Outer space  

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Flammarion engraving, a wood engraving by an unknown artist, so named because its first documented appearance is in Camille Flammarion's 1888 book L'atmosphère: météorologie populaire ("The Atmosphere: Popular Meteorology").
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Flammarion engraving, a wood engraving by an unknown artist, so named because its first documented appearance is in Camille Flammarion's 1888 book L'atmosphère: météorologie populaire ("The Atmosphere: Popular Meteorology").
Magnum Chaos (c. 1524 ) by Lorenzo Lotto   "Before the ocean and the earth appeared— before the skies had overspread them all— the face of Nature in a vast expanse was naught but Chaos uniformly waste. It was a rude and undeveloped mass, that nothing made except a ponderous weight; and all discordant elements confused, were there congested in a shapeless heap." (trans. Brookes More)
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Magnum Chaos (c. 1524 ) by Lorenzo Lotto
"Before the ocean and the earth appeared— before the skies had overspread them all— the face of Nature in a vast expanse was naught but Chaos uniformly waste. It was a rude and undeveloped mass, that nothing made except a ponderous weight; and all discordant elements confused, were there congested in a shapeless heap." (trans. Brookes More)

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Train wreck at Montparnasse (October 22, 1895) by Studio Lévy and Sons.
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Train wreck at Montparnasse (October 22, 1895) by Studio Lévy and Sons.

Outer space is the void that exists between celestial bodies, including the Earth. It is not completely empty, but consists of a hard vacuum containing a low density of particles: predominantly a plasma of hydrogen and helium, as well as electromagnetic radiation, magnetic fields, and neutrinos. Theory suggests that it also contains dark matter and dark energy. In the space between galaxies, matter density can be as low as a few atoms of hydrogen per cubic meter. The baseline temperature, as set by background radiation left over from the Big Bang, is only 3 Kelvin; in contrast, temperatures in the coronae of stars can reach over a million Kelvin. Plasma with an extremely low density and high temperature, such as warm-hot intergalactic medium and intracluster medium, accounts for most of the baryonic (ordinary) matter in outer space; local concentrations have evolved into stars and galaxies. Intergalactic outer space takes up most of the volume of the universe, but even galaxies and star systems consist almost entirely of empty space. As of yet, space travel has been limited to the vicinity of the Solar System; the remainder of outer space remains inaccessible to humans other than by passive observation with telescopes.

There is no firm boundary where space begins. However the Kármán line, at an altitude of 100 kilometres above sea level, is conventionally used as the start of outer space for the purpose of space treaties and aerospace records keeping. The framework for international space law was established by the Outer Space Treaty, which was passed by the United Nations in 1963. This treaty precludes any claims of national sovereignty and permits all states to explore outer space freely. In 1979, the Moon Treaty made the surfaces of objects such as planets, as well as the orbital space around these bodies, the jurisdiction of the international community. Additional resolutions regarding outer space have been drafted by the United Nations, but these have not precluded the deployment of weapons into outer space.

Discovery

In 350 BC, 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.Template:Sfn As late as the 17th century, the French philosopher René Descartes argued that the entirety of space must be filled.Template:Sfn

In ancient China, there were various schools of thought concerning the nature of the heavens, some of which bear a resemblance to the modern understanding. In the 2nd century CE, 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".Template:Sfn

The Italian scientist Galileo Galilei knew that air had weight 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 his pupil Evangelista Torricelli to create an apparatus that would produce a vacuum in 1643. At the time this experiment 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.Template:Sfn 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 inflate, then deflate upon descent.Template:Sfn

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.Template:Sfn

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.Template:Sfn 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 cause resistance to the motions of heavenly bodies.Template:Sfn 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.Template:Sfn This concept of an aether originated with ancient Greek philosophers, including Aristotle, who conceived of it as the medium through which the heavenly bodies moved.Template:Sfn

The concept of a universe filled with a luminiferous aether remained in vogue among some scientists until the early 20th century. This form of aether was viewed as the medium through which light could propagate.Template:Sfn 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 reference.Template:SfnTemplate:Sfn

The first professional astronomer to support the concept of an infinite universe was the Englishman Thomas Digges in 1576.Template:Sfn 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 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.Template:Sfn The distance to the Andromeda galaxy was determined in 1923 by American astronomer Edwin Hubble by measuring the brightness of cepheid variables in that galaxy, a new technique discovered by Henrietta Leavitt. This established that the Andromeda galaxy, and by extension all galaxies, lay well outside the Milky Way.

The modern concept of outer space is based on the Big Bang cosmology, first proposed in 1931 by the Belgian physicist Georges Lemaître. This theory holds that the observable universe originated from a very compact form that has since undergone continuous expansion. 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.Template:Sfn 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 term outward space was used as early as 1842 by the English poet Lady Emmeline Stuart-Wortley in her poem "The Maiden of Moscow", although she employed it in a terrestrial context.Template:Sfn The expression outer space was used as an astronomical term by Alexander von Humboldt in 1845.Template:Sfn It was later popularized in the writings of HG Wells in 1901. The shorter term space is actually older, first used to mean the region beyond Earth's sky in John Milton's Paradise Lost in 1667.

See also




Unless indicated otherwise, the text in this article is either based on Wikipedia article "Outer space" or another language Wikipedia page thereof used under the terms of the GNU Free Documentation License; or on original research by Jahsonic and friends. See Art and Popular Culture's copyright notice.

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