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Tuesday, October 3, 2023

Asteroid

From Wikipedia, the free encyclopedia

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

Images of visited asteroids illustrating their difference: 243 Ida with its moon Dactyl (the 1–2 km sized dot to the right); 433 Eros, the first asteroid orbited and landed on (2001); and Ceres, a considerably larger asteroid and dwarf planet 1,000 km across

An asteroid is a minor planet—an object that is neither a true planet nor a comet—that orbits within the inner Solar System. They are rocky, metallic or icy bodies with no atmosphere. Sizes and shapes of asteroids vary significantly, ranging from 1-meter rocks to a dwarf planet almost 1000 km in diameter.

Of the roughly one million known asteroids the greatest number are located between the orbits of Mars and Jupiter, approximately 2 to 4 AU from the Sun, in the main asteroid belt. Asteroids are generally classified to be of three types: C-type, M-type, and S-type. These were named after and are generally identified with carbonaceous, metallic, and silicaceous compositions, respectively. The size of asteroids varies greatly; the largest, Ceres, is almost 1,000 km (600 mi) across and qualifies as a dwarf planet. The total mass of all the asteroids combined is only 3% that of Earth's Moon. The majority of main belt asteroids follow slightly elliptical, stable orbits, revolving in the same direction as the Earth and taking from three to six years to complete a full circuit of the Sun.

Asteroids have been historically observed from Earth; the Galileo spacecraft provided the first close observation of an asteroid. Several dedicated missions to asteroids were subsequently launched by NASA and JAXA, with plans for other missions in progress. NASA's NEAR Shoemaker studied Eros, and Dawn observed Vesta and Ceres. JAXA's missions Hayabusa and Hayabusa2 studied and returned samples of Itokawa and Ryugu, respectively. OSIRIS-REx studied Bennu, collecting a sample in 2020 which was delivered back to Earth in 2023. NASA's Lucy, launched in 2021, will study ten different asteroids, two from the main belt and eight Jupiter trojans. Psyche, scheduled for launch in 2023, will study a metallic asteroid of the same name.

Near-Earth asteroids can threaten all life on the planet; an asteroid impact event resulted in the Cretaceous–Paleogene extinction. Different asteroid deflection strategies have been proposed; the Double Asteroid Redirection Test spacecraft, or DART, was launched in 2021 and intentionally impacted Dimorphos in September 2022, successfully altering its orbit by crashing into it.

History of observations

Only one asteroid, 4 Vesta, which has a relatively reflective surface, is normally visible to the naked eye. When favorably positioned, 4 Vesta can be seen in dark skies. Rarely, small asteroids passing close to Earth may be visible to the naked eye for a short amount of time. As of April 2022, the Minor Planet Center had data on 1,199,224 minor planets in the inner and outer Solar System, of which about 614,690 had enough information to be given numbered designations.

Discovery of Ceres

In 1772, German astronomer Johann Elert Bode, citing Johann Daniel Titius, published a numerical procession known as the Titius–Bode law (now discredited). Except for an unexplained gap between Mars and Jupiter, Bode's formula seemed to predict the orbits of the known planets. He wrote the following explanation for the existence of a "missing planet":

This latter point seems in particular to follow from the astonishing relation which the known six planets observe in their distances from the Sun. Let the distance from the Sun to Saturn be taken as 100, then Mercury is separated by 4 such parts from the Sun. Venus is 4 + 3 = 7. The Earth 4 + 6 = 10. Mars 4 + 12 = 16. Now comes a gap in this so orderly progression. After Mars there follows a space of 4 + 24 = 28 parts, in which no planet has yet been seen. Can one believe that the Founder of the universe had left this space empty? Certainly not. From here we come to the distance of Jupiter by 4 + 48 = 52 parts, and finally to that of Saturn by 4 + 96 = 100 parts.

Bode's formula predicted another planet would be found with an orbital radius near 2.8 astronomical units (AU), or 420 million km, from the Sun. The Titius–Bode law got a boost with William Herschel's discovery of Uranus near the predicted distance for a planet beyond Saturn. In 1800, a group headed by Franz Xaver von Zach, editor of the German astronomical journal Monatliche Correspondenz (Monthly Correspondence), sent requests to 24 experienced astronomers (whom he dubbed the "celestial police"), asking that they combine their efforts and begin a methodical search for the expected planet. Although they did not discover Ceres, they later found the asteroids 2 Pallas, 3 Juno and 4 Vesta.

One of the astronomers selected for the search was Giuseppe Piazzi, a Catholic priest at the Academy of Palermo, Sicily. Before receiving his invitation to join the group, Piazzi discovered Ceres on 1 January 1801. He was searching for "the 87th [star] of the Catalogue of the Zodiacal stars of Mr la Caille", but found that "it was preceded by another". Instead of a star, Piazzi had found a moving star-like object, which he first thought was a comet:

The light was a little faint, and of the colour of Jupiter, but similar to many others which generally are reckoned of the eighth magnitude. Therefore I had no doubt of its being any other than a fixed star. [...] The evening of the third, my suspicion was converted into certainty, being assured it was not a fixed star. Nevertheless before I made it known, I waited till the evening of the fourth, when I had the satisfaction to see it had moved at the same rate as on the preceding days.

Piazzi observed Ceres a total of 24 times, the final time on 11 February 1801, when illness interrupted his work. He announced his discovery on 24 January 1801 in letters to only two fellow astronomers, his compatriot Barnaba Oriani of Milan and Bode in Berlin. He reported it as a comet but "since its movement is so slow and rather uniform, it has occurred to me several times that it might be something better than a comet". In April, Piazzi sent his complete observations to Oriani, Bode, and French astronomer Jérôme Lalande. The information was published in the September 1801 issue of the Monatliche Correspondenz.

By this time, the apparent position of Ceres had changed (mostly due to Earth's motion around the Sun), and was too close to the Sun's glare for other astronomers to confirm Piazzi's observations. Toward the end of the year, Ceres should have been visible again, but after such a long time it was difficult to predict its exact position. To recover Ceres, mathematician Carl Friedrich Gauss, then 24 years old, developed an efficient method of orbit determination. In a few weeks, he predicted the path of Ceres and sent his results to von Zach. On 31 December 1801, von Zach and fellow celestial policeman Heinrich W. M. Olbers found Ceres near the predicted position and thus recovered it. At 2.8 AU from the Sun, Ceres appeared to fit the Titius–Bode law almost perfectly; however, Neptune, once discovered in 1846, was 8 AU closer than predicted, leading most astronomers to conclude that the law was a coincidence. Piazzi named the newly discovered object Ceres Ferdinandea, "in honor of the patron goddess of Sicily and of King Ferdinand of Bourbon".

Further search

Three other asteroids (2 Pallas, 3 Juno, and 4 Vesta) were discovered by von Zach's group over the next few years, with Vesta found in 1807. No new asteroids were discovered until 1845. Amateur astronomer Karl Ludwig Hencke started his searches of new asteroids in 1830, and fifteen years later, while looking for Vesta, he found the asteroid later named 5 Astraea. It was the first new asteroid discovery in 38 years. Carl Friedrich Gauss was given the honor of naming the asteroid. After this, other astronomers joined; 15 asteroids were found by the end of 1851. In 1868, when James Craig Watson discovered the 100th asteroid, the French Academy of Sciences engraved the faces of Karl Theodor Robert Luther, John Russell Hind, and Hermann Goldschmidt, the three most successful asteroid-hunters at that time, on a commemorative medallion marking the event.

In 1891, Max Wolf pioneered the use of astrophotography to detect asteroids, which appeared as short streaks on long-exposure photographic plates. This dramatically increased the rate of detection compared with earlier visual methods: Wolf alone discovered 248 asteroids, beginning with 323 Brucia, whereas only slightly more than 300 had been discovered up to that point. It was known that there were many more, but most astronomers did not bother with them, some calling them "vermin of the skies", a phrase variously attributed to Eduard Suess and Edmund Weiss. Even a century later, only a few thousand asteroids were identified, numbered and named.

19th and 20th centuries

In the past, asteroids were discovered by a four-step process. First, a region of the sky was photographed by a wide-field telescope, or astrograph. Pairs of photographs were taken, typically one hour apart. Multiple pairs could be taken over a series of days. Second, the two films or plates of the same region were viewed under a stereoscope. A body in orbit around the Sun would move slightly between the pair of films. Under the stereoscope, the image of the body would seem to float slightly above the background of stars. Third, once a moving body was identified, its location would be measured precisely using a digitizing microscope. The location would be measured relative to known star locations.

These first three steps do not constitute asteroid discovery: the observer has only found an apparition, which gets a provisional designation, made up of the year of discovery, a letter representing the half-month of discovery, and finally a letter and a number indicating the discovery's sequential number (example: 1998 FJ₇₄). The last step is sending the locations and time of observations to the Minor Planet Center, where computer programs determine whether an apparition ties together earlier apparitions into a single orbit. If so, the object receives a catalogue number and the observer of the first apparition with a calculated orbit is declared the discoverer, and granted the honor of naming the object subject to the approval of the International Astronomical Union.

Naming

2013 EC, shown here in radar images, has a provisional designation

By 1851, the Royal Astronomical Society decided that asteroids were being discovered at such a rapid rate that a different system was needed to categorize or name asteroids. In 1852, when de Gasparis discovered the twentieth asteroid, Benjamin Valz gave it a name and a number designating its rank among asteroid discoveries, 20 Massalia. Sometimes asteroids were discovered and not seen again. So, starting in 1892, new asteroids were listed by the year and a capital letter indicating the order in which the asteroid's orbit was calculated and registered within that specific year. For example, the first two asteroids discovered in 1892 were labeled 1892A and 1892B. However, there were not enough letters in the alphabet for all of the asteroids discovered in 1893, so 1893Z was followed by 1893AA. A number of variations of these methods were tried, including designations that included year plus a Greek letter in 1914. A simple chronological numbering system was established in 1925.

Currently all newly discovered asteroids receive a provisional designation (such as 2002 AT4) consisting of the year of discovery and an alphanumeric code indicating the half-month of discovery and the sequence within that half-month. Once an asteroid's orbit has been confirmed, it is given a number, and later may also be given a name (e.g. 433 Eros). The formal naming convention uses parentheses around the number—e.g. (433) Eros—but dropping the parentheses is quite common. Informally, it is also common to drop the number altogether, or to drop it after the first mention when a name is repeated in running text. In addition, names can be proposed by the asteroid's discoverer, within guidelines established by the International Astronomical Union.

Symbols

The first asteroids to be discovered were assigned iconic symbols like the ones traditionally used to designate the planets. By 1855 there were two dozen asteroid symbols, which often occurred in multiple variants.

In 1851, after the fifteenth asteroid, Eunomia, had been discovered, Johann Franz Encke made a major change in the upcoming 1854 edition of the Berliner Astronomisches Jahrbuch (BAJ, Berlin Astronomical Yearbook). He introduced a disk (circle), a traditional symbol for a star, as the generic symbol for an asteroid. The circle was then numbered in order of discovery to indicate a specific asteroid. The numbered-circle convention was quickly adopted by astronomers, and the next asteroid to be discovered (16 Psyche, in 1852) was the first to be designated in that way at the time of its discovery. However, Psyche was given an iconic symbol as well, as were a few other asteroids discovered over the next few years. 20 Massalia was the first asteroid that was not assigned an iconic symbol, and no iconic symbols were created after the 1855 discovery of 37 Fides.

Terminology

A composite image, to the same scale, of the asteroids imaged at high resolution prior to 2012. They are, from largest to smallest: 4 Vesta, 21 Lutetia, 253 Mathilde, 243 Ida and its moon Dactyl, 433 Eros, 951 Gaspra, 2867 Šteins, 25143 Itokawa.

Vesta (left), with Ceres (center) and the Moon (right) shown to scale

The first discovered asteroid, Ceres, was originally considered a new planet. It was followed by the discovery of other similar bodies, which with the equipment of the time appeared to be points of light like stars, showing little or no planetary disc, though readily distinguishable from stars due to their apparent motions. This prompted the astronomer Sir William Herschel to propose the term asteroid, coined in Greek as ἀστεροειδής, or asteroeidēs, meaning 'star-like, star-shaped', and derived from the Ancient Greek ἀστήρ astēr 'star, planet'. In the early second half of the 19th century, the terms asteroid and planet (not always qualified as "minor") were still used interchangeably.

Traditionally, small bodies orbiting the Sun were classified as comets, asteroids, or meteoroids, with anything smaller than one meter across being called a meteoroid. The term asteroid never had a formal definition, with the broader term small Solar System bodies being preferred by the International Astronomical Union (IAU). As no IAU definition exists, asteroid can be defined as "an irregularly shaped rocky body orbiting the Sun that does not qualify as a planet or a dwarf planet under the IAU definitions of those terms".

