Atmosphere of Titan

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Atmosphere of Titan

True-color image of layers of haze in Titan's atmosphere
General information
Chemical species	Molar fraction
Composition
Nitrogen	94.2%
Methane	5.65%
Hydrogen	0.099%

The atmosphere of Titan is the layer of gases surrounding Titan, the largest moon of Saturn. It is the only thick atmosphere of a natural satellite in the Solar System. Titan's lower atmosphere is primarily composed of nitrogen (94.2%), methane (5.65%), and hydrogen (0.099%). There are trace amounts of other hydrocarbons, such as ethane, diacetylene, methylacetylene, acetylene, propane, PAHs and of other gases, such as cyanoacetylene, hydrogen cyanide, carbon dioxide, carbon monoxide, cyanogen, acetonitrile, argon and helium. The isotopic study of nitrogen isotopes ratio also suggest acetonitrile may be present in quantities exceeding hydrogen cyanide and cyanoacetylene. The surface pressure is about 50% higher than Earth at 1.5 bars which is near the triple point of methane and allows there to be gaseous methane in the atmosphere and liquid methane on the surface. The orange color as seen from space is produced by other more complex chemicals in small quantities, possibly tholins, tar-like organic precipitates.

Observational history

The presence of a significant atmosphere was first suspected by Spanish astronomer Josep Comas i Solà, who observed distinct limb darkening on Titan in 1903, and confirmed by Gerard P. Kuiper in 1944 using a spectroscopic technique that yielded an estimate of an atmospheric partial pressure of methane of the order of 100 millibars (10 kPa). Subsequent observations in the 1970s showed that Kuiper's figures had been significant underestimates; methane abundances in Titan's atmosphere were ten times higher, and the surface pressure was at least double what he had predicted. The high surface pressure meant that methane could only form a small fraction of Titan's atmosphere. In 1980, Voyager 1 made the first detailed observations of Titan's atmosphere, revealing that its surface pressure was higher than Earth's, at 1.5 bars (about 1.48 times that of Earth's).

The joint NASA/ESA Cassini-Huygens mission provided a wealth of information about Titan, and the Saturn system in general, since entering orbit on July 1, 2004. It was determined that Titan's atmospheric isotopic abundances were evidence that the abundant nitrogen in the atmosphere came from materials in the Oort cloud, associated with comets, and not from the materials that formed Saturn in earlier times. It was determined that complex organic chemicals could arise on Titan, including polycyclic aromatic hydrocarbons, propylene, and methane.

The Dragonfly mission by NASA is planning to land a large aerial vehicle on Titan in 2034. The mission will study Titan's habitability and prebiotic chemistry at various locations. The drone-like aircraft will perform measurements of geologic processes, and surface and atmospheric composition.

Overview

Observations from the Voyager space probes have shown that the Titanean atmosphere is denser than Earth's, with a surface pressure about 1.48 times that of Earth's. Titan's atmosphere is about 1.19 times as massive as Earth's overall, or about 7.3 times more massive on a per surface area basis. It supports opaque haze layers that block most visible light from the Sun and other sources and renders Titan's surface features obscure. The atmosphere is so thick and the gravity so low that humans could fly through it by flapping "wings" attached to their arms. Titan's lower gravity means that its atmosphere is far more extended than Earth's; even at a distance of 975 km, the Cassini spacecraft had to make adjustments to maintain a stable orbit against atmospheric drag. The atmosphere of Titan is opaque at many wavelengths and a complete reflectance spectrum of the surface is impossible to acquire from the outside. It was not until the arrival of Cassini–Huygens in 2004 that the first direct images of Titan's surface were obtained. The Huygens probe was unable to detect the direction of the Sun during its descent, and although it was able to take images from the surface, the Huygens team likened the process to "taking pictures of an asphalt parking lot at dusk".

Vertical structure

Diagram of Titan's atmosphere

Titan's vertical atmospheric structure is similar to Earth. They both have a troposphere, stratosphere, mesosphere, and thermosphere. However, Titan's lower surface gravity creates a more extended atmosphere, with scale heights of 15-50 km in comparison to 5-8 km on Earth. Voyager data, combined with data from Huygens and radiative-convective models provide increased understanding of Titan's atmospheric structure.

