The asteroid belt is a ring-shaped region in the Solar System—located roughly between the orbits of the planets Mars and Jupiter—that is occupied by a great many solid, irregularly shaped asteroids that are of many sizes much smaller than planets. This asteroid belt is also called 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.

About half the mass of the belt is contained in the four largest asteroids: Ceres, Vesta, Pallas, and Hygiea. The total mass of the asteroid belt is approximately 4% that of the Moon, or 22% that of Pluto.

Ceres, the only object in the asteroid belt large enough to be called a dwarf planet, is about 950 km in diameter, whereas Vesta, Pallas, and Hygiea have mean diameters of less than 600 km. 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. 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. 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. 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. 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.

The Hungaria group is a dynamical group of asteroids in the asteroid belt. The asteroids orbit the Sun with a semi-major axis between 1.78 and 2.00 AU. They are the innermost dense concentration of asteroids in the Solar System—the near-Earth asteroids are much more sparse—and derive their name from their largest member, 434 Hungaria. The Hungaria group includes the Hungaria family, a collisional asteroid family which dominates its population. Most Hungarias are E-type asteroids, which means they have extremely bright enstatite surfaces.

The 4:1 resonance Kirkwood gap (at 2.06 AU) marks the outer boundary of the Hungaria family, while interactions with Mars determine the inner boundary. For comparison, the majority of asteroids are in the core region of the asteroid belt, which lies between the 4:1 gap (at 2.06 AU) and the 2:1 gap (at 3.27 AU).

E-belt

The Hungaria asteroids are thought to be the remains of the hypothetical E-belt asteroid population. The dispersal of most of that hypothetical E-belt might have been caused by the outwards migration of the giant planets of the Solar System, and these dispersed E-belt asteroids might in turn have been the impactors of the Late Heavy Bombardment.

The Flora family is a prominent family of stony asteroids located in the inner region of the asteroid belt. It is one of the largest families, with more than 13,000 known members, or approximately 3.5% of all main-belt asteroids. The origin and properties of this family are relatively poorly understood. It is a very broad family which gradually fades into the surrounding background population.

While the largest members, 8 Flora and 43 Ariadne, are located near the edge, there are several distinct groupings within the family, possibly created by later, secondary collisions. Due to this complex internal structure and the poorly defined boundaries, the Flora family has also been described as an asteroid clan. Only few interlopers have been identified. This family may be the source of the impactor that formed the Chicxulub crater, the likely culprit in the extinction of the dinosaurs.

Vesta contributes an estimated 9% of the mass of the asteroid belt. It is probably slightly larger than Pallas, but is about 25% more massive. Vesta is the only known remaining rocky protoplanet (with a differentiated interior) of the kind that formed the terrestrial planets.

Vesta is the brightest asteroid visible from Earth. Its maximum distance from the Sun is slightly greater than the minimum distance of Ceres from the Sun, though its orbit lies entirely within that of Ceres.

The Vesta family is a family of asteroids. The family is located in the inner asteroid belt, in the vicinity of its namesake and principal body, 4 Vesta. It is one of the largest asteroid families, and consists of mostly bright V-type asteroids; so-called "vestoids".

The Vestian asteroids consist of 4 Vesta, and many small asteroids below 10 km diameter. The family originated from an impact on asteroid 4 Vesta, with the giant south-polar crater the likely impact site. The Vesta family also includes a few J-type asteroids (related to the V-type), which are thought to have come from the deeper layers of Vesta's crust.

A collisional family of asteroids in the inner region of the asteroid belt. Phocaea asteroids are of stony S-type composition. Several Phocaean asteroids are also Mars-crossers.

The Phocaea family region contains other collisional families such as the recently identified carbonaceous, Tamara family, named after its potentially largest member 326 Tamara. Several clumps around 290 Bruna (Bruna family), 1192 Prisma and 6246 Komurotoru, as well as (17628) 1996 FB5, (19536) 1999 JM4 and (26142) 1994 PL1 have also been detected.

It is located in the inner region of the asteroid belt, and is part of the Nysa–Polana complex; the largest cluster of asteroid families in the asteroid belt, with nearly 20,000 members.

Asteroids in this complex are typically divided into the stony Nysa and carbonaceous Polana subgroups, two mineralogically different families.

A dynamical group of asteroids held in the 2.5 AU region by the 1:3 orbital resonance with Jupiter, which results in their being close to a 4:1 resonance with Earth. An object in this resonance has its orbital eccentricity steadily increased by gravitational interactions with Jupiter until it eventually has a close encounter with an inner planet that breaks the resonance.

