According to the
International Astronomical Union
The International Astronomical Union (IAU; , UAI) is an international non-governmental organization (INGO) with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreach, education, and developmen ...
(IAU), a minor planet is an
astronomical object
An astronomical object, celestial object, stellar object or heavenly body is a naturally occurring physical entity, association, or structure that exists within the observable universe. In astronomy, the terms ''object'' and ''body'' are of ...
in direct
orbit
In celestial mechanics, an orbit (also known as orbital revolution) is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an ...
around the
Sun
The Sun is the star at the centre of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light a ...
that is exclusively classified as neither a
planet
A planet is a large, Hydrostatic equilibrium, rounded Astronomical object, astronomical body that is generally required to be in orbit around a star, stellar remnant, or brown dwarf, and is not one itself. The Solar System has eight planets b ...
nor a
comet
A comet is an icy, small Solar System body that warms and begins to release gases when passing close to the Sun, a process called outgassing. This produces an extended, gravitationally unbound atmosphere or Coma (cometary), coma surrounding ...
. Before 2006, the IAU officially used the term ''minor planet'', but that year's meeting
reclassified minor planets and comets into
dwarf planet
A dwarf planet is a small planetary-mass object that is in direct orbit around the Sun, massive enough to be hydrostatic equilibrium, gravitationally rounded, but insufficient to achieve clearing the neighbourhood, orbital dominance like the ...
s and
small Solar System bodies
A small Solar System body (SSSB) is an object in the Solar System that is neither a planet, a dwarf planet, nor a natural satellite. The term was first IAU definition of planet, defined in 2006 by the International Astronomical Union (IAU) as fo ...
(SSSBs).
[Press release, IAU 2006 General Assembly: Result of the IAU Resolution votes]
International Astronomical Union, August 24, 2006. Accessed May 5, 2008. In contrast to the eight official planets of the
Solar System
The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Sola ...
, all minor planets fail to
clear their orbital neighborhood.
[
Minor planets include ]asteroid
An asteroid is a minor planet—an object larger than a meteoroid that is neither a planet nor an identified comet—that orbits within the Solar System#Inner Solar System, inner Solar System or is co-orbital with Jupiter (Trojan asteroids). As ...
s (near-Earth object
A near-Earth object (NEO) is any small Solar System body orbiting the Sun whose closest approach to the Sun ( perihelion) is less than 1.3 times the Earth–Sun distance (astronomical unit, AU). This definition applies to the object's orbit a ...
s, Earth trojan
An Earth trojan is an asteroid that orbits the Sun in the vicinity of the Earth–Sun Lagrange points (leading 60°) or (trailing 60°), thus having an orbit similar to Earth's. Only two Earth trojans have so far been discovered. The name "tr ...
s, Mars trojan
The Mars trojans are a group of trojan 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. One t ...
s, Mars-crossers, main-belt asteroids
The asteroid belt is a torus-shaped region in the Solar System, centered on the Sun and roughly spanning the space between the orbits of the planets Jupiter and Mars. It contains a great many solid, irregularly shaped bodies called asteroi ...
and Jupiter trojan
The Jupiter trojans, commonly called trojan asteroids or simply trojans, are a large group of asteroids that share the planet Jupiter's orbit around the Sun. Relative to Jupiter, each Trojan (celestial body), trojan Libration point orbit, librat ...
s), as well as distant minor planet
A distant minor planet, or ''distant object'', is any minor planet found beyond Jupiter in the outer Solar System that is not commonly thought of as an "asteroid". The umbrella term is used by IAU's Minor Planet Center (MPC), which is responsib ...
s (Uranus trojan
A Uranus trojan is an asteroid that shares an orbit with Uranus
Uranus is the seventh planet from the Sun. It is a gaseous cyan-coloured ice giant. Most of the planet is made of water, ammonia, and methane in a Supercritical fluid, supercr ...
s, Neptune trojan
Neptune trojans are bodies that orbit the Sun near one of the stable Lagrangian points of Neptune, similar to the trojans of other planets. They therefore have approximately the same orbital period as Neptune and follow roughly the same orbita ...
s, centaurs
A centaur ( ; ; ), occasionally hippocentaur, also called Ixionidae (), is a creature from Greek mythology with the upper body of a human and the lower body and legs of a horse that was said to live in the mountains of Thessaly. In one version o ...
and trans-Neptunian objects
A trans-Neptunian object (TNO), also written transneptunian object, is any minor planet in the Solar System that orbits the Sun at a greater average distance than Neptune, which has an orbital semi-major axis of 30.1 astronomical units (AU).
