Asteroid

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433 Eros photographed by NEAR Shoemaker
Galileo image of 243 Ida (the dot to the right is its moon Dactyl)
Dawn image of the dwarf planet Ceres
OSIRIS-REx image of 101955 Bennu, a rubble-pile asteroid
Images of visited asteroids illustrating their differences: (top row) 433 Eros and 243 Ida with its moon Dactyl, (bottom row) Ceres and 101955 Bennu. Sizes are not to scale.

An asteroid is a minor planet—an object that is neither a true planet nor a comet—that orbits within the inner Solar System. They are rocky, metallic, or icy bodies with no atmosphere. The size and shape of asteroids vary significantly, ranging from small rubble piles under a kilometer across to Ceres, a dwarf planet almost 1000 km in diameter.

Of the roughly one million known asteroids,

silicaceous compositions, respectively. The size of asteroids varies greatly; the largest, Ceres, is almost 1,000 km (600 mi) across and qualifies as a dwarf planet. The total mass of all the asteroids combined is only 3% that of Earth's Moon. The majority of main belt asteroids follow slightly elliptical, stable orbits, revolving in the same direction as the Earth and taking from three to six years to complete a full circuit of the Sun.[2]

Asteroids have been historically observed from Earth; the

Ceres. JAXA's missions Hayabusa and Hayabusa2 studied and returned samples of Itokawa and Ryugu, respectively. OSIRIS-REx studied Bennu, collecting a sample in 2020 which was delivered back to Earth in 2023. NASA's Lucy, launched in 2021, is tasked with studying ten different asteroids, two from the main belt and eight Jupiter trojans. Psyche, launched in October 2023, aims to study a metallic asteroid of the same name
.

Near-Earth asteroids can threaten all life on the planet; an asteroid impact event resulted in the Cretaceous–Paleogene extinction. Different asteroid deflection strategies have been proposed; the Double Asteroid Redirection Test spacecraft, or DART, was launched in 2021 and intentionally impacted Dimorphos in September 2022, successfully altering its orbit by crashing into it.

History of observations

Despite their large numbers, asteroids are a relatively recent discovery, with the first known asteroid—Ceres—only being identified in 1801.[3] Only one asteroid, 4 Vesta, which has a relatively reflective surface, is normally visible to the naked eye. When favorably positioned, 4 Vesta can be seen in dark skies. Rarely, small asteroids passing close to Earth may be visible to the naked eye for a short amount of time.[4] As of April 2022, the Minor Planet Center had data on 1,199,224 minor planets in the inner and outer Solar System, of which about 614,690 had enough information to be given numbered designations.[5]

Discovery of Ceres

In 1772, German astronomer Johann Elert Bode, citing Johann Daniel Titius, published a numerical procession known as the Titius–Bode law (now discredited). Except for an unexplained gap between Mars and Jupiter, Bode's formula seemed to predict the orbits of the known planets.[6][7] He wrote the following explanation for the existence of a "missing planet":

This latter point seems in particular to follow from the astonishing relation which the known six planets observe in their distances from the Sun. Let the distance from the Sun to Saturn be taken as 100, then Mercury is separated by 4 such parts from the Sun. Venus is 4 + 3 = 7. The Earth 4 + 6 = 10. Mars 4 + 12 = 16. Now comes a gap in this so orderly progression. After Mars there follows a space of 4 + 24 = 28 parts, in which no planet has yet been seen. Can one believe that the Founder of the universe had left this space empty? Certainly not. From here we come to the distance of Jupiter by 4 + 48 = 52 parts, and finally to that of Saturn by 4 + 96 = 100 parts.[8]

Bode's formula predicted another planet would be found with an orbital radius near 2.8 astronomical units (AU), or 420 million km, from the Sun.[7] The Titius–Bode law got a boost with William Herschel's discovery of Uranus near the predicted distance for a planet beyond Saturn.[6] In 1800, a group headed by Franz Xaver von Zach, editor of the German astronomical journal Monatliche Correspondenz (Monthly Correspondence), sent requests to 24 experienced astronomers (whom he dubbed the "celestial police"),[7] asking that they combine their efforts and begin a methodical search for the expected planet.[7] Although they did not discover Ceres, they later found the asteroids 2 Pallas, 3 Juno and 4 Vesta.[7]

