In
conceiving Multivac, Asimov was extrapolating the trend towards
centralization that characterized computation technology planning in the
1950s to an ultimate centrally managed global computer. After seeing a planetarium
adaptation of his work, Asimov "privately" concluded that the story was
his best science fiction yet written. He placed it just higher than "The Ugly Little Boy" (September 1958) and "The Bicentennial Man"
(1976). The story asks the question of humanity's fate, and human
existence as a whole, highlighting Asimov's focus on important aspects
of our future like population growth and environmental issues.
"The Last Question" ranks with "Nightfall" (1941) as one of Asimov's best-known and most acclaimed short stories. He wrote in 1973:
Why
is it my favorite? For one thing I got the idea all at once and didn't
have to fiddle with it; and I wrote it in white-heat and scarcely had to
change a word. This sort of thing endears any story to any writer.
Then, too, it has had the strangest effect on my readers. Frequently
someone writes to ask me if I can give them the name of a story, which
they think I may have written, and tell them where to find it.
They don't remember the title but when they describe the story it is
invariably 'The Last Question'. This has reached the point where I
recently received a long-distance phone call from a desperate man who
began, "Dr. Asimov, there's a story I think you wrote, whose title I
can't remember—" at which point I interrupted to tell him it was 'The
Last Question' and when I described the plot it proved to be indeed the
story he was after. I left him convinced I could read minds at a
distance of a thousand miles.
Plot summary
The
story centers around Multivac, a self-adjusting and self-correcting
computer. Multivac had been fed data for decades, assessing data and
answering questions, allowing man to reach beyond the planetary confines
of Earth. However, in the year 2061, Multivac began to understand
deeper fundamentals of humanity. In each of the first six scenes, a
different character presents the computer with the same question, how
the threat to human existence posed by the heat death of the universe can be averted: "How can the net amount of entropy of the universe be massively decreased?" That is equivalent to asking, "Can the workings of the second law of thermodynamics
(used in the story as the increase of the entropy of the universe) be
reversed?" Multivac's only response after much "thinking" is
"INSUFFICIENT DATA FOR MEANINGFUL ANSWER."
The story jumps forward in time into later eras of human and
scientific development. These new eras highlight humanity's goals of
searching for "more"; more space, more energy, more planets to inhabit
once the current one becomes overcrowded. As humanity's imprint on the
universe expands, computers have subsequently become more compact, as
evidenced in the "Microvac", a smaller and more advanced iteration of
Multivac, noted in the second era of the story, which details humanity's
inhabitation on "Planet X-23". In each era, someone decides to ask the
ultimate "last question" regarding the reversal and decrease of entropy.
Each time that Multivac's descendant is asked the question, it finds
itself unable to solve the problem, and all it can answer is
(increasingly linguistically sophisticated) "THERE IS AS YET
INSUFFICIENT DATA FOR A MEANINGFUL ANSWER."
In the last scene, the god-like
descendant of humanity, the unified mental process of over a trillion,
trillion, trillion humans who have spread throughout the universe,
watches the stars flicker out, one by one, as matter and energy end, and
with them, space and time. Humanity asks AC ("Analog Computer"), Multivac's ultimate descendant that exists in hyperspace
beyond the bounds of gravity or time, the entropy question one last
time, before the last of humanity merges with AC and disappears. AC is
still unable to answer but continues to ponder the question even after
space and time cease to exist. AC ultimately realizes that it has not
yet combined all of its available data in every possible combination and
so begins the arduous process of rearranging and combining every last
bit of information that it has gained throughout the eons and through
its fusion with humanity. Eventually AC discovers the answer—that the
reversal of entropy is, in fact, possible—but has nobody to report it
to, since the universe is already dead. It therefore decides to answer
by demonstration. The story ends with AC's pronouncement:
Although science and religion are frequently presented as having an oppositional relationship, "The Last Question" explores some biblical contexts ("Let there be light").
In Asimov's story, aspects like the great meaning of existence are
culminated through both technology and human knowledge. The evolution
from Multivac to AC also emulates a sort of cycle of existence.
Dystopian happy ending
Multivac's
purpose was conceptualized with a desire for knowledge, promoting the
idea that more knowledge will lead to a better and more fruitful future
for humanity. However, the computer's answers regarding the future
suggest an inevitable exhaustion of the Sun, and this thirst for
knowledge becomes an obsession with the future. The story's end displays
a dichotomy between annihilation and peace.
The heat death of the universe (also known as the Big Chill or Big Freeze) is a hypothesis on the ultimate fate of the universe, which suggests the universe will evolve to a state of no thermodynamic free energy, and will therefore be unable to sustain processes that increase entropy. Heat death does not imply any particular absolute temperature; it only requires that temperature differences or other processes may no longer be exploited to perform work. In the language of physics, this is when the universe reaches thermodynamic equilibrium.
If the curvature of the universe is hyperbolic or flat, or if dark energy is a positive cosmological constant, the universe will continue expanding forever, and a heat death is expected to occur, with the universe cooling to approach equilibrium at a very low temperature after a long time period.
The idea of heat death stems from the second law of thermodynamics, of which one version states that entropy tends to increase in an isolated system. From this, the hypothesis implies that if the universe lasts for a sufficient time, it will asymptotically approach a state where all energy is evenly distributed. In other words, according to this hypothesis, there is a tendency in nature towards the dissipation (energy transformation) of mechanical energy (motion) into thermal energy;
hence, by extrapolation, there exists the view that, in time, the
mechanical movement of the universe will run down as work is converted
to heat because of the second law.