T ...
), most of which reside in the Kuiper belt
The Kuiper belt ( ) is a circumstellar disc in the outer Solar System, extending from the orbit of Neptune at 30 astronomical units (AU) to approximately 50 AU from the Sun. It is similar to the asteroid belt, but is far larger—20 times ...
and the scattered disc
The scattered disc (or scattered disk) is a distant circumstellar disc in the Solar System that is sparsely populated by icy small Solar System bodies, which are a subset of the broader family of trans-Neptunian objects. The scattered-disc obj ...
. , there are known objects, divided into 740,000 numbered, with only one of them recognized as a dwarf planet
A dwarf planet is a small planetary-mass object that is in direct orbit around the Sun, massive enough to be hydrostatic equilibrium, gravitationally rounded, but insufficient to achieve clearing the neighbourhood, orbital dominance like the ...
(secured discoveries) and 652,085 unnumbered minor planets, with only five of those officially recognized as a dwarf planet.
The first minor planet to be discovered was Ceres in 1801, though it was called a 'planet' at the time and an 'asteroid' soon after; the term ''minor planet'' was not introduced until 1841, and was considered a subcategory of 'planet' until 1932. The term planetoid has also been used, especially for larger, planetary objects such as those the IAU has called dwarf planet
A dwarf planet is a small planetary-mass object that is in direct orbit around the Sun, massive enough to be hydrostatic equilibrium, gravitationally rounded, but insufficient to achieve clearing the neighbourhood, orbital dominance like the ...
s since 2006.[Planet, asteroid, minor planet: A case study in astronomical nomenclature, David W. Hughes, Brian G. Marsden, ''Journal of Astronomical History and Heritage'' 10, #1 (2007), pp. 21–30. ] Historically, the terms ''asteroid'', ''minor planet'', and ''planetoid'' have been more or less synonymous.[ This terminology has become more complicated by the discovery of numerous minor planets beyond the orbit of ]Jupiter
Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a Jupiter mass, mass more than 2.5 times that of all the other planets in the Solar System combined a ...
, especially trans-Neptunian objects that are generally not considered asteroids.[Asteroid]
, ''MSN Encarta'', Microsoft. Accessed May 5, 2008
2009-11-01. A minor planet seen releasing gas may be dually classified as a comet.
Objects are called dwarf planets if their own gravity is sufficient to achieve hydrostatic equilibrium
In fluid mechanics, hydrostatic equilibrium, also called hydrostatic balance and hydrostasy, is the condition of a fluid or plastic solid at rest, which occurs when external forces, such as gravity, are balanced by a pressure-gradient force. I ...
and form an ellipsoid
An ellipsoid is a surface that can be obtained from a sphere by deforming it by means of directional Scaling (geometry), scalings, or more generally, of an affine transformation.
An ellipsoid is a quadric surface; that is, a Surface (mathemat ...
al shape. All other minor planets and comets are called ''small Solar System bodies''.[ The IAU stated that the term ''minor planet'' may still be used, but the term ''small Solar System body'' will be preferred. However, for purposes of numbering and naming, the traditional distinction between minor planet and comet is still used.
]
Populations
Hundreds of thousands of minor planets have been discovered within the Solar System and thousands more are discovered each month. The Minor Planet Center
The Minor Planet Center (MPC) is the official body for observing and reporting on minor planets under the auspices of the International Astronomical Union (IAU). Founded in 1947, it operates at the Smithsonian Astrophysical Observatory.
Funct ...
has documented over 213 million observations and 794,832 minor planets, of which 541,128 have orbits known well enough to be assigned permanent official numbers. Of these, 21,922 have official names. , the lowest-numbered unnamed minor planet is , and the highest-numbered named minor planet is 594913 ꞌAylóꞌchaxnim.
There are various broad minor-planet populations:
* Asteroid
An asteroid is a minor planet—an object larger than a meteoroid that is neither a planet nor an identified comet—that orbits within the Solar System#Inner Solar System, inner Solar System or is co-orbital with Jupiter (Trojan asteroids). As ...
s; traditionally, most have been bodies in the inner Solar System.[
** ]Near-Earth asteroids
A near-Earth object (NEO) is any small Solar System body orbiting the Sun whose closest approach to the Sun (perihelion) is less than 1.3 times the Earth–Sun distance (astronomical unit, AU). This definition applies to the object's orbit aro ...
