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Saturday, April 11, 2020

Commuter rail

From Wikipedia, the free encyclopedia
The DART is part of Dublin's commuter rail network. It is operated by a fleet of Electric Multiple Units.
 
A CPTM train on the Coral Line in São Paulo, Brazil.
 
A renovated MI 79 of the Réseau Express Régional in Paris.
 
AM class electric multiple unit used in Auckland, New Zealand
 
Commuter rail, or suburban rail, is a passenger rail transport service that primarily operates within a metropolitan area, connecting commuters to a central city from adjacent suburbs or commuter towns. Generally commuter rail systems are considered heavy rail, using electrified or diesel trains. Distance charges or zone pricing may be used. 

The term can refer to systems with a wide variety of different features and service frequencies, but is often used in contrast to rapid transit or light rail

Similar non-English terms include Treno suburbano in Italian, Cercanías in Spanish, Rodalies in Catalan, Proastiakos in Greek, Train de banlieue in French, Příměstský vlak or Esko in Czech, Elektrichka in Russian, Pociąg podmiejski in Polish and Pendeltåg in Swedish. 

Some services share similarities with both commuter rail and high-frequency rapid transit, such as the German S-Bahn, the Réseau Express Régional in Paris, many Japanese commuter systems, and some Australasian suburban networks. Some services, like British commuter rail, share tracks with other passenger services and freight

In the United States, commuter rail often refers to services that operate a higher frequency during peak periods and a lower frequency off-peak. Since the creation of GO Transit's commuter service in 1967, commuter rail services and route length have been expanding in North America. In the US, commuter rail is sometimes referred to as regional rail.

Characteristics

Mumbai Suburban Railway carries more than 7.24 million commuters on a daily basis
 
GO Transit serves the Golden Horseshoe region surrounding Toronto. Its train services are transitioning from a peak direction commuter railway to a Regional Express Network.
 
Most commuter (or suburban) trains are built to main line rail standards, differing from light rail or rapid transit (metro rail) systems by:
  • being larger
  • providing more seating and less standing room, owing to the longer distances involved
  • having (in most cases) a lower frequency of service
  • having scheduled services (i.e. trains run at specific times rather than at specific intervals)
  • serving lower-density suburban areas, typically connecting suburbs to the city center
  • sharing track or right-of-way with intercity or freight trains
  • not fully grade separated (containing at-grade crossings with crossing gates)
  • being able to skip certain stations as an express service due to normally being driver controlled

Train schedule

Compared to rapid transit (or metro rail), commuter/suburban rail often has lower frequency, following a schedule rather than fixed intervals, and fewer stations spaced further apart. They primarily serve lower density suburban areas (non inner-city), and often share right-of-way with intercity or freight trains. Some services operate only during peak hours and others uses fewer departures during off peak hours and weekends. Average speeds are high, often 50 km/h (30 mph) or higher. These higher speeds better serve the longer distances involved. Some services include express services which skip some stations in order to run faster and separate longer distance riders from short-distance ones.

The general range of commuter trains' distance varies between 15 and 200 km (10 and 125 miles). Sometimes long distances can be explained by that the train runs between two or several cities (e.g. S-Bahn in the Ruhr area of Germany). Distances between stations may vary, but are usually much longer than those of urban rail systems. In city centers the train either has a terminal station or passes through the city centre with notably fewer station stops than those of urban rail systems. Toilets are often available on-board trains and in stations.

Track

Their ability to coexist with freight or intercity services in the same right-of-way can drastically reduce system construction costs. However, frequently they are built with dedicated tracks within that right-of-way to prevent delays, especially where service densities have converged in the inner parts of the network.

Most such trains run on the local standard gauge track. Some systems may run on a narrower or broader gauge. Examples of narrow gauge systems are found in Japan, Indonesia, Malaysia, Thailand, Switzerland, in the Brisbane (Queensland Rail's City network) and Perth (Transperth) systems in Australia, in some systems in Sweden, and on the Genoa-Casella line in Italy. Some countries and regions, including Finland, India, Pakistan, Russia, Brazil and Sri Lanka, as well as San Francisco (BART) in the US and Melbourne and Adelaide in Australia, use broad gauge track.

Distinction between other modes of rail

Metro

Metro rail or rapid transit usually covers a smaller inner-urban area ranging outwards to between 12 km to 20 km (or 8 to 14 miles), has a higher train frequency and runs on separate tracks (underground or elevated), whereas commuter rail often shares tracks, technology and the legal framework within mainline railway systems.

