Outer Space

Outer space, often simply referred to as space, is the vast, largely empty expanse that lies beyond the confines of Earth's atmosphere and stretches between celestial bodies. It is characterized by an almost complete vacuum, a state of incredibly low particle density, where the dominant components are hydrogen and helium plasma. This ethereal realm is further permeated by a symphony of electromagnetic radiation, elusive cosmic rays, ghostly neutrinos, pervasive magnetic fields, and the fine particulate matter known as cosmic dust. The baseline temperature of this cosmic void is dictated by the lingering echo of the Big Bang, a frigid 2.7 kelvins, which translates to a chilling −270 degrees Celsius or −455 degrees Fahrenheit.

Within this seemingly barren expanse, a significant portion of the universe's ordinary, or baryonic, matter resides in the plasma that exists between galaxies. This "warm–hot intergalactic medium" is astonishingly diffuse, containing less than one hydrogen atom per cubic meter, yet it possesses a kinetic temperature soaring into the millions of kelvins. However, even within this vastness, concentrations of matter have coalesced under the relentless pull of gravity, forming the intricate structures we recognize as stars and galaxies. Despite the immense volume occupied by galaxies and star systems, they are themselves almost entirely composed of empty space. The prevailing constituents of the observable universe's mass-energy budget remain enigmatic, attributed to dark matter and dark energy, forces whose nature continues to elude our complete understanding.

The precise demarcation of where Earth's atmosphere yields to the vacuum of outer space is not a sharply defined boundary. Conventionally, the Kármán line, situated at an altitude of 100 kilometers (62 miles) above sea level, serves as the accepted threshold, particularly for the purposes of space treaties and the recording of aerospace achievements. Regions within the upper stratosphere and mesosphere are sometimes colloquially referred to as "near space." The legal framework governing human activity in this domain is largely shaped by the Outer Space Treaty, which entered into force in 1967, asserting that outer space is the province of all humankind and prohibiting national sovereignty claims within it. Despite these accords, the testing of anti-satellite weapons in Earth orbit underscores the complex interplay between international law and geopolitical realities.

The notion that space between Earth and the Moon might be a vacuum emerged in the 17th century, fueled by observations of decreasing air pressure with altitude. The sheer immensity of outer space, however, wasn't truly grasped until the 20th century, when the distance to the Andromeda Galaxy was first accurately measured. Physical exploration of this frontier commenced in earnest later that century, beginning with high-altitude balloon flights and progressing to crewed rocket launches. The historic flight of Yuri Gagarin of the Soviet Union in 1961, who achieved Earth orbit, marked a pivotal moment. The prohibitive cost of spaceflight, however, has largely confined human missions to low Earth orbit and the Moon, while uncrewed probes have ventured to every known planet in the Solar System. The inherent hazards of vacuum and radiation present formidable challenges to human exploration. Furthermore, the physiological effects of microgravity, including muscle atrophy and bone loss, demand significant countermeasures.

Terminology

The informal term "space" to denote the region beyond Earth's sky predates the more formal "outer space." Its earliest recorded use in this context appears in John Milton's epic poem, Paradise Lost, published in 1667. The phrase "outward space" surfaced in a poem by Lady Emmeline Stuart-Wortley in 1842, but it was Alexander von Humboldt who first applied the term "outer space" in an astronomical context in 1845. The phrase gained wider currency through the writings of H. G. Wells after 1901. Theodore von Kármán, a pioneering aerospace engineer, introduced the concept of "free space" to describe altitudes above Earth where atmospheric drag becomes negligible, thereby distinguishing it from airspace and establishing a legal domain beyond national jurisdiction. This definition is now widely recognized as the Kármán line.

The term "spaceborne" signifies existence or operation within outer space, particularly when carried by a spacecraft. Similarly, "space-based" refers to something that is located or operates in outer space or on a celestial body.

Formation and State

The ultimate extent of the universe remains a profound mystery; it may very well be infinite. Current cosmological models, particularly the Big Bang theory, posit that the universe began approximately 13.8 billion years ago in an extraordinarily hot and dense state that underwent rapid expansion. Roughly 380,000 years after this initial inflationary period, the universe cooled sufficiently for protons and electrons to combine, forming hydrogen atoms. This epoch, known as recombination, marked the decoupling of matter and energy, allowing photons to traverse space unimpeded as the universe continued its expansion. The matter that persisted from the initial expansion has since coalesced under gravity, giving rise to stars, galaxies, and other cosmic structures, leaving behind the vast, deep vacuum we now identify as outer space. The finite speed of light inherently limits our ability to observe the entirety of the universe.

