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Outer space
For centuries, the greatest minds in history believed that nature abhorred a vacuum, a principle known as horror vacui. Aristotle, the Greek philosopher of 350 BCE, argued that empty space could not exist, a view that dominated Western thought for two millennia. Even as late as the 17th century, the French philosopher René Descartes insisted that the entirety of space must be filled with some substance. This belief persisted until 1643, when Evangelista Torricelli, a pupil of Galileo Galilei, constructed the first mercury barometer. Torricelli demonstrated that air has weight and that atmospheric pressure decreases with altitude, creating a partial vacuum. The scientific community was shaken by this discovery, which suggested that the space between the Earth and the Moon might indeed be empty. In 1650, German scientist Otto von Guericke constructed the first vacuum pump, further refuting the ancient principle. He correctly noted that the atmosphere of the Earth surrounds the planet like a shell, with density gradually declining with altitude. This conclusion led him to assert that there must be a vacuum between the Earth and the Moon, a radical idea that challenged the very fabric of reality as understood by his contemporaries.
The Infinite Scale
The true scale of the universe remained a mystery until the 19th century, when astronomers began to measure the vast distances between stars. In 1838, the German astronomer Friedrich Bessel successfully measured the distance to the nearby star 61 Cygni, showing it had a parallax of just 0.31 arcseconds. This corresponded to a distance of over 10 light years, a figure that was difficult for the human mind to comprehend. The scale of the universe expanded even further in 1923, when American astronomer Edwin Hubble determined the distance to the Andromeda Galaxy by measuring the brightness of cepheid variables. This established that the Andromeda Galaxy, and by extension all galaxies, lay well outside the Milky Way. 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. The modern concept of outer space is based on the Big Bang cosmology, first proposed in 1931 by the Belgian physicist Georges Lemaître. This theory holds that the universe originated from a state of extreme energy density that has since undergone continuous expansion. The earliest known estimate of the temperature of outer space was by the Swiss physicist Charles É. Guillaume in 1896, who 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, and German physicist Erich Regener used the total measured energy of cosmic rays to estimate an intergalactic temperature of 2.8 K in 1933.
Common questions
What is the definition of outer space according to the Kármán line?
The Kármán line is set at an altitude of 100 kilometers 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. The Fédération Aéronautique Internationale and the United Nations use this 100 kilometer altitude internationally.
Who first measured the distance to the nearby star 61 Cygni and when did this occur?
The German astronomer Friedrich Bessel successfully measured the distance to the nearby star 61 Cygni in 1838. This measurement showed the star had a parallax of just 0.31 arcseconds, corresponding to a distance of over 10 light years. This discovery helped astronomers begin to measure the vast distances between stars in the 19th century.
What happens to human fluids when exposed to the pressure of the Armstrong line?
At the Armstrong line, which is an altitude of around 100 kilometers, fluids in the throat and lungs boil away due to the matching vapor pressure of water at the temperature of the human body. This condition is called ebullism and occurs when pressure drops below 6.3 kilopascals. Exposed bodily liquids such as saliva, tears, and liquids in the lungs boil away, causing the body to bloat to twice its normal size.
When was the Outer Space Treaty passed and which countries signed it in 1967?
The Outer Space Treaty was passed by the United Nations General Assembly in 1963 and signed in 1967 by the Union of Soviet Socialist Republics, the United States of America, and the United Kingdom. As of 2017, 105 state parties have either ratified or acceded to the treaty. The treaty states that outer space is free for all nation states to explore and is not subject to claims of national sovereignty.
What is the warm hot intergalactic medium and how does it form?
The warm hot intergalactic medium is a rarefied plasma organized in a galactic filamentary structure that heats up to temperatures of 105 K to 107 K when gas falls from voids. This plasma contains filaments of higher density, about one atom per cubic meter, which is 5, 200 times the average density of the universe. Computer simulations and observations indicate that up to half of the atomic matter in the universe might exist in this warm, hot, rarefied state.
When did Voyager 1 become the first man-made object to leave the Solar System?
In August 2012, Voyager 1 became the first man-made object to leave the Solar System and enter interstellar space. This event marked a significant milestone after the spacecraft had performed fly-bys of the Moon and other planets. The placing of artificial satellites in Earth orbit has produced numerous benefits, including relay of long-range communications and precise navigation.
