Natural rubber called India rubber or caoutchouc, as produced, consists of polymers of the organic compound isoprene, with minor impurities of other organic compounds, plus water. Thailand and Indonesia are two of the leading rubber producers. Forms of polyisoprene that are used as natural rubbers are classified as elastomers. Rubber is harvested in the form of the latex from the rubber tree or others; the latex is a sticky, milky colloid drawn off by making incisions in the bark and collecting the fluid in vessels in a process called "tapping". The latex is refined into rubber ready for commercial processing. In major areas, latex is allowed to coagulate in the collection cup; the coagulated lumps are processed into dry forms for marketing. Natural rubber is used extensively in many applications and products, either alone or in combination with other materials. In most of its useful forms, it has a large stretch ratio and high resilience, is waterproof; the major commercial source of natural rubber latex is the Pará rubber tree, a member of the spurge family, Euphorbiaceae.
This species is preferred. A properly managed tree responds to wounding by producing more latex for several years. Congo rubber a major source of rubber, came from vines in the genus Landolphia. Dandelion milk contains latex; the latex exhibits the same quality as the natural rubber from rubber trees. In the wild types of dandelion, latex content varies greatly. In Nazi Germany, research projects tried to use dandelions as a base for rubber production, but failed. In 2013, by inhibiting one key enzyme and using modern cultivation methods and optimization techniques, scientists in the Fraunhofer Institute for Molecular Biology and Applied Ecology in Germany developed a cultivar, suitable for commercial production of natural rubber. In collaboration with Continental Tires, IME began a pilot facility. Many other plants produce forms of latex rich in isoprene polymers, though not all produce usable forms of polymer as as the Pará; some of them require more elaborate processing to produce anything like usable rubber, most are more difficult to tap.
Some produce other desirable materials, for example chicle from Manilkara species. Others that have been commercially exploited, or at least showed promise as rubber sources, include the rubber fig, Panama rubber tree, various spurges, the related Scorzonera tau-saghyz, various Taraxacum species, including common dandelion and Russian dandelion, most for its hypoallergenic properties, guayule; the term gum rubber is sometimes applied to the tree-obtained version of natural rubber in order to distinguish it from the synthetic version. The first use of rubber was by the indigenous cultures of Mesoamerica; the earliest archeological evidence of the use of natural latex from the Hevea tree comes from the Olmec culture, in which rubber was first used for making balls for the Mesoamerican ballgame. Rubber was used by the Maya and Aztec cultures – in addition to making balls Aztecs used rubber for other purposes such as making containers and to make textiles waterproof by impregnating them with the latex sap.
The Pará rubber tree is indigenous to South America. Charles Marie de La Condamine is credited with introducing samples of rubber to the Académie Royale des Sciences of France in 1736. In 1751, he presented a paper by François Fresneau to the Académie that described many of rubber's properties; this has been referred to as the first scientific paper on rubber. In England, Joseph Priestley, in 1770, observed that a piece of the material was good for rubbing off pencil marks on paper, hence the name "rubber", it made its way around England. In 1764 François Fresnau discovered. Giovanni Fabbroni is credited with the discovery of naphtha as a rubber solvent in 1779. South America remained the main source of latex rubber used during much of the 19th century; the rubber trade was controlled by business interests but no laws expressly prohibited the export of seeds or plants. In 1876, Henry Wickham smuggled 70,000 Pará rubber tree seeds from Brazil and delivered them to Kew Gardens, England. Only 2,400 of these germinated.
Seedlings were sent to India, British Ceylon, Dutch East Indies and British Malaya. Malaya was to become the biggest producer of rubber. In the early 1900s, the Congo Free State in Africa was a significant source of natural rubber latex gathered by forced labor. King Leopold II's colonial state brutally enforced production quotas. Tactics to enforce the rubber quotas included removing the hands of victims to prove they had been killed. Soldiers came back from raids with baskets full of chopped-off hands. Villages that resisted were razed to encourage better compliance locally. See Atrocities in the Congo Free State for more information on the rubber trade in the Congo Free State in the late 1800s and early 1900s. Liberia and Nigeria started production. In India, commercial cultivation was introduced by British planters, although the experimental efforts to grow rubber on a commercial scale were initiated as early as 1873 at the Calcutta Botanical Gardens; the first commercial Hevea plantations were established at Thattekadu in Kerala in 1902.