When found, asteroids were seen as a class of objects distinct from comets, and there was no unified term for the two until small Solar System body was coined in 2006. The main difference between an asteroid and a comet is that a comet shows a coma due to sublimation of near-surface ices by solar radiation. A few objects have ended up being dual-listed because they were first classified as minor planets but later showed evidence of cometary activity. Conversely, some (perhaps all) comets are eventually depleted of their surface volatile ices and become asteroid-like. A further distinction is that comets typically have more eccentric orbits than most asteroids; "asteroids" with notably eccentric orbits are probably dormant or extinct comets.

For almost two centuries, from the discovery of Ceres in 1801 until the discovery of the first centaur, 2060 Chiron in 1977, all known asteroids spent most of their time at or within the orbit of Jupiter, though a few such as 944 Hidalgo ventured far beyond Jupiter for part of their orbit. When astronomers started finding more small bodies that permanently resided further out than Jupiter, now called centaurs, they numbered them among the traditional asteroids. There was debate over whether these objects should be considered asteroids or given a new classification. Then, when the first trans-Neptunian object (other than Pluto), 15760 Albion, was discovered in 1992, and especially when large numbers of similar objects started turning up, new terms were invented to sidestep the issue: Kuiper-belt object, trans-Neptunian object, scattered-disc object, and so on. They inhabit the cold outer reaches of the Solar System where ices remain solid and comet-like bodies are not expected to exhibit much cometary activity; if centaurs or trans-Neptunian objects were to venture close to the Sun, their volatile ices would sublimate, and traditional approaches would classify them as comets and not asteroids.

The innermost of these are the Kuiper-belt objects, called "objects" partly to avoid the need to classify them as asteroids or comets. They are thought to be predominantly comet-like in composition, though some may be more akin to asteroids. Furthermore, most do not have the highly eccentric orbits associated with comets, and the ones so far discovered are larger than traditional comet nuclei. (The much more distant Oort cloud is hypothesized to be the main reservoir of dormant comets.) Other recent observations, such as the analysis of the cometary dust collected by the Stardust probe, are increasingly blurring the distinction between comets and asteroids, suggesting "a continuum between asteroids and comets" rather than a sharp dividing line.

The minor planets beyond Jupiter's orbit are sometimes also called "asteroids", especially in popular presentations.^[e] However, it is becoming increasingly common for the term asteroid to be restricted to minor planets of the inner Solar System.^[32] Therefore, this article will restrict itself for the most part to the classical asteroids: objects of the asteroid belt, Jupiter trojans, and near-Earth objects.

When the IAU introduced the class small Solar System bodies in 2006 to include most objects previously classified as minor planets and comets, they created the class of dwarf planets for the largest minor planets—those that have enough mass to have become ellipsoidal under their own gravity. According to the IAU, "the term 'minor planet' may still be used, but generally, the term 'Small Solar System Body' will be preferred." Currently only the largest object in the asteroid belt, Ceres, at about 975 km (606 mi) across, has been placed in the dwarf planet category.

Formation

Many asteroids are the shattered remnants of planetesimals, bodies within the young Sun's solar nebula that never grew large enough to become planets. It is thought that planetesimals in the asteroid belt evolved much like the rest of objects in the solar nebula until Jupiter neared its current mass, at which point excitation from orbital resonances with Jupiter ejected over 99% of planetesimals in the belt. Simulations and a discontinuity in spin rate and spectral properties suggest that asteroids larger than approximately 120 km (75 mi) in diameter accreted during that early era, whereas smaller bodies are fragments from collisions between asteroids during or after the Jovian disruption. Ceres and Vesta grew large enough to melt and differentiate, with heavy metallic elements sinking to the core, leaving rocky minerals in the crust.

In the Nice model, many Kuiper-belt objects are captured in the outer asteroid belt, at distances greater than 2.6 AU. Most were later ejected by Jupiter, but those that remained may be the D-type asteroids, and possibly include Ceres.

Distribution within the Solar System

Various dynamical groups of asteroids have been discovered orbiting in the inner Solar System. Their orbits are perturbed by the gravity of other bodies in the Solar System and by the Yarkovsky effect. Significant populations include:

Asteroid belt

The majority of known asteroids orbit within the asteroid belt between the orbits of Mars and Jupiter, generally in relatively low-eccentricity (i.e. not very elongated) orbits. This belt is estimated to contain between 1.1 and 1.9 million asteroids larger than 1 km (0.6 mi) in diameter, and millions of smaller ones. These asteroids may be remnants of the protoplanetary disk, and in this region the accretion of planetesimals into planets during the formative period of the Solar System was prevented by large gravitational perturbations by Jupiter.

Contrary to popular imagery, the asteroid belt is mostly empty. The asteroids are spread over such a large volume that reaching an asteroid without aiming carefully would be improbable. Nonetheless, hundreds of thousands of asteroids are currently known, and the total number ranges in the millions or more, depending on the lower size cutoff. Over 200 asteroids are known to be larger than 100 km, and a survey in the infrared wavelengths has shown that the asteroid belt has between 700,000 and 1.7 million asteroids with a diameter of 1 km or more. The absolute magnitudes of most of the known asteroids are between 11 and 19, with the median at about 16.

The total mass of the asteroid belt is estimated to be 2.39×10²¹ kg, which is just 3% of the mass of the Moon; the mass of the Kuiper Belt and Scattered Disk is over 100 times as large. The four largest objects, Ceres, Vesta, Pallas, and Hygiea, account for maybe 62% of the belt's total mass, with 39% accounted for by Ceres alone.

Trojans

Trojans are populations that share an orbit with a larger planet or moon, but do not collide with it because they orbit in one of the two Lagrangian points of stability, L₄ and L₅, which lie 60° ahead of and behind the larger body.

In the Solar System, most known trojans share the orbit of Jupiter. They are divided into the Greek camp at L₄ (ahead of Jupiter) and the Trojan camp at L₅ (trailing Jupiter). More than a million Jupiter trojans larger than one kilometer are thought to exist, of which more than 7,000 are currently catalogued. In other planetary orbits only nine Mars trojans, 28 Neptune trojans, two Uranus trojans, and two Earth trojans, have been found to date. A temporary Venus trojan is also known. Numerical orbital dynamics stability simulations indicate that Saturn and Uranus probably do not have any primordial trojans.

Near-Earth asteroids

Near-Earth asteroids, or NEAs, are asteroids that have orbits that pass close to that of Earth. Asteroids that actually cross Earth's orbital path are known as Earth-crossers. As of April 2022, a total of 28,772 near-Earth asteroids were known; 878 have a diameter of one kilometer or larger.

A small number of NEAs are extinct comets that have lost their volatile surface materials, although having a faint or intermittent comet-like tail does not necessarily result in a classification as a near-Earth comet, making the boundaries somewhat fuzzy. The rest of the near-Earth asteroids are driven out of the asteroid belt by gravitational interactions with Jupiter.

Many asteroids have natural satellites (minor-planet moons). As of October 2021, there were 85 NEAs known to have at least one moon, including three known to have two moons. The asteroid 3122 Florence, one of the largest potentially hazardous asteroids with a diameter of 4.5 km (2.8 mi), has two moons measuring 100–300 m (330–980 ft) across, which were discovered by radar imaging during the asteroid's 2017 approach to Earth.

Near-Earth asteroids are divided into groups based on their semi-major axis (a), perihelion distance (q), and aphelion distance (Q):

The Atiras or Apoheles have orbits strictly inside Earth's orbit: an Atira asteroid's aphelion distance (Q) is smaller than Earth's perihelion distance (0.983 AU). That is, Q < 0.983 AU, which implies that the asteroid's semi-major axis is also less than 0.983 AU.
The Atens have a semi-major axis of less than 1 AU and cross Earth's orbit. Mathematically, a < 1.0 AU and Q > 0.983 AU. (0.983 AU is Earth's perihelion distance.)
The Apollos have a semi-major axis of more than 1 AU and cross Earth's orbit. Mathematically, a > 1.0 AU and q < 1.017 AU. (1.017 AU is Earth's aphelion distance.)
The Amors have orbits strictly outside Earth's orbit: an Amor asteroid's perihelion distance (q) is greater than Earth's aphelion distance (1.017 AU). Amor asteroids are also near-earth objects so q < 1.3 AU. In summary, 1.017 AU < q < 1.3 AU. (This implies that the asteroid's semi-major axis (a) is also larger than 1.017 AU.) Some Amor asteroid orbits cross the orbit of Mars.

Martian moons

Phobos

Deimos

It is unclear whether Martian moons Phobos and Deimos are captured asteroids or were formed due to impact event on Mars. Phobos and Deimos both have much in common with carbonaceous C-type asteroids, with spectra, albedo, and density very similar to those of C- or D-type asteroids. Based on their similarity, one hypothesis is that both moons may be captured main-belt asteroids. Both moons have very circular orbits which lie almost exactly in Mars's equatorial plane, and hence a capture origin requires a mechanism for circularizing the initially highly eccentric orbit, and adjusting its inclination into the equatorial plane, most probably by a combination of atmospheric drag and tidal forces, although it is not clear whether sufficient time was available for this to occur for Deimos. Capture also requires dissipation of energy. The current Martian atmosphere is too thin to capture a Phobos-sized object by atmospheric braking. Geoffrey A. Landis has pointed out that the capture could have occurred if the original body was a binary asteroid that separated under tidal forces.

Phobos could be a second-generation Solar System object that coalesced in orbit after Mars formed, rather than forming concurrently out of the same birth cloud as Mars.

Another hypothesis is that Mars was once surrounded by many Phobos- and Deimos-sized bodies, perhaps ejected into orbit around it by a collision with a large planetesimal. The high porosity of the interior of Phobos (based on the density of 1.88 g/cm³, voids are estimated to comprise 25 to 35 percent of Phobos's volume) is inconsistent with an asteroidal origin. Observations of Phobos in the thermal infrared suggest a composition containing mainly phyllosilicates, which are well known from the surface of Mars. The spectra are distinct from those of all classes of chondrite meteorites, again pointing away from an asteroidal origin. Both sets of findings support an origin of Phobos from material ejected by an impact on Mars that reaccreted in Martian orbit, similar to the prevailing theory for the origin of Earth's moon.

Characteristics

Size distribution

Asteroids vary greatly in size, from almost 1000 km for the largest down to rocks just 1 meter across, below which an object is classified as a meteoroid. The three largest are very much like miniature planets: they are roughly spherical, have at least partly differentiated interiors, and are thought to be surviving protoplanets. The vast majority, however, are much smaller and are irregularly shaped; they are thought to be either battered planetesimals or fragments of larger bodies.

The dwarf planet Ceres is by far the largest asteroid, with a diameter of 940 km (580 mi). The next largest are 4 Vesta and 2 Pallas, both with diameters of just over 500 km (300 mi). Vesta is the brightest of the four main-belt asteroids that can, on occasion, be visible to the naked eye. On some rare occasions, a near-Earth asteroid may briefly become visible without technical aid; see 99942 Apophis.

The mass of all the objects of the asteroid belt, lying between the orbits of Mars and Jupiter, is estimated to be (2394±6)×10¹⁸ kg, ≈ 3.25% of the mass of the Moon. Of this, Ceres comprises 938×10¹⁸ kg, about 40% of the total. Adding in the next three most massive objects, Vesta (11%), Pallas (8.5%), and Hygiea (3–4%), brings this figure up to a bit over 60%, whereas the next seven most-massive asteroids bring the total up to 70%. The number of asteroids increases rapidly as their individual masses decrease.

The number of asteroids decreases markedly with increasing size. Although the size distribution generally follows a power law, there are 'bumps' at about 5 km and 100 km, where more asteroids than expected from such a curve are found. Most asteroids larger than approximately 120 km in diameter are primordial (surviving from the accretion epoch), whereas most smaller asteroids are products of fragmentation of primordial asteroids. The primordial population of the main belt was probably 200 times what it is today.

Largest asteroids

42 of the largest objects in the asteroid belt captured by ESO's Very Large Telescope

Eros, Vesta and Ceres size comparison

Three largest objects in the asteroid belt, Ceres, Vesta, and Pallas, are intact protoplanets that share many characteristics common to planets, and are atypical compared to the majority of irregularly shaped asteroids. The fourth-largest asteroid, Hygiea, appears nearly spherical although it may have an undifferentiated interior, like the majority of asteroids. The four largest asteroids constitute half the mass of the asteroid belt.

