Outer space, or simply space, is the void that exists between celestial bodies, including the Earth.[1] 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. Observations and theory suggest that it also contains dark matter and dark energy. The baseline temperature, as set by the background radiation left over from the Big Bang, is only 3 Kelvin (K); in contrast, temperatures in the coronae of stars can reach over a million Kelvin. Plasma with an extremely low density (less than one hydrogen atom per cubic meter) and high temperature (millions of Kelvin) in the space between galaxies accounts for most of the baryonic (ordinary) matter in outer space; local concentrations have condensed into stars and galaxies. Intergalactic space takes up most of the volume of the Universe, but even galaxies and star systems consist almost entirely of empty space.
There is no firm boundary where space begins. However the Kármán line, at an altitude of 100 km (62 mi) 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 1967. 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 the peaceful uses of outer space have been drafted by the United Nations, but these have not precluded the deployment of weapons into outer space, including the live testing of anti-satellite weapons.
Humans began the physical exploration of space during the twentieth century with the advent of high-altitude balloon flights, followed by the development of single and multi-stage rocket launchers. Earth orbit was achieved by Yuri Gagarin in 1961 and unmanned spacecraft have since reached all of the planets in the Solar System. Achieving orbit requires a minimum velocity of 28,400 km/h (17,600 mph); much faster than any conventional aircraft. Outer space represents a challenging environment for human exploration because of the dual hazards of vacuum and radiation. Microgravity has a deleterious effect on human physiology, resulting in muscle atrophy and bone loss. As of yet space travel has been limited to low Earth orbit and the Moon for manned flight, and the vicinity of the Solar System for unmanned; the remainder of outer space remains inaccessible to humans other than by passive observation with telescopes.
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.[2] 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.[3] As late as the 17th century, the French philosopher René Descartes argued that the entirety of space must be filled.[4]
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".[5]
The Italian scientist Galileo Galilei knew that air had mass and so was subject to gravity. In 1640, he demonstrated that an established force resisted the formation of a vacuum. However, it would remain for his pupil Evangelista Torricelli to create an apparatus that would produce a 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.[6] 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.[7]
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.[9] 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.[10] 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.[11] 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.[12]
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.[13] 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.[14][15]
The first professional astronomer to support the concept of an infinite Universe was the Englishman Thomas Digges in 1576.[16] 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.[17] 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.[18] This established that the Andromeda galaxy, and by extension all galaxies, lay well outside the Milky Way.[19]
The modern concept of outer space is based on the "Big Bang" cosmology, first proposed in 1931 by the Belgian physicist Georges Lemaître.[20] 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.[21] 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 outer space was used as early as 1842 by the English poet Lady Emmeline Stuart-Wortley in her poem "The Maiden of Moscow".[22] The expression outer space was used as an astronomical term by Alexander von Humboldt in 1845.[23] It was later popularized in the writings of H. G. Wells in 1901.[24] 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.[25]
Environment
Stars, planets and moons retain their atmospheres by gravitational attraction. Atmospheres have no clearly delineated boundary: the density of atmospheric gas gradually decreases with distance from the object until it becomes indistinguishable from the surrounding environment.[30] The Earth's atmospheric pressure drops to about 3.2 × 10−2 Pa at 100 kilometres (62 miles) of altitude,[31] compared to 100 kPA for the International Union of Pure and Applied Chemistry (IUPAC) definition of standard pressure. Beyond 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.[32]
On the Earth, temperature is defined in terms of the kinetic activity of the surrounding atmosphere. However the temperature of the vacuum cannot be measured in this way. Instead, the temperature is determined by measurement of the radiation. 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).[33] Some regions of outer space can contain highly energetic particles that have a much higher temperature than the CMB, such as the corona of the Sun where temperatures can range over 1.2–2.6 MK.[34]