In years the plantation expanded to Karnataka, Tamil Nadu and the Andaman and Nicobar Islands of India. India today is the
Nitrogen dioxide is the chemical compound with the formula NO2. It is one of several nitrogen oxides. NO2 is an intermediate in the industrial synthesis of nitric acid, millions of tons of which are produced each year, used in the production of fertilizers. At higher temperatures it is a reddish-brown gas that has a characteristic sharp, biting odor and is a prominent air pollutant. Nitrogen dioxide is a bent molecule with C2v point group symmetry. Nitrogen dioxide is a reddish-brown gas above 21.2 °C with a pungent, acrid odor, becomes a yellowish-brown liquid below 21.2 °C, converts to the colorless dinitrogen tetroxide below −11.2 °C. The bond length between the nitrogen atom and the oxygen atom is 119.7 pm. This bond length is consistent with a bond order between two. Unlike ozone, O3, the ground electronic state of nitrogen dioxide is a doublet state, since nitrogen has one unpaired electron, which decreases the alpha effect compared with nitrite and creates a weak bonding interaction with the oxygen lone pairs.
The lone electron in NO2 means that this compound is a free radical, so the formula for nitrogen dioxide is written as •NO2. The reddish-brown color is a consequence of preferential absorption of light in the blue, although the absorption extends throughout the visible and into the infrared. Absorption of light at wavelengths shorter than about 400 nm results in photolysis. Nitrogen dioxide arises via the oxidation of nitric oxide by oxygen in air: 2 NO + O2 → 2 NO2Nitrogen dioxide is formed in most combustion processes using air as the oxidant. At elevated temperatures nitrogen combines with oxygen to form nitric oxide: O2 + N2 → 2 NOIn the laboratory, NO2 can be prepared in a two-step procedure where dehydration of nitric acid produces dinitrogen pentoxide, which subsequently undergoes thermal decomposition: 2 HNO3 → N2O5 + H2O 2 N2O5 → 4 NO2 + O2The thermal decomposition of some metal nitrates affords NO2: 2 Pb2 → 2 PbO + 4 NO2 + O2Alternatively, reduction of concentrated nitric acid by metal.
4 HNO3 + Cu → Cu2 + 2 NO2 + 2 H2OOr by adding concentrated nitric acid over tin. 4 HNO3 + Sn → H2O + H2SnO3 + 4 NO2 NO2 exists in equilibrium with the colourless gas dinitrogen tetroxide: 2 NO2 ⇌ N2O4The equilibrium is characterized by ΔH = −57.23 kJ/mol, exothermic. NO2 is favored at higher temperatures, while at lower temperatures, dinitrogen tetroxide predominates. Dinitrogen tetroxide can be obtained as a white solid with melting point −11.2 °C. NO2 is paramagnetic due to its unpaired electron; the chemistry of nitrogen dioxide has been investigated extensively. At 150 °C, NO2 decomposes with release of oxygen via an endothermic process: 2 NO2 → 2 NO + O2 As suggested by the weakness of the N–O bond, NO2 is a good oxidizer, it will combust, sometimes explosively, with many compounds, such as hydrocarbons. It hydrolyses to give nitric acid and nitrous acid: 2 NO2 + H2O → HNO2 + HNO3This reaction is one step in the Ostwald process for the industrial production of nitric acid from ammonia; this reaction is negligibly slow at low concentrations of NO2 characteristic of the ambient atmosphere, although it does proceed upon NO2 uptake to surfaces.