One of the astronomers selected for the search was

Mr la Caille",[6] but found that "it was preceded by another".[6] Instead of a star, Piazzi had found a moving star-like object, which he first thought was a comet:[10]

The light was a little faint, and of the colour of Jupiter, but similar to many others which generally are reckoned of the eighth magnitude. Therefore I had no doubt of its being any other than a fixed star. [...] The evening of the third, my suspicion was converted into certainty, being assured it was not a fixed star. Nevertheless before I made it known, I waited till the evening of the fourth, when I had the satisfaction to see it had moved at the same rate as on the preceding days.[6]

Piazzi observed Ceres a total of 24 times, the final time on 11 February 1801, when illness interrupted his work. He announced his discovery on 24 January 1801 in letters to only two fellow astronomers, his compatriot Barnaba Oriani of Milan and Bode in Berlin.[3] He reported it as a comet but "since its movement is so slow and rather uniform, it has occurred to me several times that it might be something better than a comet".[6] In April, Piazzi sent his complete observations to Oriani, Bode, and French astronomer Jérôme Lalande. The information was published in the September 1801 issue of the Monatliche Correspondenz.[10]

By this time, the apparent position of Ceres had changed (mostly due to Earth's motion around the Sun), and was too close to the Sun's glare for other astronomers to confirm Piazzi's observations. Toward the end of the year, Ceres should have been visible again, but after such a long time it was difficult to predict its exact position. To recover Ceres, mathematician

patron goddess of Sicily and of King Ferdinand of Bourbon".[8]

Further search

Sizes of the first ten discovered asteroids, compared to the Moon

Three other asteroids (

Hermann Goldschmidt, the three most successful asteroid-hunters at that time, on a commemorative medallion marking the event.[12]

In 1891,

Max Wolf pioneered the use of astrophotography to detect asteroids, which appeared as short streaks on long-exposure photographic plates.[12] This dramatically increased the rate of detection compared with earlier visual methods: Wolf alone discovered 248 asteroids, beginning with 323 Brucia,[13] whereas only slightly more than 300 had been discovered up to that point. It was known that there were many more, but most astronomers did not bother with them, some calling them "vermin of the skies",[14] a phrase variously attributed to Eduard Suess[15] and Edmund Weiss.[16]
Even a century later, only a few thousand asteroids were identified, numbered and named.

19th and 20th centuries

Cumulative discoveries of just the near-Earth asteroids known by size, 1980–2022

In the past, asteroids were discovered by a four-step process. First, a region of the sky was photographed by a wide-field telescope or astrograph. Pairs of photographs were taken, typically one hour apart. Multiple pairs could be taken over a series of days. Second, the two films or plates of the same region were viewed under a stereoscope. A body in orbit around the Sun would move slightly between the pair of films. Under the stereoscope, the image of the body would seem to float slightly above the background of stars. Third, once a moving body was identified, its location would be measured precisely using a digitizing microscope. The location would be measured relative to known star locations.[17]

These first three steps do not constitute asteroid discovery: the observer has only found an apparition, which gets a provisional designation, made up of the year of discovery, a letter representing the half-month of discovery, and finally a letter and a number indicating the discovery's sequential number (example: 1998 FJ74). The last step is sending the locations and time of observations to the Minor Planet Center, where computer programs determine whether an apparition ties together earlier apparitions into a single orbit. If so, the object receives a catalogue number and the observer of the first apparition with a calculated orbit is declared the discoverer, and granted the honor of naming the object subject to the approval of the International Astronomical Union.[18]

