253 Mathilde, a C-type asteroid measuring about 50 km (30 mi) across, covered in craters half that size. Photograph taken in 1997 by the NEAR Shoemaker probe.
2014 JO25 imaged by radar during its 2017 Earth flyby
Diagram of the Solar System's asteroid belt

Asteroids are minor planets, especially of the inner Solar System. Larger asteroids have also been called planetoids. These terms have historically been applied to any astronomical object orbiting the Sun that did not resemble a planet-like disc and was not observed to have characteristics of an active comet such as a tail. As minor planets in the outer Solar System were discovered they were typically found to have volatile-rich surfaces similar to comets. As a result, they were often distinguished from objects found in the main asteroid belt.[1] In this article, the term "asteroid" refers to the minor planets of the inner Solar System including those co-orbital with Jupiter.

There exist millions of asteroids, many thought to be the shattered remnants of planetesimals, bodies within the young Sun's solar nebula that never grew large enough to become planets.[2] The vast majority of known asteroids orbit within the main asteroid belt located between the orbits of Mars and Jupiter, or are co-orbital with Jupiter (the Jupiter trojans). However, other orbital families exist with significant populations, including the near-Earth objects. Individual asteroids are classified by their characteristic spectra, with the majority falling into three main groups: C-type, M-type, and S-type. These were named after and are generally identified with carbon-rich, metallic, and silicate (stony) compositions, respectively. The sizes of asteroids varies greatly; the largest, Ceres, is almost 1,000 km (625 mi) across.

Asteroids are differentiated from comets and meteoroids. In the case of comets, the difference is one of composition: while asteroids are mainly composed of mineral and rock, comets are primarily composed of dust and ice. Furthermore, asteroids formed closer to the sun, preventing the development of cometary ice.[3] The difference between asteroids and meteoroids is mainly one of size: meteoroids have a diameter of one meter or less, whereas asteroids have a diameter of greater than one meter.[4] Finally, meteoroids can be composed of either cometary or asteroidal materials.[5]

Only one asteroid, 4 Vesta, which has a relatively reflective surface, is normally visible to the naked eye, and this only in very dark skies when it is favorably positioned. Rarely, small asteroids passing close to Earth may be visible to the naked eye for a short time.[6] As of October 2017, the Minor Planet Center had data on almost 745,000 objects in the inner and outer Solar System, of which almost 504,000 had enough information to be given numbered designations.[7]

The United Nations declared 30 June as International Asteroid Day to educate the public about asteroids. The date of International Asteroid Day commemorates the anniversary of the Tunguska asteroid impact over Siberia, Russian Federation, on 30 June 1908.[8][9]

In April 2018, the B612 Foundation reported "It's 100 percent certain we'll be hit [by a devastating asteroid], but we're not 100 percent sure when."[10][11] Also in 2018, physicist Stephen Hawking,in his final book Brief Answers to the Big Questions, considered an asteroid collision to be the biggest threat to the planet.[12][13][14] In June 2018, the US National Science and Technology Council warned that America is unprepared for an asteroid impact event, and has developed and released the "National Near-Earth Object Preparedness Strategy Action Plan" to better prepare.[15][16][17][18][19] According to expert testimony in the United States Congress in 2013, NASA would require at least five years of preparation before a mission to intercept an asteroid could be launched.[20]


Sizes of the first ten asteroids to be discovered, compared to the Moon
243 Ida and its moon Dactyl. Dactyl is the first satellite of an asteroid to be discovered.

The first asteroid to be discovered, Ceres, was originally considered to be a new planet.[note 1] This 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",[note 2] 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 nineteenth century, the terms "asteroid" and "planet" (not always qualified as "minor") were still used interchangeably. [note 3]

Overview of discovery timeline:[24]

Historical methods

Asteroid discovery methods have dramatically improved over the past two centuries.

In the last years of the 18th century, Baron Franz Xaver von Zach organized a group of 24 astronomers to search the sky for the missing planet predicted at about 2.8 AU from the Sun by the Titius-Bode law, partly because of the discovery, by Sir William Herschel in 1781, of the planet Uranus at the distance predicted by the law.[28] This task required that hand-drawn sky charts be prepared for all stars in the zodiacal band down to an agreed-upon limit of faintness. On subsequent nights, the sky would be charted again and any moving object would, hopefully, be spotted. The expected motion of the missing planet was about 30 seconds of arc per hour, readily discernible by observers.

