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As asteroids (from ancient Greek ἀστεροειδής asteroeidḗs , German , starlike ' ), minor planets or planetoids are astronomical Little Body referred, based on Kepler's orbit around the sun move and larger than meteoroids ( millimeters to meters ), but smaller than the dwarf planet (about one thousand kilometers ) are.

The term asteroid is often used as a synonym of minor planet used, but mainly refers only to objects within the Neptune ground and is not from the IAU defined term. Beyond the orbit of Neptune, such bodies are also called trans- Neptunian objects (TNO). According to the more recent definition, the term minor planet encompasses the “classic” asteroids and the TNO.

So far 989,137 asteroids are known in the solar system (as of July 16, 2020), with several thousand new discoveries being added every month and the actual number is likely to be in the millions. In contrast to the dwarf planets, asteroids have by definition too low a mass to come into hydrostatic equilibrium and assume an approximately round shape, and are therefore generally irregularly shaped bodies. Very few are more than a few hundred kilometers in diameter.

Large asteroids in the asteroid belt are the objects (2) Pallas , (3) Juno , (4) Vesta , (5) Astraea , (6) Hebe , (7) Iris , (10) Hygiea and (15) Eunomia .

Asteroid (243) Ida with the moon Dactyl , photographed from the Galileo probe
Asteroid (433) Eros , photographed from the NEAR Shoemaker probe


The term asteroid refers to the size of the objects. Asteroid literally means "star-like". Almost all of them are so small that in the telescope they appear like the point of light from a star . The planets, on the other hand, appear as small disks with a certain spatial extent.

The term small planet or asteroid comes from the fact that the objects in the firmament move like planets relative to the stars. Asteroids are not planets and are not considered dwarf planets , because due to their small size, gravity is too weak to shape them approximately into a sphere. Together with comets and meteoroids , asteroids belong to the class of small bodies . Meteoroids are smaller than asteroids, but there is no clear limit between them and asteroids, neither in size nor in composition.

Dwarf planets

Since the 26th General Assembly of the International Astronomical Union (IAU) and its definition on August 24, 2006, the large, round objects, whose shape is in hydrostatic equilibrium , no longer belong to the asteroids, but to the dwarf planets .

(1) Ceres (975 km in diameter) is the largest object in the asteroid belt and is the only object to be counted among the dwarf planets. (2) Pallas and (4) Vesta are large objects in the asteroid belt, but both are not round and therefore not dwarf planets by definition.

In the Kuiper Belt there are, in addition to Pluto (2390 km in diameter), which used to be a planet and now a dwarf planet, other dwarf planets: (136199) Eris (2326 km), (136472) Makemake (1430 × 1502 km), (136108) Haumea ( elliptical, approximately 1920 × 1540 × 990 km), (50000) Quaoar (1110 km), and (90482) Orcus (917 km).

The object (90377) Sedna , about 995 km in size, discovered at the end of 2003 beyond the Kuiper Belt should also be classified as a dwarf planet.

The history of asteroid exploration

Assumed minor planet and the "sky police"

As early as 1760, the German scholar Johann Daniel Titius developed a simple mathematical formula ( Titius-Bode series ) according to which the solar distances of the planets correspond to a simple numerical sequence. According to this sequence, however, there should be another planet between Mars and Jupiter at a distance of 2.8 AU from the sun  . A downright hunt began on this apparently still undiscovered planet towards the end of the 18th century. For a coordinated search, the Sky Police was founded in 1800 as the first international research project . The organizer was Baron Franz Xaver von Zach , who was at the Gotha observatory at the time. The starry sky has been divided into 24 sectors that have been systematically searched by astronomers across Europe. The name " Phaeton " had already been reserved for the planet .

The search was unsuccessful in that the first minor planet (Ceres) was discovered by chance at the beginning of 1801. However, the Sky Police soon proved itself in several ways: with the recovery of the minor planet that had been lost from sight, with improved communication about celestial discoveries and with the successful search for other minor planets between 1802 and 1807.

The discovery of the first minor planets

Giuseppe Piazzi

On New Year's Eve of 1801, the astronomer and theologian Giuseppe Piazzi discovered a faintly luminous celestial body that was not shown on any star map while surveying the constellation Taurus in the telescope of the observatory in Palermo ( Sicily ) . Piazzi had heard of Zach's research project and observed the object over the following nights, suspecting that he had found the planet he was looking for. He sent his observation results to Zach, initially calling it a new comet . However, Piazzi fell ill and could not continue his observations. Much time passed before his observations were published. The celestial body had meanwhile moved further towards the sun and could not be found again at first.

