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. 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, 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. 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. 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.
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. 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."
- 1 History of observation
- 2 Origin
- 3 Characteristics
- 4 Collisions
- 5 Families and groups
- 6 Exploration
- 7 See also
- 8 References
- 9 Further reading
- 10 External links
History of observation
In 1596, Johannes Kepler predicted “Between Mars and Jupiter, I place a planet” in his Mysterium Cosmographicum. While analyzing Tycho Brahe's data, Kepler thought that there was too large a gap between the orbits of Mars and Jupiter.
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 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."
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.
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.
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". 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 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 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.
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.
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: "[...] 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 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.
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. 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 quantities.
In 1802, shortly after discovering Pallas, Olbers suggested to Herschel that Ceres and Pallas were fragments of a much larger planet that once occupied the Mars–Jupiter region, this planet having suffered an internal explosion or a cometary impact many million years before. Over time, however, this hypothesis has fallen from favor. The large amount of energy required to destroy a planet, combined with the belt's low combined mass, which is only about 4% of the mass of the Moon, do not support the hypothesis. Further, the significant chemical differences between the asteroids become difficult to explain if they come from the same planet. Today, most scientists accept that, rather than fragmenting from a progenitor planet, the asteroids never formed a planet at all.
In general, in 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 further condensed 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 the formation of the planets.
Planetesimals within the region which would become the asteroid belt were too strongly perturbed by Jupiter's gravity to form a planet. Instead they continued to orbit the Sun as before, occasionally colliding. 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 planet-sized bodies. 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 the asteroid belt, dynamically exciting the region's population and increasing their velocities relative to each other.
During the early history of the Solar System, the asteroids melted to some degree, allowing elements within them to be partially or completely differentiated by mass. Some of the progenitor bodies may even have undergone periods of explosive volcanism and formed magma oceans. However, because of the relatively small size of the bodies, the period of melting was necessarily brief (compared to the much larger planets), and had generally ended about 4.5 billion years ago, in 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 4 Vesta suggested that it, and by extension the rest of the asteroid belt, had formed rather quickly, within ten million years of the Solar System's origin.
The asteroids are not samples of the primordial Solar System. 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. 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 the original asteroid belt may have contained mass equivalent to the Earth. Primarily because of gravitational perturbations, most of the material was ejected from the belt within about a million years of formation, leaving behind less than 0.1% of the original mass. Since their formation, the size distribution of the asteroid belt has remained relatively stable: there has been no significant increase or decrease in the typical dimensions of the main-belt asteroids.
The 4:1 orbital resonance with Jupiter, at a radius 2.06 AU, can be considered the inner boundary of the asteroid belt. Perturbations by Jupiter send bodies straying there into unstable orbits. Most bodies formed inside 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. 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 it was announced that 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, there was insufficient outgassing of water during the Earth's formative period to form the oceans, requiring an external source such as a cometary bombardment.
Contrary to popular imagery, the asteroid belt is mostly empty. The asteroids are spread over such a large volume that it would be improbable to reach an asteroid without aiming carefully. 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. 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 0.7–1.7 million asteroids with a diameter of 1 km or more. The apparent magnitudes of most of the known asteroids are 11–19, with the median at about 16.
The total mass of the asteroid belt is estimated to be 2.8×1021 to 3.2×1021 kilograms, which is just 4% of the mass of the Moon. The four largest objects, Ceres, 4 Vesta, 2 Pallas, and 10 Hygiea, account for half of the belt's total mass, with almost one-third accounted for by Ceres alone.
The current belt consists primarily of three categories of asteroids: C-type or carbonaceous asteroids, S-type or silicate asteroids, and M-type or metallic asteroids.
Carbonaceous asteroids, as their name suggests, are carbon-rich. They dominate the asteroid belt's outer regions. Together they comprise over 75% of the visible asteroids. They are redder in hue than the other asteroids and have a very low albedo. Their surface composition is similar to carbonaceous chondrite meteorites. Chemically, their spectra match the primordial composition of the early Solar System, with only the lighter elements 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 a relatively high albedo, and form about 17% of the total asteroid population.
