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105-641: Shooting star refers to a meteor . Shooting star may also refer to: Meteor A meteor or shooting star is the visible passage of a meteoroid, comet, or asteroid entering Earth's atmosphere . At a speed typically in excess of 20 km/s (72,000 km/h; 45,000 mph), aerodynamic heating of that object produces a streak of light, both from the glowing object and the trail of glowing particles that it leaves in its wake. Meteors typically become visible when they are about 100 km (62 mi) above sea level. A series of many meteors appearing seconds or minutes apart and appearing to originate from

210-440: A certain amount of meteoroids are shed. The meteoroids spread out along the entire orbit of the comet to form a meteoroid stream, also known as a "dust trail" (as opposed to a comet's "dust tail" caused by the very small particles that are quickly blown away by solar radiation pressure). The frequency of fireball sightings increases by about 10–30% during the weeks of vernal equinox . Even meteorite falls are more common during

315-632: A cloud of interstellar dust and gas collapsed under the influence of gravity to form a rotating disc of material that then conglomerated to form the Sun and planets. During the first few million years of the Solar System's history, an accretion process of sticky collisions caused the clumping of small particles, which gradually increased in size. Once the clumps reached sufficient mass, they could draw in other bodies through gravitational attraction and become planetesimals. This gravitational accretion led to

420-488: A common origin in the breakup of a larger body. Graphical displays of these element pairs, for members of the asteroid belt, show concentrations indicating the presence of an asteroid family. There are about 20 to 30 associations that are likely asteroid families. Additional groupings have been found that are less certain. Asteroid families can be confirmed when the members display similar spectral features. Smaller associations of asteroids are called groups or clusters. Some of

525-462: A cosmic origin. After reviewing historical records, Heinrich Wilhelm Matthias Olbers predicted the storm's return in 1867, drawing other astronomers' attention to the phenomenon. Hubert A. Newton 's more thorough historical work led to a refined prediction of 1866, which proved correct. With Giovanni Schiaparelli 's success in connecting the Leonids (as they are called) with comet Tempel-Tuttle ,

630-738: A deviation for any object causing a meteor. Objects smaller than meteoroids are classified as micrometeoroids and interplanetary dust . The Minor Planet Center does not use the term "meteoroid". Almost all meteoroids contain extraterrestrial nickel and iron. They have three main classifications: iron, stone, and stony-iron. Some stone meteoroids contain grain-like inclusions known as chondrules and are called chondrites . Stony meteoroids without these features are called " achondrites ", which are typically formed from extraterrestrial igneous activity; they contain little or no extraterrestrial iron. The composition of meteoroids can be inferred as they pass through Earth's atmosphere from their trajectories and

735-539: A few hundred micrometres . This fine material is produced, at least in part, from collisions between asteroids, and by the impact of micrometeorites upon the asteroids. Due to the Poynting–Robertson effect , the pressure of solar radiation causes this dust to slowly spiral inward toward the Sun. The combination of this fine asteroid dust, as well as ejected cometary material, produces the zodiacal light . This faint auroral glow can be viewed at night extending from

840-518: A few metres. The asteroid material is so thinly distributed that numerous uncrewed spacecraft have traversed it without incident. Nonetheless, collisions between large asteroids occur and can produce an asteroid family , whose members have similar orbital characteristics and compositions. Individual asteroids within the belt are categorized by their spectra , with most falling into three basic groups: carbonaceous ( C-type ), silicate ( S-type ), and metal-rich ( M-type ). The asteroid belt formed from

945-484: A generic large crater-forming projectile in a manner "to imply that we do not know the precise nature of the impacting body ... whether it is a rocky or metallic asteroid, or an icy comet for example". Meteoroids also hit other bodies in the Solar System. On such stony bodies as the Moon or Mars that have little or no atmosphere, they leave enduring craters. Meteoroid collisions with solid Solar System objects, including

1050-632: A low albedo . Their surface compositions are similar to carbonaceous chondrite meteorites . Chemically, their spectra match the primordial composition of the early Solar System, with hydrogen, helium, and volatiles removed. S-type ( silicate -rich) asteroids are more common toward the inner region of the belt, within 2.5 AU of the Sun. The spectra of their surfaces reveal the presence of silicates and some metal, but no significant carbonaceous compounds. This indicates that their materials have been significantly modified from their primordial composition, probably through melting and reformation. They have

1155-454: A major source of the Earth's oceans because the deuterium-hydrogen ratio is too low for classical comets to have been the principal source. Most asteroids within the asteroid belt have orbital eccentricities of less than 0.4, and an inclination of less than 30°. The orbital distribution of the asteroids reaches a maximum at an eccentricity around 0.07 and an inclination below 4°. Thus, although

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1260-418: A mean radius of 10 km are expected to occur about once every 10 million years. A collision may fragment an asteroid into numerous smaller pieces (leading to the formation of a new asteroid family ). Conversely, collisions that occur at low relative speeds may also join two asteroids. After more than 4 billion years of such processes, the members of the asteroid belt now bear little resemblance to

