Outer space

For other uses, see Outer space (disambiguation).

The interface between the Earth’s surface and outer space. TheKármán line at a height of 100 km (62 mi) is shown. The layersof the atmosphere are drawn to scale. Objects within them, suchas the International Space Station, are not.

Outer space, or just space, is the void that exists be-tween celestial bodies, including the Earth.[1] It is notcompletely empty, but consists of a hard vacuum con-taining a low density of particles, predominantly a plasmaof hydrogen and helium as well as electromagnetic radi-ation, magnetic fields, neutrinos, dust and cosmic rays.The baseline temperature, as set by the background radia-tion from the Big Bang, is 2.7 Kelvin (K) (−454.81 °F).[2]Plasma with a number density of less than one hydrogenatom per cubic metre and a temperature of millions ofkelvin in the space between galaxies accounts for mostof the baryonic (ordinary) matter in outer space; local

concentrations have condensed into stars and galaxies. Inmost galaxies, observations provide evidence that 90%of the mass is in an unknown form, called dark matter,which interacts with other matter through gravitationalbut not electromagnetic forces.[3][4] Data indicates thatthe majority of the mass-energy in the observable Uni-verse is a poorly understood vacuum energy of spacewhich astronomers label dark energy.[5][6] Intergalacticspace takes up most of the volume of the Universe, buteven galaxies and star systems consist almost entirely ofempty space.There is no firm boundary where space begins. Howeverthe Kármán line, at an altitude of 100 km (62 mi) abovesea level,[7][8] is conventionally used as the start of outerspace in space treaties and for aerospace records keep-ing. The framework for international space law was es-tablished by the Outer Space Treaty, which was passedby the United Nations in 1967. This treaty precludesany claims of national sovereignty and permits all statesto freely explore outer space. Despite the drafting ofUN resolutions for the peaceful uses of outer space, anti-satellite weapons have been tested in Earth orbit.Humans began the physical exploration of space duringthe 20th century with the advent of high-altitude balloonflights, followed by manned rocket launches. Earth orbitwas first achieved by Yuri Gagarin of the Soviet Union in1961 and unmanned spacecraft have since reached all ofthe known planets in the Solar System. Due to the highcost of getting into space, manned spaceflight has beenlimited to low Earth orbit and the Moon.Outer space represents a challenging environment for hu-man exploration because of the dual hazards of vacuumand radiation. Microgravity also has a negative effect onhuman physiology that causes both muscle atrophy andbone loss. In addition to these health and environmen-tal issues, the economic cost of putting objects, includinghumans, into space is high.

1 Discovery

In 350 BC, Greek philosopher Aristotle suggested thatnature abhors a vacuum, a principle that became knownas the horror vacui. This concept built upon a 5th-century BC ontological argument by the Greek philoso-pher Parmenides, who denied the possible existence of avoid in space.[9] Based on this idea that a vacuum couldnot exist, in the West it was widely held for many cen-

1

2 1 DISCOVERY

turies that space could not be empty.[10] As late as the17th century, the French philosopher René Descartes ar-gued that the entirety of space must be filled.[11]

In ancient China, there were various schools of thoughtconcerning the nature of the heavens, some of which beara resemblance to the modern understanding. In the 2ndcentury, astronomer Zhang Heng became convinced thatspace must be infinite, extending well beyond the mecha-nism that supported the Sun and the stars. The survivingbooks of the Hsüan Yeh school said that the heavens wereboundless, “empty and void of substance”. Likewise, the“sun, moon, and the company of stars float in the emptyspace, moving or standing still”.[12]

The Italian scientist Galileo Galilei knew that air hadmass and so was subject to gravity. In 1640, he demon-strated that an established force resisted the formationof a vacuum. However, it would remain for his pupilEvangelista Torricelli to create an apparatus that wouldproduce a vacuum in 1643. This experiment resulted inthe first mercury barometer and created a scientific sensa-tion in Europe. The French mathematician Blaise Pascalreasoned that if the column of mercury was supported byair then the column ought to be shorter at higher altitudewhere the air pressure is lower.[13] In 1648, his brother-in-law, Florin Périer, repeated the experiment on the Puyde Dôme mountain in central France and found that thecolumn was shorter by three inches. This decrease inpressure was further demonstrated by carrying a half-fullballoon up a mountain and watching it gradually expand,then contract upon descent.[14]

The original Magdeburg hemispheres (lower left) used to demon-strate Otto von Guericke’s vacuum pump (right)

In 1650, German scientist Otto von Guericke constructedthe first vacuum pump: a device that would further refutethe principle of horror vacui. He correctly noted thatthe atmosphere of the Earth surrounds the planet like ashell, with the density gradually declining with altitude.He concluded that there must be a vacuum between theEarth and the Moon.[15]

Back in the 15th century, German theologian NicolausCusanus speculated that the Universe lacked a center anda circumference. He believed that the Universe, while notinfinite, could not be held as finite as it lacked any boundswithin which it could be contained.[16] These ideas led tospeculations as to the infinite dimension of space by theItalian philosopher Giordano Bruno in the 16th century.He extended the Copernican heliocentric cosmology tothe concept of an infinite Universe filled with a substancehe called aether, which did not cause resistance to themo-tions of heavenly bodies.[17] English philosopher WilliamGilbert arrived at a similar conclusion, arguing that thestars are visible to us only because they are surrounded bya thin aether or a void.[18] This concept of an aether orig-inated with ancient Greek philosophers, including Aris-totle, who conceived of it as the medium through whichthe heavenly bodies moved.[19]