M-type (metal-rich) asteroids form about 10% 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, there are also some silicate compounds that can produce 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 semi-major axis of about 2.7 AU. It is not yet clear 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.
One mystery of the asteroid belt is the relative rarity of V-type, or basaltic asteroids. 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 olivine. Observations, however, suggest that 99 percent of 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). However, 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 the further discovery in 2007 of two asteroids in the outer belt, 7472 Kumakiri and (10537) 1991 RY16, with differing basaltic composition that could not have originated from Vesta. These latter 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, the surface temperature of an asteroid can vary considerably as the sides are alternately exposed to solar radiation and then to the stellar background.
Several otherwise unremarkable bodies in the outer belt show cometary activity. Because their orbits cannot be explained through capture of classical comets, it is thought that many of the outer asteroids may be icy, with the ice occasionally exposed to sublimation through small impacts. Main-belt comets may have been 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 of around 0.07 and an inclination below 4°. Thus although 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°. This "core" region contains approximately 93.4% of all numbered minor planets within the Solar System.
The semi-major 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 at positions where their period of revolution about 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. Asteroids that become located in the gap orbits (either primordially because of the migration of Jupiter's orbit, or due to prior perturbations or collisions) are gradually nudged into different, random orbits with a larger or smaller semi-major axis.
The gaps are not seen in a simple snapshot of the locations of the asteroids at any one time because asteroid orbits are elliptical, and many asteroids still cross through the radii corresponding to the gaps. The actual spatial density of asteroids in these gaps does not differ significantly from the neighboring regions.
The main gaps occur at the 3:1, 5:2, 7:3, and 2:1 mean-motion resonances with Jupiter. An asteroid in the 3:1 Kirkwood gap would orbit the Sun three times for each Jovian orbit, for instance. Weaker resonances occur at other semi-major axis values, with fewer asteroids found than nearby. (For example, an 8:3 resonance for asteroids with a semi-major axis of 2.71 AU.)
The main or core population of the asteroid belt is sometimes divided into three zones, based on the most prominent Kirkwood gaps:
- Zone I lies between the 4:1 resonance (2.06 AU) and 3:1 resonance (2.5 AU) Kirkwood gaps.
- Zone II continues from the end of Zone I out to the 5:2 resonance gap (2.82 AU).
- Zone III extends from the outer edge of Zone II to the 2:1 resonance gap (3.28 AU).
The asteroid belt may also be divided into the inner and outer belts, with the inner belt formed by asteroids orbiting nearer to Mars than the 3:1 Kirkwood gap (2.5 AU), and the outer belt formed by those asteroids closer to Jupiter's orbit. (Some authors subdivide the inner and outer belts at the 2:1 resonance gap (3.3 AU), whereas others suggest inner, middle, and outer belts; also see diagram).
The high population of the asteroid belt makes for a very active environment, where collisions between asteroids occur frequently (on astronomical time scales). Collisions between main-belt bodies with 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 the original population.
Along with the asteroid bodies, the asteroid belt also contains bands of dust with particle radii of up to 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 the direction of the Sun along the plane of the ecliptic. Asteroid particles that produce the 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 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 the 50,000 meteorites found on Earth to date, 99.8 percent are believed to have originated in the asteroid belt.
Families and 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, all of which indicate a common origin in the breakup of a larger body. Graphical displays of these elements, for members of the asteroid belt, show concentrations indicating the presence of an asteroid family. There are about 20–30 associations that are almost certainly asteroid families. Additional groupings have been found that are less certain. Asteroid families can be confirmed when the members display common spectral features. Smaller associations of asteroids are called groups or clusters.
Some of the most prominent families in the asteroid belt (in order of increasing semi-major axes) are the Flora, Eunoma, 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 a billion years ago. The largest asteroid to be a true member of a family (as opposed to an interloper in the case of Ceres with the Gefion family) is 4 Vesta. 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.