Outside of 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.[35] 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.[36] Cosmic rays can damage electronic components and pose a health threat to space travelers.[37]
Effect on human bodies
See also: Space exposure and Weightlessness
As a consequence of rapid decompression, any oxygen dissolved in the blood would empty into the lungs to try to equalize the partial pressure gradient. Once the deoxygenated blood arrived at the brain, humans and animals will lose consciousness after a few seconds and die of hypoxia within minutes.[41] Blood and other body fluids boil when the pressure drops below 6.3 kPa, and this condition is called ebullism.[42] 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.[43][44] Swelling and ebullism can be reduced by containment in a flight suit. Shuttle astronauts wear a fitted elastic garment called the Crew Altitude Protection Suit (CAPS) which prevents ebullism at pressures as low as 2 kPa.[45] Space suits are needed at 8 km (5.0 mi) to provide enough oxygen for breathing and to prevent water loss, while above 20 km (12 mi) they are essential to prevent ebullism.[46] 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 blood could still cause decompression sickness and gas embolisms if not managed.[47]
Because humans are optimized for life in Earth gravity, exposure to weightlessness has been shown to have deleterious effects on the 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.[48] 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.[49]
For long duration space travel, radiation can pose an acute health hazard. Exposure to radiation sources such as 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 increase in the risk of cancer, plus damage to the eyes, nervous system, lungs and the gastrointestinal tract.[50] On a round-trip Mars mission lasting three years, nearly the entire body would be traversed by high energy nuclei, each of which can cause ionization damage to cells. Fortunately, most such particles are significantly attenuated by the shielding provided by the aluminum walls of a spacecraft, and can be further diminished by water containers and other barriers. However, the impact of the cosmic rays upon the shielding produces additional radiation that can affect the crew. Further research will be needed to assess the radiation hazards and determine suitable countermeasures.[51]
Boundary
- 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 roughly 100 km (62 mi), as Theodore von Kármán calculated, a vehicle would have to travel faster than orbital velocity in order to derive sufficient aerodynamic lift from the atmosphere to support itself.[52]
- The United States designates people who travel above an altitude of 50 miles (80 km) as astronauts.[53]
- NASA's mission control uses 76 mi (122 km) as their re-entry altitude (termed the Entry Interface), which roughly marks the boundary where atmospheric drag becomes noticeable (depending on the ballistic coefficient of the vehicle), thus leading shuttles to switch from steering with thrusters to maneuvering with air surfaces.[54]
Legal status
Main article: Space law
Beginning in 1958, outer space has been the subject of multiple resolutions by the United Nations General Assembly. 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.[58] 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 and China.[59] The 1979 Moon Treaty turned the jurisdiction of all heavenly bodies (including the orbits around such bodies) over to the international community. However, this treaty has not been ratified by any nation that currently practices manned spaceflight.[60]
In 1976 eight equatorial states (Ecuador, Colombia, Brazil, Congo, Zaire, Uganda, Kenya, and Indonesia) met in Bogotá, Colombia. They made the "Declaration of the First Meeting of Equatorial Countries," also known as "the Bogotá Declaration", where they made a claim to control the segment of the geosyncronous orbital path corresponding to each country.[61] These claims are not internationally accepted.[62]
Space versus orbit
A spacecraft enters orbit when it has enough horizontal velocity for its centripetal acceleration due to gravity to be 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 7,900 m/s (28,400 km/h; 17,700 mph);[63] by contrast, the fastest 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.[64]
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.[65] Spacecraft with a perigee below about 2,000 km (1,200 mi) are subject to drag from the Earth's atmospheric, which will cause the orbital altitude to decrease. 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 will start to burn up in the atmosphere.[66] 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).[67]
Earth's gravity reaches out far past the Van Allen radiation belt and keeps the Moon in orbit at an average distance of 384,403 km (238,857 mi). The region of space where the gravity of a planet tends to dominate the motion of objects in the presence of other perturbing bodies (such as another planet) is known as the Hill sphere. For Earth, this sphere has a radius of about 1,500,000 km (930,000 mi).[68]
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