, those whose orbits take them inside the orbit of Mars. Further subclassification of these, based on orbital distance, is used:
*** Apohele asteroids orbit inside of Earth's perihelion distance and thus are contained entirely within the orbit of Earth.
*** Aten asteroids, those that have a semimajor axis of less than Earth's and an aphelion (furthest distance from the Sun) greater than 0.983 AU.
*** Apollo asteroids
The Apollo asteroids are a group of near-Earth asteroids named after 1862 Apollo, discovered by German astronomer Karl Reinmuth in the 1930s. They are Earth-crosser asteroid, Earth-crossing asteroids that have an orbital semimajor axis, semi-majo ...
are those asteroids with a semimajor axis greater than Earth's while having a perihelion distance of 1.017 AU or less. Like Aten asteroids, Apollo asteroids are Earth-crossers.
*** Amor asteroids are those near-Earth
Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...
asteroids that approach the orbit of Earth from beyond but do not cross it. Amor asteroids are further subdivided into four subgroups, depending on where their semimajor axis falls between Earth's orbit and the asteroid belt.
** Earth trojan
An Earth trojan is an asteroid that orbits the Sun in the vicinity of the Earth–Sun Lagrange points (leading 60°) or (trailing 60°), thus having an orbit similar to Earth's. Only two Earth trojans have so far been discovered. The name "tr ...
s, asteroids sharing Earth's orbit and gravitationally locked to it. As of 2022, two Earth trojans are known: 2010 TK7 and 2020 XL5.
** Mars trojan
The Mars trojans are a group of trojan 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. One t ...
s, asteroids sharing Mars's orbit and gravitationally locked to it. As of 2007, eight such asteroids are known.
** Asteroid belt
The asteroid belt is a torus-shaped region in the Solar System, centered on the Sun and roughly spanning the space between the orbits of the planets Jupiter and Mars. It contains a great many solid, irregularly shaped bodies called asteroids ...
, whose members follow roughly circular orbits between Mars and Jupiter. These are the original and best-known group of asteroids.
** Jupiter trojan
The Jupiter trojans, commonly called trojan asteroids or simply trojans, are a large group of asteroids that share the planet Jupiter's orbit around the Sun. Relative to Jupiter, each Trojan (celestial body), trojan Libration point orbit, librat ...
s, asteroids sharing Jupiter's orbit and gravitationally locked to it. Numerically they are estimated to equal the main-belt asteroids.
* Distant minor planet
A distant minor planet, or ''distant object'', is any minor planet found beyond Jupiter in the outer Solar System that is not commonly thought of as an "asteroid". The umbrella term is used by IAU's Minor Planet Center (MPC), which is responsib ...
s, an umbrella term for minor planets in the outer Solar System.
** Centaurs
A centaur ( ; ; ), occasionally hippocentaur, also called Ixionidae (), is a creature from Greek mythology with the upper body of a human and the lower body and legs of a horse that was said to live in the mountains of Thessaly. In one version o ...
, bodies in the outer Solar System between Jupiter and Neptune. They have unstable orbits due to the gravitational influence of the giant planets, and therefore must have come from elsewhere, probably outside Neptune.
** Neptune trojan
Neptune trojans are bodies that orbit the Sun near one of the stable Lagrangian points of Neptune, similar to the trojans of other planets. They therefore have approximately the same orbital period as Neptune and follow roughly the same orbita ...
s, bodies sharing Neptune's orbit and gravitationally locked to it. Although only a handful are known, there is evidence that Neptune trojans are more numerous than either the asteroids in the asteroid belt or the Jupiter trojans.
** Trans-Neptunian objects
A trans-Neptunian object (TNO), also written transneptunian object, is any minor planet in the Solar System that orbits the Sun at a greater average distance than Neptune, which has an orbital semi-major axis of 30.1 astronomical units (AU).
T ...
, bodies at or beyond the orbit of Neptune
Neptune is the eighth and farthest known planet from the Sun. It is the List of Solar System objects by size, fourth-largest planet in the Solar System by diameter, the third-most-massive planet, and the densest giant planet. It is 17 t ...
, the outermost planet.