Ceres is the only asteroid that appears to have a plastic shape under its own gravity and hence the only one that is a dwarf planet. It has a much higher absolute magnitude than the other asteroids, of around 3.32, and may possess a surface layer of ice. Like the planets, Ceres is differentiated: it has a crust, a mantle and a core. No meteorites from Ceres have been found on Earth.

Vesta, too, has a differentiated interior, though it formed inside the Solar System's frost line, and so is devoid of water; its composition is mainly of basaltic rock with minerals such as olivine. Aside from the large crater at its southern pole, Rheasilvia, Vesta also has an ellipsoidal shape. Vesta is the parent body of the Vestian family and other V-type asteroids, and is the source of the HED meteorites, which constitute 5% of all meteorites on Earth.

Pallas is unusual in that, like Uranus, it rotates on its side, with its axis of rotation tilted at high angles to its orbital plane. Its composition is similar to that of Ceres: high in carbon and silicon, and perhaps partially differentiated. Pallas is the parent body of the Palladian family of asteroids.

Hygiea is the largest carbonaceous asteroid and, unlike the other largest asteroids, lies relatively close to the plane of the ecliptic. It is the largest member and presumed parent body of the Hygiean family of asteroids. Because there is no sufficiently large crater on the surface to be the source of that family, as there is on Vesta, it is thought that Hygiea may have been completely disrupted in the collision that formed the Hygiean family and recoalesced after losing a bit less than 2% of its mass. Observations taken with the Very Large Telescope's SPHERE imager in 2017 and 2018, revealed that Hygiea has a nearly spherical shape, which is consistent both with it being in hydrostatic equilibrium, or formerly being in hydrostatic equilibrium, or with being disrupted and recoalescing.

Internal differentiation of large asteroids is possibly related to their lack of natural satellites, as satellites of main belt asteroids are mostly believed to form from collisional disruption, creating a rubble pile structure.

Attributes of largest asteroids
Name	Orbital radius (AU)	Orbital period (years)	Inclination to ecliptic	Orbital eccentricity	Diameter (km)	Diameter (% of Moon)	Mass (×10¹⁸ kg)	Mass (% of Ceres)	Density (g/cm³)	Rotation period (hr)
Ceres	2.77	4.60	10.6°	0.079	964×964×892 (mean 939.4)	27%	938	100%	2.16±0.01	9.07
Vesta	2.36	3.63	7.1°	0.089	573×557×446 (mean 525.4)	15%	259	28%	3.46 ± 0.04	5.34
Pallas	2.77	4.62	34.8°	0.231	550×516×476 (mean 511±4)	15%	204±3	21%	2.92±0.08	7.81
Hygiea	3.14	5.56	3.8°	0.117	450×430×424 (mean 433±8)	12%	87±7	9%	2.06±0.20	13.8

Rotation

Measurements of the rotation rates of large asteroids in the asteroid belt show that there is an upper limit. Very few asteroids with a diameter larger than 100 meters have a rotation period less than 2.2 hours. For asteroids rotating faster than approximately this rate, the inertial force at the surface is greater than the gravitational force, so any loose surface material would be flung out. However, a solid object should be able to rotate much more rapidly. This suggests that most asteroids with a diameter over 100 meters are rubble piles formed through the accumulation of debris after collisions between asteroids.

Color

Asteroids become darker and redder with age due to space weathering. However evidence suggests most of the color change occurs rapidly, in the first hundred thousand years, limiting the usefulness of spectral measurement for determining the age of asteroids.

Surface features

Except for the "big four" (Ceres, Pallas, Vesta, and Hygiea), asteroids are likely to be broadly similar in appearance, if irregular in shape. 50 km (31 mi) 253 Mathilde is a rubble pile saturated with craters with diameters the size of the asteroid's radius. Earth-based observations of 300 km (190 mi) 511 Davida, one of the largest asteroids after the big four, reveal a similarly angular profile, suggesting it is also saturated with radius-size craters. Medium-sized asteroids such as Mathilde and 243 Ida, that have been observed up close, also reveal a deep regolith covering the surface. Of the big four, Pallas and Hygiea are practically unknown. Vesta has compression fractures encircling a radius-size crater at its south pole but is otherwise a spheroid.

Dawn spacecraft revealed that Ceres has a heavily cratered surface, but with fewer large craters than expected. Models based on the formation of the current asteroid belt had suggested Ceres should possess 10 to 15 craters larger than 400 km (250 mi) in diameter. The largest confirmed crater on Ceres, Kerwan Basin, is 284 km (176 mi) across. The most likely reason for this is viscous relaxation of the crust slowly flattening out larger impacts.

Composition

Asteroids are classified by their characteristic emission spectra, with the majority falling into three main groups: C-type, M-type, and S-type. These were named after and are generally identified with carbonaceous (carbon-rich), metallic, and silicaceous (stony) compositions, respectively. The physical composition of asteroids is varied and in most cases poorly understood. Ceres appears to be composed of a rocky core covered by an icy mantle, where Vesta is thought to have a nickel-iron core, olivine mantle, and basaltic crust. Thought to be the largest undifferentiated asteroid, 10 Hygiea seems to have a uniformly primitive composition of carbonaceous chondrite, but it may actually be a differentiated asteroid that was globally disrupted by an impact and then reassembled. Other asteroids appear to be the remnant cores or mantles of proto-planets, high in rock and metal. Most small asteroids are believed to be piles of rubble held together loosely by gravity, although the largest are probably solid. Some asteroids have moons or are co-orbiting binaries: rubble piles, moons, binaries, and scattered asteroid families are thought to be the results of collisions that disrupted a parent asteroid, or possibly a planet.

In the main asteroid belt, there appear to be two primary populations of asteroid: a dark, volatile-rich population, consisting of the C-type and P-type asteroids, with albedos less than 0.10 and densities under 2.2 g/cm³, and a dense, volatile-poor population, consisting of the S-type and M-type asteroids, with albedos over 0.15 and densities greater than 2.7. Within these populations, larger asteroids are denser, presumably due to compression. There appears to be minimal macro-porosity (interstitial vacuum) in the score of asteroids with masses greater than 10×10¹⁸ kg.

Composition is calculated from three primary sources: albedo, surface spectrum, and density. The last can only be determined accurately by observing the orbits of moons the asteroid might have. So far, every asteroid with moons has turned out to be a rubble pile, a loose conglomeration of rock and metal that may be half empty space by volume. The investigated asteroids are as large as 280 km in diameter, and include 121 Hermione (268×186×183 km), and 87 Sylvia (384×262×232 km). Few asteroids are larger than 87 Sylvia, none of them have moons. The fact that such large asteroids as Sylvia may be rubble piles, presumably due to disruptive impacts, has important consequences for the formation of the Solar System: computer simulations of collisions involving solid bodies show them destroying each other as often as merging, but colliding rubble piles are more likely to merge. This means that the cores of the planets could have formed relatively quickly.

Water

Scientists hypothesize that some of the first water brought to Earth was delivered by asteroid impacts after the collision that produced the Moon. In 2009, the presence of water ice was confirmed on the surface of 24 Themis using NASA's Infrared Telescope Facility. The surface of the asteroid appears completely covered in ice. As this ice layer is sublimating, it may be getting replenished by a reservoir of ice under the surface. Organic compounds were also detected on the surface.The presence of ice on 24 Themis makes the initial theory plausible.

In October 2013, water was detected on an extrasolar body for the first time, on an asteroid orbiting the white dwarf GD 61. On 22 January 2014, European Space Agency (ESA) scientists reported the detection, for the first definitive time, of water vapor on Ceres, the largest object in the asteroid belt. The detection was made by using the far-infrared abilities of the Herschel Space Observatory. The finding is unexpected because comets, not asteroids, are typically considered to "sprout jets and plumes". According to one of the scientists, "The lines are becoming more and more blurred between comets and asteroids."

Findings have shown that solar winds can react with the oxygen in the upper layer of the asteroids and create water. It has been estimated that "every cubic metre of irradiated rock could contain up to 20 litres"; study was conducted using an atom probe tomography, numbers are given for the Itokawa S-type asteroid.

Acfer 049, a meteorite discovered in Algeria in 1990, was shown in 2019 to have an ultraporous lithology (UPL): porous texture that could be formed by removal of ice that filled these pores, this suggests that UPL "represent fossils of primordial ice".

Organic compounds

Asteroids contain traces of amino acids and other organic compounds, and some speculate that asteroid impacts may have seeded the early Earth with the chemicals necessary to initiate life, or may have even brought life itself to Earth (an event called "panspermia"). In August 2011, a report, based on NASA studies with meteorites found on Earth, was published suggesting DNA and RNA components (adenine, guanine and related organic molecules) may have been formed on asteroids and comets in outer space.

In November 2019, scientists reported detecting, for the first time, sugar molecules, including ribose, in meteorites, suggesting that chemical processes on asteroids can produce some fundamentally essential bio-ingredients important to life, and supporting the notion of an RNA world prior to a DNA-based origin of life on Earth, and possibly, as well, the notion of panspermia.

Classification

Asteroids are commonly categorized according to two criteria: the characteristics of their orbits, and features of their reflectance spectrum.

Orbital classification

Many asteroids have been placed in groups and families based on their orbital characteristics. Apart from the broadest divisions, it is customary to name a group of asteroids after the first member of that group to be discovered. Groups are relatively loose dynamical associations, whereas families are tighter and result from the catastrophic break-up of a large parent asteroid sometime in the past. Families are more common and easier to identify within the main asteroid belt, but several small families have been reported among the Jupiter trojans. Main belt families were first recognized by Kiyotsugu Hirayama in 1918 and are often called Hirayama families in his honor.

About 30–35% of the bodies in the asteroid belt belong to dynamical families, each thought to have a common origin in a past collision between asteroids. A family has also been associated with the plutoid dwarf planet Haumea.

Some asteroids have unusual horseshoe orbits that are co-orbital with Earth or another planet. Examples are 3753 Cruithne and 2002 AA29. The first instance of this type of orbital arrangement was discovered between Saturn's moons Epimetheus and Janus. Sometimes these horseshoe objects temporarily become quasi-satellites for a few decades or a few hundred years, before returning to their earlier status. Both Earth and Venus are known to have quasi-satellites.

Such objects, if associated with Earth or Venus or even hypothetically Mercury, are a special class of Aten asteroids. However, such objects could be associated with the outer planets as well.

Spectral classification

In 1975, an asteroid taxonomic system based on color, albedo, and spectral shape was developed by Chapman, Morrison, and Zellner. These properties are thought to correspond to the composition of the asteroid's surface material. The original classification system had three categories: C-types for dark carbonaceous objects (75% of known asteroids), S-types for stony (silicaceous) objects (17% of known asteroids) and U for those that did not fit into either C or S. This classification has since been expanded to include many other asteroid types. The number of types continues to grow as more asteroids are studied.

The two most widely used taxonomies now used are the Tholen classification and SMASS classification. The former was proposed in 1984 by David J. Tholen, and was based on data collected from an eight-color asteroid survey performed in the 1980s. This resulted in 14 asteroid categories. In 2002, the Small Main-Belt Asteroid Spectroscopic Survey resulted in a modified version of the Tholen taxonomy with 24 different types. Both systems have three broad categories of C, S, and X asteroids, where X consists of mostly metallic asteroids, such as the M-type. There are also several smaller classes.

The proportion of known asteroids falling into the various spectral types does not necessarily reflect the proportion of all asteroids that are of that type; some types are easier to detect than others, biasing the totals.

Problems

Originally, spectral designations were based on inferences of an asteroid's composition. However, the correspondence between spectral class and composition is not always very good, and a variety of classifications are in use. This has led to significant confusion. Although asteroids of different spectral classifications are likely to be composed of different materials, there are no assurances that asteroids within the same taxonomic class are composed of the same (or similar) materials.

Active asteroids

Asteroid (101955) Bennu seen ejecting particles by the OSIRIS-REx

Active asteroids are objects that have asteroid-like orbits but show comet-like visual characteristics. That is, they show comae, tails, or other visual evidence of mass-loss (like a comet), but their orbit remains within Jupiter's orbit (like an asteroid). These bodies were originally designated main-belt comets (MBCs) in 2006 by astronomers David Jewitt and Henry Hsieh, but this name implies they are necessarily icy in composition like a comet and that they only exist within the main-belt, whereas the growing population of active asteroids shows that this is not always the case.

The first active asteroid discovered is 7968 Elst–Pizarro. It was discovered (as an asteroid) in 1979 but then was found to have a tail by Eric Elst and Guido Pizarro in 1996 and given the cometary designation 133P/Elst-Pizarro. Another notable object is 311P/PanSTARRS: observations made by the Hubble Space Telescope revealed that it had six comet-like tails. The tails are suspected to be streams of material ejected by the asteroid as a result of a rubble pile asteroid spinning fast enough to remove material from it.