1365-538: A mean semi-major axis of 1.9 AU) is the Hungaria family of minor planets. They are named after the main member, 434 Hungaria ; the group contains at least 52 named asteroids. The Hungaria group is separated from the main body by the 4:1 Kirkwood gap and their orbits have a high inclination. Some members belong to the Mars-crossing category of asteroids, and gravitational perturbations by Mars are likely

1470-418: A meteoroid would be between 100 μm and 10 m (33 ft) across. In 2010, following the discovery of asteroids below 10 m in size, Rubin and Grossman proposed a revision of the previous definition of meteoroid to objects between 10 μm (0.00039 in) and one meter (3 ft 3 in) in diameter in order to maintain the distinction. According to Rubin and Grossman, the minimum size of an asteroid

1575-582: A more rigid definition. It defines a fireball as a meteor that would have a magnitude of −3 or brighter if seen at zenith . This definition corrects for the greater distance between an observer and a meteor near the horizon. For example, a meteor of magnitude −1 at 5 degrees above the horizon would be classified as a fireball because, if the observer had been directly below the meteor, it would have appeared as magnitude −6. Fireballs reaching apparent magnitude −14 or brighter are called bolides . The IAU has no official definition of "bolide", and generally considers

1680-485: A much larger planet that once occupied the Mars–Jupiter region, with this planet having suffered an internal explosion or a cometary impact many million years before, while Odesan astronomer K. N. Savchenko suggested that Ceres, Pallas, Juno, and Vesta were escaped moons rather than fragments of the exploded planet. The large amount of energy required to destroy a planet, combined with the belt's low combined mass, which

1785-482: A planet," in his Mysterium Cosmographicum , stating his prediction that a planet would be found there. While analyzing Tycho Brahe 's data, Kepler thought that too large a gap existed between the orbits of Mars and Jupiter to fit his own model of where planetary orbits should be found. In an anonymous footnote to his 1766 translation of Charles Bonnet 's Contemplation de la Nature , the astronomer Johann Daniel Titius of Wittenberg noted an apparent pattern in

1890-430: A population of comets had been discovered within the asteroid belt beyond the snow line, which may have provided a source of water for Earth's oceans. According to some models, outgassing of water during the Earth's formative period was insufficient to form the oceans, requiring an external source such as a cometary bombardment. The outer asteroid belt appears to include a few objects that may have arrived there during

1995-484: A quarter that of ice, to nickel-iron rich dense rocks. The study of meteorites also gives insights into the composition of non-ephemeral meteoroids. Most meteoroids come from the asteroid belt , having been perturbed by the gravitational influences of planets, but others are particles from comets , giving rise to meteor showers . Some meteoroids are fragments from bodies such as Mars or the Moon , that have been thrown into space by an impact. Meteoroids travel around

2100-455: A relatively high albedo and form about 17% of the total asteroid population. M-type (metal-rich) asteroids are typically found in the middle of the main belt, and they make up much of the remainder of the total population. Their spectra resemble that of iron-nickel. Some are believed to have formed from the metallic cores of differentiated progenitor bodies that were disrupted through collision. However, some silicate compounds also can produce

2205-573: A second. Meteors were not known to be an astronomical phenomenon until early in the nineteenth century. Before that, they were seen in the West as an atmospheric phenomenon, like lightning, and were not connected with strange stories of rocks falling from the sky. In 1807, Yale University chemistry professor Benjamin Silliman investigated a meteorite that fell in Weston, Connecticut . Silliman believed

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2310-400: A similar appearance. For example, the large M-type asteroid 22 Kalliope does not appear to be primarily composed of metal. Within the asteroid belt, the number distribution of M-type asteroids peaks at a semimajor axis of about 2.7 AU. Whether all M-types are compositionally similar, or whether it is a label for several varieties which do not fit neatly into the main C and S classes

2415-412: A size scale larger than 10 cm (3.9 in) to several meters meteor visibility is due to the atmospheric ram pressure (not friction) that heats the meteoroid so that it glows and creates a shining trail of gases and melted meteoroid particles. The gases include vaporised meteoroid material and atmospheric gases that heat up when the meteoroid passes through the atmosphere. Most meteors glow for about

2520-407: A specific stream of space debris . A number of specific meteors have been observed, largely by members of the public and largely by accident, but with enough detail that orbits of the meteoroids producing the meteors have been calculated. The atmospheric velocities of meteors result from the movement of Earth around the Sun at about 30 km/s (67,000 mph), the orbital speeds of meteoroids, and

2625-612: A typical asteroid has a relatively circular orbit and lies near the plane of the ecliptic , some asteroid orbits can be highly eccentric or travel well outside the ecliptic plane. Sometimes, the term "main belt" is used to refer only to the more compact "core" region where the greatest concentration of bodies is found. This lies between the strong 4:1 and 2:1 Kirkwood gaps at 2.06 and 3.27 AU, and at orbital eccentricities less than roughly 0.33, along with orbital inclinations below about 20°. As of 2006 , this "core" region contained 93% of all discovered and numbered minor planets within

