Whale vocalization

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Whale_vocalization

Whales use a variety of sounds for communication and sensation. The mechanisms used to produce sound vary from one family of cetaceans to another. Marine mammals, including whales, dolphins, and porpoises, are much more dependent on sound than land mammals due to the limited effectiveness of other senses in water. Sight is less effective for marine mammals because of the particulate way in which the ocean scatters light. Smell is also limited, as molecules diffuse more slowly in water than in air, which makes smelling less effective. However, the speed of sound is roughly four times greater in water than in the atmosphere at sea level. As sea mammals are so dependent on hearing to communicate and feed, environmentalists and cetologists are concerned that they are being harmed by the increased ambient noise in the world's oceans caused by ships, sonar and marine seismic surveys.

The word "song" is used to describe the pattern of regular and predictable sounds made by some species of whales, notably the humpback whale. This is included with or in comparison with music, and male humpback whales have been described as "inveterate composers" of songs that are "'strikingly similar' to human musical traditions". It has been suggested that humpback songs communicate male fitness to female whales. The click sounds made by sperm whales and dolphins are not strictly song, but the clicking sequences have been suggested to be individualized rhythmic sequences that communicate the identity of a single whale to other whales in its group. These clicking sequences reportedly allow the groups to coordinate foraging activities.

Production of sound

Humans produce voiced sounds by passing air through the larynx. Within the larynx, when the vocal cords are brought close together, the passing air will force them to alternately close and open, separating the continuous airstream into discrete pulses of air that are heard as a vibration. This vibration is further modified by speech organs in the oral and nasal cavities, creating sounds which are used in human speech.

Cetacean sound production differs markedly from this mechanism. The precise mechanism differs in the two major suborders of cetaceans: the Odontoceti (toothed whales, including dolphins) and the Mysticeti (baleen whales, including the largest whales such as the blue whale).

Odontocete whales

Process in a dolphin echolocation: in green the sounds generated by the dolphin, in red from the fish.

 

Idealized dolphin head showing the regions involved in sound production. This image was redrawn from Cranford (2000).

Odontocetes produce rapid bursts of high-frequency clicks that are thought to be primarily for echolocation. Specialized organs in an odontocete produce collections of clicks and buzzes at frequencies from 0.2 to 150 kHz to obtain sonic information about its environment. Lower frequencies are used for distance echolocation, due to the fact that shorter wavelengths do not travel as far as longer wavelengths underwater. Higher frequencies are more effective at shorter distances, and can reveal more detailed information about a target. Echoes from clicks convey not only the distance to the target, but also the size, shape, speed, and vector of its movement. Additionally, echolocation allows the odontocete to easily discern the difference between objects that are different in material composition, even if visually identical, by their different densities. Individuals also appear to be able to isolate their own echoes during pod feeding activity without interference from other pod members' echolocations.

Whistles are used for communication, and four- to six-month-old calves develop unique sounds that they use most frequently throughout their lives. Such "signature whistles" are distinctive to the individual and may serve as a form of identification among other odontocetes. Though a large pod of dolphins will produce a wide range of different noises, very little is known about the meaning of the sound. Frankel quotes one researcher who says listening to a school of odontocetes is like listening to a group of children at a school playground.

The multiple sounds odontocetes make are produced by passing air through a structure in the head called the phonic lips. The structure is analogous to the human nasal cavity, but the phonic lips act similarly to human vocal cords, which in humans are located in the larynx. As the air passes through this narrow passage, the phonic lip membranes are sucked together, causing the surrounding tissue to vibrate. These vibrations can, as with the vibrations in the human larynx, be consciously controlled with great sensitivity. The vibrations pass through the tissue of the head to the melon, which shapes and directs the sound into a beam of sound useful in echolocation. Every toothed whale except the sperm whale has two sets of phonic lips and is thus capable of making two sounds independently. Once the air has passed the phonic lips it enters the vestibular sac. From there, the air may be recycled back into the lower part of the nasal complex, ready to be used for sound creation again, or passed out through the blowhole.

The French name for phonic lips, museau de singe, translates literally as "monkey's muzzle", which the phonic lip structure is supposed to resemble. New cranial analysis using computed axial and single photon emission computed tomography scans in 2004 showed, at least in the case of bottlenose dolphins, that air might be supplied to the nasal complex from the lungs by the palatopharyngeal sphincter, enabling the sound creation process to continue for as long as the dolphin is able to hold its breath.

Mysticete whales

Baleen whales (formally called mysticetes) do not have phonic lip structure. Instead, they have a larynx that appears to play a role in sound production, but it lacks vocal cords, and scientists remain uncertain as to the exact mechanism. The process, however, cannot be completely analogous to humans, because whales do not have to exhale in order to produce sound. It is likely that they recycle air around the body for this purpose. Cranial sinuses may also be used to create the sounds, but again, researchers are currently unsure how.

Vocal plasticity and acoustic behavior

There are at least nine separate blue whale acoustic populations worldwide. Over the last 50 years blue whales have changed the way they are singing. Calls are progressively getting lower in frequency. For example, the Australian pygmy blue whales are decreasing their mean call frequency rate at approximately 0.35 Hz/year.

The migration patterns of blue whales remains unclear. Some populations appear to be resident in habitats of year-round high productivity in some years, while others undertake long migrations to high-latitude feeding grounds, but the extent of migrations and the components of the populations that undertake them are poorly known.

Sound levels

The frequency of baleen whale sounds ranges from 10 Hz to 31 kHz. A list of typical levels is shown in the table below.

Source	Broadband source level (dB re 1 ${\displaystyle \mu }$Pa at 1m)
Fin whale moans	155–186
Blue whale moans	155–188
Gray whale moans	142–185
Bowhead whale tonals, moans and song	128–189

Purpose of whale-created sounds

While the complex sounds of the humpback whale (and some blue whales) are believed to be primarily used in sexual selection, there are simpler sounds that are created by other species of whales that have an alternative use and are used all year round. Whale watchers have watched mother whales lift their young towards the surface in a playful motion, while making a noise that resembles cooing in humans. This cooing-like noise made by whales seems designed to relax their young and is one of several distinct everyday noises whales are known to make. Unlike some fish such as sharks and dolphins, a toothed whale's sense of smell is absent, causing them to rely heavily on echolocation, both for hunting prey and for navigating the ocean under darkness. This requires the whales to produce noise year round to ensure they are able to navigate around any obstacles they may face such as sunken ships or other animals.

