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Monday, January 24, 2011


Solar & Alternative Energy
Probing UV photo-oxidation on oxide surfaces
Christof Wöll
UV light directly oxidizes carbon monoxide on rutile titanium dioxide surfaces.
10 January 2011, SPIE Newsroom. DOI: 10.1117/2.1201012.003336
http://spie.org/images/Graphics/Newsroom/Imported-2010/003336/003336_10_big.jpgPhotochemistry at oxide surfaces is important for solar-energy conversion.1,2 The most promising candidate for solar-powered water splitting (i.e., hydrogen production) and, thus, benign energy production is titania (TiO2)-based photocatalysis. Since the initial work of Fujishima and Honda,2numerous improvements related to water splitting have been reported.3 However, even after nearly 40 years of research, major issues concerning the photoactivity of this important oxide are still debated. For example, a convincing explanation as to why the anatase polymorph of TiO2 shows much more photocatalytic activity than its rutile form, by an order of magnitude,4,5 is unavailable.3
Even simple reactions, such as carbon monoxide (CO) oxidation (see Figure1), are not understood because of the almost exclusive focus on oxide-powder samples. Such samples are structurally undefined, while all possible surface orientations—often with a high defect density—are probed simultaneously.
The surface-science approach to understanding heterogeneous catalysis has been quite successful.6 Consequently, progress toward unraveling fundamental principles governing oxide-surface chemistry urgently necessitates detailed experimental and theoretical studies on model systems.
http://spie.org/Images/Graphics/Newsroom/Imported-2010/003336/003336_10_fig1.jpg
Figure 1.Photo-oxidation of carbon monoxide (CO) adsorbed on a rutile titania (TiO2) (110) surface.7 C: Carbon. O: Oxygen. O2: Molecular oxygen. O2c: Bridging oxygen. CO2: Carbon dioxide. Ti5c: Fivefold coordinated titanium.
Especially for TiO2, surface-oxide research within the past two decades has proceeded along a different direction than that of metal surfaces. Primarily driven by the tremendous success of microscopic interrogation, particularly by scanning-tunneling microscopy (STM), a wealth of structural information has become available about processes on the atomic scale, e.g., reactions occurring at defect sites on rutile8–15 and, to a lesser extent, anatase.16–18By employing density functional theory (DFT), many STM observations have been explained.9
http://spie.org/Images/Graphics/Newsroom/Imported-2010/003336/003336_10_fig2.jpg
Figure 2.Reflection-absorption IR spectroscopy data recorded for CO adsorbed on a rutile TiO2(110) substrate. (A) Prior to UV exposure. (B)–(F) Increasing exposure to UV photons.7
However, identification of chemical intermediates by STM/ DFT is subject to some pitfalls (e.g., reliable identification of hydroxyl species at TiO2 surfaces remains a challenge). It has become clear that spectroscopic methods, and in particular IR vibrational spectroscopy, are indispensable for establishing a reliable foundation of chemistry on oxide surfaces, as has been demonstrated with respect to chemistry on metal surfaces through the surface-science approach.
Unfortunately, specific optical properties of oxides lead to severe problems for applications of reflection-absorption IR spectroscopy (IRRAS), the standard experimental method in this field. Sensitivity to molecular vibrations within adsorbates on oxides is reduced by two orders of magnitude with respect to metals. Consequently, despite the availability of a large set of IR data recorded in transmission mode for powders, data for well-defined oxide model systems is virtually unavailable. This is one of the primary reasons why our understanding of chemistry and photochemistry on oxide surfaces is still relatively limited.
We recently overcame these intensity problems by employing a novel, carefully optimized apparatus, where an IR spectrometer is directly attached to an ultrahigh-vacuum chamber.19 We detected (for the first time) the internal CO-stretch vibration of a TiO2single crystal surface through IRRAS7(see Figure 2) and consequently demonstrated that application of IRRAS on oxide surfaces offers huge potential for other molecular adsorbate species.20When adsorbed CO is exposed to UV photons in the presence of molecular oxygen (O2), photo-oxidation proceeds without any intermediates. Activated O2 reacts directly with CO, yielding carbon dioxide.7 Furthermore, we determined the photo cross section of the photo-oxidation reaction and found that it agrees with previous data.7 In very recent measurements on anatase single crystals, we found that the corresponding cross section is much larger. This parallels observations reported for powders and indicates that the surprisingly large photo cross sections seen for anatase powders do not result from either special types of surface-active sites or a particular form of defects characteristic for anatase, as proposed previously. In contrast, they originate from special features of the (bulk) electronic structure, specifically the presence of an indirect band gap.21
These results have important implications for our fundamental understanding of photochemical energy conversion in general, as well as for fabrication of materials with high photochemical cross sections. For nanoparticles with diameters in the sub-10nm region, the electronic structure of particles will be strongly disturbed with respect to the bulk. Consequently, the longer lifetimes of electronic excitations (electron-hole pairs) of anatase are predicted to be reduced to the values characteristic of rutile, since the indirect band gap will no longer reduce the electron-hole recombination rate. We next plan to look at other photochemical reactions on TiO2and zinc oxide surfaces, with particular emphasis on doping effects.