• Troposphere: This is the layer where a lot of the weather occurs on Titan. Since methane condenses out of Titan's atmosphere at high altitudes, its abundance increases below the tropopause at an altitude of 32 km, leveling off at a value of 4.9% between 8 km and the surface. Methane rain, haze rainout, and varying cloud layers are found in the troposphere.
• Stratosphere: The atmospheric composition in the stratosphere is 98.4% nitrogen—the only dense, nitrogen-rich atmosphere in the Solar System aside from Earth's—with the remaining 1.6% composed mostly of methane (1.4%) and hydrogen (0.1–0.2%). The main tholin haze layer lies in the stratosphere at about 100-210 km. In this layer of the atmosphere there is a strong temperature inversion caused by the haze due to a high ratio of shortwave to infrared opacity.
• Mesosphere: A detached haze layer is found at about 450-500 km, within the mesosphere. The temperature at this layer is similar to that of the thermosphere because of the cooling of hydrogen cyanide (HCN) lines.
• Thermosphere: Particle production begins in the thermosphere This was concluded after finding and measuring heavy ions and particles. This was also Cassini's closest approach in Titan's atmosphere.
• Ionosphere: Titan's ionosphere is also more complex than Earth's, with the main ionosphere at an altitude of 1,200 km but with an additional layer of charged particles at 63 km. This splits Titan's atmosphere to some extent into two separate radio-resonating chambers. The source of natural extremely-low-frequency (ELF) waves on Titan, as detected by Cassini–Huygens, is unclear as there does not appear to be extensive lightning activity.

Atmospheric composition and chemistry

Titan's atmospheric chemistry is diverse and complex. Each layer of the atmosphere has unique chemical interactions occurring within that are then interacting with other sub layers in the atmosphere. For instance, the hydrocarbons are thought to form in Titan's upper atmosphere in reactions resulting from the breakup of methane by the Sun's ultraviolet light, producing a thick orange smog. The table below highlights the production and loss mechanisms of the most abundant photochemically produced molecules in Titan's atmosphere.

Chemistry in Titan's atmosphere

Molecule	Production	Loss
Hydrogen	Methane photolysis	Escape
Carbon Monoxide	${\displaystyle {\ce {O + CH3->H2CO + H}}}$ ${\displaystyle {\ce {H2CO + h\nu->CO + H2/2H}}}$	${\displaystyle {\ce {CO + OH->CO2 + H}}}$
Ethane	${\displaystyle {\ce {2CH3 + M->C2H6 + M}}}$	Condensation
Acetylene	${\displaystyle {\ce {C2H + CH4->C2H2 + CH3}}}$	${\displaystyle {\ce {C2H2 + h\nu->C2H + H}}}$ Condensation
Propane	${\displaystyle {\ce {CH3 + C2H5 + M->C3H8 + M}}}$	Condensation
Ethylene	${\displaystyle {\ce {CH + CH4->C2H4 + H}}}$ ${\displaystyle {\ce {CH2 + CH3->C2H4 + H}}}$	${\displaystyle {\ce {C2H4 + h\nu->C2H2 + H2/2H}}}$
Hydrogen Cyanide	${\displaystyle {\ce {N + CH3->H2CN + H}}}$ ${\displaystyle {\ce {H2CN + H->HCN + H2}}}$	Condensation
Carbon Dioxide	${\displaystyle {\ce {CO + OH->CO2 + H}}}$	Condensation
Methylacetylene	${\displaystyle {\ce {CH + C2H4->CH3CCH + H}}}$	${\displaystyle {\ce {CH3CCH + h\nu->C3H3 + H}}}$ ${\displaystyle {\ce {H + CH3CCH->C3H5}}}$
Diacetylene	${\displaystyle {\ce {C2H + C2H2->C4H2 + H}}}$	${\displaystyle {\ce {C4H2 + h\nu->C4H + H}}}$

A cloud imaged in false color over Titan's north pole.