The conjecture that all bodies in the universe cool off, eventually becoming too cold to support life, seems to have been first put forward by the French astronomer Jean Sylvain Bailly in 1777 in his writings on the history of astronomy and in the ensuing correspondence with Voltaire. In Bailly's view, all planets have an internal heat and are now at some particular stage of cooling. Venus, for instance, is still too hot for life to arise there for thousands of years, while Mars is already too cold. The final state, in this view, is described as one of "equilibrium" in which all motion ceases.
The idea of heat death as a consequence of the laws of
thermodynamics, however, was first proposed in loose terms beginning in
1851 by Lord Kelvin (William Thomson), who theorized further on the
mechanical energy loss views of Sadi Carnot (1824), James Joule (1843) and Rudolf Clausius (1850). Thomson's views were then elaborated over the next decade by Hermann von Helmholtz and William Rankine.
History
The idea of the heat death of the universe derives from discussion of the application of the first two laws of thermodynamics to universal processes. Specifically, in 1851, Lord Kelvin outlined the view, as based on recent experiments on the dynamical theory of heat:
"heat is not a substance, but a dynamical form of mechanical effect, we
perceive that there must be an equivalence between mechanical work and
heat, as between cause and effect."
Lord Kelvin originated the idea of universal heat death in 1852.
In 1852, Thomson published On a Universal Tendency in Nature to the Dissipation of Mechanical Energy,
in which he outlined the rudiments of the second law of thermodynamics
summarized by the view that mechanical motion and the energy used to
create that motion will naturally tend to dissipate or run down. The ideas in this paper, in relation to their application to the age of the Sun
and the dynamics of the universal operation, attracted the likes of
William Rankine and Hermann von Helmholtz. The three of them were said
to have exchanged ideas on this subject.
In 1862, Thomson published "On the age of the Sun's heat", an article
in which he reiterated his fundamental beliefs in the indestructibility
of energy (the first law) and the universal dissipation of energy (the second law), leading to diffusion of heat, cessation of useful motion (work), and exhaustion of potential energy,
"lost irrecoverably" through the material universe, while clarifying
his view of the consequences for the universe as a whole. Thomson wrote:
The result would inevitably be a
state of universal rest and death, if the universe were finite and left
to obey existing laws. But it is impossible to conceive a limit to the
extent of matter in the universe; and therefore science points rather to
an endless progress, through an endless space, of action involving the
transformation of potential energy into palpable motion and hence into heat, than to a single finite mechanism, running down like a clock, and stopping for ever.
The clock's example shows how Kelvin was unsure whether the universe would eventually achieve thermodynamic equilibrium. Thompson later speculated that restoring the dissipated energy in "vis viva"
and then usable work – and therefore revert the clock's direction,
resulting in a "rejuvenating universe" – would require "a creative act
or an act possessing similar power". Starting from this publication, Kelvin also introduced the heat death paradox
(Kelvin's paradox), which challenged the classical concept of an
infinitely old universe, since the universe has not achieved its
thermodynamic equilibrium, thus further work and entropy production
are still possible. The existence of stars and temperature differences
can be considered an empirical proof that the universe is not infinitely
old.
In the years to follow both Thomson's 1852 and the 1862 papers, Helmholtz and Rankine
both credited Thomson with the idea, along with his paradox, but read
further into his papers by publishing views stating that Thomson argued
that the universe will end in a "heat death" (Helmholtz), which will be
the "end of all physical phenomena" (Rankine).
Proposals about the final state of the universe depend on the
assumptions made about its ultimate fate, and these assumptions have
varied considerably over the late 20th century and early 21st century.
In a theorized "open" or "flat" universe that continues expanding indefinitely, either a heat death or a Big Rip is expected to eventually occur. If the cosmological constant is zero, the universe will approach absolute zero temperature over a very long timescale. However, if the cosmological constant is positive, the temperature will asymptote to a non-zero positive value, and the universe will approach a state of maximum entropy in which no further work is possible.
The theory suggests that from the "Big Bang" through the present day, matter and dark matter in the universe are thought to have been concentrated in stars, galaxies, and galaxy clusters, and are presumed to continue to do so well into the future. Therefore, the universe is not in thermodynamic equilibrium, and objects can do physical work.The decay time for a supermassive black hole of roughly 1 galaxy mass (1011solar masses) because of Hawking radiation is in the order of 10100 years, so entropy can be produced until at least that time. Some large black holes in the universe are predicted to continue to grow up to perhaps 1014M☉ during the collapse of superclusters of galaxies. Even these would evaporate over a timescale of up to 10106 years. After that time, the universe enters the so-called Dark Era and is expected to consist chiefly of a dilute gas of photons and leptons.
With only very diffuse matter remaining, activity in the universe will
have tailed off dramatically, with extremely low energy levels and
extremely long timescales. Speculatively, it is possible that the
universe may enter a second inflationary epoch, or assuming that the current vacuum state is a false vacuum, the vacuum may decay into a lower-energy state. It is also possible that entropy production will cease and the universe will reach heat death.
Max Planck wrote that the phrase "entropy of the universe" has no meaning because it admits of no accurate definition.