*** The Kuiper belt
The Kuiper belt ( ) is a circumstellar disc in the outer Solar System, extending from the orbit of Neptune at 30 astronomical units (AU) to approximately 50 AU from the Sun. It is similar to the asteroid belt, but is far larger—20 times ...
, objects inside an apparent population drop-off approximately 55 AU from the Sun.
**** Classical Kuiper belt objects like Makemake
Makemake ( minor-planet designation: 136472 Makemake) is a dwarf planet and the largest of what is known as the classical population of Kuiper belt objects, with a diameter approximately that of Saturn's moon Iapetus, or 60% that of Pluto. It ...
, also known as cubewanos, are in primordial, relatively circular orbits that are not in resonance with Neptune.
**** Resonant Kuiper belt objects.
***** Plutinos, bodies like that are in a 2:3 resonance with Neptune.
*** Scattered disc
The scattered disc (or scattered disk) is a distant circumstellar disc in the Solar System that is sparsely populated by icy small Solar System bodies, which are a subset of the broader family of trans-Neptunian objects. The scattered-disc obj ...
objects like Eris, with aphelia outside the Kuiper belt. These are thought to have been scattered by Neptune.
**** Resonant scattered disc objects.
*** Detached objects such as Sedna, with both an aphelion and a perihelion
An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. The line of apsides (also called apse line, or major axis of the orbit) is the line connecting the two extreme values.
Apsides perta ...
outside the Kuiper belt.
**** Sednoids, detached objects with a perihelion greater than 75 AU (Sedna, , and Leleākūhonua).
*** The Oort cloud
The Oort cloud (pronounced or ), sometimes called the Öpik–Oort cloud, is scientific theory, theorized to be a cloud of billions of Volatile (astrogeology), icy planetesimals surrounding the Sun at distances ranging from 2,000 to 200,000 A ...
, a hypothetical population thought to be the source of long-period comets and that may extend to 50,000 AU from the Sun.
Naming conventions
All astronomical bodies in the Solar System need a distinct designation. The naming of minor planets runs through a three-step process. First, a provisional designation is given upon discovery—because the object still may turn out to be a false positive or become lost later on—called a ''provisionally designated minor planet''. After the observation arc
In observational astronomy, the observation arc (or arc length) of a Solar System body is the time period between its earliest and latest observations, used for tracing the body's path. It is usually given in days or years. The term is mostly use ...
is accurate enough to predict its future location, a minor planet is formally designated and receives a number. It is then a ''numbered minor planet''. Finally, in the third step, it may be named by its discoverers. However, only a small fraction of all minor planets have been named. The vast majority are either numbered or have still only a provisional designation. Example of the naming process:
* – provisional designation upon discovery on 24 April 1932
* – formal designation, receives an official number
* 1862 Apollo – named minor planet, receives a name, the alphanumeric code is dropped
Provisional designation
A newly discovered minor planet is given a provisional designation. For example, the provisional designation consists of the year of discovery (2002) and an alphanumeric code indicating the half-month of discovery and the sequence within that half-month. Once an asteroid's orbit has been confirmed, it is given a number, and later may also be given a name (e.g. 433 Eros). The formal naming convention uses parentheses around the number, but dropping the parentheses is quite common. Informally, it is common to drop the number altogether or to drop it after the first mention when a name is repeated in running text.
Minor planets that have been given a number but not a name keep their provisional designation, e.g. (29075) 1950 DA
( provisional designation ) is a risk-listed asteroid, classified as a near-Earth object and potentially hazardous asteroid of the Apollo group, approximately in diameter. It once had the highest known probability of impacting Earth. In 2002 ...
. Because modern discovery techniques are finding vast numbers of new asteroids, they are increasingly being left unnamed. The earliest discovered to be left unnamed was for a long time (3360) 1981 VA, now 3360 Syrinx. In November 2006 its position as the lowest-numbered unnamed asteroid passed to (now 3708 Socus), and in May 2021 to . On rare occasions, a small object's provisional designation may become used as a name in itself: the then-unnamed gave its "name" to a group of objects that became known as classical Kuiper belt objects ("cubewanos") before it was finally named 15760 Albion
15760 Albion ( provisional designation ) was the first trans-Neptunian object to be discovered after Pluto and Charon. Measuring about 108–167 kilometres in diameter, it was discovered in 1992 by David C. Jewitt and Jane X. Luu at the Maun ...
in January 2018.
