Theory 
Asteroids| CONTENT - All about asteroids |
Asteroids began as minor planets in 1801 as Father Gioacchino Giuseppe Maria Ubaldo Nicolò Piazzi, astronomer to the King of Two Sicilies, discovered Ceres. Piazzi had followed up on mathematical predictions that there should be a planet between Mars and Jupiter. Piazzi called his discovery Ceres after the Roman goddess of harvests and corn. She also was considered the patron goddess of Sicily. Further similar bodies were found in the same region and their number quickly amounted to several hundred by 19th century's end. A half-century after Ceres' detection, there were only 15 known asteroids as with astronomers' equipment, techniques and interest in hunting asteroids, by 1868 the number had reached 100. By 1923 it was 1,000. They eventually became known as asteroids, or minor planets. By 1850, astronomers decided to demote the minor planets to 'asteroids' (a Greek term for 'starlike'). Today, it is more than half a million. Most of these minor planets are located between Mars and Jupiter orbits in what is called the asteroid belt. Jupiter generally is parting the solar system with respect to the dust into a inner and an outer system. Further objects were later found in other solar system locations. Both terms "minor planet" and "asteroid" are similar when used about these bodies. A new usage among astronomers is to name asteroids both by their number and their name (both being attributed and managed by the International Astronomical Union (IAU)), bringing, for example, to "1 Ceres", instead of "Ceres" only. The number of the asteroids or minor planets refers to the chronology of the discovery of those
 |  | Ceres and Vesta compared in size to other bodies of the solar system (left), Ceres, compared to the size of Florida (right). picture site 'Amateur Astronomy' based upon a picture NASA (left), picture site 'Amateur Astronomy' (right) |
Most asteroids are found in what is called the asteroid belt. The asteroid belt is located between Mars and Jupiter orbits. Asteroids found there are thought to have originated like planetesimals -those blocks of materials which accreted to form planets. Those planetesimals were prevented from accreting further by Jupiter gravitational influence when the gas giant formed. Further collisions between them eventually produced this asteroids' flurry which is populating the asteroid belt nowaday. All the asteroids in the asteroid belt combined, however, represent a mass equivalent to half our Moon only. It might that the migration of Jupiter and Saturn outwards, some 4 billion years ago, might have sent bodies from the Kuiper Belt into the inner solar system, where they mingled with the existing asteroid belt. Such bodies are more like primitive comets than rocks, which composes the majority of the components of the asteroid belt. The asteroid belt is featuring the so-called 'Kirkwood gaps', which were discovered in the 1860's and which are gaps of a lesser density of asteroids as they were created through the gravitational interactions occurring with the gas giant planets of the solar system. The side of the Kirkwood gaps located on the side of the gas giants is less populated in asteroids still. Some more asteroids exist. 20 so-called Atira asteroids have their orbits entirely inside that of Earth's due to Jupiter's gravitational influence. Theorized Vatira asteroids are those have their orbites inside Venus or Mercury's. One, 2020 AV2, was found in January 2020. Vatiras more likely it will end up crashing on one of those two planets as some likely were thrown there after a gravitional encounter with some other planet . Centaurs (best known is Chiron) are objects orbiting about the gas giants and the Kuiper Belt. Most centaurs are comets as astronomers were not certain whether centaurs are asteroids flung out from the inner solar system or comets traveling in toward the Sun from afar, so their dual nature awared them their name from the creature in Greek mythology whose head and torso are human and legs are those of a horse. Centaurs are in fact coming from deeper out in the solar system, thus they may have been active in the past, and may be active again in the future. Centaurs and 'scattered disk objects' orbit in an unstable belt. Ultimately, gravity from the giant planets will fling them either closer to the Sun, turning them maybe into short-period comets on time scales of 1 to 100 million years, or farther away from their current locations. Centaurs generally to be either blue-gray or reddish in hue as most of the blue-gray objects are dark, a telltale sign of comets and areddish object is more likely to be an asteroid. Trojans are asteroids located at a planet's two Lagrangian points hence preceding and trailing it, each side of it. They mostly exist around Jupiter as they were found too at Neptune, Mars or even two Saturn's moons. More recently, stress has been put about asteroids whose perihelion (nearest point of their orbit) or whose orbit itself is close to Earth, that is Near-Earth Asteroids (NEAs). The general public