Magnetic field

Titan has no magnetic field, although studies in 2008 showed that Titan retains remnants of Saturn's magnetic field on the brief occasions when it passes outside Saturn's magnetosphere and is directly exposed to the solar wind. This may ionize and carry away some molecules from the top of the atmosphere. Titan's internal magnetic field is negligible, and perhaps even nonexistent. Its orbital distance of 20.3 Saturn radii does place it within Saturn's magnetosphere occasionally. However, the difference between Saturn's rotational period (10.7 hours) and Titan's orbital period (15.95 days) causes a relative speed of about 100 km/s between the Saturn's magnetized plasma and Titan. That can actually intensify reactions causing atmospheric loss, instead of guarding the atmosphere from the solar wind.

Chemistry of the ionosphere

In November 2007, scientists uncovered evidence of negative ions with roughly 13 800 times the mass of hydrogen in Titan's ionosphere, which are thought to fall into the lower regions to form the orange haze which obscures Titan's surface. The smaller negative ions have been identified as linear carbon chain anions with larger molecules displaying evidence of more complex structures, possibly derived from benzene. These negative ions appear to play a key role in the formation of more complex molecules, which are thought to be tholins, and may form the basis for polycyclic aromatic hydrocarbons, cyanopolyynes and their derivatives. Remarkably, negative ions such as these have previously been shown to enhance the production of larger organic molecules in molecular clouds beyond our Solar System, a similarity which highlights the possible wider relevance of Titan's negative ions.

Titan's South Pole Vortex—a swirling HCN gas cloud (November 29, 2012).

Atmospheric circulation

There is a pattern of air circulation found flowing in the direction of Titan's rotation, from west to east. In addition, seasonal variation in the atmospheric circulation have also been detected. Observations by Cassini of the atmosphere made in 2004 also suggest that Titan is a "super rotator", like Venus, with an atmosphere that rotates much faster than its surface. The atmospheric circulation is explained by a big Hadley circulation that is occurring from pole to pole.

Methane cycle

Energy from the Sun should have converted all traces of methane in Titan's atmosphere into more complex hydrocarbons within 50 million years — a short time compared to the age of the Solar System. This suggests that methane must be somehow replenished by a reservoir on or within Titan itself. Most of the methane on Titan is in the atmosphere. Methane is transported through the cold trap at the tropopause. Therefore the circulation of methane in the atmosphere influences the radiation balance and chemistry of other layers in the atmosphere. If there is a reservoir of methane on Titan, the cycle would only be stable over geologic timescales.

Trace organic gases in Titan's atmosphere—HNC (left) and HC₃N (right).

Evidence that Titan's atmosphere contains over a thousand times more methane than carbon monoxide would appear to rule out significant contributions from cometary impacts, because comets are composed of more carbon monoxide than methane. That Titan might have accreted an atmosphere from the early Saturnian nebula at the time of formation also seems unlikely; in such a case, it ought to have atmospheric abundances similar to the solar nebula, including hydrogen and neon. Many astronomers have suggested that the ultimate origin for the methane in Titan's atmosphere is from within Titan itself, released via eruptions from cryovolcanoes.

Polar clouds, made of methane, on Titan (left) compared with

polar clouds on Earth (right).

Daytime and Twilight (sunrise/sunset) Skies

Sky brightness models of a sunny day on Titan. The Sun is seen setting from noon until after dusk at 3 wavelengths: 5 μm, near-infrared (1-2 μm), and visible. Each image shows a "rolled-out" version of the sky as seen from the surface of Titan. The left side shows the Sun, while the right side points away from the Sun. The top and bottom of the image are the zenith and the horizon respectively. The solar zenith angle represents the angle between the Sun and zenith (0°), where 90° is when the Sun reaches the horizon.

 

Sky brightness and viewing conditions are expected to be quite different from Earth and Mars due to Titan's farther distance from the Sun (~10 AU) and complex haze layers in its atmosphere. The sky brightness model videos show what a typical sunny day may look like standing on the surface of Titan based on radiative transfer models.