However, the mathematician Gauss had developed a numerical method that made it possible to determine the orbits of planets or comets from just a few positions using the method of least squares . After Gauss had read Piazzi's publications, he calculated the orbit of the celestial body and sent the result to Gotha. Heinrich Wilhelm Olbers then rediscovered the object on December 31, 1801, which was finally given the name Ceres . In 1802, Olbers discovered another celestial body, which he named Pallas . Juno was discovered in 1803 and Vesta in 1807 .

However, it took 38 years for the fifth asteroid, Astraea, to be discovered in 1845. The asteroids discovered by then were not yet referred to as such - they were considered full-fledged planets at that time. So it happened that the planet Neptune was not counted as the eighth, but as the thirteenth planet when it was discovered in 1846. From 1847, however, further discoveries followed so quickly that it was soon decided to introduce a new object class of celestial bodies for the numerous but all of them quite small celestial bodies that orbit the sun between Mars and Jupiter: the asteroids , the so-called small planets . The number of large planets thus dropped to eight. By 1890, a total of over 300 asteroids had been discovered.

Photographic search methods, radar measurements

After 1890, the use of photography in astronomy brought significant advances. The asteroids, which until then had been painstakingly found by comparing telescope observations with sky maps, were now revealed by traces of light on the photographic plates. Due to the higher light sensitivity of the photographic emulsions compared to the human eye , in combination with long exposure times with tracking of the telescope, almost in fast motion, extremely weak objects could be detected. With the use of the new technology, the number of asteroids discovered rose rapidly.

A century later, around 1990, digital photography triggered another leap in development in the form of CCD camera technology, which is further increased by the possibilities of computer-aided evaluation of electronic recordings. Since then, the number of asteroids found each year has multiplied again.

Once the orbit of an asteroid has been determined, the size of the celestial body can be determined by examining its brightness and reflectivity, the albedo . For this purpose, measurements are carried out with visible light frequencies and in the infrared range . However, this method is associated with uncertainties, since the surfaces of the asteroids have different chemical structures and reflect the light to different degrees.

More precise results can be obtained using radar observations . Refer to radio telescopes are used, which, converted as a transmitter, strong radio waves emit in the direction of asteroids. By measuring the transit time of the waves reflected by the asteroids, their exact distance can be determined. Further evaluation of the radio waves provides data on shape and size. For example, the observation of the asteroids (4769) Castalia and (4179) Toutatis provided real “radar images” .

Automated surveys

Since the 1990s, new and further developed technologies, as well as continued improvements in the performance of detectors and electronic data processing, have enabled a number of automated search programs with different objectives. These surveys are a major contributor to the discovery of new asteroids.

A number of search programs focus on near-earth asteroids e.g. B. LONEOS , LINEAR , NEAT , NeoWise , Spacewatch , Catalina Sky Survey and Pan-STARRS . They play a major role in the fact that new asteroids are found almost daily, the number of which had reached over 900,000 in mid-July 2020.

In the near future the number of known asteroids will increase again dramatically, as surveys with increased sensitivity are planned for the next few years, for example Gaia and LSST . According to model calculations, the Gaia space probe alone is expected to discover up to a million previously unknown asteroids.

Space probe observations

Image of some asteroids explored by space probes

A number of asteroids could be examined more closely using space probes :

More missions are planned, including:


The names of the asteroids are composed of a prefixed number and a name. The number used to indicate the order in which the celestial body was discovered. Today it is a purely numerical form of counting, since it is only given when the orbit of the asteroid is secured and the object can be found again at any time; this can definitely only take place years after the initial observation. Of the 791,194 asteroids known to date, 523,824 have a number (as of January 11, 2019).

The discoverer has the right to propose a name within ten years after the numbering. However, this has to be confirmed by a commission of the IAU, as there are guidelines for the names of astronomical objects . Accordingly, numerous asteroids exist with numbers but without names, especially in the upper ten thousand.

New discoveries, for which a path could not yet be calculated with sufficient accuracy, are marked with the year of discovery and a combination of letters, for example 2003 UB 313 . The letter combination consists of the first letter for the half of the month (beginning with A and continuing to Y without I) and a continuous letter (A to Z without I). If more than 25 minor planets are discovered in one half of the month - which is the rule today - the letter combination starts from the beginning, followed by a consecutive number increased by one for each run.