The concept of a Universe filled with a luminiferousaether remained in vogue among some scientists until theearly 20th century. This form of aether was viewed asthe medium through which light could propagate.[20] In1887, the Michelson–Morley experiment tried to detectthe Earth’s motion through this medium by looking forchanges in the speed of light depending on the directionof the planet’s motion. However, the null result indicatedsomething was wrong with the concept. The idea of theluminiferous aether was then abandoned. It was replacedby Albert Einstein's theory of special relativity, whichholds that the speed of light in a vacuum is a fixed con-stant, independent of the observer’s motion or frame ofreference.[21][22]

The first professional astronomer to support the con-cept of an infinite Universe was the Englishman ThomasDigges in 1576.[23] But the scale of the Universe re-mained unknown until the first successful measurementof the distance to a nearby star in 1838 by the Germanastronomer Friedrich Bessel. He showed that the star 61Cygni had a parallax of just 0.31 arcseconds (comparedto the modern value of 0.287″). This corresponds to adistance of over 10 light years.[24] The distance to theAndromeda Galaxy was determined in 1923 by Amer-ican astronomer Edwin Hubble by measuring the bright-ness of cepheid variables in that galaxy, a new techniquediscovered by Henrietta Leavitt.[25] This established thatthe Andromeda galaxy, and by extension all galaxies, laywell outside the Milky Way.[26]

The earliest known estimate of the temperature of outerspace was by the Swiss physicist Charles É. Guillaume in1896. Using the estimated radiation of the background

3

stars, he concluded that spacemust be heated to a temper-ature of 5–6 K. British physicist Arthur Eddington madea similar calculation to derive a temperature of 3.18° in1926. 1933 German physicist Erich Regener used thetotal measured energy of cosmic rays to estimate an in-tergalactic temperature of 2.8 K.[27]

The modern concept of outer space is based on the “BigBang” cosmology, first proposed in 1931 by the Belgianphysicist Georges Lemaître.[28] This theory holds thatthe observable Universe originated from a very compactform that has since undergone continuous expansion. Thebackground energy released during the initial expansionhas steadily decreased in density, leading to a 1948 pre-diction by American physicts Ralph Alpher and RobertHerman of a temperature of 5 K for the temperature ofspace.[27]

The term outer space was used as in 1842 by the Englishpoet Lady Emmeline Stuart-Wortley in her poem “TheMaiden of Moscow”.[29] The expression outer space wasused as an astronomical term byAlexander vonHumboldtin 1845.[30] It was later popularized in the writings of H.G. Wells in 1901.[31] The shorter term space is actuallyolder, first used to mean the region beyond Earth’s sky inJohn Milton's Paradise Lost in 1667.[32]

2 Formation and state

This is an artist’s concept of the metric expansion of space, wherea volume of the Universe is represented at each time interval bythe circular sections. At left is depicted the rapid inflation fromthe initial state, followed thereafter by steady expansion to thepresent day, shown at right.

Main article: Big Bang

According to the Big Bang theory, the Universe origi-nated in an extremely hot and dense state about 13.8 bil-lion years ago[33] and began expanding rapidly. About380,000 years later the Universe had cooled sufficientlyto allow protons and electrons to combine and formhydrogen—the so-called recombination epoch. Whenthis happened, matter and energy became decoupled, al-lowing photons to travel freely through space.[34] The

matter that remained following the initial expansion hassince undergone gravitational collapse to create stars,galaxies and other astronomical objects, leaving behinda deep vacuum that forms what is now called outerspace.[35] As light has a finite velocity, this theory alsoconstrains the size of the directly observable Universe.[34]This leaves open the question as to whether the Universeis finite or infinite.The present day shape of the Universe has been de-termined from measurements of the cosmic microwavebackground using satellites like theWilkinsonMicrowaveAnisotropy Probe. These observations indicate that theobservable Universe is flat, meaning that photons on par-allel paths at one point will remain parallel as they travelthrough space to the limit of the observable Universe,except for local gravity.[36] The flat Universe, combinedwith the measured mass density of the Universe and theaccelerating expansion of the Universe, indicates thatspace has a non-zero vacuum energy, which is called darkenergy.[37]

Estimates put the average energy density of the Universeat the equivalent of 5.9 protons per cubic meter, includ-ing dark energy, dark matter, and baryonic matter (or-dinary matter composed of atoms). The atoms accountfor only 4.6% of the total energy density, or a densityof one proton per four cubic meters.[38] The density ofthe Universe, however, is clearly not uniform; it rangesfrom relatively high density in galaxies—including veryhigh density in structures within galaxies, such as plan-ets, stars, and black holes—to conditions in vast voidsthat have much lower density, at least in terms of vis-ible matter.[39] Unlike the matter and dark matter, thedark energy seems not to be concentrated in galaxies: al-though dark energy may account for a majority of themass-energy in the Universe, dark energy’s influence is 5orders of magnitude smaller than the influence of gravityfrom matter and dark matter within the Milky Way.[40]