better knows the NEOs (Near-Earth Objects) which encompass both asteroids and comets with such hazardous trajectories. Generally speaking known asteroids are now numbering about several hundred thousands, and thousands more are discovered each year. This is due to increase as NEOs research programs will spot more (they will be smaller and smaller in size). A question is that of knowing what the accurate nature of the objects which are populating the Kuiper Belt. The Kuiper Belt is, too, an area which is harbouring the leftovers from the formation of the solar system. Two questions however are still remaining, like whether all the objects orbiting there may be called asteroids, or what the proportion is, among the objects of the Kuiper Belt, of large-dimensions planetoid bodies, like Quaoar, or Varuna, which already have been discovered there. S-type asteroid C/2014 S3 (PANSTARRS) is the first object to be discovered on a long-period cometary orbit that has the characteristics of a pristine inner Solar System asteroid. It is one of the potential building blocks of the rocky planets, such as the Earth, that was expelled from the inner Solar System and preserved in the deep freeze of the Oort Cloud for billions of years. C/2014 S3 (PANSTARRS) was dubbed the 'first rocky comet.' Tailless long-period comets are dubbed a Manx comet, after the tailless cat. Some are asteroids
 | click to the location of the asteroid belt in the solar system |
check a table of data about asteroids
 | The 21-mile (34-km) wide asteroid Eros as seen for its southern hemisphere by the Near Earth Asteroid Rendezvous (NEAR) mission in 2001; the southern 'pole" of the asteroid is at the top. picture NASA |
Asteroids comes in a variety, with some rubble piles of loosely held gravel or others mostly made of iron and all sorts in-between. Iron-rich olivine is a constituent of large asteroids that have undergone more heating, or a differentiation process. Asteroids are rotating on themselves on the order of days or hours as they are very cold or hundreds of degrees below zero. Different parts of a asteroid have different densities, which may result from the merger of two colliding objects as that is hinting to the mechanisms of collisions between asteroids or how the planets form. When the shape of the asteroid is very irregular the heat is not radiated evenly and this creates a tiny, but continuous, torque on the body and changes its spin rate. Most asteroids have a Moon-like surface with craters and regolith, this rock dust yielded through the impacts at the surface. Some have been found binary or having a moon of their own. Dactyl, for example, was found about Ida or asteroid 24 Themis is coming along with two small fragments one inert and the other with a comet-like tail. 16 percent of asteroids are triple or binary systems. How common asteroid binary systems are in the asteroid belt remains ill-understood. The most probable formation scenario of such asteroids is a breakup due to fast rotation. Sublimation of the asteroids' surface yields a tail along with getting both constituents further from each other. Most asteroids are under 210 mi wide (340 km)and the vast majority 60-mile (100-kilometer)-wide or smaller. Largest and better known are Ceres, Vesta, Pallas, and Juno. Ceres, now termed a 'dwarf planet' by the International Astronomical Union, is about 590 mi wide (960 km), by far the largest and most massive body in the asteroid belt, and representing almost one third of the total mass of it. Ceres is round, because it's large enough for gravity to mold its shape into a sphere. One day on Ceres lasts 9 hours as the asteroid is nearly round is shape. The interior of Ceres might have been differentiaited -layered- like those of terrestrial planets. It might feature a rocky inner core, a icy mantle, and a thin, dusty outer crust. Ceres is featuring too a bright spot, brighter than its surroundings yet still very dark, and with a feeble albedo. That spot is remaining a mystery. The larger minor planets are roughly spherical, as smaller asteroids are usually irregular in shape. As water was long thought a rarity in the Asteroid Belt due to its inhabitants beeing too close to Sun, seemingly large asteroid 24 Themis was found with water-ice and carbon-based organic compounds on the surface, adding credibility to that asteroids, like comets, might have played a role into the beginnings of life at Earth or other celestial bodies. 