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Asteroid

2007 Schools Wikipedia Selection. Related subjects: Space (Astronomy)

   253 Mathilde, a C-type asteroid.
   Enlarge
   253 Mathilde, a C-type asteroid.

   Asteroids, also called minor planets or planetoids, are a class of
   astronomical object. The term asteroid is generally used to indicate a
   diverse group of small celestial bodies that drift in the solar system
   in orbit around the Sun. Asteroid (Greek for "star-like") is the word
   used most in the English literature for minor planets, which has been
   the term preferred by the International Astronomical Union; some other
   languages prefer planetoid (Greek: "planet-like"), because it more
   accurately describes what they are. In late August 2006, the IAU
   introduced the term " small solar system bodies" (SSSBs), which
   includes most objects thusfar classified as minor planets, as well as
   comets. At the same time they introduced the term dwarf planet for the
   largest minor planets. This article deals specifically with the minor
   planets that orbit in the inner solar system (roughly up to the orbit
   of Jupiter). For other types of objects, such as comets,
   Trans-Neptunian objects, and Centaurs, see Small solar system body.

   The first asteroid to be discovered, Ceres, is the largest asteroid
   known to date and is now classified as a dwarf planet. All others are
   currently classified as small solar system bodies. The vast majority of
   asteroids are found within the main asteroid belt, with elliptical
   orbits between those of Mars and Jupiter. It is thought that these
   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 or are found in pairs known as binary systems.

Asteroids in the solar system

   Left to right: 4 Vesta, 1 Ceres, Earth's Moon.
   Enlarge
   Left to right: 4 Vesta, 1 Ceres, Earth's Moon.

   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 September 17, 2006, from a total of 341,328 registered minor
   planets, 136,563 have orbits known well enough to be given permanent
   official numbers. Of these, 13,479 have official names. The
   lowest-numbered but unnamed minor planet is (3708) 1974 FV1.; the
   highest-numbered named minor planet (other than the dwarf planet 136199
   Eris) is 135268 Haignere.
   Location of the Main Belt asteroids
   Enlarge
   Location of the Main Belt asteroids

   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
   3.0-3.6×10^21 kg, or about 4% of the mass of our moon. Of this, 1 Ceres
   comprises 0.95×10^21 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
   rapidly as their individual masses decrease.

Asteroid classification

   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.
   Enlarge
   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, 8% of known 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 - Metallic

   However, 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).

Asteroid discovery

   243 Ida and its moon Dactyl, the first satellite of an asteroid to be
   discovered.
   Enlarge
   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 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, and 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 (usually
   an Astrograph). 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: 1998 FJ[74]).

   The final step of discovery is to send the locations and time of
   observations to Brian Marsden of the Minor Planet Centre. 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

   2004 FH is the centre dot being followed by the sequence; the object
   that flashes by during the clip is a satellite.
   Enlarge
   2004 FH is the centre dot being followed by the sequence; the object
   that flashes by during the clip is a satellite.

   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 71,770 asteroids, as of November
   9, 2006. Between all of the automated systems, 4286 near-Earth
   asteroids have been discovered including over 600 more than 1 km in
   diameter.

Naming asteroids

Overview: naming conventions

   A newly discovered asteroid is given a provisional designation
   consisting of the year of discovery and an alphanumeric code (such as
   2002 AT[4]). Once its 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.

   Asteroids that have been given a number but not a name keep their
   provisional designation, e.g. (29075) 1950 DA. As modern discovery
   techniques are discovering vast numbers of new asteroids, they are
   increasingly being left unnamed. The first asteroid to be left unnamed
   was for a long time (3360) 1981 VA, now 3360 Syrinx; as of November
   2006, this distinction is now held by (3708) 1974 FV[1]. On rare
   occasions, an asteroid's provisional designation may become used as a
   name in itself: the still unnamed (15760) 1992 QB[1] gave its name to a
   group of asteroids which became known as cubewanos.

Numbering asteroids

   Asteroids are awarded with an official number once their orbits are
   confirmed. With the increasing rapidity of asteroid discovery,
   asteroids are currently being awarded six-figure numbers. The switch
   from five figures to six figures arrived with the publication of the
   Minor Planet Circular (MPC) of October 19, 2005, which saw the highest
   numbered asteroid jump from 99947 to 118161. This change caused a small
   " Y2k"-like crisis for various automated data services, since 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, the problem 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.

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 Variant symbol of Ceres
          Sickle variant symbol of Ceres Other sickle variant symbol of
          Ceres
          2 Pallas Old symbol of Pallas Variant symbol of Pallas
          3 Juno Old symbol of Juno Other symbol of Juno
          4 Vesta Old symbol of 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.

Asteroid exploration

   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.
   Enlarge
   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 1
   Ceres and 4 Vesta in 2011-2015, with its mission possibly then extended
   to 2 Pallas.

   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).

Asteroids in fiction

   A 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). In this context, asteroids serve the
   same role in space travel stories as reefs and underwater rocks in the
   older genre of sea-faring adventure 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.

   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.
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