It has also been proven that whales are extremely social creatures. The noises that are made throughout the entire year (the main sounds being whistles, clicks, and pulsed calls) are used to communicate with other members of their pod. Each sound a whale makes could mean something different. The clicking noises whales make are used for navigation.   

The question of whether whales sometimes sing purely for aesthetic enjoyment, personal satisfaction, or 'for art's sake', is considered by some to be "an untestable question".

Song of the humpback whale

Two groups of whales, the humpback whale and the subspecies of blue whale found in the Indian Ocean, are known to produce a series of repetitious sounds at varying frequencies known as whale song. Marine biologist Philip Clapham describes the song as "probably the most complex in the animal kingdom."

Male humpback whales perform these vocalizations often during the mating season, and so it is believed the purpose of songs is to aid mate selection.

Interest in whale song was aroused by researchers Roger Payne and Scott McVay after the songs were brought to their attention by a Bermudian named Frank Watlington who was working for the US government at the SOFAR station listening for Russian submarines with underwater hydrophones off the coast of the island. Payne released the best-selling Songs of the Humpback Whale in 1970, and the whale songs were quickly incorporated into human music by, among others, singer Judy Collins.

The songs follow a distinct hierarchical structure. The base units of the song (sometimes loosely called the "notes") are single uninterrupted emissions of sound that last up to a few seconds. These sounds vary in frequency from 20 Hz to upward of 24 kHz (the typical human range of hearing is 20 Hz to 20 kHz). The units may be frequency modulated (i.e., the pitch of the sound may go up, down, or stay the same during the note) or amplitude modulated (get louder or quieter). However, the adjustment of bandwidth on a spectrogram representation of the song reveals the essentially pulsed nature of the FM sounds.

A collection of four or six units is known as a sub-phrase, lasting perhaps ten seconds (see also phrase (music)). A collection of two sub-phrases is a phrase. A whale will typically repeat the same phrase over and over for two to four minutes. This is known as a theme. A collection of themes is known as a song. The whale song will last up to 30 or so minutes, and will be repeated over and over again over the course of hours or even days. This "Russian doll" hierarchy of sounds suggests a syntactic structure that is more human-like in its complexity than other forms of animal communication like bird songs, which have only linear structure.

All the whales in an area sing virtually the same song at any point in time and the song is constantly and slowly evolving over time. For example, over the course of a month a particular unit that started as an upsweep (increasing in frequency) might slowly flatten to become a constant note. Another unit may get steadily louder. The pace of evolution of a whale's song also changes—some years the song may change quite rapidly, whereas in other years little variation may be recorded.

Idealized schematic of the song of a humpback whale.
Redrawn from Payne, et al. (1983)

 

Humpback whale, sound spectrum and time plots

Whales occupying the same geographical areas (which can be as large as entire ocean basins) tend to sing similar songs, with only slight variations. Whales from non-overlapping regions sing entirely different songs.

As the song evolves, it appears that old patterns are not revisited. An analysis of 19 years of whale songs found that while general patterns in song could be spotted, the same combination never recurred.

Humpback whales may also make stand-alone sounds that do not form part of a song, particularly during courtship rituals. Finally, humpbacks make a third class of sound called the feeding call. This is a long sound (5 to 10 s duration) of near constant frequency. Humpbacks generally feed cooperatively by gathering in groups, swimming underneath shoals of fish and all lunging up vertically through the fish and out of the water together. Prior to these lunges, whales make their feeding call. The exact purpose of the call is not known.

Some scientists have proposed that humpback whale songs may serve an echolocative purpose, but this has been subject to disagreement.

Other whale sounds

Humpback whales have also been found to make a range of other social sounds to communicate such as "grunts", "groans", "thwops", "snorts" and "barks".

In 2009, researchers found that blue whale song has been deepening in its tonal frequency since the 1960s. While noise pollution has increased ambient ocean noise by over 12 decibels since the mid-20th century, researcher Mark McDonald indicated that higher pitches would be expected if the whales were straining to be heard.

Killer whales have been observed to produce long range calls that are stereotyped and high frequency travelling distances from 10–16 km (6.2–9.9 mi) as well as short range calls that can travel distances from 5–9 km (3.1–5.6 mi). Short range calls are reported during social and resting periods while long range are more commonly reported during foraging and feeding.

Most other whales and dolphins produce sounds of varying degrees of complexity. Of particular interest is the Beluga (the "sea canary") which produces an immense variety of whistles, clicks and pulses.

Research in Whale Vocalization

It was previously thought that most baleen whales make sounds at about 15–20 hertz. However, a team of marine biologists, led by Mary Ann Daher of the Woods Hole Oceanographic Institution, reported in New Scientist in December 2004 that they had been tracking a whale in the North Pacific for 12 years that was "singing" at 52 Hz. Scientists have been unable to explain this phenomenon. 52 Hz is a very low sound, it is audible through human ears as a low moaning sound. It was not expected that this whale was a new species, more so this whale indicated that a currently known species potentially has a much wider vocal range than previously thought. There is disagreement in the scientific community regarding the uniqueness of the whale's vocalization and whether it is a member of a hybrid whale such as the well documented blue and fin whale hybrids.

Human interaction

Blue whales stop producing foraging D calls once a mid-frequency sonar is activated, even though the sonar frequency range (1–8 kHz) far exceeds their sound production range (25–100 Hz).

 

Voyager Golden Records carried whale songs into outer space with other sounds representing planet Earth.