The spatial geometry of the observable universe, as revealed by precise measurements of the cosmic microwave background from missions like the Wilkinson Microwave Anisotropy Probe, appears to be "flat". This implies that parallel light paths, once established, remain parallel throughout their journey to the edge of our observable cosmic horizon, barring local gravitational influences. This flatness, coupled with the universe's measured mass density and its accelerating expansion, strongly suggests the existence of a non-zero vacuum energy, commonly referred to as dark energy.

The average energy density of the present-day universe is estimated to be equivalent to about 5.9 protons per cubic meter, encompassing dark energy, dark matter, and baryonic matter. Of this, ordinary matter constitutes a mere 4.6%, translating to roughly one proton per four cubic meters. This density is far from uniform; it is considerably higher within galaxies, particularly in dense structures like planets, stars, and black holes, and significantly lower in the vast voids between them. Unlike matter and dark matter, dark energy does not appear to be concentrated within galaxies. While it comprises the majority of the universe's mass-energy, its influence is considerably weaker than the gravitational forces exerted by matter and dark matter within our own Milky Way galaxy.

Environment

Outer space is the closest approximation to a perfect vacuum that we know of. Its near-total absence of friction allows celestial bodies to glide along their orbits unimpeded. Yet, the vacuum of intergalactic space is not entirely devoid of matter, containing a sparse distribution of hydrogen atoms, typically numbering a few per cubic meter. In stark contrast, the air we breathe contains approximately 10^25 molecules per cubic meter. This extreme low density of matter permits electromagnetic radiation to traverse vast cosmic distances without significant scattering. The mean free path of a photon in intergalactic space stretches to an astounding 10 billion light-years. Nevertheless, the processes of absorption and scattering by dust and gas, collectively known as extinction, play a crucial role in astronomical observations across galactic and intergalactic scales.

Celestial bodies like stars, planets, and moons retain their atmospheres through the force of gravitational attraction. These atmospheric envelopes do not possess a sharp upper boundary; rather, their density gradually diminishes with increasing altitude until they become indistinguishable from the surrounding outer space. For Earth, atmospheric pressure drops to about 0.032 Pascals at an altitude of 100 kilometers (62 miles), a stark contrast to the standard atmospheric pressure at sea level. Above this altitude, the influence of isotropic gas pressure wanes significantly, yielding dominance to the radiation pressure exerted by the Sun and the dynamic pressure of the solar wind. The [thermosphere], in this region, exhibits substantial variations in pressure, temperature, and composition, largely driven by space weather phenomena.

The temperature of outer space is typically quantified by the kinetic energy of its constituent particles, much like temperature is understood on Earth. However, it's crucial to distinguish this kinetic temperature from the temperature of the radiation itself, as these two are not always in thermodynamic equilibrium. The entire observable universe is bathed in photons originating from the Big Bang, forming the cosmic microwave background radiation (CMB). While not directly observed yet, a comparable abundance of neutrinos likely constitutes the cosmic neutrino background. The current black body temperature of the CMB is approximately 2.7 kelvins (−270 °C; −455 °F). The kinetic temperatures of gases in space can vary dramatically. For instance, the Boomerang Nebula registers a frigid 1 kelvin (−272 °C; −458 °F), while the solar corona blazes at over 1.2 million kelvins.

Magnetic fields permeate the space surrounding numerous celestial objects. Within spiral galaxies, the process of star formation can catalyze small-scale dynamos, generating turbulent magnetic fields of roughly 5–10 microgauss. The [Davis–Greenstein effect] is thought to align elongated cosmic dust grains with a galaxy's magnetic field, leading to weak optical polarization, an observation that confirms the presence of ordered magnetic fields in several nearby galaxies. In active galactic nuclei of elliptical galaxies, magneto-hydrodynamic processes are responsible for the formation of their characteristic jets and radio lobes. Evidence of non-thermal radio sources has been detected even in the most distant, high-redshift objects, indicating the pervasive presence of magnetic fields throughout the cosmos.

Beyond the protective embrace of a planetary atmosphere and magnetic field, there are few impediments to the passage of energetic subatomic particles, known as cosmic rays, through space. These particles possess energies spanning from approximately 10^6 electronvolts to an astonishing 10^20 electronvolts for ultra-high-energy cosmic rays. The peak flux of cosmic rays occurs around 10^9 electronvolts, with a composition of roughly 87% protons, 12% helium nuclei, and 1% heavier nuclei. At higher energies, the flux of electrons is significantly lower, constituting only about 1% of that of protons. Cosmic rays pose a considerable threat, capable of damaging delicate electronic components and posing a significant health hazard to space travelers.

Intriguingly, astronauts returning from extravehicular activity in low Earth orbit have reported a distinct, burned, metallic odor clinging to their suits and equipment, often likened to the fumes of arc welding. This peculiar scent is attributed to the presence of atomic oxygen in low Earth orbit. The olfactory experience of space can vary significantly, with different regions potentially harboring unique scents, such as the distinct alcohols detected in molecular clouds.

