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By AARON SKETCHLEY (aaronsketch@HOTdelete_thisMAIL.com) 2015.07.10 Ver 3.6

Official Setting information is in darkgreen. Extended Universe information is in steelblue.

The Solar System - Inner Planets

0 AU
  • The Sun
  • Vulcanoid Asteroids
  • (P) Mercury
  • Mercury-crosser Asteroids
    0.5 AU
  • (P) Venus
    - Henry Beggs Satellite City
  • Venus-crosser Asteroids
  • Apohele asteroid
  • The Atens
    1 AU
    (P) Earth
    - Macross City
    - Kármán line (Earth Orbit)
    - Space Colony Clusters [Bunches]
  • - Earth-Moon Lagrange Points
  • Moon
    - Apollo Base Colony
  • Earth Trojans
  • Near-Earth object
  • Earth-crosser Asteroids
  • Apollo Asteroids
  • The Amors
    1.5 AU
  • (P) Mars
    - H.G. Wells City
    - Sera Base
  • Phobos
  • Deimos
  • Mars Trojans
  • Mars-crosser Asteroids
  • 2.0 AU
  • The Asteroid Belt
    - Hungaria family
    - Flora family
    - Vesta family
  • Vesta
    - Phocaea family
    - Nysa family
    2.5 AU
    - Alinda family
    - Maria family
    - Eunomia family
  • Juno
    - Pallas family
  • Pallas
    - Gefion family
  • (D) Ceres
    - Ceres Base

    - Koronis family
    3.0 AU
    - Eos family
    - Hygiea family
    - Themis family
    - Griqua family
    - Cybele family
    3.5 AU
  • Don Quixote

  • Outer Solar System
  • Trans-Neptunian & Farthest Regions
  • Only objects of note or larger than 500 km have been included in the table of contents. For more information on comets, please use the following links: list of periodic comets and list of non-periodic comets.

    Sol
    The Sun is the star at the center of the Solar System and is by far the most important source of energy for life on Earth. It is a nearly perfect spherical ball of hot plasma, with internal convective motion that generates a magnetic field via a dynamo process. Its diameter is about 109 times that of Earth, and it has a mass about 330,000 times that of Earth, accounting for about 99.86% of the total mass of the Solar System. Chemically, about three quarters of the Sun's mass consists of hydrogen, whereas the rest is mostly helium, and much smaller quantities of heavier elements, including oxygen, carbon, neon and iron.

    The Sun formed approximately 4.567 billion years ago from the gravitational collapse of matter within a region of a large molecular cloud. Most of this matter gathered in the center, whereas the rest flattened into an orbiting disk that became the Solar System. The Sun is roughly middle age and has not changed dramatically for four billion years, and will remain fairly stable for four billion more. However, after hydrogen fusion in its core has stopped, the Sun will undergo severe changes and become a red giant. It is calculated that the Sun will become sufficiently large to engulf the current orbits of Mercury, Venus, and possibly Earth.

    The Sun does not have a definite boundary, and in its outer parts its density decreases exponentially with increasing distance from its center. For the purpose of measurement, however, the Sun's radius is considered to be the distance from its center to the edge of the photosphere, the apparent visible surface of the Sun.

    Vulcanoid Asteroids
    The vulcanoid are a hypothetical population of asteroids in a stable orbit with a semi-major axis less than that of Mercury. This does not include objects like sungrazing comets which, although they have a perihelion inside the orbit of Mercury, have a far greater semi-major axis. So far, no vulcanoids have been discovered, and it is not yet clear if any exist. If they do exist, the vulcanoids could easily evade detection because they would be very small and drowned out by the bright glare of the nearby Sun. Due to their proximity to the Sun, searches from the ground can only be carried out during twilight or solar eclipses. Any vulcanoids must be between about 100 m and 6 km in diameter and are probably located in nearly circular orbits near the outer edge of the gravitationally stable zone, 0.06 to 0.21.

    The vulcanoids, should they be found, may provide scientists with material from the first period of planet formation, as well as insights into the conditions prevalent in the early Solar System. Although every other gravitationally stable region in the Solar System has been found to contain objects, non-gravitational forces (such as the Yarkovsky effect) or the influence of a migrating planet in the early stages of the Solar System's development may have depleted this area of any asteroids that may have been there.

    There may be no more than 300-900 vulcanoids larger than 1 km in radius remaining, if any. The gravitational stability of the vulcanoid zone is due in part to the fact that there is only one neighbouring planet. In that respect it can be compared to the Kuiper belt.[18]

    Mercury
    Mercury is the smallest and closest planet to the Sun. Because it has almost no atmosphere to retain heat, Mercury's surface experiences the greatest temperature variation of the planets in the Solar System, ranging from 100 K (-173°C) at night to 700 K (427°C) during the day at some equatorial regions. The poles are constantly below 180 K (-93°C). Mercury's axis has the smallest tilt of any of the Solar System's planets, but it has the largest orbital eccentricity. At aphelion, Mercury is about 1.5 times as far from the Sun as it is at perihelion. Mercury's surface is heavily cratered and similar in appearance to the Moon, indicating that it has been geologically inactive for billions of years.