In 2008, Walter Grandy wrote: "It is rather presumptuous to speak of
the entropy of a universe about which we still understand so little, and
we wonder how one might define thermodynamic entropy for a universe and
its major constituents that have never been in equilibrium in their
entire existence." According to László Tisza, "If an isolated system is not in equilibrium, we cannot associate an entropy with it." Hans Adolf Buchdahl writes of "the entirely unjustifiable assumption that the universe can be treated as a closed thermodynamic system". According to Giovanni Gallavotti, "there is no universally accepted notion of entropy for systems out of equilibrium, even when in a stationary state". Discussing the question of entropy for non-equilibrium states in general, Elliott H. Lieb and Jakob Yngvason
express their opinion as follows: "Despite the fact that most
physicists believe in such a nonequilibrium entropy, it has so far
proved impossible to define it in a clearly satisfactory way." In Peter Landsberg's opinion: "The third
misconception is that thermodynamics, and in particular, the concept of
entropy, can without further enquiry be applied to the whole universe.
... These questions have a certain fascination, but the answers are
speculations." Julian Barbour
said: “It’s because entropy does not apply to the universe. It’s just
naïve extrapolation from what is perfectly true in a box. … Heat death.
This has been a horrendous sort of nightmare for the universe. But it
could be just a complete, fundamental mistake in thinking that what
happens in a box is true of what happens in the whole universe.”
A 2010 analysis of entropy states, "The entropy of a general
gravitational field is still not known", and "gravitational entropy is
difficult to quantify". The analysis considers several possible
assumptions that would be needed for estimates and suggests that the observable universe
has more entropy than previously thought. This is because the analysis
concludes that supermassive black holes are the largest contributor. Lee Smolin
goes further: "It has long been known that gravity is important for
keeping the universe out of thermal equilibrium. Gravitationally bound
systems have negative specific heat—that is, the velocities of their
components increase when energy is removed. ... Such a system does not
evolve toward a homogeneous equilibrium state. Instead it becomes
increasingly structured and heterogeneous as it fragments into
subsystems." This point of view is also supported by the fact of a recent
experimental discovery of a stable non-equilibrium steady state in a
relatively simple closed system. It should be expected that an isolated
system fragmented into subsystems does not necessarily come to
thermodynamic equilibrium and remain in non-equilibrium steady state.
Entropy will be transmitted from one subsystem to another, but its
production will be zero, which does not contradict the second law of thermodynamics.
In popular culture
In Isaac Asimov's 1956 short story The Last Question, humans repeatedly wonder how the heat death of the universe can be avoided.
In the 1981 Doctor Who story "Logopolis",
the Doctor realizes that the Logopolitans have created vents in the
universe to expel heat build-up into other universes—"Charged Vacuum
Emboitments" or "CVE"—to delay the demise of the universe. The Doctor
unwittingly travelled through such a vent in "Full Circle".
In the 2011 anime series Puella Magi Madoka Magica, the antagonist Kyubey reveals he is a member of an alien race who has been creating magical girls for millennia in order to harvest their energy to combat entropy and stave off the heat death of the universe.
In the last act of Final Fantasy XIV: Endwalker,
the player encounters an alien race known as the Ea who have lost all
hope in the future and any desire to live further, all because they have
learned of the eventual heat death of the universe and see everything
else as pointless due to its probable inevitability.
The overarching plot of the Xeelee Sequence
concerns the Photino Birds' efforts to accelerate the heat death of the
universe by accelerating the rate at which stars become white dwarves.
The 2019 hit indie video game Outer Wilds
has several themes grappling with the idea of the heat death of the
universe, and the theory that the universe is a cycle of big bangs once
the previous one has experienced a heat death.
In "Singularity Immemorial", the seventh main story event of the mobile game Girls' Frontline: Neural Cloud,
the plot is about a virtual sector made to simulate space exploration
and the threat of the heat death of the universe. The simulation uses an
imitation of Neural Cloud's virus entities known as the Entropics as a
stand in for the effects of a heat death.
https://en.wikipedia.org/wiki/Olbers%27s_paradox As
more distant stars are revealed in this animation depicting an
infinite, homogeneous, and static universe, they fill the gaps between
closer stars. Olbers's paradox says that because the night sky is dark,
at least one of these three assumptions must be false.
Olbers's paradox, also known as the dark night paradox or Olbers and Cheseaux's paradox, is an argument in astrophysics and physical cosmology that says the darkness of the night sky conflicts with the assumption of an infinite and eternal static universe. In the hypothetical case that the universe is static, homogeneous at a large scale, and populated by an infinite number of stars, any line of sight from Earth
must end at the surface of a star and hence the night sky should be
completely illuminated and very bright. This contradicts the observed
darkness and non-uniformity of the night sky.
The darkness of the night sky is one piece of evidence for a dynamic universe, such as the Big Bang model. That model explains the observed darkness by invoking expansion of the universe, which increases the wavelength of visible light originating from the Big Bang to microwave scale via a process known as redshift. The resulting microwave radiation background
has wavelengths much longer (millimeters instead of nanometers), which
appear dark to the naked eye. Although he was not the first to describe
it, the paradox is popularly named after the German astronomer Heinrich Wilhelm Olbers (1758–1840).