However, the classification as a metro or rapid rail can be difficult as both may typically cover a metropolitan area exclusively, run on separate tracks in the centre, and often feature purpose-built rolling stock. The fact that the terminology is not standardised across countries (even across English-speaking countries) further complicates matters. This distinction is most easily made when there are two (or more) systems such as New York's subway and the LIRR and Metro-North Railroad, Paris' Métro and RER along with Transilien, London's tube lines of the Underground and the Overground, (future) Crossrail, Thameslink along with other commuter rail operators, Madrid's Metro and Cercanías, Barcelona's Metro and Rodalies, and Tokyo's subway and the JR lines along with various privately owned and operated commuter rail systems.

S-Trains

An S-Train is a type of hybrid urban-suburban rail serving a metropolitan region. The most well-known S-train systems are the S-Bahn systems in Germany and Austria. Other well-known examples of S-train systems include the S-tog in Copenhagen and S-Bahn/RER systems in Switzerland. In Germany, the S-Bahn is regarded as a train category of its own, and exists in many large cities and in some other areas, with differing service and technical standards from city to city. Most S-Bahns typically behave like commuter rail with most trackage not separated from other trains, and long lines with trains running between cities and suburbs rather than within a city. The distances between stations however, are usually short. In larger systems there is usually a high frequency metro-like central corridor in the city center where all the lines converge into. Typical examples of large city S-Bahns include Munich and Frankfurt. S-Bahns also exist in some mid-size cities like Rostock and Magdeburg but behave more like typical commuter rail with lower frequencies and very little exclusive trackage. In Berlin, the S-Bahn systems arguably fulfill all considerations of a true metro system (despite the existence of U-Bahns as well) – the trains run on tracks that are entirely separated from other trains, short distances between stations, high frequency and uses tunnels but do run a bit further out from the city centre, compared with U-Bahn. In Hamburg and Copenhagen, other, diesel driven trains, do continue where the S-Bahn ends ("A-Bahn" in Hamburg area, and "L-tog" in Copenhagen).

Regional rail

Regional rail usually provides rail services between towns and cities, rather than purely linking major population hubs in the way inter-city rail does. Regional rail operates outside major cities. Unlike Inter-city, it stops at most or all stations between cities. It provides a service between smaller communities along the line, and also connections with long-distance services at interchange stations located at junctions or at larger towns along the line. Alternative names are "local train" or "stopping train". Examples include the former BR's Regional Railways, France's TER (Transport express régional), Germany's DB Regio and South Korea's Tonggeun services.

Regional rail does not exist in this sense in the United States, so the term "regional rail" has become synonymous with commuter rail there, although the two are more clearly defined in Europe.

Inter-city rail

 
In some European countries the distinction between commuter trains and long-distance/intercity trains is very hard to make, because of the relatively short distances involved. For example, so-called "intercity" trains in Belgium and the Netherlands carry many commuters and their equipment, range and speeds are similar to those of commuter trains in some larger countries. In the United Kingdom there is no real division of organisation and brand name between commuter, regional and inter-city trains, making it hard to categorize train connections.

Russian commuter trains, on the other hand, frequently cover areas larger than Belgium itself, although these are still short distances by Russian standards. They have a different ticketing system from long-distance trains, and in major cities they often operate from a separate section of the train station. 

The easiest way to identify these "inter-city" services is that they tend to operate as express services - only linking the main stations in the cities they link, not stopping at any other stations. However, this term is used in Australia (Sydney for example) to describe the regional trains operating beyond the boundaries of the suburban services, even though some of these "inter-city" services stop all stations similar to German regional services. In this regard, the German service delineations and corresponding naming conventions are clearer and better used for academic purposes.

High-speed rail

Sometimes high-speed rail can serve daily use of commuters. The Japanese Shinkansen high speed rail system is heavily used by commuters in the Greater Tokyo Area. They commute between 100 and 200 km by Shinkansen. To meet the demand of commuters, JR sells commuter discount passes and operates 16-car bilevel E4 Series Shinkansen trains at rush hour, providing a capacity of 1,600 seats. Several lines in China, such as the Beijing–Tianjin Intercity Railway and the Shanghai–Nanjing High-Speed Railway, serve a similar role with many more under construction or planned.