    Mercury is gravitationally locked and rotates in a way that is unique in the Solar System. As seen relative to the fixed stars, it rotates on its axis exactly three times for every two revolutions it makes around the Sun. As seen from the Sun, in a frame of reference that rotates with the orbital motion, it appears to rotate only once every two Mercurian years. An observer on Mercury would therefore see only one day every two years.

    Because Mercury orbits the Sun within Earth's orbit, it can appear in Earth's sky in the morning or the evening, but not in the middle of the night. Also, like Venus and the Moon, it displays a complete range of phases as it moves around its orbit relative to Earth. Although Mercury can appear as a bright object when viewed from Earth, its proximity to the Sun makes it more difficult to see than Venus.

    Mercury-crosser Asteroids
    A Mercury crosser is an asteroid whose orbit crosses that of Mercury. The Mercury crossers proper have aphelia outside Mercury's and perihelia inside Mercury's. As of 28 February 2015, there are 237 Mercury crossers known.

    All Mercury grazers have semi-major axes larger than Mercury's, and hence are outer grazers, i.e. have perihelia within Mercury's aphelion but not within its perihelion. As of May 2012, there are 409 Mercury grazers known.


    2004 JG6

    2004 JG6 is one of the closest orbiting objects to the Sun.

    1999 KW4

    1999 KW4 is a binary asteroid. The shapes of the two bodies and their dynamics are complex. Among other bizarre properties, the equatorial regions of Alpha are very close to breakup: raising a particle a meter above the surface would put it into orbit around the object. The gravitational effects between the moon and the asteroid create a gigantic mountain extending laterally around the entire asteroid.

    2002 VE68

    2002 VE68 is a temporary quasi-satellite of Venus. It was the first quasi-satellite to be discovered around a major planet in the Solar System. In a frame of reference rotating with Venus, it appears to travel around it during one Venusian year but it actually orbits the Sun, not Venus.

    Atira

    Atira was the first asteroid known to have an orbit entirely within that of Earth. It is the largest of the 12 currently known Atiras and one of the larger Aten asteroids.

    1998 WT24

    1998 WT24 has frequent close encounters with Mercury, Venus, and Earth. It is also one of the best studied potentially hazardous asteroids and was the 10th Aten to be numbered. It is classified a type E/Xe asteroid. 1998 WT24 has an orbit that not only crosses Earth's orbit, but is roughly parallel to it at its furthest. This means that during favourable years it is easy to reach from Earth. Several times this decade, it will be an easier target than Earth's own Moon. It is high on the list of possible Near-Earth Asteroid targets for future flybys, landings, mining, or orbit manipulations.

    Venus
    Venus is the second planet from the Sun. It is a terrestrial planet and is sometimes called Earth's "sister planet", because of their similar size, mass, proximity to the Sun and bulk composition. It is radically different from Earth in other respects. It has the densest atmosphere of the four terrestrial planets, consisting of more than 96% carbon dioxide. The atmospheric pressure at the planet's surface is 92 times that of Earth's. With a mean surface temperature of 735 K (462°C), Venus is by far the hottest planet in the Solar System, even though Mercury is closer to the Sun.

    Venus has no carbon cycle that puts carbon into rock, nor does it seem to have any organic life to absorb carbon in biomass. Venus is shrouded by an opaque layer of highly reflective clouds of sulfuric acid, preventing its surface from being seen from space in visible light. It may have had oceans in the past, but these would have vaporized as the temperature rose due to a runaway greenhouse effect. The water has most probably photodissociated, and, because of the lack of a planetary magnetic field, the free hydrogen has been swept into interplanetary space by the solar wind. Venus's surface is a dry desertscape interspersed with slab-like rocks and periodically refreshed by volcanism.

    Venus-crosser Asteroids
    A Venus-crosser is an asteroid whose orbit crosses that of Venus. There are 2,479 Venus-crosser and 86 outer-grazers known.

    Apohele asteroid
    Apohele asteroids, or alternatively Interior-Earth Objects (IEOs) or Atira asteroids, are a class of near-Earth asteroids. They have not only their perihelion within Earth's orbit, but also their aphelion; that is, their entire orbit is within Earth's. As of January 2015, there are 15 suspected Apoheles, of which eight have well-known orbits, of which five have been determined with sufficient precision to receive a permanent number; 28 more objects have aphelia smaller than Earth's aphelion.

    There is no standard name for the class. The name Apohele is the Hawaiian word for orbit; it was chosen partially because of its similarity to the words aphelion (apoapsis) and helios. Other authors adopted the designation Inner Earth Objects (IEOs). Still others, following the general practice to name a new class of asteroids for the first recognized member of that class, use the designation Atira asteroids. Atiras do not cross Earth's orbit and are not immediate impact threats, but their orbits may be perturbed outward by a close approach to either Mercury or Venus and become Earth-crossing asteroids in the future.


    2000 BD19

    2000 BD19 is an asteroid with the smallest perihelion of any numbered asteroid. It has the third largest aphelion of any numbered Aten asteroid . Its orbital elements indicate that may be an extinct comet. It hasn't been seen displaying cometary activity so far.

