13 Nov 2013

Asteroid



Asteroids are minor planets, especially those of the inner Solar System. The larger ones have also been called planetoids. These terms have historically been applied to any astronomical object orbiting the Sun that did not show the disk of a planet and was not observed to have the characteristics of an active comet, but as minor planets in the outer Solar System were discovered, their volatile-based surfaces were found to more closely resemble comets and so were often distinguished from traditional asteroids. Thus the term asteroid has come increasingly to refer specifically to the small bodies of the inner Solar System out to the orbit of Jupiter. They are grouped with the outer bodies—centaurs, Neptune trojans, and trans-Neptunian objects—as minor planets, which is the term preferred in astronomical circles. In this article the term "asteroid" refers to the minor planets of the inner Solar System.
There are 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. The large majority of known asteroids orbit in the asteroid belt between the orbits of Mars and Jupiter or co-orbital with Jupiter (the Jupiter Trojans). However, other orbital families exist with significant populations, including the near-Earth asteroids. Individual asteroids are classified by their characteristic spectra, with the majority falling into three main groups: C-type, S-type, and M-type. These were named after and are generally identified with carbon-rich, stony, and metallic compositions, respectively.
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. As of September 2013, the Minor Planet Center had data on more than one million objects, of which 625,106 had enough information to be given numbered designations.

Naming


A newly discovered asteroid is given 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 common to drop the number altogether, or to drop it after the first mention when a name is repeated in running text.

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.

AsteroidSymbolYear
1 Ceres Old planetary symbol of Ceres Variant symbol of Ceres Other sickle variant symbol of CeresCeres' scythe, reversed to double as the letter C1801
2 Pallas Old symbol of Pallas Variant symbol of PallasAthena's (Pallas') spear1801
3 Juno Old symbol of Juno Other symbol of Juno Symbol 3.jpgA star mounted on a scepter,
for Juno, the Queen of Heaven
1804
4 Vesta Modern astrological symbol of Vesta Old symbol of Vesta Old planetary symbol of Vesta 4 Vesta Unsimplified Symbol.svgThe altar and sacred fire of Vesta1807
5 Astraea5 Astraea symbol alternate.svg 5 Astraea Symbol.svgA scale, or an inverted anchor, symbols of justice1845
6 Hebe6 Hebe Astronomical Symbol.svgHebe's cup1847
7 Iris7 Iris Astronomical Symbol.svgA rainbow (iris) and a star1847
8 Flora8 Flora Astronomical Symbol.svgA flower (flora) (specifically the Rose of England)1847
9 Metis9 Metis symbol.svgThe eye of wisdom and a star1848
10 Hygiea10 Hygeia symbol alternate.svg 10 Hygiea Astronomical Symbol.svgHygiea's serpent and a star, or the Rod of Asclepius1849
11 Parthenope11 Parthenope symbol alternate.svg 11 Parthenope symbol.svgA harp, or a fish and a star; symbols of the sirens1850
12 Victoria12 Victoria symbol.svgThe laurels of victory and a star1850
13 EgeriaAstronomical symbol of 13 EgeriaA shield, symbol of Egeria's protection, and a star1850
14 IreneAstronomical symbol of 14 IreneA dove carrying an olive branch (symbol of
irene 'peace') with a star on its head, or
an olive branch, a flag of truce, and a star
1851
15 Eunomia15 Eunomia symbol.svgA heart, symbol of good order
(eunomia), and a star
1851
16 Psyche16 Psyche symbol.svgA butterfly's wing, symbol of
the soul (psyche), and a star
1852
17 Thetis17 Thetis symbol.pngA dolphin, symbol of Thetis, and a star1852
18 Melpomene18 Melpomene symbol.svgThe dagger of Melpomene, and a star1852
19 Fortuna19 Fortuna symbol.svgThe wheel of fortune and a star1852
26 Proserpina26 Proserpina symbol.svgProserpina's pomegranate1853
28 Bellona28 Bellona symbol.svgBellona's whip and lance1854
29 Amphitrite29 Amphitrite symbol.svgThe shell of Amphitrite and a star1854
35 Leukothea35 Leukothea symbol.pngA lighthouse beacon, symbol of Leucothea1855
37 Fides37 Fides symbol.svgThe cross of faith (fides)1855
 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 (although he assigned ① to the fifth, Astraea, while continuing to designate the first four only with their existing iconic symbols). 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 also given an iconic symbol, as were a few other asteroids discovered over the next few years (see chart above). 20 Massalia was the first asteroid that was not assigned a symbol, and no additional iconic symbols were created after the 1855 discovery of 37 Fides. That year Astraea's number was increased to ⑤, but the first four asteroids, Ceres to Vesta, were not listed by their numbers until the 1867 edition. The circle became a pair of parentheses, and the parentheses were sometimes omitted altogether over the next few decades, leading to the modern convention.

Discovery
 

The first asteroid to be discovered, Ceres, was found in 1801 by Giuseppe Piazzi, and was originally considered to be a new planet. 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", coined in Greek as ἀστεροειδής asteroeidēs 'star-like, star-shaped', from 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; for example, the Annual of Scientific Discovery for 1871, page 316, reads "Professor J. Watson has been awarded by the Paris Academy of Sciences, the astronomical prize, Lalande foundation, for the discovery of eight new asteroids in one year. The planet Lydia (No. 110), discovered by M. Borelly at the Marseilles Observatory [...] M. Borelly had previously discovered two planets bearing the numbers 91 and 99 in the system of asteroids revolving between Mars and Jupiter".

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