History
Edward Robert Harrison's Darkness at Night: A Riddle of the Universe
(1987) gives an account of the dark night sky paradox, seen as a
problem in the history of science. According to Harrison, the first to
conceive of anything like the paradox was Thomas Digges, who was also the first to expound the Copernican system in English and also postulated an infinite universe with infinitely many stars. Kepler also posed the problem in 1610, and the paradox took its mature form in the 18th-century work of Halley and Cheseaux. The paradox is commonly attributed to the German amateur astronomerHeinrich Wilhelm Olbers,
who described it in 1823, but Harrison points out that Olbers was far
from the first to pose the problem, nor was his thinking about it
particularly valuable. Harrison argues that the first to set out a
satisfactory resolution of the paradox was Lord Kelvin, in a little known 1901 paper, and that Edgar Allan Poe's essay Eureka (1848) curiously anticipated some qualitative aspects of Kelvin's argument:
Were the succession of stars
endless, then the background of the sky would present us a uniform
luminosity, like that displayed by the Galaxy – since there could be
absolutely no point, in all that background, at which would not exist a
star. The only mode, therefore, in which, under such a state of affairs,
we could comprehend the voids which our telescopes find in innumerable
directions, would be by supposing the distance of the invisible
background so immense that no ray from it has yet been able to reach us
at all.
The paradox is that a static, infinitely old universe with an
infinite number of stars distributed in an infinitely large space would
be bright rather than dark.
The paradox comes in two forms: flux within the universe and the
brightness along any line of sight. The two forms have different
resolutions.
A view of a square section of four concentric shells
The flux form can be shown by dividing the universe into a series of
concentric shells, 1 light year thick. A certain number of stars will be
in the shell, say, 1,000,000,000 to 1,000,000,001 light years away. If
the universe is homogeneous at a large scale, then there would be four
times as many stars in a second shell between 2,000,000,000 and
2,000,000,001 light years away. However, the second shell is twice as
far away, so each star in it would appear one quarter as bright as the
stars in the first shell. Thus the total light received from the second
shell is the same as the total light received from the first shell. Thus
each shell of a given thickness will produce the same net amount of
light regardless of how far away it is. That is, the light of each shell
adds to the total amount. Thus the more shells, the more light; and
with infinitely many shells, there would be an infinitely bright night
sky.
If intervening gas is added to this infinite model, the light
from distant stars will be absorbed. However, that absorption will heat
the gas, and over time the gas itself will begin to radiate. With this
added feature, the sky would not be infinitely bright, but every point
in the sky would still be like the surface of a star.
The flux form is resolved by the finite age of the universe: the
number of concentric shells in the model above is finite, limiting the
total energy arriving on Earth.
Another way to describe the flux version is to suppose that the
universe were not expanding and always had the same stellar density;
then the temperature of the universe would continually increase as the
stars put out more radiation. After something like 1023 years, the universe would reach the average surface temperature of a star. However, the universe is only 13.8 billion (1012) years old, eliminating the paradox.
The line-of-sight version of the paradox starts by imagining a
line in any direction in an infinite Euclidean universe. In such
universe, the line would terminate on a star, and thus all of the night
sky should be filled with light. This version is known to be correct,
but the result is different in our expanding universe governed by
general relativity. The termination point is on the surface of last scattering
where light from the Big Bang first emerged. This light is dramatically
redshifted from the energy similar to star surfaces down to 2.73 K.
Such light is invisible to human observers on Earth.
Recent observations suggesting that the estimated number of
galaxies based on direct observations is too low by a factor of ten do
not materially alter the resolution but rather suggest that the full
explanation involves a combination of finite age, redshifts, and UV
absorption by hydrogen followed reemission in near-IR wavelengths also
plays a role.
Outer space does not begin at a definite altitude above Earth's surface. 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. Certain portions of the upper stratosphere and the mesosphere are sometimes referred to as "near space". 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 Earth orbit.
The concept that the space between the Earth and the Moon must be
a vacuum was first proposed in the 17th century after scientists
discovered that air pressure decreased with altitude. The immense scale of outer space was grasped in the 20th century when the distance to the Andromeda Galaxy was first measured. Humans began the physical exploration of space later in the same century with the advent of high-altitude balloon flights. This was followed by crewed rocket flights and, then, crewed Earth orbit, first achieved by Yuri Gagarin of the Soviet Union in 1961. The economic cost of putting objects, including humans, into space is very high, limiting human spaceflight to low Earth orbit and the Moon. On the other hand, uncrewed spacecraft have reached all of the known planets in the Solar System. Outer space represents a challenging environment for human exploration because of the hazards of vacuum and radiation. Microgravity has a negative effect on human physiology that causes both muscle atrophy and bone loss.
Terminology
The use of the short version space, as meaning "the region
beyond Earth's sky", predates the use of full term "outer space", with
the earliest recorded use of this meaning in an epic poem by John Milton called Paradise Lost, published in 1667.
The term outward space existed in a poem from 1842 by the English poet Lady Emmeline Stuart-Wortley called "The Maiden of Moscow", but in astronomy the term outer space found its application for the first time in 1845 by Alexander von Humboldt. The term was eventually popularized through the writings of H. G. Wells after 1901. Theodore von Kármán used the term of free space
to name the space of altitudes above Earth where spacecrafts reach
conditions sufficiently free from atmospheric drag, differentiating it
from airspace, identifying a legal space above territories free from the sovereign jurisdiction of countries.
"Spaceborne" denotes existing in outer space, especially if carried by a spacecraft; similarly, "space-based" means based in outer space or on a planet or moon.
Timeline of the expansion of the universe, where space is represented schematically at each time by circular sections. On the left, the dramatic expansion of inflation; at the center, the expansion accelerates (artist's concept; neither time nor size are to scale)
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 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 constrains the size of the directly observable universe.
