Minor planet

Various visited minor planets and their diversity:

• Pluto and Ceres
• Vesta, Lutetia, and Arrokoth
• Itokawa, Bennu, and Dinkinesh and its satellite Selam

Sizes are not to scale.

According to the

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.^[4] The term planetoid has also been used, especially for larger, planetary objects such as those the IAU has called dwarf planets since 2006.^[5][6] Historically, the terms asteroid, minor planet, and planetoid have been more or less synonymous.^[5][7] This terminology has become more complicated by the discovery of numerous minor planets beyond the orbit of Jupiter, especially trans-Neptunian objects that are generally not considered asteroids.^[7] 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 and form an ellipsoidal shape. All other minor planets and comets are called small Solar System bodies.^[1] The IAU stated that the term minor planet may still be used, but the term small Solar System body will be preferred.^[8] 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

• Asteroids; traditionally, most have been bodies in the inner Solar System.^[7]
  • Near-Earth asteroids, those whose orbits take them inside the orbit of Mars. Further subclassification of these, based on orbital distance, is used:^[13]
    • 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.
    • Earth-crossers
      .
    • Amor asteroids are those near-Earth 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.
  • 2020 XL₅.^[14]
  • Mars trojans, asteroids sharing Mars's orbit and gravitationally locked to it. As of 2007, eight such asteroids are known.^[15][16]
  • Asteroid belt, whose members follow roughly circular orbits between Mars and Jupiter. These are the original and best-known group of asteroids.
  • Jupiter trojans, asteroids sharing Jupiter's orbit and gravitationally locked to it. Numerically they are estimated to equal the main-belt asteroids.
• Distant minor planets, an umbrella term for minor planets in the outer Solar System.
  • Centaurs, 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.^[17]
  • Neptune trojans, 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.^[18]
  • Trans-Neptunian objects, bodies at or beyond the orbit of Neptune
    , the outermost planet.
    • The Kuiper belt, objects inside an apparent population drop-off approximately 55 AU from the Sun.
      • Makemake
        , also known as cubewanos, are in primordial, relatively circular orbits that are not in resonance with Neptune.
      • Resonant Kuiper belt objects.
        • Plutinos, bodies like Pluto
          that are in a 2:3 resonance with Neptune.
    • aphelia
      outside the Kuiper belt. These are thought to have been scattered by Neptune.
      • Resonant scattered disc objects.
    • perihelion
      outside the Kuiper belt.
      • Sednoids, detached objects with a perihelion greater than 75 AU (Sedna, 2012 VP113, and Leleākūhonua).
    • The Oort cloud, 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

• 1932 HA – provisional designation upon discovery on 24 April 1932
• (1862) 1932 HA – 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 2002 AT4 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.

The first few asteroids were named after figures from Greek and Roman mythology, 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.^[21] The first to be given an entirely non-Classical name was 45 Eugenia, named after Empress Eugénie de Montijo, the wife of Napoleon III. For some time only female (or feminized) names were used; Alexander von Humboldt 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

24680 Alleven, and 26858 Misterrogers

.

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

Lucy d'Escoffier Crespo da Silva, because she died shortly after the discovery, at age 22.^[24][25]

Names were adapted to various languages from the beginning.

1 Ceres, 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), 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) for 3 Juno, Ἑστία (Hestia) for 4 Vesta, Χλωρίς (Chloris) for 8 Flora, and Πίστη (Pistis) for 37 Fides. 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',^[26] Pallas is 智神星 'wisdom goddess planet', etc.^{[citation needed}

]

Commission 15[27] of the International Astronomical Union 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.^[28] 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.^[29]

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

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 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.^[33] 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.^[34] In addition to impact, there are a variety of other rich geological effects on the surface of minor planets,^[35] such as mass wasting on slopes and impact crater walls,^[36] large-scale linear features associated with graben,^[37] and electrostatic transport of dust.^[38] 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) of unknown thickness. Compared to other atmosphere-free bodies in the solar system (e.g. the Moon), minor planets have weaker gravity fields and are less capable of retaining fine-grained material, resulting in a somewhat larger surface soil layer size.^[39] 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.^[40] 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.^[41] These boulders 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

• Groups of minor planets
• List of minor planets
• Dwarf planet
• Quasi-satellite
• Small Solar System body
• List of minor planets and comets visited by spacecraft

Notes