3 Environment

Outer space is the closest known approximation to aperfect vacuum. It has effectively no friction, allowingstars, planets and moons to move freely along their idealorbits. However, even the deep vacuum of intergalacticspace is not devoid of matter, as it contains a fewhydrogen atoms per cubic meter.[41] By comparison, theair we breathe contains about 1025 molecules per cubicmeter.[42][43] The sparse density of matter in outer spacemeans that electromagnetic radiation can travel great dis-tances without being scattered: the mean free path ofa photon in intergalactic space is about 1023 km, or 10billion light years.[44] In spite of this, extinction, whichis the absorption and scattering of photons by dust andgas, is an important factor in galactic and intergalacticastronomy.[45]

4 3 ENVIRONMENT

Part of the Hubble Ultra-Deep Field image showing a typicalsection of space containing galaxies interspersed by deep vacuum.Given the finite speed of light, this view covers the last 13 billionyears of the history of outer space.

Stars, planets and moons retain their atmospheres bygravitational attraction. Atmospheres have no clearlydelineated boundary: the density of atmospheric gasgradually decreases with distance from the object un-til it becomes indistinguishable from the surroundingenvironment.[46] The Earth’s atmospheric pressure dropsto about 0.032 Pa at 100 kilometres (62 miles) ofaltitude,[47] compared to 100,000 Pa for the InternationalUnion of Pure and Applied Chemistry (IUPAC) defini-tion of standard pressure. Beyond this altitude, isotropicgas pressure rapidly becomes insignificant when com-pared to radiation pressure from the Sun and the dynamicpressure of the solar wind. The thermosphere in thisrange has large gradients of pressure, temperature andcomposition, and varies greatly due to space weather.[48]

The temperature of the vacuum is measured in terms ofthe kinetic activity of the gas, as it is on Earth. However,the radiation that fills the vacuum has a different tem-perature than the kinetic temperature of the gas, mean-ing that the gas and radiation are not in thermodynamicequilibrium.[49][50] All of the observable Universe is filledwith photons that were created during the Big Bang,which is known as the cosmic microwave background ra-diation (CMB). (There is quite likely a correspondinglylarge number of neutrinos called the cosmic neutrinobackground.[51]) The current black body temperature ofthe background radiation is about 3 K (−270 °C; −454°F).[52] The gas temperatures in outer space are alwaysat least the temperature of the CMB but can be muchhigher. For example, the corona of the Sun has tempera-tures which range over 1.2–2.6 million K.[53]

Outside of a protective atmosphere and magnetic field,there are few obstacles to the passage through space

of energetic subatomic particles known as cosmic rays.These particles have energies ranging from about 106 eVup to an extreme 1020 eV of ultra-high-energy cosmicrays.[54] The peak flux of cosmic rays occurs at energiesof about 109 eV, with approximately 87% protons, 12%helium nuclei and 1% heavier nuclei. In the high en-ergy range, the flux of electrons is only about 1% of thatof protons.[55] Cosmic rays can damage electronic com-ponents and pose a health threat to space travelers.[56]According to astronauts, like Don Pettit, space has aburned/metallic odor that clings to their suits and equip-ment, similar to the scent of an arc welding torch.[57][58]

Despite the harsh environment, several life forms havebeen found that can withstand extreme space conditionsfor extended periods. Species of lichen carried on theESA BIOPAN facility survived exposure for ten days in2007.[59] Seeds of Arabidopsis thaliana and Nicotianatabacum germinated after being exposed to space for 1.5years.[60] A strain of bacillus subtilis has survived 559days when exposed to low-Earth orbit or a simulatedmartian environment.[61] The lithopanspermia hypothe-sis suggests that rocks ejected into outer space from life-harboring planets may successfully transport life forms toanother habitable world. A conjecture is that just such ascenario occurred early in the history of the Solar Sys-tem, with potentially microorganism-bearing rocks beingexchanged between Venus, Earth, and Mars.[62]

3.1 Effect on human bodies

See also: Space exposure and WeightlessnessSudden exposure to very low pressure, such as dur-

Because of the hazards of a vacuum, astronauts must wear apressurized space suit while off-Earth and outside their space-craft.

ing a rapid decompression, can cause pulmonary baro-trauma—a rupture of the lungs, due to the large pressure

5

differential between inside and outside of the chest.[63]Even if the victim’s airway is fully open, the flow of airthrough the windpipe may be too slow to prevent therupture.[64] Rapid decompression can rupture eardrumsand sinuses, bruising and blood seep can occur in softtissues, and shock can cause an increase in oxygen con-sumption that leads to hypoxia.[63]

As a consequence of rapid decompression, any oxygendissolved in the blood will empty into the lungs to tryto equalize the partial pressure gradient. Once the de-oxygenated blood arrives at the brain, humans and ani-mals will lose consciousness after a few seconds and dieof hypoxia within minutes.[65] Blood and other body flu-ids boil when the pressure drops below 6.3 kPa, and thiscondition is called ebullism.[66] The steam may bloat thebody to twice its normal size and slow circulation, buttissues are elastic and porous enough to prevent rupture.Ebullism is slowed by the pressure containment of bloodvessels, so some blood remains liquid.[67][68] Swelling andebullism can be reduced by containment in a flight suit.The Crew Altitude Protection Suit (CAPS), a fitted elas-tic garment designed in the 1960s for Shuttle astronauts,prevents ebullism at pressures as low as 2 kPa.[69] Spacesuits are needed at 8 km (5.0 mi) to provide enough oxy-gen for breathing and to prevent water loss, while above20 km (12 mi) they are essential to prevent ebullism.[70]Most space suits use around 30–39 kPa of pure oxygen,about the same as on the Earth’s surface. This pressure ishigh enough to prevent ebullism, but evaporation of nitro-gen dissolved in the blood could still cause decompressionsickness and gas embolisms if not managed.[71]