    Apophis

    Apophis is an Sq-type asteroid. It is a near-Earth asteroid that caused a brief period of concern in December 2004 because initial observations indicated a probability of up to 2.7% that it would hit Earth on April 13, 2029. During the short time when it had been most concerning, Apophis set the record for highest rating on the Torino Scale, reaching level 4.

    On April 13, 2029, Apophis will pass Earth within the orbits of geosynchronous communication satellites, but will come no closer than 31,300 km above Earth's surface. The 2029 pass will be much closer than had first been predicted. The pass in late March 2036 will be no closer than about 23 million km, and will most likely miss Earth by something closer to 56 million km.


    Sekhmet

    Sekhmet is believed to be an S-type asteroid.

    The Atens
    The Aten asteroids are a group of near-Earth asteroids, named after the first of the group to be discovered. They are defined by having a semi-major axis of less than one AU. Atens also have an aphelion (furthest distance from the Sun) greater than 0.983 AU. Asteroids' orbits can be highly eccentric. An Aten orbit need not be entirely contained within Earth's orbit, as nearly all known Aten asteroids have their aphelion greater than one AU, although their semi-major axis is less than one AU. Observation of objects inferior to the Earth's orbit is difficult and this difficulty may be the cause of some sampling bias in the apparent preponderance of eccentric Atens. Aten asteroids account for only about 6% of the known near-Earth asteroid population. Many more Apollo-class asteroids are known than Aten-class asteroids, possibly because of the sampling bias.

    As of January 2015, there are only 14 known Apoheles, and 898 known Aten asteroids.


    Aten

    Aten is an S-type asteroid, and was the first asteroid found to have a semi-major orbital axis of less than one AU. A new category of asteroids was thus created, the Atens.

    2003 YN107

    2003 YN107 is a very small near-Earth object moving in a 1:1 mean-motion resonance with Earth. As such it is co-orbital to Earth.

    3753 Cruithne

    Cruithne is an Aten asteroid in orbit around the Sun in 1:1 orbital resonance with Earth, making it a co-orbital object. It is a minor planet in solar orbit that, relative to Earth, orbits in a bean-shaped orbit that ultimately effectively describes a horseshoe, and which can transition into a quasi-satellite orbit. It has been incorrectly called "Earth's second moon". Cruithne does not orbit Earth and at times it is on the other side of the Sun. Its orbit takes it inside the orbit of Mercury and outside the orbit of Mars. Cruithne orbits the Sun in about 1 year but it takes 770 years for the series to complete a horseshoe-shaped movement around the Earth.

    2002 AA29

    2002 AA29 is a small near-Earth asteroid. It revolves about the Sun on an almost circular orbit very similar to that of the Earth. This lies for the most part inside the Earth's orbit, which it crosses near the asteroid's furthest point from the Sun. A further characteristic is that its mean orbital period about the Sun is exactly one sidereal year. This means that it is locked into a relationship with the Earth, since such an orbit is only stable under particular conditions. As yet only a few asteroids of this sort are known, locked into a 1:1 resonance with the Earth. It follows a so-called horseshoe orbit along the path of the Earth.

    Earth
    It is the densest the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets. Earth formed approximately 4.54 billion years ago, and life appeared on its surface within one billion years. Earth's biosphere then significantly altered the atmospheric and other basic physical conditions, which enabled the proliferation of organisms as well as the formation of the ozone layer, which together with Earth's magnetic field blocked harmful solar radiation, and permitted formerly ocean-confined life to move safely to land. The physical properties of the Earth, as well as its geological history and orbit, have allowed life to persist.

    Earth's crust is divided into several rigid segments, or tectonic plates, that migrate across the surface over periods of many millions of years. About 71% of the surface is covered by salt water oceans, with the remainder consisting of continents and islands which together have many lakes and other sources of water that contribute to the hydrosphere. Earth's poles are mostly covered with ice, which is the solid ice of the Antarctic ice sheet and the sea ice that is the polar ice packs. The planet's interior remains active, with a solid iron inner core, a liquid outer core that generates the magnetic field, and a thick layer of relatively solid mantle.

    Earth interacts with other objects in space, especially the Sun and the Moon. The Earth's axis of rotation is tilted 23.4° away from the perpendicular of its orbital plane, producing seasonal variations on the planet's surface with a period of one tropical year. The Moon's gravitational interaction with Earth stimulates ocean tides, stabilizes the axial tilt, and gradually slows the planet's rotation.

    Three-quarters of the Earth's atmosphere's mass is contained within the first 11 km of the planet's surface. This lowest layer is called the troposphere. Energy from the Sun heats this layer, and the surface below, causing expansion of the air. This lower density air then rises, and is replaced by cooler, higher density air. The result is atmospheric circulation that drives the weather and climate through redistribution of heat energy. The primary atmospheric circulation bands consist of the trade winds in the equatorial region below 30° latitude and the westerlies in the mid-latitudes between 30° and 60°. Ocean currents are also important factors in determining climate, particularly the thermohaline circulation that distributes heat energy from the equatorial oceans to the polar regions.