2730-546: Is a torus -shaped region in the Solar System , centered on the Sun and roughly spanning the space between the orbits of the planets Jupiter and Mars . It contains a great many solid, irregularly shaped bodies called asteroids or minor planets . The identified objects are of many sizes, but much smaller than planets , and, on average, are about one million kilometers (or six hundred thousand miles) apart. This asteroid belt

2835-414: Is a compositional trend of asteroid types by increasing distance from the Sun, in the order of S, C, P, and the spectrally-featureless D-types . Carbonaceous asteroids , as their name suggests, are carbon-rich. They dominate the asteroid belt's outer regions, and are rare in the inner belt. Together they comprise over 75% of the visible asteroids. They are redder in hue than the other asteroids and have

2940-456: Is also called the main asteroid belt or main belt to distinguish it from other asteroid populations in the Solar System. The asteroid belt is the smallest and innermost known circumstellar disc in the Solar System. Classes of small Solar System bodies in other regions are the near-Earth objects , the centaurs , the Kuiper belt objects, the scattered disc objects, the sednoids , and

3045-489: Is called meteoric dust or just meteor dust. Meteor dust particles can persist in the atmosphere for up to several months. These particles might affect climate, both by scattering electromagnetic radiation and by catalyzing chemical reactions in the upper atmosphere. Meteoroids or their fragments achieve dark flight after deceleration to terminal velocity . Dark flight starts when they decelerate to about 2–4 km/s (4,500–8,900 mph). Larger fragments fall further down

3150-558: Is given by what can be discovered from Earth-bound telescopes, so the distinction between meteoroid and asteroid is fuzzy. Some of the smallest asteroids discovered (based on absolute magnitude H ) are 2008 TS 26 with H = 33.2 and 2011 CQ 1 with H = 32.1 both with an estimated size of one m (3 ft 3 in). In April 2017, the IAU adopted an official revision of its definition, limiting size to between 30 μm (0.0012 in) and one meter in diameter, but allowing for

3255-421: Is not yet clear. One mystery is the relative rarity of V-type (Vestoid) or basaltic asteroids in the asteroid belt. Theories of asteroid formation predict that objects the size of Vesta or larger should form crusts and mantles, which would be composed mainly of basaltic rock, resulting in more than half of all asteroids being composed either of basalt or of olivine . However, observations suggest that 99% of

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3360-436: Is only about 4% of the mass of Earth's Moon, does not support these hypotheses. Further, the significant chemical differences between the asteroids become difficult to explain if they come from the same planet. A modern hypothesis for the asteroid belt's creation relates to how, in general for the Solar System, planetary formation is thought to have occurred via a process comparable to the long-standing nebular hypothesis ;

3465-490: The Greek asteroeides , meaning "star-like". Upon completing a series of observations of Ceres and Pallas, he concluded, Neither the appellation of planets nor that of comets can with any propriety of language be given to these two stars ... They resemble small stars so much as hardly to be distinguished from them. From this, their asteroidal appearance, if I take my name, and call them Asteroids; reserving for myself, however,

3570-420: The Greek βολίς ( bolis ) which can mean a missile or to flash . If the magnitude of a bolide reaches −17 or brighter it is known as a superbolide . A relatively small percentage of fireballs hit Earth's atmosphere and then pass out again: these are termed Earth-grazing fireballs . Such an event happened in broad daylight over North America in 1972 . Another rare phenomenon is a meteor procession , where

3675-540: The Oort cloud objects. About 60% of the main belt mass is contained in the four largest asteroids: Ceres , Vesta , Pallas , and Hygiea . The total mass of the asteroid belt is estimated to be 3% that of the Moon . Ceres, the only object in the asteroid belt large enough to be a dwarf planet , is about 950 km in diameter, whereas Vesta, Pallas, and Hygiea have mean diameters less than 600 km. The remaining mineralogically classified bodies range in size down to

3780-459: The Phoenicids can make atmospheric entry at as slow as about 11 km/s. On January 17, 2013, at 05:21 PST, a one-meter-sized comet from the Oort cloud entered Earth atmosphere over California and Nevada . The object had a retrograde orbit with perihelion at 0.98 ± 0.03  AU . It approached from the direction of the constellation Virgo (which was in the south about 50° above

3885-598: The University of Palermo , Sicily, found a tiny moving object in an orbit with exactly the radius predicted by this pattern. He dubbed it "Ceres", after the Roman goddess of the harvest and patron of Sicily. Piazzi initially believed it to be a comet, but its lack of a coma suggested it was a planet. Thus, the aforementioned pattern predicted the semimajor axes of all eight planets of the time (Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, and Uranus). Concurrent with

3990-438: The gravity well of Earth. Meteors become visible between about 75 to 120 km (250,000 to 390,000 ft) above Earth. They usually disintegrate at altitudes of 50 to 95 km (160,000 to 310,000 ft). Meteors have roughly a fifty percent chance of a daylight (or near daylight) collision with Earth. Most meteors are, however, observed at night, when darkness allows fainter objects to be recognized. For bodies with