Humans evolved for life in Earth gravity, and exposureto weightlessness has been shown to have deleterious ef-fects on the health of the human body. Initially, morethan 50% of astronauts experience spacemotion sickness.This can cause nausea and vomiting, vertigo, headaches,lethargy, and overall malaise. The duration of spacesickness varies, but it typically lasts for 1–3 days, afterwhich the body adjusts to the new environment. Longerterm exposure to weightlessness results in muscle atrophyand deterioration of the skeleton, or spaceflight osteope-nia. These effects can be minimized through a regimenof exercise.[72] Other effects include fluid redistribution,slowing of the cardiovascular system, decreased produc-tion of red blood cells, balance disorders, and a weaken-ing of the immune system. Lesser symptoms include lossof body mass, nasal congestion, sleep disturbance, andpuffiness of the face.[73]

For long duration space travel, radiation can pose an acutehealth hazard. Exposure to radiation sources such ashigh-energy, ionizing cosmic rays can result in fatigue,nausea, vomiting, as well as damage to the immune sys-tem and changes to the white blood cell count. Overlonger durations, symptoms include an increased risk ofcancer, plus damage to the eyes, nervous system, lungsand the gastrointestinal tract.[74] On a round-trip Marsmission lasting three years, nearly the entire body would

be traversed by high energy nuclei, each of which cancause ionization damage to cells. Fortunately, most suchparticles are significantly attenuated by the shielding pro-vided by the aluminium walls of a spacecraft, and can befurther diminished by water containers and other barri-ers. However, the impact of the cosmic rays upon theshielding produces additional radiation that can affect thecrew. Further research will be needed to assess the radia-tion hazards and determine suitable countermeasures.[75]

4 Boundary

SpaceShipOne completed the first manned private spaceflight in2004, reaching an altitude of 100.12 km (62.21 mi).[76]

There is no clear boundary between Earth’s atmosphereand space, as the density of the atmosphere gradually de-creases as the altitude increases. There are several stan-dard boundary designations, namely:

• The Fédération Aéronautique Internationale has es-tablished the Kármán line at an altitude of 100 km(62 mi) as a working definition for the boundarybetween aeronautics and astronautics. This is usedbecause at an altitude of about 100 km (62 mi), asTheodore von Kármán calculated, a vehicle wouldhave to travel faster than orbital velocity in orderto derive sufficient aerodynamic lift from the atmo-sphere to support itself.[7][8]

• The United States designates people who travelabove an altitude of 50 miles (80 km) asastronauts.[77]

• NASA's Space Shuttle used 400,000 feet (76 mi,122 km) as its re-entry altitude (termed the EntryInterface), which roughlymarks the boundary whereatmospheric drag becomes noticeable, thus begin-ning the process of switching from steering withthrusters to maneuvering with air surfaces.[78]

In 2009, scientists at the University of Calgary reporteddetailed measurements with a Supra-Thermal Ion Im-ager (an instrument that measures the direction and speed

6 6 EARTH ORBIT

of ions), which allowed them to establish a boundary at118 km (73 mi) above Earth. The boundary representsthe midpoint of a gradual transition over tens of kilome-ters from the relatively gentle winds of the Earth’s atmo-sphere to the more violent flows of charged particles inspace, which can reach speeds well over 268 m/s (600mph).[79][80]

The altitude where the atmospheric pressure matches thevapor pressure of water at the temperature of the humanbody is called the Armstrong line, named after Americanphysician Harry G. Armstrong. It is located at an alti-tude of around 19.14 km (11.89 mi). At or above theArmstrong line, fluids in the throat and lungs will boilaway. More specifically, exposed bodily liquids such assaliva, tears, and the liquids wetting the alveoli within thelungs will boil away. Hence, at this altitude the humanbody requires a pressure suit, or a pressurized capsule, tosurvive.[81]

5 Legal status

Main article: Space lawThe Outer Space Treaty provides the basic framework

2008 launch of the SM-3 missile used to destroy American spysatellite USA-193

for international space law. It covers the legal use ofouter space by nation states, and includes in its defini-tion 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 nationalsovereignty. It also prohibits the deployment of nuclearweapons in outer space. The treaty was passed by theUnited Nations General Assembly in 1963 and signed in1967 by the USSR, the United States of America and theUnited Kingdom. As of January 1, 2008 the treaty hasbeen ratified by 98 states and signed by an additional 27states.[82]

Beginning in 1958, outer space has been the subject ofmultiple resolutions by the United Nations General As-sembly. Of these, more than 50 have been concerning theinternational co-operation in the peaceful uses of outerspace and preventing an arms race in space.[83] Four addi-tional space law treaties have been negotiated and draftedby the UN’s Committee on the Peaceful Uses of OuterSpace. Still, there remains no legal prohibition against de-ploying conventional weapons in space, and anti-satelliteweapons have been successfully tested by the US, USSRand China.[84] The 1979 Moon Treaty turned the juris-diction of all heavenly bodies (including the orbits aroundsuch bodies) over to the international community. How-ever, this treaty has not been ratified by any nation thatcurrently practices manned spaceflight.[85]