    Above the troposphere, the atmosphere is usually divided into the stratosphere, mesosphere, and thermosphere. Beyond these, the exosphere thins out into the magnetosphere, where the Earth's magnetic fields interact with the solar wind. Within the stratosphere is the ozone layer, a component that partially shields the surface from ultraviolet light and thus is important for life on Earth.

    Earth Trojans
    An Earth trojan is an asteroid that orbits the Sun in the vicinity of the Earth-Sun Lagrangian points L4 (leading 60°) and L5 (trailing 60°), which means they have an orbit similar to Earth's. Several other small objects have been found on an orbital path associated with Earth. Although these objects are in 1:1 orbital resonance, they are not Earth trojans because they do not librate around a definite Sun-Earth Lagrangian point.

    Near-Earth object
    A near-Earth object (NEO) is a Solar System object whose orbit brings it into proximity with Earth. All NEOs have a closest approach to the Sun (perihelion) of less than 1.3 AU. They include more than ten thousand near-Earth asteroids (NEAs), near-Earth comets, a number of solar-orbiting spacecraft, and meteoroids large enough to be tracked in space before striking the Earth.

    Those NEOs that are asteroids (NEA) have orbits that lie partly between 0.983 and 1.3 astronomical units away from the Sun. Some near-Earth asteroids' orbits intersect that of Earth's so they pose a collision danger. There have been 872 near-Earth asteroids larger than 1 km discovered, of which 153 are potentially hazardous asteroids (PHAs). Potentially hazardous objects (PHOs) are currently defined based on parameters that measure the object's potential to make threatening close approaches to the Earth.

    Some NEOs are of high interest because they can be physically explored with lower mission velocity even than the Moon, due to their combination of low velocity with respect to Earth and small gravity, so they may present interesting scientific opportunities both for direct geochemical and astronomical investigation, and as potentially economical sources of extraterrestrial materials for human exploitation. This makes them an attractive target for exploration.

    Earth-crosser Asteroids
    An Earth-crosser is a near-Earth asteroid whose orbit crosses that of Earth as observed from the ecliptic pole of Earth's orbit. Those Earth-crossers whose semi-major axes are smaller than Earth's are Aten asteroids; the remaining ones are Apollo asteroids.

    An asteroid with an Earth-crossing orbit is not necessarily in danger of colliding with Earth. The orbit of an Earth-crossing asteroid may not even intersect with that of Earth. This apparent contradiction arises because many asteroids have highly inclined orbits, so although they may have a perihelion less than that of Earth, their paths can never cross. An asteroid for which there is some possibility of a collision with Earth at a future date and which is above a certain size is classified as a Potentially Hazardous Asteroid (PHA). The concept of PHA is intended to replace the now abandoned strict definition of ECA (Earth-crossing asteroid) which existed for a few years. Determining if an asteroid was an ECA required calculation of its orbits millennia into the future, including planetary gravitational perturbations, to assess whether a collision with Earth was possible and this has proved to be impractical.


    2010 SO16

    2010 SO16 is a near-Earth asteroid. It has a horseshoe orbit that allows it to stably share Earth's orbital neighborhood without colliding with it. It is one of a handful of known asteroids with an Earth-following orbit. In its ~350 yr horseshoe cycle, it never approaches Earth more closely than about 0.15 AU, alternately trailing and leading.

    Apollo Asteroids
    The Apollo asteroids are a group of near-Earth asteroids named after 1862 Apollo. They are Earth-crosser asteroids that have orbital semi-major axis greater than that of the Earth (> 1 AU) but perihelion distances less than the Earth's aphelion distance. Some can get very close to the Earth, making them a potential threat to our planet (the closer their semi-major axis is to Earth's, the less eccentricity is needed for the orbits to cross).

    There are 5,766 known Apollo-class asteroids of which 832 are numbered. Near-Earth asteroids are not numbered until they have been observed at two or more oppositions.


    2006 FV35

    2006 FV35 is a small near-Earth asteroid, and a quasi-satellite of Earth. It is also notable for having a low delta-v requirement for rendezvous.

    Asclepius

    Asclepius is a small asteroid that makes close orbital passes with Earth.

    2005 YU55

    2005 YU55 is a C-type, and a potentially hazardous asteroid.

    1998 KY26

    KY26 is a small near-Earth asteroid. With a rotation period of 10.7 minutes it has one of the shortest sidereal days of any known object in the Solar System, and cannot possibly be a rubble pile. It is also one of the most easily accessible objects in the Solar System, and its orbit frequently brings it on a path very similar to the optimum Earth-Mars transfer orbit. This, coupled with the fact that it is water rich, makes it an attractive target for further study and a potential source of water for future missions to Mars.

    Toro

    Toro is an S-type asteroid that orbits the Sun in a 5:8 resonance with Earth, and near a 5:13 resonance with Venus.

    Castalia

    Castalia is an S-type asteroid, and was the first asteroid to be modeled by radar imaging. Castalia has a peanut shape, suggesting two approximately 800-meter-diameter pieces held together by their weak mutual gravity.

    Geographos

    Geographos is an S-type asteroid, and is the most elongated object in the solar system.

    Phaethon

    Phaethon is a B-type asteroid, with an unusual orbit that brings it closer to the Sun than any other named asteroid.