Researchers use hydrophones (often adapted from their original military use in tracking submarines) to ascertain the exact location of the origin of whale noises. Their methods also allow them to detect how far through an ocean a sound travels. Research by Dr. Christopher Clark of Cornell University conducted using military data showed that whale noises travel for thousands of kilometres. As well as providing information about song production, the data allows researchers to follow the migratory path of whales throughout the "singing" (mating) season. An important finding is that whales, in a process called the Lombard effect, adjust their song to compensate for background noise pollution. Moreover, there is evidence that blue whales stop producing foraging D calls once a mid-frequency sonar is activated, even though the sonar frequency range (1–8 kHz) far exceeds their sound production range (25–100 Hz).

Prior to the introduction of human noise production, Clark says the noises may have travelled right from one side of an ocean to the other, agreeing with a thirty-year-old concept blaming large-scale shipping. His research indicates that ambient noise from boats is doubling with each decade. This has the effect of reducing the range at which whale noises can be heard. Environmentalists fear that such boat activity is putting undue stress on the animals as well as making it difficult to find a mate.

In the past decade, many effective automated methods, such as signal processing, data mining, and machine learning techniques have been developed to detect and classify whale vocalizations.

Media

See also: List of whale vocalizations

 

A humpback whale song

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Common humpback whale vocalizations on a windy day

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An orca's song

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An orca's song, at a distance

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Vocalizations of an orca

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A fin whale song

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A blue whale song

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Recorded in the Atlantic (1)

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A blue whale song

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Recorded in the Atlantic (2)

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A blue whale song

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Recorded in North Eastern Pacific

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A blue whale song

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Recorded in the South Pacific

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A blue whale song

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Recorded in the West Pacific

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An Atlantic minke whale

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Selected discography

• Songs of the Humpback Whale (SWR 118) was originally released in 1970 by CRM Records from recordings made by Roger Payne, Frank Watlington, and others. The LP was later re-released by Capitol Records, published in a flexible format in the National Geographic Society magazine, Volume 155, Number 1, in January 1979, re-released by Living Music/Windham Hill/BMG Records on CD in 1992, and remastered on CD by BGO-Beat Goes On in 2001.
• Deep Voices: The Second Whale Record (Capitol/EMI Records 0777 7 11598 1 0) was released on LP in 1977 from additional recordings made by Roger Payne, and re-released on CD in 1995 by Living Music/Windham Hill/BMG Records. It includes recordings of humpbacks, blues, and rights.
• Northern Whales (MGE 19) was released by Music Gallery Editions from recordings made by Pierre Ouellet, John Ford, and others affiliated with Interspecies Music and Communication Research. It includes recordings of belugas, narwhals, orca, and bearded seals.
• Sounds of the Earth: Humpback Whales (Oreade Music) was released on CD in 1999.
• Rapture of the Deep: Humpback Whale Singing (Compass Recordings) was released on CD in 2001.
• Songlines: Songs of the East Australian Humpback whales. was released in 2009.

History

Whaling Captain Wm. H. Kelly was the first person known to recognize whale singing for what it was, while on the brig Eliza in the Sea of Japan in 1881.

 

Hall-effect thruster

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Hall-effect_thruster 

 

6 kW Hall thruster in operation at the NASA Jet Propulsion Laboratory

In spacecraft propulsion, a Hall-effect thruster (HET) is a type of ion thruster in which the propellant is accelerated by an electric field. Hall-effect thrusters (based on the discovery by Edwin Hall) are sometimes referred to as Hall thrusters or Hall-current thrusters. Hall-effect thrusters use a magnetic field to limit the electrons' axial motion and then use them to ionize propellant, efficiently accelerate the ions to produce thrust, and neutralize the ions in the plume. The Hall-effect thruster is classed as a moderate specific impulse (1,600 s) space propulsion technology and has benefited from considerable theoretical and experimental research since the 1960s.

Hall thrusters operate on a variety of propellants, the most common being xenon and krypton. Other propellants of interest include argon, bismuth, iodine, magnesium, zinc and adamantane.

Hall thrusters are able to accelerate their exhaust to speeds between 10 and 80 km/s (1,000–8,000 s specific impulse), with most models operating between 15 and 30 km/s. The thrust produced depends on the power level. Devices operating at 1.35 kW produce about 83 mN of thrust. High-power models have demonstrated up to 5.4 N in the laboratory. Power levels up to 100 kW have been demonstrated for xenon Hall thrusters.

As of 2009, Hall-effect thrusters ranged in input power levels from 1.35 to 10 kilowatts and had exhaust velocities of 10–50 kilometers per second, with thrust of 40–600 millinewtons and efficiency in the range of 45–60 percent. The applications of Hall-effect thrusters include control of the orientation and position of orbiting satellites and use as a main propulsion engine for medium-size robotic space vehicles.

History

Hall thrusters were studied independently in the United States and the Soviet Union. They were first described publicly in the US in the early 1960s. However, the Hall thruster was first developed into an efficient propulsion device in the Soviet Union. In the US, scientists focused on developing gridded ion thrusters.

Two types of Hall thrusters were developed in the Soviet Union:

• thrusters with wide acceleration zone, SPT (Russian: СПД, стационарный плазменный двигатель; English: SPT, Stationary Plasma Thruster) at Design Bureau Fakel
• thrusters with narrow acceleration zone, DAS (Russian: ДАС, двигатель с анодным слоем; English: TAL, Thruster with Anode Layer), at the Central Research Institute for Machine Building (TsNIIMASH).

Soviet and Russian SPT thrusters

The SPT design was largely the work of A. I. Morozov. The first SPT to operate in space, an SPT-50 aboard a Soviet Meteor spacecraft, was launched December 1971. They were mainly used for satellite stabilization in north–south and in east–west directions. Since then until the late 1990s 118 SPT engines completed their mission and some 50 continued to be operated. Thrust of the first generation of SPT engines, SPT-50 and SPT-60 was 20 and 30 mN respectively. In 1982, SPT-70 and SPT-100 were introduced, their thrusts being 40 and 83 mN, respectively. In the post-Soviet Russia high-power (a few kilowatts) SPT-140, SPT-160, SPT-200, T-160 and low-power (less than 500 W) SPT-35 were introduced.

Soviet and Russian TAL-type thrusters include the D-38, D-55, D-80, and D-100.

