Asteroid belt

The asteroid belt 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.

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).

[15] In an anonymous footnote to his 1766 translation of Charles Bonnet's Contemplation de la Nature,[16] the astronomer Johann Daniel Titius of Wittenberg[17][18] noted an apparent pattern in the layout of the planets, now known as the Titius-Bode Law.

[19] On January 1, 1801, Giuseppe Piazzi, chairman of astronomy at the University of Palermo, Sicily, found a tiny moving object in an orbit with exactly the radius predicted by this pattern.

[20] Thus, the aforementioned pattern predicted the semimajor axes of all eight planets of the time (Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, and Uranus).

[24][25] Upon completing a series of observations of Ceres and Pallas, he concluded,[26] Neither the appellation of planets nor that of comets can with any propriety of language be given to these two stars ...

[23] Despite Herschel's coinage, for several decades it remained common practice to refer to these objects as planets[16] and to prefix their names with numbers representing their sequence of discovery: 1 Ceres, 2 Pallas, 3 Juno, 4 Vesta.

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 first English use seems to be in the 1850 translation (by Elise Otté) of Alexander von Humboldt's Cosmos:[29] "[...] 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:[30] "The orbits of the asteroids are placed in a wide belt of space, extending between the extremes of [...]".

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.

[40] 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; 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.

[41] 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.

As Jupiter migrated inward following its formation, these resonances would have swept across the asteroid belt, dynamically exciting the region's population and increasing their velocities relative to each other.

[44] In regions where the average velocity of the collisions was too high, the shattering of planetesimals tended to dominate over accretion,[45] preventing the formation of a planet.

They have undergone considerable evolution since their formation, including internal heating (in the first few tens of millions of years), surface melting from impacts, space weathering from radiation, and bombardment by micrometeorites.

[54] 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.

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 the early history of the Solar System.

[58][59] In 2006, 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.

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.

[74] 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.

[80] This hypothesis was reinforced by the further discovery in 2007 of two asteroids in the outer belt, 7472 Kumakiri and (10537) 1991 RY16, with a differing basaltic composition that could not have originated from Vesta.

[82] However, due to rotation, the surface temperature of an asteroid can vary considerably as the sides are alternately exposed to solar radiation then to the stellar background.

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.

[87] Subsequently, asteroids primarily migrate into these gap orbits due to the Yarkovsky effect,[72] but may also enter because of perturbations or collisions.

[94] In 1918, the Japanese astronomer Kiyotsugu Hirayama noticed that the orbits of some of the asteroids had similar parameters, forming families or groups.

[99] The main belt evolution after the Late Heavy Bombardment was likely affected by the passages of large Centaurs and trans-Neptunian objects (TNOs).

Some members belong to the Mars-crossing category of asteroids, and gravitational perturbations by Mars are likely a factor in reducing the total population of this group.

[104] The Veritas family formed about 8.3 million years ago; evidence includes interplanetary dust recovered from ocean sediment.