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, and seeds of Arabidopsis thaliana and Nicotiana tabacum germinated after being exposed to space for 1.5 years. A strain of Bacillus subtilis has survived 559 days when exposed to low Earth orbit or a simulated Martian environment. However, for humans, the lack of pressure in space is the most immediate dangerous characteristic. Pressure decreases above Earth, reaching a level at an altitude of around 100 kilometers 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. At or above the Armstrong line, fluids in the throat and lungs boil away. More specifically, exposed bodily liquids such as saliva, tears, and liquids in the lungs boil away. Hence, at this altitude, human survival requires a pressure suit, or a pressurized capsule. Out in space, sudden exposure of an unprotected human to very low pressure, such as during a rapid decompression, can cause pulmonary barotrauma, a rupture of the lungs, due to the large pressure differential between inside and outside the chest. Even if the subject's airway is fully open, the flow of air through the windpipe may be too slow to prevent the rupture. Rapid decompression can rupture eardrums and sinuses, bruising and blood seep can occur in soft tissues, and shock can cause an increase in oxygen consumption that leads to hypoxia. As a consequence of rapid decompression, oxygen dissolved in the blood empties into the lungs to try to equalize the partial pressure gradient. Once the deoxygenated blood arrives at the brain, humans lose consciousness after a few seconds and die of hypoxia within minutes. Blood and other body fluids boil when the pressure drops below 6.3 kilopascals, and this condition is called ebullism. The steam may bloat the body to twice its normal size and slow circulation, but tissues are elastic and porous enough to prevent rupture. Ebullism is slowed by the pressure containment of blood vessels, so some blood remains liquid. Swelling and ebullism can be reduced by containment in a pressure suit. The Crew Altitude Protection Suit, a fitted elastic garment designed in the 1960s for astronauts, prevents ebullism at pressures as low as 6.3 kilopascals. Supplemental oxygen is needed at 16 kilopascals to provide enough oxygen for breathing and to prevent water loss, while above 16 kilopascals pressure suits are essential to prevent ebullism. Most space suits use around 34 kilopascals 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. Weightlessness and radiation also pose significant threats. Humans evolved for life in Earth gravity, and exposure to weightlessness has been shown to have deleterious effects on human health. Initially, more than 50% of astronauts experience space motion sickness. This can cause nausea and vomiting, vertigo, headaches, lethargy, and overall malaise. The duration of space sickness varies, but it typically lasts for 1, 3 days, after which the body adjusts to the new environment. Longer-term exposure to weightlessness results in muscle atrophy and deterioration of the skeleton, or spaceflight osteopenia. These effects can be minimized through a regimen of exercise. Other effects include fluid redistribution, slowing of the cardiovascular system, decreased production of red blood cells, balance disorders, and a weakening of the immune system. Lesser symptoms include loss of body mass, nasal congestion, sleep disturbance, and puffiness of the face. During long-duration space travel, radiation can pose an acute health hazard. Exposure to high-energy, ionizing cosmic rays can result in fatigue, nausea, vomiting, as well as damage to the immune system and changes to the white blood cell count. Over longer durations, symptoms include an increased risk of cancer, plus damage to the eyes, nervous system, lungs and the gastrointestinal tract. On a round-trip Mars mission lasting three years, a large fraction of the cells in an astronaut's body would be traversed and potentially damaged by high energy nuclei. The energy of such particles is significantly diminished by the shielding provided by the walls of a spacecraft and can be further diminished by water containers and other barriers. The impact of the cosmic rays upon the shielding produces additional radiation that can affect the crew. Further research is needed to assess the radiation hazards and determine suitable countermeasures.
The Legal Frontier
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 80 kilometers out to 100 kilometers. In 2009, measurements of the direction and speed of ions in the atmosphere were made from a sounding rocket. The altitude of 80 kilometers 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 7.8 kilometers per second. High-altitude aircraft, such as high-altitude balloons have reached altitudes above Earth of up to 50 kilometers. Up until 2021, the United States designated people who travel above an altitude of 80 kilometers as astronauts. 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. 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 50 to 100 kilometers. For safety reasons, commercial aircraft are typically limited to altitudes of 15 kilometers, and air navigation services only extend to 18 kilometers. The upper limit of the range is the Kármán line, where astrodynamics must take over from aerodynamics in order to achieve flight. This range includes the stratosphere, mesosphere and lower thermosphere layers of the Earth's atmosphere. Larger ranges for near space are used by some authors, such as 100 kilometers. These extend to the altitudes where orbital flight in very low Earth orbits becomes practical. Spacecraft have entered into a highly elliptical orbit with a perigee as low as 100 kilometers, surviving for multiple orbits. At an altitude of 100 kilometers, 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. The Kármán line, established by the Fédération Aéronautique Internationale, and used internationally by the United Nations, is set at an altitude of 100 kilometers 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, which he calculated to be at an altitude of about 100 kilometers. This distinguishes altitudes below 100 kilometers as the region of aerodynamics and airspace, and above 100 kilometers as the space of astronautics and free space. 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. 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. Spacecraft have flown over foreign countries as low as 100 kilometers, as in the example of the Space Shuttle. 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. 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, the United States of America, and the United Kingdom. As of 2017, 105 state parties have either ratified or acceded to the treaty. An additional 25 states signed the treaty, without ratifying it. Since 1958, outer space has been the subject of multiple United Nations resolutions. Of these, more than 50 have been concerning the international co-operation in the peaceful uses of outer space and preventing an arms race in space. Four additional space law treaties have been negotiated and drafted by the UN's Committee on the Peaceful Uses of Outer Space. Still, there remains no legal prohibition against deploying conventional weapons in space, and anti-satellite weapons have been successfully tested by the USA, USSR, China, and in 2019, India. The 1979 Moon Treaty turned the jurisdiction of all heavenly bodies (including the orbits around such bodies) over to the international community. The treaty has not been ratified by any nation that currently practices human spaceflight. 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. These claims are not internationally accepted. An increasing issue of international space law and regulation has been the dangers of the growing number of space debris.