By smashing into the asteroid Dimorphos, NASA's Double Asteroid Redirection Test spacecraft made it an active asteroid. Scientists had proposed that some active asteroids are the result of impact events, but no one had ever observed the activation of an asteroid. The DART mission activated Dimorphos under precisely known and carefully observed impact conditions, enabling the detailed study of the formation of an active asteroid for the first time.Observations show that Dimorphos lost approximately 1 million kilograms after the collision. Impact produced a dust plume that temporarily brightened the Didymos system and developed a 10,000-kilometer (6,200 mi)-long dust tail that persisted for several months.

Observation and exploration

Until the age of space travel, objects in the asteroid belt could only be observed with large telescopes, their shapes and terrain remaining a mystery. The best modern ground-based telescopes and the Earth-orbiting Hubble Space Telescope can only resolve a small amount of detail on the surfaces of the largest asteroids. Limited information about the shapes and compositions of asteroids can be inferred from their light curves (variation in brightness during rotation) and their spectral properties. Sizes can be estimated by timing the lengths of star occultations (when an asteroid passes directly in front of a star). Radar imaging can yield good information about asteroid shapes and orbital and rotational parameters, especially for near-Earth asteroids. Spacecraft flybys can provide much more data than any ground or space-based observations; sample-return missions gives insights about regolith composition.

Ground-based observations

As asteroids are rather small and faint objects, the data that can be obtained from ground-based observations (GBO) are limited. By means of ground-based optical telescopes the visual magnitude can be obtained; when converted into the absolute magnitude it gives a rough estimate of the asteroid's size. Light-curve measurements can also be made by GBO; when collected over a long period of time it allows an estimate of the rotational period, the pole orientation (sometimes), and a rough estimate of the asteroid's shape. Spectral data (both visible-light and near-infrared spectroscopy) gives information about the object's composition, used to classify the observed asteroids. Such observations are limited as they provide information about only the thin layer on the surface (up to several micrometers). As planetologist Patrick Michel writes:

Mid- to thermal-infrared observations, along with polarimetry measurements, are probably the only data that give some indication of actual physical properties. Measuring the heat flux of an asteroid at a single wavelength gives an estimate of the dimensions of the object; these measurements have lower uncertainty than measurements of the reflected sunlight in the visible-light spectral region. If the two measurements can be combined, both the effective diameter and the geometric albedo—the latter being a measure of the brightness at zero phase angle, that is, when illumination comes from directly behind the observer—can be derived. In addition, thermal measurements at two or more wavelengths, plus the brightness in the visible-light region, give information on the thermal properties. The thermal inertia, which is a measure of how fast a material heats up or cools off, of most observed asteroids is lower than the bare-rock reference value but greater than that of the lunar regolith; this observation indicates the presence of an insulating layer of granular material on their surface. Moreover, there seems to be a trend, perhaps related to the gravitational environment, that smaller objects (with lower gravity) have a small regolith layer consisting of coarse grains, while larger objects have a thicker regolith layer consisting of fine grains. However, the detailed properties of this regolith layer are poorly known from remote observations. Moreover, the relation between thermal inertia and surface roughness is not straightforward, so one needs to interpret the thermal inertia with caution.

Near-Earth asteroids that come into close vicinity of the planet can be studied in more details with radar; it provides information about the surface of the asteroid (for example can show the presence of craters and boulders). Such observations were conducted by the Arecibo Observatory in Puerto Rico (305 meter dish) and Goldstone Observatory in California (70 meter dish). Radar observations can also be used for accurate determination of the orbital and rotational dynamics of observed objects.

Space-based observations

Both space and ground-based observatories conducted asteroid search programs; the space-based searches are expected to detect more objects because there is no atmosphere to interfere and because they can observe larger portions of the sky. NEOWISE observed more than 100,000 asteroids of the main belt, Spitzer Space Telescope observed more than 700 near-Earth asteroids. These observations determined rough sizes of the majority of observed objects, but provided limited detail about surface properties (such as regolith depth and composition, angle of repose, cohesion, and porosity).

Asteroids were also studied by the Hubble Space Telescope, such as tracking the colliding asteroids in the main belt, break-up of an asteroid, observing an active asteroid with six comet-like tails, and observing asteroids that were chosen as targets of dedicated missions.

Space probe missions

According to Patrick Michel

The internal structure of asteroids is inferred only from indirect evidence: bulk densities measured by spacecraft, the orbits of natural satellites in the case of asteroid binaries, and the drift of an asteroid's orbit due to the Yarkovsky thermal effect. A spacecraft near an asteroid is perturbed enough by the asteroid's gravity to allow an estimate of the asteroid's mass. The volume is then estimated using a model of the asteroid's shape. Mass and volume allow the derivation of the bulk density, whose uncertainty is usually dominated by the errors made on the volume estimate. The internal porosity of asteroids can be inferred by comparing their bulk density with that of their assumed meteorite analogues, dark asteroids seem to be more porous (>40%) than bright ones. The nature of this porosity is unclear.

Dedicated missions

The first asteroid to be photographed in close-up was 951 Gaspra in 1991, followed in 1993 by 243 Ida and its moon Dactyl, all of which were imaged by the Galileo probe en route to Jupiter. Other asteroids briefly visited by spacecraft en route to other destinations include 9969 Braille (by Deep Space 1 in 1999), 5535 Annefrank (by Stardust in 2002), 2867 Šteins and 21 Lutetia (by the Rosetta probe in 2008), and 4179 Toutatis (China's lunar orbiter Chang'e 2, which flew within 3.2 km (2 mi) in 2012).

The first dedicated asteroid probe was NASA's NEAR Shoemaker, which photographed 253 Mathilde in 1997, before entering into orbit around 433 Eros, finally landing on its surface in 2001. It was the first spacecraft to successfully orbit and land on an asteroid. From September to November 2005, the Japanese Hayabusa probe studied 25143 Itokawa in detail and returned samples of its surface to Earth on 13 June 2010, the first asteroid sample-return mission. In 2007, NASA launched the Dawn spacecraft, which orbited 4 Vesta for a year, and observed the dwarf planet Ceres for three years.

Hayabusa2, a probe launched by JAXA 2014, orbited its target asteroid 162173 Ryugu for more than a year and took samples that were delivered to Earth in 2020. The spacecraft is now on an extended mission and expected to arrive at a new target in 2031.

NASA launched the OSIRIS-REx in 2016, a sample return mission to asteroid 101955 Bennu. In 2021, the probe departed the asteroid with a sample from its surface. Sample delivery to Earth is expected on September 24, 2023. The spacecraft will continue on an extended mission, designated OSIRIS-APEX, to explore near-Earth asteroid Apophis in 2029.

In 2021, NASA launched Double Asteroid Redirection Test (DART), a mission to test technology for defending Earth against potential hazardous objects. DART deliberately crashed into the minor-planet moon Dimorphos of the double asteroid Didymos in September 2022 to assess the potential of a spacecraft impact to deflect an asteroid from a collision course with Earth. In October, NASA declared DART a success, confirming it had shortened Dimorphos' orbital period around Didymos by about 32 minutes.

Asteroid-dedicated space probes
Hayabusa2
Dawn
Lucy
Psyche

Planned missions

Currently, several asteroid-dedicated missions are planned by NASA, JAXA, ESA, and CNSA.

NASA's Lucy, launched in 2021, would visit eight asteroids, one from the main belt and seven Jupiter trojans; it is the first mission to trojans. The main mission would start in 2027.

ESA's Hera, planned for launch in 2024, will study the results of the DART impact. It will measure the size and morphology of the crater, and momentum transmitted by the impact, to determine the efficiency of the deflection produced by DART.

NASA's Psyche would be launched in 2023 or 2024 to study the large metallic asteroid of the same name.

JAXA's DESTINY+ is a mission for a flyby of the Geminids meteor shower parent body 3200 Phaethon, as well as various minor bodies. Its launch is planned for 2024.

CNSA's Tianwen-2 is planned to launch in 2025. It will use solar electric propulsion to explore the co-orbital near-Earth asteroid 469219 Kamoʻoalewa and the active asteroid 311P/PanSTARRS. The spacecraft will collect samples of the regolith of Kamo'oalewa.

Asteroid mining

The concept of asteroid mining was proposed in 1970s. Matt Anderson defines successful asteroid mining as "the development of a mining program that is both financially self-sustaining and profitable to its investors". It has been suggested that asteroids might be used as a source of materials that may be rare or exhausted on Earth, or materials for constructing space habitats. Materials that are heavy and expensive to launch from Earth may someday be mined from asteroids and used for space manufacturing and construction.

As resource depletion on Earth becomes more real, the idea of extracting valuable elements from asteroids and returning these to Earth for profit, or using space-based resources to build solar-power satellites and space habitats, becomes more attractive. Hypothetically, water processed from ice could refuel orbiting propellant depots.

From the astrobiological perspective, asteroid prospecting could provide scientific data for the search for extraterrestrial intelligence (SETI). Some astrophysicists have suggested that if advanced extraterrestrial civilizations employed asteroid mining long ago, the hallmarks of these activities might be detectable.

Mining Ceres is also considered a possibility. As the largest body in the asteroid belt, Ceres could become the main base and transport hub for future asteroid mining infrastructure, allowing mineral resources to be transported to Mars, the Moon, and Earth. Because of its small escape velocity combined with large amounts of water ice, it also could serve as a source of water, fuel, and oxygen for ships going through and beyond the asteroid belt. Transportation from Mars or the Moon to Ceres would be even more energy-efficient than transportation from Earth to the Moon.

Threats to Earth

Frequency of bolides, small asteroids roughly 1 to 20 meters in diameter impacting Earth's atmosphere

There is increasing interest in identifying asteroids whose orbits cross Earth's, and that could, given enough time, collide with Earth. The three most important groups of near-Earth asteroids are the Apollos, Amors, and Atens.

The near-Earth asteroid 433 Eros had been discovered as long ago as 1898, and the 1930s brought a flurry of similar objects. In order of discovery, these were: 1221 Amor, 1862 Apollo, 2101 Adonis, and finally 69230 Hermes, which approached within 0.005 AU of Earth in 1937. Astronomers began to realize the possibilities of Earth impact.

Two events in later decades increased the alarm: the increasing acceptance of the Alvarez hypothesis that an impact event resulted in the Cretaceous–Paleogene extinction, and the 1994 observation of Comet Shoemaker-Levy 9 crashing into Jupiter. The U.S. military also declassified the information that its military satellites, built to detect nuclear explosions, had detected hundreds of upper-atmosphere impacts by objects ranging from one to ten meters across.

All of these considerations helped spur the launch of highly efficient surveys, consisting of charge-coupled device (CCD) cameras and computers directly connected to telescopes. As of 2011, it was estimated that 89% to 96% of near-Earth asteroids one kilometer or larger in diameter had been discovered. A list of teams using such systems includes:

Lincoln Near-Earth Asteroid Research (LINEAR)
Near-Earth Asteroid Tracking (NEAT)
Spacewatch
Lowell Observatory Near-Earth-Object Search (LONEOS)
Catalina Sky Survey (CSS)
Pan-STARRS
NEOWISE
Asteroid Terrestrial-impact Last Alert System (ATLAS)
Campo Imperatore Near-Earth Object Survey (CINEOS)
Japanese Spaceguard Association
Asiago-DLR Asteroid Survey (ADAS)

As of 29 October 2018, the LINEAR system alone had discovered 147,132 asteroids. Among the surveys, 19,266 near-Earth asteroids have been discovered including almost 900 more than 1 km (0.6 mi) in diameter.

In April 2018, the B612 Foundation reported "It is 100 percent certain we'll be hit [by a devastating asteroid], but we're not 100 percent sure when." In June 2018, the National Science and Technology Council warned that the United States is unprepared for an asteroid impact event, and has developed and released the "National Near-Earth Object Preparedness Strategy Action Plan" to better prepare. According to expert testimony in the United States Congress in 2013, NASA would require at least five years of preparation before a mission to intercept an asteroid could be launched.

The United Nations declared 30 June to be International Asteroid Day to educate the public about asteroids. The date of International Asteroid Day commemorates the anniversary of the Tunguska asteroid impact over Siberia, on 30 June 1908.