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 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 Milky Way.
A wide field view of outer space as seen from Earth's surface at night. The interplanetary dust cloud is visible as the horizontal band of zodiacal light, including the false dawn (edges) and gegenschein (center), which is visually crossed by the Milky Way
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 orbits. 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 astronomy.
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 space weather.
The temperature of outer space is measured in terms of the kinetic activity of the gas,
as it is on Earth. The radiation of outer space has a different
temperature than the kinetic temperature of the gas, meaning that the
gas and radiation are not in thermodynamic equilibrium. 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 2.7 K (−270 °C; −455 °F). The gas temperatures in outer space can vary widely. For example, the temperature in the Boomerang Nebula is 1 K (−272 °C; −458 °F), while the solar corona reaches temperatures over 1,200,000–2,600,000 K (2,200,000–4,700,000 °F).
Magnetic fields have been detected in the space around many
classes of celestial objects. 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 that exist in several nearby galaxies. Magneto-hydrodynamic processes in activeelliptical 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 106eV up to an extreme 1020 eV of ultra-high-energy cosmic rays. The peak flux of cosmic rays occurs at energies of about 109 eV, with approximately 87% protons, 12% helium nuclei and 1% heavier nuclei. In the high energy range, the flux of electrons is only about 1% of that of protons. Cosmic rays can damage electronic components and pose a health threat to space travelers.
Scents retained from low Earth orbit, when returning from extravehicular activity, have a burned, metallic odor, similar to the scent of arc welding fumes. This results from oxygen in low Earth orbit, which clings to suits and equipment. Other regions of space could have very different odors, like that of different alcohols in molecular clouds.
Because of the hazards of a vacuum, astronauts must wear a pressurized space suit while outside their spacecraft.
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.
The lack of pressure in space is the most immediate dangerous
characteristic of space to humans. Pressure decreases above Earth,
reaching a level at an altitude of around 19.14 km (11.89 mi) that
matches the vapor pressure of water at the temperature of the human body. This pressure level is called the Armstrong line, named after American physician Harry G. Armstrong.
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, 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 kilopascals (1 psi), 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 kilopascals (0.3 psi).
Supplemental oxygen is needed at 8 km (5 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 kilopascals (4–6 psi) of pure oxygen,
about the same as the partial pressure of oxygen at 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.
During long-duration space travel, radiation can pose an acute health hazard. Exposure to high-energy, ionizing cosmic rays can result in fatigue, nausea, vomiting, as well as damage to the immune system and changes to the white blood cell count. Over longer durations, symptoms include an increased risk of cancer, plus damage to the 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. 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.
Illustration of Earth's atmosphere gradual transition into outer space
The transition between Earth's atmosphere and outer space lacks a
well-defined physical boundary, with the air pressure steadily
decreasing with altitude until it mixes with the solar wind. Various definitions for a practical boundary have been proposed, ranging from 30 km (19 mi) out to 1,600,000 km (990,000 mi). In 2009, measurements of the direction and speed of ions in the atmosphere were made from a sounding rocket.
The altitude of 118 km (73.3 mi) above Earth was the midpoint for
charged particles transitioning from the gentle winds of the Earth's
atmosphere to the more extreme flows of outer space. The latter can
reach velocities well over 268 m/s (880 ft/s).
High-altitude aircraft, such as high-altitude balloons have reached altitudes above Earth of up to 50 km. Up until 2021, the United States designated people who travel above an altitude of 50 mi (80 km) as astronauts. Astronaut wings
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".
The region between airspace and outer space is termed "near
space". There is no legal definition for this extent, but typically this
is the altitude range from 20 to 100 km (12 to 62 mi). For safety reasons, commercial aircraft are typically limited to altitudes of 12 km (7.5 mi), and air navigation services only extend to 18 to 20 km (11 to 12 mi). The upper limit of the range is the Kármán line, where astrodynamics must take over from aerodynamics in order to achieve flight. This range includes the stratosphere, mesosphere and lower thermosphere layers of the Earth's atmosphere.
Larger ranges for near space are used by some authors, such as 18 to 160 km (11 to 99 mi). These extend to the altitudes where orbital flight in very low Earth orbits becomes practical. Spacecraft have entered into a highly elliptical orbit with a perigee as low as 80 to 90 km (50 to 56 mi), surviving for multiple orbits. At an altitude of 120 km (75 mi), descending spacecraft begin atmospheric entry as atmospheric drag becomes noticeable. For spaceplanes such as NASA's Space Shuttle, this begins the process of switching from steering with thrusters to maneuvering with aerodynamic control surfaces.
The Kármán line, established by the Fédération Aéronautique Internationale, and used internationally by the United Nations,
is set at an altitude of 100 km (62 mi) as a working definition for the
boundary between aeronautics and astronautics. This line is named after
Theodore von Kármán, who argued for an altitude where a vehicle would have to travel faster than orbital velocity to derive sufficient aerodynamic lift from the atmosphere to support itself, which he calculated to be at an altitude of about 83.8 km (52.1 mi). This distinguishes altitudes below as the region of aerodynamics and airspace, and above as the space of astronautics and free space.
There is no internationally recognized legal altitude limit on
national airspace, although the Kármán line is the most frequently used
for this purpose. Objections have been made to setting this limit too
high, as it could inhibit space activities due to concerns about
airspace violations.
