white) in the tail of comet
Hale–Bopp are being shaped by pressure from solar radiation and the
solar wind, respectively
Interplanetary space is defined by the solar wind, a continuous stream
of charged particles emanating from the
Sun that creates a very
tenuous atmosphere (the heliosphere) for billions of kilometers into
space. This wind has a particle density of 5–10 protons/cm3 and is
moving at a velocity of 350–400 km/s
(780,000–890,000 mph). 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
The volume of interplanetary space is a nearly total vacuum, with a
mean free path of about one astronomical unit at the orbital distance
of the Earth. However, this space is not completely empty, and is
sparsely filled with cosmic rays, which include ionized atomic nuclei
and various subatomic particles. There is also gas, plasma and
dust, small meteors, and several dozen types of organic
molecules discovered to date by microwave spectroscopy. A
cloud of interplanetary dust is visible at night as a faint band
called the zodiacal light.
Interplanetary space contains the magnetic field generated by the
Sun. There are also magnetospheres generated by planets
such as Jupiter, Saturn, Mercury and the
Earth that have their own
magnetic fields. These are shaped by the influence of the solar wind
into the approximation of a teardrop shape, with the long tail
extending outward behind the planet. These magnetic fields can trap
particles from the solar wind and other sources, creating belts of
charged particles such as the Van Allen radiation belts. Planets
without magnetic fields, such as Mars, have their atmospheres
gradually eroded by the solar wind.
Main article: Interstellar medium
"Interstellar space" redirects here. For the album, see Interstellar
Bow shock formed by the magnetosphere of the young star LL Orionis
(center) as it collides with the
Orion Nebula flow
Interstellar space is the physical space within a galaxy beyond the
influence each star has upon the encompassed plasma. The
contents of interstellar space are called the interstellar medium.
Approximately 70% of the mass of the interstellar medium consists of
lone hydrogen atoms; most of the remainder consists of helium atoms.
This is enriched with trace amounts of heavier atoms formed through
stellar nucleosynthesis. These atoms are ejected into the interstellar
medium by stellar winds or when evolved stars begin to shed their
outer envelopes such as during the formation of a planetary
nebula. The cataclysmic explosion of a supernova
generates an expanding shock wave consisting of ejected materials that
further enrich the medium. The density of matter in the
interstellar medium can vary considerably: the average is around 106
particles per m3, but cold molecular clouds can hold
108–1012 per m3.
A number of molecules exist in interstellar space, as can tiny
0.1 μm dust particles. The tally of molecules
discovered through radio astronomy is steadily increasing at the rate
of about four new species per year. Large regions of higher density
matter known as molecular clouds allow chemical reactions to occur,
including the formation of organic polyatomic species. Much of this
chemistry is driven by collisions. Energetic cosmic rays penetrate the
cold, dense clouds and ionize hydrogen and helium, resulting, for
example, in the trihydrogen cation. An ionized helium atom can then
split relatively abundant carbon monoxide to produce ionized carbon,
which in turn can lead to organic chemical reactions.
The local interstellar medium is a region of space within
100 parsecs (pc) of the Sun, which is of interest both for its
proximity and for its interaction with the Solar System. This volume
nearly coincides with a region of space known as the Local Bubble,
which is characterized by a lack of dense, cold clouds. It forms a
cavity in the
Orion Arm of the
Milky Way galaxy, with dense molecular
clouds lying along the borders, such as those in the constellations of
Ophiuchus and Taurus. (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
Local Bubble contains dozens of warm interstellar clouds
with temperatures of up to 7,000 K and radii of
When stars are moving at sufficiently high peculiar velocities, their
astrospheres can generate bow shocks as they collide with the
interstellar medium. For decades it was assumed that the
Sun had a bow
shock. In 2012, data from
Interstellar Boundary Explorer
Interstellar Boundary Explorer (IBEX) and
NASA's Voyager 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 and the astropause (called
the heliopause in the Solar System).