Chicxulub impact

The Chicxulub crater is an impact crater buried underneath the Yucatán Peninsula in Mexico. Its center is offshore near the communities of Chicxulub Puerto and Chicxulub Pueblo, after which the crater is named. It was formed when a large asteroid, about 10 kilometers (6.2 miles) in diameter, struck the Earth. The crater is estimated to be 180 kilometers (110 miles) in diameter and 20 kilometers (12 miles) in depth. It is one of the largest confirmed impact structures on Earth, and the only one whose peak ring is intact and directly accessible for scientific research.

In the late 1970s, geologist Walter Alvarez and his father, Nobel Prize–winning scientist Luis Walter Alvarez, put forth their theory that the Cretaceous–Paleogene extinction was caused by an impact event. The main evidence of such an impact was contained in a thin layer of clay present in the K–Pg boundary in Gubbio, Italy. The Alvarezes and colleagues reported that it contained an abnormally high concentration of iridium, a chemical element rare on earth but common in asteroids. Iridium levels in this layer were as much as 160 times above the background level. It was hypothesized that the iridium was spread into the atmosphere when the impactor was vaporized and settled across the Earth's surface among other material thrown up by the impact, producing the layer of iridium-enriched clay. At the time, consensus was not settled on what caused the Cretaceous–Paleogene extinction and the boundary layer, with theories including a nearby supernova, climate change, or a geomagnetic reversal.The Alvarezes' impact hypothesis was rejected by many paleontologists, who believed that the lack of fossils found close to the K–Pg boundary—the "three-meter problem"—suggested a more gradual die-off of fossil species.

There is broad consensus that the Chicxulub impactor was an asteroid with a carbonaceous chondrite composition, rather than a comet. The impactor was around 10 kilometers (6.2 miles) in diameter—large enough that, if set at sea level, it would have reached taller than Mount Everest.

Asteroid deflection strategies

Various collision avoidance techniques have different trade-offs with respect to metrics such as overall performance, cost, failure risks, operations, and technology readiness. There are various methods for changing the course of an asteroid/comet. These can be differentiated by various types of attributes such as the type of mitigation (deflection or fragmentation), energy source (kinetic, electromagnetic, gravitational, solar/thermal, or nuclear), and approach strategy (interception, rendezvous, or remote station).

Strategies fall into two basic sets: fragmentation and delay. Fragmentation concentrates on rendering the impactor harmless by fragmenting it and scattering the fragments so that they miss the Earth or are small enough to burn up in the atmosphere. Delay exploits the fact that both the Earth and the impactor are in orbit. An impact occurs when both reach the same point in space at the same time, or more correctly when some point on Earth's surface intersects the impactor's orbit when the impactor arrives. Since the Earth is approximately 12,750 km in diameter and moves at approx. 30 km per second in its orbit, it travels a distance of one planetary diameter in about 425 seconds, or slightly over seven minutes. Delaying, or advancing the impactor's arrival by times of this magnitude can, depending on the exact geometry of the impact, cause it to miss the Earth.

"Project Icarus" was one of the first projects designed in 1967 as a contingency plan in case of collision with 1566 Icarus. The plan relied on the new Saturn V rocket, which did not make its first flight until after the report had been completed. Six Saturn V rockets would be used, each launched at variable intervals from months to hours away from impact. Each rocket was to be fitted with a single 100-megaton nuclear warhead as well as a modified Apollo Service Module and uncrewed Apollo Command Module for guidance to the target. The warheads would be detonated 30 meters from the surface, deflecting or partially destroying the asteroid. Depending on the subsequent impacts on the course or the destruction of the asteroid, later missions would be modified or cancelled as needed. The "last-ditch" launch of the sixth rocket would be 18 hours prior to impact.

Fiction

Asteroids and the asteroid belt are a staple of science fiction stories. Asteroids play several potential roles in science fiction: as places human beings might colonize, resources for extracting minerals, hazards encountered by spacecraft traveling between two other points, and as a threat to life on Earth or other inhabited planets, dwarf planets, and natural satellites by potential impact.

Sensory deprivation

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Sensory_deprivation

Sensory deprivation or perceptual isolation is the deliberate reduction or removal of stimuli from one or more of the senses. Simple devices such as blindfolds or hoods and earmuffs can cut off sight and hearing, while more complex devices can also cut off the sense of smell, touch, taste, thermoception (heat-sense), and the ability to know which way is down. Sensory deprivation has been used in various alternative medicines and in psychological experiments (e.g. with an isolation tank). When deprived of sensation, the brain attempts to restore sensation in the form of hallucinations.

Short-term sessions of sensory deprivation are described as relaxing and conducive to meditation; however, extended or forced sensory deprivation can result in extreme anxiety, hallucinations, bizarre thoughts, and depression.

A related phenomenon is perceptual deprivation, also called the Ganzfeld effect. In this case a constant uniform stimulus is used instead of attempting to remove the stimuli; this leads to effects which have similarities to sensory deprivation.

Sensory deprivation techniques were developed by some of the armed forces within NATO, as a means of interrogating prisoners within international treaty obligations. The European Court of Human Rights ruled that the use of the five techniques by British security forces in Northern Ireland amounted to a practice of inhuman and degrading treatment. It was also used in prisons such as Guantanamo.

Restricted environmental stimulation therapy (REST)

There are many different numbers of basic methods of restricted environmental stimulation, including therapy (REST), chamber REST, and flotation REST.

Chamber REST

In chamber REST, the subject lies on a bed in a completely dark and sound-reducing (on average, 80 dB) room for up to 24 hours. Their movement is restricted by the experimental instructions, but not by any mechanical restraints. Food, drink, and toilet facilities are provided in the room and are at the discretion of the tester, who can communicate with the participants using an open intercom. Subjects are allowed to leave the room before the 24 hours are complete; however, fewer than 10% actually do because they find the chamber so relaxing. Chamber REST affects psychological functioning (thinking, perception, memory, motivation, and mood) and psychophysiological processes.

Flotation REST

In flotation REST, the room contains a tank or pool. The flotation medium consists of a skin-temperature solution of water and Epsom salts at a specific gravity that allows for the patient to float supine without the worry of safety. In fact, to turn over while in the solution requires "major deliberate effort." Fewer than 5% of the subjects tested leave before the session duration ends, which is usually around an hour for flotation REST.

Spas sometimes provide commercial float tanks for use in relaxation. Flotation therapy has been academically studied in the US and in Sweden with published results showing reductions of both pain and stress. The relaxed state also involves lowered blood pressure, lowered levels of cortisol, and maximal blood flow. Besides physiological effects, REST seems to have positive effects on well-being and performance.

Chamber versus flotation REST

Several differences exist between flotation and chamber REST. For example, with the presence of a medium in flotation REST, the subject has reduced tactile stimulation while experiencing weightlessness. The addition of Epsom salts to attain the desired specific gravity may have a therapeutic effect on hypertonic muscles. Since one of the main results of chamber REST is a state of relaxation, the effects of chamber REST on arousal are less clear-cut, which can be attributed to the nature of the solution.

Also, due to the inherent immobilization that is experienced in flotation REST (by not being able to roll over), which can become uncomfortable after several hours, the subject is unable to experience the session durations of chamber REST. This may not allow the subject to experience the changes in attitudes and thinking that are associated with chamber REST. Additionally, the research questions asked between each technique are different. Chamber REST questions stemmed from research that began in the 1950s and explored a variety of questions about the need for stimulation, the nature of arousal, and its relationship with external stimulation. Practitioners in this area have explored its utility in the treatment of major psychiatric dysfunctions such as substance abuse. On the contrary, flotation REST was seen as more of a recreational tool as it was tested more for its use with stress-related disorders, pain reduction, and insomnia.

Numerous studies have debated which method is a more effective treatment process, however, only one has explored this statistically. Nineteen subjects, all of whom used chamber or flotation REST to induce relaxation or treat smoking, obesity, alcohol intake or chronic pain were analyzed. The statistic of interest, d, is a measure of the size of the treatment effect. For reference, d=0.5 is considered a moderate effect and d=0.8 a large effect. The 19 subjects who underwent chamber REST had d=0.53 and six flotation REST subjects showed d=0.33. Additionally, when examining subjects undergoing REST treatment and REST in conjunction with another treatment method, there was little difference. However, Flotation REST has the advantage of a lower duration required (45 minutes as opposed to 24 hours).

Sensory deprivation as a philosophical thought experiment

Sensory deprivation has been used to help support arguments by philosophers on how minds work. One example is the floating man argument proposed by Ibn Sīnā, whose primary objective is to affirm the existence of the human soul.

Floating man argument

Ibn Sīnā, one of the most important philosophers of the medieval period, investigated the existence of the self and explored the self's nature. Like many others, he proposed an argument to support his claim regarding the relationship between the mind and the body. He based his investigation on the Floating Man argument where, he proposes, a man floating in the air or a vacuum where he cannot perceive anything, not even the substance of air. This man is unable to see anything external; his arms and legs are separated from the rest of his body; they do not meet or touch. In other words, the man is experiencing extreme sensory deprivation in order to separate what physical body and any perception of stimuli that a person can experience from what consciousness might be in Ibn Sīnā's thought experiment. The man later reflects on his existence. He will not question that he exists, but he will not be able to affirm if his legs, arms, or internal organs exist. He guarantees that his essence exists, but he will not have awareness of the length or depth of himself. Therefore, in the thought experiment, what the man can affirm to exist is the man's self and what he cannot affirm does not make part of his essence, like an arm or a toe. The argument concludes then that since the man can affirm his existence while being subjected to extreme sensory deprivation, his soul is something different from his physical body. His soul is then said to be an immaterial substance separate from his body. This is considered a dualist argument in the philosophy of mind as it separates the mind from the body to affirm the existence of oneself.

Dualism

Dualism presupposes that the world is made up of physical (perceived through the senses), and immaterial (not perceived through the senses) substances.

René Descartes was the philosopher who proposed Cartesian dualism, also called substance dualism, since it claims the existence of two kinds of “substances”: mental states and material stuff that takes up space. For Descartes, the mind is an entity, different from a physical entity since the mind, in Descartes' point of view, can exist independently, that is, without a physical body. For this reason, he concluded that the mind is a substance.

Other uses

The use of REST has been explored in aiding in the cessation of smoking. In studies ranging between 12 months and five years, 25% of REST patients achieved long-term abstinence. REST, when combined with other effective smoking cessation methods (for example: behavior modification) resulted in long-term abstinence of 50%. Also, when combined with weekly support groups, REST resulted in 80% of patients achieving long-term abstinence. Comparatively, the use of a nicotine patch alone has a success rate of 5%.

Alcoholism has also been the target of research associated with REST. In conjunction with anti-alcohol educational messages, patients who underwent two hours of REST treatment reduced alcohol consumption by 56% in the first two weeks after treatment. The reduction in consumption was maintained during follow-ups conducted three and six months after the first treatment. It is, however, possible that this is caused by the placebo effect.

In addition, REST has been tested to determine its effect on users of other drugs. A University of Arizona study used chamber REST as a complement to traditional outpatient substance abuse treatment and found that four years later, 43% of the patients were still sober and drug-free. Eight months later, no one in the control group remained clean.

Psychedelic effects

Studies have been conducted to test the effect of sensory deprivation on the brain. One study took 19 volunteers, all of whom tested in the lower and upper 20th percentiles on a questionnaire that measures the tendency of healthy people to see things not really there, and placed them in a pitch-black, soundproof booth for 15 minutes, after which they completed another test that measures psychosis-like experiences, originally used to study recreational drug users. Five subjects reported seeing hallucinations of faces; six reported seeing shapes/faces not actually there; four noted a heightened sense of smell, and two reported sensing a "presence of evil" in the room. People who scored lower on the first test experienced fewer perceptual distortions; however, they still reported seeing a variety of hallucinations. Many studies have been conducted to understand the main causes of the hallucinations, and considerable evidence has been accumulated indicating that long periods of isolation aren't directly related to the level of experienced hallucinations.

Schizophrenics appear to tend to experience fewer hallucinations while in REST as compared to non-psychotic individuals. A possible explanation for this could be that non-psychotic individuals are normally exposed to a greater degree of sensory stimulation in everyday life, and in REST, the brain attempts to re-create a similar level of stimulation, producing the hallucinatory events. According to a 2009 study published in the Journal of Nervous and Mental Disease, the hallucinations are caused by the brain misidentifying the source of what it is currently experiencing, a phenomenon called faulty source monitoring. A study conducted on individuals who underwent REST while under the effects of Phencyclidine (PCP) showed a lower incidence of hallucination in comparison to participants who did not take PCP. The effects of PCP also appeared to be reduced while undergoing REST. The effects PCP has on reducing occurrences of hallucinatory events provide a potential insight into the mechanisms behind these events.

