A.E. van Vogt
A asteroid is a Celestial object whose size varies few tens of meters to several kilometers in diameter and who, unlike a Comet, turns around the Sun on an orbit slightly elliptic (cf Lois of Kepler). The asteroids belong to our Solar system and are not the satellite S of a Planet.
It is supposed that the asteroids are remainders of the Disque protoplanétaire which did not gather out of planets during its formation.
Observation with the naked eye
Though one now succeeded in identifying, at January 6th, 2007, 362 447 asteroids, those are almost impossible to observe with the naked eye. They are much smaller than planets and far from luminous. The asteroid 4 Vesta is the exception since 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 more or less 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 compared to the bottom, which, him, appears stable. Certain catalogs index the position of the asteroids and it is then easier to point the telescope at the good place.
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 New Year's Day day before 1801, this last observed the constellation of the Taureau, when he 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 Roman goddess which makes leave the sap the ground and which makes push the starts-up in spring, and which was also the protective goddess of Sicily.
All that was very surprising because 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 Loi of Titius-Bode.
Between 1802 and 1807, three other objects were discovered: Pallas, Junon and Vesta. These four bodies were then regarded as planets with whole share. Then research was abandoned until the discovery of Astrée by Karl Ludwig Hencke in 1845. In July 1868, hundred asteroids were known. The thousandths discovered approved took place in November 1921 (969 Leocadia) and it ten-thousandths in October 1989 ((21030) 1989 TZ11). 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
Until 1998, the asteroids were discovered using a process in four stages:
- First of all, an area of the sky was photographed using a Télescope 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 Stéréoscope. 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 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.
- : Note: these the first three stages do not constitute a discovery of an asteroid: the observer found only a appearance .
- the final stage of the discovery was to send the position and the hour of discovered to Brian G. Marsden of the Minor Planet Center which, using computer 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.
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),
- the team of Lowell Observatory Near-Earth Object Search (LONEOS),
- Catalina Sky Survey,
- the Japanese association of space monitoring,
- Asiago DLR Asteroid Survey.
System LINEAR had discovered alone more 37 000 asteroids in February 2004.
See also: Designation of the asteroids
The Minor Planet Center is in charge of the management of the Désignation of the asteroids. When the orbit of an asteroid is confirmed, 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). It receives also a permanent number (example: (26308) 1998 SM165), then sometimes, and later, a name (example: 1 Cérès). The first received the names of characters of the Greek Mythologie or Roman, then following their exhaustion, one used others of them, like the famous names of characters or of the wives of the discoverer or even of characters of televised series and favorite desserts. These last years, the rate/rhythm of discovered asteroid is such as the asteroids without names are majority. Some groups of asteroids have names having a common topic. For example, the Centaures are named according to the Centaure S of mythology and the Troyens are named according to the heroes of the Trojan War. In March 2006, on 129 436 asteroids, the named last was (99942) Apophis, and the first asteroid without name was (3360) 1981 GOES.
Exploration of the asteroids
The first images close to an asteroid are the work of the Sonde Galileo sent towards 951 Gaspra and 243 Ida in 1991.
The probe NEAR Shoemaker ( Near Earth Asteroid Rendezvous - Shoemaker ) was launched on February 17th, 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, the probe NEAR Shoemaker were finally posed on the asteroid on February 12th, 2001 without suffering damage. The last signal of the probe was received on February 28th, 2001.
The girdles known as '' principal '' or “jovio-Martian”, between the orbits of Mars and Jupiter, distant from two to four astronomical units of the Sun, is the principal grouping.: more 20 000 objects were indexed there to date. The influence of the gravitational Jupiter field prevented them 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 Résonance.
The Trojan asteroids form the second groups most important (2075 at November 2nd, 2006).
They are located on the Orbite of another Planet, to both points of Lagrange, L4 and L5.
The near total of Troyens are on the orbit of Jupiter. Only two another planets have some Trojan, Mars: only one of confirmed and Neptune, four! The another planets do not seem to undoubtedly have some because of the influence either of the Sun or of close planets likely to disturb the points of Lagrange.
The asteroids géocroiseurs are asteroids to which the orbit is relatively close to that of the Earth. The Amor S, of which 433 Eros form part, the Aten S and the Apollo S are the independent 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 or NEO Near Earth Objects in English.
