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An asteroid is a heavenly object whose size varies few tens of meters to several kilometers in diameter and who turns around the Sun. Arbitrarily, the objects of less than 50 m in diameter are called météoroïdes. The asteroids form part of our solar system and are not the satellites of a planet. It is supposed that the asteroids are remainders of the disc protoplanétaire which did not gather out of planets during its formation.
Observation with the naked?il of the asteroids
editThough one now succeeded in identifying tens of thousands of them, the asteroids remain almost impossible to observe with the naked?il. They are well too small comparatively with planets and thus far from luminous. The asteroid 4 Vesta is the exception, it is the only one which it is sometimes possible to observe without optical apparatus. Its luminosity not being however very large, it should be known where to pose the glance!
An asteroid resembles a star which shines in the night sky. The best means to leave to hunting to the asteroids with its binoculars or its telescope is to observe the spangled bottom several nights of sharp and to detect the luminous points which move vis-a-vis at the stable bottom. Certain catalogues index the position of the asteroids and it is then easier to point the telescope at the good place. Then good luck!
The discovery of the first asteroids
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The study of the asteroids was forsaken a long time by the astronomers. We know them since now more than two hundred years, but they were regarded as the rejects of the solar system. It is known now that the asteroids are an important key of the comprehension of the formation of the solar system and for this reason the astronomers show a greater interest towards these objects.
The first asteroid was discovered completely by chance by Giuseppe Piazzi, director, at the time, of the observatory of Palermo, in Sicily. The day before of the New Year's Day 1801, this last observed the constellation of the Bull, when it saw an object not identified moving very slowly on the spangled bottom. It followed the displacement of this object during several nights. His/her colleague, Carl Friedrich Gauss, used these observations to determine the exact distance from this unknown object to the Earth. Its calculations placed the star between the planets Mars and Jupiter. Piazzi named it Cérès, of the name of the Greek goddess who makes leave the sap the ground and which makes push the young growths in spring.
All that was very surprising bus before, in 1766, the physicist, astronomer and Prussian biologist Johann Daniel Titius had predicted the existence of a planet on this orbit under the terms of the law of Titius-Bode.
Between 1802 and 1807, three other bodies were discovered: Pallas, Junon and Vesta. Then research was abandoned until the discovery of Astrée by Karl L Hencke in 1845. In July 1868, 100 asteroids were known. The 1000E discovered approved took place in November 1921 (969 Leocadia) and the 10 000E in October 1989 ((21030) 1989 TZ11). In July 2004, there were 85 117 approved asteroids. In general, the order of the dates of discovered does not correspond to the order of classification of the asteroids, because the granting of a number depends on the establishment of a reliable orbit.
Modern methods of detection of the asteroids
editUntil 1998, the asteroids were discovered using a process in four stages. First of all, an area of the sky was photographed using a telescope with broad field. Pairs of photographs were taken with regular intervals - typically an hour - and this, over one duration of several days. Secondly, two films of the same area are observed in a stereoscope. Any body orbits about it around The Sun will then have moved slightly. In the stereoscope, the image of this body will then seem floating slightly on the bottom of stars. Thirdly, once that a moving object was identified, its position was measured precisely by using a microscope, the position being measured relative with that of a known star.
These the first three stages do not constitute a discovery of an asteroid: the observer found only one appearance. The final stage of the discovery was to send the position and the hour of discovered to Brian G Marsden of Minor Planet Center which, using data-processing programs, calculates if this appearance is connected to other appearances on the same orbit. If it is the case, the observer of the final appearance is declared the discoverer and obtains the honor to name the asteroid. The name suggested must nevertheless be approved by the international astronomical Union.
The appearance receives a designation, made up of the year of discovery, a code of two letters representing the week of discovery, and of a number so more than the one discovered one took place in this week (example: 1998 FJ74). When the orbit of an asteroid is confirmed, it receives a permanent number (example: (26308) 1998 SM165), then, later, a name (example: 1 Cérès). The first asteroids are named according to characters of mythology gréco-Roman, but as these names quickly became exhausted, others were then used: famous names of characters or the wives of the discoverer or even of the characters of televised series and the favorite desserts. These last years, the rate/rhythm of discovered asteroid is such as the asteroids without names are in a majority. Some groups of asteroids have names having a common topic. For example, the Centaurs are named according to the Centaurs of mythology and Troyens are named according to the heroes of the Trojan War. In July 2004, out of 85 117 asteroids, the named last was 78433 Gertrudolf, and the first asteroid without name was (3360) 1981 GOES.
Since 1998, the majority of the asteroids are discovered using automated systems which include/understand cameras CCC and computers connected directly to the telescopes. Here some of the teams using such systems:
- the team of Lincoln Near-Earth Asteroid Research (LINEAR),
- the team of Near-Earth Asteroid Alignment (NEAT),
- Spacewatch,
- the team of Lowell Observatory Near-Earth Object Search,
- Catalina Sky Survey,
- the Japanese association of space monitoring,
- Asiago DLR Asteroid Survey.
System LINEAR had discovered alone more than 37 000 asteroids in February 2004.
Exploration of the asteroids
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The first images brought closer an asteroid are the?uvre Galileo probe sent towards 951 Gaspra and 243 Ida in 1991.