The first asteroid was discovered in 1801 by Giuseppe Piazzi at the Palermo observatory in Sicily . Piazzi baptized the heavenly body with the name "Ceres Ferdinandea". The Roman goddess Ceres is the patron saint of the island of Sicily. With the second name, Piazzi wanted to honor King Ferdinand IV , the ruler of Italy and Sicily. This displeased the international research community and the second name was dropped. The official name of the asteroid is therefore (1) Ceres .

In the further discoveries the nomenclature was retained and the asteroids were named after Roman and Greek goddesses ; these were (2) Pallas , (3) Juno , (4) Vesta , (5) Astraea , (6) Hebe , and so on.

As more and more asteroids were discovered, astronomers ran out of ancient deities. Asteroids were named after the wives of the discoverers, in honor of historical or public figures, cities and fairy tale characters. Examples are the asteroids (21) Lutetia , (216) Cleopatra , (719) Albert , (1773) Rumpelstilz , (5535) Annefrank , (17744) Jodiefoster .

In addition to names from Greco-Roman mythology , names of deities from other cultures are also used, especially for newly discovered, larger objects such as (20,000) Varuna , (50,000) Quaoar and (90377) Sedna .

Asteroid moons do not have a permanent number in addition to their name and are not considered asteroids or small bodies , as they do not orbit the sun independently.


At first the astronomers assumed that the asteroids were the result of a cosmic catastrophe in which a planet between Mars and Jupiter broke apart and left fragments on its orbit. However, it turned out that the total mass of the asteroids present in the main belt is much smaller than that of the Earth's moon . Estimates of the total mass of the minor planets vary between 0.1 and 0.01 percent of the earth's mass (the moon is about 1.23 percent of the earth's mass). It is therefore assumed that the asteroids represent a residual population of planetesimals from the formation phase of the solar system. The gravity of Jupiter, whose mass increased the fastest, prevented the formation of a larger planet from the asteroid material. The planetesimals were disturbed in their orbits, repeatedly colliding violently with one another and breaking. Some were diverted into orbits that put them on a collision course with the planets. The impact craters on the planetary moons and the inner planets are evidence of this . The largest asteroids have been heated strongly after their formation (mainly by the radioactive decay of aluminum - isotope 26 Al and possibly the iron isotope 60 Fe) and melted inside. Heavy elements , such as nickel and iron , are deposited inside as a result of the effect of gravity, the lighter compounds, such as silicates , remain in the outer areas. This led to the formation of differentiated bodies with a metallic core and a silicate coat. Some of the differentiated asteroids broke up in further collisions, with fragments falling into the Earth's area of ​​attraction as meteorites .

Classification schemes of asteroids

Carboniferous chondrite

The spectroscopic examination of the asteroids showed that their surfaces are chemically composed differently. There was a similar division into different spectral or taxonomic classes.

Classification scheme according to Tholen

In 1984 David J. Tholen published a classification scheme with 14 classes for the classification of asteroids based on their spectral properties, which in turn are summarized in 3 groups (C, S and X):

The classification scheme was supplemented by Tholen in 1989:

U addition indicates unusual spectrum; far from the center of the cluster
: Addition shows "noisy" data
:: Addition indicates very "noisy" data
--- Displays data that are too "noisy" for classification to be possible (basically all classes would be possible)
I Conflicting data

After Tholen, up to four letters can be assigned, for example "SCTU".

An asteroid with such an addition is for example (2340) Hathor , which would be sorted into the spectral class "CSU" according to Tholen (according to SMASSII as Sq). For example, the letter “I” is entered in the JPL Small-Body Database for the asteroid (515) Athalia , according to SMASSII the asteroid is classified as “Cb”.


In the past, scientists assumed that the asteroids were monolithic boulders, i.e. compact structures. The low densities of several asteroids as well as the presence of huge impact craters indicate, however, that many asteroids are loosely structured and are more like rubble piles than loose "heaps of rubble" that are only held together by gravity . Loosely structured bodies can absorb the forces that occur in a collision without being destroyed. Compact bodies, on the other hand, are torn apart by the shock waves during larger impact events. In addition, the large asteroids only have low rotational speeds. A rapid rotation around its own axis would otherwise lead to the centrifugal forces tearing the body apart (see also: YORP effect ) . Today it is assumed that the majority of the asteroids over 200 meters in size are such cosmic rubble heaps.