A few objects are cross-listed as both comets and asteroids, such as 4015 Wilson–Harrington, which is also listed as 107P/Wilson–Harrington.
Numbering
Minor planets are awarded an official number once their orbits are confirmed. With the increasing rapidity of discovery, these are now six-figure numbers. The switch from five figures to six figures arrived with the publication of the Minor Planet Circular
The Minor Planet Center (MPC) is the official body for observing and reporting on minor planets under the auspices of the International Astronomical Union (IAU). Founded in 1947, it operates at the Smithsonian Astrophysical Observatory.
Function ...
(MPC) of October 19, 2005, which saw the highest-numbered minor planet jump from 99947 to 118161.
Naming
The first few asteroids were named after figures from Greek
Greek may refer to:
Anything of, from, or related to Greece, a country in Southern Europe:
*Greeks, an ethnic group
*Greek language, a branch of the Indo-European language family
**Proto-Greek language, the assumed last common ancestor of all kno ...
and Roman mythology
Roman mythology is the body of myths of ancient Rome as represented in the literature and visual arts of the Romans, and is a form of Roman folklore. "Roman mythology" may also refer to the modern study of these representations, and to th ...
, but as such names started to dwindle the names of famous people, literary characters, discoverers' spouses, children, colleagues, and even television characters were used.
Gender
: The first asteroid to be given a non-mythological name was 20 Massalia, named after the Greek name for the city of Marseille
Marseille (; ; see #Name, below) is a city in southern France, the Prefectures in France, prefecture of the Departments of France, department of Bouches-du-Rhône and of the Provence-Alpes-Côte d'Azur Regions of France, region. Situated in the ...
. The first to be given an entirely non-Classical name was 45 Eugenia, named after Empress Eugénie de Montijo
Eugénie de Montijo (; born María Eugenia Ignacia Agustina de Palafox y Kirkpatrick; 5 May 1826 – 11 July 1920) was Second French Empire, Empress of the French from her marriage to Napoleon III on 30 January 1853 until he was overthrown on 4 ...
, the wife of Napoleon III
Napoleon III (Charles-Louis Napoléon Bonaparte; 20 April 18089 January 1873) was President of France from 1848 to 1852 and then Emperor of the French from 1852 until his deposition in 1870. He was the first president, second emperor, and last ...
. For some time only female (or feminized) names were used; Alexander von Humboldt
Friedrich Wilhelm Heinrich Alexander von Humboldt (14 September 1769 – 6 May 1859) was a German polymath, geographer, natural history, naturalist, List of explorers, explorer, and proponent of Romanticism, Romantic philosophy and Romanticism ...
was the first man to have an asteroid named after him, but his name was feminized to 54 Alexandra. This unspoken tradition lasted until 334 Chicago was named; even then, female names showed up in the list for years after.
Eccentric
: As the number of asteroids began to run into the hundreds, and eventually, in the thousands, discoverers began to give them increasingly frivolous names. The first hints of this were 482 Petrina and 483 Seppina, named after the discoverer's pet dogs. However, there was little controversy about this until 1971, upon the naming of 2309 Mr. Spock (the name of the discoverer's cat). Although the IAU
The International Astronomical Union (IAU; , UAI) is an international non-governmental organization (INGO) with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreach, education, and developmen ...
subsequently discouraged the use of pet names as sources, eccentric asteroid names are still being proposed and accepted, such as 4321 Zero, 6042 Cheshirecat, 9007 James Bond, 13579 Allodd and 24680 Alleven, and 26858 Misterrogers.
Discoverer's name
: A well-established rule is that, unlike comets, minor planets may not be named after their discoverer(s). One way to circumvent this rule has been for astronomers to exchange the courtesy of naming their discoveries after each other. Rare exceptions to this rule are 1927 Suvanto and 96747 Crespodasilva. 1927 Suvanto was named after its discoverer, Rafael Suvanto, posthumously by the Minor Planet Center. He died four years after the discovery in the last days of the Finnish winter war
The Winter War was a war between the Soviet Union and Finland. It began with a Soviet invasion of Finland on 30 November 1939, three months after the outbreak of World War II, and ended three and a half months later with the Moscow Peac ...
of 1939-40. 96747 Crespodasilva was named after its discoverer, Lucy d'Escoffier Crespo da Silva, because she died shortly after the discovery, at age 22.