Soviet-built thrusters were introduced to the West in 1992 after a team of electric propulsion specialists from NASA's Jet Propulsion Laboratory, Glenn Research Center, and the Air Force Research Laboratory, under the support of the Ballistic Missile Defense Organization, visited Russian laboratories and experimentally evaluated the SPT-100 (i.e., a 100 mm diameter SPT thruster). Over 200 Hall thrusters have been flown on Soviet/Russian satellites in the past thirty years. No failures have ever occurred on orbit. Hall thrusters continue to be used on Russian spacecraft and have also flown on European and American spacecraft. Space Systems/Loral, an American commercial satellite manufacturer, now flies Fakel SPT-100's on their GEO communications spacecraft.

Since their introduction to the West in the early 1990s, Hall thrusters have been the subject of a large number of research efforts throughout the United States, France, Italy, Japan, and Russia (with many smaller efforts scattered in various countries across the globe). Hall thruster research in the US is conducted at several government laboratories, universities and private companies. Government and government funded centers include NASA's Jet Propulsion Laboratory, NASA's Glenn Research Center, the Air Force Research Laboratory (Edwards AFB, CA), and The Aerospace Corporation. Universities include the US Air Force Institute of Technology, University of Michigan, Stanford University, The Massachusetts Institute of Technology, Princeton University, Michigan Technological University, and Georgia Tech. A considerable amount of development is being conducted in industry, such as IHI Corporation in Japan, Aerojet and Busek in the US, SNECMA in France, LAJP in Ukraine, SITAEL in Italy, and Satrec Initiative in South Korea.

The first use of Hall thrusters on lunar orbit was the European Space Agency (ESA) lunar mission SMART-1 in 2003.

Hall thrusters were first demonstrated on a western satellite on the Naval Research Laboratory (NRL) STEX spacecraft, which flew the Russian D-55. The first American Hall thruster to fly in space was the Busek BHT-200 on TacSat-2 technology demonstration spacecraft. The first flight of an American Hall thruster on an operational mission, was the Aerojet BPT-4000, which launched August 2010 on the military Advanced Extremely High Frequency GEO communications satellite. At 4.5 kW, the BPT-4000 is also the highest power Hall thruster ever flown in space. Besides the usual stationkeeping tasks, the BPT-4000 is also providing orbit raising capability to the spacecraft. The X-37B has been used as a testbed for the Hall thruster for the AEHF satellite series. Several countries worldwide continue efforts to qualify Hall thruster technology for commercial uses. The SpaceX Starlink constellation, the largest satellite constellation in the world, uses Hall thrusters. They are also included in the design of the Psyche spacecraft for asteroid exploration.

Principle of operation

The essential working principle of the Hall thruster is that it uses an electrostatic potential to accelerate ions up to high speeds. In a Hall thruster, the attractive negative charge is provided by an electron plasma at the open end of the thruster instead of a grid. A radial magnetic field of about 100–300 G (0.01–0.03 T) is used to confine the electrons, where the combination of the radial magnetic field and axial electric field cause the electrons to drift in azimuth thus forming the Hall current from which the device gets its name.

Hall thruster. Hall thrusters are largely axially symmetric. This is a cross-section containing that axis.

A schematic of a Hall thruster is shown in the adjacent image. An electric potential of between 150 and 800 volts is applied between the anode and cathode.

The central spike forms one pole of an electromagnet and is surrounded by an annular space, and around that is the other pole of the electromagnet, with a radial magnetic field in between.

The propellant, such as xenon gas, is fed through the anode, which has numerous small holes in it to act as a gas distributor. As the neutral xenon atoms diffuse into the channel of the thruster, they are ionized by collisions with circulating high-energy electrons (typically 10–40 eV, or about 10% of the discharge voltage). Most of the xenon atoms are ionized to a net charge of +1, but a noticeable fraction (~20%) have +2 net charge.

The xenon ions are then accelerated by the electric field between the anode and the cathode. For discharge voltages of 300 V, the ions reach speeds of around 15 km/s (9.3 mps) for a specific impulse of 1,500 seconds (15 kN·s/kg). Upon exiting, however, the ions pull an equal number of electrons with them, creating a plasma plume with no net charge.

The radial magnetic field is designed to be strong enough to substantially deflect the low-mass electrons, but not the high-mass ions, which have a much larger gyroradius and are hardly impeded. The majority of electrons are thus stuck orbiting in the region of high radial magnetic field near the thruster exit plane, trapped in E×B (axial electric field and radial magnetic field). This orbital rotation of the electrons is a circulating Hall current, and it is from this that the Hall thruster gets its name. Collisions with other particles and walls, as well as plasma instabilities, allow some of the electrons to be freed from the magnetic field, and they drift towards the anode.

About 20–30% of the discharge current is an electron current, which does not produce thrust, thus limiting the energetic efficiency of the thruster; the other 70–80% of the current is in the ions. Because the majority of electrons are trapped in the Hall current, they have a long residence time inside the thruster and are able to ionize almost all of the xenon propellant, allowing mass use of 90–99%. The mass use efficiency of the thruster is thus around 90%, while the discharge current efficiency is around 70%, for a combined thruster efficiency of around 63% (= 90% × 70%). Modern Hall thrusters have achieved efficiencies as high as 75% through advanced designs.

Compared to chemical rockets, the thrust is very small, on the order of 83 mN for a typical thruster operating at 300 V and 1.5 kW. For comparison, the weight of a coin like the U.S. quarter or a 20-cent Euro coin is approximately 60 mN. As with all forms of electrically powered spacecraft propulsion, thrust is limited by available power, efficiency, and specific impulse.

However, Hall thrusters operate at the high specific impulses that are typical for electric propulsion. One particular advantage of Hall thrusters, as compared to a gridded ion thruster, is that the generation and acceleration of the ions takes place in a quasi-neutral plasma, so there is no Child-Langmuir charge (space charge) saturated current limitation on the thrust density. This allows much smaller thrusters compared to gridded ion thrusters.

Another advantage is that these thrusters can use a wider variety of propellants supplied to the anode, even oxygen, although something easily ionized is needed at the cathode.