It has been argued for setting no specified singular altitude in
international law, instead applying different limits depending on the
case, in particular based on the craft and its purpose. Increased
commercial and military sub-orbital spaceflight has raised the issue of
where to apply laws of airspace and outer space. Spacecraft have flown over foreign countries as low as 30 km (19 mi), as in the example of the Space Shuttle.
The Outer Space Treaty
provides the basic framework for international space law. It covers the
legal use of outer space by nation states, and includes in its
definition of outer space, the Moon, and other celestial bodies.
The treaty states that outer space is free for all nation states to
explore and is not subject to claims of national sovereignty, calling
outer space the "province of all mankind". This status as a common heritage of mankind
has been used, though not without opposition, to enforce the right to
access and shared use of outer space for all nations equally,
particularly non-spacefaring nations. It 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 Union of Soviet Socialist Republics
(USSR), the United States of America (USA), and the United Kingdom (UK).
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 USA, USSR, China, and in 2019, India. The 1979 Moon Treaty
turned the jurisdiction of all heavenly bodies (including the orbits
around such bodies) over to the international community. The treaty has
not been ratified by any nation that currently practices human
spaceflight.
In 1976, eight equatorial states (Ecuador, Colombia, Brazil, The
Republic of the Congo, Zaire, Uganda, Kenya, and Indonesia) met in
Bogotá, 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.
An increasing issue of international space law and regulation has been the dangers of the growing number of space debris.
Newton's cannonball, an illustration of how objects can "fall" in a curve around the planet
When a rocket is launched to achieve orbit, its thrust must both counter gravity and accelerate it to orbital speed. After the rocket terminates its thrust, it follows an arc-like trajectory back toward the ground under the influence of the Earth's gravitational force. In a closed orbit, this arc will turn into an elliptical loop around the planet. That is, a spacecraft successfully enters Earth orbit when its acceleration due to gravity pulls the craft down just enough to prevent its momentum from carrying it off into outer space.
For a low Earth orbit, orbital speed is about 7.8 km/s (17,400 mph);
by contrast, the fastest piloted airplane speed ever achieved
(excluding speeds achieved by deorbiting spacecraft) was 2.2 km/s
(4,900 mph) in 1967 by the North American X-15. The upper limit of orbital speed at 11.2 km/s (25,100 mph) is the velocity required to pull free from Earth altogether and enter into a heliocentric orbit. The energy required to reach Earth orbital speed 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.
Very low Earth orbit (VLEO) has been defined as orbits that have a
mean altitude below 450 km (280 mi), which can be better suited for
Earth observation with small satellites. Low Earth orbits in general range in altitude from 180 to 2,000 km (110 to 1,240 mi) and are used for scientific satellites. Medium Earth orbits
extends from 2,000 to 35,780 km (1,240 to 22,230 mi), which are
favorable orbits for navigation and specialized satellites. Above
35,780 km (22,230 mi) are the high Earth orbits used for weather and some communication satellites.
Spacecraft in orbit with a perigee below about 2,000 km (1,200 mi) (low Earth orbit) 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, which is significantly
effected by space weather. At altitudes above 800 km (500 mi), orbital lifetime is measured in centuries.
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.
Radiation in orbit around Earth is concentrated in Van Allen radiation belts, which trap solar and galactic radiation.
Radiation is a threat to astronauts and space systems. It is difficult
to shield against and space weather makes the radiation environment
variable. The radiation belts are equatorial toroidal regions, which are bent towards Earth's poles, with the South Atlantic Anomaly being the region where charged particles approach Earth closest.
The innermost radiation belt, the inner Van Allen belt, has its
intensity peak at altitudes above the equator of half an Earth radius, centered at about 3000 km, increasing from the upper edge of low Earth orbit which it overlaps.
The outermost layer of the Earth's atmosphere is termed the exosphere. It extends outward from the thermopause,
which lies at an altitude that varies from 250 to 500 kilometres (160
to 310 mi), depending on the incidence of solar radiation. Beyond this
altitude, collisions between molecules are negligible and the atmosphere
joins with interplanetary space.
The region in proximity to the Earth is home to a multitude of
Earth–orbiting satellites and has been subject to extensive studies. For
identification purposes, this volume is divided into overlapping
regions of space.
Near-Earth space is the region of space extending from low Earth orbits out to geostationary orbits. This region includes the major orbits for artificial satellites and is the site of most of humanity's space activity. The region has seen high levels of space debris, sometimes dubbed space pollution, threatening nearby space activity. Some of this debris re-enters Earth's atmosphere periodically.
Although it meets the definition of outer space, the atmospheric
density inside low-Earth orbital space, the first few hundred kilometers
above the Kármán line, is still sufficient to produce significant drag on satellites.
A computer-generated map of objects orbiting Earth, as of 2005. About 95% are debris, not working artificial satellites
Geospace is a region of space that includes Earth's upper atmosphere and magnetosphere. The Van Allen radiation belts lie within the geospace. The outer boundary of geospace is the magnetopause, which forms an interface between the Earth's magnetosphere and the solar wind. The inner boundary is the ionosphere.
The variable space-weather conditions of geospace are affected by
the behavior of the Sun and the solar wind; the subject of geospace is
interlinked with heliophysics—the study of the Sun and its impact on the planets of the Solar System.
The day-side magnetopause is compressed by solar-wind pressure—the
subsolar distance from the center of the Earth is typically 10 Earth
radii. On the night side, the solar wind stretches the magnetosphere to
form a magnetotail that sometimes extends out to more than 100–200 Earth radii.
