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Being essentially empty, outer space allows the earliest (redder) galaxies to be viewed without obstruction, as in the Webb's First Deep Field image.

Outer space, or simply space, is the expanse that exists beyond Earth's atmosphere and between celestial bodies.[1] It contains ultra-low levels of particle densities, constituting a near-perfect vacuum[2] of predominantly hydrogen and helium plasma, permeated by electromagnetic radiation, cosmic rays, neutrinos, magnetic fields and dust. The baseline temperature of outer space, as set by the background radiation from the Big Bang, is 2.7 kelvins (−270 °C; −455 °F).[3]

The plasma between galaxies is thought to account for about half of the baryonic (ordinary) matter in the universe, having a number density of less than one hydrogen atom per cubic metre and a kinetic temperature of millions of kelvins.[4] Local concentrations of matter 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. Most of the remaining mass-energy in the observable universe is made up of an unknown form, dubbed dark matter and dark energy.[5][6][7]

Outer space does not begin at a definite altitude above Earth's surface. The Kármán line, an altitude of 100 km (62 mi) above sea level,[8][9] is conventionally used as the start of outer space in space treaties and for aerospace records keeping. Certain portions of the upper stratosphere and the mesosphere are sometimes referred to as "near space". The framework for international space law was established by the Outer Space Treaty, which entered into force on 10 October 1967. This treaty precludes any claims of national sovereignty and permits all states to freely explore outer space. Despite the drafting of UN resolutions for the peaceful uses of outer space, anti-satellite weapons have been tested in Earth orbit.

The concept that the space between the Earth and the Moon must be a vacuum was first proposed in the 17th century after scientists discovered that air pressure decreased with altitude. The immense scale of outer space was grasped in the 20th century when the distance to the Andromeda Galaxy was first measured. Humans began the physical exploration of space later in the same century with the advent of high-altitude balloon flights. This was followed by crewed rocket flights and, then, crewed Earth orbit, first achieved by Yuri Gagarin of the Soviet Union in 1961. The economic cost of putting objects, including humans, into space is very high, limiting human spaceflight to low Earth orbit and the Moon. On the other hand, uncrewed spacecraft have reached all of the known planets in the Solar System. Outer space represents a challenging environment for human exploration because of the hazards of vacuum and radiation. Microgravity has a negative effect on human physiology that causes both muscle atrophy and bone loss.

Terminology

[edit]

The use of the short version space, as meaning "the region beyond Earth's sky", predates the use of full term "outer space", with the earliest recorded use of this meaning in an epic poem by John Milton called Paradise Lost, published in 1667.[10][11]

The term outward space existed in a poem from 1842 by the English poet Lady Emmeline Stuart-Wortley called "The Maiden of Moscow",[12] but in astronomy the term outer space found its application for the first time in 1845 by Alexander von Humboldt.[13] The term was eventually popularized through the writings of H. G. Wells after 1901.[14] Theodore von Kármán used the term of free space to name the space of altitudes above Earth where spacecrafts reach conditions sufficiently free from atmospheric drag, differentiating it from airspace, identifying a legal space above territories free from the sovereign jurisdiction of countries. This definition of the boundary to outer space became known as the Kármán line.[15]

"Spaceborne" denotes existing in outer space, especially if carried by a spacecraft;[16][17] similarly, "space-based" means based in outer space or on a planet or moon.[18]

Formation and state

[edit]
Main article: Big Bang
An artist's concept of the expanding universe opening up from the viewer's left, facing the viewer in a 3/4 pose.
Timeline of the expansion of the universe, where space is represented schematically at each time by circular sections. On the left, the dramatic expansion of inflation; at the center, the expansion accelerates (artist's concept; neither time nor size are to scale)

The size of the whole universe is unknown, and it might be infinite in extent.[19] According to the Big Bang theory, the very early universe was an extremely hot and dense state about 13.8 billion years ago[20] 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.[21] 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.[22] As light has a finite velocity, this theory constrains the size of the directly observable universe.[21]

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.[23] 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.[24]

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.[25] 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.[26] 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.[27]

Environment

[edit]
Main articles: Space environment, Space weather, and Space weathering
A wide field view of outer space as seen from Earth's surface at night. The interplanetary dust cloud is visible as the horizontal band of zodiacal light, including the false dawn[28] (edges) and gegenschein (center), which is visually crossed by the Milky Way

Outer space is the closest known approximation to a perfect vacuum. It has effectively no friction, allowing stars, planets, and moons to move freely along their orbits. The deep vacuum of intergalactic space is not devoid of matter, as it contains a few hydrogen atoms per cubic meter.[29] By comparison, the air humans breathe contains about 1025 molecules per cubic meter.[30][31] 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.[32] 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.[33]

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.[34] The Earth's atmospheric pressure drops to about 0.032 Pa at 100 kilometres (62 miles) of altitude,[35] 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.[36]

The temperature of outer space is measured in terms of the kinetic activity of the gas,[37] 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.[38][39] 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.[40]) The current black body temperature of the background radiation is about 2.7 K (−270 °C; −455 °F).[41] The gas temperatures in outer space can vary widely. For example, the temperature in the Boomerang Nebula is 1 K (−272 °C; −458 °F),[42] while the solar corona reaches temperatures over 1,200,000–2,600,000 K (2,200,000–4,700,000 °F).[43]

Magnetic fields have been detected in the space around many classes of celestial objects. Star formation in spiral galaxies can generate small-scale dynamos, creating turbulent magnetic field strengths of around 5–10 μG. The Davis–Greenstein effect causes elongated dust grains to align themselves with a galaxy's magnetic field, resulting in weak optical polarization. This has been used to show ordered magnetic fields that exist in several nearby galaxies. Magneto-hydrodynamic processes in 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.[44]

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.[45] 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.[46] Cosmic rays can damage electronic components and pose a health threat to space travelers.[47]

Scents retained from low Earth orbit, when returning from extravehicular activity, have a burned, metallic odor, similar to the scent of arc welding fumes. This results from oxygen in low Earth orbit, which clings to suits and equipment.[48][49][50] Other regions of space could have very different odors, like that of different alcohols in molecular clouds.[51]

Human access

[edit]

Effect on biology and human bodies

[edit]
Main articles: Effect of spaceflight on the human body, Space medicine, and Bioastronautics
See also: Astrobiology, Astrobotany, Plants in space, and Animals in space
The lower half shows a blue planet with patchy white clouds. The upper half has a man in a white spacesuit and maneuvering unit against a black background.
Because of the hazards of a vacuum, astronauts must wear a pressurized space suit while outside their spacecraft.

Despite the harsh environment, several life forms have been found that can withstand extreme space conditions for extended periods. Species of lichen carried on the ESA BIOPAN facility survived exposure for ten days in 2007.[52] Seeds of Arabidopsis thaliana and Nicotiana tabacum germinated after being exposed to space for 1.5 years.[53] A strain of Bacillus subtilis has survived 559 days when exposed to low Earth orbit or a simulated Martian environment.[54]

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.[55] Because bacteria can survive for millions of years, it is at least theoretically possible for Galactic-scale panspermia to occur.[56]

Vacuum

[edit]
Main article: Uncontrolled decompression

The lack of pressure in space is the most immediate dangerous characteristic of space to humans. Pressure decreases above Earth, reaching a level at an altitude of around 19.14 km (11.89 mi) that matches the vapor pressure of water at the temperature of the human body. This pressure level is called the Armstrong line, named after American physician Harry G. Armstrong.[57] 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.[58]

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.[59] Even if the subject's airway is fully open, the flow of air through the windpipe may be too slow to prevent the rupture.[60] 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.[61]

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.[62] Blood and other body fluids boil when the pressure drops below 6.3 kilopascals (1 psi), and this condition is called ebullism.[63] 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.[64][65]

Swelling and ebullism can be reduced by containment in a pressure suit. The Crew Altitude Protection Suit (CAPS), a fitted elastic garment designed in the 1960s for astronauts, prevents ebullism at pressures as low as 2 kilopascals (0.3 psi).[66] 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.[67] Most space suits use around 30–39 kilopascals (4–6 psi) of pure oxygen, about the same as the partial pressure of oxygen at the Earth's surface. This pressure is high enough to prevent ebullism, but evaporation of nitrogen dissolved in the blood could still cause decompression sickness and gas embolisms if not managed.[68]

Weightlessness and radiation

[edit]
Main articles: Weightlessness and Radiobiology

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.[69] 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.[70]

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.[71] 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.[72] 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.[73]

Boundary

[edit]
For the furthest reaches of space, see observable universe.
Illustration of Earth's atmosphere gradual transition into outer space

The transition between Earth's atmosphere and outer space lacks a well-defined physical boundary, with the air pressure steadily decreasing with altitude until it mixes with the solar wind. Various definitions for a practical boundary have been proposed, ranging from 30 km (19 mi) out to 1,600,000 km (990,000 mi).[15] In 2009, measurements of the direction and speed of ions in the atmosphere were made from a sounding rocket. The altitude of 118 km (73.3 mi) above Earth was the midpoint for charged particles transitioning from the gentle winds of the Earth's atmosphere to the more extreme flows of outer space. The latter can reach velocities well over 268 m/s (880 ft/s).[74][75]

High-altitude aircraft, such as high-altitude balloons have reached altitudes above Earth of up to 50 km.[76] Up until 2021, the United States designated people who travel above an altitude of 50 mi (80 km) as astronauts.[77] Astronaut wings are now only awarded to spacecraft crew members that "demonstrated activities during flight that were essential to public safety, or contributed to human space flight safety".[78]

The region between airspace and outer space is termed "near space". There is no legal definition for this extent, but typically this is the altitude range from 20 to 100 km (12 to 62 mi).[79] For safety reasons, commercial aircraft are typically limited to altitudes of 12 km (7.5 mi), and air navigation services only extend to 18 to 20 km (11 to 12 mi).[79] The upper limit of the range is the Kármán line, where astrodynamics must take over from aerodynamics in order to achieve flight.[80] This range includes the stratosphere, mesosphere and lower thermosphere layers of the Earth's atmosphere.[81]

Larger ranges for near space are used by some authors, such as 18 to 160 km (11 to 99 mi).[82] These extend to the altitudes where orbital flight in very low Earth orbits becomes practical.[82] Spacecraft have entered into a highly elliptical orbit with a perigee as low as 80 to 90 km (50 to 56 mi), surviving for multiple orbits.[83] At an altitude of 120 km (75 mi),[83] descending spacecraft begin atmospheric entry as atmospheric drag becomes noticeable. For spaceplanes such as NASA's Space Shuttle, this begins the process of switching from steering with thrusters to maneuvering with aerodynamic control surfaces.[84]

The Kármán line, established by the Fédération Aéronautique Internationale, and used internationally by the United Nations,[15] is set at an altitude of 100 km (62 mi) as a working definition for the boundary between aeronautics and astronautics. This line is named after Theodore von Kármán, who argued for an altitude where a vehicle would have to travel faster than orbital velocity to derive sufficient aerodynamic lift from the atmosphere to support itself,[8][9] which he calculated to be at an altitude of about 83.8 km (52.1 mi).[76] This distinguishes altitudes below as the region of aerodynamics and airspace, and above as the space of astronautics and free space.[15]

There is no internationally recognized legal altitude limit on national airspace, although the Kármán line is the most frequently used for this purpose. Objections have been made to setting this limit too high, as it could inhibit space activities due to concerns about airspace violations.[83] It has been argued for setting no specified singular altitude in international law, instead applying different limits depending on the case, in particular based on the craft and its purpose. Increased commercial and military sub-orbital spaceflight has raised the issue of where to apply laws of airspace and outer space.[82][80] Spacecraft have flown over foreign countries as low as 30 km (19 mi), as in the example of the Space Shuttle.[76]

[edit]
Main article: Space law
Conventional anti-satellite weapons such as the SM-3 missile remain legal under the law of armed conflict, even though they create hazardous space debris

The Outer Space Treaty provides the basic framework for international space law. It covers the legal use of outer space by nation states, and includes in its definition of outer space, the Moon, and other celestial bodies. The treaty states that outer space is free for all nation states to explore and is not subject to claims of national sovereignty, calling outer space the "province of all mankind". This status as a common heritage of mankind has been used, though not without opposition, to enforce the right to access and shared use of outer space for all nations equally, particularly non-spacefaring nations.[85] It prohibits the deployment of nuclear weapons in outer space. The treaty was passed by the United Nations General Assembly in 1963 and signed in 1967 by the Union of Soviet Socialist Republics (USSR), the United States of America (USA), and the United Kingdom (UK). As of 2017, 105 state parties have either ratified or acceded to the treaty. An additional 25 states signed the treaty, without ratifying it.[86][87]

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.[88] Four additional space law treaties have been negotiated and drafted by the UN's Committee on the Peaceful Uses of Outer Space. Still, there remains no legal prohibition against deploying conventional weapons in space, and anti-satellite weapons have been successfully tested by the USA, USSR, China,[89] and in 2019, India.[90] The 1979 Moon Treaty turned the jurisdiction of all heavenly bodies (including the orbits around such bodies) over to the international community. The treaty has not been ratified by any nation that currently practices human spaceflight.[91]