In 1976, eight equatorial states (Ecuador, Colombia,Brazil, Congo, Zaire, Uganda, Kenya, and Indonesia) metin Bogotá, Colombia. They made the “Declaration ofthe First Meeting of Equatorial Countries,” also knownas “the Bogotá Declaration”, where they made a claim tocontrol the segment of the geosynchronous orbital pathcorresponding to each country.[86] These claims are notinternationally accepted.[87]

6 Earth orbit

A spacecraft enters orbit when it has enough horizontalvelocity for its centripetal acceleration due to gravity to beless than or equal to the centrifugal acceleration due to thehorizontal component of its velocity. For a low Earth or-bit, this velocity is about 7,800m/s (28,100 km/h; 17,400mph);[88] by contrast, the fastest manned airplane speedever achieved (excluding speeds achieved by deorbitingspacecraft) was 2,200 m/s (7,900 km/h; 4,900 mph) in1967 by the North American X-15.[89]

To achieve an orbit, a spacecraft must travel faster thana sub-orbital spaceflight. The energy required to reachEarth orbital velocity at an altitude of 600 km (370 mi)is about 36 MJ/kg, which is six times the energy neededmerely to climb to the corresponding altitude.[90] Space-craft with a perigee below about 2,000 km (1,200 mi)are subject to drag from the Earth’s atmosphere,[91] whichwill cause the orbital altitude to decrease. The rate of or-bital decay depends on the satellite’s cross-sectional areaand mass, as well as variations in the air density of theupper atmosphere. Below about 300 km (190 mi), de-cay becomes more rapid with lifetimes measured in days.

7.2 Interplanetary space 7

Once a satellite descends to 180 km (110 mi), it will startto burn up in the atmosphere.[92] The escape velocity re-quired to pull free of Earth’s gravitational field altogetherand move into interplanetary space is about 11,200 m/s(40,300 km/h; 25,100 mph).[93]

Earth’s gravity reaches out far past the Van Allen radia-tion belt and keeps the Moon in orbit at an average dis-tance of 384,403 km (238,857 mi). The region of spacewhere the gravity of a planet tends to dominate the mo-tion of objects in the presence of other perturbing bodies(such as another planet) is known as the Hill sphere. ForEarth, this sphere has a radius of about 1,500,000 km(930,000 mi).[94]

7 Regions

Space is a partial vacuum: its different regions are de-fined by the various atmospheres and “winds” that domi-nate within them, and extend to the point at which thosewinds give way to those beyond. Geospace extends fromEarth’s atmosphere to the outer reaches of Earth’s mag-netic field, whereupon it gives way to the solar wind ofinterplanetary space.[95] Interplanetary space extends tothe heliopause, whereupon the solar wind gives way tothe winds of the interstellar medium.[96] Interstellar spacethen continues to the edges of the galaxy, where it fadesinto the intergalactic void.[97]

7.1 Geospace

Aurora australis observed from the Space Shuttle Discovery, onSTS-39, May 1991 (orbital altitude: 260 km)

Geospace is the region of outer space near Earth.Geospace includes the upper region of the atmosphereand the magnetosphere.[95] The Van Allen radiation beltlies within the geospace. The outer boundary of geospaceis the magnetopause, which forms an interface betweenthe planet’s magnetosphere and the solar wind. The innerboundary is the ionosphere.[98] As the physical propertiesand behavior of near Earth space is affected by the behav-ior of the Sun and space weather, the field of geospace is

interlinked with heliophysics; the study of the Sun and itsimpact on the Solar System planets.[99]

The volume of geospace defined by the magnetopause iscompacted in the direction of the Sun by the pressure ofthe solar wind, giving it a typical subsolar distance of10 Earth radii from the center of the planet. However,the tail can extend outward to more than 100–200 Earthradii.[100] TheMoon passes through the geospace tail dur-ing roughly four days each month, during which time thesurface is shielded from the solar wind.[101]

Geospace is populated by electrically charged particles atvery low densities, the motions of which are controlled bythe Earth’s magnetic field. These plasmas form amediumfrom which storm-like disturbances powered by the solarwind can drive electrical currents into the Earth’s upperatmosphere. During geomagnetic storms two regions ofgeospace, the radiation belts and the ionosphere, can be-come strongly disturbed. These storms increase fluxes ofenergetic electrons that can permanently damage satel-lite electronics, disrupting telecommunications and GPStechnologies, and can also be a hazard to astronauts, evenin low Earth orbit. They also create aurorae seen near themagnetic poles.[102]

Although it meets the definition of outer space, the at-mospheric density within the first few hundred kilometersabove the Kármán line is still sufficient to produce signif-icant drag on satellites.[92] This region contains materialleft over from previous manned and unmanned launchesthat are a potential hazard to spacecraft. Some of thisdebris re-enters Earth’s atmosphere periodically.[103]

7.1.1 Cislunar space

The region outside Earth’s atmosphere and extending outto just beyond theMoon’s orbit, including the Lagrangianpoints, is sometimes referred to as cis-lunar space.[104]