Some Alindas have perihelia very close to Earth's orbit, resulting in a series of close encounters at almost exactly four-year intervals, due to the 4:1 near resonance. One consequence is that some of these asteroids make repeated relatively close approaches to Earth.

A collisional asteroid family located in the inner parts of the intermediate asteroid belt, near the 1:3 Kirkwood gap. The family consist of several thousand stony S-type asteroids. The family consists of 2,940 known members.

The Eunomia or Eunomian family is a large asteroid family of S-type asteroids. It is the most prominent family in the intermediate asteroid belt, and the 6th-largest family with nearly six thousand known members, or approximately 1.4% of all asteroids in the asteroid belt.

The family contains relatively large numbers of small objects. Since most of these smaller objects are "eroded" away over time due to secondary collisions, gravitational perturbations, and the Yarkovsky effect, this indicates that the Eunomia family was created relatively recently (on an astronomical timescale).

The interlopers share the same orbital elements as the true Eunomia family members, but can not have come from the same breakup because of spectral (hence, compositional) differences.

It was the third asteroid found, but was initially considered to be a planet; it was reclassified as an asteroid and minor planet during the 1850s. Juno's small size and irregular shape preclude it from being designated a dwarf planet.

Juno is one of the larger asteroids, and is the second-most-massive S-type asteroid after 15 Eunomia. Even so, Juno has only 3% the mass of Ceres. Amongst S-type asteroids, Juno is unusually reflective, which may be indicative of distinct surface properties.

The bodies in the clump are all small. (32326) 2000 QO₆₂—the brightest of those clearly in the visible clump—would have a diameter of about 6 km, given the same albedo as 3 Juno. This indicates that it is probably a so-called cratering family composed of ejecta from impacts on 3 Juno.

Its shape departs from that of an ellipsoid. Pallas is thought to have undergone at least some degree of thermal alteration and partial differentiation, suggesting that it is a remnant protoplanet.

Pallas's orbit is unusually highly inclined, and its orbital eccentricity is nearly as large as that of Pluto. It is in a near-1:1 orbital resonance with Ceres. Pallas also has a near-18:7 resonance and an approximate 5:2 resonance with Jupiter. Furthermore, Pallas has a very high axial tilt. This means that every Palladian summer and winter, large parts of the surface are in constant sunlight or constant darkness for about an Earth year, with areas near the poles experiencing continuous sunlight for as long as two years.

A small asteroid family of B-type asteroids at very high inclinations in the intermediate asteroid belt. The remaining bodies are far smaller than Pallas. This, along with the preponderance of the otherwise rare B spectral type among its members, indicates that this is likely a cratering family composed of ejecta from impacts on Pallas.

An S-type asteroid family located the in intermediate asteroid belt. It had previously been known as the Ceres family. It is still known as Minerva family, named after then thought parent body 93 Minerva, until it was identified to be an interloper into its own family.

The largest object in the main asteroid belt. Ceres is the only object in the asteroid belt known to be currently rounded by its own gravity.

Ceres appears to be partially differentiated into a muddy (ice-rock) mantle, with a crust that is 60% rock and 40% ice or less than 30% ice.] It probably no longer has an internal ocean of liquid water, but there is brine that can flow through the outer mantle and reach the surface. The surface is a mixture of water ice and various hydrated minerals such as carbonates and clay.

Emissions of water vapor were detected from several regions of Ceres. Any atmosphere, however, would be the minimal kind known as an exosphere.

Ceres is in a near-1:1 mean-motion orbital resonance with Pallas. However, a true resonance between the two would be unlikely; due to their small masses relative to their large separations. Ceres is able to capture other asteroids into temporary 1:1 resonant orbital relationships (making them temporary trojans) for periods up to 2 million years or more; fifty such objects have been identified.

Although not as actively discussed as a potential home for microbial extraterrestrial life as Mars, Europa, Enceladus, or Titan, there is evidence that Ceres' icy mantle was once a watery subterranean ocean . The remote detection of organic compounds and the presence of water with 20% carbon by mass in its near surface, could provide conditions favorable to organic chemistry.

The Koronis or Koronian family—also known as the Lacrimosa family—is a very large asteroid family of stony asteroids, located in the outer region of the asteroid belt. They are thought to have been formed at least two billion years ago in a catastrophic collision between two larger bodies. The Koronis family travels in a cluster along the same orbit.

This family has two subfamilies. The Karin family was formed remarkably recently in a catastrophic collision (destroying the parent body). The Koronis family, with 246 members, is the other. It formed in non-catastrophic collision with 158 Koronis.