Human Access and Effects

Effect on Biology and Human Bodies

Despite the extreme conditions, life has demonstrated remarkable resilience, with certain species capable of enduring space environments for extended periods. In 2007, lichen species aboard the ESA's BIOPAN facility survived ten days of direct exposure to space. Seeds of Arabidopsis thaliana and Nicotiana tabacum successfully germinated after 1.5 years in space, and a strain of Bacillus subtilis endured 559 days under simulated Martian conditions and in low Earth orbit.

The hypothesis of lithopanspermia posits that rocks ejected from life-bearing planets could potentially transport microbial life to other habitable worlds. It is theorized that such exchanges may have occurred early in the Solar System's history between Earth, Venus, and Mars. Given the longevity of bacteria, the concept of galactic-scale panspermia remains a theoretical possibility.

Vacuum

The most immediate and perilous threat posed by space to unprotected humans is its lack of pressure. As altitude increases above Earth, atmospheric pressure diminishes. At approximately 19.14 kilometers (11.89 miles), the pressure drops to a level that matches the vapor pressure of water at human body temperature. This critical altitude is known as the Armstrong line. Above this point, exposed bodily fluids, such as saliva, tears, and the fluids within the lungs, would begin to boil away. Consequently, survival at or above the Armstrong line necessitates the use of a pressure suit or a pressurized capsule.

Sudden exposure to the near-vacuum of space, as might occur during a rapid decompression, can inflict pulmonary barotrauma, a potentially fatal rupture of the lungs caused by the immense pressure differential between the inside and outside of the chest. Even with an open airway, the rate of air expulsion may be insufficient to prevent lung damage. Rapid decompression can also rupture eardrums and sinuses, cause bruising and blood seepage in soft tissues, and trigger shock, leading to increased oxygen consumption and hypoxia.

The rapid decrease in pressure causes dissolved oxygen in the blood to escape into the lungs, attempting to equalize the partial pressure gradient. This deoxygenated blood reaching the brain results in a loss of consciousness within seconds and death from hypoxia within minutes. If the pressure drops below 6.3 kilopascals (1 psi), body fluids will begin to boil, a phenomenon known as ebullism. While the resulting steam can cause the body to swell significantly, the elasticity of tissues typically prevents rupture. Ebullism is somewhat mitigated by the containment provided by blood vessels, allowing some blood to remain liquid.

The use of a pressure suit can mitigate swelling and ebullism. The Crew Altitude Protection Suit (CAPS), developed in the 1960s, was designed to prevent ebullism at pressures as low as 2 kilopascals (0.3 psi). Supplemental oxygen is crucial at altitudes above 8 kilometers (5 miles) to ensure adequate breathing and prevent water loss. Above 20 kilometers (12 miles), pressure suits become essential to prevent ebullism. Standard space suits operate at pressures around 30–39 kilopascals (4–6 psi) of pure oxygen, comparable to the partial pressure of oxygen at sea level. While this pressure prevents ebullism, the evaporation of dissolved nitrogen from the blood can still lead to decompression sickness and air embolism if not managed carefully.

Weightlessness and Radiation

Humans, having evolved under Earth's gravity, are susceptible to the detrimental effects of weightlessness. A significant portion of astronauts, over 50%, experience space motion sickness, characterized by nausea, vomiting, vertigo, headaches, and lethargy, typically lasting for a few days. Prolonged exposure to weightlessness leads to muscle atrophy and skeletal deterioration, or spaceflight osteopenia, which can be mitigated through rigorous exercise regimens. Other physiological changes include fluid redistribution, a slowing of the cardiovascular system, reduced red blood cell production, balance disorders, and a weakened immune system. Lesser symptoms can include weight loss, nasal congestion, sleep disturbances, and facial puffiness.

During extended space missions, radiation presents a significant acute health hazard. Exposure to high-energy, ionizing cosmic rays can induce fatigue, nausea, vomiting, compromise the immune system, and alter white blood cell counts. Over longer durations, the risks escalate to include an increased incidence of cancer and damage to the eyes, nervous system, lungs, and the gastrointestinal tract. A hypothetical three-year round-trip mission to Mars could expose an astronaut's cells to a substantial barrage of high-energy nuclei. While the shielding provided by spacecraft walls, water containers, and other barriers can diminish the energy of these particles, their impact generates secondary radiation that also affects the crew. Further research is imperative to fully assess these radiation hazards and develop effective countermeasures.