Nanorods in an electric field create liquid crystals for transformation optics


Nanotechnology
Nanorods in an electric field create liquid crystals for transformation optics
Oleg D. Lavrentovich, Andrii B. Golovin, Jie Xiang and Yuriy A. Nastishin
Condensing and aligning metal nanorods in a fluid is a useful technique in fabricating reconfigurable metamaterials for applications such as invisibility cloaking and solar-energy collectors.
14 January 2011, SPIE Newsroom. DOI: 10.1117/2.1201012.003315
Advances in optical engineering are often associated with the development of new materials. One example is liquid crystals, organic fluids formed by rodlike molecules that align parallel to each other. Tailoring the temperature range of their existence, refractive indices, surface alignment, and dielectric and elastic properties made possible a multibillion-dollar liquid-crystal display industry. A newly emerging field is that of optical metamaterials for ‘transformation’ optics.1In these materials, the index of refraction varies from point to point in a broad range, including negative and zero values. It would be ideal if we could fabricate these materials with 3D reconfigurable architecture, as that would allow us to control light propagation with a finesse http://spie.org/images/Graphics/Newsroom/Imported-2010/003315/003315_10_big.jpgpreviously unknown, making possible applications such as sub-wavelength imaging, invisibility cloaking, and light concentrators.
The challenge is that to direct the waves of light into desired trajectories, the local dielectric and magnetic properties of a metamaterial should vary so widely in space that no single natural substance can fit the bill. The metamaterials need to be constructed artificially, by selecting proper metallic and dielectric elements and then arranging them in space into predesigned architectures. The building units must be smaller than the wavelength of light to avoid scattering. Some metastructures can be fabricated by lithography, but this technique is of limited applicability for complex 3D and reconfigurable architectures. We propose an alternative approach,2,3 using a dispersion of metal nanoparticles in a fluid and controlling their concentration and orientation by a nonuniform electric field.
Metal nanorods (NRs) can be dispersed in a dielectric fluid such as water or toluene. The electric field polarizes the NRs and aligns them along the field lines. If the field is nonuniform, it also creates a force moving the NRs, usually toward the region of stronger field. The particle does not need to be charged: the two ends of the rod with the two polarization charges of the same absolute value but opposite signs find themselves in slightly different fields. The effect is called dielectrophoresis. Previously, it was used to manipulate supramicron particles.4 In optical metamaterials, however, one prefers to work with much smaller (nanometer-scale) particles, to avoid light scattering. Since the dielectrophoretic force is proportional to the particle' volume, it was not clear whether it would be strong enough to manipulate the nanoparticles. Our work shows that the dielectrophoretic effect is in fact a very potent way to control the spatial location and orientation of nanoparticles and thus the optical performance of the metamaterial.
We use a dispersion of gold (Au) NRs in toluene. Their diameter is 10–20nm and length 40–80nm. The volume fraction is low, 0.01–0.1%, and thus the dispersion is spatially uniform and isotropic, i.e., the NRs are distributed homogeneously in space far apart from each other and adopt arbitrary orientations. A nonuniform field transforms this fluid into a liquid crystal, as illustrated in Figure 1 in an experiment with two mutually perpendicular metal wires (2) and (3) in a glass cell (1). The dielectrophoretic force attracts the NRs to the electrode (2), where they align parallel to the electric field lines and condense into a cloud. The concentration of NRs is high near the