7.2 Interplanetary space

Main article: Interplanetary mediumInterplanetary space, the space around the Sun andplanets of the Solar System, is the region dominatedby the interplanetary medium, which extends out to theheliopause where the influence of the galactic environ-ment starts to dominate over the magnetic field and parti-cle flux from the Sun.[96] Interplanetary space is definedby the solar wind, a continuous stream of charged par-ticles emanating from the Sun that creates a very tenu-ous atmosphere (the heliosphere) for billions of kilome-ters into space. This wind has a particle density of 5–10protons/cm3 and is moving at a velocity of 350–400 km/s(780,000–890,000 mph).[105] The distance and strengthof the heliopause varies depending on the activity level ofthe solar wind.[106] The discovery since 1995 of extrasolarplanets means that other stars must possess their own in-terplanetary media.[107]

8 7 REGIONS

The sparse plasma (blue) and dust (white) in the tail of cometHale–Bopp are being shaped by pressure from solar radiationand the solar wind, respectively

The volume of interplanetary space is a nearly total vac-uum, with a mean free path of about one astronomicalunit at the orbital distance of the Earth. However, thisspace is not completely empty, and is sparsely filled withcosmic rays, which include ionized atomic nuclei andvarious subatomic particles. There is also gas, plasmaand dust, small meteors, and several dozen types oforganic molecules discovered to date by microwave spec-troscopy.[108] A cloud of interplanetary dust is visible atnight as a faint band called the zodiacal light.[109]

Interplanetary space contains themagnetic field generatedby the Sun.[105] There are also magnetospheres generatedby planets such as Jupiter, Saturn, Mercury and the Earththat have their own magnetic fields. These are shapedby the influence of the solar wind into the approximationof a teardrop shape, with the long tail extending outwardbehind the planet. Thesemagnetic fields can trap particlesfrom the solar wind and other sources, creating belts ofmagnetic particles such as the Van Allen radiation belt.Planets without magnetic fields, such as Mars, have theiratmospheres gradually eroded by the solar wind.[110]

7.3 Interstellar space

Main article: Interstellar medium“Interstellar space” redirects here. For the album, seeInterstellar Space.Interstellar space is the physical space within a galaxybeyond the influence of each star on the plasma.[97] Thecontents of interstellar space are called the interstellarmedium. Approximately 70% of the mass of the inter-stellar medium consists of lone hydrogen atoms; most ofthe remainder consists of helium atoms. This is enrichedwith trace amounts of heavier atoms formed throughstellar nucleosynthesis. These atoms are ejected into theinterstellar medium by stellar winds or when evolved starsbegin to shed their outer envelopes such as during the

Bow shock formed by the magnetosphere of the young star LLOrionis (center) as it collides with the Orion Nebula flow

formation of a planetary nebula.[111] The cataclysmic ex-plosion of a supernova will generate an expanding shockwave consisting of ejected materials.[112] The density ofmatter in the interstellar medium can vary considerably:the average is around 106 particles per m3, but coldmolecular clouds can hold 108–1012 per m3.[49][111]

A number of molecules exist in interstellar space, as cantiny, 0.1 μm dust particles.[113] The tally of molecules dis-covered through radio astronomy is steadily increasing atthe rate of about four new species per year. Large re-gions of higher density matter known as molecular cloudsallow chemical reactions to occur, including the forma-tion of organic polyatomic species. Much of this chem-istry is driven by collisions. Energetic cosmic rays pene-trate the cold, dense clouds and ionize hydrogen and he-lium, resulting, for example, in the trihydrogen cation.An ionized helium atom can then split relatively abun-dant carbon monoxide to produce ionized carbon, whichin turn can lead to organic chemical reactions.[114]

The local interstellar medium is a region of space within100 parsecs (pc) of the Sun, which is of interest bothfor its proximity and for its interaction with the SolarSystem. This volume nearly coincides with a region ofspace known as the Local Bubble, which is characterizedby a lack of dense, cold clouds. It forms a cavity in theOrion Arm of the Milky Way galaxy, with dense molec-ular clouds lying along the borders, such as those in theconstellations of Ophiuchus and Taurus. (The actual dis-tance to the border of this cavity varies from 60 to 250 pcor more.) This volume contains about 104–105 stars andthe local interstellar gas counterbalances the astrospheresthat surround these stars, with the volume of each spherevarying depending on the local density of the interstellarmedium. The Local Bubble contains dozens of warm in-terstellar clouds with temperatures of up to 7,000 K andradii of 0.5–5 pc.[115]

When stars are moving at sufficiently high peculiar veloc-ities, their astrospheres can generate bow shocks as they

9

collide with the interstellar medium. For decades it wasassumed that the Sun had a bow shock. In 2012, datafrom Interstellar Boundary Explorer (IBEX) and NASA’sVoyager probes showed that the Sun’s bow shock doesnot exist. Instead, these authors argue that a subsonicbow wave defines the transition from the solar wind flowto the interstellar medium.[116][117] A bow shock is thethird boundary of an astrosphere after the terminationshock and the astropause (called the heliopause in the So-lar System).[117]

7.4 Intergalactic space

A star forming region in the Large Magellanic Cloud, perhapsthe closest Galaxy to Earth’s Milky Way