A very large asteroid family located in the outer region of the asteroid belt. The family of K-type asteroids is believed to have formed as a result of an ancient catastrophic collision. The family's parent body is the asteroid 221 Eos. Prior to the breakup, the parent body was partly segregated with denser materials moving toward the core. Since the breakup, the family members have undergone space weathering.

The inner orbit of the family is bracketed by the 7/3 mean-motion resonance with Jupiter at 2.96 AU. The orbital range also includes the 9/4 mean-motion resonance with Jupiter at 3.03 AU. Most of the family members lie within the latter orbital distance.

The rotation rates of the Eos family asteroids are randomly distributed. This randomization resulted from subsequent collisions with other bodies, implying that the asteroids retain some "memory" of the rotation rate of the parent body. Evolutionary models of this spread in the rotation rate of the Eos family implies that this group may be comparable to the age of the Solar System.

Not all fragments of the original parent body have remained in the orbital zone occupied by the Eos family. Spectroscopic analysis has shown that some of these asteroids are now located in the 9:4 mean-motion resonance with Jupiter. These fugitives appear relatively young compared to the other family members.

Hygiea has a nearly spherical shape.

Aside from being the smallest of the "big four", Hygiea—like Ceres—has a relatively low density, which is more comparable to the icy satellites of Jupiter or Saturn, than to the terrestrial planets or the stony asteroids. Aqueous alteration products have been detected on its surface, which could indicate the presence of water ice in the past which was heated sufficiently to melt.

Hygiea is the main member of the Hygiean asteroid family that constitutes about 1% of asteroids in the main belt. Because the impact craters on Hygiea today are too small to contain the volume of ejected material, it is thought that Hygiea was completely disrupted by the impact, and that the majority of the debris recoalesced after the pieces that formed the rest of the family had escaped. Hygiea contains almost all the mass (over 98%) of the family.

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10 Hygiea is a major Unified government position and comparatively large Unified Forces base in the outer main belt. It is self-sufficient (food and manufacturing), and is the center of the outer main belt mining and manufacturing complex.

finish writing this description (1 Factory Satellite in orbit, and in charge of mining and manufacturing in this part of the belt.)

The Hygiea—or Hygiean—family of asteroids is a grouping of dark, carbonaceous C-type and B-type asteroids in outer asteroid belt, the largest member of which is 10 Hygiea. About 1% of all known asteroids in the asteroid belt belong to this family.

The members are much smaller than 10 Hygiea, so Hygiea contains about 94–98% of the mass in the family. Aside from the 3 largest, the members have diameters of less than 30 km.

The Hygiea family is thought to be of the cratering type, implying that it was formed as a result of a giant impact that excavated the much smaller family members from Hygiea. The family contains a significant number of objects of the otherwise rare B spectral type. There are some indications that this family is relatively quite old.

A family of carbonaceous asteroids located in the outer portion of the asteroid belt. It is one of the largest families with, and consists of a well-defined core of larger bodies surrounded by a region of smaller ones.

One of the ten most-massive asteroids, and the 7th-largest asteroid. Davida comprises an estimated 1.5% of the total mass of the asteroid belt.

There are at least two promontories and at least one flat facet. The facet is presumably a 150-km global-scale crater.

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The Unified Government presence is limited to a listening post/inner solar system observation facilities, and a handful of small mining settlements. The Unified Government facilities are centred on Batel—also the location of the "main" starport—and Nissa. The mining and refining industries are centred on Nissa and Alona. All three cities are surrounded by a handful of satellite settlements—all centred on mining. Efforts have been made to slow down the rotation rate (the figure on the left is the real world value).

While the Unified Forces personnel assigned to Sylvia find it extremely remote and boring, the civilian miners love the quiet and laid-back atmosphere that comes from being so far off the beaten track. The settlements are not self-sufficient, and rely on a steady flow of water, food, and other resources from the outside.

A Hecuba-gap asteroid stays in a 2:1 mean motion resonance with the gas giant Jupiter, which may gradually perturb its orbit over a long period, until it either intersect with the orbit of Mars or Jupiter itself.

Depending on the dynamical stability of an asteroid's orbit in the Hecuba gap, three subgroups have been proposed. These are the marginally unstable Griqua asteroids, the stable Zhongguo asteroids, and an unnamed, strongly unstable population of asteroids.

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The Unified Government presence is limited to routine patrols and random training missions.