In 1976, eight equatorial states (Ecuador, Colombia, Brazil, The Republic of the Congo, Zaire, Uganda, Kenya, and Indonesia) met in Bogotá, Colombia: with their "Declaration of the First Meeting of Equatorial Countries", or the Bogotá Declaration, they claimed control of the segment of the geosynchronous orbital path corresponding to each country.[92] These claims are not internationally accepted.[93]

An increasing issue of international space law and regulation has been the dangers of the growing number of space debris.[94]

Earth orbit

[edit]
Main articles: Geocentric orbit and Orbital decay
Newton's cannonball, an illustration of how objects can "fall" in a curve around the planet

When a rocket is launched to achieve orbit, its thrust must both counter gravity and accelerate it to orbital speed. After the rocket terminates its thrust, it follows an arc-like trajectory back toward the ground under the influence of the Earth's gravitational force. In a closed orbit, this arc will turn into an elliptical loop around the planet. That is, a spacecraft successfully enters Earth orbit when its acceleration due to gravity pulls the craft down just enough to prevent its momentum from carrying it off into outer space.[95]

For a low Earth orbit, orbital speed is about 7.8 km/s (17,400 mph);[96] by contrast, the fastest piloted airplane speed ever achieved (excluding speeds achieved by deorbiting spacecraft) was 2.2 km/s (4,900 mph) in 1967 by the North American X-15.[97] The upper limit of orbital speed at 11.2 km/s (25,100 mph) is the velocity required to pull free from Earth altogether and enter into a heliocentric orbit.[98] The energy required to reach Earth orbital speed at an altitude of 600 km (370 mi) is about 36 MJ/kg, which is six times the energy needed merely to climb to the corresponding altitude.[99]

Very low Earth orbit (VLEO) has been defined as orbits that have a mean altitude below 450 km (280 mi), which can be better suited for Earth observation with small satellites.[100] Low Earth orbits in general range in altitude from 180 to 2,000 km (110 to 1,240 mi) and are used for scientific satellites. Medium Earth orbits extends from 2,000 to 35,780 km (1,240 to 22,230 mi), which are favorable orbits for navigation and specialized satellites. Above 35,780 km (22,230 mi) are the high Earth orbits used for weather and some communication satellites.[101]

Spacecraft in orbit with a perigee below about 2,000 km (1,200 mi) (low Earth orbit) are subject to drag from the Earth's atmosphere,[102] which decreases the orbital altitude. The rate of orbital decay depends on the satellite's cross-sectional area and mass, as well as variations in the air density of the upper atmosphere, which is significantly effected by space weather.[103] At altitudes above 800 km (500 mi), orbital lifetime is measured in centuries.[104] 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.[105]

Radiation in orbit around Earth is concentrated in Van Allen radiation belts, which trap solar and galactic radiation. Radiation is a threat to astronauts and space systems. It is difficult to shield against and space weather makes the radiation environment variable. The radiation belts are equatorial toroidal regions, which are bent towards Earth's poles, with the South Atlantic Anomaly being the region where charged particles approach Earth closest.[106][107] The innermost radiation belt, the inner Van Allen belt, has its intensity peak at altitudes above the equator of half an Earth radius,[108] centered at about 3000 km,[109] increasing from the upper edge of low Earth orbit which it overlaps.[110][111][112]

Regions

[edit]
See also: Location of Earth

Regions near the Earth

[edit]

The outermost layer of the Earth's atmosphere is termed the exosphere. It extends outward from the thermopause, which lies at an altitude that varies from 250 to 500 kilometres (160 to 310 mi), depending on the incidence of solar radiation. Beyond this altitude, collisions between molecules are negligible and the atmosphere joins with interplanetary space.[113] The region in proximity to the Earth is home to a multitude of Earth–orbiting satellites and has been subject to extensive studies. For identification purposes, this volume is divided into overlapping regions of space.[114][115][116][117]

Near-Earth space is the region of space extending from low Earth orbits out to geostationary orbits.[114] This region includes the major orbits for artificial satellites and is the site of most of humanity's space activity. The region has seen high levels of space debris, sometimes dubbed space pollution, threatening nearby space activity.[114] Some of this debris re-enters Earth's atmosphere periodically.[118] Although it meets the definition of outer space, the atmospheric density inside low-Earth orbital space, the first few hundred kilometers above the Kármán line, is still sufficient to produce significant drag on satellites.[105]

A computer-generated map of objects orbiting Earth, as of 2005. About 95% are debris, not working artificial satellites[119]

Geospace is a region of space that includes Earth's upper atmosphere and magnetosphere.[115] 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.[120][121]

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.[122] 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.[123][124] For roughly four days of each month, the lunar surface is shielded from the solar wind as the Moon passes through the magnetotail.[125]

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.[126] Magnetic storms can be a hazard to astronauts, even in low Earth orbit. They create aurorae seen at high latitudes in an oval surrounding the geomagnetic poles.[127]

Earth and the Moon as seen from cislunar space on the 2022 Artemis 1 mission

XGEO space is a concept used by the USA to refer to the space of high Earth orbits, with the 'X' being some multiple of geosynchronous orbit (GEO) at approximately 35,786 km (22,236 mi).[116] Hence, the L2 Earth-Moon Lagrange point at 448,900 km (278,934 mi) is approximately 10.67 XGEO.[128] Translunar space is the region of lunar transfer orbits, between the Moon and Earth.[129] Cislunar space is a region outside of Earth that includes lunar orbits, the Moon's orbital space around Earth and the Earth-Moon Lagrange points.[117]

The region where a body's gravitational potential remains dominant against gravitational potentials from other bodies, is the body's sphere of influence or gravity well, mostly described with the Hill sphere model.[130] In the case of Earth this includes all space from the Earth to a distance of roughly 1% of the mean distance from Earth to the Sun,[131] or 1.5 million km (0.93 million mi). Beyond Earth's Hill sphere extends along Earth's orbital path its orbital and co-orbital space. This space is co-populated by groups of co-orbital Near-Earth Objects (NEOs), such as horseshoe librators and Earth trojans, with some NEOs at times becoming temporary satellites and quasi-moons to Earth.[132]

Deep space is defined by the United States government as all of outer space which lies further from Earth than a typical low-Earth-orbit, thus assigning the Moon to deep-space.[133] Other definitions vary the starting point of deep-space from, "That which lies beyond the orbit of the moon," to "That which lies beyond the farthest reaches of the Solar System itself."[134][135][136] The International Telecommunication Union responsible for radio communication, including with satellites, defines deep-space as, "distances from the Earth equal to, or greater than, 2 million km (1.2 million mi),"[137] which is about five times the Moon's orbital distance, but which distance is also far less than the distance between Earth and any adjacent planet.[138]

Near-Earth space showing the low-Earth (blue), medium Earth (green), and high Earth (red) orbits. The last extends beyond the radius of geosynchronous orbits

Interplanetary space

[edit]
Main article: Interplanetary medium
At lower left, a white coma stands out against a black background. Nebulous material streams away to the top and left, slowly fading with distance.
The sparse plasma (blue) and dust (white) in the tail of comet Hale–Bopp are being shaped by pressure from solar radiation and the solar wind, respectively.

Interplanetary space within the Solar System is dominated by the gravitation of the Sun, outside the gravitational spheres of influence of the planets.[139] Interplanetary space extends well beyond the orbit of the outermost planet Neptune, all the way out to where the influence of the galactic environment starts to dominate over the Sun and its solar wind producing the heliopause at 110 to 160 AU.[140] The heliopause deflects away low-energy galactic cosmic rays, and its distance and strength varies depending on the activity level of the solar wind.[141][142] The solar wind is a continuous stream of charged particles emanating from the Sun which creates a very tenuous atmosphere (the heliosphere) for billions of kilometers into space. This wind has a particle density of 5–10 protons/cm3 and is moving at a velocity of 350–400 km/s (780,000–890,000 mph).[143]

The region of interplanetary space is a nearly total vacuum, with a mean free path of about one astronomical unit at the orbital distance of the Earth. This space is not completely empty, but is sparsely filled with cosmic rays, which include ionized atomic nuclei and various subatomic particles. There is gas, plasma and dust,[144] small meteors, and several dozen types of organic molecules discovered to date by microwave spectroscopy.[145] Collectively, this matter is termed the interplanetary medium.[140] A cloud of interplanetary dust is visible at night as a faint band called the zodiacal light.[146]

Interplanetary space contains the magnetic field generated by the Sun.[143] There are magnetospheres generated by planets such as Jupiter, Saturn, Mercury and the Earth that have their own magnetic fields. These are shaped by the influence of the solar wind into the approximation of a teardrop shape, with the long tail extending outward behind the planet. These magnetic fields can trap particles from the solar wind and other sources, creating belts of charged particles such as the Van Allen radiation belts. Planets without magnetic fields, such as Mars, have their atmospheres gradually eroded by the solar wind.[147]

Interstellar space

[edit]
"Interstellar space" redirects here. For the album, see Interstellar Space.
Main article: Interstellar medium
Patchy orange and blue nebulosity against a black background, with a curved orange arc wrapping around a star at the center.
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 outside of the bubbles of plasma known as astrospheres, formed by stellar winds originating from individual stars.[148] It is the space between the stars or stellar systems within a nebula or galaxy.[149] Interstellar space contains an interstellar medium of sparse matter and radiation. The boundary between an astrosphere and interstellar space is known as an astropause. For the Sun, the astrosphere and astropause are called the heliosphere and heliopause, respectively.[150]

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.[151] The cataclysmic explosion of a supernova propagates shock waves of stellar ejecta outward, distributing it throughout the interstellar medium, including the heavy elements previously formed within the star's core.[152] The density of matter in the interstellar medium can vary considerably: the average is around 106 particles per m3,[153] but cold molecular clouds can hold 108–1012 per m3.[38][151]

A number of molecules exist in interstellar space, which can form dust particles as tiny as 0.1 μm.[154] 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.[155]

The local interstellar medium is a region of space within 100 pc of the Sun, which is of interest both for its proximity and for its interaction with the Solar System. This volume nearly coincides with a region of space known as the Local Bubble, which is characterized by a lack of dense, cold clouds. It forms a cavity in the Orion Arm of the Milky Way Galaxy, with dense molecular clouds lying along the borders, such as those in the constellations of Ophiuchus and Taurus. The actual distance to the border of this cavity varies from 60 to 250 pc or more. This volume contains about 104–105 stars and the local interstellar gas counterbalances the astrospheres that surround these stars, with the volume of each sphere varying depending on the local density of the interstellar medium. The Local Bubble contains dozens of warm interstellar clouds with temperatures of up to 7,000 K and radii of 0.5–5 pc.[156]

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.[157][158] A bow shock is a third boundary characteristic of an astrosphere, lying outside the termination shock and the astropause.[158]

Intergalactic space

[edit]
Main articles: Warm–hot intergalactic medium, Intracluster medium, and Intergalactic dust
Structure of the Universe
Distribution of Matter in a cubic section of the universe. The blue fiber-like structures represent matter, while the empty regions show the cosmic voids

Intergalactic space is the physical space between galaxies. Studies of the large-scale distribution of galaxies show that the universe has a foam-like structure, with groups and clusters of galaxies lying along filaments that occupy about a tenth of the total space. The remainder forms cosmic voids that are mostly empty of galaxies. Typically, a void spans a distance of 7–30 megaparsecs.[159]

Surrounding and stretching between galaxies is the intergalactic medium (IGM). This rarefied plasma[160] is organized in a galactic filamentary structure.[161] The diffuse photoionized gas contains filaments of higher density, about one atom per cubic meter,[162] which is 5–200 times the average density of the universe.[163] The IGM is inferred to be mostly primordial in composition, with 76% hydrogen by mass, and enriched with higher mass elements from high-velocity galactic outflows.[164]

As gas falls into the intergalactic medium from the voids, it heats up to temperatures of 105 K to 107 K.[4] 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.[163][165][166] 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).[167]

Overview of different scales of space as regions around Earth
Earth-Moon System
Inner Solar System with Near-Earth objects
Solar System and Oort cloud
Nearest stars
Local Interstellar Cloud and neighbouring interstellar medium
Star associations and interstellar medium map of the Local Bubble
Molecular clouds around the Sun inside the Orion-Cygnus Arm
Orion-Cygnus Arm and neighbouring arms
Orion-Cygnus Arm inside the Milky Way
The Sun within the structure of the Milky Way
Satellite galaxies of the Milky Way in Local Group
Virgo SCl in Laniakea SCl
Laniakea SCl in Pisces–Cetus Supercluster Complex
Observable Universe of the Universe

History of discovery

[edit]
Further information: Timeline of knowledge about galaxies, clusters of galaxies, and large-scale structure

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.[168] 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.[169] As late as the 17th century, the French philosopher René Descartes argued that the entirety of space must be filled.[170]

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".[171]

The Italian scientist Galileo Galilei knew that air has mass and so was subject to gravity. In 1640, he demonstrated that an established force resisted the formation of a vacuum. It would remain for his pupil Evangelista Torricelli to create an apparatus that would produce a partial vacuum in 1643. This experiment resulted in the first mercury barometer and created a scientific sensation in Europe. Torricelli suggested that since air has weight, then air pressure should decrease with altitude.[172] The French mathematician Blaise Pascal proposed an experiment to test this hypothesis.[173] 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.[174]