Languages
: Names were adapted to various languages from the beginning. 1 Ceres
Ceres ( minor-planet designation: 1 Ceres) is a dwarf planet in the middle main asteroid belt between the orbits of Mars and Jupiter. It was the first known asteroid, discovered on 1 January 1801 by Giuseppe Piazzi at Palermo Astronomical O ...
, ''Ceres'' being its Anglo-Latin name, was actually named ''Cerere'', the Italian form of the name. German, French, Arabic, and Hindi use forms similar to the English, whereas Russian uses a form, ''Tserera'', similar to the Italian. In Greek, the name was translated to Δήμητρα (Demeter
In ancient Greek religion and Greek mythology, mythology, Demeter (; Attic Greek, Attic: ''Dēmḗtēr'' ; Doric Greek, Doric: ''Dāmā́tēr'') is the Twelve Olympians, Olympian goddess of the harvest and agriculture, presiding over cro ...
), the Greek equivalent of the Roman goddess Ceres. In the early years, before it started causing conflicts, asteroids named after Roman figures were generally translated in Greek; other examples are Ἥρα (Hera
In ancient Greek religion, Hera (; ; in Ionic Greek, Ionic and Homeric Greek) is the goddess of marriage, women, and family, and the protector of women during childbirth. In Greek mythology, she is queen of the twelve Olympians and Mount Oly ...
) for 3 Juno, Ἑστία (Hestia
In ancient Greek religion and mythology, Hestia (; ) is the virgin goddess of the hearth and the home. In myth, she is the firstborn child of the Titans Cronus and Rhea, and one of the Twelve Olympians.
In Greek mythology, newborn Hestia, alo ...
) for 4 Vesta
Vesta (minor-planet designation: 4 Vesta) is one of the largest objects in the asteroid belt, with a mean diameter of . It was discovered by the German astronomer Heinrich Wilhelm Matthias Olbers on 29 March 1807 and is named after Vesta (mytho ...
, Χλωρίς ( Chloris) for 8 Flora, and Πίστη (Pistis
In Greek mythology, ''Pistis'' (; Ancient Greek: Πίστις) was the personification of good faith, trust and reliability. In Christianity and in the New Testament, ''pistis'' is typically translated as "faith". The word is mentioned together ...
) for 37 Fides
37 Fides () is a large main-belt asteroid. It was discovered by German astronomer Karl Theodor Robert Luther on October 5, 1855, and named after Fides, the Roman goddess of loyalty. Fides was the last of the main-belt asteroids to be assigned ...
. In Chinese, the names are not given the Chinese forms of the deities they are named after, but rather typically have a syllable or two for the character of the deity or person, followed by 神 'god(dess)' or 女 'woman' if just one syllable, plus 星 'star/planet', so that most asteroid names are written with three Chinese characters. Thus Ceres is 穀神星 'grain goddess planet', Pallas is 智神星 'wisdom goddess planet', etc.
Physical properties of comets and minor planets
Commission 15 of the International Astronomical Union
The International Astronomical Union (IAU; , UAI) is an international non-governmental organization (INGO) with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreach, education, and developmen ...
is dedicated to the Physical Study of Comets & Minor Planets.
Archival data on the physical properties of comets and minor planets are found in the PDS Asteroid/Dust Archive. This includes standard asteroid physical characteristics such as the properties of binary systems, occultation timings and diameters, masses, densities, rotation periods, surface temperatures, albedoes, spin vectors, taxonomy, and absolute magnitudes and slopes. In addition, European Asteroid Research Node (E.A.R.N.), an association of asteroid research groups, maintains a Data Base of Physical and Dynamical Properties of Near Earth Asteroids.