In contrast to the planets , many asteroids do not have nearly circular orbits. Apart from most of the main belt asteroids and the Cubewanos in the Kuiper belt , they mostly have very eccentric orbits, the planes of which are in many cases strongly inclined towards the ecliptic . Their relatively high eccentricities make them rail cruisers ; these are objects that pass the orbits of one or more planets during their orbit. However, the gravity of Jupiter ensures that asteroids, with a few exceptions, only move inside or outside of its orbit.

On the basis of their orbits, asteroids are also assigned to several asteroid families, which are characterized by similar values ​​of the major semi-axis, eccentricity and inclination of their orbit. The asteroids of a family presumably descend from the same body of origin. In 2015, David Nesvorný listed five main families. About 45% of all asteroids in the main belt can be assigned to such a family based on the given criteria.

Asteroids within Mars orbit

Several different groups of asteroids move within Mars' orbit, all of which, with a few exceptions, consist of objects less than five kilometers in size (but mostly much smaller). Some of these objects are Mercury and Venus cruisers, several of which only move within the earth's orbit, some can also cross them. Others, however, only move outside the earth's orbit.

The existence of the group of asteroids known as volcanoes has not yet been proven. These asteroids are said to move on orbits close to the Sun within Mercury's .

Near-Earth asteroids

Types of near-earth orbits

Asteroids whose orbits come close to the orbit of the earth are called near-earth asteroids , also NEAs (Near Earth Asteroids). Usually a perihelion smaller than 1.3 AU is used as a delimitation criterion. Because of a theoretical risk of collision with the earth, they have been systematically searched for for several years. Well-known search programs include Lincoln Near Earth Asteroid Research (LINEAR), the Catalina Sky Survey , Pan-STARRS , NEAT and LONEOS .

  • Cupid type: The objects of this type of asteroid cross the orbit of Mars towards Earth . However, they do not cross the earth's orbit. One representative is the (433) Eros , discovered in 1898 , which approaches the earth's orbit up to 0.15  AU . In the years 1900 and 1931, Eros passed close to the earth to precisely measure the solar system. The namesake of the group, who discovered Amor in 1932 (1221) , has a typical orbit of 1.08 to 2.76  AU . The largest representative of this group is the asteroid (1036) Ganymede with a diameter of 38 kilometers . All Cupid-type asteroids have their perihelion relatively close to the Earth, but their aphelion can be both within the orbit of Mars and far outside of the orbit of Jupiter.
  • Apohele-Type : These objects belong to a subgroup of the Aten-type, the aphelion of which lies within the earth's orbit and therefore does not cross it (Aten asteroids typically have their aphelion outside the earth's orbit).
  • Earth Orbit Cruisers : These are objects whose orbit crosses that of Earth, which implies the likelihood of a collision.
    • Apollo-type: Asteroids of this type have a half-axis orbit larger than one AU , with some of their members having very eccentric orbits that can cross Earth's orbit. Some can even get inside Venus' orbit in the perihelion passage . The group is named after the Apollo, discovered by K. Reinmuth in 1932 (1862) with an orbit from 0.65 to 2.29  AU . Discovered in 1937 (69230) Hermes passed the earth at only 1½ times the lunar distance and was then considered lost until it was finally found again in 2003. The largest Apollo asteroid is (1866) Sisyphus .
    • Aten-Type: These are near-Earth asteroids whose orbital half-axis typically has a length of less than one AU. However, their aphelion is in all cases outside the earth's orbit. Therefore, aten asteroids with eccentric orbits can cross the earth's orbit from within. The group was named after Aten, discovered in 1976 (2062) . Other representatives of the group are (99942) Apophis , (2340) Hathor and (3753) Cruithne .
    • Arjuna asteroids : Objects in this group have an Earth-like orbit. This group mostly includes asteroids of the Apollo, Amor or Aten groups.

Asteroids between Mars and Jupiter

The asteroid belt
Length of the orbit half-axes of the asteroids between Mars and Jupiter against their orbital inclination (red: main belt objects, blue: other asteroid groups); Clearly visible: the Kirkwood gaps, the Hildas at 4 AU and the Trojans at around 5.2 AU.

About 90 percent of known asteroids move between the orbits of Mars and Jupiter . You fill the gap in the Titius-Bode series . The largest objects here are (1) Ceres , (2) Pallas , (4) Vesta and (10) Hygiea .