    Itokawa

    Itokawa is an S-type asteroid. It was the first asteroid to be the target of a sample return mission, and the smallest asteroid photographed by a spacecraft. Itokawa may be a contact binary formed by two or more smaller asteroids that have gravitated toward each other and stuck together. There is a surprising lack of impact craters and a very rough surface studded with boulders, described by the mission team as a 'rubble pile'.

    The Amors
    The Amor asteroids are a group of near-Earth asteroids named after the asteroid 1221 Amor. They approach the orbit of Earth from beyond, but do not cross it. Most Amors do cross the orbit of Mars. The two moons of Mars, Deimos and Phobos, may be Amor asteroids that were captured by Mars's gravity. There are 3,729 Amor asteroids currently known. 580 of them are numbered, and 75 of them are named.


    433 Eros

    Eros is an S-type near-Earth asteroid, the second-largest near-Earth asteroid after Ganymed. It is a potential Earth impactor, comparable in size to the impactor that created Chicxulub crater and led to the extinction of the dinosaurs. Eros was one of the first asteroids to be visited by a spacecraft, and the first to be orbited and soft-landed on. NASA spacecraft NEAR Shoemaker entered orbit around Eros in 2000, and came to rest on its surface in 2001.

    Apollo

    Apollo is a Q-type asteroid. It is the namesake of the Apollo asteroids. Apollo is a potentially hazardous asteroid (PHA) because its minimum orbit intersection distance is less than 0.05 AU and its diameter is greater than 150 meters.

    Mars
    Mars is the fourth planet from the Sun and the second smallest planet in the Solar System, after Mercury. It is often referred to as the "Red Planet" because the iron oxide prevalent on its surface gives it a reddish appearance. Mars is a terrestrial planet with a thin atmosphere, having surface features reminiscent both of the impact craters of the Moon and the volcanoes, valleys, deserts, and polar ice caps of Earth. The rotational period and seasonal cycles of Mars are likewise similar to those of Earth, as is the tilt that produces the seasons. Mars is the site of Olympus Mons, the largest volcano and second-highest known mountain in the Solar System, and of Valles Marineris, one of the largest canyons in the Solar System. The smooth Borealis basin in the northern hemisphere covers 40% of the planet and may be a giant impact feature.

    Geological evidence gathered by unmanned missions suggests that Mars once had large-scale water coverage on its surface at some earlier stage of its life. There are large quantities of water ice at the poles and at mid-latitudes. Observations have revealed possible flowing water during the warmest months on Mars.

    Mars Trojans
    The Mars trojans are a group of objects that share the orbit of the planet Mars around the Sun. They can be found around the two Lagrangian points 60° ahead of and behind Mars. The origin of the Mars trojans is not well understood.

    Mars-crosser Asteroids
    A Mars-crosser is an asteroid whose orbit crosses that of Mars. A grazer is an object with a perihelion below the aphelion of Mars (1.67 AU) but above the Martian perihelion (1.38 AU). The JPL SBDB lists 13,500 Mars-crossing asteroids (MCAs).


    1990 MU

    1990 MU is one of the largest known Earth-Crossing Asteroids.

    1994 CC

    1994 CC is orbited by two satellites.

    Didymos

    Didymos is an Xk-type asteroid. Didymos is the most easily reachable asteroid of its size from Earth.

    1950 DA

    1950 DA is an E or M type asteroid. Among asteroids more than 1 km in diameter, it is notable for having the highest known probability of impacting Earth.

    Sisyphus

    Sisyphus is an S-type binary asteroid. It is the largest of the Earth-crossing asteroids, and is comparable in size to the Chicxulub object, whose impact may have killed off the dinosaurs.

    Eric

    Eric is a B-type asteroid.

    Amor

    Amor is a C (or S?) type asteroid, and is the namesake of the Amor asteroids.

    2001 SN263

    2001 SN263 is a C-type asteroid orbited by two satellites.

    3671 Dionysus

    Dionysus is a B-type binary asteroid.

    Mithra

    Mithra is an S-type asteroid. It has a bizarre shape - it is the most highly bifurcated object in the solar system, with two distinct lobes. It is a contact binary asteroid.

    Ganymed

    Ganymed is the largest near-Earth asteroid. It is an S-type asteroid.

    Asteroid Belt
    The asteroid belt is the region of 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. About half the mass of the belt is contained in the four largest asteroids, the three intact protoplanets Ceres, Vesta, and Pallas, and Hygiea. 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 form 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, the smaller precursors of the planets, which in turn formed 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.

    Kirkwood Gaps

    A Kirkwood gap is a gap or dip in the distribution of the semi-major axes (or equivalently of the orbital periods) of the orbits of main-belt asteroids. They correspond to the locations of orbital resonances with Jupiter. More recently, a relatively small number of asteroids have been found to possess high eccentricity orbits which do lie within the Kirkwood gaps. These orbits slowly increase their eccentricity on a timescale of tens of millions of years, and will eventually break out of the resonance due to close encounters with a major planet.

    The most prominent Kirkwood gaps are located at mean orbital radii of:[citation needed]

    Hungaria family
    The Hungaria family is a group of asteroids that 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. 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 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). Most Hungarias are E-type asteroids, which means they have extremely bright enstatite surfaces.