The high-speed services linking Zürich, Bern and Basel in Switzerland (200 km/h (120 mph)) have brought the Central Business Districts (CBDs) of these three cities within 1 hour of each other. This has resulted in unexpectedly high demand for new commuter trips between the three cities and a corresponding increase in suburban rail passengers accessing the high-speed services at the main city-centre stations (or Hauptbahnhof). The Regional-Express commuter service between Munich and Nuremberg in Germany go in (200 km/h (120 mph)) along a 300 km/h high-speed line.

The regional trains StockholmUppsala, Stockholm–Västerås, Stockholm–Eskilstuna and GothenburgTrollhättan in Sweden reach 200 km/h (120 mph) and have many daily commuters.

Train types

Commuter/suburban trains are usually optimized for maximum passenger volume, in most cases without sacrificing too much comfort and luggage space, though they seldom have all the amenities of long-distance trains. Cars may be single- or double-level, and aim to provide seating for all. Compared to intercity trains, they have less space, fewer amenities and limited baggage areas.

Multiple unit type

An electric multiple unit on the Yamanote Line operated by JR East in Tokyo, Japan
 
Commuter rail trains are usually composed of multiple units, which are self-propelled, bidirectional, articulated passenger rail cars with driving motors on each (or every other) bogie. Depending on local circumstances and tradition they may be powered either by diesel engines located below the passenger compartment (diesel multiple units) or by electricity picked up from third rails or overhead lines (electric multiple units). Multiple units are almost invariably equipped with control cabs at both ends, which is why such units are so frequently used to provide commuter services, due to the associated short turn-around time.

Locomotive hauled services

An Altamont Corridor Express train operating along the San Francisco Bay; a MPI F40PH-2C locomotive hauls a consist of Bombardier Bi-Level VI coaches.
 
Locomotive hauled services are used in some countries or locations. This is often a case of asset sweating, by using a single large combined fleet for intercity and regional services. Loco hauled services are usually run in push-pull formation, that is, the train can run with the locomotive at the "front" or "rear" of the train (pushing or pulling). Trains are often equipped with a control cab at the other end of the train from the locomotive, allowing the train operator to operate the train from either end. The motive power for locomotive-hauled commuter trains may be either electric or diesel-electric, although some countries, such as Germany and some of the former Soviet-bloc countries, also use diesel-hydraulic locomotives.

Seat plans

In the US and some other countries, a three-and-two seat plan is used. However, few people sit in the middle seat on these trains because they feel crowded and uncomfortable.

In Japan and South Korea, longitudinal (sideways window-lining) seating is widely used in many commuter rail trains to increase capacity in rush hours. Carriages are usually not organized to increase seating capacity (although in some trains at least one carriage would feature more doors to facilitate easier boarding and alighting and bench seats so that they can be folded up during rush hour to provide more standing room) even in the case of commuting longer than 50 km and commuters in the Greater Tokyo Area and the Seoul metropolitan area have to stand in the train for more than an hour.

Commuter rail systems around the world

Africa

A Metrorail train pulling out of Kalk Bay station in Cape Town
 
Currently there are not many examples of commuter rail in Africa. Metrorail operates in the major cities of South Africa, and there are some commuter rail services in Algeria, Botswana, Kenya, Morocco, Egypt and Tunisia. In Algeria, SNTF operates commuter rail lines between the capital Algiers and its southern and eastern suburbs. They also serve to connect Algiers' main universities to each other. The Dar es Salaam commuter rail offers intracity services in Dar es Salaam, Tanzania. In Botswana, the (Botswana Railways) "BR Express" has a commuter train between Lobatse and Gaborone.

Asia

East Asia

A Tokyo Metro 16000 series train operating a through service on the JR East Joban Line, an example of high-density commuter rail in Japan.
 
In Japan, commuter rail systems have extensive network and frequent service and are heavily used. In many cases, Japanese commuter rail is operationally more like a typical metro system (with very high operating frequencies, an emphasis on standing passengers, short station spacing) than it is like commuter rail in other countries. Japanese commuter rail also tends to be heavily interlined with subway lines, with commuter rail trains continuing into the subway network, and then out onto different commuter rail systems on the other side of the city. Many Japanese commuter systems operate several levels of express trains to reduce the travel time to distant locations, often using station bypass tracks instead of dedicated express tracks. It is notable that the larger Japanese commuter rail systems are owned and operated by for-profit private railway companies, without public subsidy.