Such surface reaction is thought to produce gaseous HNO2 in indoor environments. Nitric acid decomposes to nitrogen dioxide by the overall reaction: 4 HNO3 → 4 NO2 + 2 H2O + O2The nitrogen dioxide so formed confers the characteristic yellow color exhibited by this acid. NO2 is used to generate anhydrous metal nitrates from the oxides: MO + 3 NO2 → M2 + NO Alkyl and metal iodides give the corresponding nitrites: 2 CH3I + 2 NO2 → 2 CH3NO2 + I2TiI4 + 4 NO2 → Ti4 + 2 I2 NO2 is introduced into the environment by natural causes, including entry from the stratosphere, bacterial respiration and lightning; these sources make NO2 a trace gas in the atmosphere of Earth, where it plays a role in absorbing sunlight and regulating the chemistry of the troposphere in determining ozone concentrations. NO2 is used as an intermediate in the manufacturing of nitric acid, as a nitrating agent in manufacturing of chemical explosives, as a polymerization inhibitor for acrylates, as a flour bleaching agent, and as a room temperature sterilization agent.
It is used as an oxidizer in rocket fuel, for example in red fuming nitric acid. For the general public, the most prominent sources of NO2 are internal combustion engines burning fossil fuels. Outdoors, NO2 can be a result of traffic from motor vehicles. Indoors, exposure arises from cigarette smoke, butane and kerosene heaters and stoves. Workers in industries where NO2 is used are exposed and are at risk for occupational lung diseases, NIOSH has set exposure limits and safety standards. Astronauts in the Apollo–Soyuz Test Project were killed when NO2 was accidentally vented into the cabin. Agricultural workers can be exposed to NO2 arising from grain decomposing in silos. Nitrogen dioxide was produced by atmospheric nuclear tests, was responsible for the reddish colour of mushroom clouds. Gaseous NO2 diffuses into the epithelial lining fluid of the res
The Viking program consisted of a pair of American space probes sent to Mars, Viking 1 and Viking 2. Each spacecraft was composed of two main parts: an orbiter designed to photograph the surface of Mars from orbit, a lander designed to study the planet from the surface; the orbiters served as communication relays for the landers once they touched down. The Viking program grew from NASA's earlier more ambitious, Voyager Mars program, not related to the successful Voyager deep space probes of the late 1970s. Viking 1 was launched on August 20, 1975, the second craft, Viking 2, was launched on September 9, 1975, both riding atop Titan IIIE rockets with Centaur upper stages. Viking 1 entered Mars orbit on June 19, 1976, with Viking 2 following suit on August 7. After orbiting Mars for more than a month and returning images used for landing site selection, the orbiters and landers detached; the Viking 1 lander touched down on the surface of Mars on July 20, 1976, was joined by the Viking 2 lander on September 3.
The orbiters continued imaging and performing other scientific operations from orbit while the landers deployed instruments on the surface. The project cost US$1 billion in 1970s dollars, equivalent to about 5 billion USD in 2018 dollars; the mission was considered successful and is credited with helping to form most of the body of knowledge about Mars through the late 1990s and early 2000s. Obtain high-resolution images of the Martian surface Characterize the structure and composition of the atmosphere and surface Search for evidence of life on Mars The primary objectives of the two Viking orbiters were to transport the landers to Mars, perform reconnaissance to locate and certify landing sites, act as communications relays for the landers, to perform their own scientific investigations; each orbiter, based on the earlier Mariner 9 spacecraft, was an octagon 2.5 m across. The fueled orbiter-lander pair had a mass of 3527 kg. After separation and landing, the lander had a mass of the orbiter 900 kg.