A glass display case holds a mechanical device with a lever arm, plus two metal hemispheres attached to draw ropes.
The original Magdeburg hemispheres (left) used to demonstrate Otto von Guericke's vacuum pump (right)

In 1650, German scientist Otto von Guericke constructed the first vacuum pump: a device that would further refute the principle of horror vacui. He correctly noted that the atmosphere of the Earth surrounds the planet like a shell, with the density gradually declining with altitude. He concluded that there must be a vacuum between the Earth and the Moon.[175]

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.[176] 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.[177] 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.[178] 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.[179]

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.[180] 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.[181][182]

The first professional astronomer to support the concept of an infinite universe was the Englishman Thomas Digges in 1576.[183] 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.[184] 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.[185] 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.[186] This established that the Andromeda Galaxy, and by extension all galaxies, lay well outside the Milky Way.[187] With this Hubble formulated the Hubble constant, which allowed for the first time a calculation of the age of the Universe and size of the Observable Universe, starting at 2 billion years and 280 million light-years. This became increasingly precise with better measurements, until 2006 when data of the Hubble Space Telescope allowed a very accurate calculation of the age of the Universe and size of the Observable Universe.[188]

The modern concept of outer space is based on the "Big Bang" cosmology, first proposed in 1931 by the Belgian physicist Georges Lemaître.[189] This theory holds that the universe originated from a state of extreme energy density that has since undergone continuous expansion.[190]

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.[191] 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.[191]

Exploration

[edit]
Main articles: Space exploration and Human presence in space
See also: Astronautics, Spaceflight, and Human spaceflight
South is up in the first image of Earth taken by a person,[192] probably by Bill Anders (during the 1968 Apollo 8 mission)

For most of human history, space was explored by observations made from the Earth's surface—initially with the unaided eye and then with the telescope. Before reliable rocket technology, the closest that humans had come to reaching outer space was through balloon flights. In 1935, the American Explorer II crewed balloon flight reached an altitude of 22 km (14 mi).[193] This was greatly exceeded in 1942 when the third launch of the German A-4 rocket climbed to an altitude of about 80 km (50 mi). In 1957, the uncrewed satellite Sputnik 1 was launched by a Russian R-7 rocket, achieving Earth orbit at an altitude of 215–939 kilometres (134–583 mi).[194] This was followed by the first human spaceflight in 1961, when Yuri Gagarin was sent into orbit on Vostok 1. The first humans to escape low Earth orbit were Frank Borman, Jim Lovell and William Anders in 1968 on board the American Apollo 8, which achieved lunar orbit[195] and reached a maximum distance of 377,349 km (234,474 mi) from the Earth.[196]

The first spacecraft to reach escape velocity was the Soviet Luna 1, which performed a fly-by of the Moon in 1959.[197] 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.[198] The first fly-by of Mars was by Mariner 4 in 1964. Since that time, uncrewed spacecraft have successfully examined each of the Solar System's planets, as well their moons and many minor planets and comets. They remain a fundamental tool for the exploration of outer space, as well as for observation of the Earth.[199] In August 2012, Voyager 1 became the first man-made object to leave the Solar System and enter interstellar space.[200]

Application

[edit]
See also: Space science, Benefits of space exploration, Earth observation, Commercialization of space, and Space habitation
View from International Space Station, showing the yellow-green airglow of Earth's ionosphere with the Milky Way in the background.

Outer space has become an important element of global society. It provides multiple applications that are beneficial to the economy and scientific research.

The placing of artificial satellites in Earth orbit has produced numerous benefits and has become the dominating sector of the space economy. They allow relay of long-range communications like television, provide a means of precise navigation, and permit direct monitoring of weather conditions and remote sensing of the Earth. The latter role serves a variety of purposes, including tracking soil moisture for agriculture, prediction of water outflow from seasonal snow packs, detection of diseases in plants and trees, and surveillance of military activities.[201] They facilitate the discovery and monitoring of climate change influences.[202] Satellites make use of the significantly reduced drag in space to stay in stable orbits, allowing them to efficiently span the whole globe, compared to for example stratospheric balloons or high-altitude platform stations, which have other benefits.[203]

The absence of air makes outer space an ideal location for astronomy at all wavelengths of the electromagnetic spectrum. This is evidenced by the pictures sent back by the Hubble Space Telescope, allowing light from more than 13 billion years ago—almost to the time of the Big Bang—to be observed.[204] 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.[205] 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.[206]

Concept for a space-based solar power system to beam energy down to Earth[207]

The deep vacuum of space could make it an attractive environment for certain industrial processes, such as those requiring ultraclean surfaces.[208] Like asteroid mining, space manufacturing would require a large financial investment with little prospect of immediate return.[209] An important factor in the total expense is the high cost of placing mass into Earth orbit: $9,000–$31,000 per kg, according to a 2006 estimate (allowing for inflation since then).[210] The cost of access to space has declined since 2013. Partially reusable rockets such as the Falcon 9 have lowered access to space below $3,500 per kg. 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.[211]

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.[212]

From the Earth's surface, the ultracold temperature of outer space can be used as a renewable cooling technology for various applications on Earth through passive daytime radiative cooling.[213][214] This enhances longwave infrared (LWIR) thermal radiation heat transfer through the atmosphere's infrared window into outer space, lowering ambient temperatures.[215][216] Photonic metamaterials can be used to suppress solar heating.[217]

See also

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References

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[edit]
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Outer space

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Outer space, also known simply as , is the region beyond Earth's atmosphere where the density of matter drops to negligible levels, conventionally beginning at the approximately 100 kilometers above , marking the transition from to . This near-vacuum environment features extremely low particle density, enabling unhindered propagation of but exposing objects to intense cosmic rays and temperatures ranging from near in shadowed voids to extreme heat near stars, primarily transferred via rather than conduction or . The contents of outer space encompass the solar system—with its sun, planets, asteroids, and comets—extending to interstellar space filled with diffuse gas and dust, stars clustered in galaxies like the Milky Way, and vast cosmic structures including galaxy clusters and superclusters, all expanding within the observable universe estimated at 93 billion light-years in diameter. Human exploration commenced with the Soviet Union's launch of Sputnik 1 on October 4, 1957, the first artificial satellite to orbit Earth, followed by crewed missions culminating in NASA's Apollo 11 landing on the Moon in 1969, which returned 382 kilograms of lunar samples and demonstrated technologies foundational to subsequent endeavors. Contemporary efforts include sustained human presence aboard the since 2000, enabling microgravity research in biology, physics, and , alongside NASA's aiming to reestablish lunar landings by 2026 to prepare for Mars transit, bolstered by commercial partnerships such as SpaceX's reusable rockets and Dragon spacecraft for crew and cargo transport. These achievements underscore outer space's role in advancing scientific understanding of cosmology, planetary formation, and fundamental physics, while highlighting challenges like orbital accumulation and the physiological toll of long-duration exposure on astronauts.

Terminology and Definition

Historical and Etymological Context

The English term "" originates from the Latin spatium, signifying an extent, interval, or expanse, which entered around 1300 via espace. Initially denoting physical room or duration, its application broadened in scientific discourse to encompass the vast, near-vacuous region surrounding celestial bodies. In early astronomical usage, "" evoked the celestial realm beyond Earth's perceptible domain, with the poet employing it in (1667) to depict the "spacious" and starry voids, marking one of the earliest literary associations with extraterrestrial emptiness. The modifier "outer" prefixed to "space" distinguished the extraterrestrial void from terrestrial or atmospheric confines, with the compound "outer space" first attested in 1845 by Prussian explorer and naturalist , who used it in to describe regions beyond planetary atmospheres in an astronomical context. This terminology gained traction amid 19th-century advances in and measurements, which empirically confirmed interstellar distances and the rarity of in interplanetary gaps, as quantified by Friedrich Bessel's 1838 parallax determination of at 10.3 light-years. further popularized "outer space" in his 1901 novel The First Men in the Moon, embedding it in that mirrored emerging rocketry concepts, such as Konstantin Tsiolkovsky's 1903 equation for from Earth's gravity well. Conceptually, the historical framing of outer space evolved from ancient geocentrism, where (circa 350 BCE) envisioned a plenum of quintessence filling , rejecting as logically impossible per horror vacui principles. The (1543) shifted paradigms toward heliocentric models with infinite extent, bolstered by Galileo's 1610 sidereal observations revealing the Milky Way's stellar composition rather than nebulous aether. Newton's Principia (1687) posited absolute space as an immutable, sensorless container for motion and gravitation, enabling predictive without invoking filled mediums. By the , experiments and the 1887 Michelson-Morley null result empirically validated outer space's near-emptiness, aligning terminology with causal realities of sparse plasma densities averaging 1 atom per cubic centimeter in interstellar voids. Outer space has no universally accepted legal boundary with Earth's airspace under international law, as the 1967 Outer Space Treaty, which governs activities in space, does not specify a demarcation line. This absence creates ambiguity in distinguishing sovereign airspace—governed by the 1944 Chicago Convention, which grants states complete sovereignty up to undefined altitudes—from outer space, treated as a global commons where no national appropriation is permitted. The United Nations Committee on the Peaceful Uses of Outer Space (COPUOS) has debated definitions since the 1960s, but no consensus has emerged due to technical, legal, and geopolitical challenges. The , at 100 kilometers (62 miles) above mean sea level, serves as a de facto scientific and regulatory boundary in many contexts, named after aerospace engineer who calculated it in the 1950s as the altitude where aerodynamic lift becomes insufficient for sustained flight, requiring orbital velocity instead. The (FAI), the international body for aeronautical records, adopted this threshold in 1960 for awarding spaceflight credentials, influencing practices like NASA's . However, alternatives exist; the U.S. Air Force historically used 80 kilometers (50 miles) for X-15 pilot wings, reflecting varying physical interpretations of atmospheric drag cessation. Legally, nations adopt pragmatic definitions for domestic regulation without international harmonization; for instance, the U.S. maintains that no fixed delimitation is necessary, as practical issues have not arisen from the ambiguity. ’s Space Activities Act of 1998 (amended 2002) sets the boundary at 100 kilometers for licensing launches and returns, while the aligns with the for operational purposes. This patchwork underscores risks in suborbital tourism and , where crossing altitudes could invoke conflicting regimes—civil rules below versus space liability conventions above—prompting calls for a fixed line to clarify , , and protocols. Ongoing COPUOS efforts, including proposals as of 2022, aim to address these gaps amid rising commercial space activities.

Physical Characteristics

Vacuum, Density, and Matter Distribution

Outer space constitutes a near-, with pressures typically ranging from 10^{-14} Pa in interplanetary regions to below 10^{-17} Pa in interstellar and intergalactic voids, orders of magnitude lower than Earth's of about 10^5 Pa. This low pressure arises from the vast distances between matter concentrations and the absence of significant gravitational confinement for gases beyond planetary atmospheres. However, the is imperfect, containing sparse particles such as ions, electrons, cosmic rays, photons from the , and neutrinos, which collectively contribute to a residual energy density dominated by radiation and on cosmic scales. The of matter in outer space varies dramatically by region, reflecting the hierarchical structure of . In interplanetary space within the solar system, particle densities near 1 AU from the Sun average 5 to 10 protons and electrons per cubic centimeter (5 × 10^6 to 10^7 per m³), primarily from plasma. Interstellar medium densities in the local galactic neighborhood range from 0.1 to 1 atom per cm³ (10^5 to 10^6 per m³) in diffuse clouds, dropping to 10^{-4} atoms per cm³ in warm ionized phases. On the universal scale, the average baryonic matter equates to roughly 0.25 atoms per cubic meter, corresponding to a of about 4 × 10^{-28} kg/m³, as inferred from measurements. Matter distribution in outer space is profoundly inhomogeneous, clumped into dense structures amid expansive emptiness due to gravitational amplifying primordial fluctuations from cosmic . Baryonic , comprising , gas, and , concentrates in galaxies—each containing 10^8 to 10^12 solar masses—organized into clusters and superclusters connected by filamentary webs, while comprising only ~5% of the total . These filaments enclose cosmic voids, vast underdense regions spanning tens to hundreds of megaparsecs that occupy approximately 80% of the 's volume but harbor fewer than 10% of its galaxies, with densities approaching 10% of the cosmic mean. This large-scale structure, mapped by surveys like the , aligns with predictions from models, where non-baryonic (~25% of ) enhances clustering without direct electromagnetic emission. Intergalactic medium within voids and filaments consists of highly diffuse, hot plasma at temperatures of 10^5 to 10^7 , detected via absorption and Sunyaev-Zel'dovich effects.