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Griqua asteroids
The Griqua asteroids (also known as the "Griquas") are a dynamical group of asteroids with marginally unstable orbits. The Griquas are located in the Hecuba gap, and stay in a 2:1 mean motion resonance with the gas giant Jupiter. The group has an estimated lifetime of 100 to 500 million years.

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Zhongguo asteroids
While the Griquas are asteroids in a marginally unstable orbit, the Zhongguo asteroids (or "Zhongguos") are in a rather stable 1:2 resonance with Jupiter. The group of 26 identified members, with a lifetime of more than 530 million (or even 1 billion) years.

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Strongly unstable group
Related but distinct from the "long-lived" Griquas and Zhongguos, is the group of "unstable" Hecuba-gap asteroids. They show life-times of less than 70 million years. It is also thought that approximately 25% of the short-lived population are "extremely unstable", with lifetimes of less than 2 million years.

Some asteroids have such a high eccentricity with a perihelion of less than 1.3 AU, that qualifies them as a near-Earth object.

Sylvia is very dark in color and probably has a very primitive composition. Its density is very low, indicating that the asteroid is porous to very porous; 25%–60% may be empty space. This suggests a loose rubble pile structure.

Sylvia is the first asteroid known to possess more than one moon. The orbital planes of both satellites and the equatorial plane of the primary asteroid are all well-aligned; all planes are aligned within about 1° of each other, suggestive of satellite formation in or near the equatorial plane of the primary.

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The Unified Government presence is limited to a listening post/inner solar system observation facilities, and a handful of small mining settlements. The Unified Government facilities are centred on Aemilia—also the location of the "main" starport—and the mining and refining industries are centred on Fabricius, with a handful of satellite settlements also centred on mining.

While the Unified Forces personnel assigned to Sylvia find it extremely remote and boring, the civilian miners love the quiet and laid-back atmosphere that comes from being so far off the beaten track. The settlements are not self-sufficient, and rely on a steady flow of water, food, and other resources from the outside.

A dynamical group of asteroids, named after the asteroid 65 Cybele. Considered by some as the last outpost of an extended asteroid belt, the group consists of nearly 2,000 members, and a few collisional families.

The dynamical Cybele group is located adjacent to the outermost asteroid belt, beyond the Hecuba gap, and inside the orbital region of the Hilda asteroids, which are themselves followed by the Jupiter trojans further out. The group is thought to have formed from the breakup of a larger object in the distant past.

Three known asteroid families exist within the Cybele group: the Sylvia family (255 members), the Huberta family (26 members) and the Ulla family (26 members). A potential fourth family is a small cluster with the parent body 2000 EK. A fifth family, named after 522 Helga, was identified in 2015.

An exceptionally eccentric asteroid, classified as a near-Earth object, as well as a centaur and extinct comet.

Infrared observations, however, revealed a faint coma and tail around the object. It is still unknown whether the observed activity is persistent or an outburst, resulting from the excavation of sub-surface CO₂ ice due to a recent impact of a smaller body.

The Hilda asteroids are a dynamical group of more than 4,000 asteroids located beyond the asteroid belt, in a 3:2 orbital resonance with Jupiter. It is not a true asteroid family, since the members are not physically related, but rather share similar orbital elements.

Hildas move in their elliptical orbits so that their aphelia put them opposite Jupiter (at L₃), or 60° ahead of or behind Jupiter at the L₄ and L₅ Lagrangian points. Over three successive orbits, each Hilda asteroid approaches all of these three points in sequence. A Hilda's orbit has a semi-major axis between 3.7 and 4.2 AU.

Two collisional families exist within the Hilda group: the Hilda family and the Schubart family.

Hildas' surface colors often correspond to the D-type and P-type. D-type and P-type asteroids have surface colors (mineralogies) similar to those of cometary nuclei.

The Hildas taken together constitute a dynamic triangular figure in the triangular libration points of Jupiter—the "Hildas Triangle". The "asteroidal stream" within the sides of the triangle is about 1 AU wide, and in the apexes this value is 20–40% greater. The Hildas Triangle has proven to be dynamically stable over a long time span.

At the apexes of the triangle—corresponding to the points L₄ and L₅ of Jupiter's orbit—the Hildas approach the Trojans. At the mid-sides of the triangle, they are close to the asteroids of the external part of the asteroid belt. The velocity dispersion of the Hildas is more evident than that of the Jupiter Trojans in the regions where they intersect. The dispersion of Trojans in inclination is twice that of the Hildas. Due to this, as much as one quarter of the Trojans cannot intersect with the Hildas, and at all times many Trojans are located outside Jupiter's orbit.