4095-403: The strewn field . The visible light produced by a meteor may take on various hues, depending on the chemical composition of the meteoroid, and the speed of its movement through the atmosphere. As layers of the meteoroid abrade and ionize, the colour of the light emitted may change according to the layering of minerals. Colours of meteors depend on the relative influence of the metallic content of

4200-597: The 1850 translation (by Elise Otté ) of Alexander von Humboldt's Cosmos : "[...] and the regular appearance, about the 13th of November and the 11th of August, of shooting stars, which probably form part of a belt of asteroids intersecting the Earth's orbit and moving with planetary velocity". Another early appearance occurred in Robert James Mann 's A Guide to the Knowledge of the Heavens : "The orbits of

4305-590: The 50,000 meteorites found on Earth to date, 99.8 percent are believed to have originated in the asteroid belt. In 1918, the Japanese astronomer Kiyotsugu Hirayama noticed that the orbits of some of the asteroids had similar parameters, forming families or groups. Approximately one-third of the asteroids in the asteroid belt are members of an asteroid family. These share similar orbital elements , such as semi-major axis , eccentricity , and orbital inclination as well as similar spectral features, which indicate

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4410-541: The Late Heavy Bombardment was likely affected by the passages of large Centaurs and trans-Neptunian objects (TNOs). Centaurs and TNOs that reach the inner Solar System can modify the orbits of main belt asteroids, though only if their mass is of the order of 10   M ☉ for single encounters or, one order less in case of multiple close encounters. However, Centaurs and TNOs are unlikely to have significantly dispersed young asteroid families in

4515-476: The Moon, Mercury , Callisto , Ganymede , and most small moons and asteroids , create impact craters, which are the dominant geographic features of many of those objects. On other planets and moons with active surface geological processes, such as Earth, Venus , Mars , Europa , Io , and Titan , visible impact craters may become eroded , buried, or transformed by tectonics over time. In early literature, before

4620-432: The Solar System, the asteroids melted to some degree, allowing elements within them to be differentiated by mass. Some of the progenitor bodies may even have undergone periods of explosive volcanism and formed magma oceans. Because of the relatively small size of the bodies, though, the period of melting was necessarily brief compared to the much larger planets, and had generally ended about 4.5 billion years ago, in

4725-445: The Solar System. The JPL Small-Body Database lists over 1 million known main-belt asteroids. The semimajor axis of an asteroid is used to describe the dimensions of its orbit around the Sun, and its value determines the minor planet's orbital period . In 1866, Daniel Kirkwood announced the discovery of gaps in the distances of these bodies' orbits from the Sun. They were located in positions where their period of revolution about

4830-444: The Sun in a variety of orbits and at various velocities. The fastest move at about 42 km/s (94,000 mph) through space in the vicinity of Earth's orbit. This is escape velocity from the Sun, equal to the square root of two times Earth's speed, and is the upper speed limit of objects in the vicinity of Earth, unless they come from interstellar space. Earth travels at about 29.6 km/s (66,000 mph), so when meteoroids meet

4935-460: The Sun was an integer fraction of Jupiter's orbital period. Kirkwood proposed that the gravitational perturbations of the planet led to the removal of asteroids from these orbits. When the mean orbital period of an asteroid is an integer fraction of the orbital period of Jupiter, a mean-motion resonance with the gas giant is created that is sufficient to perturb an asteroid to new orbital elements . Primordial asteroids entered these gaps because of

5040-404: The asteroid belt, dynamically exciting the region's population and increasing their velocities relative to each other. In regions where the average velocity of the collisions was too high, the shattering of planetesimals tended to dominate over accretion, preventing the formation of a planet. Instead, they continued to orbit the Sun as before, occasionally colliding. During the early history of

5145-692: The asteroids are placed in a wide belt of space, extending between the extremes of [...]". The American astronomer Benjamin Peirce seems to have adopted that terminology and to have been one of its promoters. Over 100 asteroids had been located by mid-1868, and in 1891, the introduction of astrophotography by Max Wolf accelerated the rate of discovery. A total of 1,000 asteroids had been found by 1921, 10,000 by 1981, and 100,000 by 2000. Modern asteroid survey systems now use automated means to locate new minor planets in ever-increasing numbers. On 22 January 2014, European Space Agency (ESA) scientists reported

5250-448: The astronomer Karl Ludwig Hencke detected a fifth object ( 5 Astraea ) and, shortly thereafter, new objects were found at an accelerating rate. Counting them among the planets became increasingly cumbersome. Eventually, they were dropped from the planet list (as first suggested by Alexander von Humboldt in the early 1850s) and Herschel's coinage, "asteroids", gradually came into common use. The discovery of Neptune in 1846 led to