The Cosmic Web
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. Surrounding and stretching between galaxies is the intergalactic medium. This rarefied plasma is organized in a galactic filamentary structure. The diffuse photoionized gas contains filaments of higher density, about one atom per cubic meter, which is 5, 200 times the average density of the universe. 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. As gas falls into the intergalactic medium from the voids, it heats up to temperatures of 105 K to 107 K. At these temperatures, it is called the warm, hot intergalactic medium. Although the plasma is very hot by terrestrial standards, 105 K is often called warm in astrophysics. Computer simulations and observations indicate that up to half of the atomic matter in the universe might exist in this warm, hot, rarefied state. When gas falls from the filamentary structures of the WHIM into the galaxy clusters at the intersections of the cosmic filaments, it can heat up even more, reaching temperatures of 108 K and above in the so-called intracluster medium. Interstellar space is the physical space outside of the bubbles of plasma known as astrospheres, formed by stellar winds originating from individual stars. It is the space between the stars or stellar systems within a nebula or galaxy. 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. Approximately 70% of the mass of the interstellar medium consists of lone hydrogen atoms; most of the remainder consists of helium atoms. This is enriched with trace amounts of heavier atoms formed through stellar nucleosynthesis. These atoms are ejected into the interstellar medium by stellar winds or when evolved stars begin to shed their outer envelopes such as during the formation of a planetary nebula. The cataclysmic explosion of a supernova propagates shock waves of stellar ejecta outward, distributing it throughout the interstellar medium, including the heavy elements previously formed within the star's core. The density of matter in the interstellar medium can vary considerably: the average is around 106 particles per m3, but cold molecular clouds can hold 108, 1012 per m3. A number of molecules exist in interstellar space, which can form dust particles as tiny as 0.1 micrometers. The tally of molecules discovered through radio astronomy is steadily increasing at the rate of about four new species per year. Large regions of higher density matter known as molecular clouds allow chemical reactions to occur, including the formation of organic polyatomic species. Much of this chemistry is driven by collisions. Energetic cosmic rays penetrate the cold, dense clouds and ionize hydrogen and helium, resulting, for example, in the trihydrogen cation. An ionized helium atom can then split relatively abundant carbon monoxide to produce ionized carbon, which in turn can lead to organic chemical reactions. The local interstellar medium is a region of space within 100 parsecs 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 parsecs 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 parsecs. 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 and NASA's Voyager probes showed that the Sun's bow shock does not exist. Instead, these authors argue that a subsonic bow wave defines the transition from the solar wind flow to the interstellar medium. A bow shock is a third boundary characteristic of an astrosphere, lying outside the termination shock and the astropause.
The Human Footprint
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 kilometers. This was greatly exceeded in 1942 when the third launch of the German A-4 rocket climbed to an altitude of about 189 kilometers. In 1957, the uncrewed satellite Sputnik 1 was launched by a Russian R-7 rocket, achieving Earth orbit at an altitude of 215 kilometers. 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 and reached a maximum distance of 400,000 kilometers from the Earth. The first spacecraft to reach escape velocity was the Soviet Luna 1, which performed a fly-by of the Moon in 1959. In 1961, Venera 1 became the first planetary probe. It revealed the presence of the solar wind and performed the first fly-by of Venus, although contact was lost before reaching Venus. The first successful planetary mission was the 1962 fly-by of Venus by Mariner 2. The first fly-by of Mars was by Mariner 4 in 1964. Since that time, 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. In August 2012, Voyager 1 became the first man-made object to leave the Solar System and enter interstellar space. 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. They facilitate the discovery and monitoring of climate change influences. 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. 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. Not every location in space is ideal for a telescope. The interplanetary zodiacal dust emits a diffuse near-infrared radiation that can mask the emission of faint sources such as extrasolar planets. Moving an infrared telescope out past the dust increases its effectiveness. Likewise, a site like the Daedalus crater on the far side of the Moon could shield a radio telescope from the radio frequency interference that hampers Earth-based observations. The deep vacuum of space could make it an attractive environment for certain industrial processes, such as those requiring ultraclean surfaces. Like asteroid mining, space manufacturing would require a large financial investment with little prospect of immediate return. An important factor in the total expense is the high cost of placing mass into Earth orbit: $10,000 to $20,000 per kg, according to a 2006 estimate (allowing for inflation since then). The cost of access to space has declined since 2013. Partially reusable rockets such as the Falcon 9 have lowered access to space below $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. 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. 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. This enhances longwave infrared thermal radiation heat transfer through the atmosphere's infrared window into outer space, lowering ambient temperatures. Photonic metamaterials can be used to suppress solar heating.