Interrogation

Sensory deprivation has been used to disorientate subjects during interrogation, brainwashing, and torture. In particular, the five techniques of wall-standing; hooding; subjection to noise; deprivation of sleep; deprivation of food and drink were used by the security forces in Northern Ireland in the early 1970s. After the Parker Report of 1972, these techniques were formally abandoned by the United Kingdom as aids to the interrogation of paramilitary suspects.

The Irish government on behalf of the men who had been subject to the five methods took a case to the European Commission of Human Rights (Ireland v. United Kingdom, 1976 Y.B. Eur. Conv. on Hum. Rts. 512, 748, 788-94 (European Commission of Human Rights)). The Commission stated that it "considered the combined use of the five methods to amount to torture."This consideration was overturned on appeal, when in 1978 the European Court of Human Rights (ECtHR) examined the United Nations' definition of torture. The court subsequently ruled that the five techniques "did not occasion suffering of the particular intensity and cruelty implied by the word torture," however they did amount "to a practice of inhuman and degrading treatment," which is a breach of the European Convention on Human Rights, Article 3.

In their judgment the court states that:

These methods, sometimes termed "disorientation" or "sensory deprivation" techniques, were not used in any cases other than the fourteen so indicated above. It emerges from the Commission's establishment of the facts that the techniques consisted of:

wall-standing: forcing the detainees to remain for periods of some hours in a stress position, described by those who underwent it as being "spreadeagled against the wall, with their fingers put high above the head against the wall, the legs spread apart and the feet back, causing them to stand on their toes with the weight of the body mainly on the fingers";
hooding: putting a black or navy colored bag over the detainees' heads and, at least initially, keeping it there all the time except during interrogation;
subjection to noise: pending their interrogations, holding the detainees in a room where there was a continuous loud and hissing noise;
deprivation of sleep: pending their interrogations, depriving the detainees of sleep;
deprivation of food and drink: subjecting the detainees to a reduced diet during their stay at the center and pending interrogation.

Threshold potential

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Threshold_potential

In electrophysiology, the threshold potential is the critical level to which a membrane potential must be depolarized to initiate an action potential. In neuroscience, threshold potentials are necessary to regulate and propagate signaling in both the central nervous system (CNS) and the peripheral nervous system (PNS).

Most often, the threshold potential is a membrane potential value between –50 and –55 mV, but can vary based upon several factors. A neuron's resting membrane potential (–70 mV) can be altered to either increase or decrease likelihood of reaching threshold via sodium and potassium ions. An influx of sodium into the cell through open, voltage-gated sodium channels can depolarize the membrane past threshold and thus excite it while an efflux of potassium or influx of chloride can hyperpolarize the cell and thus inhibit threshold from being reached.

Discovery

Initial experiments revolved around the concept that any electrical change that is brought about in neurons must occur through the action of ions. The German physical chemist Walther Nernst applied this concept in experiments to discover nervous excitability, and concluded that the local excitatory process through a semi-permeable membrane depends upon the ionic concentration. Also, ion concentration was shown to be the limiting factor in excitation. If the proper concentration of ions was attained, excitation would certainly occur. This was the basis for discovering the threshold value.

Along with reconstructing the action potential in the 1950s, Alan Lloyd Hodgkin and Andrew Huxley were also able to experimentally determine the mechanism behind the threshold for excitation. It is known as the Hodgkin–Huxley model. Through use of voltage clamp techniques on a squid giant axon, they discovered that excitable tissues generally exhibit the phenomenon that a certain membrane potential must be reached in order to fire an action potential. Since the experiment yielded results through the observation of ionic conductance changes, Hodgkin and Huxley used these terms to discuss the threshold potential. They initially suggested that there must be a discontinuity in the conductance of either sodium or potassium, but in reality both conductances tended to vary smoothly along with the membrane potential.

They soon discovered that at threshold potential, the inward and outward currents, of sodium and potassium ions respectively, were exactly equal and opposite. As opposed to the resting membrane potential, the threshold potential's conditions exhibited a balance of currents that were unstable. Instability refers to the fact that any further depolarization activates even more voltage-gated sodium channels, and the incoming sodium depolarizing current overcomes the delayed outward current of potassium. At resting level, on the other hand, the potassium and sodium currents are equal and opposite in a stable manner, where a sudden, continuous flow of ions should not result. The basis is that at a certain level of depolarization, when the currents are equal and opposite in an unstable manner, any further entry of positive charge generates an action potential. This specific value of depolarization (in mV) is otherwise known as the threshold potential.

Physiological function and characteristics

The threshold value controls whether or not the incoming stimuli are sufficient to generate an action potential. It relies on a balance of incoming inhibitory and excitatory stimuli. The potentials generated by the stimuli are additive, and they may reach threshold depending on their frequency and amplitude. Normal functioning of the central nervous system entails a summation of synaptic inputs made largely onto a neuron's dendritic tree. These local graded potentials, which are primarily associated with external stimuli, reach the axonal initial segment and build until they manage to reach the threshold value. The larger the stimulus, the greater the depolarization, or attempt to reach threshold. The task of depolarization requires several key steps that rely on anatomical factors of the cell. The ion conductances involved depend on the membrane potential and also the time after the membrane potential changes.

Resting membrane potential

The phospholipid bilayer of the cell membrane is, in itself, highly impermeable to ions. The complete structure of the cell membrane includes many proteins that are embedded in or completely cross the lipid bilayer. Some of those proteins allow for the highly specific passage of ions, ion channels. Leak potassium channels allow potassium to flow through the membrane in response to the disparity in concentrations of potassium inside (high concentration) and outside the cell (low). The loss of positive(+) charges of the potassium(K+) ions from the inside of the cell results in a negative potential there compared to the extracellular surface of the membrane. A much smaller "leak" of sodium(Na+) into the cell results in the actual resting potential, about –70 mV, being less negative than the calculated potential for K+ alone, the equilibrium potential, about –90 mV. The sodium-potassium ATPase is an active transporter within the membrane that pumps potassium (2 ions) back into the cell and sodium (3 ions) out of the cell, maintaining the concentrations of both ions as well as preserving the voltage polarization.

Depolarization

However, once a stimulus activates the voltage-gated sodium channels to open, positive sodium ions flood into the cell and the voltage increases. This process can also be initiated by ligand or neurotransmitter binding to a ligand-gated channel. More sodium is outside the cell relative to the inside, and the positive charge within the cell propels the outflow of potassium ions through delayed-rectifier voltage-gated potassium channels. Since the potassium channels within the cell membrane are delayed, any further entrance of sodium activates more and more voltage-gated sodium channels. Depolarization above threshold results in an increase in the conductance of Na sufficient for inward sodium movement to swamp outward potassium movement immediately.^[3] If the influx of sodium ions fails to reach threshold, then sodium conductance does not increase a sufficient amount to override the resting potassium conductance. In that case, subthreshold membrane potential oscillations are observed in some type of neurons. If successful, the sudden influx of positive charge depolarizes the membrane, and potassium is delayed in re-establishing, or hyperpolarizing, the cell. Sodium influx depolarizes the cell in attempt to establish its own equilibrium potential (about +52 mV) to make the inside of the cell more positive relative to the outside.

Variations

The value of threshold can vary according to numerous factors. Changes in the ion conductances of sodium or potassium can lead to either a raised or lowered value of threshold. Additionally, the diameter of the axon, density of voltage activated sodium channels, and properties of sodium channels within the axon all affect the threshold value. Typically in the axon or dendrite, there are small depolarizing or hyperpolarizing signals resulting from a prior stimulus. The passive spread of these signals depend on the passive electrical properties of the cell. The signals can only continue along the neuron to cause an action potential further down if they are strong enough to make it past the cell's membrane resistance and capacitance. For example, a neuron with a large diameter has more ionic channels in its membrane than a smaller cell, resulting in a lower resistance to the flow of ionic current. The current spreads quicker in a cell with less resistance, and is more likely to reach the threshold at other portions of the neuron.

The threshold potential has also been shown experimentally to adapt to slow changes in input characteristics by regulating sodium channel density as well as inactivating these sodium channels overall. Hyperpolarization by the delayed-rectifier potassium channels causes a relative refractory period that makes it much more difficult to reach threshold. The delayed-rectifier potassium channels are responsible for the late outward phase of the action potential, where they open at a different voltage stimulus compared to the quickly activated sodium channels. They rectify, or repair, the balance of ions across the membrane by opening and letting potassium flow down its concentration gradient from inside to outside the cell. They close slowly as well, resulting in an outward flow of positive charge that exceeds the balance necessary. It results in excess negativity in the cell, requiring an extremely large stimulus and resulting depolarization to cause a response.

Tracking techniques

Threshold tracking techniques test nerve excitability, and depend on the properties of axonal membranes and sites of stimulation. They are extremely sensitive to the membrane potential and changes in this potential. These tests can measure and compare a control threshold (or resting threshold) to a threshold produced by a change in the environment, by a preceding single impulse, an impulse train, or a subthreshold current. Measuring changes in threshold can indicate changes in membrane potential, axonal properties, and/or the integrity of the myelin sheath.

Threshold tracking allows for the strength of a test stimulus to be adjusted by a computer in order to activate a defined fraction of the maximal nerve or muscle potential. A threshold tracking experiment consists of a 1-ms stimulus being applied to a nerve in regular intervals. The action potential is recorded downstream from the triggering impulse. The stimulus is automatically decreased in steps of a set percentage until the response falls below the target (generation of an action potential). Thereafter, the stimulus is stepped up or down depending on whether the previous response was lesser or greater than the target response until a resting (or control) threshold has been established. Nerve excitability can then be changed by altering the nerve environment or applying additional currents. Since the value of a single threshold current provides little valuable information because it varies within and between subjects, pairs of threshold measurements, comparing the control threshold to thresholds produced by refractoriness, supernormality, strength-duration time constant or "threshold electrotonus" are more useful in scientific and clinical study.

Tracking threshold has advantages over other electrophysiological techniques, like the constant stimulus method. This technique can track threshold changes within a dynamic range of 200% and in general give more insight into axonal properties than other tests. Also, this technique allows for changes in threshold to be given a quantitative value, which when mathematically converted into a percentage, can be used to compare single fiber and multifiber preparations, different neuronal sites, and nerve excitability in different species.

"Threshold electrotonus"

A specific threshold tracking technique is threshold electrotonus, which uses the threshold tracking set-up to produce long-lasting subthreshold depolarizing or hyperpolarizing currents within a membrane. Changes in cell excitability can be observed and recorded by creating these long-lasting currents. Threshold decrease is evident during extensive depolarization, and threshold increase is evident with extensive hyperpolarization. With hyperpolarization, there is an increase in the resistance of the internodal membrane due to closure of potassium channels, and the resulting plot "fans out". Depolarization produces has the opposite effect, activating potassium channels, producing a plot that "fans in".

The most important factor determining threshold electrotonus is membrane potential, so threshold electrotonus can also be used as an index of membrane potential. Furthermore, it can be used to identify characteristics of significant medical conditions through comparing the effects of those conditions on threshold potential with the effects viewed experimentally. For example, ischemia and depolarization cause the same "fanning in" effect of the electrotonus waveforms. This observation leads to the conclusion that ischemia may result from over-activation of potassium channels.

Clinical significance

The role of the threshold potential has been implicated in a clinical context, namely in the functioning of the nervous system itself as well as in the cardiovascular system.

Febrile seizures

A febrile seizure, or "fever fit", is a convulsion associated with a significant rise in body temperature, occurring most commonly in early childhood. Repeated episodes of childhood febrile seizures are associated with an increased risk of temporal lobe epilepsy in adulthood.

With patch clamp recording, an analogous state was replicated in vitro in rat cortical neurons after induction of febrile body temperatures; a notable decrease in threshold potential was observed. The mechanism for this decrease possibly involves suppression of inhibition mediated by the GABA_B receptor with excessive heat exposure.

ALS and diabetes

Abnormalities in neuronal excitability have been noted in amyotrophic lateral sclerosis and diabetes patients. While the mechanism ultimately responsible for the variance differs between the two conditions, tests through a response to ischemia indicate a similar resistance, ironically, to ischemia and resulting paresthesias. As ischemia occurs through inhibition of the sodium-potassium pump, abnormalities in the threshold potential are hence implicated.

Arrythmia

Since the 1940s, the concept of diastolic depolarization, or "pacemaker potential", has become established; this mechanism is a characteristic distinctive of cardiac tissue. When the threshold is reached and the resulting action potential fires, a heartbeat results from the interactions; however, when this heartbeat occurs at an irregular time, a potentially serious condition known as arrythmia may result.