5355-460: The atmosphere and hits the ground without being destroyed. Meteorites are sometimes, but not always, found in association with hypervelocity impact craters ; during energetic collisions, the entire impactor may be vaporized, leaving no meteorites. Geologists use the term, "bolide", in a different sense from astronomers to indicate a very large impactor . For example, the USGS uses the term to mean

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5460-636: The atmosphere as a meteor and has impacted the ground. In 1961, the International Astronomical Union (IAU) defined a meteoroid as "a solid object moving in interplanetary space, of a size considerably smaller than an asteroid and considerably larger than an atom". In 1995, Beech and Steel, writing in the Quarterly Journal of the Royal Astronomical Society , proposed a new definition where

5565-417: The atmosphere head-on (which only occurs when meteors are in a retrograde orbit such as the Leonids , which are associated with the retrograde comet 55P/Tempel–Tuttle ) the combined speed may reach about 71 km/s (160,000 mph) (see Specific energy#Astrodynamics ). Meteoroids moving through Earth's orbital space average about 20 km/s (45,000 mph), but due to Earth's gravity meteors such as

5670-399: The atmosphere, and thus ionization trails can be found in the upper atmosphere more or less continuously. When radio waves are bounced off these trails, it is called meteor burst communications . Meteor radars can measure atmospheric density and winds by measuring the decay rate and Doppler shift of a meteor trail. Most meteoroids burn up when they enter the atmosphere. The left-over debris

5775-568: The belt's total mass, with 39% accounted for by Ceres alone. The present day belt consists primarily of three categories of asteroids: C-type carbonaceous asteroids, S-type silicate asteroids, and a hybrid group of X-type asteroids. The hybrid group have featureless spectra, but they can be divided into three groups based on reflectivity, yielding the M-type metallic, P-type primitive, and E-type enstatite asteroids. Additional types have been found that do not fit within these primary classes. There

5880-640: The case of Ceres with the Gefion family .) The Vesta family is believed to have formed as the result of a crater-forming impact on Vesta. Likewise, the HED meteorites may also have originated from Vesta as a result of this collision. Three prominent bands of dust have been found within the asteroid belt. These have similar orbital inclinations as the Eos, Koronis, and Themis asteroid families, and so are possibly associated with those groupings. The main belt evolution after

5985-430: The celestial police, discovered a second object in the same region, Pallas. Unlike the other known planets, Ceres and Pallas remained points of light even under the highest telescope magnifications instead of resolving into discs. Apart from their rapid movement, they appeared indistinguishable from stars . Accordingly, in 1802, William Herschel suggested they be placed into a separate category, named "asteroids", after

6090-573: The cosmic origin of meteors was firmly established. Still, they remain an atmospheric phenomenon and retain their name "meteor" from the Greek word for "atmospheric". A fireball is a brighter-than-usual meteor that also becomes visible when about 100 km from sea level. The International Astronomical Union (IAU) defines a fireball as "a meteor brighter than any of the planets" ( apparent magnitude −4 or greater). The International Meteor Organization (an amateur organization that studies meteors) has

6195-449: The curve 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, having survived 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. The absolute magnitudes of most of

6300-652: 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 was 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". In 1802, shortly after discovering Pallas, Olbers suggested to Herschel and Carl Gauss that Ceres and Pallas were fragments of

6405-419: The direction of the Sun along the plane of the ecliptic . Asteroid particles that produce visible zodiacal light average about 40 μm in radius. The typical lifetimes of main-belt zodiacal cloud particles are about 700,000 years. Thus, to maintain the bands of dust, new particles must be steadily produced within the asteroid belt. It was once thought that collisions of asteroids form a major component of

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6510-454: The discovery of Ceres, an informal group of 24 astronomers dubbed the " celestial police " was formed under the invitation of Franz Xaver von Zach with the express purpose of finding additional planets; they focused their search for them in the region between Mars and Jupiter where the Titius–Bode law predicted there should be a planet. About 15 months later, Heinrich Olbers , a member of

6615-457: The discrediting of the Titius–Bode law in the eyes of scientists because its orbit was nowhere near the predicted position. To date, no scientific explanation for the law has been given, and astronomers' consensus regards it as a coincidence. The expression "asteroid belt" came into use in the early 1850s, although pinpointing who coined the term is difficult. The first English use seems to be in

6720-433: The early evening, increasing chances of eyewitness reports. This explains a part, but perhaps not all of the seasonal variation. Research is in progress for mapping the orbits of the meteors to gain a better understanding of the phenomenon. In a range of countries networks of sky observing installations have been set up to monitor meteors. A meteorite is a portion of a meteoroid or asteroid that survives its passage through

6825-412: The early history of the Solar System. The Hungaria asteroids lie closer to the Sun than the 4:1 resonance, but are protected from disruption by their high inclination. When the asteroid belt was first formed, the temperatures at a distance of 2.7 AU from the Sun formed a " snow line " below the freezing point of water. Planetesimals formed beyond this radius were able to accumulate ice. In 2006,