Temperature, Radiation, and Energy Profiles

The temperature of outer space varies significantly by region and is primarily determined by the local balance of absorbed and emitted rather than conduction or , due to the near-vacuum conditions. In deep interstellar and intergalactic , far from significant sources of heating, objects reach with the (CMB), a uniform field with a temperature of 2.725 K. This value, precisely measured as 2.72548 ± 0.00057 K from ground-based and satellite observations, represents the cooled remnant of the hot early , providing the baseline thermal bath across the . In contrast, denser regions of the , such as molecular clouds, exhibit gas temperatures as low as 10 K due to , while warmer neutral regions average around 100 K from stellar heating and ionization. Within the Solar System's interplanetary space, solar radiation dominates the energy input, creating a radial . Exposed surfaces on or dust particles experience equilibrium temperatures governed by the Stefan-Boltzmann law, T=(F(1−A)4σϵ)1/4T = \left( \frac{F (1 - A)}{4 \sigma \epsilon} \right)^{1/4}, where FF is the solar flux (approximately 1366 W/m² at 1 AU), AA is , σ\sigma is the Stefan-Boltzmann constant, and ϵ\epsilon is ; this yields daytime temperatures up to 120–150°C on sun-facing sides, while shadowed regions drop toward 3–50 K, modulated by albedo and conduction. The , a plasma of protons and electrons extending to the heliopause at about 120 AU, maintains kinetic temperatures of 10⁵–10⁶ K, though its low density (∼5 particles/cm³ near Earth, dropping to ∼0.002/cm³ beyond) results in negligible collisional heating for macroscopic objects. Radiation in outer space encompasses both electromagnetic (EM) waves and high-energy particles, with profiles shaped by cosmic and local sources. The CMB dominates the EM spectrum at microwave wavelengths (peak at 160.2 GHz), contributing an energy density of u=aT4≈4.2×10−14u = a T^4 \approx 4.2 \times 10^{-14} J/m³, where a=7.566×10−16a = 7.566 \times 10^{-16} J/m³K⁴ is the radiation constant. Stellar and galactic light forms the interstellar radiation field, peaking in the infrared (∼10–100 μm) with intensities of ∼10⁻⁶ to 10⁻⁴ erg/cm²/s/sr, varying by galactic position due to dust absorption and re-emission. Ultraviolet and X-ray radiation arises from hot stars, supernovae remnants, and active galactic nuclei, while gamma rays trace high-energy processes like pion decay in cosmic ray interactions. Particle radiation, primarily galactic cosmic rays (89% protons, accelerating to relativistic speeds in supernova shocks), delivers a flux of ∼1 particle/cm²/s above 1 GeV, posing penetration risks equivalent to 0.3–1 Sv/year in unshielded deep space, far exceeding Earth's geomagnetic protection. Energy profiles in outer space reflect the dominance of radiative and kinetic forms over in the . Radiative energy flux follows inverse-square dilution from point sources like stars, with the providing isotropic and minimal directional variation (ΔT/T ∼ 10⁻⁵). resides in sparse plasmas (e.g., ram pressure ∼2 nPa near ) and cosmic rays, whose rivals that of the interstellar (∼1 μG) and turbulence, sustaining galactic dynamos via first-principles . In voids, density, inferred from the Λ ≈ 10⁻⁵² m⁻², equates to ∼5 GeV/m³ or 10⁻⁹ J/m³, uniform and non-diluting with expansion, though its quantum origins remain unresolved empirically. These profiles underpin thermal design, requiring active control to avoid extremes from -270°C (-limited) to +120°C (solar-heated).

Expansion Dynamics and Fundamental Laws

The expansion of the universe manifests as the recessional motion of distant galaxies proportional to their distance, formalized in Hubble's law: v=H0dv = H_0 d, where vv is the recessional velocity, dd is the proper distance, and H0H_0 is the Hubble constant representing the current expansion rate. This empirical relation was established by Edwin Hubble through spectroscopic observations of galaxies in 1929, revealing redshifted spectral lines interpreted as Doppler shifts due to expansion rather than peculiar motions. Supporting evidence includes the cosmic microwave background (CMB) uniformity and galaxy distribution patterns consistent with isotropic expansion on scales exceeding 100 megaparsecs. Theoretically, cosmic expansion derives from Einstein's general relativity applied to a homogeneous, isotropic universe via the Friedmann-Lemaître-Robertson-Walker (FLRW) metric. Alexander Friedmann derived the governing equations in 1922, with the first Friedmann equation expressing the Hubble parameter squared as H2=(a˙a)2=8πG3ρ−kc2a2+Λc23H^2 = \left(\frac{\dot{a}}{a}\right)^2 = \frac{8\pi G}{3}\rho - \frac{k c^2}{a^2} + \frac{\Lambda c^2}{3}, where a(t)a(t) is the scale factor, ρ\rho the total energy density, kk the curvature parameter, GG the gravitational constant, cc the speed of light, and Λ\Lambda the cosmological constant. The second equation, a¨a=−4πG3(ρ+3pc2)+Λc23\frac{\ddot{a}}{a} = -\frac{4\pi G}{3}\left(\rho + \frac{3p}{c^2}\right) + \frac{\Lambda c^2}{3}, dictates acceleration, where pp is pressure; deceleration occurs for matter-dominated eras (p≈0p \approx 0), but acceleration arises when dominated by components with negative pressure, such as a cosmological constant where p=−ρc2p = -\rho c^2. Observations of Type Ia supernovae in 1998 by the Supernova Cosmology Project and High-Z Supernova Search Team revealed that distant supernovae appear fainter than expected in a decelerating model, indicating accelerated expansion beginning approximately 5-6 billion years ago, attributed to comprising about 68% of the universe's energy budget in the . The current Hubble constant value remains contentious, with local measurements using Cepheid variables and supernovae yielding H0≈73−76H_0 \approx 73-76 km/s/Mpc, while CMB analyses from Planck infer H0≈67H_0 \approx 67 km/s/Mpc, a discrepancy known as the Hubble tension persisting as of 2025 despite efforts with the . This tension, exceeding 5 sigma, challenges assumptions and prompts investigations into systematic errors or new physics, such as evolving or modified gravity, though no consensus resolution exists.

Cosmological Origins and Evolution

Big Bang and Initial Conditions

The theory describes the 's origin as an extremely hot and dense state that expanded rapidly approximately 13.8 billion years ago, evolving into the vast expanse of outer observed today. This model posits that all , , , and time emerged from this , with the cooling and expanding over time to form the cosmic vacuum and sparse distribution characteristic of outer . Empirical support includes the observed of distant galaxies, indicating universal expansion consistent with , where recession velocity increases with distance. A key pillar of evidence is the (CMB) radiation, discovered in 1965 by Arno Penzias and Robert Wilson using a that detected uniform emission across the sky at about 2.725 K. This relic radiation originates from the epoch of recombination around 380,000 years after the , when the universe cooled sufficiently for protons and electrons to form neutral , allowing photons to decouple and propagate freely. The CMB's near-perfect blackbody spectrum and tiny temperature fluctuations (on the order of 1 part in 100,000) provide direct snapshots of the early universe's density variations, which seeded the formation of cosmic structures. Measurements from the Planck satellite refined the universe's age to 13.8 billion years and confirmed the CMB's isotropy, aligning with predictions. Initial conditions of the involved a quark-gluon plasma at temperatures exceeding 10^12 K in the first microseconds, followed by producing light elements like (75% by mass) and (25%) within . The theory of cosmic , proposed by in 1980, addresses fine-tuning issues such as the horizon and flatness problems by positing an exponential expansion phase around 10^{-36} seconds after the singularity, driven by a hypothetical field, which stretched quantum fluctuations to cosmic scales and homogenized the . This phase ended with reheating, transitioning to the hot phase, setting the causal conditions for the large-scale uniformity of outer space while allowing perturbations that led to galaxy formation. While resolves empirical puzzles like the observed spatial flatness (Ω ≈ 1 from data), it remains theoretically speculative, requiring specific initial field configurations without direct observational confirmation beyond indirect support.

Structure Formation and Large-Scale Evolution

Following the , the expanded from a hot, dense state approximately 13.8 billion years ago, initially featuring tiny density perturbations on the order of 10^{-5} relative to the mean density, as imprinted in the (CMB) radiation measured by satellites like WMAP and Planck. These , originating from quantum effects during cosmic —a rapid expansion phase around 10^{-36} to 10^{-32} seconds after the —served as seeds for gravitational instability, whereby regions of slightly higher density attracted more , amplifying contrasts over time. After recombination at about 380,000 years, when the cooled enough for neutral atoms to form, photons decoupled, allowing perturbations to grow freely under gravity without radiation pressure opposition. Non-baryonic (CDM), comprising roughly 27% of the universe's , played a pivotal role by clustering first due to its collisionless nature and lack of electromagnetic interactions, forming extended halos that provided gravitational scaffolds. Baryonic matter, making up about 5%, subsequently fell into these potential wells, with cooling and fragmentation leading to and galaxy assembly starting around 100-400 million years post-Big Bang, as evidenced by high-redshift observations of early galaxies by the . This process adhered to the criterion for , where perturbations exceeded a scale dependent on and , transitioning from linear growth (proportional to the scale factor during matter domination) to nonlinear collapse. Structure formation proceeded hierarchically in the , with small halos merging into larger ones over cosmic time, culminating in galaxies, clusters, and superclusters; simulations demonstrate that dwarf galaxies formed by z ≈ 10-20, while Milky Way-mass systems assembled primarily between z ≈ 1-4 (corresponding to 0.8-12 billion years ago). Mergers, often involving gas inflows triggering starbursts, shaped morphologies, with major mergers doubling occurring about three times per massive galaxy over the past 10 billion years. Empirical support comes from N-body simulations matching large-scale surveys, reproducing power spectra of density fields where the two-point ξ(r) ≈ (r / 8 h^{-1} Mpc)^{-1.8} on scales of 1-10 Mpc. On large scales, these processes yielded the cosmic web: a filamentary network of walls and nodes (clusters) enclosing vast voids, spanning hundreds of megaparsecs, as mapped by redshift surveys like the revealing coherent structures up to ~100 Mpc/h. Dark matter's dominance ensured early structure growth before baryons, with its particle properties—such as mass and velocity dispersion—dictating cutoff scales for smallest halos and overall evolution. Since around z ≈ 0.5 (5 billion years ago), dark energy's accelerating expansion has slowed merger rates, preserving the web's topology while voids expand faster, consistent with observed acquisition via tidal torques during hierarchical buildup. This evolution aligns with CMB acoustic peaks and galaxy clustering statistics, validating gravitational instability as the primary driver without invoking exotic mechanisms beyond standard cosmology.

Regions of Outer Space

Near-Earth and Cislunar Regions

Near-Earth space includes (LEO) from approximately 160 to 2,000 kilometers altitude, (MEO) from 2,000 to 35,786 kilometers, and (GEO) at 35,786 kilometers, where objects remain fixed relative to 's surface. space comprises the three-dimensional volume between and the , extending roughly 384,400 kilometers, governed primarily by the gravitational influences of both bodies and including Earth-Moon Lagrange points. This region hosts over 40,000 tracked objects as of 2025, including approximately 11,000 active satellites, with the majority concentrated in LEO due to commercial constellations like exceeding 7,800 satellites. , comprising defunct satellites, rocket stages, and fragments, numbers in the tens of thousands larger than 10 centimeters, with statistical models estimating over 1 million objects larger than 1 centimeter, posing collision risks that could exacerbate the debris population through cascading events. The Van Allen radiation belts encircle in MEO, with the inner belt—primarily protons from interactions—spanning 1,000 to 6,000 kilometers altitude and the outer belt—dominated by electrons from —extending from 13,000 to 60,000 kilometers, trapping high-energy particles that fluctuate with solar activity and damage electronics and biological tissues. Discovered in via data, these belts necessitate shielding for spacecraft transiting the region. In cislunar space, orbital dynamics deviate from simple Keplerian paths due to the Earth-Moon system's perturbations, enabling stable periodic orbits and points like L1 and L2 for long-duration missions with periods from days to weeks. Object density remains low compared to near-Earth orbits, though increasing mission traffic raises concerns for , with gravitational influences supporting trajectories for lunar access but complicating persistent . Radiation levels vary, influenced by and galactic cosmic rays, with less geomagnetic shielding beyond GEO.

Interplanetary Space

Interplanetary space refers to the region within the solar system extending from the outer boundaries of planetary magnetospheres to the heliopause, approximately 120 from the Sun, where the solar wind's influence diminishes. This volume is filled by the , a dilute plasma environment shaped primarily by the dynamic outflow from the Sun. Unlike near-Earth space, which is modulated by geomagnetic fields, interplanetary space features radial propagation of solar material with minimal planetary interference beyond ~5 . The interplanetary medium's composition is dominated by the , a magnetized plasma consisting mainly of protons, electrons, and trace heavier ions ejected from the Sun's corona at supersonic speeds. At 1 AU, typical solar wind parameters include particle densities of around 5 ions per cubic centimeter, temperatures of approximately 10^5 , and radial flow speeds averaging 400 km/s, though fast streams from can exceed 800 km/s while slow streams fall below 300 km/s. Neutral components, such as interstellar hydrogen atoms, contribute a minor density of about 0.2 atoms per cubic centimeter, while interplanetary —microscopic silicates and organics totaling roughly 10^{-23} g/cm³ in —scatters to produce the but poses collision risks to . Galactic cosmic rays, high-energy protons and nuclei accelerated by distant supernovae, permeate this region at fluxes modulated by the , delivering doses hazardous for unshielded human missions on timescales beyond months. The interplanetary magnetic field (IMF), embedded in the solar wind, exhibits a Parker spiral configuration due to the Sun's 25-day rotation at the equator, with field lines stretching outward in a flattened helix. At Earth's orbit, the IMF strength averages 5 nanotesla (nT), decreasing as 1/r with heliocentric distance r, though transient enhancements from coronal mass ejections can amplify it by factors of 10 or more, driving space weather effects like geomagnetic storms. These ejections propagate as plasma shocks through interplanetary space, compressing the ambient medium and altering particle fluxes, as observed by probes like Voyager 2 during its transit to Jupiter in 1979. Empirical measurements from spacecraft such as Parker Solar Probe confirm that wave-particle interactions sustain the solar wind's heat against adiabatic cooling, maintaining causal balance between coronal origins and interplanetary expansion.