    Flora family
    The Flora or Florian family of asteroids is a large grouping of S-type asteroids in the inner main belt, whose origin and properties are relatively poorly understood at present. Roughly 4-5% of all main belt asteroids belong to this family. Because of its poorly defined boundaries, and the location of Flora itself near the edge, this asteroid group has also sometimes been called the Ariadne(an) family. The Flora family of asteroids may be the source of the Chicxlub (Cretaceous?Paleogene) impactor, the likely culprit in the extinction of the dinosaurs.

    The Flora family is very broad and gradually fades into the background population (which is particularly dense in this part of space) in such a way that its boundaries are very poorly defined. There are also several non-uniformities or lobes within the family, one cause of which may have been later secondary collisions between family members. Hence, it is a classical example of a so-called 'asteroid clan'. Curiously, the largest members, Flora and Ariadne, are located near the edge of the family. The reason for this unusual mass distribution within the family is unknown at present. The Flora family members are considered good candidates for being the parent bodies of the L chondrite meteorites, which contribute about 38% of all meteorites impacting the Earth.


    Eichsfeldia

    Eichsfeldia is a C-type asteroid and is probably composed of primitive carbonaceous material. Although Eichsfeldia has an orbit similar to the Vesta family asteroids, it was found to be an unrelated interloper on the basis of its non-matching spectral type.

    Unitas

    Unitas is an S-type asteroid. Although Unitas has an orbit similar to the Vesta family asteroids, it was found to be an unrelated interloper on the basis of its non-matching spectral type.

    Vesta family
    The Vesta or Vestian family of asteroids is a large and prominent grouping of mostly V-type asteroids ("vestoids") in the inner asteroid belt in the vicinity of Vesta. The family originated from an impact on asteroid Vesta, with the giant south-polar crater the likely impact site. The family are thought to be the source of the HED meteorites.

    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, and are similar to the diogenite meteorites. There are 235 core members, and 6,051 objects (about 6% of the total) lying within the Vesta-family region.

    4 Vesta
    Vesta is the second-most-massive object in the asteroid belt after the dwarf planet Ceres, and it contributes an estimated 9% of the mass of the asteroid belt. The less-massive Pallas is slightly larger, making Vesta third in volume. Vesta is the last remaining rocky protoplanet (with a differentiated interior) of the kind that formed the terrestrial planets.

    Numerous fragments of Vesta were ejected by collisions one and two billion years ago that left two enormous craters occupying much of Vesta's southern hemisphere. Debris from these events has fallen to Earth as howardite-eucrite-diogenite (HED) meteorites, which have been a rich source of information about Vesta.


    Amalthea

    Amalthea is an S-type asteroid. Amalthea is thought to be a fragment from the mantle of a Vesta-sized, 300-600 km diameter parent body that broke up around one billion years ago, with the other major remnant being Metis.

    Iris

    Iris is an S-type asteroid.

    Metis

    Metis is an S-type asteroid, and may be the core remnant of a large asteroid that was destroyed by an ancient collision. Metis was once considered to be a member of an asteroid family known as the Metis family, but more recent searches for prominent families did not recognize any such group, nor is a clump evident in the vicinity of Metis by visual inspection of proper orbital element diagrams.

    Phocaea family
    The Phocaea asteroids are a group of asteroids that orbit the Sun between 2.25 and 2.5 AU. Asteroids in this group have orbits with eccentricities greater than 0.1 and inclinations between 18 and 32.


    Nausikaa

    Nausikaa is a large main-belt S-type asteroid.

    Nysa family
    The Nysa or Nysian asteroids (also known as the Hertha family or the Polana family) are a group of asteroids in the Main Belt orbiting the sun between 2.41 and 2.5 AU. Asteroids in this family have eccentricities between 0.12 and 0.21 and inclinations of 1.4 to 4.3. The family derives its name from its most massive member, Nysa. It has also been known as the Hertha (Herthian) family.

    Asteroids in the Nysa family are typically divided into two, mineralogically different subgroups: Nysa and Polana. Aside from Nysa and Hertha, asteroids in the Nysian subgroup are S-type asteroids. Asteroids in the Polanian subgroup, like Polana, are F-type asteroids.


    Hebe

    Hebe is a large main-belt asteroid, containing around half a percent of the mass of the belt. It has an apparently high bulk density, which suggests an extremely solid body that has not been impacted by collisions, which is not typical of asteroids of its size. Hebe is probably the parent body of the H chondrite meteorites, which account for about 40% of all meteorites striking Earth.

    A small moon around Hebe has been reported. It was nicknamed "Jebe". However, the discovery of Hebe's moon has not been confirmed.


    Lutetia

    Lutetia is an M-type asteroid, with a high average density, meaning that it is made of metal-rich rock. The surface of Lutetia is covered by numerous impact craters and intersected by fractures, scarps and grooves thought to be surface manifestations of internal fractures. The composition of Lutetia suggests that it formed in the inner Solar System, among the terrestrial planets, and was ejected into the asteroid belt through an interaction with one of them.

    Fortuna

    Fortuna is a G-type asteroid - a darkly colored surface that is heavily space-weathered with the composition of primitive organic compounds, including tholins.