electrode and low at the periphery of the cell. Consequently, the refractive index of the medium changes from a high value at the periphery (equal to the refractive index of pure toluene) to a smaller value near the electrode (2). The effect is most pronounced for light polarized perpendicularly to the electrode (2) and less so for light polarized parallel to it. This optical anisotropy is caused by the aligned assembly of the NRs, which resembles a liquid crystal formed by orientationally ordered rodlike molecules. The density variation represents a major difference compared with the normal liquid crystal, since all material properties, such as the refractive index, also change from point to point with the NR concentration.
Figure 1.Field-induced birefringent cloud of gold nanorods (Au NRs) concentrated and aligned in a high-electric-field region. A, P: Axes of the analyzer and polarizer. U, f: Amplitude and frequency of the applied voltage.
The most interesting effect is produced when the sample represents a circular glass capillary: see (1) in Figure 2. One electrode is a metal wire (2) running along the axis. The second electrode is a transparent indium tin oxide layer (3) at the outer surface of the capillary. The field-induced radial pattern of NRs with the concentration that decreases from the electrode to the periphery (4) resembles the theoretical optical cloak proposed by Vladimir Shalaev's group.5 By applying the electric field, we reduce the visibility of the central wire in light polarized normally to the cylinder: see Figure 2 and video.6
Figure 2.(a) Field-induced radial gradient structure of an Au NR dispersion. The visibility of the central wire (b) is reduced when the field is on (c).
The refractive index of toluene with the condensed cloud of NRs decreases when one moves from the NR-depleted periphery of the cylinder to the NR-rich region near the central electrode (2). The radial variation of the refractive index in the cloud (4) causes the light rays to bend around the wire (2), so its shadow is mitigated, representing an imperfect version of the cloaking effect. Theoretically, the refractive index at the inner surface of a cloaking shell should be 0. Our experiments2,3 have not yet reached this limit, as the estimated field-induced change in the refractive index is 0.1. The task is to increase it further, by a factor of 10.
To make a 3D reconfigurable metamaterial, we need to learn how to reversibly intertwine the metal and dielectric elements at the scale of nanometers. Our approach based on the dielectrophoretic effect is only the first step forward. It shows that the architecture of metal and dielectric nanoscale elements can be controlled by the nonuniform electric field. The next steps would be to increase the concentration of NRs, to explore whether and how the dielectrophoretic forces can be supplemented by other forces such as electrophoretic and induced-charge electrophoretic ones, for a better control of the 3D architecture. The issues of light scattering and losses in the metal-dielectric composites also need to be addressed. We plan to apply the dielectrophoretic technique to NRs that are preassembled into chainlike and raftlike clusters7 to increase the initial concentration of NRs and to assemble clouds with differently oriented NRs in them. These tasks are formidable, but the rewards hold a major promise for optical engineering of the future.
This work was supported by Air Force Office of Scientific Research grant FA9550-10-1-0527, Department of Defense Multidisciplinary University Research Initiative grant FA9550-06-1-0337, and Department of Energy DE-FG02-06ER46331. We thank N. A. Kotov and P. Palffy-Muhoray for providing us with Au NR dispersions, and A. Agarwal, J. Fontana, P. Luchette, H.-S. Park, L. Tortora, B. Senyuk, H. Wonderly, and L. Qiu for help in sample preparation. We thank V. M. Shalaev, P. Palffy-Muhoray, C. Y. Lee, A. V. Kildishev, S. V. Shiyanovskii, and V. P. Drachev for fruitful discussions.