Commuter rail systems have been inaugurated in several cities in China such as Beijing, Shanghai, Zhengzhou, Wuhan, Changsha and the Pearl River Delta. With plans for large systems in northeastern Zhejiang, Jingjinji, and Yangtze River Delta areas. The level of service varies considerably from line to line ranging high to near high speeds. More developed and established lines such as the Guangshen Railway have more frequent metro like service. Hong Kong MTR's East Rail Line, West Rail Line and Tung Chung Line were built to commuter rail standards but are operated as a metro system.

In Taiwan, Western Line in Taipei-Taoyuan Metropolitan Area, Taichung Metropolitan Area, Tainan-Kaohsiung Metropolitan Area as well as Neiwan-Liujia Line in Hsinchu Area is considered commuter rail. 

Other examples in East Asia include Seoul Metropolitan Subway of which some lines are suburban lines operated by Korail in South Korea.

Southeast Asia

Electric multiple unit of KRL Commuterline Serpong Line train at Palmerah Station, Jakarta

In Indonesia, the KRL Commuterline is the largest commuter rail system in the country, serving Jakarta metropolitan area. It connects the Jakarta city center with surrounding cities and sub-urbans in Banten and West Java provinces, including Depok, Bogor, Tangerang, Bekasi, Serpong and Maja. In July 2015, KA Commuter Jabodetabek served more than 850,000 passengers per day, which is almost triple of the 2011 figures, but still less than 3.5% of all Jabodetabek commutes. Other commuter rail systems in Indonesia include the Metro Surabaya Commuter Line, Prambanan Ekspres Commuter, Solo Ekspres, Kedung Sepur, Greater Bandung Commuter, and Cut Meutia.

In the Philippines, the Philippine National Railways has two commuter rail systems currently operational; the PNR Metro Commuter Line in the Greater Manila Area and the PNR Bicol Commuter in the Bicol Region. A new commuter rail line in Metro Manila, the North–South Commuter Railway, is currently under construction. Its North section is set to be partially opened by 2021.

In Malaysia, the KTM Komuter serves Kuala Lumpur and the surrounding Klang Valley area.

In Thailand, the Greater Bangkok Commuter rail and the Airport Rail Link serve the Bangkok Metropolitan Region. The SRT Red Lines, a new commuter line in Bangkok, started construction in 2009. It is currently slated to be opened by 2020.

Another commuter rail system in Southeast Asia is the Yangon Circular Railway in Myanmar.

South Asia

In India, commuter rail systems are present in major cities. Mumbai Suburban Railway, the oldest suburban rail system in Asia, carries more than 7.24 million commuters on a daily basis which constitutes more than half of the total daily passenger capacity of the Indian Railways itself. Kolkata Suburban Railway is the biggest Suburban Railway network in India covering 348 stations. The Chennai Suburban Railway along with MRTS is another railway of comparison where more than 1 million people travel daily to different areas in Chennai. Other commuter railways in India include Hyderabad MMTS, Delhi Suburban Railway, Pune Suburban Railway and Lucknow-Kanpur Suburban Railway.

West Asia

In Iran, SYSTRA has done a "Tehran long term urban rail study". SYSTRA proposed 4 express lines similar to RER suburban lines in Paris. Tehran Metro is going to construct express lines. For instance, the Rahyab Behineh, a consultant for Tehran Metro, is studying Tehran Express Line 2. Tehran Metro currently has a commuter line between Tehran and Karaj. Isfahan has two lines to its suburbs Baharestan and Fuladshahr under construction, and a third line to Shahinshahr is planned.

Europe

Type X60 at Stockholm Central in Sweden
 
Major metropolitan areas in most European countries are usually served by extensive commuter/suburban rail systems. Well-known examples include BG Voz in Belgrade (Serbia), S-Bahn in Germany and German-speaking areas of Switzerland and Austria, Proastiakos in Greece, RER in France and Belgium, suburban lines in Milan (Italy), Cercanías and Rodalies (Catalonia) in Spain, CP Urban Services in Portugal, Esko in Prague and Ostrava (Czech Republic), HÉV in Budapest (Hungary) and DART in Dublin (Ireland).

In Russia, Ukraine and some other countries of the former Soviet Union, electrical multiple unit passenger suburban trains called Elektrichka are widespread.