Use of medications

A variety of drugs can present prolongation of the QT interval as a side effect. Prolongation of this interval is a result of a delay in sodium and calcium channel inactivation; without proper channel inactivation, the threshold potential is reached prematurely and thus arrhythmia tends to result. These drugs, known as pro-arrhythmic agents, include antimicrobials, antipsychotics, methadone, and, ironically, antiarrhythmic agents. The use of such agents is particularly frequent in intensive care units, and special care must be exercised when QT intervals are prolonged in such patients: arrhythmias as a result of prolonged QT intervals include the potentially fatal torsades de pointes, or TdP.

Role of diet

Diet may be a variable in the risk of arrhythmia. Polyunsaturated fatty acids, found in fish oils and several plant oils, serve a role in the prevention of arrhythmias. By inhibiting the voltage-dependent sodium current, these oils shift the threshold potential to a more positive value; therefore, an action potential requires increased depolarization. Clinically therapeutic use of these extracts remains a subject of research, but a strong correlation is established between regular consumption of fish oil and lower frequency of hospitalization for atrial fibrillation, a severe and increasingly common arrythmia.

Mahdi

From Wikipedia, the free encyclopedia

The Mahdi (Arabic: ٱلْمَهْدِيّ, romanized: al-Mahdī, lit. 'the Guided') is a Final Leader in Islamic eschatology who is believed to appear at the end of times to rid the world of evil and injustice. He is said to be a descendant of Muhammad who will appear shortly before the Prophet ʿĪsā (Jesus Christ) and will lead the Muslims to rule the entire world.

Though the Mahdi is not referenced in the Quran, and is absent from several canonical compilations of hadith – including the two most-revered Sunni hadith collections: Sahih al-Bukhari and Sahih Muslim – he is mentioned in other hadith literature. Many Sunni theologians have therefore questioned the orthodoxy of the Mahdi. The doctrine of the mahdi seems to have gained traction during the confusion and unrest of the religious and political upheavals of the first and second centuries of Islam. Among the first references to the Mahdi appear in the late 7th century, when the revolutionary Mukhtar ibn Abi Ubayd (c. 622 – 687) declared Muhammad ibn al-Hanafiyya, a son of caliph Ali (r. 656–661), to be the Mahdi. Although the concept of a Mahdi is not an essential doctrine in Islam, it is popular among Muslims. It has been a part of the ʿaqīdah (creed) of Muslims for 1,400 years. Over centuries, there have been a vast number of Mahdi claimants.

The Mahdi features in both Shi'a and Sunni branches of Islam, though they differ extensively on his attributes and status. Among Twelver Shi'as, the Mahdi is believed to be Muhammad al-Mahdi, son of the eleventh Imam, Hasan al-Askari (d. 874), who is said to be in occultation (ghayba) by divine will. This is rejected by most Sunnis, who assert that the Mahdi has not been born yet.

Etymology

The term Mahdi is derived from the Arabic root h-d-y (ه-د-ي), commonly used to mean "divine guidance". Although the root appears in the Qur'an at multiple places and in various contexts, the word Mahdi never occurs in the book. The associated verb is hada, which means to guide. However, Mahdi can be read in active voice, where it means the one who guides, as well as passive voice, where it means the one who is guided. In the doctrinal sense, Mahdi is the title of the end-times eschatological redeemer in most Islamic sects.

Historical development

Pre-Islamic ideas

Some historians suggest that the term itself was probably introduced into Islam by southern Arabian tribes who had settled in Syria in the mid-7th century. They believed that the Mahdi would lead them back to their homeland and re-establish the Himyarite Kingdom. They also believed that he would eventually conquer Constantinople. It has also been suggested that the concept of the Mahdi may have been derived from earlier messianic Judeo-Christian beliefs. Accordingly, traditions were introduced to support certain political interests, especially anti-Abbasid sentiments. These traditions about the Mahdi appeared only at later times in ḥadīth collections such as Jami' at-Tirmidhi and Sunan Abu Dawud, but are absent from the early works of Bukhari and Muslim.

Origin

The term al-Mahdi was employed from the beginning of Islam, but only as an honorific epithet ("the guide") and without any messianic significance. As an honorific, it was used in some instances to describe Muhammad (by Hassan ibn Thabit), Abraham, al-Husayn, and various Umayyad caliphs (هداة مهديون, hudat mahdiyyun). During the Second Muslim Civil War (680–692), after the death of Mu'awiya I (r. 661–680), the term acquired a new meaning of a ruler who would restore Islam to its perfect form and restore justice after oppression. Abd Allah ibn al-Zubayr, who laid claim to the caliphate against the Umayyads and found temporary success during the civil war, presented himself in this role. Although the title Mahdi was not applied to him, his career as the anti-caliph significantly influenced the future development of the concept. A hadith was promulgated in which Muhammad prophesies the coming of a just ruler.

There will arise a difference after the death of a caliph, and a man of the people of Medina will go forth fleeing to Mecca. Then some of the people of Mecca will come to him and will make him rise in revolt against his will ... An expedition will be sent against him from Syria but will be swallowed up ... in the desert between Mecca and Medina. When the people see this, the righteous men ... of Syria and ... Iraq will come to him and pledge allegiance to him. Thereafter a man of the Quraysh will arise whose maternal uncles are of Kalb. He will send an expedition against them, but they will defeat them ... He will then divide the wealth and act among them according to the Sunna of their Prophet. Islam will settle down firmly on the ground ... He will stay seven years and then die, and the Muslims will pray over him.

Refusing to recognize the new caliph, Yazid I (r. 680–683), after Mu'awiya's death in 680, Ibn al-Zubayr had fled to the Meccan sanctuary. From there he launched anti-Umayyad propaganda, calling for a shura of the Quraysh to elect a new caliph. Those opposed to the Umayyads were paying him homage and asking for the public proclamation of his caliphate, forcing Yazid to send an army to dislodge him in 683. After defeating rebels in the nearby Medina, the army besieged Mecca but was forced to withdraw as a result of Yazid's sudden death shortly afterward. Ibn al-Zubayr was recognized caliph in Arabia, Iraq, and parts of Syria, where Yazid's son and successor Mu'awiya II (r. 683–684) held power in Damascus and adjoining areas. The hadith hoped to enlist support against an expected Umayyad campaign from Syria. The Umayyads did indeed send another army to Mecca in 692, but contrary to the hadith's prediction was successful in removing Ibn al-Zubayr. The hadith lost relevance soon afterward, but resurfaced in the Basran hadith circles a generation later, this time removed from its original context and understood as referring to a future restorer.

Around the time when Ibn al-Zubayr was trying to expand his dominion, the pro-Alid revolutionary al-Mukhtar al-Thaqafi took control of the Iraqi garrison town of Kufa in the name of Ali's son Muhammad ibn al-Hanafiyya, whom he proclaimed as the Mahdi in the messianic sense. The association of the name Muhammad with the Mahdi seems to have originated with Ibn al-Hanafiyya, who also shared the epithet Abu al-Qasim with Muhammad, the Islamic prophet. Among the Umayyads, the caliph Sulayman ibn Abd al-Malik (r. 715–717) encouraged the belief that he was the Mahdi, and other Umayyad rulers, like Umar II (r. 717–720), have been addressed as such in the panegyrics of Jarir (d. 728) and al-Farazdaq (d. 728–730).

Early discussions about the identity of the Mahdi by religious scholars can be traced back to the time after the Second Fitna. These discussions developed in different directions and were influenced by traditions (hadith) attributed to Muhammad. In Umayyad times, scholars and traditionists not only differed on which caliph or rebel leader should be designated as Mahdi but also on whether the Mahdi is a messianic figure and if signs and predictions of his time had been satisfied. In Medina, among the conservative religious circles, the belief in Umar II being the Mahdi was widespread. Said ibn al-Musayyib (d. 715) is said to identify Umar II as the Mahdi long before his reign. The Basran, Abu Qilabah, supported the view that Umar II was the Mahdi. Hasan al-Basri (d. 728) opposed the concept of a Muslim Messiah but believed that if there was the Mahdi, it was Umar II.

By the time of the Abbasid Revolution in 750, Mahdi was already a known concept. Evidence shows that the first Abbasid caliph Saffah (r. 750–754) assumed the title of "the Mahdi" for himself.

Shi'a Islam

In Shi'a Islam, the eschatological Mahdi was commonly given the epithet al-Qa'im (القائم),which can be translated as 'he who will rise,' signifying his rise against tyranny in the end of time. Distinctively Shi'a is the notion of temporary absence or occultation of the Mahdi, whose life has been prolonged by divine will. An intimately related Shi'a notion is that of raj'a (lit. 'return'), which often means the return to life of (some) Shi'a Imams, particularly Husayn ibn Ali, to exact their revenge on their oppressors.

Traditions that predicted the occultation and rise of a future imam were already in circulation for a century before the death of the eleventh Imam in 260 (874 CE), and possibly as early as the seventh-century CE. These traditions were appropriated by various Shi'a sects in different periods, including the now-extinct sects of Nawusites and Waqifites. For instance, these traditions were cited by the now-extinct Kaysanites, who denied the death of Ibn al-Hanafiyya, and held that he was in hiding in the Razwa mountains near Medina. This likely originated with two groups of his supporters, namely, southern Arabian settlers and local recent converts in Iraq, who seem to have spread the notions now known as occultation and raj'a. Later on, these traditions were also employed by the Waqifites to argue that Musa al-Kazim, the seventh Imam, had not died but was in occultation.

In parallel, traditions predicting the occultation of a future imam also persisted in the writings of the mainstream Shi'a, who later formed the Twelvers. Based on this material, the Twelver doctrine of occultation crystallized in the first half of the fourth (tenth) century, in the works of Ibrahim al-Qummi (d. 919), Ya'qub al-Kulayni (d. 941), and Ibn Babawayh (d. 991), among others. This period also saw a transition in Twelver arguments from a traditionist to a rationalist approach in order to vindicate the occultation of the twelfth Imam.

The Twelver authors also aim to establish that the description of Mahdi in Sunni sources applies to the twelfth Imam. Their efforts gained momentum in the seventh (thirteenth) century when some notable Sunni scholars endorsed the Shi'a view of the Mahdi,including the Shafi'i traditionist Muhammad ibn Yusuf al-Gandji. Since then, Amir-Moezzi writes, there is Sunni support from time to time for the Twelvers' view of Mahdi. There has also been some support for the mahdiship of the twelfth Imam in Sufi circles, for instance, by the Egyptian Sufi al-Sha'rani.

Before the rise of the Fatimid Caliphate, as a major Isma'ili Shi'a dynasty, the terms Mahdi and Qa'im were used interchangeably for the messianic imam anticipated in Shi'a traditions. With the rise of the Fatimids in the tenth century CE, however, al-Qadi al-Nu'man argued that some of these predictions had materialized by the first Fatimid caliph, Abdallah al-Mahdi Billah, while the rest would be fulfilled by his successors. Henceforth, their literature referred to the awaited eschatological imam only as Qa'im (instead of Mahdi). In Zaydi view, imams are not endowed with superhuman qualities, and expectations for their mahdiship are thus often marginal. One exception is the now-extinct Husaynites in Yemen, who denied the death of al-Husayn ibn al-Qasim al-Iyani and awaited his return.

In Islamic doctrine

Sunni Islam

In Sunni Islam, the Mahdi doctrine is not theologically important and remains as a popular belief instead. Of the six canonical Sunni hadith compilations, three—Abu Dawood, Ibn Maja, and Tirmidhi—contain traditions on the Mahdi; the compilations of Bukhari and Muslim—considered the most authoritative by the Sunnis and the earliest of the six—do not, nor does Nasai. Some Sunnis, including the philosopher and historian Ibn Khaldun (d. 1406), and reportedly also Hasan al-Basri (d. 728), an influential early theologian and exegete, deny the Mahdi being a separate figure, holding that Jesus will fulfill this role and judge over mankind; Mahdi is thus considered a title for Jesus when he returns.Others, like the historian and the Qur'an commentator Ibn Kathir (d. 1373), elaborated a whole apocalyptic scenario which includes prophecies about the Mahdi, Jesus, and the Dajjal (the antichrist) during the end times.

The common opinion among the Sunnis is that the Mahdi is an expected ruler to be sent by God before the end times to re-establish righteousness. He is held to be from among the descendants of Muhammad through his daughter Fatima and her husband Ali, and his physical characteristics including a broad forehead and curved nose. He will eradicate injustice and evil from the world. He will be from the Hasanid branch of Muhammad's descendants, as opposed to the Shi'a belief that he is of the Husaynid line. The Mahdi's name would be Muhammad and his father's name would be Abd Allah. Abu Dawood quotes Muhammad as saying: "The Mahdi will be from my family, from the descendants of Fatimah". Another hadith states:

Even if only one day remains [until the doomsday], God will lengthen this day until He calls forth a man from me, or from the family of my house, his name matching mine and his father's name matching that of my father. He will fill the Earth with equity and justice just as it had previously been filled with injustice and oppression.