Boundary

The transition from Earth's atmosphere to outer space is not marked by a distinct physical boundary; rather, atmospheric pressure gradually diminishes with altitude, eventually merging with the solar wind. Various definitions for a practical boundary have been proposed, ranging from 30 km (19 mi) to as far out as 1,600,000 km (990,000 mi). In 2009, measurements from a sounding rocket indicated that at an altitude of 118 km (73.3 mi), ions in the atmosphere were transitioning between the gentle atmospheric winds and the more energetic flows of outer space, which can exceed velocities of 268 m/s (880 ft/s).

High-altitude aircraft, including high-altitude balloons, have reached altitudes of up to 50 km. Until 2021, the United States designated individuals traveling above 50 miles (80 km) as astronauts. However, the criteria for awarding astronaut wings have since been revised to recognize contributions essential to public safety or human spaceflight.

The region between regulated airspace and outer space is termed "near space." While lacking a precise legal definition, it is generally understood to encompass altitudes from 20 to 100 km (12 to 62 mi). For safety reasons, commercial aircraft typically operate below 12 km (7.5 mi), and air traffic control services extend only to about 18 to 20 km (11 to 12 mi). The upper limit of this near-space region coincides with the Kármán line, beyond which astrodynamics takes precedence over [aerodynamics] for flight. This altitude range includes the stratosphere, [mesosphere], and lower [thermosphere] layers of Earth's atmosphere.

Some researchers employ broader definitions for near space, extending it from 18 to 160 km (11 to 99 mi). This range encompasses altitudes where orbital flight in very low Earth orbit becomes feasible. Spacecraft have entered highly elliptical orbits with perigees as low as 80 to 90 km (50 to 56 mi), sustaining multiple orbits. At altitudes around 120 km (75 mi), descending spacecraft begin their atmospheric entry as atmospheric drag becomes a significant factor. For vehicles like NASA's Space Shuttle, this marks the transition from thruster-based steering to maneuvering with aerodynamic control surfaces.

The Kármán line, defined by the Fédération Aéronautique Internationale and recognized internationally by the United Nations, is set at 100 km (62 mi) as a working definition for the boundary between aeronautics and astronautics. This altitude was calculated by Theodore von Kármán as the point where a vehicle would need to exceed orbital velocity to generate sufficient aerodynamic lift to remain airborne. This line delineates the realm of aerodynamics and airspace below it from the domain of astronautics and free space above it.

There is no universally agreed-upon legal altitude limit for national airspace, though the Kármán line is frequently used as a reference. Some argue that setting this limit too high could impede space activities due to concerns over airspace violations. There is also a case to be made for a more flexible approach, with different altitude limits applied based on the specific craft and its intended purpose, especially given the rise of commercial and military sub-orbital spaceflight. Spacecraft have traversed foreign airspace at altitudes as low as 30 km (19 mi), as demonstrated by the Space Shuttle.

Legal Status

The Outer Space Treaty forms the foundational framework for international space law, encompassing the legal use of outer space, the Moon, and other celestial bodies. It declares outer space to be freely accessible for exploration by all nation-states, prohibiting claims of national sovereignty and designating it as the "province of all mankind." This principle of common heritage of mankind has been invoked to uphold equitable access and utilization of outer space, though not without contention. The treaty also prohibits the deployment of nuclear weapons in outer space. Ratified by the United Nations General Assembly in 1963 and signed by the USSR, USA, and UK in 1967, it has since been ratified or acceded to by 105 states.

Since 1958, numerous United Nations resolutions have addressed outer space, with over 50 focusing on international cooperation for peaceful uses and the prevention of an arms race in space. Four additional space law treaties have been developed by the UN's Committee on the Peaceful Uses of Outer Space. However, a legal prohibition against the deployment of conventional weapons in space remains absent, and anti-satellite weapons have been successfully tested by the USA, USSR, China, and India. The 1979 Moon Treaty, which aimed to place the jurisdiction of all celestial bodies under international control, has not been ratified by any nation currently engaged in human spaceflight.

In 1976, eight equatorial nations convened in Bogotá, Colombia, issuing the Bogotá Declaration. This declaration asserted claims over segments of the geosynchronous orbital path corresponding to each nation, claims that lack international recognition.

The increasing proliferation of space debris has emerged as a significant concern for international space law and regulation.

Earth Orbit

Achieving orbit requires a rocket to not only overcome Earth's gravitational force but also to accelerate to a specific orbital speed. Once the engines cease firing, the spacecraft follows a curved trajectory influenced by gravity. In a closed orbit, this path forms an elliptical loop around the planet. Thus, a spacecraft enters Earth orbit when its centripetal acceleration due to gravity is precisely balanced to keep it from flying off into space.