A star-forming region in the Large Magellanic Cloud, perhaps the
Galaxy to Earth's Milky Way
Main articles: Warm–hot intergalactic medium, Intracluster medium,
and Intergalactic dust
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 (10–40) h−1 Mpc, where h is the Hubble
constant in units of 100 km s−1 Mpc−1.
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 (ICM).
Exploration and applications
Main articles: Space exploration, Space colonization, and Space
The first image taken of the entire
Earth by astronauts was shot
Apollo 8 mission
For the majority of human history, space was explored by observations
made from the Earth's surface—initially with the unaided eye and
then with the telescope. Prior to the advent of reliable rocket
technology, the closest that humans had come to reaching outer space
was through the use of balloon flights. In 1935, the U.S. Explorer II
manned balloon flight reached an altitude of 22 km
(14 mi). This was greatly exceeded in 1942 when the
third launch of the German A-4 rocket climbed to an altitude of about
80 km (50 mi). In 1957, the unmanned satellite
Sputnik 1 was
launched by a Russian R-7 rocket, achieving
Earth orbit at an altitude
of 215–939 kilometres (134–583 mi). This was
followed by the first human spaceflight in 1961, when
Yuri Gagarin was
sent into orbit on Vostok 1. The first humans to escape low-Earth
orbit were Frank Borman,
Jim Lovell and
William Anders in 1968 on
board the U.S. Apollo 8, which achieved lunar orbit and
reached a maximum distance of 377,349 km (234,474 mi) from
The first spacecraft to reach escape velocity was the Soviet Luna 1,
which performed a fly-by of the
Moon in 1959. In 1961,
Venera 1 became the first planetary probe. It revealed the presence of
the solar wind and performed the first fly-by of Venus, although
contact was lost before reaching Venus. The first successful planetary
mission was the 1962 fly-by of
Venus by Mariner 2. The
first fly-by of
Mars was by
Mariner 4 in 1964. Since that time,
unmanned spacecraft have successfully examined each of the Solar
System's planets, as well their moons and many minor planets and
comets. They remain a fundamental tool for the exploration of outer
space, as well as observation of the Earth. In August
Voyager 1 became the first man-made object to leave the Solar
System and enter interstellar space.
The absence of air makes outer space an ideal location for astronomy
at all wavelengths of the electromagnetic spectrum. This is evidenced
by the spectacular pictures sent back by the Hubble Space Telescope,
allowing light from more than 13 billion years ago—almost to
the time of the Big Bang—to be observed. However, 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 out past the dust increases its
effectiveness. Likewise, a site like the Daedalus crater
on the far side of the
Moon could shield a radio telescope from the
radio frequency interference that hampers Earth-based
Unmanned spacecraft in
Earth orbit are an essential technology of
modern civilization. They allow direct monitoring of weather
conditions, relay long-range communications like television, provide a
means of precise navigation, and allow remote sensing of the Earth.
The latter role serves a wide variety of purposes, including tracking
soil moisture for agriculture, prediction of water outflow from
seasonal snow packs, detection of diseases in plants and trees, and
surveillance of military activities.
The deep vacuum of space could make it an attractive environment for
certain industrial processes, such as those requiring ultraclean
surfaces. However, like asteroid mining, space
manufacturing requires significant investment with little prospect of
immediate return. An important factor in the total
expense is the high cost of placing mass into
$7,000–25,000 per kg in inflation-adjusted dollars, according to a
2006 estimate. Proposed concepts for addressing this
issue include non-rocket spacelaunch, momentum exchange tethers, and
Interstellar travel for a human crew remains at present only a
theoretical possibility. The distances to the nearest stars will
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
Deuterium and He3, would require 36 years to reach the
Alpha Centauri system. Other proposed interstellar propulsion
systems include light sails, ramjets, and beam-powered propulsion.
More advanced propulsion systems could use antimatter as a fuel,
potentially reaching relativistic velocities.
Outer space portal
Earth's location in the universe
List of government space agencies
List of topics in space
Outline of space science
Space and survival
Timeline of knowledge about the interstellar and intergalactic medium
Solar System exploration
Timeline of spaceflight
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Helium hydride ion
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