Environmental properties
Environmental characteristics have three aspects: space environment, surface environment and internal environment, including geological, optical, thermal and radiological environmental properties, etc., which are the basis for understanding the basic properties of minor planets, carrying out scientific research, and are also an important reference basis for designing the payload of exploration missions
Radiation environment
Without the protection of an atmosphere and its own strong magnetic field, the minor planet's surface is directly exposed to the surrounding radiation environment. In the cosmic space where minor planets are located, the radiation on the surface of the planets can be divided into two categories according to their sources: one comes from the sun, including electromagnetic radiation
In physics, electromagnetic radiation (EMR) is a self-propagating wave of the electromagnetic field that carries momentum and radiant energy through space. It encompasses a broad spectrum, classified by frequency or its inverse, wavelength ...
from the sun, and ionizing radiation
Ionizing (ionising) radiation, including Radioactive decay, nuclear radiation, consists of subatomic particles or electromagnetic waves that have enough energy per individual photon or particle to ionization, ionize atoms or molecules by detaching ...
from the solar wind
The solar wind is a stream of charged particles released from the Sun's outermost atmospheric layer, the Stellar corona, corona. This Plasma (physics), plasma mostly consists of electrons, protons and alpha particles with kinetic energy betwee ...
and solar energy particles; the other comes from the sun outside the solar system, that is, galactic cosmic rays, etc.
Optical environment
Usually during one rotation period of a minor planet, the albedo
Albedo ( ; ) is the fraction of sunlight that is Diffuse reflection, diffusely reflected by a body. It is measured on a scale from 0 (corresponding to a black body that absorbs all incident radiation) to 1 (corresponding to a body that reflects ...
of a minor planet will change slightly due to its irregular shape and uneven distribution of material composition. This small change will be reflected in the periodic change of the planet's light curve, which can be observed by ground-based equipment, so as to obtain the planet's magnitude
Magnitude may refer to:
Mathematics
*Euclidean vector, a quantity defined by both its magnitude and its direction
*Magnitude (mathematics), the relative size of an object
*Norm (mathematics), a term for the size or length of a vector
*Order of ...
, rotation period
In astronomy, the rotation period or spin period of a celestial object (e.g., star, planet, moon, asteroid) has two definitions. The first one corresponds to the '' sidereal rotation period'' (or ''sidereal day''), i.e., the time that the objec ...
, rotation axis orientation, shape, albedo distribution, and scattering properties. Generally speaking, the albedo
Albedo ( ; ) is the fraction of sunlight that is Diffuse reflection, diffusely reflected by a body. It is measured on a scale from 0 (corresponding to a black body that absorbs all incident radiation) to 1 (corresponding to a body that reflects ...
of minor planets is usually low, and the overall statistical distribution is bimodal, corresponding to C-type (average 0.035) and S-type (average 0.15) minor planets. In the minor planet exploration mission, measuring the albedo and color changes of the planet surface is also the most basic method to directly know the difference in the material composition of the planet surface.
Geological environment
The geological environment on the surface of minor planets is similar to that of other unprotected celestial bodies, with the most widespread geomorphological feature present being impact craters: however, the fact that most minor planets are rubble pile
In astronomy, a rubble pile is a celestial body that consists of numerous pieces of debris that have coalesced under the influence of gravity. Rubble piles have low density because there are large cavities between the various chunks that make the ...
structures, which are loose and porous, gives the impact action on the surface of minor planets its unique characteristics. On highly porous minor planets, small impact events produce spatter blankets similar to common impact events: whereas large impact events are dominated by compaction and spatter blankets are difficult to form, and the longer the planets receive such large impacts, the greater the overall density. In addition, statistical analysis of impact craters is an important means of obtaining information on the age of a planet surface. Although the Crater Size-Frequency Distribution (CSFD) method of dating commonly used on minor planet surfaces does not allow absolute ages to be obtained, it can be used to determine the relative ages of different geological bodies for comparison. In addition to impact, there are a variety of other rich geological effects on the surface of minor planets, such as mass wasting
Mass wasting, also known as mass movement, is a general term for the movement of rock (geology), rock or soil down slopes under the force of gravity. It differs from other processes of erosion in that the debris transported by mass wasting is no ...
on slopes and impact crater walls, large-scale linear features associated with graben
In geology, a graben () is a depression (geology), depressed block of the Crust (geology), crust of a planet or moon, bordered by parallel normal faults.
Etymology
''Graben'' is a loan word from German language, German, meaning 'ditch' or 't ...
, and electrostatic transport of dust. By analysing the various geological processes on the surface of minor planets, it is possible to learn about the possible internal activity at this stage and some of the key evolutionary information about the long-term interaction with the external environment, which may lead to some indication of the nature of the parent body's origin. Many of the larger planets are often covered by a layer of soil (regolith
Regolith () is a blanket of unconsolidated, loose, heterogeneous superficial deposits covering solid rock. It includes dust, broken rocks, and other related materials and is present on Earth, the Moon, Mars, some asteroids, and other terrestria ...