Propellants

Xenon

Xenon has been the typical choice of propellant for many electric propulsion systems, including Hall thrusters. Xenon propellant is used because of its high atomic weight and low ionization potential. Xenon is relatively easy to store, and as a gas at spacecraft operating temperatures does not need to be vaporized before usage, unlike metallic propellants such as bismuth. Xenon's high atomic weight means that the ratio of energy expended for ionization per mass unit is low, leading to a more efficient thruster.

Krypton

Krypton is another choice of propellant for Hall thrusters. Xenon has an ionization potential of 12.1298 eV, while krypton has an ionization potential of 13.996 eV. This means that thrusters utilizing krypton need to expend a slightly higher energy per mole to ionize, which reduces efficiency. Additionally, krypton is a lighter ion, so the unit mass per ionization energy is further reduced compared to xenon. However, xenon can be more than ten times as expensive as krypton per kilogram, making krypton a more economical choice for building out satellite constellations like that of SpaceX's Starlink, whose Hall thrusters are fueled with krypton.

Variants

Cylindrical Hall thrusters

An Exotrail ExoMG - nano (60W) Hall Effect Thruster firing in a vacuum chamber

Although conventional (annular) Hall thrusters are efficient in the kilowatt power regime, they become inefficient when scaled to small sizes. This is due to the difficulties associated with holding the performance scaling parameters constant while decreasing the channel size and increasing the applied magnetic field strength. This led to the design of the cylindrical Hall thruster. The cylindrical Hall thruster can be more readily scaled to smaller sizes due to its nonconventional discharge-chamber geometry and associated magnetic field profile. The cylindrical Hall thruster more readily lends itself to miniaturization and low-power operation than a conventional (annular) Hall thruster. The primary reason for cylindrical Hall thrusters is that it is difficult to achieve a regular Hall thruster that operates over a broad envelope from ~1 kW down to ~100 W while maintaining an efficiency of 45-55%.

External discharge Hall thruster

Sputtering erosion of discharge channel walls and pole pieces that protect the magnetic circuit causes failure of thruster operation. Therefore, annular and cylindrical Hall thrusters have limited lifetime. Although magnetic shielding has been shown to dramatically reduce discharge channel wall erosion, pole piece erosion is still a concern. As an alternative, an unconventional Hall thruster design called external discharge Hall thruster or external discharge plasma thruster (XPT) has been introduced. The external discharge Hall thruster does not possess any discharge channel walls or pole pieces. Plasma discharge is produced and sustained completely in the open space outside the thruster structure, and thus erosion-free operation is achieved.

Applications

An illustration of the Gateway in orbit around the Moon. The orbit of the Gateway will be maintained with Hall thrusters.

Hall thrusters have been flying in space since December 1971, when the Soviet Union launched an SPT-50 on a Meteor satellite. Over 240 thrusters have flown in space since that time, with a 100% success rate. Hall thrusters are now routinely flown on commercial LEO and GEO communications satellites, where they are used for orbital insertion and stationkeeping.

The first Hall thruster to fly on a western satellite was a Russian D-55 built by TsNIIMASH, on the NRO's STEX spacecraft, launched on October 3, 1998.

The solar electric propulsion system of the European Space Agency's SMART-1 spacecraft used a Snecma PPS-1350-G Hall thruster. SMART-1 was a technology demonstration mission that orbited the Moon. This use of the PPS-1350-G, starting on September 28, 2003, was the first use of a Hall thruster outside geosynchronous earth orbit (GEO). Like most Hall thruster propulsion systems used in commercial applications, the Hall thruster on SMART-1 could be throttled over a range of power, specific impulse, and thrust. It has a discharge power range of 0.46–1.19 kW, a specific impulse of 1,100–1,600 s and thrust of 30–70 mN.

Many small satellites of the SpaceX Starlink cluster use krypton-fueled Hall thrusters for position-keeping and deorbiting.

Tiangong space station is fitted with Hall-effect thrusters. Tianhe core module is propelled by both chemical thrusters and four ion thrusters, which are used to adjust and maintain the station's orbit. The development of the Hall-effect thrusters is considered a sensitive topic in China, with scientists "working to improve the technology without attracting attention". Hall-effect thrusters are created with manned mission safety in mind with effort to prevent erosion and damage caused by the accelerated ion particles. A magnetic field and specially designed ceramic shield was created to repel damaging particles and maintain integrity of the thrusters. According to the Chinese Academy of Sciences, the ion drive used on Tiangong has burned continuously for 8,240 hours without a glitch, indicating their suitability for the Chinese space station’s designated 15-year lifespan. This is the world's first Hall thrusters on a human-rated mission.

The Jet Propulsion Laboratory (JPL) granted exclusive commercial licensing to Apollo Fusion- led by Mike Cassidy, for its Magnetically Shielded Miniature, or MaSMi Hall thruster technology. In January 2021, Apollo Fusion announced they had secured a contract with York Space Systems for an order of its latest iteration named the "Apollo Constellation Engine".

In late 2022, the NASA mission to the asteroid Psyche will utilize xenon gas Hall Thrusters. The electricity will come from the craft's 75 square meter solar panels.

NASA's first Hall thrusters on a human-rated mission will be a combination of 6kW Hall thrusters provided by Busek and NASA Advanced Electric Propulsion System (AEPS) Hall thrusters. They will serve as the primary propulsion on Maxar's Power and Propulsion Element (PPE) for the Lunar Gateway under NASA's Artemis program. The high specific impulse of Hall thrusters will allow for efficient orbit raising and station keep for the Lunar Gateway's polar near-rectilinear halo orbit.

In development

The highest power Hall-effect thruster in development is the University of Michigan's 100 kW X3 Nested Channel Hall Thruster. The thruster is approximately 80 cm in diameter and weighs 230 kg, and has demonstrated a thrust of 5.4 N. 

Other high power thrusters include NASA's 40 kW Advanced Electric Propulsion System (AEPS), meant to propel large-scale science missions and cargo transportation in deep space.

LGBT sex education

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/LGBT_sex_education 

 

Poster carried in a gay pride march in Portland, Maine

LGBT sex education is a sex education program within a school, university, or community center that addresses prominent sexual health topics among LGBT groups. Within schools, topics on LGBT sexual health are usually integrated into the general sex education courses.

