Vacuum pump andbell jar for vacuum experiments, used in science education during the early 20th century, on display in the Schulhistorische Sammlung ('School Historical Museum'),Bremerhaven, Germany
Avacuum (pl.:vacuums orvacua) isspace devoid ofmatter. The word is derived from the Latin adjectivevacuus (neutervacuum) meaning "vacant" or "void". An approximation to such vacuum is a region with a gaseouspressure much less thanatmospheric pressure.[1] Physicists often discuss ideal test results that would occur in aperfect vacuum, which they sometimes simply call "vacuum" orfree space, and use the termpartial vacuum to refer to an actual imperfect vacuum as one might have in alaboratory or inspace. Inengineering and applied physics on the other hand, vacuum refers to any space in which the pressure is considerably lower than atmospheric pressure.[2] The Latin termin vacuo is used to describe an object that is surrounded by a vacuum.
Thequality of a partial vacuum refers to how closely it approaches a perfect vacuum. Other things equal, lower gaspressure means higher-quality vacuum. For example, a typicalvacuum cleaner produces enoughsuction to reduce air pressure by around 20%.[3] But higher-quality vacuums are possible.Ultra-high vacuum chambers, common in chemistry, physics, and engineering, operate below one trillionth (10−12) of atmospheric pressure (100 nPa), and can reach around 100 particles/cm3.[4]Outer space is an even higher-quality vacuum, with the equivalent of just a few hydrogen atoms per cubic meter on average in intergalactic space.[5]
Vacuum has been a frequent topic of philosophical debate since ancientGreek times, but was not studied empirically until the 17th century.Clemens Timpler (1605) philosophized about the experimental possibility of producing a vacuum in small tubes.[6]Evangelista Torricelli produced the first laboratory vacuum in 1643, and other experimental techniques were developed as a result of his theories of atmospheric pressure. A Torricellian vacuum is created by filling with mercury a tall glass container closed at one end, and then inverting it in a bowl to contain the mercury (see below).[7]
Vacuum became a valuable industrial tool in the 20th century with the introduction ofincandescent light bulbs andvacuum tubes, and a wide array of vacuum technologies has since become available. The development ofhuman spaceflight has raised interest in the impact of vacuum on human health, and on life forms in general.
Historically, there has been much dispute over whether such a thing as a vacuum can exist. AncientGreek philosophers debated the existence of a vacuum, or void, in the context ofatomism, which positedvoid and atom as the fundamental explanatory elements of physics.Lucretius argued for the existence of vacuum in the first century BC andHero of Alexandria tried unsuccessfully to create an artificial vacuum in the first century AD.[9]
FollowingPlato, however, even the abstract concept of a featureless void faced considerable skepticism: it could not be apprehended by the senses, it could not, itself, provide additional explanatory power beyond the physical volume with which it was commensurate and, by definition, it was quite literally nothing at all, which cannot rightly be said to exist.Aristotle believed that no void could occur naturally, because the denser surrounding material continuum would immediately fill any incipient rarity that might give rise to a void. In hisPhysics, book IV, Aristotle offered numerous arguments against the void: for example, that motion through a medium which offered no impediment could continuead infinitum, there being no reason that something would come to rest anywhere in particular.