First asteroid image (Ceres and Vesta) from Mars – viewed by Curiosity (20 April 2014).

The first object, Ceres, was not discovered by a member of the group, but rather by accident in 1801 by Giuseppe Piazzi, director of the observatory of Palermo in Sicily. He discovered a new star-like object in Taurus and followed the displacement of this object during several nights. Later that year, Carl Friedrich Gauss used these observations to calculate the orbit of this unknown object, which was found to be between the planets Mars and Jupiter. Piazzi named it after Ceres, the Roman goddess of agriculture.[28]

Three other asteroids (2 Pallas, 3 Juno, and 4 Vesta) were discovered over the next few years, with Vesta found in 1807. After eight more years of fruitless searches, most astronomers assumed that there were no more and abandoned any further searches.[citation needed]

However, Karl Ludwig Hencke persisted, and began searching for more asteroids in 1830. Fifteen years later, he found 5 Astraea, the first new asteroid in 38 years. He also found 6 Hebe less than two years later. After this, other astronomers joined in the search and at least one new asteroid was discovered every year after that (except the wartime years 1944 and 1945). Notable asteroid hunters of this early era were J. R. Hind, Annibale de Gasparis, Robert Luther, H. M. S. Goldschmidt, Jean Chacornac, James Ferguson, Norman Robert Pogson, E. W. Tempel, J. C. Watson, C. H. F. Peters, A. Borrelly, J. Palisa, the Henry brothers and Auguste Charlois.

In 1891, Max Wolf pioneered the use of astrophotography to detect asteroids, which appeared as short streaks on long-exposure photographic plates. This dramatically increased the rate of detection compared with earlier visual methods: Wolf alone discovered 248 asteroids, beginning with 323 Brucia, 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, calling them "vermin of the skies",[29] a phrase variously attributed to Eduard Suess[30] and Edmund Weiss.[31] Even a century later, only a few thousand asteroids were identified, numbered and named.

Manual methods of the 1900s and modern reporting

Until 1998, 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. Any 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.[32]

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 of discovery is to send 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.

Computerized methods

2004 FH is the center dot being followed by the sequence; the object that flashes by during the clip is an artificial satellite.
Cumulative discoveries of just the near-Earth asteroids known by size, 1980–2017

There is increasing interest in identifying asteroids whose orbits cross Earth's, and that could, given enough time, collide with Earth (see Earth-crosser asteroids). The three most important groups of near-Earth asteroids are the Apollos, Amors, and Atens. Various asteroid deflection strategies have been proposed, as early as the 1960s.

The near-Earth asteroid 433 Eros had been discovered as long ago as 1898, and the 1930s brought a flurry of similar objects. In order of discovery, these were: 1221 Amor, 1862 Apollo, 2101 Adonis, and finally 69230 Hermes, which approached within 0.005 AU of Earth in 1937. Astronomers began to realize the possibilities of Earth impact.

Two events in later decades increased the alarm: the increasing acceptance of the Alvarez hypothesis that an impact event resulted in the Cretaceous–Paleogene extinction, and the 1994 observation of Comet Shoemaker-Levy 9 crashing into Jupiter. The U.S. military also declassified the information that its military satellites, built to detect nuclear explosions, had detected hundreds of upper-atmosphere impacts by objects ranging from one to ten meters across.

All these considerations helped spur the launch of highly efficient surveys that consist of charge-coupled device (CCD) cameras and computers directly connected to telescopes. As of 2011, it was estimated that 89% to 96% of near-Earth asteroids one kilometer or larger in diameter had been discovered.[33] A list of teams using such systems includes:[34] [35]

As of 29 October 2018, the LINEAR system alone has discovered 147,132 asteroids.[36] Among all the surveys, 19,266 near-Earth asteroids have been discovered[37] including almost 900 more than 1 km (0.6 mi) in diameter.[38]