There is some debate about whether LGBT sex education should be included in sex education curricula. Advocates of LGBT sex education say that the inclusion of LGBT issues into sex education programs would reduce homophobic bullying, improve the health of LGBT people, and decrease instances of problems common in LGBT students such as depression and low self-esteem; opponents argue that LGBT sex education programs would force a political point of view on students, misuse tax money, and disrespect religious values. As of 2014, only 5% of middle and high school students in the United States reported receiving "positive discussions of LGBT-related topics" within their health classes.

Background

LGBT sex education is currently not covered in many schools. Research has also posited that students often do not find existing LGBT sex education programs to be effective. Teachers have differing views on the subject of homosexuality, and these personal opinions can impact LGBT sex education when it is implemented.

Research

Multiple studies have concluded that LGBT sex education is often not encompassed in school sex education courses and that most students do not receive effective instruction in LGBT sex issues. In a study conducted by Ellis and High in the UK (2004), 384 students were surveyed; they found that 24% had not received instruction in LGBT sex issues. The CDC Division of Adolescent and School Health's study revealed that 48% of schools in the US covered LGBT topics. According to research reported by Burston and Hart in 2001, 45% of surveyed students believed that they did not cover LGBT sex education sufficiently in school. Research has also shown that there can be an implicit assumption that all students are heterosexual in sex education classes. The LGBT students in Eleanor Formby's 2011 study of sex education said that they do not always feel welcomed by sex education classes or at school. Sex education courses commonly idealize marriage (not acknowledging that many countries outlaw same-sex marriage) thereby presenting a heterosexual view of sex and relationships as normal. Studies have suggested that sex education programs often do not cover safe sex practices for LGBT individuals.

However, there are some sex education curricula that do cover LGBT issues. For example, the Unitarian Universalist Association of Congregations provides a sex education program called Our Whole Lives, which includes discussion of sexual orientation and presents homosexuality and heterosexuality as equally valid. Our Whole Lives offers programs designed for a range of developmental stages, from Kindergarten-level through adulthood, and follows the "Guidelines for Comprehensive Sexuality Education" that the Sexuality Information and Education Council of the United States (SIECUS) endorses. The United Church of Christ also supports Our Whole Lives.

Issues with sex education programs

Research has illustrated that some sex education courses present LGBT issues in a negative light—portraying LGBT individuals and LGBT sexuality as something wrong, sick, and abnormal. According to the American Civil Liberties Union, "abstinence-only" approaches to sex education can also be alienating to LGBT students because these programs assume that marriage is a possibility and a desire for all students; however, same-sex marriage is illegal in many countries. Further, promoting marriage as the goal for LGBT students reproduces a homonormative standard, marginalizing those without access to or interest in marriage. Ellis and High's survey research in 2004 (including 384 students) revealed that 59% of young people who did receive LGBT sex education found it to be ineffective.

Teachers

Teachers have been identified as a hindrance to LGBT sex education in some studies. Teachers always have their own opinions about homosexuality, and, according to these studies, if teachers have negative views toward LGBT individuals this can come through in their teaching—causing LGBT students to feel unaccepted and unsafe. According to Ellis and High (2004), when LGBT students receive information about LGBT sexuality with negative undertones they are left feeling significantly worse and more unsafe than if homosexuality were left out of the curriculum. Researchers have documented multiple self-proclaimed "LGBT-friendly" teachers whose classrooms actually foster prejudiced lessons. Such teachers are also highly likely to ignore instances of homophobic bullying directed at LGBT youth within their classes. Burston and Hart (2001) reported that teachers sometimes believe that they should not take a side on the issue of homosexuality, and therefore should not interfere when homophobia occurs in the classroom. According to Formby (2011), even phrasing that subtly casts homosexuality in a negative light can have a detrimental effect on LGBT students' experience of sex education.

There have also been issues around teachers feeling free to teach sex education that equally emphasizes both heterosexual and homosexual health information. Deana Morrow's study (1993) reported that some teachers said they were afraid they might be fired if they discussed LGBT issues. This fear is linked to the historical misconnection of homosexuality to molestation in the United States; this supposedly natural linkage has been debunked multiple times. Regardless, straight and LGBT teachers alike still experience allegations of molestation when they engage in discussions surrounding sexuality, particularly those discussions that are LGBT related. Teachers can also feel hindered because the school environment is inhospitable to homosexuality; in Burston and Hart's 2001 study, some even said that they were under the impression that the school would not allow them to teach LGBT sex education.

Classmates can also be non-receptive toward LGBT issues in current sex education courses, and students are often hesitant to talk about homosexuality, according to Buston and Hart (2001).

At home

Since sex education has been present in health education in schools, many parents expect their children to learn about sex there. Studies show that most families do not engage in conversation about sex in the home, and when they do it is often from a heteronormative perspective. The assumptions of being heterosexual can make LGBT people feel ashamed or lacking support from their family. Lack of conversation and knowledge received in the home for LGBT people can often lead them to receive their information for outside sources that contain false or misleading information. The same study showed that many parents don't have a solid knowledge base on same-sex or LGBT topics, nor do they know of resources to direct their children towards.

Proposed LGBT sex education programs

Advocates for LGBT sex education have suggested adjustments to current sex education practices in schools. One common place for improvement that researchers have identified is the angle from which sex education is approached in general. Buston and Hart (2001), Ellis and High (2004), and others have recommended that teachers frame sex education in terms of relationships rather than merely reproduction, which can lead to the exclusion of LGBT students. Ellis and High mention that sexual orientation might be more appropriately taught as "an aspect of culture and identity" (Ellis and High 2004, pg. 11). Other researchers such as Morrow (1993) believe that in order for sex education to be effective, it must present LGB as just as natural and legitimate as heterosexuality. Advocates for LGBT sex education ask that LGBT sexual health issues be given equal weight in the curriculum accordingly. They also say that more resources concerning LGBT sexual health issues need to be made available to students. According to UCLA's Center for the Study of Women's Policy Brief 11 (2012), LBGT students may not be willing to reach out for guidance themselves.