For low Earth orbit, the required speed is approximately 7.8 km/s (17,400 mph), far exceeding the speed of the fastest piloted aircraft. The ultimate speed required to escape Earth's gravity entirely and enter a heliocentric orbit is 11.2 km/s (25,100 mph). The energy required to reach orbital speed at an altitude of 600 km (370 mi) is substantial, six times that needed merely to ascend to that height.

Very low Earth orbit (VLEO), defined as orbits below 450 km (280 mi), is increasingly favored for Earth observation using smaller satellites. Low Earth orbits generally range from 180 to 2,000 km (110 to 1,240 mi) and are commonly used for scientific satellites. Medium Earth orbits extend from 2,000 to 35,780 km (1,240 to 22,230 mi), suitable for navigation and specialized satellites, while high Earth orbits, above 35,780 km (22,230 mi), are utilized for weather and communication satellites.

Spacecraft in orbits with a perigee below approximately 2,000 km (1,200 mi) experience drag from Earth's atmosphere, causing their orbital altitude to decrease. The rate of this orbital decay depends on the satellite's size, mass, and variations in atmospheric density influenced by space weather. At altitudes above 800 km (500 mi), satellites can remain in orbit for centuries. However, below about 300 km (190 mi), decay accelerates, with lifetimes measured in days. At an altitude of 180 km (110 mi), a satellite will typically vaporize within hours upon re-entry into the atmosphere.

Radiation in Earth's orbit is concentrated within the Van Allen radiation belts, which trap energetic particles from solar and galactic sources. This radiation poses a significant threat to both astronauts and space systems, and its intensity can fluctuate dramatically due to space weather. The radiation belts are toroidal regions encircling Earth's equator, dipping closest to the surface in the South Atlantic Anomaly. The inner Van Allen belt reaches its peak intensity at altitudes roughly half an Earth radius above the equator, centered around 3,000 km, and overlaps with the upper reaches of low Earth orbit.

Regions

Near the Earth

The outermost layer of Earth's atmosphere is the [exosphere], extending outward from the [thermopause], which fluctuates between 250 and 500 kilometers (160 to 310 mi) depending on solar radiation. Beyond this altitude, molecular collisions become exceedingly rare, and the atmosphere seamlessly merges with interplanetary space. This region close to Earth is densely populated with Earth-orbiting satellites and has been the subject of extensive study. For organizational purposes, this volume is subdivided into distinct, often overlapping regions.

Near-Earth space encompasses the region extending from low Earth orbits out to geostationary orbits. This volume hosts the primary orbits for artificial satellites and is the locus of most human space activity. It is also burdened by a significant amount of space debris, sometimes termed space pollution, which poses a threat to ongoing operations. Some of this debris periodically re-enters Earth's atmosphere. While technically outer space, the residual atmospheric density in low-Earth orbital space, the first few hundred kilometers above the Kármán line, still exerts a noticeable drag on satellites.

Geospace is a region encompassing Earth's upper atmosphere and its magnetosphere. The Van Allen radiation belts are situated within geospace, and its outer boundary is defined by the [magnetopause], the interface between Earth's magnetosphere and the solar wind. The inner boundary is the ionosphere.

The dynamic and often turbulent conditions of geospace are heavily influenced by the Sun's activity and the solar wind. Geospace research is intrinsically linked to [heliophysics], the study of the Sun and its interactions with the planets. The sunward-facing magnetopause is compressed by solar wind pressure, typically lying about 10 Earth radii from the planet's center. On the opposite side, the solar wind stretches the magnetosphere into a vast [magnetotail] that can extend beyond 100–200 Earth radii. For approximately four days each month, the Moon passes through this magnetotail, temporarily shielded from the solar wind.

Geospace is characterized by electrically charged particles at extremely low densities, their movements governed by Earth's magnetic field. These plasmas can generate electrical currents that flow into Earth's upper atmosphere, driven by solar wind disturbances. [Geomagnetic storms] can disrupt both the radiation belts and the ionosphere, leading to increased fluxes of energetic electrons that can damage satellite electronics, interfere with radio communications, and affect GPS accuracy. Magnetic storms can also pose a hazard to astronauts, even in low Earth orbit, and are responsible for the spectacular [aurorae] observed at high latitudes.

The term XGEO space is used by the United States to describe high Earth orbits, where 'X' represents a multiple of geosynchronous orbit (GEO) at approximately 35,786 km (22,236 mi). For instance, the L2 Earth-Moon Lagrange point at 448,900 km (278,934 mi) is roughly 10.67 XGEO. Translunar space refers to the region encompassing lunar transfer orbits between the Earth and the Moon.