Main Belt asteroids

Most of the objects whose orbital half-axes lie between the orbits of Mars and Jupiter are part of the main asteroid belt. They have an inclination of less than 20 ° and eccentricities of less than 0.25. Most of them were created by collisions of larger asteroids in this zone and therefore form groups with similar chemical compositions. Their orbits are limited by the so-called Kirkwood gaps , which are caused by orbital resonances to Jupiter. This allows the main belt to be divided into three zones:

  • Inner main belt: This zone is limited by the 4: 1 and 3: 1 resonance, lies between approximately 2.06 and 2.5 AU and mostly contains V- and S-class asteroids rich in silica.
  • Middle main belt: Objects in this group have orbital axes between 2.5 and 2.8 AU. C-type asteroids dominate there. The dwarf planet Ceres also moves in this zone, which lies between the 3: 1 resonance (Hestia gap) and the 5: 2 resonance.
  • Outer main belt: This area is bounded to the outside by the Hecuba gap (2: 1 resonance) at around 3.3 AU. Class D and P objects often appear in this area.

Asteroids outside the main belt

Outside the asteroid belt, there are isolated smaller groups of asteroids whose orbits are mostly in resonance with Jupiter's orbit and are thereby stabilized. There are also other groups that have similar lengths of the orbital half-axes as the main belt asteroids, but have significantly more inclined orbits (sometimes over 25 °) or other unusual orbital elements:

  • Hungaria group: This group has orbital half axes from 1.7 to 2 AU and is in 9: 2 resonance to Jupiter. With an average eccentricity of 0.08, they have almost circular orbits, but these are very stronglyinclinedtowards the ecliptic (17 ° to 27 °). The namesake for the Hungaria group is the asteroid (434) Hungaria.
  • Phocaea group: Objects with a mean orbit radius between 2.25 and 2.5 AU, eccentricities of more than 0.1 and inclinations between 18 ° and 32 °.
  • Alinda type: This group moves in 3: 1 resonance to Jupiter and in 1: 4 resonance to Earth with orbital half-axes around 2.5 AU. The orbits of these objects are disrupted by the resonance to Jupiter, which clears this area of ​​asteroids (this is where the Hestia Gap is). As a result, the eccentricities of these objects are constantly increased until the resonance is resolved when approaching one of the inner planets. Some Alinda asteroids have their perihelion near or within the Earth's orbit. A representative of this group is the asteroid (4179) Toutatis .
  • Pallas family: A group of B-class asteroids with semi-axes of 2.7 to 2.8 AU and relatively high orbital inclinations of over 30 °. The family is made up of fragments that were thrown out of Pallas in clashes.
  • Cybele Group: Objects in this group move beyond the Hecuba Gap outside the main belt at distances between 3.27 and 3.7 AU and group around the 7: 4 resonance to Jupiter. They have eccentricities of less than 0.3 and orbital inclinations of less than 25 °.
  • Hilda group: The Hildas move in a 3: 2 orbital resonance with the planet Jupiter. What they have in common is a mean distance from the sun between 3.7 and 4.2 AU, an eccentricity of less than 0.3 and an inclination less than 20 °.

Asteroids outside Jupiter's orbit

  • Centaurs: A group of asteroids known as centaurs move on eccentric orbits between the planets Jupiter and Neptune . The first representative discovered was (2060) Chiron . The centaurs probably come from the Kuiper belt and have been diverted onto unstable orbits by gravitational disturbances.

  • Damocloids: A group of objects named after the asteroid (5335) Damocles . They usually have their aphelion beyond the Uranus orbit, but perihelion in the inner solar system. Their comet-like orbits are very eccentric and strongly inclined towards the ecliptic. Their circulation is declining in some cases . The known objects are around eight kilometers in size and resemble comet nuclei, but have neither a halo nor a tail.

Trans-Neptunian objects, Kuiper belt objects

Orbits of the Trans-Neptunian Objects. (blue: Cubewanos, green: resonant KBOs, black: SDOs)

In the outer solar system, beyond the orbit of Neptune , the move trans-Neptunian objects, most of which as part of the Kuiper belt are considered (Kuiper belt objects; KBO). The largest asteroids or planetoids to date have been discovered there. The objects in this zone can be divided into three groups based on their path properties:

  • Resonant KBOs: The orbits of these objects are in resonance with Neptune. The most famous representatives are the Plutinos , to which the largest known dwarf planet (134340) Pluto and also (90482) Orcus belong.
  • Cubewanos : These objects move in almost circular orbits with inclinations below 30 ° at a distance between 42 and 50 AU around the sun. Well-known representatives are (20,000) Varuna and (50,000) Quaoar as well as the namesake of the group (15760) QB 1 .
  • Scattered KBOs : Celestial bodies in this group have very eccentric orbits, the aphelion of which can be more than 25,000 AU away, while the perihelion is usually 35 AU. Part of this group is the most massive known dwarf planet (136199) Eris .