Researchers have recommended that teachers in sex education programs avoid framing homosexuality as something that is fundamentally connected to sexually transmitted diseases and refrain from practices that are potentially detrimental to LGBT students, such as referring to partners as specifically "him" or "her" (better to use the more flexible "they"). Gowen and Winges-Yanez (2014) suggest through their focus groups on LGBT teens that there are several problems with the way sex education is taught. The teens cited silencing, heterocentricity, and pathologizing of LGBT individuals as common practices. When asked how they would improve sex education, the group said inclusive sex education would include discussion of LGBT issues, learning how to access resources, STI or STD prevention, relationships, and anatomy. Advocates for LGBT rights also say that teachers need to abandon any reluctance to take a side in the debate about homosexuality.

There are also alternative sexual education programs for LGBT people, such as that of an online sexual education course. According to a study evaluating the effectiveness of an online, interactive sexual education program for LGBT people, all subsections recorded statistically significant improvement of knowledge. Some of the topics included safe sex practices, healthy relationships, pleasure and sexually transmitted infections. This type of program also created an online community for people taking the course to ask questions and interact with each other. This social aspect of the program also created a sense of normalcy and acceptance. Online programs could offer a means of education for those who cannot receive it in school. There are also various online LGBT sites on the internet that offer educational leaflets or information.

Novels including LGBT relationships can be a useful tool in an LGBT inclusive sexual education course or as a way for youth and teens to learn about LGBT relationships and issue in a different type of way. Novels that include LGBT relationships can aid in normalizing queer relationships potentially creating a more accepting and inclusive atmosphere for LGBT youth. It can also supplement information learned by reinforcing it in the form of a story. Many LGBT youth use young adult novels that include LGBT relationships as sources of information, especially if they don't receive sex education in school. Sexually explicit young adult novels can provide details about sexual intercourse, intimacy and sexual identity that LGBT youth can relate to allowing them to explore their own sexual identity.

A study of LGBT youth asked them what their current curriculum is teaching them and what they would want to see in a new curriculum. Some of the responses included a more inclusive curriculum that described different people with different gender identities, sexual orientations and ethnicity, "how-to" information that related to LGBT people relationships, and specific sections related to LGBT risks, problems and behaviors. They also mentioned the use of internet information and resources as a way of creating a community for extended education and support.

Support for LGBT sex education

Proponents of incorporating LGBT sex education into school curricula commonly present several arguments. According to the Huffington Post, some supporters claim that failing to include LGBT issues in sex education programs will overlook a significant number of students who identify as LGBT; the Center for American Progress (CAP) says that this can cause them to feel marginalized and removed from the lesson because it doesn't pertain to them. LGBT sex education advocates also argue that leaving LGBT safe sex instruction out of the curriculum will increase the likelihood of health problems for LGBT students. Supporters say that since LGBT people are particularly at risk for HIV/AIDS, it is especially important to provide them with sexual health information. According to researcher Eleanor Formby (2011), lesbian women are a high-risk group for sexually transmitted diseases (STDs), because many do not know that they can be susceptible to STDs or how to engage in safe sex. Therefore, it is important that they receive lesbian sex education. LGBT sex education advocates suggest that because LGBT students aren't taught sex education that pertains to them in school, they feel unprepared for sex, unable to talk about it openly, and have to learn about it by themselves—which can result in negative health outcomes. Sanchez (2012) argues that LGBT students are unlikely to reach out to resources that could give them good information on their own, which furthers the need for LGBT sex education in schools. LGBT youth are also at higher risk of engaging in high-risk behavior such as higher rates of suicide attempts, substance use and high risk sexual behavior. Since many of these high risk actions among LGBT youth have been correlated with depression, emotional distress and victimization experiences from non- LGBT people, LGBT sensitive sex and HIV education in schools could reduce this high-risk behavior by normalizing LGBT people and also providing support services to LGBT youth.

Parade attendees wave rainbow flags at the 2012 Washington, D.C. Capital Pride parade

LGBT sex education supporters have also argued that the inclusion of LGBT topics in the curriculum can decrease instances of bullying in schools by familiarizing students with the range of sexual orientations and reducing harmful stereotypes. The Center for American Progress argues that LGBT sex education results in a decrease in homophobic comments. According to the Huffington Post, supporters say that educating young people about LGBT individuals could help them have a more positive attitude toward their gay peers. The Center for American Progress (2013) says that LGBT sex education would therefore reduce common problems LGBT students face as a result of negative attitudes; these include mental health issues like depression, the risk of suicide, drug abuse, self-esteem issues, and poorer academic performance due to stress caused by discrimination. They argue that covering homosexuality in sex education programs helps students feel more secure at school.

Several studies have also shown that heteronormative and negative attitudes toward LGBT people are associated with lower rates of academic success. In schools, heteronormative and non inclusive culture can poorly effect motivation, health, and learning habits in students who identify as LGBTQ+. Andreas Gegenfurtner and Markus Gebhardt have shared findings which suggest that tolerance and acceptance toward sexual minorities were reported to be more positive when people are more highly educated and less religious. Similar findings within their study have shown a positive correlation between academic success among LGBTQ+ students and inclusive school environments.

According to Jen Gilbert, associate professor of education at York University, LGBT kids often do not have queer parents who they can ask for sexuality-related advice, nor access to LGBT adults. LGBT sex education could potentially fill this gap and provide LGBT students with elders well-versed in their specific needs and equipped with affirming information, that students are otherwise unable to receive at home or in school.

Finally, proponents of LGBT sex education have said that curricula that explore all facets of sexuality would be beneficial to straight students as well, because they claim that it presents a more accurate picture of the world and human sexuality. A study of Gay/Straight Alliances in Utah found that peer-facilitated discussions concerning the spectrum of sexuality and gender identities benefited both straight and LGBT students. It exposed them to the reality of relationships outside of the heteronormative images that dominate media (as well as sex education), and even positively impacted all involved students' academic performance. Proponents also argue that offering LGBT-inclusive sex education can be of major assistance to any questioning students that might be in the class.