Naming

2013 EC, shown here in radar images, has a provisional designation

By 1851, the Royal Astronomical Society decided that asteroids were being discovered at such a rapid rate that a different system was needed to categorize or name asteroids. In 1852, when de Gasparis discovered the twentieth asteroid, Benjamin Valz gave it a name and a number designating its rank among asteroid discoveries, 20 Massalia. Sometimes asteroids were discovered and not seen again. So, starting in 1892, new asteroids were listed by the year and a capital letter indicating the order in which the asteroid's orbit was calculated and registered within that specific year. For example, the first two asteroids discovered in 1892 were labeled 1892A and 1892B. However, there were not enough letters in the alphabet for all of the asteroids discovered in 1893, so 1893Z was followed by 1893AA. A number of variations of these methods were tried, including designations that included year plus a Greek letter in 1914. A simple chronological numbering system was established in 1925.[12][19]

Currently all newly discovered asteroids receive a provisional designation (such as 2002 AT4) consisting of the year of discovery 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—e.g. (433) Eros—but dropping the parentheses is quite common. Informally, it is also common to drop the number altogether, or to drop it after the first mention when a name is repeated in running text.[20] In addition, names can be proposed by the asteroid's discoverer, within guidelines established by the International Astronomical Union.[21]

Symbols

The first asteroids to be discovered were assigned iconic symbols like the ones traditionally used to designate the planets. By 1855 there were two dozen asteroid symbols, which often occurred in multiple variants.[22]

In 1851, after the fifteenth asteroid, Eunomia, had been discovered, Johann Franz Encke made a major change in the upcoming 1854 edition of the Berliner Astronomisches Jahrbuch (BAJ, Berlin Astronomical Yearbook). He introduced a disk (circle), a traditional symbol for a star, as the generic symbol for an asteroid. The circle was then numbered in order of discovery to indicate a specific asteroid. The numbered-circle convention was quickly adopted by astronomers, and the next asteroid to be discovered (16 Psyche, in 1852) was the first to be designated in that way at the time of its discovery. However, Psyche was given an iconic symbol as well, as were a few other asteroids discovered over the next few years. 20 Massalia was the first asteroid that was not assigned an iconic symbol, and no iconic symbols were created after the 1855 discovery of 37 Fides.[a][23]

Terminology

The first discovered asteroid, Ceres, was originally considered a new planet.[b] It was followed by the discovery of other similar bodies, which with the equipment of the time appeared to be points of light like stars, showing little or no planetary disc, though readily distinguishable from stars due to their apparent motions. This prompted the astronomer Sir William Herschel to propose the term asteroid,[c] coined in Greek as ἀστεροειδής, or asteroeidēs, meaning 'star-like, star-shaped', and derived from the Ancient Greek ἀστήρ astēr 'star, planet'. In the early second half of the 19th century, the terms asteroid and planet (not always qualified as "minor") were still used interchangeably.[d]

Traditionally, small bodies orbiting the Sun were classified as

small Solar System bodies being preferred by the International Astronomical Union (IAU).[29] As no IAU definition exists, asteroid can be defined as "an irregularly shaped rocky body orbiting the Sun that does not qualify as a planet or a dwarf planet under the IAU definitions of those terms".[30]

When found, asteroids were seen as a class of objects distinct from comets, and there was no unified term for the two until small Solar System body was coined in 2006. The main difference between an asteroid and a comet is that a comet shows a coma due to sublimation of near-surface ices by solar radiation. A few objects have ended up being dual-listed because they were first classified as minor planets but later showed evidence of cometary activity. Conversely, some (perhaps all) comets are eventually depleted of their surface volatile ices and become asteroid-like. A further distinction is that comets typically have more eccentric orbits than most asteroids; "asteroids" with notably eccentric orbits are probably dormant or extinct comets.[31]

For almost two centuries, from the discovery of

scattered-disc object
, and so on. They inhabit the cold outer reaches of the Solar System where ices remain solid and comet-like bodies are not expected to exhibit much cometary activity; if centaurs or trans-Neptunian objects were to venture close to the Sun, their volatile ices would sublimate, and traditional approaches would classify them as comets and not asteroids.