Other Languages
Afrikaans: Asteroïde
Alemannisch: Asteroid
العربية: كويكب
aragonés: Asteroide
অসমীয়া: গ্ৰহাণু
asturianu: Asteroide
Avañe'ẽ: Mbyjaveve
azərbaycanca: Asteroid
تۆرکجه: گزگنچه
বাংলা: গ্রহাণু
Bahasa Banjar: Asteruid
Bân-lâm-gú: Sió-he̍k-chheⁿ
башҡортса: Астероид
беларуская: Астэроід
беларуская (тарашкевіца)‎: Астэроід
भोजपुरी: एस्टेरॉइड्स
български: Астероид
Boarisch: Asteroid
bosanski: Asteroidi
brezhoneg: Asteroidenn
català: Asteroide
Чӑвашла: Астероид
čeština: Asteroid
Cymraeg: Asteroid
dansk: Asteroide
Deutsch: Asteroid
eesti: Asteroid
Ελληνικά: Αστεροειδής
emiliàn e rumagnòl: Asteròid
español: Asteroide
Esperanto: Asteroido
euskara: Asteroide
فارسی: سیارک
Fiji Hindi: Chhota tara
français: Astéroïde
Frysk: Asteroïde
Gaeilge: Astaróideach
Gaelg: Roltageagh
galego: Asteroide
한국어: 소행성
հայերեն: Աստղակերպ
hornjoserbsce: Asteroid
hrvatski: Asteroidi
Ilokano: Asteroid
Bahasa Indonesia: Asteroid
interlingua: Asteroide
íslenska: Smástirni
italiano: Asteroide
עברית: אסטרואיד
Jawa: Asteroid
Kapampangan: Asteroyde
къарачай-малкъар: Астероид
қазақша: Астероид
Kiswahili: Asteroidi
Kreyòl ayisyen: Astewoyid
kurdî: Asteroîd
Кыргызча: Астероид
Latina: Asteroides
latviešu: Asteroīds
Lëtzebuergesch: Asteroid
лезги: Астероид
lietuvių: Asteroidas
Limburgs: Planetoïed
lumbaart: Asteroide
magyar: Kisbolygó
македонски: Астероид
Malagasy: Asterôida
മലയാളം: ഛിന്നഗ്രഹം
Malti: Asterojde
मराठी: लघुग्रह
მარგალური: ასტეროიდი
Bahasa Melayu: Asteroid
Minangkabau: Asteroid
Mirandés: Asteróide
မြန်မာဘာသာ: ဥက္ကာပျံ
Nederlands: Planetoïde
日本語: 小惑星
Napulitano: Asteroide
нохчийн: Астероид
Nordfriisk: Asteroiid
norsk: Asteroide
norsk nynorsk: Asteroide
occitan: Asteroïde
oʻzbekcha/ўзбекча: Asteroid
پنجابی: تارے ورگا
Patois: Astaraid
Piemontèis: Asteròid
Plattdüütsch: Asteroid
polski: Planetoida
português: Asteroide
Qaraqalpaqsha: Asteroid
Ripoarisch: Asteroid
română: Asteroid
Runa Simi: Puriq quyllurcha
русиньскый: Астероід
русский: Астероид
саха тыла: Астероид
Scots: Asteroid
Seeltersk: Planetoiden
shqip: Asteroidet
sicilianu: Astiròidi
සිංහල: ග්‍රහක
Simple English: Asteroid
slovenčina: Asteroid
slovenščina: Asteroid
српски / srpski: Астероид
srpskohrvatski / српскохрватски: Asteroid
Basa Sunda: Astéroid
suomi: Asteroidi
svenska: Asteroid
Tagalog: Asteroyd
தமிழ்: சிறுகோள்
Taqbaylit: Azunyur
татарча/tatarça: Астероидлар
తెలుగు: గ్రహశకలం
тоҷикӣ: Сайёрак
Türkçe: Asteroit
Türkmençe: Asteroid
тыва дыл: Астероид
українська: Астероїд
اردو: سیارچہ
vèneto: Asteroide
Tiếng Việt: Tiểu hành tinh
文言: 小行星
West-Vlams: Asteroïde
Winaray: Asteroyd
吴语: 小行星
ייִדיש: אסטערויד
粵語: 小行星
žemaitėška: Asteruoids
中文: 小行星