Asteroid
For the arcade game, see Asteroids (computer game).
An
asteroid is a predominantly rocky body that orbits around its star. In our
solar system, asteroids are the best known class of
minor planet (or
planetoid). They are much smaller than even the small major
planets such as Mercury or Mars. The vast majority of the asteroids are found within the main
asteroid belt, with
elliptical orbits between those of
Mars and
Jupiter. It is thought that asteroids are remnants of the
protoplanetary disc, and in this region the incorporation of protoplanetary remnants into the planets was prevented by large gravitational perturbations induced by
Jupiter during the formative period of the solar system. Some asteroids have
moons.
Hundreds of thousands of asteroids have been discovered within the solar system and the present rate of discovery is about 5000 per month. As of
July 23,
2006, from a total of 338,186 registered minor planets, 134,339 have orbits known well enough to be given
permanent official numbers. Of these, 13,242
have official names (trivia: about 650 of these names require
diacritics). The lowest-numbered but unnamed minor planet is
(3360) 1981 VA; the highest-numbered named minor planet is
129342 Ependes .
Current estimates put the total number of asteroids above 1 km in diameter in the solar system to be between 1.1 and 1.9 million
. The largest asteroid in the inner solar system is
1 Ceres, with a diameter of 900-1000 km. Two other large inner solar system belt asteroids are
2 Pallas and
4 Vesta; both have diameters of ~500 km. Vesta is the only main belt asteroid that is sometimes visible to the naked eye (in some very rare occasions, a near-Earth asteroid may be visible without technical aid; see
99942 Apophis).
The mass of all the asteroids of the Main Belt is estimated to be about 3.0-3.6 kg
, or about 4% of the mass of our moon. Of this,
1 Ceres comprises 950 kg, some 32% of the total. Adding in the next three most massive asteroids,
4 Vesta (9%),
2 Pallas (7%), and
10 Hygiea (3%), bring this figure up to 51%; while the three after that,
511 Davida (1.2%),
704 Interamnia (1.0%), and
3 Juno (0.9%), only add another 3% to the total mass. The number of asteroids then increases
exponentially as their individual masses decrease.
See also a
List of noteworthy asteroids in our Solar System, or a sequentially-ordered
List of asteroids.
Asteroids are commonly classified into groups based on the characteristics of their orbits and on the details of the
spectrum of sunlight they reflect.
Orbit groups and families
Many asteroids have been placed in groups and families based on their orbital characteristics. It is customary to name a group of asteroids after the first member of that group to be discovered. Groups are relatively loose dynamical associations, whereas families are much "tighter" and result from the catastrophic break-up of a large parent asteroid sometime in the past.
For a full listing of known asteroid groups and families, see
minor planet and
asteroid family.
Spectral classification
|
This picture of 433 Eros shows the view looking from one end of the asteroid across the gouge on its underside and toward the opposite end. Features as small as 35 m across can be seen. |
In 1975, an asteroid
taxonomic system based on
colour,
albedo, and
spectral shape was developed by
Clark R. Chapman,
David Morrison, and
Ben Zellner. These properties are thought to correspond to the composition of the asteroid's surface material. Originally, they classified only three types of asteroids:
*
C-type asteroids - carbonaceous, 75% of known asteroids
*
S-type asteroids - silicaceous, 17% of known asteroids
*
M-type asteroids - metallic, most of the remaining asteroids
This list has since been expanded to include a number of other asteroid types. The number of types continues to grow as more asteroids are studied. See
Asteroid spectral types for more detail or
:Category:Asteroid spectral classes for a list.
Note that the proportion of known asteroids falling into the various spectral types does not necessarily reflect the proportion of all asteroids that are of that type; some types are easier to detect than others, biasing the totals.
Problems with spectral classification
Originally, spectral designations were based on inferences of an asteroid's composition:
* C -
Carbonaceous* S -
Silicaceous* M -
MetallicHowever, the correspondence between spectral class and composition is not always very good, and there are a variety of classifications in use. This has led to significant confusion. While asteroids of different spectral classifications are likely to be composed of different materials, there are no assurances that asteroids within the same taxonomic class are composed of similar materials.
At present, the spectral classification based on several coarse resolution spectroscopic surveys in the 1990s is still the standard. Scientists have been unable to agree on a better taxonomic system, largely due to the difficulty of obtaining detailed measurements consistently for a large sample of asteroids (e.g. finer resolution spectra, or non-spectral data such as densities would be very useful).
|
243 Ida and its moon Dactyl, the first satellite of an asteroid to be discovered. |
Historical discovery methods
Asteroid discovery methods have drastically 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 as a consequence of the discovery, by Sir
William Herschel in 1781, of the planet
Uranus at the distance "predicted" by the law. 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 discernable by observers.
