'Asteroids', also called minor planets or planetoids, are a class of
astronomical objects. The term asteroid is generally used to indicate a diverse group of small celestial bodies in the
solar system that orbit around the
Sun. ''Asteroid'' (Greek for "star-like") is the most commonly used word 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 thus far 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
accretion of
planetesimals into a larger planet or planets during the formative period of the solar system was prevented by large gravitational perturbations by
Jupiter. Some asteroids have
moons or are found in co-orbiting pairs known as
binary systems.
Asteroids in the solar system
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 31,
2007, from a total of 378,546 registered minor planets, 160,508 have orbits known well enough to be given
permanent official numbers.
[1] Of these, 13,889
[2] have official names.
[3] The lowest-numbered but unnamed minor planet is ;
[4] the highest-numbered named minor planet is
159351 Leonpascal.
[5]
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.
[6] The largest asteroid in the inner solar system is
1 Ceres, with diameters of 975×909 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,
[7][8] or about 4% of the mass of our moon. Of this,
Ceres comprises 0.95 kg, some 32% of the total. Adding in the next three most massive asteroids,
4 Vesta (9%),
2 Pallas (7%), and
10 Hygiea (3%), brings 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 according to two criteria: the characteristics of their orbits, and features of their reflectance
spectrum.
Orbit groups and families
Main articles: Asteroid family,
Minor planet
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.
[9] 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
★
L-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 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:
[10]
★ 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.
Historical 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 discernible 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, the
Henry brothers 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".
Manual methods of the 1900s and modern reporting
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.
[11]
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.
Computerized methods
2004 FH is the centre dot being followed by the sequence; the object that flashes by during the clip is an artificial satellite.
There is increasing interest in identifying asteroids whose orbits cross
Earth's, 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
Atens. Various
asteroid deflection strategies have been proposed, as early as the 1960s.
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' hypothesis that an
impact event resulted in the
Cretaceous–Tertiary extinction, 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:
[12]
★ 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 84,764 asteroids, as of
August 28,
2007.
[13] Between all of the automated systems, 4711 near-Earth asteroids have been discovered
[14] 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 ). 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 . On rare occasions, a small body's
provisional designation may become used as a name in itself: the still unnamed gave its name to a group of
Kuiper belt objects 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.
Sources for names
Main articles: Meanings of asteroid 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
4321 Zero,
6042 Cheshirecat,
9007 James Bond,
13579 Allodd,
24680 Alleven,
128036 Rafaelnadal or
26858 Misterrogers.
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 died shortly after the discovery, at age 22.
[15][16] A few objects are also cross-listed as both comets and asteroids, such as
4015 Wilson-Harrington =
107P/Wilson-Harrington.
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(s).
[17]
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,
[19] 35 Leukothea,
[20] and
37 Fides[21]) 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.
[22]
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.
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 may return samples of its surface to earth. The Hayabusa mission has been plagued with difficulties, including the failure of two of its three control wheels, rendering it difficult to maintain its orientation to the sun to collect solar energy. 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), or materials for constructing
space habitats (see
Colonization of the asteroids). Materials that are heavy and expensive to launch from earth may someday be mined from asteroids and used for
space manufacturing and construction.
Asteroids in fiction
Main articles: Asteroids in fiction
Asteroids and asteroid belts are a staple of science fiction stories. Asteroids play several potential roles in science fiction: as places which human beings might colonize; as resources for extracting minerals; as a hazard encountered by spaceships travelling between two other points; and as a threat to life on Earth due to potential impacts.
Notes
1. Minor Planet Statistics
2. Minor Planet Names
3. trivia: about 650 of these names require diacritics
4. Discovery Circumstances: Numbered Minor Planets (1)-(5000)
5. Discovery Circumstances: Numbered Minor Planets (155001)-(160000)
6. New study reveals twice as many asteroids as previously believed
7. Hidden Mass in the Asteroid Belt, , G. A., Krasinsky, Icarus, 2002
8.
9. Surface properties of asteroids: A synthesis of polarimetry, radiometry, and spectrophotometry, Chapman, C. R., Morrison, D., & Zellner, B., , , Icarus, 1975
10. Meteorites and Their Parent Planets, , , McSween Jr., Harry Y., , , ISBN 0-521-58751-4
11. Carolyn Shoemaker
12. Near Earth Object Program
13. Minor Planet Discover Sites
14. Unusual Minor Planets
15. Citation from MPC 55988
16. MIT News Office: Lucy Crespo da Silva, 22, a senior, dies in fall
17. On the Symbolic Notation of the Asteroids, , B. A., Gould, Astronomical Journal, 1852
18. When Did the Asteroids Become Minor Planets
19. Beobachtung der Bellona, nebst Nachrichten über die Bilker Sternwarte, , J. F., Encke, Astronomische Nachrichten, 1854
20. Name und Zeichen des von Herrn R. Luther zu Bilk am 19. April entdeckten Planeten, , G., Rümker, Astronomische Nachrichten, 1855
21. Schreiben des Herrn Dr. R. Luther, Directors der Sternwarte zu Bilk, an den Herausgeber, , R., Luther, Astronomische Nachrichten, 1856
22. When Did the Asteroids Become Minor Planets
See also
★
Asteroid belt
★
Asteroid mining
★
BOOTES (Burst Observer and Optical Transient Exploring System)
★
★
★
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 Center
★
Near-Earth object
★
Impact event
★
Pronunciation of asteroid names
External links
★
Alphabetical list of minor planet names (ASCII) (Minor Planet Center)
★
Near Earth Asteroid Tracking (NEAT)
★
Asteroids Page at
NASA's Solar System Exploration
★
When Did the Asteroids Become Minor Planets?
★
Asteroid Simulator with Moon and Earth
★
Everything you wanted to know about comets and asteroids — Provided by ''
New Scientist''.
★
Alphabetical and numerical lists of minor planet names (Unicode) (Institute of Applied Astronomy)
★
Known Asteroid Impacts & Their Effects
★
Future Asteroid Interception Research
★
Near Earth Objects Dynamic Site
★
Asteroids Dynamic Site Up-to date
osculating orbital elements and
proper orbital elements
★
Asteroid naming statistics
★
Spaceguard UK
★
Large amount of information on asteroid groups collected by Gérard Faure, translation Richard Miles.
★
Near Earth Objects and Asteroids: Are We Whistling in the Dark?
''(asteroid navigator) | First asteroid | ...''
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