Asteroid belt

The asteroids of the inner Solar System and Jupiter: The donut-shaped asteroid belt is located between the orbits of Jupiter and Mars.
  Jupiter trojans
  Orbits of planets
  Asteroid belt
  Hilda asteroids (Hildas)
  Near-Earth objects (selection)
The relative masses of the top twelve asteroids known compared to the remaining mass of all the other asteroids in the belt.
By far the largest object within the belt is Ceres. The total mass of the asteroid belt is significantly less than Pluto's, and approximately twice that of Pluto's moon Charon.

The asteroid belt is the circumstellar disc in the Solar System located roughly between the orbits of the planets Mars and Jupiter. It is occupied by numerous irregularly shaped bodies called asteroids or minor planets. The asteroid belt is also termed the main asteroid belt or main belt to distinguish it from other asteroid populations in the Solar System such as near-Earth asteroids and trojan asteroids.[1] About half the mass of the belt is contained in the four largest asteroids: Ceres, Vesta, Pallas, and Hygiea.[1] The total mass of the asteroid belt is approximately 4% that of the Moon, or 22% that of Pluto, and roughly twice that of Pluto's moon Charon (whose diameter is 1200 km).

Ceres, the asteroid belt's only dwarf planet, is about 950 km in diameter, whereas 4 Vesta, 2 Pallas, and 10 Hygiea have mean diameters of less than 600 km.[2][3][4][5] The remaining bodies range down to the size of a dust particle. The asteroid material is so thinly distributed that numerous unmanned spacecraft have traversed it without incident.[6] Nonetheless, collisions between large asteroids do occur, and these can produce an asteroid family whose members have similar orbital characteristics and compositions. Individual asteroids within the asteroid 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 the primordial solar nebula as a group of planetesimals.[7] Planetesimals are the smaller precursors of the protoplanets. Between Mars and Jupiter, however, gravitational perturbations from Jupiter imbued the protoplanets with too much orbital energy for them to accrete into a planet.[7][8] Collisions became too violent, and instead of fusing together, the planetesimals and most of the protoplanets shattered. As a result, 99.9% of the asteroid belt's original mass was lost in the first 100 million years of the Solar System's history.[9] 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.[10]

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 the Oort cloud objects.

On 22 January 2014, ESA scientists reported the detection, for the first definitive time, of water vapor on Ceres, the largest object in the asteroid belt.[11] The detection was made by using the far-infrared abilities of the Herschel Space Observatory.[12] 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."[12]

History of observation

Johannes Kepler was the person who first noticed in 1596 that there was something strange about the orbits of Mars and Jupiter.

In 1596, Johannes Kepler predicted “Between Mars and Jupiter, I place a planet” in his Mysterium Cosmographicum.[13] While analyzing Tycho Brahe's data, Kepler thought that there was too large a gap between the orbits of Mars and Jupiter.[14]

In an anonymous footnote to his 1766 translation of Charles Bonnet's Contemplation de la Nature,[15] the astronomer Johann Daniel Titius of Wittenberg[16][17] noted an apparent pattern in the layout of the planets. 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 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."[16]

When William Herschel discovered Uranus in 1781, the planet's orbit matched the law almost perfectly, leading astronomers to conclude that there had to be a planet between the orbits of Mars and Jupiter.

Giuseppe Piazzi, discoverer of Ceres, the largest object in the asteroid belt. For several decades after its discovery, Ceres was known as a planet, after which it was reclassified as an asteroid. In 2006, it was designated as a dwarf planet.

On January 1, 1801, Giuseppe Piazzi, chair of astronomy at 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.[18]

Thus, the aforementioned pattern, now known as the Titius–Bode law, predicted the semi-major axes of all eight planets of the time (Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn and Uranus).

Fifteen months later, Heinrich Olbers 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 the Greek asteroeides, meaning "star-like".[19][20] Upon completing a series of observations of Ceres and Pallas, he concluded,[21]

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, 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.[22] The burning of Lilienthal in the Napoleonic wars, where the main body of work had been done,[23] brought this first period of discovery to a close.[22]

Despite Herschel's coinage, for several decades it remained common practice to refer to these objects as planets[15] and to prefix their names with numbers representing their date of discovery: 1 Ceres, 2 Pallas, 3 Juno, 4 Vesta. However, in 1845 astronomers 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 choice of nomenclature, "asteroids", gradually came into common use.[15]

The discovery of Neptune in 1846 led to the discrediting of the Titius–Bode law in the eyes of scientists because its orbit was nowhere near the predicted position. To date, there is no scientific explanation for the law, and astronomers' consensus regards it as a coincidence.[24]

The expression "asteroid belt" came into use in the very early 1850s, although it is hard to pinpoint who coined the term. The first English use seems to be in the 1850 translation (by E. C. Otté) of Alexander von Humboldt's Cosmos:[25] "[...] 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:[26] "The orbits of 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.[27]

One hundred asteroids had been located by mid-1868, and in 1891 the introduction of astrophotography by Max Wolf accelerated the rate of discovery still further.[28] A total of 1,000 asteroids had been found by 1921,[29] 10,000 by 1981,[30] and 100,000 by 2000.[31] Modern asteroid survey systems now use automated means to locate new minor planets in ever-increasing quantities.

Other Languages
Afrikaans: Asteroïdegordel
Alemannisch: Asteroidengürtel
Avañe'ẽ: Mbyjaveve Ku'asã
azərbaycanca: Asteroid qurşağı
беларуская: Пояс астэроідаў
беларуская (тарашкевіца)‎: Пояс астэроідаў
български: Астероиден пояс
Boarisch: Asteroidngiatl
čeština: Hlavní pás
Esperanto: Asteroida zono
한국어: 소행성대
Bahasa Indonesia: Sabuk asteroid
latviešu: Asteroīdu josla
Lëtzebuergesch: Haaptasteroidenceinture
Lingua Franca Nova: Sintur de asteroides
lumbaart: Fasa Principala
македонски: Астероиден појас
Bahasa Melayu: Lingkaran asteroid
မြန်မာဘာသာ: ပင်မ ဥက္ကာ ခါးပတ်
Nederlands: Planetoïdengordel
日本語: 小惑星帯
norsk nynorsk: Asteroidebeltet
Plattdüütsch: Asteroidengördel
Ripoarisch: Asteoridejüddel
Simple English: Asteroid belt
slovenčina: Pásmo planétok
slovenščina: Asteroidni pas
srpskohrvatski / српскохрватски: Asteroidni pojas
татарча/tatarça: Астероидлар билбавы
українська: Пояс астероїдів
文言: 小行星帶
粵語: 小行星帶