For astronauts who see with visible light, the daytime sky has a distinctly dark orange color and appears uniform in all directions due to significant Mie scattering from the many high-altitude haze layers. The daytime sky is calculated to be ~100-1000 times dimmer than an afternoon on Earth, which is similar to the viewing conditions of a thick smog or dense fire smoke. The sunsets on Titan are expected to be "underwhelming events", where the Sun disappears about half-way up in the sky (~50° above the horizon) with no distinct change in color. After that, the sky will slowly darken until it reaches night. However, the surface is expected to remain as bright as the full Moon up to 1 Earth day after sunset.

In near-infrared light, the sunsets resemble a Martian sunset or dusty desert sunset. Mie scattering has a weaker influence at longer infrared wavelengths, allowing for more colorful and variable sky conditions. During the daytime, the Sun has a noticeable solar corona that transitions color from white to "red" over the afternoon. The afternoon sky brightness is ~100 times dimmer than Earth. As evening time approaches, the Sun is expected to disappear fairly close to the horizon. Titan's atmospheric optical depth is the lowest at 5 microns. So, the Sun at 5 microns may even be visible when it is below the horizon due to atmospheric refraction. Similar to images to Martian sunsets from Mars rovers, a fan-like corona is seen to develop above the Sun due to scattering from haze or dust at high-altitudes.

From outer space, Cassini images from near-infrared to UV wavelengths have shown that the twilight periods (sunrise/sunset) are brighter than the daytime on Titan. Scientists expect that planetary brightness will weaken going from the day to night side of the planetary body, known as the terminator. This paradoxical observation has not been observed on any other planetary body with a thick atmosphere. The Titanean twilight outshining the dayside is likely due to a combination of Titan's atmosphere extending hundreds of kilometers above the surface and intense forward Mie scattering from the haze. Radiative transfer models have not reproduced this effect.

Atmospheric evolution

The persistence of a dense atmosphere on Titan has been enigmatic as the atmospheres of the structurally similar satellites of Jupiter, Ganymede and Callisto, are negligible. Although the disparity is still poorly understood, data from recent missions have provided basic constraints on the evolution of Titan's atmosphere.

Layers of atmosphere, image from the Cassini spacecraft

 

Roughly speaking, at the distance of Saturn, solar insolation and solar wind flux are sufficiently low that elements and compounds that are volatile on the terrestrial planets tend to accumulate in all three phases. Titan's surface temperature is also quite low, about 94 K. Consequently, the mass fractions of substances that can become atmospheric constituents are much larger on Titan than on Earth. In fact, current interpretations suggest that only about 50% of Titan's mass is silicates, with the rest consisting primarily of various H₂O (water) ices and NH₃·H₂O (ammonia hydrates). NH₃, which may be the original source of Titan's atmospheric N₂ (dinitrogen), may constitute as much as 8% of the NH₃·H₂O mass. Titan is most likely differentiated into layers, where the liquid water layer beneath ice I_h may be rich in NH₃.

True-color image of layers of haze in Titan's atmosphere

 

Titan's atmosphere backlit by the Sun, with Saturn's rings behind. An outer haze layer merges at top with the northern polar hood.

 

Titan's winter hemisphere (top) is slightly darker in visible light due to a high-altitude haze

Tentative constraints are available, with the current loss mostly due to low gravity and solar wind aided by photolysis. The loss of Titan's early atmosphere can be estimated with the ¹⁴N–¹⁵N isotopic ratio, because the lighter ¹⁴N is preferentially lost from the upper atmosphere under photolysis and heating. Because Titan's original ¹⁴N–¹⁵N ratio is poorly constrained, the early atmosphere may have had more N₂ by factors ranging from 1.5 to 100 with certainty only in the lower factor. Because N₂ is the primary component (98%) of Titan's atmosphere, the isotopic ratio suggests that much of the atmosphere has been lost over geologic time. Nevertheless, atmospheric pressure on its surface remains nearly 1.5 times that of Earth as it began with a proportionally greater volatile budget than Earth or Mars. It is possible that most of the atmospheric loss was within 50 million years of accretion, from a highly energetic escape of light atoms carrying away a large portion of the atmosphere (hydrodynamic escape). Such an event could be driven by heating and photolysis effects of the early Sun's higher output of X-ray and ultraviolet (XUV) photons. 