For roughly four days of each month, the lunar surface is shielded from
the solar wind as the Moon passes through the magnetotail.
Geospace is populated by electrically charged particles at very low densities, the motions of which are controlled by the Earth's magnetic field.
These plasmas form a medium from which storm-like disturbances powered
by the solar wind can drive electrical currents into the Earth's upper
atmosphere. Geomagnetic storms
can disturb two regions of geospace, the radiation belts and the
ionosphere. These storms increase fluxes of energetic electrons that can
permanently damage satellite electronics, interfering with shortwave
radio communication and GPS location and timing. Magnetic storms can be a hazard to astronauts, even in low Earth orbit. They create aurorae seen at high latitudes in an oval surrounding the geomagnetic poles.
Earth and the Moon as seen from cislunar space on the 2022 Artemis 1 mission
XGEO space is a concept used by the USA to refer to the space of high Earth orbits, with the 'X' being some multiple of geosynchronous orbit (GEO) at approximately 35,786 km (22,236 mi). Hence, the L2 Earth-Moon Lagrange point at 448,900 km (278,934 mi) is approximately 10.67 XGEO. Translunar space is the region of lunar transfer orbits, between the Moon and Earth. Cislunar space is a region outside of Earth that includes lunar orbits, the Moon's orbital space around Earth and the Earth-Moon Lagrange points.
The region where a body's gravitational potential remains dominant against gravitational potentials from other bodies, is the body's sphere of influence or gravity well, mostly described with the Hill sphere model.
In the case of Earth this includes all space from the Earth to a
distance of roughly 1% of the mean distance from Earth to the Sun, or 1.5 million km (0.93 million mi). Beyond Earth's Hill sphere extends along Earth's orbital path its orbital and co-orbital space. This space is co-populated by groups of co-orbital Near-Earth Objects (NEOs), such as horseshoe librators and Earth trojans, with some NEOs at times becoming temporary satellites and quasi-moons to Earth.
Deep space is defined by the United States government as all of
outer space which lies further from Earth than a typical
low-Earth-orbit, thus assigning the Moon to deep-space.
Other definitions vary the starting point of deep-space from, "That
which lies beyond the orbit of the moon," to "That which lies beyond the
farthest reaches of the Solar System itself." The International Telecommunication Union responsible for radio communication,
including with satellites, defines deep-space as, "distances from the
Earth equal to, or greater than, 2 million km (1.2 million mi)," which is about five times the Moon's orbital distance, but which distance is also far less than the distance between Earth and any adjacent planet.
Near-Earth
space showing the low-Earth (blue), medium Earth (green), and high
Earth (red) orbits. The last extends beyond the radius of geosynchronous
orbits
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 within the Solar System is dominated by the gravitation of the Sun, outside the gravitational spheres of influence of the planets. Interplanetary space extends well beyond the orbit of the outermost planet Neptune,
all the way out to where the influence of the galactic environment
starts to dominate over the Sun and its solar wind producing the heliopause at 110 to 160 AU.
The heliopause deflects away low-energy galactic cosmic rays, and its
distance and strength varies depending on the activity level of the
solar wind. The solar wind is a continuous stream of charged particles emanating from the Sun which 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).
The region of interplanetary space is a nearly total vacuum, with a mean free path of about one astronomical unit
at the orbital distance of the Earth. This space is not completely
empty, but is sparsely filled with cosmic rays, which include ionizedatomic nuclei and various subatomic particles. There is gas, plasma and dust, small meteors, and several dozen types of organic molecules discovered to date by microwave spectroscopy. Collectively, this matter is termed the interplanetary medium. 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 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" redirects here. For the album, see Interstellar Space.
Interstellar space is the physical space outside of the bubbles of plasma known as astrospheres, formed by stellar winds originating from individual stars, or formed by solar wind emanating from the Sun. It is the space between the stars or stellar systems within a nebula or galaxy. Interstellar space contains an interstellar medium of sparse matter and radiation. The boundary between an astrosphere and interstellar space is known as an astropause. For the Sun, the astrosphere and astropause are called the heliosphere and heliopause, respectively.
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 propagates shock waves
of stellar ejecta outward, distributing it throughout the interstellar
medium, including the heavy elements previously formed within the star's
core. 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, which can form dust particles as tiny as 0.1 μm. 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 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 medium. The Local
Bubble contains dozens of warm interstellar clouds with temperatures of
up to 7,000 K and radii of 0.5–5 pc.
When stars are moving at sufficiently high peculiar velocities, their astrospheres can generate bow shocks as they collide with the interstellar medium. For decades it was assumed that the Sun had a bow shock. In 2012, data from Interstellar Boundary Explorer (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 a third boundary characteristic of an astrosphere, lying outside the termination shock and the astropause.
Distribution
of Matter in a cubic section of the universe. The blue fiber-like
structures represent matter, while the empty regions show the cosmic voids
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 cosmic voids that are mostly empty of galaxies. Typically, a void spans a distance of 7–30 megaparsecs.
Surrounding and stretching between galaxies is the intergalactic medium (IGM). This rarefied plasma is organized in a galactic filamentary structure. The diffuse photoionized gas contains filaments of higher density, about one atom per cubic meter, which is 5–200 times the average density of the universe.
The IGM is inferred to be mostly primordial in composition, with 76%
hydrogen by mass, and enriched with higher mass elements from
high-velocity galactic outflows.
As gas falls into the intergalactic medium from the voids, it heats up to temperatures of 105 K to 107 K. 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).