In Sweden, electrified commuter rail systems known as Pendeltåg are present in the cities of Stockholm and Gothenburg. The Stockholm commuter rail system, which began in 1968, is similar to the S-Bahn train systems of Munich and Frankfurt such that it may share railway tracks with inter-city trains and freight trains, but for the most part run on its own dedicated tracks, and that it is primarily used to transport passengers from nearby towns and other suburban areas into the city centre, not for transportation inside the city centre. The Gothenburg commuter rail system, which began in 1960, is similar to the Stockholm system, but does fully share tracks with long-distance trains. Other train systems that are also considered as commuter rail but not counted as pendeltåg include Roslagsbanan and Saltsjöbanan in Stockholm, Mälartåg in the Mälaren Valley, Östgötapendeln in Östergötland County, Upptåget in Uppsala County, Norrtåg in northern Norrland and Skåne Commuter Rail in Skåne County. Skåne Commuter Rail (Pågatågen) acts also as a regional rail system, as it serves cities over 100 km (62 miles) and over one hour from the principal city of Malmö.

In Norway, the Oslo commuter rail system mostly shares tracks with more long-distance trains, but also runs on some local railways without other traffic. Oslo has the largest commuter rail system in the Nordic countries in terms of line lengths and number of stations. But some lines have travel times (over an hour from Oslo) and frequencies (once per hour) which are more like regional trains. Also Bergen, Stavanger and Trondheim have commuter rail systems. These have only one or two lines each and they share tracks with other trains.

In Finland, the Helsinki commuter rail network runs on dedicated tracks from Helsinki Central railway station to Leppävaara and Kerava. The Ring Rail Line serves Helsinki Airport and northern suburbs of Vantaa and is exclusively used by the commuter rail network. On 15 December 2019 Tampere got its own commuter rail service.

In Poland, commuter rail systems exist in Tricity, Warsaw, Krakow and Katowice. There is also a similar system planned in Wrocław and Łódź.

In Romania, the first commuter trains were introduced in December 2019. They operate currently between Bucharest and Funduea or Buftea.

Americas

North and Central America

Metrolink provides commuter rail service to six counties in Southern California.
 
In the United States, Canada, Costa Rica, El Salvador and Mexico regional passenger rail services are provided by governmental or quasi-governmental agencies, with a limited number of metropolitan areas served.

Eight commuter rail systems in the United States carried over ten million trips in 2018:
North American commuter rail systems outside of the United States include:

South America

The Mitre Line is part of the extensive Buenos Aires metropolitan rail system.
 
Examples include an 899 km (559 mi) commuter system in the Buenos Aires metropolitan area, the 225 km (140 mi) long Supervia in Rio de Janeiro, the Metrotrén in Santiago, Chile, and the Valparaíso Metro in Valparaíso, Chile. Another example is Companhia Paulista de Trens Metropolitanos (CPTM) in Greater São Paulo, Brazil. CPTM has 94 stations with seven lines, numbered starting on 7 (the lines 1 to 6 and the line 15 belong to the São Paulo Metro), with a total length of 273 kilometres (170 mi).

Oceania

A Melbourne Siemens commuter train
 
The five major cities in Australia have suburban railway systems in their metropolitan areas. These networks have frequent services, with frequencies varying from every 10 to every 30 minutes on most suburban lines, and up to 3–5 minutes in peak on bundled underground lines in the city centres of Sydney, Brisbane, Perth and Melbourne. The networks in each state developed from mainline railways and have never been completely operationally separate from long distance and freight traffic, unlike metro systems in some comparable countries, but nevertheless have cohesive identities and are the backbones of their respective cities' public transport system. The suburban networks are almost completely electrified. 

The main suburban rail networks in Australia are:
New Zealand has two frequent suburban rail services comparable to those in Australia: the Auckland rail network is operated by Transdev Auckland and the Wellington rail network is operated by Transdev Wellington.

Ablation

From Wikipedia, the free encyclopedia
Ablation near the electrode in a flashtube. The high-energy electrical arc slowly erodes the glass, leaving a frosted appearance.

Ablation is removal or destruction of material from an object by vaporization, chipping, or other erosive processes. Examples of ablative materials are described below, and include spacecraft material for ascent and atmospheric reentry, ice and snow in glaciology, biological tissues in medicine and passive fire protection materials.

Biology

Biological ablation is the removal of a biological structure or functionality.