Oleg D. Lavrentovich, Andrii B. Golovin, Jie Xiang, Yuriy A. Nastishin
Liquid Crystal Institute, Kent State University
Kent, OH
Oleg D. Lavrentovich received his PhD (1984) and DSc (1990) in physics and mathematics from the National Academy of Sciences, Ukraine. He is the director of the Liquid Crystal Institute at Kent State University and a SPIE Fellow.
Andrii B. Golovin received his PhD in physics and mathematics from the National Academy of Sciences, Ukraine (1994). He is a research associate. He has conducted research in the areas of materials science, optical engineering, and laser physics.
Jie Xiang received his MS in instrument science and technology (2009) from Harbin Institute of Technology, China. He is a graduate student at the Liquid Crystal Institute and Chemical Physics Interdisciplinary Program, Kent State University.
Yuriy A. Nastishin received his PhD (1991) and DSc (2006) from the Institute of Physical Optics, Ukraine, where he leads the group on Optics of Liquid Crystals. Currently, he is a visiting scientist at the Liquid Crystal Institute, Kent State University.


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Wednesday, June 30, 2010

Outer space

Outer space

Outer space (often simply called space) is the void that exists beyond any celestial body including the Earth.[1] It is not completely empty (i.e. a perfect vacuum), but contains a low density of particles, predominantly hydrogen plasma, as well as electromagnetic radiation, magnetic fields, and neutrinos. Theoretically, it also contains dark matter and dark energy

Discovery

In 350 BC, Greek philosopher Aristotle suggested that nature abhors a vacuum, a principle that became known as the horror vacui. Based on this idea that a vacuum could not exist, it was widely held for many centuries that space could not be empty.[2] As late as the seventeenth century, the French philosopherRené Descartes argued that the entirety of space must be filled.[3] It became known to Galileo Galilei that air had weight and so was subject to gravity. He also demonstrated that there was an established force that resisted the formation of a vacuum. However, it would remain for his pupil Evangelista Torricelli to create an apparatus that would produce a vacuum. At the time this experiment created a scientific sensation in Europe. The French mathematician Blaise Pascal reasoned that if the column of mercury was suspended by air then the column ought to be shorter at higher altitude. His brother in law, Florin Périer, repeated the experiment on the Puy-de-Dôme mountain in central France and found that the column was shorter by three inches. This decrease in pressure was further demonstrated by carrying a half-full balloon up a mountain and watching it gradually inflate, then deflate upon descent. These and other experiments were used to overthrow the principle of horror vacui.[4]

Further work on the physics of the vacuum was performed by Otto von Guericke. He correctly noted that the atmosphere of the Earth surrounds the planet like a shell, with the density gradually declining with altitude. He concluded that there must be a vacuum between the Earth and the Moon.[5]

Early speculations as to the infinite dimension of space was performed in the sixteenth century by the Italian philosopher Giordano Bruno. He extended the Copernican heliocentric cosmology to the concept of an infinite universe that is filled with a substance he called aether, which did not cause resistance to the motions of heavenly bodies.[6] English philosopher William Gilbert arrived at a similar conclusion, arguing that the stars are visible to us only because they are surrounded by a thin aether or a void.[7] This concept of an aether originated with ancient Greek philosophers, including Aristotle, who conceived of it as the medium through which the heavenly bodies moved.[8]

The concept of a universe filled with a luminiferous aether remained in vogue among some scientists up until the twentieth century. This form of aether was viewed as the medium through which light could propagate. In 1887, the Michelson-Morley experiment was carried out as an attempt to detect the Earth's motion through this medium by looking for changes in the speed of light based on the direction of the planet's motion. However, the null result indicated something was wrong with the concept. Since then the idea of the luminiferous aether had essentially been abandoned, to be replaced by Albert Einstein's theory of special relativity. The latter held that the speed of light is a constant in a vacuum, regardless of the observer's motion or frame of reference.[9][10]

The first professional astronomer to support the concept of an infinite universe was the Englishman Thomas Digges in 1576.[11] However, the true scale of the universe remained unknown until the first successful measurement of the distance to a nearby star was performed in 1838 by the German astronomerFriedrich Bessel. He showed that the star 61 Cygni had a parallax of just 0.31 arcseconds (compared to the modern value of 0.287″). This corresponded to a distance of over 10 light years.[12] The distance scale to the Andromeda galaxy was determined in 1923 by American astronomer Edwin Hubble when he measured the brightness of cepheid variables within that galaxy. This established that the Andromeda galaxy, and by extension all galaxies, lay well outside the Milky Way.[13]

The modern concept of outer space is based upon the Big Bang cosmology, which was first proposed in 1931 by the Belgian physicist Georges Lemaître. This theory holds that the observable universe originated from a very compact form that has since undergone continuous expansion. Matter that remained following the initial expansion has since undergone gravitational collapse to create stars, galaxies and other astronomical objects, leaving behind a deep vacuum that forms what is now called outer space.[14]

The term outer space was first recorded by the English poet Lady Emmeline Stuart-Wortley in her poem "The Maiden of Moscow" in 1842,[15] and later popularised in the writings of HG Wells in 1901.[16] The shorter term space is actually older, first used to mean the region beyond Earth's sky in John Milton'sParadise Lost in 1667.[17]