Skirting the inner edge of the belt (ranging between 1.78 and 2.0 AU, with 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 a factor in reducing the total population of this group.
Another high-inclination group in the inner part of the asteroid belt is the Phocaea family. These are composed primarily of S-type asteroids, whereas the neighboring Hungaria family includes some E-types. The Phocaea family orbit between 2.25 and 2.5 AU from the Sun.
Skirting the outer edge of the asteroid belt is the Cybele group, orbiting between 3.3 and 3.5 AU. These have a 7:4 orbital resonance with Jupiter. The Hilda family orbit between 3.5 and 4.2 AU, and have relatively circular orbits and a stable 3:2 orbital resonance with Jupiter. There are few asteroids beyond 4.2 AU, until Jupiter's orbit. Here the two families of Trojan asteroids can be found, which, at least for objects larger than 1 km, are approximately as numerous as the asteroids of the asteroid belt.
Some asteroid families have formed recently, in astronomical terms. The Karin Cluster apparently formed about 5.7 million years ago from a collision with a progenitor asteroid 33 km in radius. The Veritas family formed about 8.3 million years ago; evidence includes interplanetary dust recovered from ocean sediment.
More recently, the Datura cluster appears to have formed about 530 thousand years ago from a collision with a main-belt asteroid. The age estimate is based on the probability of the members having their current orbits, rather than from any physical evidence. However, this cluster may have been a source for some zodiacal dust material. Other recent cluster formations, such as the Iannini cluster (circa 1–5 million years ago), may have provided additional sources of this asteroid dust.
The first spacecraft to traverse the asteroid belt was Pioneer 10, which entered the region on 16 July 1972. At the time there was some concern that the debris in the belt would pose a hazard to the spacecraft, but it has since been safely traversed by 12 spacecraft without incident. Pioneer 11, Voyagers 1 and 2 and Ulysses passed through the belt without imaging any asteroids. Galileo imaged 951 Gaspra in 1991 and 243 Ida in 1993, NEAR imaged 253 Mathilde in 1997, Cassini imaged 2685 Masursky in 2000, Stardust imaged 5535 Annefrank in 2002, New Horizons imaged 132524 APL in 2006, Rosetta imaged 2867 Šteins in September 2008 and 21 Lutetia in July 2010, and Dawn orbited Vesta between July 2011 and September 2012 and has orbited Ceres since March 2015. On its way to Jupiter, Juno traversed the asteroid belt without collecting science data. Due to the low density of materials within the belt, the odds of a probe running into an asteroid are now estimated at less than one in a billion.
Most belt asteroids imaged to date have come from brief flyby opportunities by probes headed for other targets. Only the Dawn, NEAR and Hayabusa missions have studied asteroids for a protracted period in orbit and at the surface. Dawn explored Vesta from July 2011 to September 2012, and has been orbiting Ceres since March 2015.
- Matt Williams (2015-08-23). "What is the Asteroid Belt?". Universe Today. Retrieved 2016-01-30.
- Krasinsky, G. A.; Pitjeva, E. V.; Vasilyev, M. V.; Yagudina, E. I. (July 2002). "Hidden Mass in the Asteroid Belt". Icarus. 158 (1): 98–105. Bibcode:2002Icar..158...98K. doi:10.1006/icar.2002.6837.
- Pitjeva, E. V. (2005). "High-Precision Ephemerides of Planets—EPM and Determination of Some Astronomical Constants" (PDF). Solar System Research. 39 (3): 176–186. Bibcode:2005SoSyR..39..176P. doi:10.1007/s11208-005-0033-2. Archived from the original (PDF) on July 3, 2014.
- For recent estimates of the masses of Ceres, 4 Vesta, 2 Pallas and 10 Hygiea, see the references in the infoboxes of their respective articles.