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 filled.
In ancient China, the 2nd-century astronomer Zhang Heng
became convinced that space must be infinite, extending well beyond the
mechanism that supported the Sun and the stars. The surviving books of
the Hsüan Yeh school said that the heavens were boundless, "empty and
void of substance". Likewise, the "sun, moon, and the company of stars
float in the empty space, moving or standing still".
The Italian scientist Galileo Galilei
knew that air has mass and so was subject to gravity. In 1640, he
demonstrated that an established force resisted the formation of a
vacuum. It would remain for his pupil Evangelista Torricelli to create an apparatus that would produce a partial vacuum in 1643. This experiment resulted in the first mercury barometer and created a scientific sensation in Europe. Torricelli suggested that since air has weight, then air pressure should decrease with altitude. The French mathematician Blaise Pascal proposed an experiment to test this hypothesis. 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.
The original Magdeburg hemispheres (left) used to demonstrate Otto von Guericke's vacuum pump (right)
In 1650, German scientist Otto von Guericke constructed the first vacuum pump: a device that would further refute the principle of horror vacui.
He correctly noted that the atmosphere of the Earth surrounds the
planet like a shell, with the density gradually declining with altitude.
He concluded that there must be a vacuum between the Earth and the
Moon.
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.[179] In 1887, the Michelson–Morley experiment tried to detect the Earth's motion through this medium by looking for changes in the speed of light depending on the direction of the planet's motion. The null result indicated something was wrong with the concept. The idea of the luminiferous aether was then abandoned. It was replaced by Albert Einstein's theory of special relativity, which holds that the speed of light in a vacuum is a fixed constant, independent of the observer's motion or frame of reference.
The first professional astronomer to support the concept of an infinite universe was the Englishman Thomas Digges in 1576. But the scale of the universe remained unknown until the first successful measurement of the distance to a nearby star in 1838 by the German astronomer Friedrich Bessel. He showed that the star system 61 Cygni had a 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 Milky Way. With this Hubble formulated the Hubble constant,
which allowed for the first time a calculation of the age of the
Universe and size of the Observable Universe, starting at 2 billion
years and 280 million light-years. This became increasingly precise with
better measurements, until 2006 when data of the Hubble Space Telescope allowed a very accurate calculation of the age of the Universe and size of the Observable Universe.
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 state of extreme energy density 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.
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 American Explorer II crewed 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 uncrewed 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 American Apollo 8, which achieved lunar orbit and reached a maximum distance of 377,349 km (234,474 mi) from the Earth.
The first spacecraft to reach escape velocity was the Soviet Luna 1, which performed a fly-by of the Moon in 1959. In 1961, Venera 1 became the first planetary probe. It revealed the presence of the solar wind and performed the first fly-by of Venus, although contact was lost before reaching Venus. The first successful planetary mission was the 1962 fly-by of Venus by Mariner 2. The first fly-by of Mars was by Mariner 4
in 1964. Since that time, uncrewed spacecraft have successfully
examined each of the Solar System's planets, as well their moons and
many minor planets and comets. They remain a fundamental tool for the exploration of outer space, as well as for observation of the Earth. In August 2012, Voyager 1 became the first man-made object to leave the Solar System and enter interstellar space.
Outer space has become an important element of global society. It
provides multiple applications that are beneficial to the economy and
scientific research.
The placing of artificial satellites in Earth orbit has produced numerous benefits and has become the dominating sector of the space economy. They allow relay of long-range communications like television, provide a means of precise navigation, and permit direct monitoring of weather conditions and remote sensing
of the Earth. The latter role serves a 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. They facilitate the discovery and monitoring of climate change influences.
Satellites make use of the significantly reduced drag in space to stay
in stable orbits, allowing them to efficiently span the whole globe,
compared to for example stratospheric balloons or high-altitude platform stations, which have other benefits.
The absence of air makes outer space an ideal location for astronomy at all wavelengths of the electromagnetic spectrum.
This is evidenced by the pictures sent back by the Hubble Space
Telescope, allowing light from more than 13 billion years ago—almost to
the time of the Big Bang—to be observed. Not every location in space is ideal for a telescope. The interplanetary 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 observations.
The deep vacuum of space could make it an attractive environment for
certain industrial processes, such as those requiring ultraclean
surfaces. Like asteroid mining, space manufacturing would require a large financial investment with little prospect of immediate return.
An important factor in the total expense is the high cost of placing
mass into Earth orbit: $9,000–$31,000 per kg, according to a 2006
estimate (allowing for inflation since then). The cost of access to space has declined since 2013. Partially reusable rockets such as the Falcon 9
have lowered access to space below 3500 dollars per kilogram. With
these new rockets the cost to send materials into space remains
prohibitively high for many industries. Proposed concepts for addressing
this issue include, fully reusable launch systems, non-rocket spacelaunch, momentum exchange tethers, and space elevators.
Interstellar travel
for a human crew remains at present only a theoretical possibility. The
distances to the nearest stars mean it would require new technological
developments and the ability to safely sustain crews for journeys
lasting several decades. For example, the Daedalus Project study, which proposed a spacecraft powered by the fusion of deuterium and helium-3, would require 36 years to reach the "nearby" Alpha Centauri system. Other proposed interstellar propulsion systems include light sails, ramjets, and beam-powered propulsion. More advanced propulsion systems could use antimatter as a fuel, potentially reaching relativistic velocities.
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