In the medievalMuslim world, the physicist andIslamic scholarAl-Farabi wrote a treatise rejecting the existence of the vacuum in the 10th century.[10] He concluded that air's volume can expand to fill available space, and therefore the concept of a perfect vacuum was incoherent.[11] According toAhmad Dallal,Abū Rayhān al-Bīrūnī states that "there is no observable evidence that rules out the possibility of vacuum".[12] Thesuctionpump was described by Arab engineerAl-Jazari in the 13th century, and later appeared in Europe from the 15th century.[13][14]
Europeanscholars such asRoger Bacon,Blasius of Parma andWalter Burley in the 13th and 14th century focused considerable attention on issues concerning theconcept of a vacuum. The commonly held view that nature abhorred a vacuum was calledhorror vacui. There was even speculation that even God could not create a vacuum if he wanted and the 1277Paris condemnations ofBishopÉtienne Tempier, which required there to be no restrictions on the powers of God, led to the conclusion that God could create a vacuum if he so wished.[15] From the 14th century onward increasingly departed from the Aristotelian perspective, scholars widely acknowledged that asupernatural void exists beyond the confines of the cosmos itself by the 17th century. This idea, influenced byStoic physics, helped to segregate natural and theological concerns.[16]
Almost two thousand years after Plato,René Descartes also proposed ageometrically based alternative theory of atomism, without the problematic nothing–everythingdichotomy of void and atom. Although Descartes agreed with the contemporary position, that a vacuum does not occur in nature, the success of hisnamesake coordinate system and more implicitly, the spatial–corporeal component of his metaphysics would come to define the philosophically modern notion of empty space as a quantified extension of volume. By the ancient definition however, directional information and magnitude were conceptually distinct.[citation needed]
Medievalthought experiments into the idea of a vacuum considered whether a vacuum was present, if only for an instant, between two flat plates when they were rapidly separated.[17] There was much discussion of whether the air moved in quickly enough as the plates were separated, or, asWalter Burley postulated, whether a 'celestial agent' prevented the vacuum arising.Jean Buridan reported in the 14th century that teams of ten horses could not pull openbellows when the port was sealed.[9]
In 1654,Otto von Guericke invented the firstvacuum pump[19] and conducted his famousMagdeburg hemispheres experiment, showing that, owing to atmospheric pressure outside the hemispheres, teams of horses could not separate two hemispheres from which the air had been partially evacuated.Robert Boyle improved Guericke's design and with the help ofRobert Hooke further developed vacuum pump technology. Thereafter, research into the partial vacuum lapsed until 1850 whenAugust Toepler invented theToepler pump and in 1855 whenHeinrich Geissler invented the mercury displacement pump, achieving a partial vacuum of about 10 Pa (0.1 Torr). A number of electrical properties become observable at this vacuum level, which renewed interest in further research.
While outer space provides the most rarefied example of a naturally occurring partial vacuum, the heavens were originally thought to be seamlessly filled by a rigid indestructible material calledaether. Borrowing somewhat from thepneuma ofStoic physics, aether came to be regarded as the rarefied air from which it took its name, (seeAether (mythology)). Early theories of light posited a ubiquitous terrestrial and celestial medium through which light propagated. Additionally, the concept informedIsaac Newton's explanations of bothrefraction and of radiant heat.[20] 19th century experiments into thisluminiferous aether attempted to detect a minute drag on the Earth's orbit. While the Earth does, in fact, move through a relatively dense medium in comparison to that of interstellar space, the drag is so minuscule that it could not be detected. In 1912,astronomerHenry Pickering commented: "While the interstellar absorbing medium may be simply the ether, [it] is characteristic of a gas, and free gaseous molecules are certainly there".[21] Thereafter, however, luminiferous aether was discarded.
Later, in 1930,Paul Dirac proposed a model of the vacuum as an infinite sea of particles possessing negative energy, called theDirac sea. This theory helped refine the predictions of his earlier formulatedDirac equation, and successfully predicted the existence of thepositron, confirmed two years later.Werner Heisenberg'suncertainty principle, formulated in 1927, predicted a fundamental limit within which instantaneous position andmomentum, or energy and time can be measured. This far reaching consequences also threatened whether the "emptiness" of space between particles exists.
The strictest criterion to define a vacuum is a region of space and time where all the components of thestress–energy tensor are zero. This means that this region is devoid of energy and momentum, and by consequence, it must be empty of particles and other physical fields (such as electromagnetism) that contain energy and momentum.
Ingeneral relativity, a vanishing stress–energy tensor implies, throughEinstein field equations, the vanishing of all the components of theRicci tensor. Vacuum does not mean that the curvature ofspace-time is necessarily flat: the gravitational field can still produce curvature in a vacuum in the form of tidal forces andgravitational waves (technically, these phenomena are the components of theWeyl tensor). Theblack hole (with zero electric charge) is an elegant example of a region completely "filled" with vacuum, but still showing a strong curvature.
In the theory of classical electromagnetism, free space has the following properties:
Electromagnetic radiation travels, when unobstructed, at thespeed of light, the defined value 299,792,458 m/s inSI units.[27]
Thesuperposition principle is always exactly true.[28] For example, the electric potential generated by two charges is the simple addition of the potentials generated by each charge in isolation. The value of theelectric field at any point around these two charges is found by calculating thevector sum of the two electric fields from each of the charges acting alone.