    Alinda family
    The Alinda asteroids are a group of asteroids with a semi-major axis of about 2.5 AU and an orbital eccentricity approximately between 0.4 and 0.65. These objects are held in this region by the 1:3 orbital resonance with Jupiter, which results in their being close to a 4:1 resonance with Earth. 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.

    Maria family
    The Maria or Marian asteroids are a group of asteroids that orbit the sun with distances between about 2.5 and 2.706 AU. Asteroids in this family typically have an inclination of 12 to 17°.


    Amphitrite

    Amphitrite is an S-type asteroids, probably third in diameter after Eunomia and Juno. A satellite of the asteroid is suspected to exist.

    Astraea

    Astraea is an S-type object.

    Egeria

    Egeria is a G-type asteroid. Spectral analysis of Egeria shows it to be unusually high in water content. This makes Egeria a prominent candidate for future water-mining ventures.

    Eunomia family
    The Eunomia or Eunomian family of asteroids is a large grouping of S-type asteroids. It is the most prominent family in the intermediate asteroid belt. About 5% of all asteroids in the asteroid belt belong to this family. The Eunomia family is located between the 3:1 and 8:3 resonances with Jupiter, at relatively high inclinations.

    A number of interlopers have been identified, which share the same orbital elements as the true family members, but can not have come from the same breakup because of spectral (hence, compositional) differences.


    Panopaea

    Panopaea is a dark, primitive carbonaceous C-type asteroid in contrast to the S-type asteroids of the Eunomian asteroids. The orbit of 70 Panopaea places it in a mean motion resonance with the planets Jupiter and Saturn.

    Mathilde

    Mathilde is a primitive C-type asteroid. It is very dark, with an albedo comparable to fresh asphalt. The asteroid has a number of extremely large craters. The impacts appear to have spalled large volumes off the asteroid, as suggested by the angular edges of the craters. No differences in brightness or colour were visible in the craters and there was no appearance of layering, so the asteroid's interior must be very homogeneous. There are indications of material movement along the downslope direction.

    The density is less than half that of a typical carbonaceous chondrite; this may indicate that the asteroid is very loosely packed rubble pile. Up to 50% of the interior volume of 253 Mathilde consists of open space.

    Juno
    Juno is one of the larger asteroids in the asteroid belt, being one of the two largest stony (S-type) asteroids. It is estimated to contain 1% of the total mass of the asteroid belt. Amongst S-type asteroids, Juno is unusually reflective, which may be indicative of distinct surface properties.


    Bamberga

    Bamberga is intermediate between the C-type and P-type asteroids.

    Eugenia

    Eugenia is an F-type asteroid, which means that it is very dark in colouring (darker than soot) with a carbonaceous composition. It may be a loosely-packed rubble pile, not a monolithic object.

    Oppavia

    Oppavia is a dark P-type asteroid. Oppavia was long thought to be a member of the now defunct Ceres asteroid family, but was found to be an unrelated interloper on the basis of its non-matching spectral type.

    Pandora

    Pandora is an E-type asteroids.

    Minerva

    Minerva is a C-type asteroid.

    Daphne

    Daphne is a C-type asteroid. The orbit of Daphne places it in a 9:22 mean motion resonance with the planet Mars.

    Pallas family
    The Pallas or Palladian family of asteroids is a grouping of B-type asteroids at very high inclinations in the intermediate asteroid belt. The namesake of the family is Pallas, an extremely large asteroid with a mean diameter of about 550 km. The remaining bodies are far smaller.

    Pallas
    Pallas is one of the largest asteroids in the Solar System. It is estimated to comprise 7% of the mass of the asteroid belt. It is 10-30% less massive than Vesta, placing it third among the asteroids. It is likely a remnant protoplanet.

    The Palladian surface appears to be a silicate material; the surface spectrum and estimated density resemble carbonaceous chondrite meteorites. The Palladian orbit, at 34.8°, is unusually highly inclined to the plane of the asteroid belt, and the orbital eccentricity is nearly as large as that of Pluto, making Pallas relatively inaccessible to spacecraft.

    Pallas is in a near-1:1 orbital resonance with Ceres. It also has a near-18:7 resonance (91,000-year period) and an approximate 5:2 resonance (83-year period) with Jupiter.


    Herculina

    Herculina is an S-type asteroid, that may have a moon.

    Burgundia

    Burgundia is an S-type asteroid, and was long thought to be a member of the now defunct Ceres asteroid family, but it was found to be an unrelated interloper in that group based on its non-matching composition.

    Nemesis

    Nemesis is a C-type asteroid. of carbonaceous composition. Nemesis is the largest member of the Nemesian asteroid family bearing its name.

    Gefion family
    The Gefion or Gefionian family of asteroids is a grouping of S-type asteroids in the intermediate asteroid belt. The namesake is 1272 Gefion. The family is fairly large. A search of a recent proper element database found 766 objects (about 0.8% of the total) lying within the region defined by the first table above.

    Until recently, this family was known as the Ceres (Cererean) family or the Minerva (Minervian) family after 1 Ceres (the largest asteroid) or 93 Minerva. However, spectroscopic analyses showed that these largest members were in fact interlopers in their own family, having a different spectral class from the bulk of the members.