The innermost of these are the Kuiper-belt objects, called "objects" partly to avoid the need to classify them as asteroids or comets.[32] They are thought to be predominantly comet-like in composition, though some may be more akin to asteroids.[33] Furthermore, most do not have the highly eccentric orbits associated with comets, and the ones so far discovered are larger than traditional comet nuclei. (The much more distant Oort cloud is hypothesized to be the main reservoir of dormant comets.) Other recent observations, such as the analysis of the cometary dust collected by the Stardust probe, are increasingly blurring the distinction between comets and asteroids,[34] suggesting "a continuum between asteroids and comets" rather than a sharp dividing line.[35]

The minor planets beyond Jupiter's orbit are sometimes also called "asteroids", especially in popular presentations.[e] However, it is becoming increasingly common for the term asteroid to be restricted to minor planets of the inner Solar System.[32] Therefore, this article will restrict itself for the most part to the classical asteroids: objects of the asteroid belt, Jupiter trojans, and near-Earth objects.

When the IAU introduced the class small Solar System bodies in 2006 to include most objects previously classified as minor planets and comets, they created the class of dwarf planets for the largest minor planets—those that have enough mass to have become ellipsoidal under their own gravity. According to the IAU, "the term 'minor planet' may still be used, but generally, the term 'Small Solar System Body' will be preferred."[37] Currently only the largest object in the asteroid belt, Ceres, at about 975 km (606 mi) across, has been placed in the dwarf planet category.[38][39]

Formation

Many asteroids are the shattered remnants of

solar nebula that never grew large enough to become planets.[40] It is thought that planetesimals in the asteroid belt evolved much like the rest of objects in the solar nebula until Jupiter neared its current mass, at which point excitation from orbital resonances with Jupiter ejected over 99% of planetesimals in the belt. Simulations and a discontinuity in spin rate and spectral properties suggest that asteroids larger than approximately 120 km (75 mi) in diameter accreted during that early era, whereas smaller bodies are fragments from collisions between asteroids during or after the Jovian disruption.[41] Ceres and Vesta grew large enough to melt and differentiate, with heavy metallic elements sinking to the core, leaving rocky minerals in the crust.[42]

In the Nice model, many Kuiper-belt objects are captured in the outer asteroid belt, at distances greater than 2.6 AU. Most were later ejected by Jupiter, but those that remained may be the D-type asteroids, and possibly include Ceres.[43]

Distribution within the Solar System

A top view of asteroid group location in the inner solar system
A map of planets and asteroid groups of the inner solar system. Distances from sun are to scale, object sizes are not.

Various dynamical groups of asteroids have been discovered orbiting in the inner Solar System. Their orbits are perturbed by the gravity of other bodies in the Solar System and by the Yarkovsky effect. Significant populations include:

Asteroid belt

The majority of known asteroids orbit within the asteroid belt between the orbits of Mars and Jupiter, generally in relatively low-eccentricity (i.e. not very elongated) orbits. This belt is estimated to contain between 1.1 and 1.9 million asteroids larger than 1 km (0.6 mi) in diameter,[44] and millions of smaller ones. These asteroids may be remnants of the protoplanetary disk, and in this region the accretion of planetesimals into planets during the formative period of the Solar System was prevented by large gravitational perturbations by Jupiter.

Contrary to popular imagery, the asteroid belt is mostly empty. The asteroids are spread over such a large volume that reaching an asteroid without aiming carefully would be improbable. 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,[45] and a survey in the infrared wavelengths has shown that the asteroid belt has between 700,000 and 1.7 million asteroids with a diameter of 1 km or more.[46] The absolute magnitudes of most of the known asteroids are between 11 and 19, with the median at about 16.[47]

The total mass of the asteroid belt is estimated to be 2.39×1021 kg, which is just 3% of the mass of the Moon; the mass of the Kuiper Belt and Scattered Disk is over 100 times as large.[48] The four largest objects, Ceres, Vesta, Pallas, and Hygiea, account for maybe 62% of the belt's total mass, with 39% accounted for by Ceres alone.