Interstellar and Intergalactic Voids

Interstellar space encompasses the regions between stars within a , primarily consisting of the (), a low-density plasma dominated by comprising about 90% of its mass. The ISM exhibits multiphase structure with densities spanning from roughly 10^{-4} cm^{-3} in hot ionized regions to 10^6 cm^{-3} in dense molecular clouds, reflecting variations driven by stellar feedback and radiative processes. Temperatures range from 10-100 K in cold neutral phases to millions of in coronal gas, with the low overall particle density—averaging around 1 atom per cm³—rendering it a near-vacuum compared to planetary atmospheres. This medium includes trace helium, heavier elements, dust grains, and pervasive that shape its dynamics, alongside cosmic rays propagating through the voids. Interstellar voids, as underdense regions, facilitate the travel of probes like , which entered on August 25, 2012, detecting plasma densities of about 0.06 electrons per cm³ beyond the heliopause. Empirical measurements from such missions confirm the ISM's sparsity, with mean free paths for particles exceeding light-years due to infrequent collisions. Intergalactic voids represent even vaster underdensities in the cosmic web, spanning 20-50 h^{-1} Mpc (approximately 65-160 million light-years) and comprising regions depleted of galaxies relative to average. The intergalactic medium within these voids has an extraordinarily low of about 10^{-6} particles per cm³, mostly ionized and heated to temperatures around 10^5-10^7 K by and shocks. These structures arise from initial fluctuations amplified by cosmic expansion, where underdense areas evacuate matter into surrounding filaments and walls. Prominent examples include the Boötes Void, identified in 1981 through redshift surveys and extending roughly 330 million light-years in diameter with only a few dozen galaxies observed, far below expectations for its volume. Such voids occupy over 80% of the universe's volume yet contribute minimally to its baryonic mass, highlighting the filamentary dominance of large-scale structure; their emptiness challenges models of galaxy formation by implying suppressed star formation in extreme low-density environments. Observations via galaxy surveys and cosmic microwave background anisotropies validate void properties, with intergalactic space maintaining a harder vacuum than interstellar regions due to greater separation and dilution.

Astronomical Observation and Phenomena

Observational Techniques and Instruments

Observational techniques in astronomy rely on detecting across wavelengths, supplemented by non-electromagnetic methods like detection and interferometry. Primary techniques include for spatial mapping, photometry for measuring brightness variations, for analyzing spectral lines to infer composition, temperature, and via Doppler shifts, and for precise positional measurements. Ground-based observations face limitations from Earth's atmosphere, which absorbs , X-ray, and gamma-ray while causing optical distortion through , known as "seeing," typically limiting resolution to about 1 arcsecond under good conditions. Major ground-based instruments include the twin 10-meter Keck telescopes on Mauna Kea, operational since 1993 and 1996, which use adaptive optics to correct for atmospheric distortion in real-time, achieving resolutions approaching the diffraction limit of about 0.05 arcseconds at visible wavelengths. The Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, completed in 2011 with 66 antennas, enables high-resolution radio interferometry for studying molecular gas in star-forming regions and protoplanetary disks. Radio astronomy techniques, such as very long baseline interferometry (VLBI), connect global arrays like the Event Horizon Telescope to image black hole shadows, as demonstrated by the 2019 M87* observation at 1.3 mm wavelength with angular resolution of 20 microarcseconds. Space-based instruments circumvent atmospheric interference, accessing blocked wavelengths and providing stable imaging. The , deployed in 1990, features cameras, spectrographs, and interferometers for visible and observations, capturing over 1.5 million exposures that revealed phenomena like the accelerating expansion via Type Ia supernovae in 1998. The (JWST), launched December 25, 2021, to the Sun-Earth L2 point, observes in with a 6.5-meter mirror, enabling detection of light from the 's first galaxies formed around 13.5 billion years ago. For X-rays, the , operational since 1999, detects high-energy emissions from accretion disks and supernova remnants, with sensitivity down to 0.1-10 keV energies. These instruments collectively enable multi-wavelength studies, cross-verifying data to model outer space phenomena with empirical rigor.

Key Empirical Discoveries

In 1929, published observations from the demonstrating that galaxies recede from at velocities proportional to their distances, establishing the expansion of the universe through measurements of Cepheid variable stars in the and others, with the relation now known as . This empirical finding, building on Vesto Slipher's earlier measurements, provided direct evidence against a model and supported dynamic cosmological theories. The (CMB) radiation was serendipitously detected in 1964 by Arno Penzias and Robert Wilson using a at , revealing uniform microwave emission across the sky at approximately 2.7 K, interpreted as relic radiation from the early hot, dense phase of the universe. Subsequent observations, including COBE satellite data in 1992 confirming blackbody spectrum and anisotropies, corroborated the model's predictions of primordial photon decoupling around 380,000 years post-inflation. The first confirmed exoplanets were identified in 1992 orbiting the by Aleksander Wolszczan and Dale Frail via pulsar timing variations, detecting two terrestrial-mass bodies despite the harsh radiation environment. In 1995, and announced the detection of , a Jupiter-mass orbiting a Sun-like star every 4.2 days, using , marking the first extrasolar around a main-sequence star and initiating the surge in exoplanet discoveries exceeding 5,000 by 2025. Direct imaging of supermassive s advanced in 2019 when the Event Horizon Telescope (EHT) collaboration released the first shadow image of M87*, a 6.5 billion object, resolving its silhouette against accreting plasma at 55 million light-years distance through . In 2022, EHT imaged Sagittarius A*, the 4 million at the Way's , confirming general relativity's predictions in strong-field via polarized light observations of the ring. Since its 2022 operational debut, the (JWST) has observed unexpectedly massive and bright galaxies at redshifts z > 10, such as JADES-GS-z14-0 at z=14.32 formed less than 300 million years after the , featuring complex chemistry including nitrogen-bearing molecules challenging standard galaxy formation timelines derived from Lambda-CDM simulations. These findings, including overabundant early and supermassive black holes at high redshifts, indicate potential revisions to and structure growth models, with spectroscopic confirmations of emission in primordial environments.

Human Access and Exploration

Historical Milestones in Access

The development of access to outer space transitioned from theoretical rocketry to practical achievements during the mid-20th century, driven primarily by military and geopolitical imperatives during the . Initial suborbital flights using German V-2 derivatives in the late demonstrated the feasibility of reaching altitudes above 100 km, conventionally defining the boundary of outer space per the . However, sustained orbital access required advancements in multi-stage rocketry, guidance systems, and propulsion reliability, culminating in the Space Age's onset. Key milestones in orbital and beyond-Earth access include:
  • October 4, 1957: The Soviet Union launched Sputnik 1, the first artificial satellite to orbit Earth, achieving an apogee of 947 km and demonstrating reliable access to low Earth orbit (LEO) for 21 days before reentry.
  • November 3, 1957: Sputnik 2 carried Laika, the first animal to orbit Earth, validating biological survival in space for approximately seven hours despite fatal overheating due to inadequate thermal control.
  • January 31, 1958: The United States responded with Explorer 1, its first satellite, discovering the Van Allen radiation belts and confirming independent access to LEO at inclinations up to 33 degrees.
  • April 12, 1961: Yuri Gagarin became the first human to reach outer space aboard Vostok 1, completing one orbit at an altitude of 327 km in 108 minutes, proving human viability for short-duration microgravity exposure.
  • May 5, 1961: Alan Shepard's suborbital Mercury-Redstone 3 flight marked the first U.S. astronaut in space, reaching 187 km altitude and paving the way for orbital missions.
  • February 20, 1962: John Glenn orbited Earth three times aboard Friendship 7, establishing U.S. crewed orbital capability and enduring 4.7 hours of flight.
  • March 18, 1965: Alexei Leonov performed the first extravehicular activity (EVA) during Voskhod 2, spending 12 minutes outside the spacecraft at 354 km altitude, though suit rigidity nearly prevented reentry.
  • July 20, 1969: Apollo 11 achieved the first human landing on the Moon, with Neil Armstrong and Buzz Aldrin spending 21.5 hours on the surface after a 384,000 km translunar injection, returning 21.5 kg of lunar samples.
  • April 12, 1981: The Space Shuttle Columbia's STS-1 mission introduced partially reusable orbital access, launching to 246 km altitude and landing horizontally after 54 hours, enabling 135 subsequent missions through 2011.
  • November 2, 2000: Expedition 1 crew docked with the International Space Station (ISS), initiating continuous human presence in LEO at 400 km altitude, exceeding 20 years by 2021 with over 240 individuals visiting.
  • June 21, 2004: SpaceShipOne completed the first private suborbital spaceflight to 112 km, winning the Ansari X Prize and demonstrating non-governmental access feasibility.
  • May 30, 2020: SpaceX Crew Dragon Demo-2 carried NASA astronauts Bob Behnken and Doug Hurley to the ISS, marking the first commercial crewed orbital mission and reducing reliance on Russian Soyuz vehicles.
These achievements relied on liquid-fueled rockets like the R-7 derivatives and , with launch success rates improving from under 50% in the to over 95% by the due to iterative testing and redundancy. innovations, such as SpaceX's first-stage landings starting in 2015, have lowered costs per kilogram to LEO from $54,500 in the Shuttle era to under $3,000 by 2023, enabling frequent access. Subsequent milestones include uncrewed deep-space probes like Voyager 1's 2012 interstellar entry at 121 AU, but human access remains confined to LEO and lunar vicinities, with Artemis I's 2022 uncrewed validating SLS capabilities for future crewed returns. Challenges persist in radiation shielding and for Mars trajectories exceeding 200 days.

Biological and Physiological Effects

Exposure to the space environment, characterized by microgravity, high levels of , and isolation, induces profound biological and physiological changes in humans. Microgravity eliminates the constant gravitational load on the body, leading to adaptations that mimic disuse and fluid redistribution, while cosmic and solar pose risks of cellular damage and long-term . These effects have been documented through data from astronauts on missions such as those aboard the (ISS), where durations often exceed six months. In microgravity, bodily fluids shift cephalad (toward the head), causing facial puffiness, reduced leg volume, and increased , which contributes to Spaceflight-Associated Neuro-ocular Syndrome (SANS). SANS manifests as , globe flattening, and hyperopic shifts, affecting up to 80% of long-duration ISS astronauts and potentially impairing vision permanently in some cases. Weight-bearing bones lose at rates of approximately 1-1.5% per month without countermeasures, primarily in the hips and spine, due to suppressed activity and elevated resorption driven by reduced mechanical loading. , particularly in the lower limbs and back, atrophies by 10-20% within weeks, with fast-twitch fibers disproportionately affected, leading to decreased strength and endurance despite daily exercise regimens of 2-2.5 hours. Ionizing radiation in outer space, including galactic cosmic rays (GCR) and solar particle events (SPE), delivers doses far exceeding terrestrial levels, with astronauts beyond facing annual exposures equivalent to 300-1000 mSv. This elevates lifetime cancer risk by factors of 3-5% per 1 Sv, particularly for solid tumors like and colon cancer, due to DNA double-strand breaks and genomic instability. Acute effects include radiation sickness from SPEs, while chronic exposure may induce central nervous system decrements, such as and accelerated neurodegeneration, as evidenced by animal models and limited human data. Cardiovascular risks, including endothelial damage and , are also heightened, with post-flight analyses showing stiffened arteries in astronauts. Psychological and neurobehavioral effects arise from confinement, loss, and disrupted circadian rhythms, with long-duration missions correlating to increased anxiety, depression, and disturbances in up to 20-30% of crew members. imaging post-mission reveals ventricular expansion and changes, potentially linked to fluid shifts and isolation-induced stress, which can degrade team performance and . Immune dysregulation, including T-cell suppression and latent viral reactivation (e.g., herpesviruses), further compounds vulnerability to infections, as observed in ISS crew returning with elevated inflammatory markers. Countermeasures like exercise, pharmacological interventions, and psychological support mitigate but do not fully eliminate these risks, underscoring the need for further ground-based analogs and in-flight monitoring.