Asteroids that move on planetary orbits

Asteroids that are located in the Lagrangian points of the planets are called " Trojans ". These companions were first discovered near Jupiter . They move on the orbit of Jupiter in front of or behind the planet. Jupiter Trojans are for example (588) Achilles and (1172) Aeneas . In 1990 the first Mars Trojan was discovered and named (5261) Eureka . In the period that followed, other Mars Trojans were discovered. Even Neptune has Trojan and 2011, with 2011 QF 99 discovered the first Trojan Uranus.

Some asteroids move in horseshoe orbit on a planetary orbit, such as asteroid 2002 AA 29 near Earth.

Interstellar asteroid

In October 2017 the first interstellar traveling asteroid was discovered with 1I / ʻOumuamua . It is elongated, around 400 meters long and approached the orbital plane of the planets at approximately right angles . After its orbit was deflected by around 90 ° by the gravitation of the sun, it flew past Earth on its new course in the direction of the constellation Pegasus at a distance of around 24 million kilometers on October 14, 2017.

Single objects

In the solar system, some asteroids move that have characteristics that they do not share with any other object. These include (944) Hidalgo , who moves on a highly eccentric, comet-like orbit between Saturn and the main belt, and (279) Thule , who is the only representative of a potential group of asteroids to join Jupiter in 4: 3 resonance 4.3 AU moved around the sun. Another object is (90377) Sedna , a relatively large asteroid that has an eccentric orbit far outside the Kuiper Belt, up to 900 AU from the Sun. In the meantime, however, at least five other objects with similar orbit characteristics as Sedna have been discovered; they form the new group of Sednoids .

Some characteristics such as their shape can be calculated from their light curve .

Orientation of the web rotation

Planets, asteroids, and comets typically all orbit the sun in the same direction.

A first asteroid was discovered in 2014, numbered in 2015 and named in 2019, namely (514107) Kaʻepaokaʻawela , which orbits in the opposite direction; in the co-orbit region of the planet Jupiter. In 2018 it was analyzed that (514107) Kaʻepaokaʻawela must have been captured from outside the solar system before the planets were formed.

Today it is known that about 100 other asteroids go "the wrong way round" around the sun.

Impact probability and impact

Asteroids that collide with much larger celestial bodies such as planets create impact craters . The size of the impact crater and the associated release of energy ( explosion ) is largely determined by the speed, size, mass and composition of the asteroid.

The trajectories of the asteroids in the solar system are not known precisely enough to be able to calculate in the long term whether and when exactly an asteroid will hit Earth (or another planet). When approaching other celestial bodies, the orbits of the asteroids are constantly subject to smaller changes. Therefore, only the risk of impacts is calculated on the basis of the known railway data and uncertainties . It changes continuously with new, more precise observations.

With the Turin scale and the Palermo scale, there are two common methods of assessing the risk of asteroid impact on Earth and the associated energy release and destructive power:

  • The Turin scale is clear and simple. It is divided into integer levels from 0 to 10, where 0 means  no danger and level 10 corresponds to a safe impact with great global destruction (→ Global Killer ). This scale is more likely to be used in the media as it is easier to understand than the Palermo scale.
  • The Palermo scale, on the other hand, is used more frequently in astronomy because it is more physically meaningful. It relates the probability of impact with the background risk from objects of comparable size. The Palermo scale has a logarithmic structure: A value of 0 on the Palermo scale corresponds to the simple background risk ( 1 = 10 0 ), 1 corresponds to ten times the risk (10 = 10 1 ), 2 corresponds to 100 times the risk (100 = 10 2 ) and so on.

The European Space Agency (ESA) publishes a continuously updated list of risks, in which asteroids and their probability of a collision with the earth are listed.