Inquantum mechanics andquantum field theory, the vacuum is defined as the state (that is, the solution to the equations of the theory) with the lowest possible energy (theground state of theHilbert space). Inquantum electrodynamics this vacuum is referred to as 'QED vacuum' to distinguish it from the vacuum ofquantum chromodynamics, denoted asQCD vacuum. QED vacuum is a state with no matter particles (hence the name), and nophotons. As described above, this state is impossible to achieve experimentally. (Even if every matter particle could somehow be removed from a volume, it would be impossible to eliminate all theblackbody photons.) Nonetheless, it provides a good model for realizable vacuum, and agrees with a number of experimental observations as described next.
QED vacuum has interesting and complex properties. In QED vacuum, the electric and magnetic fields have zero average values, but their variances are not zero.[33] As a result, QED vacuum containsvacuum fluctuations (virtual particles that hop into and out of existence), and a finite energy calledvacuum energy. Vacuum fluctuations are an essential and ubiquitous part of quantum field theory. Some experimentally verified effects of vacuum fluctuations includespontaneous emission and theLamb shift.[15]Coulomb's law and theelectric potential in vacuum near an electric charge are modified.[34]
Theoretically, in QCD multiple vacuum states can coexist.[35] The starting and ending ofcosmological inflation is thought to have arisen from transitions between different vacuum states. For theories obtained by quantization of a classical theory, eachstationary point of the energy in theconfiguration space gives rise to a single vacuum.String theory is believed to have a huge number of vacua – the so-calledstring theory landscape.
Outer space has very low density and pressure, and is the closest physical approximation of a perfect vacuum. But no vacuum is truly perfect, not even in interstellar space, where there are still a few hydrogen atoms per cubic meter.[5]
Stars, planets, and moons keep theiratmospheres by gravitational attraction, and as such, atmospheres have no clearly delineated boundary: the density of atmospheric gas simply decreases with distance from the object. The Earth's atmospheric pressure drops to about 32 millipascals (4.6×10−6 psi) at 100 kilometres (62 mi) of altitude,[36] theKármán line, which is a common definition of the boundary with outer space. Beyond this line, isotropic gas pressure rapidly becomes insignificant when compared toradiation pressure from theSun and thedynamic pressure of thesolar winds, so the definition of pressure becomes difficult to interpret. Thethermosphere in this range has large gradients of pressure, temperature and composition, and varies greatly due tospace weather. Astrophysicists prefer to usenumber density to describe these environments, in units of particles per cubic centimetre.
But although it meets the definition of outer space, the atmospheric density within the first few hundred kilometers above the Kármán line is still sufficient to produce significantdrag onsatellites. Most artificial satellites operate in this region, calledlow Earth orbit, and must fire their engines every couple of weeks or a few times a year (depending on solar activity).[37] The drag here is low enough that it could theoretically be overcome by radiation pressure onsolar sails, a proposed propulsion system forinterplanetary travel.[38]
The quality of a vacuum is indicated by the amount of matter remaining in the system, so that a high quality vacuum is one with very little matter left in it. Vacuum is primarily measured by itsabsolute pressure, but a complete characterization requires further parameters, such astemperature and chemical composition. One of the most important parameters is themean free path (MFP) of residual gases, which indicates the average distance that molecules will travel between collisions with each other. As the gas density decreases, the MFP increases, and when the MFP is longer than the chamber, pump, spacecraft, or other objects present, the continuum assumptions offluid mechanics do not apply. This vacuum state is calledhigh vacuum, and the study of fluid flows in this regime is called particle gas dynamics. The MFP of air at atmospheric pressure is very short, 70 nm, but at 100 mPa (≈10−3Torr) the MFP of room temperature air is roughly 100 mm, which is on the order of everyday objects such asvacuum tubes. TheCrookes radiometer turns when the MFP is larger than the size of the vanes.