Probe NEAR Shoemaker (Near Earth Asteroid Rendezvous - Shoemaker) was launched on February 17, 1996 by NASA with an aim of studying in details one of the largest asteroids géocroiseurs: 433 Eros.
After having established a complete cartography of the surface of 433 Eros between April and October 2000, and although that was not envisaged at the beginning of its mission, probe NEAR Shoemaker was finally posed on the asteroid on February 12, 2001 without suffering damage. The last signal of the probe was received on February 28, 2001.
Principal groupings
editThe principal belt
editThe belt known as principal (Asteroid belt?), between the orbits of Mars and Jupiter, distant 2 to 4 astronomical units of the Sun, is the principal grouping. The influence of the gravitational Jupiter field prevented from forming a planet. This influence of Jupiter is also at the origin of the gaps of Kirkwood which are orbits emptied by the phenomenon of orbital resonance.
Troyens
editThe Trojan asteroids form the second groups most important.
They are located on the orbit of another planet, at the two points of Lagrange, L4 and L5.
The near total of Troyens are on the Jupiter orbit although any planet can, in theory, to have some (scientists calculations state however that Troyens saturniens are not stable because of the influence of Jupiter). Only two Troyens non-joviens are known: 5261 Eureka, Trojan of Mars, and 2001 QR322, Trojan of Neptune.
The asteroids géocroiseurs
editThe asteroids géocroiseurs are asteroids to which the orbit is relatively close to that of the Earth. Amors, of which 433 Eros forms part, Atens and Apollos is the principal groups.
Only Atens and Apollos cross the orbit of the Earth and the growing interest that one them door is related to fear to see them entering in collision with this one. These cruisers are called ECA (Earth-Crossing Asteroids in English).
The European space agency (ESA) started in 2004 a long-term plan of protection of the Earth against the géocroiseurs. See public Services in the world.
The belt of Kuiper
editThe objects of the belt of Kuiper contain more ice, and are thus not to be strictly accurate asteroids. This belt is the source of about half of the comets which furrow the c?ur solar system. The first member discovered is (15760) 1992 QB1 in 1992; one counts some today a little more than 1000. English calls the asteroids of this type of the "cubewanos". Some of its members are hardly smaller than Pluton or its Charon moon. Largest identified until today Quaoar is 50000 which reaches 1280 km in diameter, that is to say more half of the diameter of Pluto which could lose its planet statute and be attached of it to this class of objects.
Centaurs
editThe Centaurs are a group of asteroids which sail around the Sun between the orbits of giant planets. The first which was discovered is 2060 Chiron, in 1977. It is generally supposed that they are asteroids or comets which were ejected their own orbits.
Denomination
editMinor Planet Center is in charge of the management of the designation of the asteroids. When the orbit of an asteroid is confirmed, one allots a number to him, then sometimes a name. The first received the names of characters of Greek or Roman mythology, then following their exhaustion, one used others of them, like those of famous people, of the discoverers, their women... Troyens are named according to the heroes of the Trojan War and the Centaurs according to the centaurs.
Classification
editThe asteroids are classified according to their optical spectrum, which corresponds to the composition of their surface. It should be noted, however, that certain types are more easily detectable than others. Thus, it is not because the proportion of asteroids of a given type is more important than they are indeed more numerous.
type C
- 75% of the known asteroids are of this type. "C" means carbonaceous. These asteroids are very dark (coefficient ofalbedo around 0,03) and similar with the meteorites carbonaceous chondrites. Their chemical composition is close to that of the Sun, except forvolatilehydrogen, helium and other gases. Their spectrum is rather blue and flat.
type S
- 17% of the asteroids are of type S, the S corresponding to silica. They are rather brilliant (albedo 0,10-0,22). They are rich in metal (iron, nickel and magnesium mainly). Their spectrum is located towards the red, similar with that of the meteorites siderolites.
type M
- This class includes the majority of the remainder of the asteroids. M means metal. They are made of alloy iron-nickel and brilliances (albedo 0,10-0,18).
There is a certain number of the rarer types, a number which increases with the liking of the new discoveries:
- type E, for enstatite,
- type R, for red,
- type V, for 4 Vesta (it is supposed that they are fragments of this larger representative).
See also spectral classification of the asteroids.
Notable asteroids
edit- This data was previously in a table. It will need to be reformatted
Number Name Diameter (km) Go back to discovery comment 243 Ida 56 X 24 X 21 September 29, 1884 Visited by Galileo S/1993 (243) 1 Dactyl 1.4 August 28, 1993 The moon ofIda 253 Mathilde 66 X 48 X 46 November 12, 1885 Visité par NEAR Shoemaker 433 Éros 13 x 13 x 33 13 août 1898 Visité par NEAR Shoemaker 624 Hektor 105 10 février 1907 Plus grand astéroïde troyen Jovien découvert 951 Gaspra 19 x 12 x 11 30 juillet 1916 Visité par Galileo 2060 Chiron 200 18 octobre 1977 1er Centaure découvert 3753 Cruithne 5 10 octobre 1986 Orbite particulière co-orbitale avec la Terre 4179 Toutatis 4.5 x 2.4 x 1.9 4 janvier 1989 Croisera de près la Terre en 2004 4769 Castalia 1.8 x 0.8 9 août 1989 Premier astéroïde dont on a eu une image 5261 Eurêka 20 juin 1990 Premier astéroïde Troyen martien découvert (point L5)