Trojans

Trojans are populations that share an orbit with a larger planet or moon, but do not collide with it because they orbit in one of the two

Lagrangian points
of stability, L4 and L5, which lie 60° ahead of and behind the larger body.

In the Solar System, most known trojans share the

Uranus trojans, and two Earth trojans, have been found to date. A temporary Venus trojan is also known. Numerical orbital dynamics stability simulations indicate that Saturn and Uranus probably do not have any primordial trojans.[50]

Near-Earth asteroids

Near-Earth asteroids, or NEAs, are asteroids that have orbits that pass close to that of Earth. Asteroids that actually cross Earth's orbital path are known as Earth-crossers. As of April 2022, a total of 28,772 near-Earth asteroids were known; 878 have a diameter of one kilometer or larger.[51]

A small number of NEAs are

extinct comets that have lost their volatile surface materials, although having a faint or intermittent comet-like tail does not necessarily result in a classification as a near-Earth comet, making the boundaries somewhat fuzzy. The rest of the near-Earth asteroids are driven out of the asteroid belt by gravitational interactions with Jupiter.[52][53]

Many asteroids have natural satellites (minor-planet moons). As of October 2021, there were 85 NEAs known to have at least one moon, including three known to have two moons.[54] The asteroid 3122 Florence, one of the largest potentially hazardous asteroids with a diameter of 4.5 km (2.8 mi), has two moons measuring 100–300 m (330–980 ft) across, which were discovered by radar imaging during the asteroid's 2017 approach to Earth.[55]

Near-Earth asteroids are divided into groups based on their

semi-major axis (a), perihelion distance (q), and aphelion distance (Q):[56][52]

  • The Atiras or Apoheles have orbits strictly inside Earth's orbit: an Atira asteroid's aphelion distance (Q) is smaller than Earth's perihelion distance (0.983 AU). That is, Q < 0.983 AU, which implies that the asteroid's semi-major axis is also less than 0.983 AU.[57]
  • The Atens have a semi-major axis of less than 1 AU and cross Earth's orbit. Mathematically, a < 1.0 AU and Q > 0.983 AU. (0.983 AU is Earth's perihelion distance.)
  • The Apollos have a semi-major axis of more than 1 AU and cross Earth's orbit. Mathematically, a > 1.0 AU and q < 1.017 AU. (1.017 AU is Earth's aphelion distance.)
  • The Amors have orbits strictly outside Earth's orbit: an Amor asteroid's perihelion distance (q) is greater than Earth's aphelion distance (1.017 AU). Amor asteroids are also near-earth objects so q < 1.3 AU. In summary, 1.017 AU < q < 1.3 AU. (This implies that the asteroid's semi-major axis (a) is also larger than 1.017 AU.) Some Amor asteroid orbits cross the orbit of Mars.

Martian moons

Phobos
Deimos

It is unclear whether Martian moons Phobos and Deimos are captured asteroids or were formed due to impact event on Mars.

equatorial plane, and hence a capture origin requires a mechanism for circularizing the initially highly eccentric orbit, and adjusting its inclination into the equatorial plane, most probably by a combination of atmospheric drag and tidal forces,[62] although it is not clear whether sufficient time was available for this to occur for Deimos.[58] Capture also requires dissipation of energy. The current Martian atmosphere is too thin to capture a Phobos-sized object by atmospheric braking.[58] Geoffrey A. Landis has pointed out that the capture could have occurred if the original body was a binary asteroid that separated under tidal forces.[61][63]

Phobos could be a second-generation Solar System object that coalesced in orbit after Mars formed, rather than forming concurrently out of the same birth cloud as Mars.[64]

Another hypothesis is that Mars was once surrounded by many Phobos- and Deimos-sized bodies, perhaps ejected into orbit around it by a collision with a large

prevailing theory
for the origin of Earth's moon.

Characteristics

Size distribution

The asteroids of the Solar System, categorized by size and number