Genetic ablation is another term for gene silencing, in which gene expression is abolished through the alteration or deletion of genetic sequence information. In cell ablation, individual cells in a population or culture are destroyed or removed. Both can be used as experimental tools, as in loss-of-function experiments.

Glaciology

In glaciology and meteorology, ablation—the opposite of accumulation—refers to all processes that remove snow, ice, or water from a glacier or snowfield. Ablation refers to the melting of snow or ice that runs off the glacier, evaporation, sublimation, calving, or erosive removal of snow by wind. Air temperature is typically the dominant control of ablation, with precipitation exercising secondary control. In a temperate climate during ablation season, ablation rates typically average around 2 mm/h. Where solar radiation is the dominant cause of snow ablation (e.g., if air temperatures are low under clear skies), characteristic ablation textures such as suncups and penitentes may develop on the snow surface.

Ablation can refer either to the processes removing ice and snow or to the quantity of ice and snow removed. 

Debris-covered glaciers have also been shown to greatly impact the ablation process. There is a thin debris layer that can be located on the top of glaciers that intensifies the ablation process below the ice. The debris-covered parts of a glacier that is experiencing ablation are sectioned into three categories which include ice cliffs, ponds, and debris. These three sections allow scientists to measure the heat digested by the debris-covered area and is calculated. The calculations are dependent on the area and net absorbed heat amounts in regards to the entire debris-covered zones. These types of calculations are done to various glaciers to understand and analyze future patterns of melting.

Moraine (glacial debris) is moved by natural processes that allow for down-slope movement of materials on the glacier body. It is noted that if the slope of a glacier is too high then the debris will continue to move along the glacier to a further location. The sizes and locations of glaciers vary around the world, so depending on the climate and physical geography the varieties of debris can differ. The size and magnitude of the debris is dependent on the area of glacier and can vary from dust-size fragments to blocks as large as a house.

There has been many experiments done to demonstrate the effect of debris on the surface of glaciers. Yoshiyuki Fujii, a professor at the National Institute of Polar Research designed an experiment that showed ablation rate was accelerated under a thin debris layer and was retarded under a thick one as compared with that of a natural snow surface. This science is significant due to the importance of long-term availability of water resources and assess glacier response to climate change. Natural resource availability is a major drive behind research conducted in regards to the ablation process and overall study of glaciers.

Laser ablation

An Nd:YAG laser drills a hole through a block of nitrile. The intense burst of infrared radiation ablates the highly absorbing rubber, releasing an eruption of plasma.

Laser ablation is greatly affected by the nature of the material and its ability to absorb energy, therefore the wavelength of the ablation laser should have a minimum absorption depth. While these lasers can average a low power, they can offer peak intensity and fluence given by:
while the peak power is
Surface ablation of the cornea for several types of eye refractive surgery is now common, using an excimer laser system (LASIK and LASEK). Since the cornea does not grow back, laser is used to remodel the cornea refractive properties to correct refraction errors, such as astigmatism, myopia, and hyperopia. Laser ablation is also used to remove part of the uterine wall in women with menstruation and adenomyosis problems in a process called endometrial ablation

Recently, researchers have demonstrated a successful technique for ablating subsurface tumors with minimal thermal damage to surrounding healthy tissue, by using a focused laser beam from an ultra-short pulse diode laser source.

Marine surface coatings

Antifouling paints and other related coatings are routinely used to prevent the buildup of microorganisms and other animals, such as barnacles for the bottom hull surfaces of recreational, commercial and military sea vessels. Ablative paints are often utilized for this purpose to prevent the dilution or deactivation of the antifouling agent. Over time, the paint will slowly decompose in the water, exposing fresh antifouling compounds on the surface. Engineering the antifouling agents and the ablation rate can produce long-lived protection from the deleterious effects of biofouling.

Medicine

In medicine, ablation is the same as removal of a part of biological tissue, usually by surgery. Surface ablation of the skin (dermabrasion, also called resurfacing because it induces regeneration) can be carried out by chemicals (chemoablation), by lasers (laser ablation), by freezing (cryoablation), or by electricity (fulguration). Its purpose is to remove skin spots, aged skin, wrinkles, thus rejuvenating it. Surface ablation is also employed in otolaryngology for several kinds of surgery, such as for snoring. Ablation therapy using radio frequency waves on the heart is used to cure a variety of cardiac arrhythmiae such as supraventricular tachycardia, Wolff–Parkinson–White syndrome (WPW), ventricular tachycardia, and more recently as management of atrial fibrillation. The term is often used in the context of laser ablation, a process in which a laser dissolves a material's molecular bonds. For a laser to ablate tissues, the power density or fluence must be high, otherwise thermocoagulation occurs, which is simply thermal vaporization of the tissues.