Close encounters with near-Earth asteroids

Radar image of the asteroid (29075) 1950 DA
  • On March 18, 2004, at 11:08 p.m. CET, the asteroid 2004 FH , a rock about 30 meters in diameter, passed the earth over the southern Atlantic at a distance of only 43,000 kilometers.
  • The asteroid 2004 FU 162 , which is only about six meters in size , approached Earth on March 31, 2004 to a distance of 6,535 kilometers.
  • The second closest approach was on December 19, 2004 by 2004 YD 5 (5 m diameter) at a distance of 35,000 km. Due to its small size of only a few meters, it would, like 2004 FU 162 , probably be counted among the meteoroids.
  • On January 29, 2008, at 9:33 a.m. CET, the asteroid 2007 TU 24 (250 m diameter) passed the earth at a distance of 538,000 kilometers.
  • On October 9, 2008, the around one meter large asteroid 2008 TS 26 passed the earth at a distance of only 6150 kilometers. Only one other currently known asteroid has come closer to Earth.
  • On March 2 and March 18, 2009 at 1:17 p.m. CET, the asteroids 2009 DD 45 (21–47 m diameter) and 2009 FH (13–29 m) passed the earth at a distance of only 70,000 and 80,000 respectively km. The two asteroids were discovered just the day before.
  • Only 15 hours before its closest approach to Earth did astronomers discover a seven-meter-large asteroid. On November 6, 2009, the rock passed the earth at a distance of 2 earth radii. He was tracked down by the Catalina Sky Survey . The asteroid with the designation 2009 VA thus reached the third closest approximation of all previously known and cataloged asteroids that did not hit the earth.
  • On January 13, 2010 at 1:46 p.m. CET, the asteroid 2010 AL 30 (10–15 m diameter) passed the earth at a distance of 130,000 kilometers. It was discovered by scientists at MIT on January 10, 2010 .
  • On September 8, 2010, two asteroids passed the earth: at 11:51 a.m. CET the asteroid 2010 RX 30 (10–62 m diameter) at a distance of 250,000 kilometers and at 11:12 p.m. CET the asteroid 2010 RF 12 (7–16 m diameter) at a distance of 80,000 kilometers. Both were discovered on September 5, 2010.
  • On November 9, 2011, the 400 m large asteroid (308635) 2005 YU 55 passed the earth at a distance of 324,600 km - i.e. within the lunar orbit.
  • On January 27, 2012, the 11 m asteroid 2012 BX 34 passed Earth at a distance of less than 60,000 km.
  • On February 15, 2013, the approx. 45 m large asteroid (367943) Duende passed the earth at a distance of almost 28,000 km, i.e. still below the orbit of the geostationary satellites.
  • On August 29, 2016, the asteroid 2016 QA2 with a diameter of about 34 m passed the earth at a distance of about 84,000 km. The asteroid was only discovered a few hours earlier.
  • On July 26, 2019, the asteroid 2019 OK with a diameter of about 100 m passed the earth at a distance of about 65,000 km. The asteroid was discovered only 12 hours earlier by the SONEAR observatory in Brazil.
  • On August 16, 2020, the asteroid 2020 QG passed Earth over the Indian Ocean at an altitude of just 3000 km. This is the very next flyby ever observed. With its approx. 3–6 m diameter, it would probably have burned up in the atmosphere if it were closer.
  • On April 13th 2029 the 270 m large asteroid (99942) Apophis will pass the earth. According to previous calculations, only about three times the diameter of the earth (about 30,000 kilometers) will lie between the earth and the asteroid. Such an event only occurs every 1,300 years, according to the University of Michigan. The probability of a collision of the earth with Apophis is at 0.023 percent from the current perspective (as of July 11, 2019) quite unlikely.
  • The asteroid (29075) 1950 DA (2 km diameter) will come very close to Earth on March 16, 2880, with the possibility of a collision. The probability of this is 0.33 percent.
  • The highest probability of a collision with the earth is currently (as of July 17, 2019) assigned to the asteroid 2010 RF 12 (8 m diameter). It will hit Earth on September 5, 2095 with a probability of 6.25 percent.

Examples of impacts on the earth

A listing of terrestrial craters can be found in the list of impact craters on earth and as a selection under Large and known impact craters .

Suspected collisions between asteroids

Science names several possible collisions between asteroids:

  • 470 million years ago (Ekaterina Korochantseva, 2007)
  • 5.8 million years ago (David Nesvorny, 2002)
  • P / 2010 A2 , 2009
  • (596) Scheila , 2010 (Dennis Bodewits, 2011)

International Asteroid Day

In 2001 the UN Committee on the Peaceful Uses of Outer Space (COPUOS) established the Action Team on Near-Earth Objects (Action Team 14). The establishment of an international asteroid warning network (IAWN) and a space mission planning advisory group (SMPAG) were recommended in 2013 . Action Team 14 fulfilled its mandate and was dissolved in 2015. On June 30, 2015, the first Asteroid Day was proclaimed.