The total launch mass was 2328 kg, of which 1445 kg were attitude control gas. The eight faces of the ring-like structure were 0.4572 m high and were alternately 1.397 and 0.508 m wide. The overall height was 3.29 m from the lander attachment points on the bottom to the launch vehicle attachment points on top. There were 3 on each of the 4 long faces and one on each short face. Four solar panel wings extended from the axis of the orbiter, the distance from tip to tip of two oppositely extended solar panels was 9.75 m. The main propulsion unit was mounted above the orbiter bus. Propulsion was furnished by a bipropellant liquid-fueled rocket engine which could be gimballed up to 9 degrees; the engine was capable of 1,323 N thrust. Attitude control was achieved by 12 small compressed-nitrogen jets. An acquisition Sun sensor, a cruise Sun sensor, a Canopus star tracker and an inertial reference unit consisting of six gyroscopes allowed three-axis stabilization. Two accelerometers were on board. Communications were accomplished through two 20 W TWTAs.
An X band downlink was added for radio science and to conduct communications experiments. Uplink was via S band. A two-axis steerable parabolic dish antenna with a diameter of 1.5 m was attached at one edge of the orbiter base, a fixed low-gain antenna extended from the top of the bus. Two tape recorders were each capable of storing 1280 megabits. A 381-MHz relay radio was available; the power to the two orbiter craft was provided by eight 1.57 × 1.23 m solar panels, two on each wing. The solar panels produced 620 W of power at Mars. Power was stored in two nickel-cadmium 30-A·h batteries; the combined area of the four panels was 15 square meters, they provided both regulated and unregulated direct current power. Two 30-amp-hour, nickel-cadmium, rechargeable batteries provided power when the spacecraft was not facing the Sun, during launch, correction maneuvers and Mars occultation. By discovering many geological forms that are formed from large amounts of water, the images from the orbiters caused a revolution in our ideas about water on Mars.
Huge river valleys were found in many areas. They showed that floods of water broke through dams, carved deep valleys, eroded grooves into bedrock, travelled thousands of kilometers. Large areas in the southern hemisphere contained branched stream networks, suggesting that rain once fell; the flanks of some volcanoes are believed to have been exposed to rainfall because they resemble those caused on Hawaiian volcanoes. Many craters look; when they were formed, ice in the soil may have melted, turned the ground into mud flowed across the surface. Material from an impact goes up down, it does not flow across the surface. Regions, called "Chaotic Terrain," seemed to have lost great volumes of water, causing large channels to be formed; the amount of water involved was estimated to ten thousand times the flow of the Mississippi River. Underground volcanism may have melted frozen ice; each lander comprised a six-sided aluminium base with alternate 1.09 and 0.56 m long sides, supported on three extended legs attac
Stratospheric Observatory for Infrared Astronomy
The Stratospheric Observatory for Infrared Astronomy is an 80/20 joint project of NASA and the German Aerospace Center to construct and maintain an airborne observatory. NASA awarded the contract for the development of the aircraft, operation of the observatory and management of the American part of the project to the Universities Space Research Association in 1996; the DSI manages the German parts of the project which are science and telescope related. SOFIA's telescope saw first light on May 26, 2010. SOFIA is the successor to the Kuiper Airborne Observatory, it will observe celestial magnetic fields, star-forming regions, comets and the galactic centre. SOFIA is based on a Boeing 747SP wide-body aircraft, modified to include a large door in the aft fuselage that can be opened in flight to allow a 2.5 m diameter reflecting telescope access to the sky. This telescope is designed for infrared astronomy observations in the stratosphere at altitudes of about 12 kilometres. SOFIA's flight capability allows it to rise above all of the water vapor in the Earth's atmosphere, which blocks some infrared wavelengths from reaching the ground.
At the aircraft's cruising altitude, 85% of the full infrared range will be available. The aircraft can travel to any point on the Earth's surface, allowing observation from the northern and southern hemispheres. Once ready for use, observing flights were expected to be flown four nights a week. Scheduled to be operational for 20 years, in its tentative budget for the fiscal year 2015 NASA announced that unless Germany's aerospace center would contribute more than agreed upon, the observatory would be grounded by 2015; the SOFIA Observatory is based at NASA's Armstrong Flight Research Center at Palmdale Regional Airport, while the SOFIA Science Center is based in Ames Research Center, in Mountain View, California. SOFIA uses a 2.5 m reflector telescope, which has an oversized, 2.7 m diameter primary mirror, as is common with most large infrared telescopes. The optical system uses a Cassegrain reflector design with a parabolic primary mirror and a remotely configurable hyperbolic secondary. In order to fit the telescope into the fuselage, the primary is shaped to an f-number as low as 1.3, while the resulting optical layout has an f-number of 19.7.