Cislunar space is the region extending outward from Earth, including lunar orbits, the Moon's orbital path around Earth, and the Earth-Moon Lagrange points. The sphere of influence of a celestial body, essentially the region where its gravitational pull is dominant, is often modeled using the Hill sphere. For Earth, this sphere extends to about 1.5 million km (0.93 million mi), or roughly 1% of the Earth-Sun distance. Beyond Earth's Hill sphere, along its orbital path, lies its orbital and co-orbital space. This region is inhabited by groups of co-orbital Near-Earth Objects, such as horseshoe librators and Earth trojans. Some of these objects can temporarily become temporary satellites or [quasi-moons] of Earth.

The United States government defines deep space as all of outer space beyond the typical range of low-Earth orbit, thus including the Moon within this category. Other definitions vary, ranging from "beyond the Moon's orbit" to "beyond the farthest reaches of the Solar System." The International Telecommunication Union defines deep space as distances from Earth equal to or greater than 2 million km (1.2 million mi), which is about five times the lunar distance but considerably less than the distance to any adjacent planet.

Interplanetary Space

Interplanetary space within the Solar System is primarily governed by the Sun's gravity, extending outward beyond the gravitational influence of the individual planets. This region stretches far beyond the orbit of Neptune, reaching the boundary where the heliopause marks the dominance of the Sun's influence over the interstellar environment, typically between 110 and 160 AU. The heliopause acts as a shield, deflecting low-energy galactic cosmic rays, and its distance and strength fluctuate with solar wind activity. The solar wind, a continuous stream of charged particles emanating from the Sun, creates a tenuous atmosphere known as the [heliosphere], extending billions of kilometers into space. This wind typically has a particle density of 5–10 protons per cubic centimeter and travels at speeds of 350–400 km/s.

The interplanetary space is a near-perfect vacuum, with a mean free path of approximately one astronomical unit at Earth's orbital distance. While not entirely empty, it is sparsely populated by cosmic rays, which include ionized atomic nuclei and various subatomic particles. It also contains gas, plasma, dust, small meteors, and a growing catalog of organic molecules detected through microwave spectroscopy. Collectively, this tenuous material is known as the interplanetary medium. A visible manifestation of this dust cloud is the faint band of [zodiacal light] observed at night.

The Sun generates a magnetic field that permeates interplanetary space. Planets like Jupiter, Saturn, Mercury, and Earth possess their own magnetospheres, shaped by the solar wind into a teardrop-like form with a long tail extending away from the Sun. These magnetospheres can trap particles, forming radiation belts like Earth's Van Allen belts. Planets lacking magnetic fields, such as Mars, experience a gradual erosion of their atmospheres by the solar wind.

Interstellar Space

Interstellar space is the physical expanse situated beyond the [astrospheres] of stars, which are vast bubbles of plasma generated by stellar winds. It is the space that lies between stars or [stellar systems] within a galaxy. This region is filled with the interstellar medium, a diffuse mixture of matter and radiation. The boundary separating an astrosphere from interstellar space is termed the [astropause]; for our Sun, these are known as the heliosphere and heliopause, respectively.

Approximately 70% of the mass of the interstellar medium consists of free hydrogen atoms, with helium accounting for most of the remainder. This primordial composition is enriched by trace amounts of heavier elements forged through [stellar nucleosynthesis] and subsequently dispersed by stellar winds or during the shedding of outer envelopes by aging stars, as seen in [planetary nebulae]. The cataclysmic explosion of a [supernova] propagates shock waves outward, scattering stellar ejecta, including heavy elements, throughout the interstellar medium. The density of matter within the interstellar medium is highly variable, averaging around 10^6 particles per cubic meter, but can reach densities of 10^8–10^12 particles per cubic meter within cold molecular clouds.

A diverse array of molecules exists in interstellar space, some of which aggregate to form dust particles as small as 0.1 micrometers. The count of molecules identified through [radio astronomy] is continuously increasing. Dense regions of matter, known as molecular clouds, provide environments conducive to chemical reactions, including the formation of complex organic molecules. These reactions are often initiated by collisions, with energetic cosmic rays ionizing hydrogen and helium, leading to the formation of species like the [trihydrogen cation]. Ionized helium atoms can then react with abundant carbon monoxide to produce ionized carbon, triggering further organic chemical reactions.

The local interstellar medium, a region within 100 [parsecs] of the Sun, is of particular interest due to its proximity and interaction with our Solar System. This volume largely coincides with the [Local Bubble], a cavity characterized by a scarcity of dense, cold clouds within the [Orion Arm] of the Milky Way Galaxy. Denser molecular clouds are found along its periphery. The Local Bubble contains an estimated 10^4–10^5 stars, and the surrounding interstellar gas counterbalances the [stellar-wind bubbles] generated by these stars. Within the bubble, dozens of warm interstellar clouds exist, with temperatures up to 7,000 K and radii of several parsecs.