Girdle of Kuiper
With the November 2nd 2006, 1016 transneptuniens objects are counted by the Minor Planet Center , for which it is necessary to add certain scattered objects. Potentially the belt of Kuiper seems well to be the greatest concentration of small bodies of the solar system.
The first member discovered of this belt is (15760) 1992 QB1 in 1992, classified thereafter in the category of the Cubewano S or traditional objects of the belt of Kuiper. The discovery of this body drew the attention of the astronomers to the objects transneptuniens .
Some of the members of the belt of Kuiper are sizes comparable with Pluton or its the moon Charon. Largest identified until today is (136199) Éris, which reaches 2400 km in diameter, that is to say more half of the diameter of Pluto. The debates thus carry from now on on the demarcation between the large objects and planets of the solar system. Thus, in August 2006, the international astronomical Union decides to create the statute of dwarf Planet, decreed at once in Pluton which loses that of planet, with (136199) Éris, objects of the belt of Kuiper and with (1) Cérès, the largest asteroid of the principal belt. Other objects of the belt of Kuiper are candidates for this new statute.
In addition, this belt is the source of about half of the Comet S which furrow our solar system.
The Centaures are a group of asteroids which sail around the Sun between the orbits of the 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.
Asteroids or planets?The technical projections helping, since 1980, the number of objects discovered increased considerably and of the very massive bodies, of the size of Pluton, then regarded as a planet, were observed. The scientists then came from there to wonder how to differentiate a planet from a large asteroid.
Also let us recall that, according to the theory of Laplace (astronomer in 1796), the planets and the Sun would have been born simultaneously from a cloud from gas and dust in rotation. Resulting from this cloud, a multitude of planetoids would be the result of an animated history, characterized by a succession of paradoxical processes of accretion and collisions. Asteroids and planets having been formed starting from the same matrix protoplanétaire, one can wonder on which physical criteria to rest to differentiate them. The very great majority of the asteroids are very irregular forms, which contrasts with the quasi-spherical shapes of planets; however, the very large asteroids, such Cérès, are also quasi-spherical. The nature of surface does not enter either counts some in differentiation. Differentiation is done primarily by the size:
an asteroid is defined implicitly as a body not exceeding 1000 km in diameter and revolving around the Sun. This diameter corresponds roughly to that of Cérès, the largest asteroid of the principal Ceinture.
However, of new objects discovered defrayed the chronicle: (20000) Varuna, (28978) Ixion, (50000) Quaoar, (90377) Sedna, (90482) Orcus and (136199) Éris. Detected either on orbits similar to Pluton, or beyond, these objects have sizes ranging between 1300 km and 2600 km and are at the border between planets and asteroids.
In August 2006, the international astronomical Union, re-examined the concept of Planet and defined a new class of objects, the dwarf planets. Thus, Pluto, Éris and Cérès was classified in the category “dwarf planet”, although Cérès continues to be also regarded as an asteroid.
The 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:
- This category gathers 75% of the known asteroids, of carbonaceous type. These asteroids are very dark (coefficient of Albédo around 0,03) and similar to the Météorite S carbonaceous Chondrite S. Their chemical composition is close to that of the Sun, except for the Hydrogène, the Hélium and other gases volatile. Their spectrum is rather blue and flat.
; Type S:
- This category gathers 17% of the asteroids, of silicic type. They are rather brilliant (albedo 0,10 to 0,22). They are rich in Métal (Fer, Nickel and Magnésium mainly). Their spectrum is located towards the red, similar to that of the Météorite S Sidérolithe S.
; Type M:
- This category includes the majority of the other asteroids, of metal type. They are made of alloy iron-nickel and brilliances (albedo 0,10 to 0,18).
There is a certain number of the rarer types, number which increases with the liking of the new discoveries:
- standard E, for enstatite,
- standard R, for red,
- standard V, for 4 Vesta (it is supposed that they are fragments of this larger representative).
See also spectral Classification of the asteroids.
Risks of impacts with the EarthWhen an asteroid or a fragment of asteroid penetrates in the atmosphere of the Earth, friction with the latter cause its combustion. If the object is rather bulky, this combustion is not complete and it strikes the surface of the Earth then. It is named then meteorite.
Risks of impacts according to the size
- Size of the body: < 10 m:
Size of the body: 10 to 100 m:
Size of the body: 100 m with 1 km:
Size of the body: > 5 km:
Notes and references of the article
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