- Yeomans, Donald K. (July 13, 2006). "JPL Small-Body Database Browser". NASA JPL. Archived from the original on 29 September 2010. Retrieved 2010-09-27.
- Brian Koberlein (2014-03-12). "Why the Asteroid Belt Doesn't Threaten Spacecraft". Universe Today. Retrieved 2016-01-30.
- "How Did The Asteroid Belt Form? Was There A Planet There?". CosmosUp. 2016-01-17. Retrieved 2016-01-30.
- Nola Taylor Redd (2012-06-11). "Asteroid Belt: Facts & Information". Space.com. Retrieved 2016-01-30.
- Beatty, Kelly (March 10, 2009). "Sculpting the Asteroid Belt". Sky & Telescope. Retrieved 2014-04-30.
- Delgrande, J. J.; Soanes, S. V. (1943). "Kirkwood's Gap in the Asteroid Orbits". Journal of the Royal Astronomical Society of Canada. 37: 187. Bibcode:1943JRASC..37..187D.
- Küppers, Michael; O’Rourke, Laurence; Bockelée-Morvan, Dominique; Zakharov, Vladimir; Lee, Seungwon; von Allmen, Paul; Carry, Benoît; Teyssier, David; Marston, Anthony; Müller, Thomas; Crovisier, Jacques; Barucci, M. Antonietta; Moreno, Raphael (2014). "Localized sources of water vapour on the dwarf planet (1) Ceres". Nature. 505 (7484): 525–527. Bibcode:2014Natur.505..525K. ISSN 0028-0836. PMID 24451541. doi:10.1038/nature12918.
- Harrington, J.D. (22 January 2014). "Herschel Telescope Detects Water on Dwarf Planet – Release 14-021". NASA. Retrieved 22 January 2014.
- "Dawn: Between Jupiter and Mars [sic], I Place a Planet" (PDF). Jet Propulsion Laboratory.
- Russell, Christopher; Raymond, Carol, eds. (2012). "The Dawn Mission to Minor Planets 4 Vesta and 1 Ceres". Springer Science+Business Media. p. 5.
- Hilton, J. (2001). "When Did the Asteroids Become Minor Planets?". US Naval Observatory (USNO). Retrieved 2007-10-01.
- "Dawn: A Journey to the Beginning of the Solar System". Space Physics Center: UCLA. 2005. Archived from the original on 2012-05-24. Retrieved 2007-11-03.
- Hoskin, Michael. "Bode's Law and the Discovery of Ceres". Churchill College, Cambridge. Retrieved 2010-07-12.
- "Call the police! The story behind the discovery of the asteroids". Astronomy Now (June 2007): 60–61.
- Harper, Douglas (2010). "Asteroid". Online Etymology Dictionary. Etymology Online. Retrieved 2011-04-15.
- DeForest, Jessica (2000). "Greek and Latin Roots". Michigan State University. Archived from the original on 12 August 2007. Retrieved 2007-07-25.
- Cunningham, Clifford (1984). "William Herschel and the First Two Asteroids". The Minor Planet Bulletin. Dance Hall Observatory, Ontario. 11: 3. Bibcode:1984MPBu...11....3C.
- Staff (2002). "Astronomical Serendipity". NASA JPL. Retrieved 2007-04-20.
- Linda T. Elkins-Tanton, Asteroids, Meteorites, and Comets, 2010:10
- "Is it a coincidence that most of the planets fall within the Titius-Bode law's boundaries?". astronomy.com. Retrieved 2014-01-22.
- von Humboldt, Alexander (1850). Cosmos: A Sketch of a Physical Description of the Universe. 1. Harper & Brothers, New York (NY). p. 44. ISBN 0-8018-5503-9.
- Mann, Robert James (1852). A Guide to the Knowledge of the Heavens. Jarrold. p. 171. and 1853, p. 216
- "Further Investigation relative to the form, the magnitude, the mass, and the orbit of the Asteroid Planets". The Edinburgh New Philosophical Journal. 5: 191. January–April 1857.: "[Professor Peirce] then observed that the analogy between the ring of Saturn and the belt of the asteroids was worthy of notice."