The volume of geospace defined by the magnetopause is compacted in the direction of the Sun by the pressure of the solar wind, giving it a typical subsolar distance of 10 Earth radii from the center of the planet. However, the tail can extend outward to more than 100–200 Earth radii.[72] The Van Allen radiation belt lies within the geospace. The region between Earth's atmosphere and the Moon is sometimes referred to as cis-lunar space. The Moon passes through geospace roughly four days each month, during which time the surface is shielded from the solar wind.[73]
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. During geomagnetic storms two regions of geospace, the radiation belts and the ionosphere, can become strongly disturbed. These storms increase fluxes of energetic electrons that can permanently damage satellite electronics, disrupting telecommunications and GPS technologies, and can also be a hazard to astronauts, even in low Earth orbit. They also create aurorae seen near the magnetic poles.[74]
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.[66] 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.[75]
Interplanetary
The volume of interplanetary space is a nearly total vacuum, with a mean free path of about one astronomical unit at the orbital distance of the Earth. However, this space is not completely empty, and is sparsely filled with cosmic rays, which include ionized atomic nuclei and various subatomic particles. There is also gas, plasma and dust, small meteors, and several dozen types of organic molecules discovered to date by microwave spectroscopy.[79]
Interplanetary space contains the magnetic field generated by the Sun.[76] 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 magnetic particles such as the Van Allen radiation belt. Planets without magnetic fields, such as Mars, have their atmospheres gradually eroded by the solar wind.[80]
Interstellar
Main article: Interstellar medium
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.[82]
When stars are moving at a sufficiently high peculiar velocity, their astrosphere can generate a bow shock as it collides with the interstellar medium. For decades it was assumed that the Sun had a bow shock. In 2012, data from IBEX and Voyagers 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.[83][84] A bow shock is the third boundary of an astrosphere after the termination shock and the astropause (called the heliopause in the Solar System).[84]
Intergalactic
Intergalactic space is the physical space between galaxies. The huge spaces between galaxy clusters are called the voids. Present estimates put the average energy density of the Universe at the equivalent of 5.9 protons per cubic meter, including dark energy, dark matter, and ordinary, baryonic matter, or atoms. The atoms account for only 4.6% of the total energy density, or a density of one proton per four cubic meters.[85] The density of the universe, however, is clearly not uniform; it ranges from relatively high density in galaxies—including very high density in structures within galaxies, such as planets, stars, and black holes—to conditions in vast voids that have much lower density, at least in terms of visible matter.[86]
Surrounding and stretching between galaxies, there is a rarefied plasma[87] that is organized in a cosmic filamentary structure.[88] This material is called the intergalactic medium (IGM). The density of the IGM is 5-200 times the average density of the Universe.[89] 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,[90] 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 gas 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.[89][91][92] 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.[93]
Exploration and applications
In order to explore the other planets, a spacecraft must first reach escape velocity, which will allow it to travel beyond Earth orbit. The first spacecraft to accomplish this feat was the Soviet Luna 1, which performed a fly-by of the Moon in 1959.[98] In 1961, Venera 1 became the first planetary probe. It revealed the presence of the solar wind and performed the first fly-by of the planet Venus, although contact was lost before reaching Venus. The first successful planetary mission was the Mariner 2 fly-by of Venus in 1962.[99] The first spacecraft to perform a fly-by of Mars was Mariner 4, which reached the planet in 1964. Since that time, unmanned spacecraft have successfully examined each of the Solar System's planets, as well their moons and many minor planets and comets. They remain a fundamental tool for the exploration of outer space, as well as observation of the Earth.[100]
The absence of air makes outer space (and the surface of the Moon) ideal locations for astronomy at all wavelengths of the electromagnetic spectrum, as evidenced by the spectacular pictures sent back by the Hubble Space Telescope, allowing light from about 13.7 billion years ago—almost to the time of the Big Bang—to be observed. However, not every location in space is ideal for a telescope. The interplanetary zodiacal dust emits a diffuse near-infrared radiation that can mask the emission of faint sources such as extrasolar planets. Moving an infrared telescope out past the dust will increase the effectiveness of the instrument.[101] 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.[102]
The deep vacuum of space could make it an attractive environment for certain industrial processes, such as those that require ultraclean surfaces.[103]
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