Rotoablation is a type of arterial cleansing that consists of inserting a tiny, diamond-tipped, drill-like device into the affected artery to remove fatty deposits or plaque. The procedure is used in the treatment of coronary heart disease to restore blood flow. 

Radiofrequency ablation (RFA) is a method of removing aberrant tissue from within the body via minimally invasive procedures.

Microwave ablation (MWA) is similar to RFA but at higher frequencies of electromagnetic radiation.
Bone marrow ablation is a process whereby the human bone marrow cells are eliminated in preparation for a bone marrow transplant. This is performed using high-intensity chemotherapy and total body irradiation. As such, it has nothing to do with the vaporization techniques described in the rest of this article.

Ablation of brain tissue is used for treating certain neurological disorders, particularly Parkinson's disease, and sometimes for psychiatric disorders as well.

Recently, some researchers reported successful results with genetic ablation. In particular, genetic ablation is potentially a much more efficient method of removing unwanted cells, such as tumor cells, because large numbers of animals lacking specific cells could be generated. Genetically ablated lines can be maintained for a prolonged period of time and shared within the research community. Researchers at Columbia University report of reconstituted caspases combined from C. elegans and humans, which maintain a high degree of target specificity. The genetic ablation techniques described could prove useful in battling cancer.

Passive fire protection

Firestopping and fireproofing products can be ablative in nature. This can mean endothermic materials, or merely materials that are sacrificial and become "spent" over time while exposed to fire, such as silicone firestop products. Given sufficient time under fire or heat conditions, these products char away, crumble, and disappear. The idea is to put enough of this material in the way of the fire that a level of fire-resistance rating can be maintained, as demonstrated in a fire test. Ablative materials usually have a large concentration of organic matter that is reduced by fire to ashes. In the case of silicone, organic rubber surrounds very finely divided silica dust (up to 380 m² of combined surface area of all the dust particles per gram of this dust). When the organic rubber is exposed to fire, it burns to ash and leaves behind the silica dust with which the product started.

Spaceflight

In spacecraft design, ablation is used to both cool and protect mechanical parts and/or payloads that would otherwise be damaged by extremely high temperatures. Two principal applications are heat shields for spacecraft entering a planetary atmosphere from space and cooling of rocket engine nozzles. Examples include the Apollo Command Module that protected astronauts from the heat of atmospheric reentry and the Kestrel second stage rocket engine designed for exclusive use in an environment of space vacuum since no heat convection is possible. 

In a basic sense, ablative material is designed so that instead of heat being transmitted into the structure of the spacecraft, only the outer surface of the material bears the majority of the heating effect. The outer surface chars and burns away -- but quite slowly, only gradually exposing new fresh protective material beneath. The heat is carried away from the spacecraft by the gases generated by the ablative process, and never penetrates the surface material, so the metallic and other sensitive structures they protect, remain at a safe temperature. As the surface burns and disperses into space, while the remaining solid material continues to insulate the craft from ongoing heat and superheated gases. The thickness of the ablative layer is calculated to be sufficient to survive the heat it will encounter on its mission. 

There is an entire branch of spaceflight research involving the search for new fireproofing materials to achieve the best ablative performance; this function is critical to protect the spacecraft occupants and payload from otherwise excessive heat loading. The same technology is used in some passive fire protection applications, in some cases by the same vendors, who offer different versions of these fireproofing products, some for aerospace and some for structural fire protection.

Meteoric iron

From Wikipedia, the free encyclopedia
 
Meteoric Iron (native iron)
TolucaMeteorite.jpg
Widmanstätten pattern on a 500g endcut from the Toluca iron meteorite
General
CategoryNative element mineral
Formula
(repeating unit)
Fe and Ni in different ratios
Space groupDifferent structures
Identification
LusterMetallic
DiaphaneityOpaque

Meteoric iron, sometimes meteoritic iron, is a native metal and early-universe protoplanetary-disk remnant found in meteorites and made from the elements iron and nickel mainly in the form of the mineral phases kamacite and taenite. Meteoric iron makes up the bulk of iron meteorites but is also found in other meteorites. Apart from minor amounts of telluric iron, meteoric iron is the only naturally occurring native metal of the element iron (in metallic form rather than in an ore) on the Earth's surface.