6930-486: The ecliptic is (in the northern hemisphere) high in the sky in the late afternoon and early evening. This means that fireball radiants with an asteroidal source are high in the sky (facilitating relatively high rates) at the moment the meteoroids "catch up" with Earth, coming from behind going in the same direction as Earth. This causes relatively low relative speeds and from this low entry speeds, which facilitates survival of meteorites. It also generates high fireball rates in

7035-497: The first 100 million years of the Solar System's history. Some fragments eventually found their way into the inner Solar System, leading to meteorite impacts with the inner planets. Asteroid orbits continue to be appreciably perturbed whenever their period of revolution about the Sun forms an orbital resonance with Jupiter. At these orbital distances, a Kirkwood gap occurs as they are swept into other orbits. In 1596, Johannes Kepler wrote, "Between Mars and Jupiter, I place

7140-409: The first few tens of millions of years), surface melting from impacts, space weathering from radiation, and bombardment by micrometeorites . Although some scientists refer to the asteroids as residual planetesimals, other scientists consider them distinct. The current asteroid belt is believed to contain only a small fraction of the mass of the primordial belt. Computer simulations suggest that

7245-484: The first tens of millions of years of formation. In August 2007, a study of zircon crystals in an Antarctic meteorite believed to have originated from Vesta suggested that it, and by extension the rest of the asteroid belt, had formed rather quickly, within 10 million years of the Solar System's origin. The asteroids are not pristine samples of the primordial Solar System. They have undergone considerable evolution since their formation, including internal heating (in

7350-528: The formation of the planets. Planetesimals within the region that would become the asteroid belt were strongly perturbed by Jupiter's gravity. Orbital resonances occurred where the orbital period of an object in the belt formed an integer fraction of the orbital period of Jupiter, perturbing the object into a different orbit; the region lying between the orbits of Mars and Jupiter contains many such orbital resonances. As Jupiter migrated inward following its formation, these resonances would have swept across

7455-539: The further discovery in 2007 of two asteroids in the outer belt, 7472 Kumakiri and (10537) 1991 RY 16 , with a differing basaltic composition that could not have originated from Vesta. These two are the only V-type asteroids discovered in the outer belt to date. The temperature of the asteroid belt varies with the distance from the Sun. For dust particles within the belt, typical temperatures range from 200 K (−73 °C) at 2.2 AU down to 165 K (−108 °C) at 3.2 AU. However, due to rotation,

7560-514: The generation of these sounds may partially explain them. For example, scientists at NASA suggested that the turbulent ionized wake of a meteor interacts with Earth's magnetic field , generating pulses of radio waves . As the trail dissipates, megawatts of electromagnetic power could be released, with a peak in the power spectrum at audio frequencies . Physical vibrations induced by the electromagnetic impulses would then be heard if they are powerful enough to make grasses, plants, eyeglass frames,

7665-457: The hearer's own body (see microwave auditory effect ), and other conductive materials vibrate. This proposed mechanism, although proven plausible by laboratory work, remains unsupported by corresponding measurements in the field. Sound recordings made under controlled conditions in Mongolia in 1998 support the contention that the sounds are real. (Also see Bolide .) A meteor shower is

7770-400: The heat of entry and force of impact. A noted 4-metre (13 ft) asteroid , 2008 TC 3 , was observed in space on a collision course with Earth on 6 October 2008 and entered Earth's atmosphere the next day, striking a remote area of northern Sudan. It was the first time that a meteoroid had been observed in space and tracked prior to impacting Earth. NASA has produced a map showing

7875-516: The horizon at the time), and collided head-on with Earth's atmosphere at 72 ± 6 km/s (161,000 ± 13,000 mph) vaporising more than 100 km (330,000 ft) above ground over a period of several seconds. When meteoroids intersect with Earth's atmosphere at night, they are likely to become visible as meteors . If meteoroids survive the entry through the atmosphere and reach Earth's surface, they are called meteorites . Meteorites are transformed in structure and chemistry by

7980-460: The known asteroids are between 11 and 19, with the median at about 16. On average the distance between the asteroids is about 965,600 km (600,000 miles), although this varies among asteroid families and smaller undetected asteroids might be even closer. The total mass of the asteroid belt is estimated to be 2.39 × 10 kg, which is 3% of the mass of the Moon. The four largest objects, Ceres, Vesta, Pallas, and Hygiea, contain an estimated 62% of

8085-415: The last few hundred years, the list includes (457175) 2008 GO 98 also known as 362P. 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

8190-472: The layout of the planets, now known as the Titius-Bode Law . If one began a numerical sequence at 0, then included 3, 6, 12, 24, 48, etc., doubling each time, and added four to each number and divided by 10, this produced a remarkably close approximation to the radii of the orbits of the known planets as measured in astronomical units , provided one allowed for a "missing planet" (equivalent to 24 in