According to the Center for American Progress (2013), the majority of parents support including homosexuality in the sex education curriculum; they report that 73% of high school parents think LGBT issues should be taught. The CAP claims that this high percentage of support indicates that LGBT topics should be incorporated.

Support within the United States

Eight states have a sexual health curriculum that affirms LGBTQ+ people. These states are California, Oregon, Nevada, Colorado, Illinois, New Jersey, Hawaii, and Connecticut.

California Healthy Youth Act

In 2016, the California Healthy Youth Act stated that all sexual health education in California districts must be inclusive of LGBTQ students. This requires schools to discuss examples of same-sex relationships and teach about different gender identities. School districts must protect LGBTQ students from harassment and prevent discrimination against them. Parents are not allowed to "opt-out" specifically from the LGBTQ-related content because this would be considered discrimination.

Opposition to LGBT sex education

Opponents of LGBT sex education argue that it is wrong to teach students about the issue of homosexuality because it is too contentious. They say that parents should have control over what their children are exposed to and taught, and allowing public schools to cover LGBT sex education would undermine this right, forcing a particular political view on students. Furthermore, many opponents of inclusive sex ed programs argue that parents are forced to lose control of what their children learn in school. This belief is especially common in households that are religiously affiliated, or identify politically with views against LGBT rights. According to The Christian Post, some parents do not want their children to study homosexuality. Critics often cite a misuse of citizens' tax dollars, claiming citizens should not have to pay for children to learn about other lifestyles that their parents do not agree with. Parents and guardians within these families commonly argue that lesbian, gay, bisexual, or transgender activity is immoral, abnormal, and unnatural.

According to Formby (2011), opponents have also argued that LGBT sex education is harmful to students because they say it exposes them to damaging information. They claim that the students should not learn about LGBT issues until they are older. Some opponents of LGBT sex education have argued that including LGBT issues in sex education programs will encourage more young people to practice homosexuality as well. LGBT sex education has also been accused of being disrespectful to certain families’ religious beliefs. The Christian Post argued that if schools elect to teach about LGBT people while neglecting religious topics, the curriculum would be unfairly balanced.

There have also been concerns that LGBT sex education wouldn't be effective because it is difficult for homophobic students to accept homosexuality, which may prevent them from being receptive to the instruction.

Opposition within the United States

There are laws prohibiting the "promotion of sexuality" (referred to as "No Promo Homo" laws"). Four states (Louisiana, Mississippi, Oklahoma, and Texas) mandate pointedly negative messages regarding all LGBT identities, when sex education is provided. Eight states (the four previously mentioned, Alabama, Arizona, South Carolina, and Utah) prohibit discussion of any topics deemed LGBT-related. According to the Guttmacher Institute's findings in 2017, "If HIV education is taught in Arizona it cannot 'promote' a 'homosexual lifestyle' or portray homosexuality in a positive manner. Mandated HIV education in Oklahoma teaches that among other behaviors that 'homosexual activity' is considered to be 'responsible for contact with the AIDS virus'." Utah, Alabama and Arizona used to have "No Homo Promo" laws but they were since repealed.

Laws and legal battles

Section 28

Main article: Section 28

Section 28 was a controversial law in the United Kingdom that barred schools from presenting homosexuality as a viable sexual orientation or basis for relationships (though the law was never used in court). It was enacted in 1988 and repealed throughout the UK by 2003. Critics of Section 28 say that the law prevented teachers from intervening in instances of homophobic bullying and greatly hindered the development of gay rights in Great Britain. According to Moran (2001), proponents of the law argued that it protected students from being harmed by gay propaganda. Recently, LGBT advocates have raised concerns that policies similar to Section 28 are appearing again in British schools. Wales has sought to challenge the implications of this section by implementing a new education curriculum Relationships and Sexuality Education (RSE) by 2022. The goals of this new curriculum will be to broaden traditional sex education and include information relating to relationships and a greater understanding of sexuality. It will also include LGBTQI topics such as gender identity and bring up issues of consent and sexual violence. The new curriculum will be required in primary and secondary schools with each containing different curriculum focuses, but it will not be required by religious schools.

Croatian textbook

In 2009, the European Committee of Social Rights found several statements in a Croatian mandatory Biology course textbook, including: "Many individuals are prone to sexual relations with persons of the same sex.... It is believed that parents are to blame because they impede their children's correct sexual development with their irregularities in family relations. Nowadays it has become evident that homosexual relations are the main culprit for increased spreading of sexually transmitted diseases (e.g. AIDS)," and "The disease [AIDS] has spread amongst promiscuous groups of people who often change their sexual partners. Such people are homosexuals because of sexual contacts with numerous partners, drug addicts...and prostitutes." The European Committee of Social Rights deemed these statements discriminatory and in violation of Croatia's obligations under the European Social Charter.

Among minority groups

CDC findings

A 2018 CDC study has maintained that Latino and black youth and young adult men who have sex with men often face stigma, discrimination, and language barriers that hinder their ability to access STD education, prevention, and treatment. As a result, they are vulnerable to high rates of HIV and other health disparities. In 2017, African Americans accounted for 43% of all new HIV diagnoses. Additionally, Hispanic/Latinos are also strongly affected. They accounted for 26% of all new HIV diagnoses. In 2017, gay and bisexual men accounted for 66% of all HIV diagnoses in the United States and 6 dependent areas.

Structural barriers

One case study has demonstrated that homophobia, racism, coupled with financial hardship and social support were associated with higher exposure to HIV among homosexual men of color. In the United States, Latino men who have sex with men (MSM) are disproportionately affected by HIV. Another study demonstrated that in a multivariable analysis, increasing age, low income, and queer identity. Additionally, people Living With HIV, MSM and transgender women are considered the "most in need" due to the stigma that prevents them from accessing high-quality health care, prevention, and sex education. According to Mattew E. Levy of The George Washington University, many systematic factors have led to the disproportionate rates of HIV among Black and Latino Men who have sex with men, including insufficient healthcare, social stigma and discrimination, incarceration, and poverty. Men of color who have sex with men experience inadequate access to culturally competent services, stigma and discrimination that impede access to services, a deficiency of services in correctional institutions, and limited services in areas where they live.