[edit]Environment

Outer space is the closest natural approximation of a perfect vacuum. It has effectively no friction, allowing stars, planets and moons to move freely along ideal gravitational trajectories. However, even in the deep vacuum of intergalactic space there are still a few hydrogen atoms per cubic meter.[18] By comparison, the air we breathe contains about 1025 molecules per cubic meter.[19] The sparse density of matter in outer space means that electromagnetic radiation can travel great distances without being scattered; the mean free path for a photon in intergalactic space is about 1023 km, or 10 billion light years.[20] The deep vacuum of space could make it an attractive environment for certain industrial processes, for instance those that require ultraclean surfaces.[21]

Stars, planets, asteroids and moons keep their atmospheres 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 1 Pa at 100 kilometres (62 mi) of altitude. This is known as the Kármán line, a common definition of the boundary with outer space. Beyond this line, isotropic gas pressure rapidly becomes insignificant when compared to radiation pressure from the Sun and the dynamic pressure of the solar wind, so the definition of pressure becomes difficult to interpret. The thermosphere in this range has large gradients of pressure, temperature and composition, and varies greatly due to space weather. Astrophysicists prefer to use number density to describe these environments, in units of particles per cubic centimetre.

[edit]Temperature

All of the observable Universe is filled with photons that were created during the Big Bang, which is known as the cosmic microwave background radiation (CMB). There is quite likely a correspondingly large number of neutrinos called the cosmic neutrino background. The current black body temperature of this photon radiation is about 3 K (−270 °C; −454 °F). Some regions of outer space can contain highly energetic particles that have a much higher temperature than the CMB.

[edit]Effect on human bodies

Contrary to popular belief,[22] a person suddenly exposed to the vacuum would not explode, freeze to death or die from boiling blood. Air would immediately leave the lungs due to the enormouspressure gradient. Any oxygen dissolved in the blood would empty into the lungs to try to equalize the partial pressure gradient. Once the deoxygenated blood arrived at the brain, death would quickly follow.

Humans and animals exposed to vacuum will lose consciousness after a few seconds and die of hypoxia within minutes. Blood and other body fluids do boil when their pressure drops below 6.3 kPa, the vapor pressure of water at body temperature.[23] 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.[24][25] Swelling and ebullism can be reduced by containment in a flight suit.Shuttle astronauts wear a fitted elastic garment called the Crew Altitude Protection Suit (CAPS) which prevents ebullism at pressures as low as 2 kPa.[26] Water vapor would also rapidly evaporate off from exposed areas such as the lungs, cornea of the eye and mouth, cooling the body. Rapid evaporative cooling of the skin will create frost, particularly in the mouth, but this is not a significant hazard. Space may be cold, but it's mostly vacuum and transfers heat ineffectively; as a result the main temperature regulation concern for space suits is how to get rid of naturally generated body heat.

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 kilometres (1.9 mi) generally compensate for the lower pressures there.[23] Above this altitude, oxygen enrichment is necessary to prevent altitude sickness, and spacesuits are necessary to prevent ebullism above 19 kilometres (12 mi).[23] Most spacesuits use only 20 kPa of pure oxygen, just enough to sustain full consciousness. This pressure is high enough to prevent ebullism, but simple evaporation of blood can still cause decompression sickness and gas embolisms if not managed.

Rapid decompression can be much more dangerous than vacuum exposure itself. Even if the victim does not hold his breath, venting through the windpipe may be too slow to prevent the fatal rupture of the delicate alveoli of the lungs.[23] 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.[27] Injuries caused by rapid decompression are called barotrauma. A pressure drop as small as 13 kPa, which produces no symptoms if it is gradual, may be fatal if it occurs suddenly.[23]

[edit]Boundary

There is no clear boundary between Earth's atmosphere and space, as the density of the atmosphere gradually decreases as the altitude increases. There are several designated scientific boundaries, namely:

In 2009, scientists at the University of Calgary reported detailed measurements with an instrument called the Supra-Thermal Ion Imager (an instrument that measures the direction and speed of ions), which allowed them to determine that space begins 118 kilometres (73 mi) above Earth. The boundary represents the midpoint of a gradual transition over tens of kilometers from the relatively gentle winds of the Earth's atmosphere to the more violent flows of charged particles in space, which can reach speeds well over 600 miles per hour (1,000 km/h).[28][29]

This was only the second time that direct measurements of charged particle flows have been conducted at this region, which is too high for balloons and too low for satellites. It was however the first study to include all the relevant elements for this kind of determination – for example, the upper atmospheric winds.