    Ceres
    Ceres is the largest object in the asteroid belt. It is estimated to comprise approximately one third of the mass of the entire asteroid belt. Ceres appears to be differentiated into a rocky core and icy mantle, and may harbor a remnant internal ocean of liquid water under the layer of ice. The surface is probably a mixture of water ice and various hydrated minerals such as carbonates and clay. Water vapor emissions have been detected from several regions of Ceres.

    Although not as actively discussed as a potential home for microbial extraterrestrial life as Mars, Titan, Europa or Enceladus, the presence of water ice has led to speculation that life may exist on Ceres, and that hypothesized ejecta could have come from Ceres to Earth.


    Thisbe

    Thisbe is a B-type asteroid.

    Koronis family
    The Koronis or Koronian family is a family of asteroids in the main belt between Mars and Jupiter. Over 300 have been found but only about 20 are larger than 20 km in diameter.


    Psyche

    Psyche is one of the ten most-massive asteroids in the asteroid belt. It contains a little less than 1% of the mass of the entire asteroid belt. It is the most massive metallic M-type asteroid.

    Eos family
    The Eos or Eoan family is a prominent family of main belt asteroids that is believed to have formed as a result of an ancient catastrophic collision. Members of the family share similar orbits. The family is named after Eos. The Eos family asteroids have semi-major axes between 2.99 and 3.03 AU, eccentricities between 0.01 and 0.13, and inclinations between 8° and 12°. Currently there are about 4,400 members known. 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.


    Patientia

    Patientia is one of the larger asteroids in the belt.

    Interamnia

    Interamnia is a very large asteroid. It is an F-type asteroids,

    Hekate

    Hekate orbits in the same region of space as the Hygiea asteroid family, though it is actually an unrelated interloper. It is listed as a member of the Hecuba group of asteroids that orbit near the 2:1 mean-motion resonance with Jupiter.

    Europa

    Europa is not round but is shaped like a triaxial ellipsoid. It is a C-type asteroids. It orbits close to the Hygiea asteroid family, but is not a member.

    Doris

    Doris is one of the largest main belt asteroids. It is an extremely irregularly shaped object.

    Lachesis

    Lachesis is a large C-type asteroid.

    Ursula

    Ursula is one of the largest asteroids from the asteroid belt.

    Hygiea family
    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 Hygiea. About 1% of all known asteroids in the asteroid belt belong to this family. This family contains quite a large number of identified interlopers.

    Themis family
    The Themis or Themistian asteroid family is a Hirayama family (having similar orbital elements) of asteroids found in the outer portion of the asteroid belt, between the orbits of Mars and Jupiter. At a mean distance of 3.13 AU from the Sun, it is one of the more populous asteroid families. It consists of a well-defined core of larger asteroids and a surrounding region of smaller ones.

    The Themis family is one of the largest and longest-recognized dynamical families of asteroids, and is made up of C-type asteroids with a composition believed to be similar to that of carbonaceous chondrites. To date, the Themis family comprises approximately 535 known asteroids. The major ones include:


    Palma

    Palma is a B-type asteroid.

    Euphrosyne

    Euphrosyne is a C-type asteroid with a primitive surface, and is a fairly dark body near the belt's outer edge. Its orbit is quite unusual, and bears a considerable resemblance to that of Pallas in its high inclination and eccentricity. This object is the namesake of a family of 323-2,066 asteroids that share similar spectral properties and orbital elements; hence they may have arisen from the same collisional event.

    Aurora

    Aurora is a C-type asteroid, is darker than soot, and has a primitive compositions consisting of carbonaecous material.

    Davida

    Davida comprises an estimated 1.5% of the total mass of the asteroid belt. It is a C-type asteroid, which means that it is dark in colouring with a carbonaceous chondrite composition.

    Hecuba

    Hecuba has an orbit near a 2:1 mean-motion resonance with the planet Jupiter, and is the namesake of the Hecuba group of asteroids. It is categorized as a stony S-type asteroid.

    Griqua family
    The Griqua asteroids are a group of asteroids in the Main Belt orbiting the sun between 3.1 and 3.27 AU. Asteroids in this group have eccentricities greater than 0.35. These asteroids are at a 2:1 resonance with Jupiter's orbit. As such, their orbits are unstable; gradually perturbed over thousands of years until their orbits intersect with that of Mars or Jupiter.

    Cybele family
    Cybele asteroids are a group of asteroids in the outer main belt with a semi-major axis between 3.27 AU and 3.7 AU, an eccentricity less than 0.3, and an inclination less than 25°. The group is named for the asteroid 65 Cybele. The group is thought to have been formed by the breakup of larger object in the distant past.

    Don Quixote
    Don Quixote has a highly inclined comet-like orbit that leads to frequent perturbations by Jupiter. Due to its comet-like orbit and albedo, Don Quixote has ever been suspected to be an extinct comet.


    REFERENCES
  • Wikipedia and the numerous contributors to it
  • The Macross Compendium's Atlas
  • The UNSDB's Colonies and Bases list by Daniel Henwood

  • © Aaron Sketchley
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