Ironically, the first asteroid,
1 Ceres, was not discovered by a member of the group, but rather by accident in 1801 by
Giuseppe Piazzi director, at the time, 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. His colleague,
Carl Friedrich Gauss, used these observations to determine the exact distance from this unknown object to the Earth. Gauss' calculations placed the object between the planets
Mars and
Jupiter. Piazzi named it after
Ceres, the Roman goddess of agriculture.
Three other asteroids (
2 Pallas,
3 Juno,
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.
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 year 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,
Paul Henry and Prosper Henry and
Auguste Charlois.
In 1891, however,
Max Wolf pioneered the use of
astrophotography to detect asteroids, which appeared as short streaks on long-exposure photographic plates. This drastically increased the rate of detection compared with previous visual methods: Wolf alone discovered 248 asteroids, beginning with
323 Brucia, whereas only slightly more than 300 had been discovered up to that point. Still, a century later, only a few thousand asteroids were identified, numbered and named. It was known that there were many more, but most astronomers did not bother with them, calling them "vermin of the skies".
Modern discovery methods
Until
1998, asteroids were discovered by a four-step process. First, a region of the sky was
photographed by a wide-field
telescope. Pairs of photographs were taken, typically one hour apart. Multiple pairs could be taken over a series of days. Second, the two
films 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 appear 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
.
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 week of discovery, and finally a letter and a number indicating the discovery's sequential number (example: ).
The final step of discovery is to send the locations and time of observations to
Brian Marsden of the
Minor Planet Center. Dr. Marsden has computer programs that compute whether an apparition ties together previous apparitions into a single orbit. If so, the object gets a number. The observer of the first apparition with a calculated orbit is declared the discoverer, and he gets the honour of naming the asteroid (subject to the approval of the
International Astronomical Union) once it is numbered.
Latest technology: detecting hazardous asteroids
[[Image:Asteroid 2004 FH.gif|thumb|right|{{2004 FH}} is the centre dot being followed by the sequence; the object that flashes by near the end is a meteor.]]There is increasing interest in identifying asteroids whose orbits cross
Earth's orbit, 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 the
Atens. Various
asteroid deflection strategies have been proposed.
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 the
Earth in 1937. Astronomers began to realize the possibilities of Earth impact.
Two events in later decades increased the level of alarm: the increasing acceptance of
Walter Alvarez' theory of
dinosaur extinction being due to an
impact event, 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 10 metres across.
All of these considerations helped spur the launch of highly efficient automated systems that consist of Charge-Coupled Device (
CCD) cameras and computers directly connected to telescopes. Since 1998, a large majority of the asteroids have been discovered by such automated systems. A list of teams using such automated systems includes:
* The
Lincoln Near-Earth Asteroid Research (LINEAR) team
* The
Near-Earth Asteroid Tracking (NEAT) team
*
Spacewatch* The
Lowell Observatory Near-Earth-Object Search (LONEOS) team
* The
Catalina Sky Survey (CSS)
* The
Campo Imperatore Near-Earth Objects Survey (CINEOS) team
* The
Japanese Spaceguard Association* The
Asiago-DLR Asteroid Survey (ADAS)
The LINEAR system alone has discovered 67,820 asteroids as of
June 13,
2006 . Between all of the automated systems, 4076 near-Earth asteroids have been discovered
including over 600 more than 1 km in diameter.
The naming format
Newly discovered asteroids are given a
provisional designation consisting of the year of discovery and an alphanumeric code, such as 2001 FH. When its orbit is confirmed, it is given a number, and later may also be given a name (e.g.
1 Ceres). The formal naming convention uses parentheses around the number (e.g.
(433) Eros), however, dropping the parentheses is quite common. Informally, especially when a name is repeated in running text, it is common to drop the number altogether, or to drop it after the first mention.
The
Minor Planet Circular (MPC) of
October 19,
2005 was a historical one, as it saw the highest numbered asteroid jump from 99947 to 118161, causing a small "
Y2k" like crisis for various automated data services —up until then, only five digits were allowed in most data formats for the asteroid number. Most services have now widened the asteroid number field. For those which did not, this has been addressed in some cases by having the leftmost digit (the ten-thousands place) use the alphabet as a digit extension. A=10, B=11,…, Z=35, a=36,…, z=61. A high number such as 120437 is thus cross-referenced as C0437 on some lists. Also, the fictional asteroid of
The Little Prince,
B612, now could be connected with the real which is listed as in the compacted lists —although it is already present as
46610 Bésixdouze (B612 in hexadecimal translates to 46610 in decimal notation).
Unnamed asteroids
Unnamed asteroids that have been given a number keep their provisional designation, e.g.