Intergalactic space is the physical space between galax-ies. The huge spaces between galaxy clusters are calledthe voids. Surrounding and stretching between galaxies,there is a rarefied plasma[118] that is organized in a galacticfilamentary structure.[119] Thismaterial is called the inter-galactic medium (IGM). The density of the IGM is 5–200times the average density of the Universe.[120] It consistsmostly of ionized hydrogen; i.e. a plasma consisting ofequal numbers of electrons and protons. As gas falls intothe intergalactic medium from the voids, it heats up totemperatures of 105 K to 107 K,[121] which is high enoughso that collisions between atoms have enough energy tocause the bound electrons to escape from the hydrogennuclei; this is why the IGM is ionized. At these tem-peratures, it is called the warm–hot intergalactic medium(WHIM). (Although the plasma is very hot by terrestrialstandards, 105 K is often called “warm” in astrophysics.)Computer simulations and observations indicate that upto half of the atomic matter in the Universe might ex-ist in this warm–hot, rarefied state.[120][122][123] When gasfalls from the filamentary structures of the WHIM into

the galaxy clusters at the intersections of the cosmic fila-ments, it can heat up evenmore, reaching temperatures of108 K and above in the so-called intraclustermedium.[124]

8 Exploration and applications

Main articles: Space exploration, Space colonization andSpace manufacturingFor the majority of human history, space was explored

The first image taken of the entire Earth by astronauts was shotduring the Apollo 8 mission

by remote observation; initially with the unaided eye andthen with the telescope. Prior to the advent of reli-able rocket technology, the closest that humans had cometo reaching outer space was through the use of balloonflights. In 1935, the U.S. Explorer II manned balloonflight had reached an altitude of 22 km (14 mi).[125] Thiswas greatly exceeded in 1942 when the third launch of theGerman A-4 rocket climbed to an altitude of about 80 km(50 mi). In 1957, the unmanned satellite Sputnik 1 waslaunched by a Russian R-7 rocket, achieving Earth orbitat an altitude of 215–939 kilometres (134–583 mi).[126]This was followed by the first human spaceflight in 1961,when Yuri Gagarin was sent into orbit on Vostok 1. Thefirst humans to escape Earth orbit were Frank Borman,Jim Lovell andWilliamAnders in 1968 on board the U.S.Apollo 8, which achieved lunar orbit[127] and reached amaximum distance of 377,349 km (234,474mi) from theEarth.[128]

The first spacecraft to reach escape velocity was the So-viet Luna 1, which performed a fly-by of the Moon in1959.[129] In 1961, Venera 1 became the first planetaryprobe. It revealed the presence of the solar wind and per-formed the first fly-by of the planet Venus, although con-tact was lost before reaching Venus. The first success-ful planetary mission was the Mariner 2 fly-by of Venusin 1962.[130] The first spacecraft to perform a fly-by ofMars was Mariner 4, which reached the planet in 1964.

10 10 REFERENCES

Since that time, unmanned spacecraft have successfullyexamined each of the Solar System’s planets, as well theirmoons and many minor planets and comets. They remaina fundamental tool for the exploration of outer space,as well as observation of the Earth.[131] In August 2012,Voyager 1 became the first man-made object to leave theSolar System and enter interstellar space.[132]

The absence of air makes outer space (and the surfaceof the Moon) ideal locations for astronomy at all wave-lengths of the electromagnetic spectrum. This is ev-idenced by the spectacular pictures sent back by theHubble Space Telescope, allowing light from more than13 billion years ago—almost to the time of the BigBang—to be observed.[133] However, not every locationin space is ideal for a telescope. The interplanetary zo-diacal dust emits a diffuse near-infrared radiation thatcan mask the emission of faint sources such as extraso-lar planets. Moving an infrared telescope out past thedust will increase the effectiveness of the instrument.[134]Likewise, a site like the Daedalus crater on the far sideof the Moon could shield a radio telescope from theradio frequency interference that hampers Earth-basedobservations.[135]

Unmanned spacecraft in Earth orbit have become an es-sential technology of modern civilization. They allow di-rect monitoring of weather conditions, relay long-rangecommunications including telephone calls and televisionsignals, provide a means of precise navigation, and al-low remote sensing of the Earth. The latter role serves awide variety of purposes, including tracking soil moisturefor agriculture, prediction of water outflow from seasonalsnow packs, detection of diseases in plants and trees, andsurveillance of military activities.[136]

The deep vacuum of space could make it an attrac-tive environment for certain industrial processes, suchas those that require ultraclean surfaces.[137] However,like asteroid mining, space manufacturing requires a sig-nificant investment with little prospect of an immediatereturn.[138] An important factor in the total expense isthe high cost of placing mass into Earth orbit: $7,000–23,000 per kg in inflation-adjusted dollars, according toa 2006 estimate.[139] Proposed concepts for addressingthis issue include non-rocket spacelaunch, momentum ex-change tethers, and space elevators.[140]

9 See also• Earth’s location in the universe

• List of government space agencies

• List of topics in space

• Outline of space science

• Panspermia

• Space and survival

• Space race

• Space station

• Space technology

• Timeline of knowledge about the interstellar and in-tergalactic medium

• Timeline of Solar System exploration

• Timeline of spaceflight

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• Fichtner, Horst; Liu, W. William (2011), “Ad-vances in Coordinated Sun-Earth System ScienceThrough Interdisciplinary Initiatives and Inter-national Programs”, written at Sopron, Hungary,in Miralles, M.P.; Almeida, J. Sánchez, TheSun, the Solar Wind, and the Heliosphere, IAGASpecial Sopron Book Series 4, Berlin: Springer,