8295-665: The liberty of changing that name, if another, more expressive of their nature, should occur. By 1807, further investigation revealed two new objects in the region: Juno and Vesta . The burning of Lilienthal in the Napoleonic wars , where the main body of work had been done, brought this first period of discovery to a close. Despite Herschel's coinage, for several decades it remained common practice to refer to these objects as planets and to prefix their names with numbers representing their sequence of discovery: 1 Ceres, 2 Pallas, 3 Juno, 4 Vesta. In 1845, though,

8400-658: The light spectra of the resulting meteor. Their effects on radio signals also give information, especially useful for daytime meteors, which are otherwise very difficult to observe. From these trajectory measurements, meteoroids have been found to have many different orbits, some clustering in streams (see meteor showers ) often associated with a parent comet , others apparently sporadic. Debris from meteoroid streams may eventually be scattered into other orbits. The light spectra, combined with trajectory and light curve measurements, have yielded various compositions and densities, ranging from fragile snowball-like objects with density about

8505-416: 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 number of asteroids in the main belt steadily increases with decreasing size. Although the size distribution generally follows a power law , there are 'bumps' in

8610-433: The main belt, although they can have perturbed some old asteroid families. Current main belt asteroids that originated as Centaurs or trans-Neptunian objects may lie in the outer belt with short lifetime of less than 4 million years, most likely orbiting between 2.8 and 3.2 AU at larger eccentricities than typical of main belt asteroids. Skirting the inner edge of the belt (ranging between 1.78 and 2.0 AU, with

8715-638: The meteor breaks up into several fireballs traveling nearly parallel to the surface of Earth. A steadily growing number of fireballs are recorded at the American Meteor Society every year. There are probably more than 500,000 fireballs a year, but most go unnoticed because most occur over the ocean and half occur during daytime. A European Fireball Network and a NASA All-sky Fireball Network detect and track many fireballs. The entry of meteoroids into Earth's atmosphere produces three main effects: ionization of atmospheric molecules, dust that

8820-474: The meteor had a cosmic origin, but meteors did not attract much attention from astronomers until the spectacular meteor storm of November 1833. People all across the eastern United States saw thousands of meteors, radiating from a single point in the sky. Careful observers noticed that the radiant , as the point is called, moved with the stars, staying in the constellation Leo. The astronomer Denison Olmsted extensively studied this storm, concluding that it had

8925-413: The meteoroid sheds, and the sound of passage. During the entry of a meteoroid or asteroid into the upper atmosphere , an ionization trail is created, where the air molecules are ionized by the passage of the meteor. Such ionization trails can last up to 45 minutes at a time. Small, sand-grain sized meteoroids are entering the atmosphere constantly, essentially every few seconds in any given region of

9030-569: The meteoroid versus the superheated air plasma, which its passage engenders: The sound generated by a meteor in the upper atmosphere, such as a sonic boom , typically arrives many seconds after the visual light from a meteor disappears. Occasionally, as with the Leonid meteor shower of 2001, "crackling", "swishing", or "hissing" sounds have been reported, occurring at the same instant as a meteor flare. Similar sounds have also been reported during intense displays of Earth's auroras . Theories on

9135-564: The migration of Jupiter's orbit. Subsequently, asteroids primarily migrate into these gap orbits due to the Yarkovsky effect , but may also enter because of perturbations or collisions. After entering, an asteroid is gradually nudged into a different, random orbit with a larger or smaller semimajor axis. The high population of the asteroid belt makes for an active environment, where collisions between asteroids occur frequently (on deep time scales). Impact events between main-belt bodies with

9240-413: The most notable asteroid collisions with Earth and its atmosphere from 1994 to 2013 from data gathered by U.S. government sensors (see below). A meteor , known colloquially as a shooting star or falling star , is the visible passage of a glowing meteoroid , micrometeoroid , comet or asteroid through Earth's atmosphere, after being heated to incandescence by collisions with air molecules in

9345-463: The most prominent families in the asteroid belt (in order of increasing semi-major axes) are the Flora , Eunomia , Koronis , Eos , and Themis families. The Flora family, one of the largest with more than 800 known members, may have formed from a collision less than 1 billion years ago. The largest asteroid to be a true member of a family is 4 Vesta. (This is in contrast to an interloper, in

9450-402: The northern hemisphere's spring season. Although this phenomenon has been known for quite some time, the reason behind the anomaly is not fully understood by scientists. Some researchers attribute this to an intrinsic variation in the meteoroid population along Earth's orbit, with a peak in big fireball-producing debris around spring and early summer. Others have pointed out that during this period

9555-411: The original asteroid belt may have contained mass equivalent to the Earth's. Primarily because of gravitational perturbations, most of the material was ejected from the belt within about 1 million years of formation, leaving behind less than 0.1% of the original mass. Since its formation, the size distribution of the asteroid belt has remained relatively stable; no significant increase or decrease in

9660-548: The original meteorite, created or modified by an impact of a meteorite is called impactite . Solar System   → Local Interstellar Cloud   → Local Bubble   → Gould Belt   → Orion Arm   → Milky Way   → Milky Way subgroup   → Local Group → Local Sheet → Virgo Supercluster → Laniakea Supercluster   → Local Hole   → Observable universe   → Universe Each arrow ( → ) may be read as "within" or "part of". Asteroid belt The asteroid belt