24 Themis however is the largest member of a group of smaller asteroids that sometimes behave like comets. More interesting is the fact that, with ice still extant at the surface of the asteroid, it likely is replenishing from a source under the crust of it since the early history of the solar system. By early 2014, water vapor was found at Ceres, with plumes of water vapor thought to shoot up periodically when portions of its icy surface warm slightly, as, when Ceres swings through the part of its orbit that is closer to the Sun, a portion of its icy surface becomes warm enough to cause water vapor to escape in plumes. Such plumes look to preferentially emanate from two dark spots on the minor planet. Astronomers now think that Vesta, as it is so much larger than the other asteroids in the Asteroid Belt, likely is to be considered a evolved object as it looks like it is featuring a layered structure (core, mantle and crust) which makes it more like terrestrials planets like the Earth or Mars. Like a planet, Vesta had sufficient radioactive material inside when it coalesced, releasing heat that melted rock allowing the 'differentiation' process endured by primitive planets when heavier rocks sank like a core and lighter ones formed a crust. Officially, Vesta is a 'minor planet,' that other name for a asteroid, a body that orbits the Sun but is not a proper planet (nor comet). Vesta may not be classified a dwarf planet, that recent entry from the International Astronomical Union (IAU) for objects in the solar system, like Ceres, for example, as it is not quite large enough. Scientists thus prefer to think of Vesta as a protoplanet (or a celestial body that almost formed into a planet) because it is a dense, layered body that orbits the Sun. It began in the same fashion as Mercury, Venus, Earth and Mars, but somehow never fully developed. As the current planets in the solar system became such by merging with other Vesta-sized objects, Vesta never found a partner and the critical time beyond the early epochs of the solar system passed. That might have had to do with the nearby presence of Jupiter which gravity disturbed the orbits of objects. Other, smaller space rocks however have collided with Vesta and knocked off bits of it whichbecame debris in the asteroid belt known as Vestoids, and even hundreds of meteorites that have ended up on Earth. As a result, Vesta is a time capsule from that earlier era, with its surface possibly the oldest planetary surface in the solar system
 | click to a picture illustrating the variety of asteroids (data in metric units). picture NASA |
Collisions may still occur in the Asteroid Belt like the one which affected 70-mile wide asteroid (596) Scheila in 2011, when hit by a 100-foot wide rock at a speed of 11,000 mph which created a crater of about 1,000 ft across. Scheila had become twice as bright as expected and immersed in a faint comet-like glow which lasterd about 1 month or 2. Such collisions between asteroids create rock fragments, from fine dust to huge boulders, that impact planets and their moons in the solar system. Collisions generally may either stick two asteroids together or shatter one of both. Collisions in the Asteroid Belt are destructive because objects?are traveling fast when they smash together, bringing to swarms of debris. Astronomers usually regarded asteroids as inactive rocks whose destinies, surfaces, shapes and sizes were determined by mutual impacts. However, this simple picture has grown more complex over the past few years. During certain parts of their orbits, some objects, once categorized as asteroids, clearly develop comet-like features that can last for many months. Others display much shorter outbursts. Icy materials further may be occasionally exposed, either by internal geological processes or by an external one, such as an impact. The process through which lunar dust is seen trapped by local magnetic fields in small regions of intense light contrast with long shadows at the terminator could also be found at asteroids and be a fundamental process there. This model provides a natural explanation for the observation of dust ponds inside craters on the asteroid Eros. Collisional events further allows astronomers to determine the inner composition of asteroids, which may, or may not, hold ice like comets. A 'asteroid family' is formed when a collision breaks apart a large parent body into fragments of various sizes. Some collisions leave giant craters. For example, the asteroid Vesta's southern hemisphere was excavated by two large impacts. Other smash-ups are catastrophic, shattering an object into numerous fragments, as was the case with the Eos asteroid family. The cast-off pieces move together in packs, traveling on the same path around the Sun, but over time the pieces become more and more spread out. Euphrosyne asteroids are asteroids distributed at the outer edge of the asteroid belt with a unusual orbital path well above the ecliptic. The asteroid after which they are named, Euphrosyne -for a ancient Greek goddess of mirth - is about 156 miles (260 kilometers) across and is one of the 10 largest asteroids in the main belt. Current-day Euphrosyne is thought to be a remnant of a massive collision about 700 million years ago that formed that family of smaller asteroids bearing its name, one of the last great collisions in the solar system. Following a protracted study in the 2010's, 38,000 main belt asteroids out of the approximately 600,000 known can be assigned to 76 families, 28 of which were new. Asteroid families are usually determined from observations of their orbits, and recently from their reflectivity. Asteroids in the same family generally have similar mineral composition and reflect similar amounts of light