Vacuum quality is subdivided into ranges according to the technology required to achieve it or measure it. These ranges were defined in ISO 3529-1:2019 as shown in the following table (100 Pa corresponds to 0.75 Torr; Torr is a non-SI unit):
Pressure range
Definition
The reasoning for the definition of the ranges is as follows (typical circumstances):
Prevailing atmospheric pressure (31 kPa to 110 kPa) to 100 Pa
low (rough) vacuum
Pressure can be achieved by simple materials (e.g. regular steel) and positive displacement vacuum pumps; viscous flow regime for gases
<100 Pa to 0.1 Pa
medium (fine) vacuum
Pressure can be achieved by elaborate materials (e.g. stainless steel) and positive displacement vacuum pumps; transitional flow regime for gases
<0.1 Pa to1×10−6 Pa
high vacuum (HV)
Pressure can be achieved by elaborate materials (e.g. stainless steel), elastomer sealings and high vacuum pumps; molecular flow regime for gases
<1×10−6 Pa to1×10−9 Pa
ultra-high vacuum (UHV)
Pressure can be achieved by elaborate materials (e.g. low-carbon stainless steel), metal sealings, special surface preparations and cleaning, bake-out and high vacuum pumps; molecular flow regime for gases
below1×10−9 Pa
extreme-high vacuum (XHV)
Pressure can be achieved by sophisticated materials (e.g. vacuum fired low-carbon stainless steel, aluminium, copper-beryllium, titanium), metal sealings, special surface preparations and cleaning, bake-out and additional getter pumps; molecular flow regime for gases
Atmospheric pressure is variable but 101.325 and 100 kilopascals (1013.25 and 1000.00 mbar) are commonstandard or reference pressures.
Deep space is generally much more empty than any artificial vacuum. It may or may not meet the definition of high vacuum above, depending on what region of space and astronomical bodies are being considered. For example, the MFP of interplanetary space is smaller than the size of the Solar System, but larger than small planets and moons. As a result, solar winds exhibit continuum flow on the scale of the Solar System, but must be considered a bombardment of particles with respect to the Earth and Moon.
Perfect vacuum is an ideal state of no particles at all. It cannot be achieved in alaboratory, although there may be small volumes which, for a brief moment, happen to have no particles of matter in them. Even if all particles of matter were removed, there would still bephotons, as well asdark energy,virtual particles, and other aspects of thequantum vacuum.
Vacuum is measured in units ofpressure, typically as a subtraction relative to ambient atmospheric pressure on Earth. But the amount of relative measurable vacuum varies with local conditions. On the surface ofVenus, where ground-level atmospheric pressure is much higher than on Earth, much higher relative vacuum readings would be possible. On the surface of the Moon with almost no atmosphere, it would be extremely difficult to create a measurable vacuum relative to the local environment.
Similarly, much higher than normal relative vacuum readings are possible deep in the Earth's ocean. Asubmarine maintaining an internal pressure of 1 atmosphere submerged to a depth of 10 atmospheres (98 metres; a 9.8-metre column of seawater has the equivalent weight of 1 atm) is effectively a vacuum chamber keeping out the crushing exterior water pressures, though the 1 atm inside the submarine would not normally be considered a vacuum.
Therefore, to properly understand the following discussions of vacuum measurement, it is important that the reader assumes the relative measurements are being done on Earth at sea level, at exactly 1 atmosphere of ambient atmospheric pressure.
TheSI unit of pressure is thepascal (symbol Pa), but vacuum is often measured intorrs, named for an Italian physicist Torricelli (1608–1647). A torr is equal to the displacement of a millimeter of mercury (mmHg) in amanometer with 1 torr equaling 133.3223684 pascals above absolute zero pressure. Vacuum is often also measured on thebarometric scale or as a percentage ofatmospheric pressure inbars oratmospheres. Low vacuum is often measured inmillimeters of mercury (mmHg) or pascals (Pa) below standard atmospheric pressure. "Below atmospheric" means that the absolute pressure is equal to the current atmospheric pressure.
In other words, most low vacuum gauges that read, for example 50.79 Torr. Many inexpensive low vacuum gauges have a margin of error and may report a vacuum of 0 Torr but in practice this generally requires a two-stage rotary vane or other medium type of vacuum pump to go much beyond (lower than) 1 torr.