) of unknown thickness. Compared to other atmosphere-free bodies in the solar system (e.g. the Moon
The Moon is Earth's only natural satellite. It Orbit of the Moon, orbits around Earth at Lunar distance, an average distance of (; about 30 times Earth diameter, Earth's diameter). The Moon rotation, rotates, with a rotation period (lunar ...
), minor planets have weaker gravity fields and are less capable of retaining fine-grained material, resulting in a somewhat larger surface soil layer size. Soil layers are inevitably subject to intense space weathering that alters their physical and chemical properties due to direct exposure to the surrounding space environment. In silicate-rich soils, the outer layers of Fe are reduced to nano-phase Fe (np-Fe), which is the main product of space weathering. For some small planets, their surfaces are more exposed as boulders of varying sizes, up to 100 metres in diameter, due to their weaker gravitational pull. These boulder
In geology, a boulder (or rarely bowlder) is a rock fragment with size greater than in diameter. Smaller pieces are called cobbles and pebbles. While a boulder may be small enough to move or roll manually, others are extremely massive. In ...
s are of high scientific interest, as they may be either deeply buried material excavated by impact action or fragments of the planet's parent body that have survived. The rocks provide more direct and primitive information about the material inside the minor planet and the nature of its parent body than the soil layer, and the different colours and forms of the rocks indicate different sources of material on the surface of the minor planet or different evolutionary processes.
Magnetic environment
Usually in the interior of the planet, the convection of the conductive fluid will generate a large and strong magnetic field
A magnetic field (sometimes called B-field) is a physical field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular ...
. However, the size of a minor planet is generally small and most of the minor planets have a "crushed stone pile" structure, and there is basically no "dynamo" structure inside, so it will not generate a self-generated dipole magnetic field like the Earth. But some minor planets do have magnetic fields—on the one hand, some minor planets have remanent magnetism
Magnetism is the class of physical attributes that occur through a magnetic field, which allows objects to attract or repel each other. Because both electric currents and magnetic moments of elementary particles give rise to a magnetic field, ...
: if the parent body had a magnetic field or if the nearby planetary body has a strong magnetic field, the rocks on the parent body will be magnetised during the cooling process and the planet formed by the fission of the parent body will still retain remanence, which can also be detected in extraterrestrial meteorites from the minor planets; on the other hand, if the minor planets are composed of electrically conductive material and their internal conductivity is similar to that of carbon- or iron-bearing meteorites, the interaction between the minor planets and the solar wind
The solar wind is a stream of charged particles released from the Sun's outermost atmospheric layer, the Stellar corona, corona. This Plasma (physics), plasma mostly consists of electrons, protons and alpha particles with kinetic energy betwee ...
is likely to be unipolar induction, resulting in an external magnetic field for the minor planet. In addition, the magnetic fields of minor planets are not static; impact events, weathering in space and changes in the thermal environment can alter the existing magnetic fields of minor planets. At present, there are not many direct observations of minor planet magnetic fields, and the few existing planets detection projects generally carry magnetometers, with some targets such as Gaspra and Braille measured to have strong magnetic fields nearby, while others such as Lutetia have no magnetic field.
See also
* Groups of minor planets
* List of minor planets
The following is a list of minor planets in ascending numerical order. Minor planets are Small Solar System bodies, small bodies in the Solar System: asteroids, Distant minor planet, distant objects, and dwarf planets, but not comets. As of 2022 ...
* Dwarf planet
A dwarf planet is a small planetary-mass object that is in direct orbit around the Sun, massive enough to be hydrostatic equilibrium, gravitationally rounded, but insufficient to achieve clearing the neighbourhood, orbital dominance like the ...
* Quasi-satellite
A quasi-satellite is an object in a specific type of co-orbital configuration (1:1 orbital resonance) with a planet (or dwarf planet) where the object stays close to that planet over many orbital periods.
A quasi-satellite's orbit around the Sun ...
* Small Solar System body
* List of minor planets and comets visited by spacecraft
The following tables list all minor planets and comets that have been visited by robotic spacecraft.
List of minor planets visited by spacecraft
A total of 19 minor planets (asteroids, dwarf planets, and Kuiper belt objects) have been visi ...
Notes
References
External links
Minor Planet Center
Logarithmic graph of asteroid discoveries from 1801-2015
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Solar System