(29075) 1950 DA.
As modern discovery techniques have discovered vast numbers of new asteroids, they are increasingly being left unnamed. The first asteroid to be left unnamed was
(3360) 1981 VA. On rare occasions, an asteroid's
provisional designation may become used as a name in itself: the still unnamed
(15760) 1992 QB₁ gave its name to a group of asteroids which became known as
cubewanos.
Sources for names
The first few asteroids were named after figures from
Graeco-Roman mythology, but as such names started to run out, others were used —famous people, literary characters, the names of the discoverer's wives, children, and even television characters.
The first asteroid to be given a non-mythological name was
20 Massalia, named after the city of
Marseilles. For some time only female (or feminized) names were used;
Alexander von Humboldt was the first man to have an asteroid named after him, but his name was feminized to
54 Alexandra. This unspoken tradition lasted until
334 Chicago was named; even then, oddly feminised names show up in the list for years afterward.
As the number of asteroids began to run into the hundreds, and eventually the thousands, discoverers began to give them increasingly frivolous names. The first hints of this were
482 Petrina and
483 Seppina, named after the discoverer's pet dogs. However, there was little controversy about this until 1971, upon the naming of
2309 Mr. Spock (which was not even named after the
Star Trek character, but after the discoverer's cat who supposedly bore a resemblance to him). Although the
IAU subsequently banned pet names as sources, eccentric asteroid names are still being proposed and accepted, such as
6042 Cheshirecat,
9007 James Bond, or
26858 Misterrogers.
For a full list, see
meanings of asteroid names.
Special naming rules
Asteroid naming is not always a free-for-all: there are some types of asteroid for which rules have developed about the sources of names. For instance
Centaurs (asteroids orbiting between Saturn and Neptune) are all named after mythological
centaurs,
Trojans after heroes from the
Trojan War, and
trans-Neptunian objects after underworld spirits.
Another well-established rule is that comets are named after their discoverer(s), whereas asteroids are not. One way to "circumvent" this rule has been for astronomers to exchange the courtesy of naming their discoveries after each other. A particular exception to this rule is
96747 Crespodasilva, which was named after its discoverer,
Lucy d'Escoffier Crespo da Silva, because she sadly died shortly after the discovery, at age 22
.
Asteroid symbols
The first few asteroids discovered were assigned symbols like the ones traditionally used to designate Earth, the Moon, the Sun and planets. The symbols quickly became ungainly, hard to draw and recognise. By the end of 1851 there were 15 known asteroids, each (except one) with its own symbol. The first four's main variants are shown here:
1 Ceres
|
Old planetary symbol of Ceres |
|
Sickle variant symbol of Ceres |
|
Other sickle variant symbol of Ceres |
:2 Pallas
:3 Juno
:4 Vesta
|
Old planetary symbol of Vesta |
|
Modern astrological symbol of Vesta |
Johann Franz Encke made a major change in the
Berliner Astronomisches Jahrbuch (BAJ, "Berlin Astronomical Yearbook") for 1854. He introduced encircled numbers instead of symbols, although his numbering began with
Astraea, the first four asteroids continuing to be denoted by their traditional symbols. This symbolic innovation was adopted very quickly by the astronomical community. The following year (1855), Astraea's number was bumped up to 5, but Ceres through Vesta would be listed by their numbers only in the 1867 edition. A few more asteroids (
28 Bellona,
35 Leukothea, and
37 Fides) would be given symbols as well as using the numbering scheme.
The circle would become a pair of parentheses, and the parentheses sometimes omitted altogether over the next few decades.
Until the age of
space travel, asteroids were merely pinpricks of light in even the largest telescopes and their shapes and terrain remained a mystery.
The first
close-up photographs of asteroid-like objects were taken in 1971 when the
Mariner 9 probe imaged
Phobos and
Deimos, the two small moons of
Mars, which are probably captured asteroids. These images revealed the irregular, potato-like shapes of most asteroids, as did subsequent images from the
Voyager probes of the small moons of the
gas giants.
|
951 Gaspra, the first asteroid to be imaged in close up. |
The first true asteroid to be photographed in close-up was
951 Gaspra in 1991, followed in 1993 by
243 Ida and its moon
Dactyl, all of which were imaged by the
Galileo probe en route to
Jupiter.
The first dedicated asteroid probe was
NEAR Shoemaker, which photographed
253 Mathilde in 1997, before entering into orbit around
433 Eros, finally landing on its surface in 2001.
Other asteroids briefly visited by spacecraft
en route to other destinations include
9969 Braille (by
Deep Space 1 in 1999), and
5535 Annefrank (by
Stardust in 2002).