A flat, dichroic mirror is used to deflect the infrared part of the beam to the Nasmyth focus where it can be analyzed. An optical mirror located behind the tertiary mirror is used for a camera guidance system; the telescope looks out of a large door in the port side of the fuselage near the airplane's tail, carried nine instruments for infrared astronomy at wavelengths from 1–655 micrometres and high-speed optical astronomy at wavelengths from 0.3–1.1 μm. The main instruments are a near infrared camera covering 1 -- 5 μm; the other four instruments include an optical photometer and infrared spectrometers with various spectral ranges. SOFIA's telescope is by far the largest to be placed in an aircraft. For each mission one interchangeable science instrument will be attached to the telescope. Two groups of general purpose instruments are available. In addition, an investigator can design and build a special purpose instrument. On April 17, 2012, two upgrades to HAWC were selected by NASA to increase the field of view with new transition edge sensor bolometer detector arrays and to add the capability of measuring the polarization of dust emission from celestial sources.
The open cavity housing the telescope will be exposed to high-speed turbulent winds. In addition, the vibrations and motions of the aircraft introduce observing difficulties; the telescope was designed to be lightweight, with a honeycomb shape milled into the back of the mirror and polymer composite material used for the telescope assembly. The mount includes a system of bearings in pressurized oil to isolate the instrument from vibration. Tracking is achieved through a system of gyroscopes, high speed cameras, magnetic torque motors to compensate for motion, including vibrations from airflow and the aircraft engines; the telescope cabin must be cooled prior to aircraft takeoff to ensure the telescope matches the external temperature to prevent thermally-induced shape changes. Prior to landing the compartment is flooded with nitrogen gas to prevent condensation of moisture on the chilled optics and instruments. DLR is responsible for the entire telescope assembly and design along with two of the nine scientific instruments used with the telescope, NASA is responsible for the aircraft.
The manufacturing of the telescope was subcontracted to European industry. The telescope is German-made; the secondary silicon carbide based mirror mechanism was manufactured by the Swiss Center for Electronics and Microtechnology. A reflective surface was applied to the mirror at a facility in Louisiana but the consortium now maintains a mirror coating facility in Moffett Field, allowing for fast recoating of the primary mirror, a process, expected to be required 1–2 times per year; the SOFIA aircraft is a modified Boeing 747SP widebody with a distinguished history. Boeing developed the SP or "Special Performance" version of the 747 for ultra long range flights, modifying the design of the 747-100 by removing sections of the fuselage and modifying othe
The Sun is the star at the center of the Solar System. It is a nearly perfect sphere of hot plasma, with internal convective motion that generates a magnetic field via a dynamo process, it is by far the most important source of energy for life on Earth. Its diameter is about 1.39 million kilometers, or 109 times that of Earth, its mass is about 330,000 times that of Earth. It accounts for about 99.86% of the total mass of the Solar System. Three quarters of the Sun's mass consists of hydrogen; the Sun is a G-type main-sequence star based on its spectral class. As such, it is informally and not accurately referred to as a yellow dwarf, it formed 4.6 billion years ago from the gravitational collapse of matter within a region of a large molecular cloud. Most of this matter gathered in the center, whereas the rest flattened into an orbiting disk that became the Solar System; the central mass became so hot and dense that it initiated nuclear fusion in its core. It is thought that all stars form by this process.