Mineralogy

The bulk of meteoric iron consists of taenite and kamacite. Taenite is a face centered cubic and kamacite a body centered cubic iron-nickel-alloy

Meteoric iron can be distinguished from telluric iron by its microstructure and perhaps by its chemical composition also, since meteoritic iron contains more nickel and less carbon.

Trace amounts of gallium and germanium in meteoric iron can be used to distinguish different meteorite types. The meteoric iron in stony iron meteorites is identical to the "gallium-germanium group" of the iron meteorites.

Overview over meteoric iron mineral phases
Mineral Formula Nickel (Mass-% Ni) Crystal structure Notes
Antitaenite γLow Spin-(Ni,Fe) 20–40 face centered cubic Only approved as a variety of taenite by the IMA
Kamacite α-(Fe,Ni); Fe0+0.9Ni0.1 5–10 body centered cubic Same structure as ferrite
Taenite γ-(Ni,Fe) 20–65 face centered cubic Same structure as austenite
Tetrataenite (FeNi) 48–57 tetragonal

Structures

Meteoric iron forms a few different structures that can be seen by etching or in thin sections of meteorites. The Widmanstätten pattern forms when meteoric iron cools and kamacite is exsolved from taenite in the form of lamellas. Plessite is a more fine-grained intergrowth of the two minerals in between the lamella of the Widmanstätten pattern. Neumann lines are fine lines running through kamacite crystals that form through impact-related deformation.

Cultural and historical usage

A lance made from a narwhal tusk with an iron head made from the Cape York meteorite.
 
Before the advent of iron smelting, meteoric iron was the only source of iron metal apart from minor amounts of telluric iron. Meteoric iron was already used before the beginning of the iron age to make cultural objects, tools and weapons.

Bronze Age

Many examples of iron working from the Bronze Age have been confirmed to be meteoritic in origin.
  • In ancient Egypt an iron metal bead was found in a graveyard near Gerzeh that contained 7.5% Ni. Dated to around 3200 BC, geochemical analysis of the Gerzeh iron beads, based on the ratio of nickel to iron and cobalt, confirms that the iron was meteoritic in origin.
  • Dated to around 2500 BC, an iron dagger from Alaca Höyük in Turkey was confirmed to be meteoritic in origin through geochemical analysis.
  • Dated to around 2300 BC, an iron pendant from Umm el-Marra in Syria was confirmed to be meteoritic in origin through geochemical analysis.
  • Dated to around 1400 BC, an iron axe from Ugarit in Syria was found to be meteoritic in origin.
  • Dated to around 1400 BC, several iron axes from the Shang Dynasty in China were also confirmed to be meteoritic in origin.
  • Dated to around 1350 BC, an iron dagger, bracelet and headrest from the tomb of Tutankhamun were confirmed to be meteoritic in origin. The Tutankhamun dagger consists of similar proportions of metals (iron, nickel and cobalt) to a meteorite discovered in the area, deposited by an ancient meteor shower.

The Americas

Africa

  • Fragments from the Gibeon meteorite were used for centuries by the Nama people of Namibia.

Asia

  • There are reports of the use of meteorites for manufacture of various items in Tibet (see Thokcha).
  • The Iron Man, a statue of Vaiśravaṇa carved from an iron meteorite., a purported Tibetan Buddhist statue, the Iron Man, was likely carved from an ataxite meteorite. It has been speculated that it may be made from a fragment of the Chinga meteorite.
Even after the invention of smelting, meteoric iron was sometimes used where this technology was not available or metal was scarce. A piece of the Cranbourne meteorite was made into a horseshoe around 1854.

Today meteoritic iron is used in niche jewellery and knife production, but most of it is used for research, educational or collecting purposes.

Atmospheric phenomena

Meteoric iron also has an effect on the Earth's atmosphere. When meteorites descend through the atmosphere outer parts are ablated. Meteoric ablation is the source of many elements in the upper atmosphere. When meteoric iron is ablated it forms a free iron atom, that can react with ozone (O3) to form FeO. This FeO may be the source of the orange spectrographic bands in the spectrum of the upper atmosphere.

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