- Hughes, David W. (2007). "A Brief History of Asteroid Spotting". BBC. Retrieved 2007-04-20.
- Moore, Patrick; Rees, Robin (2011). Patrick Moore's Data Book of Astronomy (2nd ed.). Cambridge University Press. p. 156. ISBN 0-521-89935-4.
- Manley, Scott (August 25, 2010). Asteroid Discovery from 1980 to 2010. YouTube. Retrieved 2011-04-15.
- "MPC Archive Statistics". IAU Minor Planet Center. Retrieved 2011-04-04.
- "A Brief History of Asteroid Spotting". Open2.net. Retrieved 2007-05-15.
- Masetti, M. & Mukai, K. (December 1, 2005). "Origin of the Asteroid Belt". NASA Goddard Spaceflight Center. Retrieved 2007-04-25.
- Watanabe, Susan (July 20, 2001). "Mysteries of the Solar Nebula". NASA. Retrieved 2007-04-02.
- Petit, J.-M.; Morbidelli, A. & Chambers, J. (2001). "The Primordial Excitation and Clearing of the Asteroid Belt" (PDF). Icarus. 153 (2): 338–347. Bibcode:2001Icar..153..338P. doi:10.1006/icar.2001.6702. Archived (PDF) from the original on 21 February 2007. Retrieved 2007-03-22.
- Edgar, R. & Artymowicz, P. (2004). "Pumping of a Planetesimal Disc by a Rapidly Migrating Planet" (PDF). Monthly Notices of the Royal Astronomical Society. 354 (3): 769–772. Bibcode:2004MNRAS.354..769E. arXiv: . doi:10.1111/j.1365-2966.2004.08238.x. Retrieved 2014-07-21.
- Scott, E. r. d. (March 13–17, 2006). "Constraints on Jupiter's Age and Formation Mechanism and the Nebula Lifetime from Chondrites and Asteroids". Proceedings 37th Annual Lunar and Planetary Science Conference. League City, Texas: Lunar and Planetary Society. Retrieved 2007-04-16.
- Taylor, G. J.; Keil, K.; McCoy, T.; Haack, H. & Scott, E. R. D. (1993). "Asteroid differentiation – Pyroclastic volcanism to magma oceans". Meteoritics. 28 (1): 34–52. Bibcode:1993Metic..28...34T. doi:10.1111/j.1945-5100.1993.tb00247.x.
- Kelly, Karen (2007). "U of T researchers discover clues to early solar system". University of Toronto. Retrieved 2010-07-12.
- Clark, B. E.; Hapke, B.; Pieters, C.; Britt, D. (2002). "Asteroid Space Weathering and Regolith Evolution". Asteroids III. University of Arizona: 585. Bibcode:2002aste.conf..585C. Gaffey, Michael J. (1996). "The Spectral and Physical Properties of Metal in Meteorite Assemblages: Implications for Asteroid Surface Materials". Icarus. 66 (3): 468–486. Bibcode:1986Icar...66..468G. ISSN 0019-1035. doi:10.1016/0019-1035(86)90086-2. Keil, K. (2000). "Thermal alteration of asteroids: evidence from meteorites". Planetary and Space Science. Retrieved 2007-11-08. Baragiola, R. A.; Duke, C. A.; Loeffler, M.; McFadden, L. A. & Sheffield, J. (2003). "Impact of ions and micrometeorites on mineral surfaces: Reflectance changes and production of atmospheric species in airless solar system bodies". EGS – AGU – EUG Joint Assembly: 7709. Bibcode:2003EAEJA.....7709B.
- Chapman, C. R.; Williams, J. G.; Hartmann, W. K. (1978). "The asteroids". Annual Review of Astronomy and Astrophysics. 16: 33–75. Bibcode:1978ARA&A..16...33C. doi:10.1146/annurev.aa.16.090178.000341.