Engineering and Technological Achievements

Human spaceflight began with suborbital and orbital launches using modified ballistic missiles. NASA's utilized the Redstone rocket for the first American suborbital flight on May 5, 1961, carrying to an altitude of 116.5 statute miles. Orbital capability followed with the Atlas LV-3B, enabling John Glenn's three-orbit mission on February 20, 1962. The advanced propulsion with the rocket, a three-stage vehicle standing 363 feet tall and generating 7.5 million pounds of at liftoff. First flown uncrewed on November 9, 1967, powered nine crewed lunar missions, including Apollo 11's landing on July 20, 1969. Key innovations included the for descent and ascent, employing hypergolic propellants for reliable ignition in vacuum. Reusable systems emerged with the , operational from 1981 to 2011 across 135 missions. The orbiter, with its thermal tiles and solid rocket boosters, achieved partial reusability, landing like an aircraft after deploying from a 122-foot external tank. advanced with the (MMU), allowing untethered spacewalks, as demonstrated by on February 7, 1984, during . The (ISS), assembled in orbit starting November 20, 1998, exemplifies modular engineering, spanning the size of a football field with a mass exceeding 925,000 pounds. Its systems include solar arrays generating up to 120 kilowatts and a water recovery process 93% of wastewater. Continuous human presence since November 2, 2000, has tested long-duration , including closed-loop environmental control. Recent developments emphasize full reusability to reduce costs. 's became the first orbital-class reusable rocket, with its first stage successfully landing on December 21, 2015, after deploying 11 satellites; by 2025, boosters have flown over 20 times each. The Crew Dragon capsule, qualified for human flights in 2020, features autonomous docking and abort capabilities using engines. These technologies enable frequent access to , supporting missions like Crew-1 on November 16, 2020.

Principal International Treaties

The principal international treaties governing outer space activities are administered under the auspices of the Office for Outer Space Affairs (UNOOSA) and form the core of , emphasizing peaceful use, international cooperation, and state responsibility. These treaties, negotiated during the era, establish foundational principles such as non-appropriation of celestial bodies, prohibition of nuclear weapons in space, and freedom of exploration for all states. As of 2025, over 110 countries are parties to the foundational , reflecting broad consensus on its norms, though adherence varies for subsequent agreements. The Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, including the Moon and Other Celestial Bodies—commonly known as the —was opened for signature on January 27, 1967, and entered into force on October 10, 1967. It prohibits national appropriation of outer space or celestial bodies by claim of , use, or occupation, and bans the placement of nuclear weapons or other weapons of mass destruction in or on celestial bodies. The treaty mandates that space activities benefit all countries, requires supervision of non-governmental activities by states, and obligates states to avoid harmful contamination and adverse changes to Earth's environment from space activities. Ratified by 115 states as of the latest UN records, it serves as the bedrock of international , influencing national legislation and bilateral agreements. The Agreement on the Rescue of Astronauts, the Return of Astronauts and the Return of Space Objects Launched into , opened for signature on April 22, 1968, entered into force on December 3, 1968. It expands on Article V of the by requiring states to render all possible assistance to astronauts in distress regardless of nationality, promptly return them to the launching authority, and notify the UN and launching state upon discovering space objects that have returned to outside the launching state's territory. Over 100 states have ratified it, underscoring a humanitarian commitment amid growing , though practical implementation relies on international goodwill and notification protocols. The Convention on International Liability for Damage Caused by Space Objects, adopted on March 29, 1972, and entered into force on September 1, 1972, establishes for a launching state for damage caused by its space objects on Earth's surface or to in flight, with fault-based liability applying to damage between states. Compensation must cover actual loss and aim for prompt payment, with disputes resolvable through a claims commission if fails. As of recent tallies, 102 states are parties, and it has been invoked in cases like the 1978 954 incident, where claimed damages from the for radioactive debris, resulting in a settlement outside formal adjudication. The Convention on Registration of Objects Launched into Outer Space, opened for signature on January 14, 1975, and effective from September 15, 1976, requires launching states to maintain national registries and furnish details to the UN Secretary-General, including object descriptions, launch dates, and orbital parameters, to aid identification and liability attribution. It builds on UN Resolution 1721 B (XVI) by formalizing transparency, with 75 states parties; non-registration does not absolve liability but hinders verification, prompting calls for enhanced compliance amid rising launch rates. The Agreement Governing the Activities of States on the Moon and Other Celestial Bodies, adopted December 5, 1979, and entered into force July 11, 1984, extends principles to the and celestial bodies, declaring them the common heritage of mankind, prohibiting exploitation without establishment, and requiring peaceful use with environmental protections. However, it has limited traction, with only 18 parties as of 2023, lacking by major spacefaring nations like the , , and , due to concerns over resource rights and equitable benefit-sharing provisions that could constrain commercial activities.

Criticisms, Gaps, and Proposed Reforms

The of 1967, while establishing foundational principles such as non-appropriation of celestial bodies and prohibition of nuclear weapons in orbit, has been criticized for its lack of enforceable mechanisms, rendering it ineffective against non-compliance by states. This deficiency stems from reliance on voluntary adherence through the , which has failed to persuade or compel violators, as evidenced by ongoing activities like anti-satellite tests conducted by multiple nations despite treaty prohibitions on harmful interference. Additionally, the treaty's Cold War-era framework inadequately addresses contemporary challenges, including the rise of private commercial actors responsible for over 90% of orbital launches since 2020, which were not anticipated in 1967 and thus evade direct state-centric regulation. Gaps in international persist in resource extraction, where the OST's non-appropriation creates ambiguity over property rights for mined materials, leading to unilateral domestic laws in countries like the (via the 2015 Commercial Space Launch Competitiveness Act) and , potentially sparking conflicts without a binding multilateral regime. management represents another void, as the Liability Convention of 1972 provides for compensation but lacks proactive obligations for mitigation or removal, contributing to over 36,000 tracked objects in orbit as of 2023, with projections of risks from cascading collisions unaddressed by current treaties. Furthermore, the framework overlooks autonomous systems and AI-driven operations, failing to define "harmful interference" for non-state actors deploying swarms of satellites, as seen in constellations exceeding 5,000 units by mid-2024. Enforcement uncertainty extends to non-state entities, blurring lines of responsibility and allowing "flags of convenience" where operators register under lax jurisdictions. Proposed reforms include amending the OST to incorporate verification protocols and binding liability for private entities, as advocated by legal scholars to align with technological realities like reusable launchers reducing costs by 90% since 2010. Establishing a dedicated international body for monitoring compliance and has been suggested to enforce debris mitigation standards, potentially modeled on the ’s orbital slot allocations but extended to collision avoidance. Bilateral and plurilateral initiatives, such as the signed by 40 nations as of 2024, aim to fill resource gaps through safety zones and data sharing, though critics from non-signatories like argue they undermine universal principles by favoring U.S.-led norms. Broader calls emphasize sustainable reforms, including incentives for debris removal technologies and equitable access provisions to prevent dominance by launch-capable states, which conducted 98% of missions in 2023.

Practical Applications

Scientific and Commercial Utilization

Outer space enables scientific research unhindered by Earth's atmosphere, allowing telescopes like the to measure the universe's expansion rate and contribute to the discovery of , while determining its age at approximately 13.8 billion years. The , operational since 2022, has provided observations revealing early formation and atmospheres, extending Hubble's capabilities into previously obscured wavelengths. NASA's fleet of over 80 science missions, including planetary probes, has yielded data on solar system bodies, such as the Van Allen radiation belts identified in 1958. The (ISS) facilitates microgravity experiments advancing fields like and ; protein crystal growth studies have improved drug designs for diseases including cancer, with over 450 peer-reviewed publications from ISS National Lab research by 2024. In 2024, ISS experiments included 3D metal printing and remote robotic simulations, yielding insights into manufacturing and medical procedures viable only in . Commercially, satellites underpin global communication networks, with geostationary and low-Earth orbit constellations enabling broadband internet and television broadcasting to remote areas. The (GPS), operational since 1995, supports navigation for aviation, shipping, and personal devices, processing signals from medium-Earth orbit satellites. satellites monitor climate patterns, agriculture, and disasters via , with applications in and . By 2022, operational satellites exceeded 6,700, driving the global space economy to $613 billion in , projected to reach $800 billion by amid growth in launch services and data analytics. Emerging commercial ventures include , where Blue Origin's conducted its 15th suborbital flight on October 8, 2025, carrying six passengers to the edge of space for views of . Companies like and offer similar experiences, with suborbital tickets priced around $200,000–$450,000, fostering private investment in reusable launch vehicles. Private firms also pursue satellite constellations for enhanced connectivity, contributing to a market valued at $418 billion in 2024 with a 6.7% annual growth rate.

Military and Strategic Uses

Outer space serves as a critical domain for operations, enabling capabilities such as intelligence, surveillance, and reconnaissance (ISR); positioning, navigation, and timing (PNT); secure communications; and warning that underpin joint warfighting effectiveness. Since the 1950s, the U.S. has integrated space assets to enhance , with systems like GPS providing precision guidance for weapons and forces worldwide. (SDA), which involves tracking objects in orbit to detect threats and manage congestion, acts as a strategic by improving deterrence and protecting U.S. interests against adversarial interference. The U.S. Space Force emphasizes achieving space superiority through space control activities that contest adversarial access and ensure freedom of action for friendly forces. This includes developing resilient architectures to counter hostile uses like jamming, cyberattacks, and kinetic strikes, as outlined in the 2020 Defense Space Strategy. Adversaries such as and have advanced counterspace capabilities; conducted 42 space launches by July 2025, deploying ISR satellites for , while both nations field directed-energy weapons, co-orbital killers, and electronic warfare systems intended to degrade U.S. space advantages. Anti-satellite (ASAT) weapons represent a direct kinetic , with historical tests demonstrating the ability to destroy orbiting assets and generate . The U.S. conducted its last direct-ascent ASAT test in 2008, using an SM-3 to intercept the malfunctioning at an altitude of approximately 247 kilometers, and imposed a unilateral moratorium on such destructive tests in 2022 to mitigate risks. performed a groundbreaking ASAT test in 2007, destroying the Fengyun-1C and creating over 3,000 trackable pieces, while followed with a 2021 test that fragmented Cosmos 1408, producing more than 1,500 pieces. These actions underscore the strategic escalation in space, where denial of services could cascade to disruptions in terrestrial operations reliant on space-enabled precision and connectivity.

Challenges, Risks, and Debates

Space Debris and Environmental Sustainability

Space debris encompasses non-operational human-made objects in Earth orbit, including defunct satellites, expended rocket stages, and collision fragments, posing collision risks to operational spacecraft. As of early 2025, space surveillance networks track approximately 40,000 objects larger than 10 cm, with statistical models estimating over 1 million objects exceeding 1 cm and tens of millions smaller than that, many untrackable but capable of causing damage upon impact. Primary sources include intentional anti-satellite (ASAT) tests, such as China's 2007 destruction of its Fengyun-1C satellite generating over 3,000 trackable fragments, Russia's 2021 Cosmos-1408 test producing more than 1,500 pieces, and accidental collisions or on-orbit explosions from residual propellants. A prominent example of collision risk materialized on February 10, 2009, when the operational Iridium 33 satellite struck the derelict Russian Kosmos-2251 at over 11 km/s in low Earth orbit (LEO), shattering both and creating more than 2,000 trackable debris fragments that persist and threaten other assets. This event underscored the vulnerability of crowded orbital regimes, with relative velocities amplifying even small fragments' destructive potential equivalent to high-speed projectiles. The Kessler syndrome, theorized by NASA scientist Donald Kessler in 1978, describes a tipping point where debris density in LEO triggers cascading collisions, exponentially multiplying fragments and potentially rendering orbits unusable for decades without intervention, as each impact generates shrapnel that intersects other paths. Current projections indicate LEO's debris population could double within a decade absent aggressive mitigation, exacerbated by annual launch rates surpassing 2,000 satellites since 2020. International mitigation efforts center on voluntary guidelines from the Inter-Agency Space Debris Coordination Committee (IADC) and Committee on the Peaceful Uses of Outer Space (COPUOS), endorsed in , which recommend limiting debris-releasing events during operations, passivating upper stages to prevent explosions, and ensuring 90% of mission-related objects are disposed of post-mission—either by deorbiting to burn up in the atmosphere within 25 years or relocating to graveyard orbits. National policies, such as 's 1995 standards and ESA's 2023 requirements mandating collision avoidance maneuvers and end-of-life deorbiting, build on these, yet enforcement remains inconsistent due to their non-binding status and reliance on self-reporting by operators. Active debris removal technologies, including robotic capture systems demonstrated in prototypes like ESA's e.Deorbit mission concepts, are emerging to target high-risk objects, but deployment lags behind need, with only limited tests conducted by 2025. The proliferation of large satellite constellations amplifies sustainability challenges, as mega-constellations like SpaceX's —deploying over 8,500 active satellites by October 2025—intensify orbital density in LEO, elevating conjunction probabilities despite built-in deorbit mechanisms designed for end-of-life within five years. Reentries from such fleets, averaging 1-2 satellites daily, deposit aluminum oxides into the upper atmosphere, potentially altering stratospheric chemistry and dynamics, though long-term ecological impacts require further empirical study. Sustaining access to demands shifting from passive compliance to mandatory regulations and international cooperation on debris remediation, as unchecked growth risks irreversible congestion; analyses suggest removing just 50 high-influence objects could halve collision hazards in key altitudes. Without such measures, the causal chain of launches feeding debris via inevitable failures could precipitate Kessler-like instability, prioritizing short-term commercial gains over long-term orbital habitability.