Because Callisto and Ganymede are structurally similar to Titan, it is unclear why their atmospheres are insignificant relative to Titan's. Nevertheless, the origin of Titan's N₂ via geologically ancient photolysis of accreted and degassed NH₃, as opposed to degassing of N₂ from accretionary clathrates, may be the key to a correct inference. Had N₂ been released from clathrates, ³⁶Ar and ³⁸Ar that are inert primordial isotopes of the Solar System should also be present in the atmosphere, but neither has been detected in significant quantities. The insignificant concentration of ³⁶Ar and ³⁸Ar also indicates that the ~40 K temperature required to trap them and N₂ in clathrates did not exist in the Saturnian sub-nebula. Instead, the temperature may have been higher than 75 K, limiting even the accumulation of NH₃ as hydrates. Temperatures would have been even higher in the Jovian sub-nebula due to the greater gravitational potential energy release, mass, and proximity to the Sun, greatly reducing the NH₃ inventory accreted by Callisto and Ganymede. The resulting N₂ atmospheres may have been too thin to survive the atmospheric erosion effects that Titan has withstood.

An alternative explanation is that cometary impacts release more energy on Callisto and Ganymede than they do at Titan due to the higher gravitational field of Jupiter. That could erode the atmospheres of Callisto and Ganymede, whereas the cometary material would actually build Titan's atmosphere. However, the ²H–¹H (i.e. D–H) ratio of Titan's atmosphere is (2.3±0.5)×10⁻⁴, nearly 1.5 times lower than that of comets. The difference suggests that cometary material is unlikely to be the major contributor to Titan's atmosphere. Titan's atmosphere also contains over a thousand times more methane than carbon monoxide which supports the idea that cometary material is not a likely contributor since comets are composed of more carbon monoxide than methane.

Titan - three dust storms detected in 2009-2010.

Atmospheric science

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Escape_velocity 

 

Atmospheric science is the study of the Earth's atmosphere and its various inner-working physical processes. Meteorology includes atmospheric chemistry and atmospheric physics with a major focus on weather forecasting. Climatology is the study of atmospheric changes (both long and short-term) that define average climates and their change over time, due to both natural and anthropogenic climate variability. Aeronomy is the study of the upper layers of the atmosphere, where dissociation and ionization are important. Atmospheric science has been extended to the field of planetary science and the study of the atmospheres of the planets and natural satellites of the solar system.

Experimental instruments used in atmospheric science include satellites, rocketsondes, radiosondes, weather balloons, and lasers.

The term aerology (from Greek ἀήρ, aēr, "air"; and -λογία, -logia) is sometimes used as an alternative term for the study of Earth's atmosphere; in other definitions, aerology is restricted to the free atmosphere, the region above the planetary boundary layer.

Early pioneers in the field include Léon Teisserenc de Bort and Richard Assmann.

Atmospheric chemistry

Main article: Atmospheric chemistry

Atmospheric chemistry is a branch of atmospheric science in which the chemistry of the Earth's atmosphere and that of other planets is studied. It is a multidisciplinary field of research and draws on environmental chemistry, physics, meteorology, computer modeling, oceanography, geology and volcanology and other disciplines. Research is increasingly connected with other areas of study such as climatology.

The composition and chemistry of the atmosphere is of importance for several reasons, but primarily because of the interactions between the atmosphere and living organisms. The composition of the Earth's atmosphere has been changed by human activity and some of these changes are harmful to human health, crops and ecosystems. Examples of problems which have been addressed by atmospheric chemistry include acid rain, photochemical smog and global warming. Atmospheric chemistry seeks to understand the causes of these problems, and by obtaining a theoretical understanding of them, allow possible solutions to be tested and the effects of changes in government policy evaluated.

Atmospheric dynamics

Atmospheric dynamics is the study of motion systems of meteorological importance, integrating observations at multiple locations and times and theories. Common topics studied include diverse phenomena such as thunderstorms, tornadoes, gravity waves, tropical cyclones, extratropical cyclones, jet streams, and global-scale circulations. The goal of dynamical studies is to explain the observed circulations on the basis of fundamental principles from physics. The objectives of such studies incorporate improving weather forecasting, developing methods for predicting seasonal and interannual climate fluctuations, and understanding the implications of human-induced perturbations (e.g., increased carbon dioxide concentrations or depletion of the ozone layer) on the global climate.