The Sun is middle-aged. It fuses about 600 million tons of hydrogen into helium every second, converting 4 million tons of matter into energy every second as a result; this energy, which can take between 10,000 and 170,000 years to escape from its core, is the source of the Sun's light and heat. In about 5 billion years, when hydrogen fusion in its core has diminished to the point at which the Sun is no longer in hydrostatic equilibrium, its core will undergo a marked increase in density and temperature while its outer layers expand to become a red giant, it is calculated that the Sun will become sufficiently large to engulf the current orbits of Mercury and Venus, render Earth uninhabitable. After this, it will shed its outer layers and become a dense type of cooling star known as a white dwarf, no longer produce energy by fusion, but still glow and give off heat from its previous fusion; the enormous effect of the Sun on Earth has been recognized since prehistoric times, the Sun has been regarded by some cultures as a deity.
The synodic rotation of Earth and its orbit around the Sun are the basis of solar calendars, one of, the predominant calendar in use today. The English proper name Sun may be related to south. Cognates to English sun appear in other Germanic languages, including Old Frisian sunne, Old Saxon sunna, Middle Dutch sonne, modern Dutch zon, Old High German sunna, modern German Sonne, Old Norse sunna, Gothic sunnō. All Germanic terms for the Sun stem from Proto-Germanic *sunnōn; the Latin name for the Sun, Sol, is not used in everyday English. Sol is used by planetary astronomers to refer to the duration of a solar day on another planet, such as Mars; the related word solar is the usual adjectival term used for the Sun, in terms such as solar day, solar eclipse, Solar System. A mean Earth solar day is 24 hours, whereas a mean Martian'sol' is 24 hours, 39 minutes, 35.244 seconds. The English weekday name Sunday stems from Old English and is a result of a Germanic interpretation of Latin dies solis, itself a translation of the Greek ἡμέρα ἡλίου.
The Sun is a G-type main-sequence star. The Sun has an absolute magnitude of +4.83, estimated to be brighter than about 85% of the stars in the Milky Way, most of which are red dwarfs. The Sun is heavy-element-rich, star; the formation of the Sun may have been triggered by shockwaves from more nearby supernovae. This is suggested by a high abundance of heavy elements in the Solar System, such as gold and uranium, relative to the abundances of these elements in so-called Population II, heavy-element-poor, stars; the heavy elements could most plausibly have been produced by endothermic nuclear reactions during a supernova, or by transmutation through neutron absorption within a massive second-generation star. The Sun is by far the brightest object in the Earth's sky, with an apparent magnitude of −26.74. This is about 13 billion times brighter than the next brightest star, which has an apparent magnitude of −1.46. The mean distance of the Sun's center to Earth's center is 1 astronomical unit, though the distance varies as Earth moves from perihelion in January to aphelion in July.
At this average distance, light travels from the Sun's horizon to Earth's horizon in about 8 minutes and 19 seconds, while light from the closest points of the Sun and Earth takes about two seconds less. The energy of this sunlight supports all life on Earth by photosynthesis, drives Earth's climate and weather; the Sun does not have a definite boundary, but its density decreases exponentially with increasing height above the photosphere. For the purpose of measurement, the Sun's radius is considered to be the distance from its center to the edge of the photosphere, the apparent visible surface of the Sun. By this measure, the Sun is a near-perfect sphere with an oblateness estimated at about 9 millionths, which means that its polar diameter differs from its equatorial diameter by only 10 kilometres; the tidal effect of the planets is weak and does not affect the shape of the Sun. The Sun rotates faster at its equator than at its poles; this differential rotation is caused by convective motion
The mesosphere is the layer of the Earth's atmosphere, directly above the stratosphere and directly below the thermosphere. In the mesosphere, temperature decreases; this characteristic is used to define its limits: it begins at the top of the stratosphere, ends at the mesopause, the coldest part of Earth's atmosphere with temperatures below −143 °C. The exact upper and lower boundaries of the mesosphere vary with latitude and with season, but the lower boundary is located at heights from 50 to 65 kilometres above the Earth's surface and the upper boundary is around 85 to 100 kilometres; the stratosphere and the mesosphere are collectively referred to as the "middle atmosphere", which spans heights from 10 kilometres to 100 kilometres. The mesopause, at an altitude of 80–90 km, separates the mesosphere from the thermosphere—the second-outermost layer of the Earth's atmosphere; this is around the same altitude as the turbopause, below which different chemical species are well mixed due to turbulent eddies.