Economic and Ethical Controversies

Critics of public space funding argue that expenditures on programs like NASA's, which totaled approximately $25.4 billion in fiscal year 2023, represent an that could address terrestrial issues such as and , given that the agency's budget constitutes about 0.5% of the U.S. federal budget while social programs consume roughly half. This perspective posits that space efforts yield limited immediate returns compared to direct investments on , with historical examples like the cited as economic failures due to per-mission costs exceeding $1 billion, rendering operations inefficient relative to robotic alternatives. The rise of commercial space activities has intensified economic debates, particularly around resource extraction and the absence of robust international regulations, which could precipitate terrestrial disputes over profits from or orbital slots. For instance, the U.S. Commercial Space Launch Competitiveness Act of 2015 grants private entities rights to extracted space resources without claiming sovereignty over celestial bodies, yet this conflicts with interpretations of the 1967 prohibiting national appropriation, raising concerns about first-mover advantages favoring wealthy nations and firms. Luxembourg's similar 2017 legislation has positioned it as a hub for ventures, but critics highlight potential monopolization of rare metals like platinum-group elements, exacerbating global inequalities without equitable benefit-sharing mechanisms. Ethically, commercialization challenges traditional notions of space as a , with private actors like and pursuing activities that prioritize profit over protocols designed to prevent biological contamination of other worlds. Human spaceflight in commercial contexts amplifies risks, as evidenced by calls for enhanced ethical guidelines on participant selection and research consent, given incidents like the 2024 FAA grounding of after a explosion that scattered debris, underscoring tensions between innovation and safety accountability. Furthermore, the prospect of space memorials or tourism raises dilemmas about commodifying extraterrestrial sites, potentially undermining scientific integrity and international cooperation under treaties like the Moon Agreement, which few major powers have ratified.

Existential Threats from Space

Large asteroid or comet impacts pose a potential existential risk by triggering global firestorms, atmospheric dust loading, and prolonged cooling that could disrupt agriculture and ecosystems sufficiently to collapse human civilization or cause extinction. The Chicxulub impactor, approximately 10-15 km in diameter, struck Earth 66 million years ago, leading to the extinction of about 75% of species, including non-avian dinosaurs, through mechanisms including ejecta reentry heating and sulfate aerosol-induced "impact winter." Such events occur roughly every 100 million years based on crater records and near-Earth object (NEO) population estimates, yielding an annual probability of roughly 1 in 100 million for an extinction-level impact exceeding 5 km diameter. Over the next billion years, the cumulative risk rises to 0.03-0.3%, though detection and deflection technologies, such as NASA's DART mission successful kinetic impact in 2022, mitigate near-term threats from tracked objects. Comets from the Oort cloud add uncertainty due to their hyperbolic orbits and lower observability, but their impact dynamics mirror asteroids. Gamma-ray bursts (GRBs), brief emissions of gamma radiation from collapsing massive stars or merging neutron stars, could strip Earth's if occurring within several thousand light-years and beamed toward the planet, allowing lethal ultraviolet radiation to reach the surface and precipitate mass extinctions via DNA damage and . GRBs produce nitrogen oxides that catalytically destroy , as evidenced by a 2023 detection of atmospheric NO2 enhancement from a GRB originating 2 billion light-years away. However, their narrow beaming (typically <10 degrees) and rarity within the —estimated at one per million years per —render the probability of a civilization-threatening event this century below 1 in a million, with no directed threats identified. Nearby , the explosive deaths of massive stars, emit x-rays, gamma rays, and cosmic rays that could erode the and induce radioactive fallout if within 25-50 , potentially causing selective extinctions, increased , and shifts through cosmic ray-induced formation. Geological proxies, such as iron-60 isotopes in sediments, link past supernovae around 2.6 and 8 million years ago to minor perturbations, while models suggest a 30 light-year event could deplete 30-50% of ozone, disrupting marine food chains. No stars massive enough to supernova are currently within this radius, and the galaxy's stellar distribution yields a per-century below 1 in 10 million; historical events like the Ordovician extinction 440 million years ago may correlate with such blasts at greater distances. Extreme solar activity, including coronal mass ejections (CMEs) and , threatens technological civilization through geomagnetic storms that induce currents damaging power grids, transformers, and satellites, but poses no direct existential risk to human survival absent total . The 1859 , the strongest recorded, disrupted telegraph systems; a repeat today could cost trillions in economic damage and black out continents for months due to unrepairable . The Sun's activity follows an 11-year cycle, with extreme events like the Miyake events (radiocarbon spikes from antiquity) indicating rare superflares 10-100 times stronger, yet the Sun's stability as a middle-aged G-type star limits such occurrences to once every few millennia, recoverable by pre-industrial means. Monitoring via satellites like enables days-ahead warnings, reducing severity. Overall, cosmic threats remain low-probability compared to terrestrial risks, with mitigation reliant on astronomical surveillance and redundancy in critical systems.