9765-431: The original population. Evidence suggests that most main belt asteroids between 200 m and 10 km in diameter are rubble piles formed by collisions. These bodies consist of a multitude of irregular objects that are mostly bound together by self-gravity, resulting in significant amounts of internal porosity . Along with the asteroid bodies, the asteroid belt also contains bands of dust with particle radii of up to

9870-414: The predicted basaltic material is missing. Until 2001, most basaltic bodies discovered in the asteroid belt were believed to originate from the asteroid Vesta (hence their name V-type), but the discovery of the asteroid 1459 Magnya revealed a slightly different chemical composition from the other basaltic asteroids discovered until then, suggesting a different origin. This hypothesis was reinforced by

9975-409: The primordial solar nebula as a group of planetesimals , the smaller precursors of the protoplanets . However, between Mars and Jupiter gravitational perturbations from Jupiter disrupted their accretion into a planet, imparting excess kinetic energy which shattered colliding planetesimals and most of the incipient protoplanets. As a result, 99.9% of the asteroid belt's original mass was lost in

10080-415: The result of an interaction between a planet, such as Earth, and streams of debris from a comet or other source. The passage of Earth through cosmic debris from comets and other sources is a recurring event in many cases. Comets can produce debris by water vapor drag, as demonstrated by Fred Whipple in 1951, and by breakup. Each time a comet swings by the Sun in its orbit , some of its ice vaporizes and

10185-403: The same fixed point in the sky is called a meteor shower . An estimated 25 million meteoroids, micrometeoroids and other space debris enter Earth's atmosphere each day, which results in an estimated 15,000 tonnes of that material entering the atmosphere each year. A meteorite is the remains of a meteoroid that has survived the ablation of its surface material during its passage through

10290-499: The sequence) between the orbits of Mars (12) and Jupiter (48). In his footnote, Titius declared, "But should the Lord Architect have left that space empty? Not at all." When William Herschel discovered Uranus in 1781, the planet's orbit closely matched the law, leading some astronomers to conclude that a planet had to be between the orbits of Mars and Jupiter. On January 1, 1801, Giuseppe Piazzi , chairman of astronomy at

10395-415: The significance of impact cratering was widely recognised, the terms cryptoexplosion or cryptovolcanic structure were often used to describe what are now recognised as impact-related features on Earth. Molten terrestrial material ejected from a meteorite impact crater can cool and solidify into an object known as a tektite . These are often mistaken for meteorites. Terrestrial rock, sometimes with pieces of

10500-399: The size of a grain of sand, i.e. they are usually millimeter-sized or smaller. Meteoroid sizes can be calculated from their mass and density which, in turn, can be estimated from the observed meteor trajectory in the upper atmosphere. Meteors may occur in showers , which arise when Earth passes through a stream of debris left by a comet, or as "random" or "sporadic" meteors, not associated with

10605-469: The surface temperature of an asteroid can vary considerably as the sides are alternately exposed to solar radiation then to the stellar background. Several otherwise unremarkable bodies in the outer belt show cometary activity. Because their orbits cannot be explained through the capture of classical comets, many of the outer asteroids are thought to be icy, with the ice occasionally exposed to sublimation through small impacts. Main-belt comets may have been

10710-473: The term synonymous with "fireball". Astronomers often use "bolide" to identify an exceptionally bright fireball, particularly one that explodes in a meteor air burst . They are sometimes called detonating fireballs. It may also be used to mean a fireball which creates audible sounds. In the late twentieth century, bolide has also come to mean any object that hits Earth and explodes, with no regard to its composition (asteroid or comet). The word bolide comes from

10815-453: The typical dimensions of the main-belt asteroids has occurred. The 4:1 orbital resonance with Jupiter, at a radius 2.06  astronomical units (AUs), can be considered the inner boundary of the asteroid belt. Perturbations by Jupiter send bodies straying there into unstable orbits. Most bodies formed within the radius of this gap were swept up by Mars (which has an aphelion at 1.67 AU) or ejected by its gravitational perturbations in

10920-503: The upper atmosphere, creating a streak of light via its rapid motion and sometimes also by shedding glowing material in its wake. Although a meteor may seem to be a few thousand feet from the Earth, meteors typically occur in the mesosphere at altitudes from 76 to 100 km (250,000 to 330,000 ft). The root word meteor comes from the Greek meteōros , meaning "high in the air". Millions of meteors occur in Earth's atmosphere daily. Most meteoroids that cause meteors are about

11025-412: The zodiacal light. However, computer simulations by Nesvorný and colleagues attributed 85 percent of the zodiacal-light dust to fragmentations of Jupiter-family comets, rather than to comets and collisions between asteroids in the asteroid belt. At most 10 percent of the dust is attributed to the asteroid belt. Some of the debris from collisions can form meteoroids that enter the Earth's atmosphere. Of

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