Many devices are used to measure the pressure in a vacuum, depending on what range of vacuum is needed.[39]
Hydrostatic gauges (such as the mercury columnmanometer) consist of a vertical column of liquid in a tube whose ends are exposed to different pressures. The column will rise or fall until its weight is in equilibrium with the pressure differential between the two ends of the tube. The simplest design is a closed-end U-shaped tube, one side of which is connected to the region of interest. Any fluid can be used, butmercury is preferred for its high density and low vapour pressure. Simple hydrostatic gauges can measure pressures ranging from 1 torr (100 Pa) to above atmospheric. An important variation is theMcLeod gauge which isolates a known volume of vacuum and compresses it to multiply the height variation of the liquid column. The McLeod gauge can measure vacuums as high as 10−6 torr (0.1 mPa), which is the lowest direct measurement of pressure that is possible with current technology. Other vacuum gauges can measure lower pressures, but only indirectly by measurement of other pressure-controlled properties. These indirect measurements must be calibrated via a direct measurement, most commonly a McLeod gauge.[40]
The kenotometer is a particular type of hydrostatic gauge, typically used in power plants using steam turbines. The kenotometer measures the vacuum in the steam space of the condenser, that is, the exhaust of the last stage of the turbine.[41]
Mechanical orelastic gauges depend on a Bourdon tube, diaphragm, or capsule, usually made of metal, which will change shape in response to the pressure of the region in question. A variation on this idea is thecapacitance manometer, in which the diaphragm makes up a part of a capacitor. A change in pressure leads to the flexure of the diaphragm, which results in a change in capacitance. These gauges are effective from 103 torr to 10−4 torr, and beyond.
Thermal conductivity gauges rely on the fact that the ability of a gas to conduct heat decreases with pressure. In this type of gauge, a wire filament is heated by running current through it. Athermocouple orResistance Temperature Detector (RTD) can then be used to measure the temperature of the filament. This temperature is dependent on the rate at which the filament loses heat to the surrounding gas, and therefore on the thermal conductivity. A common variant is thePirani gauge which uses a single platinum filament as both the heated element and RTD. These gauges are accurate from 10 torr to 10−3 torr, but they are sensitive to the chemical composition of the gases being measured.
Ionization gauges are used in ultrahigh vacuum. They come in two types: hot cathode and cold cathode. In thehot cathode version an electrically heated filament produces an electron beam. The electrons travel through the gauge and ionize gas molecules around them. The resulting ions are collected at a negative electrode. The current depends on the number of ions, which depends on the pressure in the gauge. Hot cathode gauges are accurate from 10−3 torr to 10−10 torr. The principle behindcold cathode version is the same, except that electrons are produced in a discharge created by a high voltage electrical discharge. Cold cathode gauges are accurate from 10−2 torr to 10−9 torr. Ionization gauge calibration is very sensitive to construction geometry, chemical composition of gases being measured, corrosion and surface deposits. Their calibration can be invalidated by activation at atmospheric pressure or low vacuum. The composition of gases at high vacuums will usually be unpredictable, so a mass spectrometer must be used in conjunction with the ionization gauge for accurate measurement.[42]
This shallow water well pump reduces atmospheric air pressure inside the pump chamber. Atmospheric pressure extends down into the well, and forces water up the pipe into the pump to balance the reduced pressure. Above-ground pump chambers are only effective to a depth of approximately 9 meters due to the water column weight balancing the atmospheric pressure.
Manifold vacuum can be used to driveaccessories onautomobiles. The best known application is thevacuum servo, used to provide power assistance for thebrakes. Obsolete applications include vacuum-drivenwindscreen wipers andAutovac fuel pumps. Some aircraft instruments (Attitude Indicator (AI) and theHeading Indicator (HI)) are typically vacuum-powered, as protection against loss of all (electrically powered) instruments, since early aircraft often did not have electrical systems, and since there are two readily available sources of vacuum on a moving aircraft, the engine and an external venturi.Vacuum induction melting uses electromagnetic induction within a vacuum.
Maintaining a vacuum in thecondenser is an important aspect of the efficient operation ofsteam turbines. A steam jetejector orliquid ring vacuum pump is used for this purpose. The typical vacuum maintained in the condenser steam space at the exhaust of the turbine (also called condenser backpressure) is in the range 5 to 15 kPa (absolute), depending on the type of condenser and the ambient conditions.
Evaporation andsublimation into a vacuum is calledoutgassing. All materials, solid or liquid, have a smallvapour pressure, and their outgassing becomes important when the vacuum pressure falls below this vapour pressure. Outgassing has the same effect as a leak and will limit the achievable vacuum. Outgassing products may condense on nearby colder surfaces, which can be troublesome if they obscure optical instruments or react with other materials. This is of great concern to space missions, where an obscured telescope or solar cell can ruin an expensive mission.