The instrument was carried by the JOULE-II rocket on January 19, 2007, and traveled to an altitude of about 124 miles (200 km). From there it collected data while it was moving through the "edge of space".[28]

[edit]Legal status

The Outer Space Treaty provides the basic framework for international space law. This treaty covers the legal use of outer space by nation states, and includes in its definition of outer space the Moonand 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. It also 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 USSR, the United States of America and the United Kingdom. As of January 1, 2008 the treaty has been signed by 98 states and ratified by an additional 27 states.[30]

Between 1958 and 2008, outer space has been the subject of multiple resolutions by the United Nations General Assembly. 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.[31] Four additional space law treaties have been negotiated and drafted by the UN's Committee on the Peaceful Uses of Outer Space. The 1979 Moon Treaty turned the jurisdiction of all heavenly bodies (including the orbits around such bodies) over to the international community. However, this treaty has not been ratified by any nation that currently practices manned spaceflight.[32]

[edit]Space versus orbit

To perform an orbit, a spacecraft must travel faster than a sub-orbital spaceflight. A spacecraft has not entered orbit until it is traveling with a sufficiently great horizontal velocity such that theacceleration due to gravity on the spacecraft is less than or equal to the centripetal acceleration being caused by its horizontal velocity (see circular motion). So to enter orbit, a spacecraft must not only reach space, but must also achieve a sufficient orbital speed (angular velocity). For a low-Earth orbit, this is about 7,900 m/s (28,400 km/h; 17,700 mph); by contrast, the fastest airplane speed ever achieved (excluding speeds achieved by deorbiting spacecraft) was 2,200 m/s (7,900 km/h; 4,900 mph) in 1967 by the North American X-15.[33]

Konstantin Tsiolkovsky was the first person to realize that, given the energy available from any available chemical fuel, a several-stage rocket would be required. The escape velocity to pull free of Earth's gravitational field altogether and move into interplanetary space is about 11,000 m/s (39,600 km/h; 24,600 mph) The energy required to reach velocity for low Earth orbit (32 MJ/kg) is about twenty times the energy required simply to climb to the corresponding altitude (10 kJ/(km·kg)).

There is a major difference between sub-orbital and orbital spaceflights. The minimum altitude for a stable orbit around Earth (that is, one without significant atmospheric drag) begins at around 350 kilometres (220 mi) above mean sea level. A common misunderstanding about the boundary to space is that orbit occurs simply by reaching this altitude. Achieving orbital speed can theoretically occur at any altitude, although atmospheric drag precludes an orbit that is too low. At sufficient speed, an airplane would need a way to keep it from flying off into space, but at present, this speed is several times greater than anything within reasonable technology.

A common misconception is that people in orbit are outside Earth's gravity because they are "floating". They are floating because they are in "free fall": they are accelerating toward Earth, along with their spacecraft, but are simultaneously moving sideways fast enough that the "fall" away from a straight-line path merely keeps them in orbit at a constant distance above Earth's surface. Earth's gravity reaches out far past the Van Allen belt and keeps the Moon in orbit at an average distance of 384,403 kilometres (238,857 mi).

[edit]Regions

Space is not a perfect vacuum: its different regions are defined by the various atmospheres and "winds" that dominate within them, and extend to the point at which those winds give way to those beyond. Geospace extends from Earth's atmosphere to the outer reaches of Earth's magnetic field, whereupon it gives way to the solar wind of interplanetary space. Interplanetary space extends to theheliopause, whereupon the solar wind gives way to the winds of the interstellar medium. Interstellar space then continues to the edges of the galaxy, where it fades into the intergalactic void.

[edit]Geospace

Aurora australis observed by Discovery, May 1991.

Geospace is the region of outer space near the Earth. Geospace includes the upper region of the atmosphere, as well as the ionosphere andmagnetosphere. The Van Allen radiation belts also lie within the geospace. The region between Earth's atmosphere and the Moon is sometimes referred to as cis-lunar space.

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 significant drag on satellites. Most artificial satellites operate in this region called low earth orbit and must fire their engines every few days to maintain orbit. The drag here is low enough that it could theoretically be overcome by radiation pressure on solar sails, a proposed propulsion system for interplanetary travel.

Geospace is populated by electrically charged particles at very low densities, the motions of which are controlled by the Earth's magnetic field. These plasmas form a medium from which storm-like disturbances powered by the solar wind can drive electrical currents into the Earth’s upper atmosphere.