In September 2005, the Japanese
Hayabusa probe started studying
25143 Itokawa in detail and will return samples of its surface to earth. Following that, the next asteroid encounters will involve the European
Rosetta probe (launched in 2004), which will study
2867 Šteins and
21 Lutetia in 2008 and 2010.
NASA is planning to launch the
Dawn Mission in
2007, which will orbit both
1 Ceres and
4 Vesta in
2011-
2015.
It has been suggested that asteroids might be used in the future as a source of materials which may be rare or exhausted on earth (
asteroid mining).
Main article:
Asteroids in fictionA common depiction of asteroids (and less often, of
Comets) in fiction is as a threat, whose impact on Earth could result with incalculable damage and loss of life
. This has a basis in scientific hypotheses regarding such impacts in the distant past as responsible for the extinction of the
Dinosaurs and other past catastrophes —though, as they seem to occur within tens of millions of years of each other, there is no special reason (other than creating a dramatic story line) to expect a new such impact at any close millennium.
Another way in which asteroids could be considered a source of danger is by depicting them as a hazard to navigation, especially threatening to ships travelling from Earth to the outer parts of the Solar System and thus needing to pass the Asteroid Belt (or make a time- and fuel-consuming detour around it). Asteroids in this context provide to space travel stories a space equivalent of reefs and underwater rocks in the older genre of sea-faring adventures stories
. And like reefs and rocks in the ocean, asteroids as navigation hazards can also be used by bold outlaws to avoid pursuit. Representations of the Asteroid Belt in film tend to make it unrealistically cluttered with dangerous rocks. In reality asteroids, even in the main belt, are spaced extremely far apart (even so, they can still be a risk to ships travelling at high speeds).
Before colonization of the asteroids became an attractive possibility, a main interest in them was theories as to their origin - specifically, the theory that the asteroids are remnants of an exploded planet. This naturally leads to SF plotlines dealing with the possibility that the planet had been inhabited, and if so - that the inhabitants caused its destruction themselves, by war or gross environmental mismanagement. A further extension is from the past of the existing asteroids to the possible future destruction of Earth or other planets and their rendering into new asteroids.
When the theme of interplanetary colonization first entered SF, the Asteroid Belt was quite low on the list of desirable real estate, far behind such planets as
Mars and
Venus (often conceived as a kind of paradise planet, until probes in the
1960s revealed the appalling temperatures and conditions under its clouds). Thus, in many stories and books the Asteroid Belt, if not a positive hazard, is still a rarely-visited backwater in a colonized Solar System.
The prospects of colonizing the Solar System planets became more dim with increasing discoveries about conditions on them. Conversely, the potential value of the asteroids increased, as a vast accumulation of mineral wealth, accessible in conditions of minimal gravity, and supplementing Earth's dwindling resources. Stories of asteroid mining became more and more numerous since the late
1940s, with the next logical step being depictions of a society on terraformed asteroids —in some cases dug under the surface, in others having dome colonies and in still others provided with an atmosphere which is kept in place by an artificial gravity. An image developed and was carried from writer to writer, of "Belters" or "Rock Rats" as rugged and independent-minded individuals, resentful of all Authority (in some books and stories of the military and political power of Earth-bound nation states, in others of the corporate power of huge companies)
. As such, this sub-genre proved naturally attractive to writers with
Libertarian tendencies
. Moreover, depictions of the Asteroid Belt as The New Frontier clearly draw (sometimes explicitly) on the considerable literature of the Nineteenth-Century
Frontier and the
Wild West. And since (in nearly all stories) the asteroids are completely lifeless until the arrival of the humans, it is a New Frontier completely free of the moral taint of the brutal dispossession of the
Native Americans in the original.
*
Asteroid belt*
Category:Asteroid groups and families*
Category:Asteroids*
List of asteroids*
List of asteroids named after important people*
List of asteroids named after places*
List of noteworthy asteroids*
Meanings of asteroid names*
Minor planet*
Minor Planet Center*
Near-Earth object*
Pronunciation of asteroid names *
Known Asteroid Impacts & Their Effects*
Alphabetical list of minor planet names (ASCII) (Minor Planet Center)
*
Alphabetical and numerical lists of minor planet names (Unicode) (Institute of Applied Astronomy)
*
Near Earth Objects Dynamic Site*
Asteroids Dynamic Site Up-to date
osculating orbital elements and
proper orbital elements*
Asteroid naming statistics*
Near Earth Asteroid Tracking (NEAT)*
Everything you wanted to know about comets and asteroids " Provided by
New Scientist.
*
Spaceguard UK*
When Did the Asteroids Become Minor Planets?*
Large amount of information on asteroid groups collected by Gérard Faure, translation Richard Miles.
*
Asteroid Simulator with Moon and Earth(asteroid navigator) | First asteroid | ...