Above this level the atmosphere becomes non-uniform. The term near space is sometimes used; this term does not have a technical definition, but refers the region of the atmosphere up to 100 km between the Armstrong limit up to the Kármán line where astrodynamics must take over from aerodynamics in order to achieve flight. The definition of near space can vary depending on the source, but in general near space comprises the altitudes above where commercial airliners fly but below orbiting satellites; some sources distinguish between the terms "near space" and "upper atmosphere," so that only the layers closest to the Karman line are called near space. Within the mesosphere, temperature decreases with increasing height, due to decreasing absorption of solar radiation by the rarefied atmosphere and increasing cooling by CO2 radiative emission; the top of the mesosphere, called the mesopause, is the coldest part of Earth's atmosphere. Temperatures in the upper mesosphere fall as low as −101 °C, varying according to latitude and season.
The main dynamic features in this region are strong zonal winds, atmospheric tides, internal atmospheric gravity waves, planetary waves. Most of these tides and waves start in the troposphere and lower stratosphere, propagate to the mesosphere. In the mesosphere, gravity-wave amplitudes can become so large that the waves become unstable and dissipate; this dissipation deposits momentum into the mesosphere and drives global circulation. This helps the Earth. Noctilucent clouds are located in the mesosphere; the upper mesosphere is the region of the ionosphere known as the D layer. The D layer is only present during the day when some ionization occurs with nitric oxide being ionized by Lyman series-alpha hydrogen radiation; the ionization is so weak that when night falls, the source of ionization is removed, the free electron and ion form back into a neutral molecule. The mesosphere has been called the "ignorosphere" because it is poorly studied relative to the stratosphere and the thermosphere. A 5 km deep sodium layer is located between 80–105 km.
Made of unbound, non-ionized atoms of sodium, the sodium layer radiates weakly to contribute to the airglow. The sodium has an average concentration of 400,000 atoms per cubic centimetre; this band is replenished by sodium sublimating from incoming meteors. Astronomers have begun utilizing this sodium band to create "guide stars" as part of the adaptive optical correction process used to produce ultra-sharp ground-based observations. Other metal layers, e.g. iron and potassium, exist in the upper mesosphere/lower thermosphere region as well. Millions of meteors enter the Earth's atmosphere, averaging 40 tons per year; the ablated material, called meteoric smoke, is thought to serve as condensation nuclei for noctilucent clouds. The mesosphere lies above altitude records for aircraft, while only the lowest few kilometers are accessible to balloons, for which the altitude record is 53.0 km. Meanwhile, the mesosphere is below the minimum altitude for orbital spacecraft due to high atmospheric drag.
It has only been accessed through the use of sounding rockets, which are only capable of taking mesospheric measurements for a few minutes per mission. As a result, it is the least-understood part of the atmosphere, resulting in the humorous moniker ignorosphere; the presence of red sprites and blue jets, noctilucent clouds, density shears within this poorly understood layer are of current scientific interest. Near space was first explored in the 1930s; the early flights flew to the edge of space without computers and with only crude life support systems. Notable people who flew in near space were Jean Piccard and his wife Jeannette, on the nearcraft The Century of Progress. Exploration was carried out by unmanned craft, although there have been skydiving attempts made from high-altitude balloons; the area is of interest for military surveillance purposes, scientific study, as well as to commercial interests for communications, tourism. Craft that fly in near space include hi
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group on the periodic table, a reactive nonmetal, an oxidizing agent that forms oxides with most elements as well as with other compounds. By mass, oxygen is the third-most abundant element in the universe, after helium. At standard temperature and pressure, two atoms of the element bind to form dioxygen, a colorless and odorless diatomic gas with the formula O2. Diatomic oxygen gas constitutes 20.8% of the Earth's atmosphere. As compounds including oxides, the element makes up half of the Earth's crust. Dioxygen is used in cellular respiration and many major classes of organic molecules in living organisms contain oxygen, such as proteins, nucleic acids and fats, as do the major constituent inorganic compounds of animal shells and bone. Most of the mass of living organisms is oxygen as a component of water, the major constituent of lifeforms. Oxygen is continuously replenished in Earth's atmosphere by photosynthesis, which uses the energy of sunlight to produce oxygen from water and carbon dioxide.