References

  1. https://www.space.com/karman-line-where-does-space-begin
  2. https://gbv.wilsoncenter.org/across-karman-faq
  3. https://van.physics.illinois.edu/ask/listing/22279
  4. https://science.nasa.gov/exoplanets/what-is-the-universe/
  5. https://www.nasa.gov/history/sputnik/index.html
  6. https://aerospace.org/article/brief-history-space-exploration
  7. https://www.nasa.gov/missions/station/iss-research/international-space-station-launching-nasa-and-humanity-into-deep-space/
  8. https://www.nasa.gov/humans-in-space/artemis/
  9. https://www.nasa.gov/blogs/spacestation/2025/08/24/spacex-dragon-cargo-mission-counts-down-to-launch/
  10. https://www.nasa.gov/international-space-station/
  11. https://www.etymonline.com/word/space
  12. https://space.stackexchange.com/questions/902/when-did-the-term-space-first-come-into-use
  13. https://www.etymonline.com/word/outer
  14. https://www.reddit.com/r/AskHistorians/comments/3ns9ib/when_did_space_become_known_as_space_and_what_was/
  15. https://www.newworldencyclopedia.org/entry/Outer_space
  16. https://www.britannica.com/science/space-exploration
  17. https://education.nationalgeographic.org/resource/history-space-exploration/
  18. https://www.unoosa.org/oosa/en/ourwork/spacelaw/treaties/introouterspacetreaty.html
  19. https://www.unoosa.org/res/oosadoc/data/documents/2022/aac_105c_22022crp/aac_105c_22022crp_24_0_html/AAC105_C2_2022_CRP24E.pdf
  20. https://www.astronomy.com/space-exploration/the-karman-line-where-does-space-begin/
  21. https://www.space.com/41286-outer-space-border-karman-line.html
  22. https://2009-2017.state.gov/s/l/22718.htm
  23. https://commons.erau.edu/cgi/viewcontent.cgi?article=1052&context=stm
  24. https://www.mks.com/n/vacuum-basics
  25. https://vacaero.com/information-resources/vac-aero-training/54916-space-simulation-chambers-part-1.html
  26. https://www.zmescience.com/feature-post/natural-sciences/physics-articles/matter-and-energy/what-is-the-vacuum-of-space/
  27. https://hypertextbook.com/facts/2000/DaWeiCai.shtml
  28. https://map.gsfc.nasa.gov/universe/uni_matter.html
  29. https://www.esa.int/Science_Exploration/Space_Science/Planck/The_cosmic_microwave_background_and_the_distribution_of_matter_in_the_Universe
  30. https://www.cfa.harvard.edu/research/topic/intergalactic-medium
  31. https://asd.gsfc.nasa.gov/archive/arcade/cmb_spectrum.html
  32. https://inspirehep.net/literature/836577
  33. https://www.space.com/how-cold-is-space
  34. https://www.sciencedirect.com/topics/earth-and-planetary-sciences/interstellar-radiation
  35. https://home.cern/science/physics/cosmic-rays-particles-outer-space
  36. https://www.nasa.gov/missions/analog-field-testing/why-space-radiation-matters/
  37. https://ned.ipac.caltech.edu/level5/Carroll2/Carroll1_3.html
  38. https://science.nasa.gov/mission/hubble/science/science-behind-the-discoveries/hubble-cosmological-redshift/
  39. https://www.history.com/articles/universe-expanding-edwin-hubble
  40. https://science.nasa.gov/mission/hubble/science/science-highlights/discovering-a-runaway-universe/
  41. https://ned.ipac.caltech.edu/level5/Peacock/Peacock3_2.html
  42. http://hyperphysics.phy-astr.gsu.edu/hbase/Astro/fried.html
  43. https://science.nasa.gov/dark-energy/
  44. https://www.nobelprize.org/uploads/2018/06/advanced-physicsprize2011.pdf
  45. https://science.nasa.gov/mission/hubble/science/science-behind-the-discoveries/hubble-constant-and-tension/
  46. https://www.scientificamerican.com/article/the-hubble-tension-is-becoming-a-hubble-crisis/
  47. https://cerncourier.com/a/the-hubble-tension/
  48. https://science.nasa.gov/universe/the-big-bang/
  49. https://www.space.com/25126-big-bang-theory.html
  50. https://www.magiscenter.com/blog/evidence-for-the-big-bang
  51. https://www.space.com/25945-cosmic-microwave-background-discovery-50th-anniversary.html
  52. https://www.esa.int/Science_Exploration/Space_Science/Cosmic_Microwave_Background_CMB_radiation
  53. https://www.nobelprize.org/uploads/2018/06/wilson-lecture-1.pdf
  54. https://www.esa.int/Science_Exploration/Space_Science/Planck/Planck_science_highlights
  55. https://www.uwa.edu.au/study/-/media/Faculties/Science/Docs/Evidence-for-the-Big-Bang.pdf
  56. https://www.ctc.cam.ac.uk/outreach/origins/inflation_zero.php
  57. https://arxiv.org/pdf/astro-ph/0210527
  58. https://www.sciencedirect.com/science/article/pii/S0370269323003623
  59. https://map.gsfc.nasa.gov/universe/bb_cosmo_struct.html
  60. https://www.esa.int/Science_Exploration/Space_Science/Planck/The_cosmic_microwave_background_and_inflation
  61. https://science.nasa.gov/dark-matter/
  62. https://www.cfa.harvard.edu/news/cosmic-evolution-galaxies
  63. https://ned.ipac.caltech.edu/level5/March02/Frenk/Frenk2_3.html
  64. https://dspace.mit.edu/bitstream/handle/1721.1/129432/1909.07976.pdf?sequence=2&isAllowed=y
  65. https://www.universetoday.com/articles/plenty-of-examples-that-giant-galaxies-like-the-milky-way-formed-through-mergers
  66. https://ned.ipac.caltech.edu/level5/Sept09/Einasto/Einasto5.html
  67. https://arxiv.org/abs/1202.6633
  68. https://academic.oup.com/mnras/article/414/3/1813/1034245
  69. https://www.jb.man.ac.uk/distance/frontiers/cmb/node12.htm
  70. https://www.esa.int/Space_Safety/Space_Debris/ESA_Space_Environment_Report_2025
  71. https://www.psu.edu/news/engineering/story/qa-what-space-between-earth-and-moon-and-why-does-it-matter
  72. https://medium.com/the-aerospace-corporation/its-international-moon-day-let-s-talk-about-cislunar-space-9d108f1a1b0b
  73. https://www.lowyinstitute.org/the-interpreter/how-low-earth-orbiting-satellites-can-help-innovation-meet-inclusion
  74. https://www.unoosa.org/res/oosadoc/data/documents/2025/aac_105c_12025crp/aac_105c_12025crp_10_0_html/AC105_C1_2025_CRP10E.pdf
  75. https://science.nasa.gov/biological-physical/stories/van-allen-belts/
  76. https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2018JA025940
  77. https://conference.sdo.esoc.esa.int/proceedings/sdc8/paper/261
  78. https://solarscience.msfc.nasa.gov/SolarWind.shtml
  79. https://www.swpc.noaa.gov/phenomena/solar-wind
  80. http://astroa.physics.metu.edu.tr/nineplanets/medium.html
  81. https://www.swpc.noaa.gov/phenomena/galactic-cosmic-rays
  82. https://ntrs.nasa.gov/api/citations/19710010992/downloads/19710010992.pdf
  83. https://solarscience.msfc.nasa.gov/people/suess/Interstellar_Probe/IMF/IMF.html
  84. https://ned.ipac.caltech.edu/level5/March01/Brinks/Brinks4.html
  85. https://fiveable.me/astrophysics-i/unit-7
  86. https://arxiv.org/html/2504.01410v1
  87. https://www.cfa.harvard.edu/research/topic/interstellar-medium-and-molecular-clouds
  88. https://home.ifa.hawaii.edu/users/jpw/classes/ast622/reading/klessen_glover_saasfe.pdf
  89. https://www.astronomy.ohio-state.edu/ryden.1/ast162_3/notes11.html
  90. https://www.astro.rug.nl/~weygaert/tim1publication/weybondgh2005.paper2.pdf
  91. https://www.americanscientist.org/article/the-cosmic-web
  92. https://academic.oup.com/mnras/article/441/4/2923/1213214
  93. https://www.skyatnightmagazine.com/space-science/bootes-void
  94. https://ned.ipac.caltech.edu/level5/Bothun2/Bothun3_5_3.html
  95. https://www.aanda.org/articles/aa/full_html/2011/10/aa17248-11/aa17248-11.html
  96. https://astronomy.stackexchange.com/questions/47279/what-are-the-differences-between-intergalactic-medium-interstellar-medium-and-i
  97. https://www.cfa.harvard.edu/research/topic/spectroscopy
  98. https://www.eso.org/public/teles-instr/technology/spectroscopy/
  99. https://courses.lumenlearning.com/suny-astronomy/chapter/observations-outside-earths-atmosphere/
  100. https://www.schoolsobservatory.org/learn/telescopes/ground
  101. https://www.space.com/biggest-telescopes-on-earth
  102. https://public.nrao.edu/radio-astronomy/the-science-of-radio-astronomy/
  103. https://science.nasa.gov/mission/hubble/observatory/design/instruments/
  104. https://lco.global/spacebook/telescopes/space-telescopes/
  105. https://science.nasa.gov/people/edwin-hubble/
  106. https://www.aps.org/apsnews/2002/07/discovery-cosmic-microwave-background
  107. https://www.astronomy.com/science/how-the-first-exoplanets-were-discovered/
  108. https://eventhorizontelescope.org/blog/astronomers-reveal-first-image-black-hole-heart-our-galaxy
  109. https://phys.org/news/2025-07-years-science-ten-cosmic-nasa.html
  110. https://www.rmg.co.uk/stories/space-astronomy/space-race-timeline
  111. https://www.usatoday.com/picture-gallery/money/2020/05/01/the-40-most-important-events-in-the-history-of-space-exploration/111607776/
  112. https://www.britannica.com/science/space-exploration/Major-milestones
  113. https://www.fai.org/page/icare-history-pioneers
  114. https://www.unoosa.org/oosa/timeline/
  115. https://www.freethink.com/space/history-of-space-exploration
  116. https://www.nasa.gov/humans-in-space/the-human-body-in-space/
  117. https://pmc.ncbi.nlm.nih.gov/articles/PMC4206856/
  118. https://www.nasa.gov/missions/station/iss-research/counteracting-bone-and-muscle-loss-in-microgravity/
  119. https://www.uclahealth.org/news/article/living-zero-gravity-causes-many-changes-body
  120. https://www.nasa.gov/hrp/radiation/space-radiation-risks/
  121. https://nap.nationalacademies.org/catalog/26155/space-radiation-and-astronaut-health-managing-and-communicating-cancer-risks
  122. https://www.nasa.gov/missions/station/iss-research/mental-well-being-in-space/
  123. https://pmc.ncbi.nlm.nih.gov/articles/PMC8696290/
  124. https://www.nature.com/articles/s41398-023-02638-5
  125. https://www.nasa.gov/specials/60counting/spaceflight.html
  126. https://airandspace.si.edu/explore/stories/tools-and-technology-apollo-program
  127. https://www.nasa.gov/wp-content/uploads/2019/08/466722main_ap_st_hist_techimpacts_10-12-09.pdf
  128. https://science.howstuffworks.com/ten-nasa-achievements.htm
  129. https://www.nasa.gov
  130. https://explorer1.jpl.nasa.gov/explore-as-one/then-and-now/satellite-evolution/
  131. https://www.nasa.gov/missions/station/20-breakthroughs-from-20-years-of-science-aboard-the-international-space-station/
  132. https://issnationallab.org/education/the-iss-engineering-feat-design/
  133. https://www.spacex.com/vehicles/falcon-9/
  134. https://www.spacex.com/updates
  135. https://www.unoosa.org/oosa/en/ourwork/spacelaw/treaties.html
  136. https://www.unoosa.org/oosa/en/ourwork/spacelaw/treaties/outerspacetreaty.html
  137. https://treaties.un.org/pages/showdetails.aspx?objid=0800000280128cbd
  138. https://2009-2017.state.gov/t/isn/5181.htm
  139. https://www.unoosa.org/oosa/en/ourwork/spacelaw/treaties/rescueagreement.html
  140. https://treaties.un.org/pages/showDetails.aspx?objid=080000028012504f
  141. https://www.unoosa.org/oosa/en/ourwork/spacelaw/treaties/liability-convention.html
  142. https://treaties.un.org/pages/showdetails.aspx?objid=08000002801098c7
  143. https://www.unoosa.org/oosa/en/ourwork/spacelaw/treaties/registration-convention.html
  144. https://treaties.un.org/pages/ViewDetails.aspx?src=TREATY&mtdsg_no=XXIV-1&chapter=24&clang=_en
  145. https://www.unoosa.org/oosa/en/ourwork/spacelaw/treaties/moon-agreement.html
  146. https://treaties.un.org/pages/ViewDetails.aspx?src=TREATY&mtdsg_no=XXIV-2&chapter=24&clang=_en
  147. https://www.cfc.forces.gc.ca/259/290/301/305/roberds.pdf
  148. https://nyujilp.org/houston-we-have-a-problem-international-laws-inability-to-regulate-space-exploration/
  149. https://digitalcommons.law.seattleu.edu/cgi/viewcontent.cgi?article=1022&context=sjteil
  150. https://warontherocks.com/2025/03/autonomy-has-outpaced-international-space-law/
  151. https://www.unoosa.org/documents/pdf/spacelaw/activities/2019/T3-2-CJ_Deficiencies_and_Pressing_Issues_in_the_Existing_Legal_Regime_of_Outer_Space.pdf
  152. https://warontherocks.com/2022/08/stabilize-friction-points-in-space-by-amending-the-1967-outer-space-treaty/
  153. https://www.oxjournal.org/loopholes-lacunae-space-law/
  154. https://eujournalfuturesresearch.springeropen.com/articles/10.1186/s40309-025-00254-8
  155. https://deloitte.wsj.com/cio/space-industry-growth-5-actions-to-guide-its-trajectory-64d36c33
  156. https://science.nasa.gov/mission/hubble/observatory/hubble-vs-webb/
  157. https://www.rmg.co.uk/stories/space-astronomy/what-has-hubble-space-telescope-discovered
  158. https://science.nasa.gov/mission/webb/
  159. https://esahubble.org/wordbank/james-webb-space-telescope/
  160. https://science.nasa.gov/science-missions/
  161. https://www.britannica.com/science/space-exploration/Science-in-space
  162. https://issnationallab.org/press-releases/2024-iss-national-lab-scientific-results/
  163. https://www.nasa.gov/missions/station/iss-research/nasas-2024-international-space-station-achievements/
  164. https://www.taoglas.com/blogs/how-does-satellite-communication-work/
  165. https://ceos.org/document_management/Publications/Data_Applications_Report/DAR_Summary-Brochure_Digital-Version_Dec2015.pdf
  166. https://www.eurisy.eu/satellite-applications/
  167. https://www.oecd.org/content/dam/oecd/en/publications/reports/2023/12/the-space-economy-in-figures_4c52ae39/fa5494aa-en.pdf
  168. https://www.spacefoundation.org/2025/07/22/the-space-report-2025-q2/
  169. https://www.deloitte.com/us/en/insights/industry/government-public-sector-services/government-trends/2025/space-industry-growth.html
  170. https://www.space.com/space-exploration/private-spaceflight/blue-origin-launches-space-nomads-ns-36-space-tourists-flight
  171. https://www.cbsnews.com/news/space-tourism-suborbital-flights-blue-orgin-virgin-galactic-spacex/
  172. https://www.gminsights.com/industry-analysis/space-economy-market
  173. https://www.eucom.mil/article/42685/the-critical-role-of-space-in-modern-warfare-and-the-imperative-of-joint-space-capabilitie
  174. https://media.defense.gov/2020/Jun/17/2002317391/-1/-1/1/2020_defense_space_strategy_summary.pdf
  175. https://www.war.gov/News/News-Stories/Article/Article/3465982/space-integral-to-the-dod-way-of-war-policy-chief-says/
  176. https://www.cna.org/reports/2021/11/Space-Domain-Awareness-as-a-Strategic-Counterweight.pdf
  177. https://www.starcom.spaceforce.mil/Portals/2/SDP%25203-100%2520Space%2520Domain%2520Awareness%2520%28November%25202023%29_pdf_safe.pdf
  178. https://www.starcom.spaceforce.mil/Portals/2/Space%2520Force%2520Doctrine%2520Document%25201%2520FINAL_4Apr25.pdf
  179. https://www.airandspaceforces.com/space-force-new-core-doctrine-warfighting/
  180. https://www.spaceforce.mil/About-Us/Fact-Sheets/Fact-Sheet-Display/Article/4297159/space-threat-fact-sheet/
  181. https://nssaspace.org/wp-content/uploads/2025/05/20250516-S2-Space-Threat-Fact-Sheet-v8-RELEASE.pdf
  182. https://www.swfound.org/publications-and-reports/2025-global-counterspace-capabilities-report
  183. https://www.swfound.org/publications-and-reports/u-s-direct-ascent-anti-satellite-testing-fact-sheet
  184. https://www.space.com/anti-satellite-weapons-asats
  185. https://www.kslaw.com/news-and-insights/anti-satellite-tests-and-the-growing-demand-for-space-debris-mitigation
  186. https://www.dia.mil/Portals/110/Documents/News/Military_Power_Publications/Challenges_Security_Space_2022.pdf
  187. https://orbitaldebris.jsc.nasa.gov/mitigation/
  188. https://keeptrack.space/debris/debris/satellite-debris
  189. https://www.space.com/kessler-syndrome-space-debris
  190. https://aerospaceamerica.aiaa.org/features/understanding-the-misunderstood-kessler-syndrome/
  191. https://www.unoosa.org/pdf/publications/st_space_49E.pdf
  192. https://www.unoosa.org/res/oosadoc/data/documents/2025/aac_105c_12025crp/aac_105c_12025crp_9_0_html/AC105_C1_2025_CRP09E.pdf
  193. https://www.esa.int/Space_Safety/Space_Debris/Mitigating_space_debris_generation
  194. https://planet4589.org/space/stats/active.html
  195. https://earthsky.org/human-world/1-to-2-starlink-satellites-falling-back-to-earth-each-day/
  196. https://www.nature.com/articles/s41598-021-89909-7
  197. https://arstechnica.com/space/2025/10/everyone-but-china-has-pretty-much-stopped-littering-in-low-earth-orbit/
  198. https://www.sciencedirect.com/science/article/abs/pii/S0377221722003447
  199. https://www.quora.com/Is-space-exploration-a-waste-of-money-when-there-are-so-many-problems-on-Earth
  200. https://theperennial.org/2763/opinion/stop-spending-money-on-space-exploration/
  201. https://onlinelibrary.wiley.com/doi/10.1111/ecaf.12672
  202. https://ig.space/commslink/the-controversial-case-for-human-space-exploration/
  203. https://andreeamosila.com/astro-economics-the-economic-potential-and-pitfalls-of-space-exploration/
  204. https://www.stimson.org/2023/the-booming-wild-west-space-economy-faces-a-rules-deficit/
  205. https://cms-lawnow.com/en/ealerts/2025/07/space-mining-breach-of-international-law-in-space
  206. https://www.congress.gov/crs-product/R48144
  207. https://www.sciencedirect.com/science/article/abs/pii/S0160791X99000032
  208. https://pmc.ncbi.nlm.nih.gov/articles/PMC3698687/
  209. https://www.nature.com/articles/s41467-023-44357-x
  210. https://aeon.co/essays/as-space-gets-more-commercial-how-can-it-be-governed-ethically
  211. https://carnegiecouncil.org/media/article/ethical-dilemmas-of-space-memorial-services
  212. https://vce.usc.edu/volume-7-issue-2/lost-in-space-the-ethics-of-the-private-space-race/
  213. https://interestingengineering.com/science/what-is-the-probability-of-a-huge-civilization-ending-asteroid-impact
  214. https://www.sciencedirect.com/science/article/pii/S0016328722000337
  215. https://www.popularmechanics.com/space/solar-system/a46283964/most-dangerous-threats-from-space-ranked/
  216. https://astrobiology.nasa.gov/news/how-deadly-would-a-nearby-gamma-ray-burst-be/
  217. https://www.science.org/content/article/cosmic-blast-seared-earth-s-atmosphere-2-billion-light-years-away
  218. https://www.esa.int/Science_Exploration/Space_Science/Space_for_you/Gamma-ray_bursts_are_we_safe
  219. https://forum.effectivealtruism.org/posts/fGCHomw45xuTHd757/an-exhaustive-list-of-cosmic-threats
  220. https://www.scientificamerican.com/article/is-earth-safe-from-a-nearby-supernova/
  221. https://earthsky.org/space/threat-of-supernovas-to-life-on-earth/
  222. https://scitechdaily.com/supernova-death-blasts-may-have-wiped-out-life-on-earth-twice/
  223. https://phys.org/news/2025-03-nearby-supernova-mass-extinction.html
  224. https://pmc.ncbi.nlm.nih.gov/articles/PMC7474607/
  225. https://www.newyorker.com/magazine/2024/03/04/what-a-major-solar-storm-could-do-to-our-planet
  226. https://www.popularmechanics.com/space/solar-system/a61679565/scientists-predict-solar-storms/
  227. https://medium.com/starts-with-a-bang/ask-ethan-how-vulnerable-is-earth-to-a-solar-flare-e8431b09fe8c
  228. https://www.astronomy.com/science/these-are-the-ways-our-world-will-end/
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