When stars possess sufficiently high [peculiar velocities], their astrospheres can generate [bow shocks] as they interact with the interstellar medium. For decades, it was assumed that the Sun possessed a bow shock. However, data from the Interstellar Boundary Explorer (IBEX) and NASA's Voyager probes indicated in 2012 that such a shock does not exist. Instead, these probes suggest a subsonic bow wave marks the transition from the solar wind to the interstellar medium. A bow shock is a distinct boundary, situated outside the [termination shock] and the astropause.

Intergalactic Space

Intergalactic space is the vast expanse separating galaxies. Observations of the large-scale distribution of galaxies reveal a cosmic structure resembling a foam, with groups and clusters of galaxies arranged along filaments that occupy approximately one-tenth of the universe's volume. The remaining space consists of immense cosmic voids, largely devoid of galaxies, with typical dimensions spanning tens of megaparsecs.

Interspersed within and connecting these galaxies is the intergalactic medium (IGM). This extremely rarefied plasma is organized into galactic filaments. Within these filaments, diffuse photoionized gas forms denser structures, containing about one atom per cubic meter, which is significantly higher than the average density of the universe. The IGM is believed to be composed primarily of primordial elements, with hydrogen constituting about 76% of its mass, enriched by heavier elements expelled from galaxies.

As gas streams into the intergalactic medium from the voids, it heats up to temperatures ranging from 10^5 K to 10^7 K. At these temperatures, it is referred to as the warm–hot intergalactic medium (WHIM). While considered "warm" in astrophysical terms, these temperatures are considerably hotter than those found on Earth. Simulations and observations suggest that up to half of the universe's atomic matter may exist in this warm, rarefied state. When gas from the WHIM filaments falls into the dense galaxy clusters located at the junctions of cosmic filaments, it can heat up further, reaching temperatures of 10^8 K and above within the intracluster medium (ICM).

History of Discovery

In 350 BCE, the Greek philosopher Aristotle proposed the principle of horror vacui, asserting that nature abhors a vacuum. This idea built upon the earlier ontological arguments of [Parmenides] in the 5th century BCE, who denied the possibility of empty space. Consequently, for many centuries in the Western world, the concept of a vacuum was largely rejected, with prominent thinkers like René Descartes maintaining in the 17th century that space must be entirely filled.

In contrast, ancient Chinese astronomers, such as the 2nd-century Zhang Heng, speculated about an infinite universe extending beyond the celestial sphere, describing the heavens as "empty and void of substance."

The Italian scientist Galileo Galilei recognized that air possesses mass and is therefore subject to gravity. His pupil, Evangelista Torricelli, successfully created an apparatus producing a partial vacuum in 1643, leading to the invention of the mercury barometer and sparking considerable scientific interest. Torricelli also posited that air pressure should decrease with altitude. The French mathematician Blaise Pascal proposed an experiment to verify this, which his brother-in-law, Florin Périer, conducted on the Puy de Dôme mountain in 1648, confirming the pressure decrease.

[Otto von Guericke], a German scientist, constructed the first vacuum pump in 1650, further challenging the notion of horror vacui. He accurately described Earth's atmosphere as a shell surrounding the planet, with density decreasing with altitude, and concluded that a vacuum must exist between Earth and the Moon.

During the 15th century, German theologian Nicolaus Cusanus speculated that the universe lacked a fixed center and circumference, suggesting it was boundless. This philosophical exploration of infinite space was further developed in the 16th century by Italian philosopher Giordano Bruno, who extended the heliocentric model to envision an infinite universe filled with a substance called aether. English philosopher William Gilbert proposed a similar concept of a thin aether or void surrounding celestial bodies. The idea of an aether as the medium for light propagation persisted until the early 20th century, but the [Michelson–Morley experiment] in 1887 yielded a null result, casting doubt on its existence. Albert Einstein's theory of special relativity, which postulates the constancy of the speed of light in a vacuum, ultimately supplanted the aether theory.

The first professional astronomer to advocate for an infinite universe was [Thomas Digges] in 1576. However, the true scale of the universe remained largely unknown until 1838, when German astronomer Friedrich Bessel accurately measured the distance to the star [61 Cygni] using the parallax method, demonstrating it was over 10 light-years away. In 1917, Heber Curtis observed that novae in spiral nebulae were significantly fainter than those within our own galaxy, suggesting these nebulae were distant "island universes." In 1923, American astronomer Edwin Hubble, using Henrietta Leavitt's discovery of cepheid variables, determined the distance to the Andromeda Galaxy, confirming its extragalactic nature. This groundbreaking work led Hubble to formulate the Hubble constant, enabling the first estimations of the universe's age and the size of the observable universe.