- Kracher, A. (2005). "Asteroid 433 Eros and partially differentiated planetesimals: bulk depletion versus surface depletion of sulfur" (PDF). Ames Laboratory. Archived (PDF) from the original on 28 November 2007. Retrieved 2007-11-08.
- Robert Piccioni (2012-11-19). "Did Asteroid Impacts Make Earth Habitable?". Guidetothecosmos.com. Retrieved 2013-05-03.
- Stiles, Lori (September 15, 2005). "Asteroids Caused the Early Inner Solar System Cataclysm". University of Arizona News. Retrieved 2007-04-18.
- Alfvén, H.; Arrhenius, G. (1976). "The Small Bodies". SP-345 Evolution of the Solar System. NASA. Archived from the original on 13 May 2007. Retrieved 2007-04-12.
- Spratt, Christopher E. (April 1990). "The Hungaria group of minor planets". Journal of the Royal Astronomical Society of Canada. 84: 123–131. Bibcode:1990JRASC..84..123S.
- Lecar, M.; Podolak, M.; Sasselov, D.; Chiang, E. (2006). "Infrared cirrus – New components of the extended infrared emission". The Astrophysical Journal. 640 (2): 1115–1118. Bibcode:2006ApJ...640.1115L. arXiv: . doi:10.1086/500287.
- Berardelli, Phil (March 23, 2006). "Main-Belt Comets May Have Been Source Of Earths Water". Space Daily. Retrieved 2007-10-27.
- Lakdawalla, Emily (April 28, 2006). "Discovery of a Whole New Type of Comet". The Planetary Society. Archived from the original on 1 May 2007. Retrieved 2007-04-20.
- Yeomans, Donald K. (April 26, 2007). "JPL Small-Body Database Search Engine". NASA JPL. Retrieved 2007-04-26. – search for asteroids in the main belt regions with a diameter >100.
- Tedesco, E. F. & Desert, F.-X. (2002). "The Infrared Space Observatory Deep Asteroid Search". The Astronomical Journal. 123 (4): 2070–2082. Bibcode:2002AJ....123.2070T. doi:10.1086/339482.
- Williams, Gareth (September 25, 2010). "Distribution of the Minor Planets". Minor Planets Center. Retrieved 2010-10-27.
- Wiegert, P.; Balam, D.; Moss, A.; Veillet, C.; Connors, M. & Shelton, I. (2007). "Evidence for a Color Dependence in the Size Distribution of Main-Belt Asteroids" (PDF). The Astronomical Journal. 133 (4): 1609–1614. Bibcode:2007AJ....133.1609W. arXiv: . doi:10.1086/512128. Retrieved 2008-09-06.
- Clark, B. E. (1996). "New News and the Competing Views of Asteroid Belt Geology". Lunar and Planetary Science. 27: 225–226. Bibcode:1996LPI....27..225C.
- Margot, J. L. & Brown, M. E. (2003). "A Low-Density M-type Asteroid in the Main Belt". Science. 300 (5627): 1939–1942. Bibcode:2003Sci...300.1939M. PMID 12817147. doi:10.1126/science.1085844.
- Lang, Kenneth R. (2003). "Asteroids and meteorites". NASA's Cosmos. Retrieved 2007-04-02.
- Mueller, M.; Harris, A. W.; Delbo, M. (2005). the MIRSI Team. "21 Lutetia and other M-types: Their sizes, albedos, and thermal properties from new IRTF measurements". Bulletin of the American Astronomical Society. 37: 627. Bibcode:2005DPS....37.0702M.
- "When is a comet not a comet?". ESA/Hubble Press Release. Retrieved 12 November 2013.
- Duffard, R. D.; Roig, F. (July 14–18, 2008). "Two New Basaltic Asteroids in the Main Belt?". Asteroids, Comets, Meteors 2008. Baltimore, Maryland. Bibcode:2008LPICo1405.8154D. arXiv: .