Asteroids have been sorted into three spectral classes. Asteroids generally are seemingly bright, with a luminosity equivalent to that of Moon as comets are darker due to their dark, soot-like coating on their icy surfaces. In the Asteroid belt further are to be found metallic asteroids (likely originating from the region of the telluric planets' formation), carbonated asteroids (in the region of formation of the gas giants), the S type asteroids, rich in silica (which originate from the region between Mars and Jupiter) and eventually D type asteroids (which come from the Kuiper belt; it is possible that the D type asteroids, which are been born at the edge of the solar system can be dislodged and drift down to the Asteroid belt). Interplanetary dust particles (IDPs) blasted into space by asteroid collisions are a important source of material accumulated on the surfaces of other asteroids:
Asteroids are too at the origins of some of the meteorites raining on Earth, like Juno, for example, one of the first asteroids discovered. As the asteroid is mostly composed of hardy silicate rock and tough enough that fragments broken off by collisions can often survive a trip through Earth's atmosphere. The L-chondrite meteorites constitute 20 percent of the meteorites today, as they originate from a dramatic collision, 470 million years ago, in the asteroid belt between two massive bodies. A rare type of meteorites is extant, called ureilites, which represent less than 10 of the nearly 1,000 known meteorites, mostly belonging to the more common type of meteorites called chondrites. Carbonaceous chondrites have bulk water contents of 15 to 20 percent as altered by it. Ureilites are containing varying amounts of the minerals called olivine and pyroxene. Scientists believe this is evidence all ureilites originated from the same source, called the ureilite parent body which likely experienced a giant collision approximately 4.5 billion years ago and caused iron-rich minerals to smelt into metallic iron, as fragments ranged 30-300 ft and subsequently endured more collisions and impacts. Even resulting smaller sand grain-sized pieces gathered further loosely together with many voids. Such meteorites are polycyclic aromatic hydrocarbons (PAHs) or amino acids rich as astronomers still do not figure whether such elements were delivered during impacts or formed directly from trapped gases as the asteroid cooled following the giant collision. Such a evolution also mixed non-ureilite types of meteorites into, sometimes accounting for 20-30 percent of some asteroids. Scientists believe a large number of the meteorites that are found on Earth originate from the protoplanet Vesta. A cataclysmic impact at the south pole of Vesta, the second most massive object in the Asteroid Belt, created an enormous crater and excavated a great deal of debris. Some of that debris ended up as other asteroids and some of it likely ended up on Earth. Iron meteorites are rich in nickel as meteorites also, generally, features a specific crystal structure as they cooled at an extremely slow rate inside asteroids when the solar system was forming. Astronomers now think that asteroids, generally, more frequently are broken and reassembled during collisions than previously thought with more mixing too. In some rare event -- in the order of once a year to 800,000 known asteroids in the Asteroid Belt -- a asteroid may loose some material due to miscellaneous cause like a too rapid rotation -- for cause, for example of the YORP ('Yarkovsky–O'Keefe–Radzievskii–Paddack') effect by which the infrared and heat from sunlight brings a angular momentum to the asteroid -- which translates into a comet-like tail of dusty debris
The radar technique has been used to observe hundreds of asteroids. Two observatory posts at Earth are able to radar-study asteroids, the 70-meter (230-foot) Goldstone antenna in California's Mojave Desert and the National Science Foundation’s 1,000-foot-diameter (305 meters) Arecibo Observatory in Puerto Rico. Goldstone is fully steerable, can see about 80 percent of the sky, can track objects several times longer per day, and can image asteroids at fine spatial resolution. The Arecibo radar is about 20 times more sensitive, can see about one-third of the sky, and can detect asteroids about twice as far away. By early 2011, Goldstone and Arecibo have observed 272 near-Earth asteroids and 14 comets with radar
Until 1991 asteroids were known from observations made on Earth only (optical, radar, Hubble). Galileo mission, a Jupiter bound mission, made first flyby ever of an asteroid (asteroid Gaspra), and grazed another, Ida. NEAR mission made a flyby of Mathilde en-route to asteroid Eros, about which it began orbiting in 1999. NEAR was successfully controlled-crashed on Eros by mission's end. Japanese mission MUSES-C is scheduled to asteroid Itokawa as NASA Dawn is leaving in 2006 to Vesta and Ceres which it will reach in 2010 and 2014
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