The most prevalent outgassing product in vacuum systems is water absorbed by chamber materials. It can be reduced by desiccating or baking the chamber, and removing absorbent materials. Outgassed water can condense in the oil ofrotary vane pumps and reduce their net speed drastically if gas ballasting is not used. High vacuum systems must be clean and free of organic matter to minimize outgassing.
Ultra-high vacuum systems are usually baked, preferably under vacuum, to temporarily raise the vapour pressure of all outgassing materials and boil them off. Once the bulk of the outgassing materials are boiled off and evacuated, the system may be cooled to lower vapour pressures and minimize residual outgassing during actual operation. Some systems are cooled well below room temperature byliquid nitrogen to shut down residual outgassing and simultaneouslycryopump the system.
Deep wells have the pump chamber down in the well close to the water surface, or in the water. A "sucker rod" extends from the handle down the center of the pipe deep into the well to operate the plunger. The pump handle acts as a heavy counterweight against both the sucker rod weight and the weight of the water column standing on the upper plunger up to ground level.
Fluids cannot generally be pulled, so a vacuum cannot be created bysuction. Suction can spread and dilute a vacuum by letting a higher pressure push fluids into it, but the vacuum has to be created first before suction can occur. The easiest way to create an artificial vacuum is to expand the volume of a container. For example, thediaphragm muscle expands the chest cavity, which causes the volume of the lungs to increase. This expansion reduces the pressure and creates a partial vacuum, which is soon filled by air pushed in by atmospheric pressure.
To continue evacuating a chamber indefinitely without requiring infinite growth, a compartment of the vacuum can be repeatedly closed off, exhausted, and expanded again. This is the principle behindpositive displacement pumps, like the manual water pump for example. Inside the pump, a mechanism expands a small sealed cavity to create a vacuum. Because of the pressure differential, some fluid from the chamber (or the well, in our example) is pushed into the pump's small cavity. The pump's cavity is then sealed from the chamber, opened to the atmosphere, and squeezed back to a minute size.
A cutaway view of aturbomolecular pump, a momentum transfer pump used to achieve high vacuum
The above explanation is merely a simple introduction to vacuum pumping, and is not representative of the entire range of pumps in use. Many variations of the positive displacement pump have been developed, and many other pump designs rely on fundamentally different principles.Momentum transfer pumps, which bear some similarities to dynamic pumps used at higher pressures, can achieve much higher quality vacuums than positive displacement pumps.Entrapment pumps can capture gases in a solid or absorbed state, often with no moving parts, no seals and no vibration. None of these pumps are universal; each type has important performance limitations. They all share a difficulty in pumping low molecular weight gases, especiallyhydrogen,helium, andneon.
The lowest pressure that can be attained in a system is also dependent on many things other than the nature of the pumps. Multiple pumps may be connected in series, called stages, to achieve higher vacuums. The choice of seals, chamber geometry, materials, and pump-down procedures will all have an impact. Collectively, these are calledvacuum technique. And sometimes, the final pressure is not the only relevant characteristic. Pumping systems differ in oil contamination, vibration, preferential pumping of certain gases, pump-down speeds, intermittent duty cycle, reliability, or tolerance to high leakage rates.
Inultra high vacuum systems, some very "odd" leakage paths and outgassing sources must be considered. The water absorption ofaluminium andpalladium becomes an unacceptable source of outgassing, and even the adsorptivity of hard metals such as stainless steel ortitanium must be considered. Some oils and greases will boil off in extreme vacuums. The permeability of the metallic chamber walls may have to be considered, and the grain direction of the metallic flanges should be parallel to the flange face.
The lowest pressures currently achievable in laboratory are about 1×10−13 torrs (13 pPa).[43] However, pressures as low as 5×10−17 torrs (6.7 fPa) have been indirectly measured in a 4 K (−269.15 °C; −452.47 °F) cryogenic vacuum system.[4] This corresponds to ≈100 particles/cm3.