During geomagnetic storms two regions of geospace, the radiation belts and the ionosphere, can become strongly disturbed. These storms increase fluxes of energetic electrons that can permanently damage satellite electronics, disrupting telecommunications and GPS technologies, and can also be a hazard to astronauts, even in low-Earth orbit. They also create aurorae seen near the magnetic poles.

Geospace contains material left over from previous manned and unmanned launches that are a potential hazard to spacecraft. Some of this debris re-enters Earth's atmosphere periodically.

The absence of air makes geospace (and the surface of the Moon) ideal locations for astronomy at all wavelengths of the electromagnetic spectrum, as evidenced by the spectacular pictures sent back by the Hubble Space Telescope, allowing light from about 13.7 billion years ago — almost to the time of the Big Bang — to be observed.

The outer boundary of geospace is the interface between the magnetosphere and the solar wind. The inner boundary is the ionosphere.[34] Alternately, geospace is the region of space between the Earth’s upper atmosphere and the outermost reaches of the Earth’s magnetic field.[35]

[edit]Interplanetary

Interplanetary space, the space around the Sun and planets of the Solar System, is the region dominated by the interplanetary medium, which extends out to the heliopause where the influence of the galactic environment starts to dominate over the magnetic field and particle flux from the Sun. Interplanetary space is defined by the solar wind, a continuous stream of charged particles emanating from the Sun that creates a very tenuous atmosphere (the heliosphere) for billions of miles into space. This wind has a particle density of 5–10 protons/cm3 and is moving at a velocity of 350–400 km/s.[36] The distance and strength of the heliopause varies depending on the activity level of the solar wind.[37] The discovery since 1995 of extrasolar planets means that other stars must possess their own interplanetary media.[38]

The volume of interplanetary space is an almost pure vacuum, with a mean free path of about one astronomical unit at the orbital distance of the Earth. However, this space is not completely empty, and is sparsely filled with cosmic rays, which include ionized atomic nuclei and various subatomic particles. There is also gas, plasma and dust, small meteors, and several dozen types of organicmolecules discovered to date by microwave spectroscopy.[39]

Interplanetary space contains the magnetic field generated by the Sun.[36] There are also magnetospheres generated by planets such as Jupiter, Saturn and the Earth that have their own magnetic fields. These are shaped by the influence of the solar wind into the approximation of a teardrop shape, with the long tail extending outward behind the planet. These magnetic fields can trap particles from the solar wind and other sources, creating belts of magnetic particles such as the Van Allen Belts. Planets without magnetic fields, such as Mars and Mercury, but excluding Venus, have their atmospheres gradually eroded by the solar wind.

[edit]Interstellar

Interstellar space is the physical space within a galaxy not occupied by stars or their planetary systems. The interstellar medium resides—by definition—in interstellar space.

[edit]Intergalactic

Intergalactic space is the physical space between galaxies. Generally free of dust and debris, intergalactic space is very close to a total vacuum. The space between galaxy clusters, called the voids, is probably nearly empty. Some theories put the average density of the Universe as the equivalent of one hydrogen atom per cubic meter.[40][41] The density of the universe, however, is clearly not uniform; it ranges from relatively high density in galaxies (including very high density in structures within galaxies, such as planets, stars, and black holes) to conditions in vast voids that have much lower density than the universe's average.

Surrounding and stretching between galaxies, there is a rarefied plasma[42] that is thought to possess a cosmic filamentary structure[43] and that is slightly denser than the average density in the universe. This material is called the intergalactic medium (IGM) and is mostly ionized hydrogen; i.e. a plasma consisting of equal numbers of electrons and protons. The IGM is thought to exist at a density of 10 to 100 times the average density of the universe (10 to 100 hydrogen atoms per cubic meter). It reaches densities as high as 1000 times the average density of the universe in rich clusters of galaxies.

The reason the IGM is thought to be mostly ionized gas is that its temperature is thought to be quite high by terrestrial standards (though some parts of it are only "warm" by astrophysical standards). As gas falls into the Intergalactic Medium from the voids, it heats up to temperatures of 105 K to 107 K, which is high enough for the bound electrons to escape from the hydrogen nuclei upon collisions. At these temperatures, it is called the Warm-Hot Intergalactic Medium (WHIM). Computer simulations indicate that on the order of half 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.