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• Frisch, Priscilla C.; Müller, Hans R.; Zank, Gary P.;Lopate, C. (May 6–9, 2002), “Galactic environmentof the Sun and stars: interstellar and interplanetarymaterial”, in Livio, Mario; Reid, I. Neill; Sparks,William B., Astrophysics of life. Proceedings of theSpace Telescope Science Institute Symposium, SpaceTelescope Science Institute symposium series 16,Baltimore, MD, USA: Cambridge University Press,Bibcode:2005asli.symp...21F, ISBN 0-521-82490-7

• Gatti, Hilary (2002), Giordano Bruno and Renais-sance science, Cornell University Press, ISBN 0-8014-8785-4

• Genz, Henning (2001), Nothingness: the science ofempty space, Da Capo Press, ISBN 0-7382-0610-5

• Ghosh, S. N. (2000), Atmospheric Science and En-vironment, Allied Publishers, ISBN 8177640437

• Grant, Edward (1981), Much ado about nothing:theories of space and vacuum from the Middle Agesto the scientific revolution, The Cambridge history ofscience series, Cambridge University Press, ISBN0-521-22983-9

• Hardesty, Von; Eisman, Gene; Krushchev, Sergei(2008), Epic Rivalry: The Inside Story of the So-viet and American Space Race, National GeographicBooks, pp. 89–90, ISBN 1-4262-0321-7

• Hariharan, P. (2003), Optical interferometry (2nded.), Academic Press, ISBN 0-12-311630-9

• Harris, Philip Robert (2008), Space enterprise: liv-ing and working offworld in the 21st century,Springer Praxis Books / Space Exploration Series,Springer, ISBN 0-387-77639-7

• Harrison, Albert A. (2002), Spacefaring: The Hu-man Dimension, University of California Press,ISBN 0-520-23677-7

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• Maor, Eli (1991), To infinity and beyond: a culturalhistory of the infinite, Princeton paperbacks, ISBN0-691-02511-8

• Mendillo, Michael (November 8–10, 2000), “Theatmosphere of the moon”, in Barbieri, Cesare;Rampazzi, Francesca, Earth-Moon Relationships,Padova, Italy at the Accademia Galileiana DiScienze Lettere Ed Arti: Springer, p. 275, ISBN0-7923-7089-9

• Needham, Joseph; Ronan, Colin (1985), TheShorter Science and Civilisation in China, ShorterScience and Civilisation in China 2, CambridgeUni-versity Press, ISBN 0-521-31536-0

• O'Leary, Beth Laura (2009), Darrin, Ann Garrison,ed., Handbook of space engineering, archaeology,and heritage, Advances in engineering, CRC Press,ISBN 1-4200-8431-3

• Olenick, Richard P.; Apostol, Tom M.; Goodstein,David L. (1986), Beyond the mechanical universe:from electricity to modern physics, Cambridge Uni-versity Press, ISBN 0-521-30430-X

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• Rauchfuss, Horst (2008), Chemical Evolution andthe Origin of Life, Translated by T. N. Mitchell,Springer, ISBN 3-540-78822-0

• Schrijver, Carolus J.; Siscoe, George L. (2010),Heliophysics: Evolving Solar Activity and the Cli-mates of Space and Earth, Cambridge UniversityPress, ISBN 0-521-11294-X

16 11 EXTERNAL LINKS

• Silk, Joseph (2000), The Big Bang (3rd ed.),Macmillan, ISBN 0-8050-7256-X

• Sparke, Linda S.; Gallagher, John S. (2007), Galax-ies in the Universe: An Introduction (2nd ed.), Cam-bridge University Press, ISBN 978-0-521-85593-8

• Spitzer, Lyman, Jr. (1978), Physical Processes in theInterstellar Medium, Wiley Classics Library, ISBN0471293350

• Stuart Wortley, Emmeline Charlotte E. (1841), Themaiden of Moscow, a poem, How and Parsons,Canto X, section XIV, lines 14–15, All Earth inmadness moved,—o'erthrown, / To outer space—driven—racked—undone!

• Thagard, Paul (1992), Conceptual revolutions,Princeton University Press, ISBN 0-691-02490-1

• Tassoul, Jean Louis; Tassoul, Monique (2004), Aconcise history of solar and stellar physics, Prince-ton University Press, ISBN 0-691-11711-X

• Tyson, Neil deGrasse; Goldsmith, Donald (2004),Origins: fourteen billion years of cosmic evolution,W. W. Norton & Company, pp. 114–115, ISBN0-393-05992-8

• Von Humboldt, Alexander (1845), Cosmos: a sur-vey of the general physical history of the Universe,New York: Harper & Brothers Publishers

• Webb, Stephen (1999), Measuring the universe: thecosmological distance ladder, Springer, ISBN 1-85233-106-2

• Wong, Wilson; Fergusson, James Gordon (2010),Military space power: a guide to the issues, Contem-porary military, strategic, and security issues, ABC-CLIO, ISBN 0-313-35680-7

11 External links

Media related to Outer space at Wikimedia Commons

• Intergalactic Space, Natural History, February 1998

• Newscientist Space

• space.com

17

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12.1 Text

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