See also


  • Comets and asteroids. (= Stars and Space. Special No. 2003/2). Spectrum of Science Verlag, Heidelberg 2003, ISBN 3-936278-36-9 .
  • William Bottke, Alberto Cellino, Paolo Paolicchi, Richard P. Binzel (Eds.): Asteroids III. (= Space Science Series ). Univ. of Arizona Press, 2002, ISBN 0-8165-2281-2 . (English)
  • Gottfried Gerstbach: The asteroids - drama and rubble in the planetary system. In: Star Messenger . Year 45/12, Vienna 2002, pp. 223-234, ( online , PDF, accessed on October 29, 2011)
  • Thorsten Dambeck: Vagabonds in the solar system. In: Image of Science . March 2008, pp. 56-61, ISSN  0006-2375
  • John S. Lewis: Mining the sky-untold riches from the asteroids, comets, and planets. Addison-Wesley, Mass. 1997, ISBN 0-201-32819-4 .
  • Thomas K. Henning: Astromineralogy. Springer, Berlin 2003, ISBN 3-540-44323-1 .
  • Thomas H. Burbine: Asteroids - Astronomical and Geological Bodies. Cambridge University Press, Cambridge 2016, ISBN 978-1-107-09684-4 .

Web links

Commons : Asteroids  - Collection of images, videos and audio files
Wiktionary: Asteroid  - explanations of meanings, word origins, synonyms, translations


Individual evidence

  1. ^ Wilhelm Pape: Concise dictionary of the Greek language. 3. Edition. 6. Imprint. Braunschweig 1914, p. 375. (
  2. ^ S. Clifford, J. Cunningham: Discovery of the origin of the word asteroid and the related terms asteroidal, planetoid, planetkin, planetule, and cometoid. In: Studia Etymologica Cracoviensia. Volume 20, 2015, pp. 47-62.
  3. Thomas H. Burbine: Asteroids - Astronomical and Geological Bodies. Cambridge University Press, Cambridge 2016, ISBN 978-1-107-09684-4 , p. Xiii, Preface, (
  4. a b NASA / JPL / SSD: How Many Solar System Bodies
  5. ^ Minor Planet Center Archive Statistics
  6. ^ Günther Glatzel: Hayabusa with asteroid dust at, Nov. 18, 2010.
  7. ^ David J. Tholen: Taxonomic Classifications of Asteroids , March 20, 1988.
  8. Jan Hattenbach: Asteroids also belong to families. In: Stars and Space. December 2018, p. 22. (Abstract)
  9. Josef Durech et al: Shape models of asteroids based on lightcurve observations with BlueEye600 robotic observatory. 2017, arXiv : 1707.03637v1
  10. The thing from another world., July 16, 2018. , accessed on July 16, 2018
  11. An interstellar origin for Jupiter's retrograde co-orbital asteroid. MNRASL,, , May 21, 2018, accessed July 16, 2018
  12. a b c d Risk Page. European Space Agency , accessed on July 17, 2019 (English, list of asteroids and their probability of colliding with the earth).
  13. a b Flicked past the earth at, Nov. 18, 2010.
  14. NASA: Two Small Asteroids to Pass Close by Earth on September 8, 2010
  15. Asteroid Yu55 on SPON
  16. Asteroid the size of a bus races just past Earth
  17. 2012 BX34, accessed January 29, 2012.
  18. Asteroid comes closer to earth than satellites , SPON of March 7, 2012.
  19. editors: 2016 QA2 raced towards the world: near disaster: asteroid discovered too late! (No longer available online.) Archived from the original on August 31, 2016 ; accessed on September 1, 2016 .
  20. Nadja Podbregar: How do you miss an asteroid? Retrieved August 6, 2019 .
  21. NASA: Tiny Asteroid Buzzes by Earth - the Closest Flyby on Record. NASA, August 16, 2020, accessed August 19, 2020 .
  22. JPL Small-Body Database Browser: 29075 (1950 DA). Jet Propulsion Laboratory , accessed July 17, 2019 .
  23. Mega-collision 470 million years ago University of Heidelberg January 19, 2007.
  24. Collision only 5.8 million years ago Rainer Kayser June 13, 2002.
  25. The wreckage of an asteroid collision Stefan Deiters May 2, 2011.
  26. ^ Action Team 14 on Near-Earth Objects: mission completed, accessed on February 17, 2017.
  27. Day of the asteroids, What to do about impacts., June 30, 2015.
This version was added to the list of articles worth reading on August 27, 2005 .