Oxygen is too chemically reactive to remain a free element in air without being continuously replenished by the photosynthetic action of living organisms. Another form of oxygen, ozone absorbs ultraviolet UVB radiation and the high-altitude ozone layer helps protect the biosphere from ultraviolet radiation. However, ozone present at the surface is a byproduct of thus a pollutant. Oxygen was isolated by Michael Sendivogius before 1604, but it is believed that the element was discovered independently by Carl Wilhelm Scheele, in Uppsala, in 1773 or earlier, Joseph Priestley in Wiltshire, in 1774. Priority is given for Priestley because his work was published first. Priestley, called oxygen "dephlogisticated air", did not recognize it as a chemical element; the name oxygen was coined in 1777 by Antoine Lavoisier, who first recognized oxygen as a chemical element and characterized the role it plays in combustion. Common uses of oxygen include production of steel and textiles, brazing and cutting of steels and other metals, rocket propellant, oxygen therapy, life support systems in aircraft, submarines and diving.
One of the first known experiments on the relationship between combustion and air was conducted by the 2nd century BCE Greek writer on mechanics, Philo of Byzantium. In his work Pneumatica, Philo observed that inverting a vessel over a burning candle and surrounding the vessel's neck with water resulted in some water rising into the neck. Philo incorrectly surmised that parts of the air in the vessel were converted into the classical element fire and thus were able to escape through pores in the glass. Many centuries Leonardo da Vinci built on Philo's work by observing that a portion of air is consumed during combustion and respiration. In the late 17th century, Robert Boyle proved. English chemist John Mayow refined this work by showing that fire requires only a part of air that he called spiritus nitroaereus. In one experiment, he found that placing either a mouse or a lit candle in a closed container over water caused the water to rise and replace one-fourteenth of the air's volume before extinguishing the subjects.
From this he surmised that nitroaereus is consumed in both combustion. Mayow observed that antimony increased in weight when heated, inferred that the nitroaereus must have combined with it, he thought that the lungs separate nitroaereus from air and pass it into the blood and that animal heat and muscle movement result from the reaction of nitroaereus with certain substances in the body. Accounts of these and other experiments and ideas were published in 1668 in his work Tractatus duo in the tract "De respiratione". Robert Hooke, Ole Borch, Mikhail Lomonosov, Pierre Bayen all produced oxygen in experiments in the 17th and the 18th century but none of them recognized it as a chemical element; this may have been in part due to the prevalence of the philosophy of combustion and corrosion called the phlogiston theory, the favored explanation of those processes. Established in 1667 by the German alchemist J. J. Becher, modified by the chemist Georg Ernst Stahl by 1731, phlogiston theory stated that all combustible materials were made of two parts.
One part, called phlogiston, was given off when the substance containing it was burned, while the dephlogisticated part was thought to be its true form, or calx. Combustible materials that leave little residue, such as wood or coal, were thought to be made of phlogiston. Air did not play a role in phlogiston theory, nor were any initial quantitative experiments conducted to test the idea. Polish alchemist and physician Michael Sendivogius in his work De Lapide Philosophorum Tractatus duodecim e naturae fonte et manuali experientia depromti described a substance contained in air, referring to it as'cibus vitae', this substance is identical with oxygen. Sendivogius, during his experiments performed between 1598 and 1604, properly recognized that the substance is equivalent to the gaseous byproduct released by the thermal decomposition of potassium nitrate. In Bugaj’s view, the isolation of oxygen and the proper association of the substance to that part of air, required for life, lends sufficient weight to the discovery of oxygen by Sendivogius.