- Than, Ker (2007). "Strange Asteroids Baffle Scientists". space.com. Retrieved 2007-10-14.
- Low, F. J.; et al. (1984). "Infrared cirrus – New components of the extended infrared emission". Astrophysical Journal Letters. 278: L19–L22. Bibcode:1984ApJ...278L..19L. doi:10.1086/184213.
- "Interview with David Jewitt". YouTube.com. 2007-01-05. Retrieved 2011-05-21.
- This value was obtained by a simple count up of all bodies in that region using data for 120437 numbered minor planets from the Minor Planet Center orbit database, dated February 8, 2006.
- Fernie, J. Donald (1999). "The American Kepler". American Scientist. 87 (5): 398. doi:10.1511/1999.5.398. Retrieved 2007-02-04.
- Liou, Jer-Chyi & Malhotra, Renu (1997). "Depletion of the Outer Asteroid Belt". Science. 275 (5298): 375–377. Bibcode:1997Sci...275..375L. PMID 8994031. doi:10.1126/science.275.5298.375. Retrieved 2007-08-01.
- McBride, N. & Hughes, D. W. (1990). "The spatial density of asteroids and its variation with asteroidal mass". Monthly Notices of the Royal Astronomical Society. 244: 513–520. Bibcode:1990MNRAS.244..513M.
- Ferraz-Mello, S. (June 14–18, 1993). "Kirkwood Gaps and Resonant Groups". proceedings of the 160th International Astronomical Union. Belgirate, Italy: Kluwer Academic Publishers. pp. 175–188. Retrieved 2007-03-28.
- Klacka, Jozef (1992). "Mass distribution in the asteroid belt". Earth, Moon, and Planets. 56 (1): 47–52. Bibcode:1992EM&P...56...47K. doi:10.1007/BF00054599.
- Backman, D. E. (March 6, 1998). "Fluctuations in the General Zodiacal Cloud Density". Backman Report. NASA Ames Research Center. Retrieved 2007-04-04.
- David Nesvorný, William F. Bottke Jr, Luke Dones & Harold F. Levison (June 2002). "The recent breakup of an asteroid in the main-belt region" (PDF). Nature. 417: 720–722. Bibcode:2002Natur.417..720N. PMID 12066178. doi:10.1038/nature00789.
- Reach, William T. (1992). "Zodiacal emission. III – Dust near the asteroid belt". Astrophysical Journal. 392 (1): 289–299. Bibcode:1992ApJ...392..289R. doi:10.1086/171428.
- Cometary origin of the zodiacal cloud and carbonaceous micrometeorites – Implications for hot debris disk
- Kingsley, Danny (May 1, 2003). "Mysterious meteorite dust mismatch solved". ABC Science. Retrieved 2007-04-04.
- "Meteors and Meteorites" (PDF). NASA. Retrieved 2012-01-12.
- Hughes, David W. (2007). "Finding Asteroids In Space". BBC. Retrieved 2007-04-20.
- Lemaitre, Anne (August 31 – September 4, 2004). "Asteroid family classification from very large catalogues". Proceedings Dynamics of Populations of Planetary Systems. Belgrade, Serbia and Montenegro: Cambridge University Press. pp. 135–144. Retrieved 2007-04-15.
- Martel, Linda M. V. (March 9, 2004). "Tiny Traces of a Big Asteroid Breakup". Planetary Science Research Discoveries. Archived from the original on 1 April 2007. Retrieved 2007-04-02.
- Drake, Michael J. (2001). "The eucrite/Vesta story". Meteoritics & Planetary Science. 36 (4): 501–513. Bibcode:2001M&PS...36..501D. doi:10.1111/j.1945-5100.2001.tb01892.x.
- Love, S. G. & Brownlee, D. E. (1992). "The IRAS dust band contribution to the interplanetary dust complex – Evidence seen at 60 and 100 microns". Astronomical Journal. 104 (6): 2236–2242. Bibcode:1992AJ....104.2236L. doi:10.1086/116399.