Before the arrival of the Mahdi, the earth would be filled with anarchy and chaos. Divisions and civil wars, moral degradation, and worldliness would be prevalent among the Muslims. Injustice and oppression would be rampant in the world. In the aftermath of the death of a king, the people would quarrel among themselves, and the as yet unrecognized Mahdi would flee from Medina to Mecca to take refuge in the Ka'ba. He would be the Mahdi recognized as ruler by the people. The Dajjal would appear and will spread corruption in the world. With an army bearing black banners, which would come to his aid from the east, the Mahdi would fight the Dajjal, and will be able to defeat him. Dressed in saffron robes with his head anointed, Jesus would descend at the point of a white minaret of the Umayyad Mosque in eastern Damascus (believed to be the Minaret of Isa) and join the Mahdi. Jesus would pray behind the Mahdi and then kill the Dajjal. The Gog and Magog would also appear wreaking havoc before their final defeat by the forces of Jesus. Although not as significant as the Dajjal and the Gog and Magog, the Sufyani, another representative of the forces of dark, also features in the Sunni traditions. He will rise in Syria before the appearance of Mahdi. When the latter appears, the Sufyani, along with his army, will either be swallowed up en route to Mecca by the earth with God's command or defeated by the Mahdi. Jesus and the Mahdi will then conquer the world and establish caliphate. The Mahdi will die after 7 to 13 years, whereas Jesus after 40 years. Their deaths would be followed by reappearance of corruption before the final end of the world.

Shia Islam

Twelver

In Twelver Shi'ism, the largest Shi'i branch, the belief in the messianic imam is not merely a part of creed, but the pivot. For the Twelver Shi'a, the Mahdi was born but disappeared, and would remain hidden from humanity until he reappears to bring justice to the world in the end of time, a doctrine known as the Occultation. This imam in occultation is the twelfth imam, Muhammad, son of the eleventh imam, Hasan al-Askari. According to the Twelvers, the Mahdi was born in Samarra around 868, though his birth was kept hidden from the public. He lived under his father's care until 874 when the latter was killed by the Abbasids.

Minor Occultation

When his father died in 874, possibly poisoned by the Abbasids, the Mahdi went into occultation by the divine command and was hidden from public view for his life was in danger from the Abbasids. Only a few of the elite among the Shi'a, known as the deputies (سفراء, sufara; sing. سفير safir) of the twelfth imam, were able to communicate with him; hence the occultation in this period is referred to as the Minor Occultation (ghayba al-sughra).

The first of the deputies is held to have been Uthman ibn Sa'id al-Amri, a trusted companion and confidant of the eleventh imam. Through him the Mahdi would answer the demands and questions of the Shi'a. He was later succeeded by his son Muhammad ibn Uthman al-Amri, who held the office for some fifty years and died in 917. His successor Husayn ibn Rawh al-Nawbakhti was in the office until his death in 938. The next deputy, Ali ibn Muhammad al-Simari, abolished the office on the orders of the imam just a few days before his death in 941.

Major Occultation

With the death of the fourth agent, thus began the Major Occultation (الغيبة الكبرى, ghayba al-kubra), in which the communication between the Mahdi and the faithful was severed. The leadership vacuum in the Twelver community was gradually filled by jurists. During the Major Occultation, the Mahdi roams the earth and is sustained by God. He is the lord of the time (صاحب الزمان sahib az-zamān) and does not age. Although his whereabouts and the exact date of his return are unknown, the Mahdi is nevertheless believed to contact some of his Shi'a if he wishes. The accounts of these encounters are numerous and widespread in the Twelver community. Shi'a scholars have argued that the longevity of the Mahdi is not unreasonable given the long lives of Khidr, Jesus, and the Dajjal, as well as secular reports about long-lived men. Along these lines, Tabatabai emphasizes the miraculous qualities of al-Mahdi, adding that his long life, while unlikely, is not impossible. He is viewed as the sole legitimate ruler of the Muslim world and the constitution of the Islamic Republic of Iran recognizes him as the head of the state.

Reappearance

Before his reappearance (ظهور, Thuhur), the world will plunge into chaos, where immorality and ignorance will be commonplace, the Qur'an will be forgotten, and religion will be abandoned. There will be plagues, earthquakes, floods, wars and death. The Sufyani will rise and lead people astray. The Mahdi will then reappear in Mecca, with the sword of Ali (dhu'l-fiqar) in his hand, between the corner of the Ka'ba and the station of Abraham.

By some accounts, he will reappear on the day of ashura (tenth of Muharram), the day the third Shi'a imam Husayn ibn Ali was slain. He will be "a young man of medium stature with a handsome face," with black hair and beard. A divine cry will call the people of the world to his aid, after which the angels, jinns, and humans will flock to the Mahdi. This is often followed shortly by another supernatural cry from the earth that invites men to join the enemies of the Mahdi, and would appeal to disbelievers and hypocrites.

The Mahdi will then go to Kufa, which will become his capital, and send troops to kill the Sufyani in Damascus. Husayn and his slain partisans are expected to resurrect to avenge their deaths, known as the doctrine of raj'a (lit. 'return'). The episode of Jesus' return in the Twelver doctrine is similar to the Sunni belief, although in some Twelver traditions it is the Mahdi who would kill the Dajjal. Those who hold enmity towards Ali ibn Abi Talib (ناصبيّ, nasibis) will be subject to jizya (poll tax) or killed if they do not accept Shi'ism.

The Mahdi is also viewed as the restorer of true Islam, and the restorer of other monotheistic religions after their distortion and abandonment. He establishes the kingdom of God on earth and Islamizes the whole world. In their true form, it is believed, all monotheistic religions are essentially identical to Islam as "submission to God." It is in this sense, according to Amir-Moezzi, that one should understand the claims that al-Mahdi will impose Islam on everyone. His rule will be paradise on earth, which will last for seventy years until his death, though other traditions state 7, 19, or 309 years.

Isma'ilism

In Isma'ilism a distinct concept of the Mahdi developed, with select Isma'ili Imams representing the Mahdi or al-Qa'im at various times. When the sixth Shi'a imam Ja'far al-Sadiq died, some of his followers held his already dead son Isma'il ibn Ja'far to be the imam asserting that he was alive and will return as the Mahdi. Another group accepted his death and acknowledged his son Muhammad ibn Isma'il as the imam instead. When he died, his followers too denied his death and believed that he was the last imam and the Mahdi. By the mid-9th century, Isma'ili groups of different persuasions had coalesced into a unified movement centered in Salamiyya in central Syria, and a network of activists was working to collect funds and amass weapons for the return of the Mahdi Muhammad ibn Isma'il, who would overthrow the Abbasids and establish his righteous caliphate. The propaganda of the Mahdi's return had a special appeal to peasants, Bedouins, and many of the later-to-be Twelver Shi'is, who were in a state of confusion (hayra) in the aftermath of the death of their 11th imam Hasan al-Askari, and resulted in many conversions.

In 899, the leader of the movement, Sa'id ibn al-Husayn, declared himself the Mahdi. This brought about schism in the unified Isma'ili community as not all adherents of the movement accepted his Mahdist claims. Those in Iraq and Arabia, known as Qarmatians after their leader Hamdan Qarmat, still held that Muhammad ibn Isma'il was the awaited Mahdi and denounced the Salamiyya-based Mahdism.In the Qarmati doctrine, the Mahdi was to abrogate the Islamic law (the Sharia) and bring forth a new message. In 931, the then Qarmati leader Abu Tahir al-Jannabi declared a Persian prisoner named Abu'l-Fadl al-Isfahani as the awaited Mahdi. The Mahdi went on to denounce Moses, Jesus, and Muhammad as liars, abolished Islam, and instituted the cult of fire. Abu Tahir had to depose him as imposter and had him executed.

Meanwhile, in Syria, Sa'id ibn al-Husayn's partisans took control of the central Syria in 903, and for a time the Friday sermon was read in the name of the "Successor, the rightly-guided Heir, the Lord of the Age, the Commander of the Faithful, the Mahdi". Eventually, the uprising was routed by the Abbasids. This forced Sa'id to flee from Syria to North Africa, where he founded the Fatimid Caliphate in Ifriqiya in 909. There he assumed the regnal name al-Mahdi Billah; as the historian Heinz Halm comments, the singular, semi-divine figure of the Mahdi was thus reduced to an adjective in a caliphal title, 'the Imam rightly guided by God' (al-imam al-mahdi bi'llah): instead of the promised messiah, al-Mahdi presented himself merely as one in a long sequence of imams descending from Ali and Fatima.

Messianic expectations associated with the Mahdi nevertheless did not materialize, contrary to the expectations of his propagandists and followers who expected him to do wonders. Al-Mahdi attempted to downplay messianism and asserted that the propaganda of Muhammad ibn Isma'il's return as the Mahdi had only been a ruse to avoid Abbasid persecution and protect the real imam predecessors of his. The Mahdi was actually a collective title of the true imams from the progeny of Ja'far al-Sadiq. In a bid to gain time, al-Mahdi also sought to shift the messianic expectations on his son, al-Qa'im: by renaming himself as Abdallah Abu Muhammad, and his son as Abu'l-Qasim Muhammad rather than his original name, Abd al-Rahman, the latter would bear the name Abu'l-Qasim Muhammad ibn Abdallah. This was the name of the Islamic prophet Muhammad, and it hand been prophesied that the Mahdi would also bear it. The Fatimids eventually dropped the millenarian rhetoric.

The Tayyibi Musta'li Isma'ili Shi'ah believe that their Occulted Imam and Mahdi is Abu'l-Qasim al-Tayyib, son of the Fatimid Caliph Al-Amir bi-Ahkam Allah.

Zaydism

In Zaydism, the concept of imamate is different from the Isma'ili and Twelver branches; a Zaydi Imam is any respectable person from the descendants of Ali and Fatima who lays claim to political leadership and struggles for its acquisition. As such, the Zaydi imamate doctrine lacks eschatological characteristics and there is no end-times redeemer in Zaydism. The title of mahdi has been applied to several Zaydi imams as an honorific over the centuries.

Ahmadiyya belief

In the Ahmadiyya belief, the prophesied eschatological figures of Christianity and Islam, the Messiah and Mahdi, actually refer to the same person. These prophecies were fulfilled in Mirza Ghulam Ahmad (1835–1908), the founder of the movement; he is held to be the Mahdi and the manifestation of Jesus. However, the historical Jesus in their view, although escaped crucifixion, nevertheless died and will not be coming back. Instead, God made Mirza Ghulam Ahmad the exact alike of Jesus in character and qualities. Similarly, the Mahdi is not an apocalyptic figure to launch global jihad and conquer the world, but a peaceful mujaddid (renewer of religion), who spreads Islam with "heavenly signs and arguments".

Mahdi claimants

Throughout history, various individuals have claimed to be or were proclaimed to be the Mahdi. Claimants have included Muhammad Jaunpuri, the founder of the Mahdavia sect; Ali Muhammad Shirazi, the founder of Bábism; Muhammad Ahmad, who established the Mahdist State in Sudan in the late 19th century. The Iranian dissident Massoud Rajavi, the leader of the MEK, also claimed to be a 'representative' of the Mahdi. The adherents of the Nation of Islam hold Wallace Fard Muhammad, the founder of the movement, to be the Messiah and the Mahdi. Adnan Oktar, a Turkish cult leader, is considered by his followers as the Mahdi.

Ibn Khaldun noted a pattern where embracing a Mahdi claimant enabled unity among tribes and/or a region, often enabled them to forcibly seize power, but the lifespan of such a force was usually limited, as their Mahdi had to conform to hadith prophesies—winning their battles and bringing peace and justice to the world before Judgement Day—which (so far) none have.

Comparative religion

Buddhism

The Mahdi figure in Islam can be likened to the Maitreya figure of Buddhism. Both are prophesied saviors sharing a messianic-like quality, and both are predicted to exert a form of world rulership.

Judaism

The prophesied savior duo of the Mahdi and the Messiah (Jesus) in Islam can be likened to the prophesied pair of the two Jewish savior figures, Mashiach ben Yosef and Mashiach ben David, respectively, in the sense that the Islamic Messiah and Masiach ben David take a central eschatological role, while the Mahdi and Mashiach ben Yosef take a peripheral role.

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Tuesday, October 3, 2023

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Mahdi

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In Islamic doctrine

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Minor Occultation

Major Occultation

Reappearance

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