Atmospheric physics

Atmospheric physics is the application of physics to the study of the atmosphere. Atmospheric physicists attempt to model Earth's atmosphere and the atmospheres of the other planets using fluid flow equations, chemical models, radiation balancing, and energy transfer processes in the atmosphere and underlying oceans. In order to model weather systems, atmospheric physicists employ elements of scattering theory, wave propagation models, cloud physics, statistical mechanics and spatial statistics, each of which incorporate high levels of mathematics and physics. Atmospheric physics has close links to meteorology and climatology and also covers the design and construction of instruments for studying the atmosphere and the interpretation of the data they provide, including remote sensing instruments.

In the United Kingdom, atmospheric studies are underpinned by the Meteorological Office. Divisions of the U.S. National Oceanic and Atmospheric Administration (NOAA) oversee research projects and weather modeling involving atmospheric physics. The U.S. National Astronomy and Ionosphere Center also carries out studies of the high atmosphere.

The Earth's magnetic field and the solar wind interact with the atmosphere, creating the ionosphere, Van Allen radiation belts, telluric currents, and radiant energy.

Climatology

Regional impacts of warm ENSO episodes (El Niño).

In contrast to meteorology, which studies short term weather systems lasting up to a few weeks, climatology studies the frequency and trends of those systems. It studies the periodicity of weather events over years to millennia, as well as changes in long-term average weather patterns, in relation to atmospheric conditions. Climatologists, those who practice climatology, study both the nature of climates – local, regional or global – and the natural or human-induced factors that cause climates to change. Climatology considers the past and can help predict future climate change.

Phenomena of climatological interest include the atmospheric boundary layer, circulation patterns, heat transfer (radiative, convective and latent), interactions between the atmosphere and the oceans and land surface (particularly vegetation, land use and topography), and the chemical and physical composition of the atmosphere. Related disciplines include astrophysics, atmospheric physics, chemistry, ecology, physical geography, geology, geophysics, glaciology, hydrology, oceanography, and volcanology.

Atmospheres on other celestial bodies

Earth's atmosphere

All of the Solar System's planets have atmospheres. This is because their gravity is strong enough to keep gaseous particles close to the surface. Larger gas giants are massive enough to keep large amounts of the light gases hydrogen and helium close by, while the smaller planets lose these gases into space. The composition of the Earth's atmosphere is different from the other planets because the various life processes that have transpired on the planet have introduced free molecular oxygen. Much of Mercury's atmosphere has been blasted away by the solar wind. The only moon that has retained a dense atmosphere is Titan. There is a thin atmosphere on Triton, and a trace of an atmosphere on the Moon.

Planetary atmospheres are affected by the varying degrees of energy received from either the Sun or their interiors, leading to the formation of dynamic weather systems such as hurricanes, (on Earth), planet-wide dust storms (on Mars), an Earth-sized anticyclone on Jupiter (called the Great Red Spot), and holes in the atmosphere (on Neptune). At least one extrasolar planet, HD 189733 b, has been claimed to possess such a weather system, similar to the Great Red Spot but twice as large.

Hot Jupiters have been shown to be losing their atmospheres into space due to stellar radiation, much like the tails of comets. These planets may have vast differences in temperature between their day and night sides which produce supersonic winds, although the day and night sides of HD 189733b appear to have very similar temperatures, indicating that planet's atmosphere effectively redistributes the star's energy around the planet.

Outer space

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The interface between the Earth's surface and outer space. The Kármán line at an altitude of 100 km (62 mi) is shown. The layers of the atmosphere are drawn to scale, whereas objects within them, such as the International Space Station, are not.

 

Outer space, or simply space, is the expanse that exists beyond Earth and between celestial bodies. Outer space is not completely empty—it is a hard vacuum containing a low density of particles, predominantly a plasma of hydrogen 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).^[1] 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 matter 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. 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 Earth orbit.