- Spratt, Christopher E. (1990). "The Hungaria group of minor planets". Journal of the Royal Astronomical Society of Canada. 84 (2): 123–131. Bibcode:1990JRASC..84..123S.
- Carvano, J. M.; Lazzaro, D.; Mothé-Diniz, T.; Angeli, C. A. & Florczak, M. (2001). "Spectroscopic Survey of the Hungaria and Phocaea Dynamical Groups". Icarus. 149 (1): 173–189. Bibcode:2001Icar..149..173C. doi:10.1006/icar.2000.6512.
- Dymock, Roger (2010). Asteroids and Dwarf Planets and How to Observe Them. Astronomers' Observing Guides. Springer. p. 24. ISBN 1-4419-6438-X. Retrieved 2011-04-04.
- Nesvorný, David; et al. (August 2006). "Karin cluster formation by asteroid impact". Icarus. 183 (2): 296–311. Bibcode:2006Icar..183..296N. doi:10.1016/j.icarus.2006.03.008.
- McKee, Maggie (January 18, 2006). "Eon of dust storms traced to asteroid smash". New Scientist Space. Retrieved 2007-04-15.
- Nesvorný; Vokrouhlický, D; Bottke, WF; et al. (2006). "The Breakup of a Main-Belt Asteroid 450 Thousand Years Ago" (PDF). Science. 312 (5779): 1490. Bibcode:2006Sci...312.1490N. PMID 16763141. doi:10.1126/science.1126175.
- Vokrouhlický; Durech, J; Michalowski, T; et al. (2009). "Datura family: the 2009 update". Astronomy&Astrophysics. 507: 495–504. doi:10.1051/0004-6361/200912696.
- Nesvorný, D.; Bottke, W. F.; Levison, H. F. & Dones, L. (2003). "Recent Origin of the Solar System Dust Bands" (PDF). The Astrophysical Journal. 591 (1): 486–497. Bibcode:2003ApJ...591..486N. doi:10.1086/374807. Retrieved 2007-04-15.
- Barucci, M. A.; Fulchignoni, M. & Rossi, A. (2007). "Rosetta Asteroid Targets: 2867 Steins and 21 Lutetia". Space Science Reviews. 128 (1–4): 67–78. Bibcode:2007SSRv..128...67B. doi:10.1007/s11214-006-9029-6.
- Greicius, Tony (July 31, 2015). "NASA's Juno Gives Starship-Like View of Earth Flyby". nasa.gov. NASA. Retrieved 4 September 2015.
- Stern, Alan (June 2, 2006). "New Horizons Crosses The Asteroid Belt". Space Daily. Retrieved 2007-04-14.
- Elkins-Tanton, Linda T. (2006). Asteroids, Meteorites, and Comets (First ed.). New York: Chelsea House. ISBN 0-8160-5195-X.
|Wikimedia Commons has media related to Main Belt asteroids.|
- Arnett, William A. (February 26, 2006). "Asteroids". The Nine Planets. Archived from the original on 18 April 2007. Retrieved 2007-04-20.
- Asteroids Page at NASA's Solar System Exploration
- Cain, Fraser. "The Asteroid Belt". Universe Today. Archived from the original on 7 March 2008. Retrieved 2008-04-01.
- "Main Asteroid Belt". Sol Company. Archived from the original on 15 May 2007. Retrieved 2007-04-20.
- Munsell, Kirk (September 16, 2005). "Asteroids: Overview". NASA's Solar System Exploration. Archived from the original on 24 May 2007. Retrieved 2007-05-26.
- Plots of eccentricity vs. semi-major axis and inclination vs. semi-major axis at Asteroid Dynamic Site
- Staff (October 31, 2006). "Asteroids". NASA. Archived from the original on 11 April 2007. Retrieved 2007-04-20.
- Staff (2007). "Space Topics: Asteroids and Comets". The Planetary Society. Archived from the original on 28 April 2007. Retrieved 2007-04-20.