Humans and animals exposed to vacuum will loseconsciousness after a few seconds and die ofhypoxia within minutes, but the symptoms are not nearly as graphic as commonly depicted in media and popular culture. The reduction in pressure lowers the temperature at which blood and other body fluids boil, but the elastic pressure of blood vessels ensures that this boiling point remains above the internal body temperature of37 °C.[44] Although the blood will not boil, the formation of gas bubbles in bodily fluids at reduced pressures, known asebullism, is still a concern. The gas may bloat the body to twice its normal size and slow circulation, but tissues are elastic and porous enough to prevent rupture.[45] Swelling and ebullism can be restrained by containment in aflight suit.Shuttle astronauts wore a fitted elastic garment called the Crew Altitude Protection Suit (CAPS) which prevents ebullism at pressures as low as 2 kPa (15 Torr).[46] Rapid boiling will cool the skin and create frost, particularly in the mouth, but this is not a significant hazard.
Animal experiments show that rapid and complete recovery is normal for exposures shorter than 90 seconds, while longer full-body exposures are fatal and resuscitation has never been successful.[47] A study by NASA on eight chimpanzees found all of them survived two and a half minute exposures to vacuum.[48] There is only a limited amount of data available from human accidents, but it is consistent with animal data. Limbs may be exposed for much longer if breathing is not impaired.[49]Robert Boyle was the first to show in 1660 that vacuum is lethal to small animals.
An experiment indicates that plants are able to survive in a low pressure environment (1.5 kPa) for about 30 minutes.[50][51]
Cold or oxygen-rich atmospheres can sustain life at pressures much lower than atmospheric, as long as the density of oxygen is similar to that of standard sea-level atmosphere. The colder air temperatures found at altitudes of up to 3 km generally compensate for the lower pressures there.[49] Above this altitude, oxygen enrichment is necessary to preventaltitude sickness in humans that did not undergo prioracclimatization, andspacesuits are necessary to prevent ebullism above 19 km.[49] Most spacesuits use only 20 kPa (150 Torr) of pure oxygen. This pressure is high enough to prevent ebullism, butdecompression sickness andgas embolisms can still occur if decompression rates are not managed.
Rapid decompression can be much more dangerous than vacuum exposure itself. Even if the victim does not hold his or her breath, venting through the windpipe may be too slow to prevent the fatal rupture of the delicatealveoli of thelungs.[49]Eardrums and sinuses may be ruptured by rapid decompression, soft tissues may bruise and seep blood, and the stress of shock will accelerate oxygen consumption leading to hypoxia.[52] Injuries caused by rapid decompression are calledbarotrauma. A pressure drop of 13 kPa (100 Torr), which produces no symptoms if it is gradual, may be fatal if it occurs suddenly.[49]
^abTadokoro, M. (1968). "A Study of the Local Group by Use of the Virial Theorem".Publications of the Astronomical Society of Japan.20 (3): 230.Bibcode:1968PASJ...20..230T.doi:10.1093/pasj/20.3.230. This source estimates a density of7×10−29 g/cm3 for theLocal Group. Adalton is1.66×10−24 g, for roughly 40 atoms per cubic meter.
^Jörg Hüttner & Martin Walter (Ed.) (2022).Clemens Timpler: Physicae seu philosophiae naturalis systema methodicum. Pars prima; complectens physicam generalem. Hildesheim / Zürich / New York: Georg Olms Verlag. pp. 28–37.ISBN978-3-487-16076-4.
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^Ishimaru, H (1989). "Ultimate Pressure of the Order of 10−13 torr in an Aluminum Alloy Vacuum Chamber".Journal of Vacuum Science and Technology.7 (3–II):2439–2442.Bibcode:1989JVSTA...7.2439I.doi:10.1116/1.575916.
^Billings, Charles E. (1973). "Chapter 1) Barometric Pressure". In Parker, James F.; West, Vita R. (eds.).Bioastronautics Data Book (Second ed.). NASA. p. 5.hdl:2060/19730006364. NASA SP-3006.
^Webb P. (1968). "The Space Activity Suit: An Elastic Leotard for Extravehicular Activity".Aerospace Medicine.39 (4):376–383.PMID4872696.
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^University of New Hampshire Experimental Space Plasma Group."What is the Interstellar Medium".The Interstellar Medium, an online tutorial. Archived fromthe original on 2006-02-17. Retrieved2006-03-15.