The modern understanding of outer space is rooted in the Big Bang cosmology, first proposed by Belgian physicist Georges Lemaître in 1931, which describes the universe originating from an extremely dense state and undergoing continuous expansion.

The earliest estimates of outer space temperature came from Swiss physicist Charles É. Guillaume in 1896, who calculated it to be around 5–6 K based on stellar radiation. Arthur Eddington proposed a similar value of 3.18 K in 1926, and Erich Regener estimated 2.8 K in 1933 based on cosmic ray energy. In 1948, Ralph Alpher and Robert Herman predicted a temperature of 5 K, aligning with the nascent Big Bang theory.

Exploration

For millennia, space exploration was confined to observations from Earth, initially with the naked eye and later with telescopes. Before the advent of reliable rocketry, high-altitude balloon flights represented the closest humans came to reaching outer space. The American balloon [Explorer II] reached 22 km (14 mi) in 1935. This was surpassed in 1942 by a German A-4 rocket reaching approximately 80 km (50 mi). The launch of the uncrewed Sputnik 1 satellite in 1957 by the Soviet Union marked the first artificial object to achieve Earth orbit. Human spaceflight began in 1961 with Yuri Gagarin's orbital mission aboard Vostok 1. The crew of Apollo 8 in 1968, comprising Frank Borman, Jim Lovell, and William Anders, were the first humans to leave low Earth orbit, achieving lunar orbit and reaching a maximum distance of 377,349 km (234,474 mi) from Earth.

The Soviet Luna 1 probe, launched in 1959, was the first spacecraft to achieve escape velocity, performing a fly-by of the Moon. [Venera 1] became the first planetary probe in 1961, providing early evidence of the solar wind. The Mariner 2 mission achieved the first successful fly-by of Venus in 1962, followed by Mariner 4's fly-by of Mars in 1964. Since then, uncrewed spacecraft have explored every planet in the Solar System, along with their moons and numerous minor planets and comets, remaining indispensable tools for space exploration and Earth observation. In August 2012, Voyager 1 became the first human-made object to venture beyond the Solar System and enter interstellar space.

Application

Outer space has become an integral component of global society, offering numerous applications that benefit the economy and advance scientific research. The deployment of artificial satellites in Earth orbit has yielded significant advantages, forming the backbone of the space economy. These satellites facilitate global long-range communications, enable precise satellite navigation, and provide crucial data for [weather satellite] monitoring and remote sensing of Earth. This remote sensing capability supports diverse applications, including agricultural soil moisture tracking, water outflow prediction, plant and tree disease detection, and surveillance of military activities, while also aiding in the discovery and monitoring of climate change influences. The reduced atmospheric drag in space allows satellites to maintain stable orbits and efficiently cover the entire globe, offering advantages over platforms like stratospheric balloons or [high-altitude platform stations].

The absence of atmospheric interference makes outer space an ideal platform for astronomical observations across the entire electromagnetic spectrum. The Hubble Space Telescope, for example, has captured light from over 13 billion years ago, offering glimpses into the universe's early history. However, not all locations in space are optimal for telescopes. The interplanetary zodiacal dust emits infrared radiation that can obscure faint signals from objects like exoplanets. Relocating infrared telescopes beyond this dust cloud enhances their observational capabilities. Similarly, a location on the far side of the Moon, such as the Daedalus crater, could shield a radio telescope from terrestrial radio frequency interference, improving observational clarity.

The extreme vacuum of space presents unique opportunities for certain industrial processes, particularly those requiring ultraclean environments. Similar to asteroid mining, space manufacturing would necessitate substantial financial investment with uncertain short-term returns. A significant factor in the overall cost is the expense of launching payloads into Earth orbit, estimated at $9,000–$ 31,000 per kilogram in 2006, though costs have decreased with advancements like reusable launch systems such as the Falcon 9. Nevertheless, the cost of space access remains a barrier for many industries. Proposed solutions include fully reusable launch systems, non-rocket spacelaunch, momentum exchange tethers, and space elevators.

[Interstellar travel] for human crews remains a theoretical concept. The immense distances to nearby stars necessitate technological advancements and the ability to sustain crews for decades-long journeys. For example, the proposed [Project Daedalus] spacecraft, powered by nuclear fusion, would require 36 years to reach [Alpha Centauri]. Other proposed propulsion systems include light sails, ramjets, and [beam-powered propulsion]. More advanced concepts envision using antimatter as fuel, potentially enabling travel at relativistic velocities.

On Earth, the ultracold temperatures of outer space can be harnessed for renewable cooling technologies through [passive daytime radiative cooling]. This process enhances heat transfer through the atmosphere's infrared window into space, lowering ambient temperatures. Specialized [photonic metamaterials] can further suppress solar heating.