Humans began the physical exploration of space during the 20th century with the advent of high-altitude balloon flights. This was followed by manned rocket flights and, then, manned Earth orbit, first achieved by Yuri Gagarin of the Soviet Union in 1961. Due to the high cost of getting into space, manned spaceflight has been limited to low Earth orbit and the Moon. On the other hand, unmanned 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 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 very high.

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.

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 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.

Environment

Part of the 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.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 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 solar corona reaches temperatures over 1.2–2.6 million K.

Magnetic fields have been detected in the space around just about every class of celestial object. Star formation in spiral galaxies can generate small-scale 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 protons. 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 welding torch.

Effect on biology and human bodies

Because of the hazards of a vacuum, astronauts must wear a pressurized space suit while off-Earth and 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.

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 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 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 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.

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.

Regions

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

Geospace

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 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 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 poles.

Although it meets the definition of outer space, the atmospheric density within the first few hundred kilometers above the Kármán line is still sufficient to produce significant drag 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 periodically.

Cislunar space

Lunar Orbital Station, one of the proposed space stations for crewed 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 space.

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 beyond cislunar space. The International Telecommunication Union responsible for radio communication (including satellites) defines the beginning of deep space at about 5 times that distance (2×10⁶ km).

Interplanetary space

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 maximum.

The volume of interplanetary space is a nearly total vacuum, with a mean free path of about one astronomical unit at the orbital distance of the Earth. This space is not completely empty, and is sparsely filled with cosmic rays, which include ionized atomic 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.

Interstellar space

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 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 the third boundary of an astrosphere after the termination shock and the astropause (called the heliopause in the Solar System).

Intergalactic space

A star-forming region in the Large Magellanic Cloud, perhaps the closest Galaxy to Earth's Milky Way

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).

Earth 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 Earth orbital 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; 25,100 mph).

Boundary

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:

• The 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 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 80 km (50 mi) as astronauts.
• NASA's Space Shuttle used 400,000 feet (122 km, 76 mi) 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 Supra-Thermal 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).

Legal status

2008 launch of the SM-3 missile used to destroy American reconnaissance satellite USA-193

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. 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. The 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 "the Bogotá Declaration", they claimed control of the segment of the geosynchronous orbital path corresponding to each country. These claims are not internationally accepted.

Discovery, exploration and applications

Discovery

In 350 BCE, Greek philosopher Aristotle suggested that nature abhors a vacuum, a principle that became known as the horror vacui. 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 had mass and so was subject to gravity. In 1640, he demonstrated that an established force resisted the formation of a vacuum. It would remain for his pupil Evangelista Torricelli to create an apparatus that would produce a partial vacuum in 1643. This experiment resulted in the first mercury barometer and created a scientific sensation in Europe. The French mathematician Blaise Pascal reasoned that if the column of mercury was supported by air, then the column ought to be shorter at higher altitude where the air pressure is lower. In 1648, his brother-in-law, Florin Périer, repeated the experiment on the Puy de Dôme mountain in central France and found that the column was shorter by three inches. This decrease in pressure was further demonstrated by carrying a half-full balloon up a mountain and watching it gradually expand, then contract upon descent.

The original Magdeburg hemispheres (lower 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.

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. 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.

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 in 1845. It was later popularized in the writings of H. G. Wells in 1901. The shorter term space is older, first used to mean the region beyond Earth's sky in John Milton's Paradise Lost in 1667.

Exploration and application

The first image taken by a human of the whole Earth, probably photographed by William Anders of Apollo 8. South is up; South America is in the middle.

 

For most of human history, space was explored by observations made from the Earth's surface—initially with the unaided eye and then with the telescope. Before reliable rocket technology, the closest that humans had come to reaching outer space was through balloon flights. In 1935, the U.S. Explorer II 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 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, 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 for observation of the Earth. In August 2012, Voyager 1 became the first man-made object to leave the Solar System and enter interstellar space.

The absence of air makes outer space an ideal location for astronomy at all wavelengths of the electromagnetic spectrum. This is evidenced by the spectacular pictures sent back by the Hubble Space Telescope, allowing light from more than 13 billion years ago—almost to the time of the Big Bang—to be observed. Not every location in space is ideal for a telescope. The interplanetary 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.

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. 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: $8,000–$25,000 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.