An asteroid is a small, solid object in our Solar System, orbiting the Sun. An asteroid is an example of a minor planet (or planetoid), which are much smaller than planets. Most asteroids are believed to be remnants of the protoplanetary disc which were not incorporated into planets during the system's formation. Some asteroids have moons. The vast majority of the asteroids are within the asteroid belt, with elliptical orbits between those of Mars and Jupiter.
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.
The term "asteroid", meaning star-like (from the Greek asteroeides, aster "star" + -eidos "form, shape"), was coined in 1802 by Sir William Herschel shortly after Olbers discovered the second one, 2 Pallas, in late March of the same year, to describe their star-like appearance; the other then-known planets all show discs, by comparison. He also applied that term to the small moons of the giant planets. The first scientific paper to use the word in its title was published in 1840 by Erman .
The exact definition of an asteroid is unsettled. The term "minor planet" (or "planetoid") carries no strong suggestion about the composition of the object or its general location in the solar system, and some argue that not every minor planet should be called an "asteroid".
One way to classify asteroids is in terms of size. A working definition is that asteroids are larger than 50 m in diameter, distinguishing them from meteoroids, which are typically boulder-sized or smaller. The distinction is made because asteroids are large enough to survive passage through Earth's atmosphere and strike Earth largely intact while the smaller meteoroids generally break up high in Earth's atmosphere.
Thus, it would be safest to use the term "asteroid" for Solar System objects that are bigger than meteoroids, smaller than planets, and made out of rock, not ice.
See Solar System for a complete taxonomy of objects in our system, and minor planet for a taxonomy of the subplanetary objects that include asteroids.
The term artificial asteroid is sometimes used to designate man-made objects which have ended up in solar orbits, such as the Mariner IV probe.
Asteroids in Earth's Solar System
Hundreds of thousands of asteroids have been discovered within Earth's solar system. As of February 24, 2005, from a total of 277,090 minor planets with calculated orbits 99,906 asteroids had been calculated well enough to be given official numbers and 12,198 of these had been officially named (598 of which have names requiring diacriticals). The first numbered but unnamed minor planet was (3360) 1981 VA ; the last numbered and named minor planet was 95959 Covadonga .
Current estimates put the total number of asteroids in the solar system at several million. The largest asteroid in our inner solar system is 1 Ceres, with a diameter of 900-1000 km. Two other large asteroids are 2 Pallas and 4 Vesta; both have diameters of ~500 km. 4 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 2004 MN4).
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
Asteroids are divided into 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.
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:
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 List of asteroid types for a complete 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:
This has led to great confusion though in that an asteroid's type is NOT indicative of its composition. 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.
However, scientists have been unable to agree on a new taxonomic system for asteroids and as a result, the spectral classification has stuck.
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 organised 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 Greek 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 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).
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 tied together previous apparitions into a single orbit. If so, the object gets a number. The observer of the final apparition is declared a 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
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 include :
The LINEAR system alone has discovered over 40,500 asteroids as of May, 2004 . Between all of the automated systems, 2701 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); Wikipedia drops the parentheses for named asteroids because it makes the text somewhat easier to read, because this convention is already quite common elsewhere, and because, frankly, it would be too much work to convert the existing pages to the formal format...
Unnamed asteroids that have been given a number keep their provisional designation, e.g. (29075) 1950 DA (for unnamed asteroids, Wikipedia does use the parentheses because the risk of confusion is much too great otherwise).
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 (3708) 1974 FV1 . 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 (even though there had been others before 2309 Mr. Spock, such as 482 Petrina ), increasingly eccentric asteroid names have been accepted, including 3494 Purple Mountain, 6042 Cheshirecat , 9007 James Bond and 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.
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.
, 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 July 2005, the Japanese Hayabusa probe will study in detail 25143 Itokawa in June 2005 and 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 teins and 21 Lutetia in 2008 and 2010. NASA is planning to launch the Dawn Mission in 2006, which will orbit both 1 Ceres and 4 Vesta in 2010-2014.
Asteroids in fiction and film
Understandably, most fictional depictions of asteroids focus on their potential risk of striking Earth. 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.
- In The Little Prince, a 1943 novel by Antoine de Saint-Exupéry, the title character lives on an asteroid. The asteroid moon Petit-Prince was named after the character.
- 'Catch that Rabbit', one of the short stories in Isaac Asimov's collection I, Robot (1950), takes place on an asteroid.
- The Japanese science fiction film The Mysterians aka Chikyu Boeigun (1957) reveals the solar system's asteroid belt as the remnants of the Mysterian's home planet, Mysteroid, after a nuclear war broke out.
- In Green Slime (1968), a masterpiece of B-movies, a rogue asteroid hurtles toward Earth. The astronauts leave Space Station Gamma 3 and place bombs on the asteroid, finding it inhabited by strange blobs of glowing slime that are drawn to the equipment. Unfortunately for everyone some of the slime was carried back on a space suit and soon evolves into tentacled creatures! See the review: . The movie inspired the classic board game Awful Green Things from Outer Space .
- In the classic science-fiction movie 2001: A Space Odyssey (1968), the Discovery has a scientifically accurate "close approach" by an asteroid whilst en route to Jupiter. The scene simply cuts briefly to a lone rock passing by the ship, with tens of thousands of kilometres to spare.
- The disaster movie Meteor (1979) depicts an asteroid named Orpheus hurtling toward Earth after its orbit is deflected by a comet.
- In The Empire Strikes Back (1980), Han Solo escapes Empire spacecraft by hiding the Millennium Falcon on an asteroid, but is then attacked by a vast monster that lives (for some unexplained reason) within the asteroid.
Arthur C. Clarke's novel 2061: Odyssey Three (1986) depicts a journey through the asteroid belt and its ominous parallels with the journey of the RMS Titanic.
Arthur C. Clarke's novel The Hammer of God (1993) depicts mankind's efforts to stop an asteroid named Kali from hitting the Earth. The film Deep Impact (1998) was based on Clarke's novel, although in the movie, the asteroid becomes a comet.
- In the LucasArts game The Dig (originally released in 1995) and its novelization, the impact-threatening asteroid Attila turns out to be an alien probe.
- In the 1998 movie Starship Troopers, aliens launch an asteroid at Earth, completely wiping out Buenos Aires. This is the opening move in the war.
- The film Armageddon (1998) is also about efforts to stop an asteroid hitting Earth. Its representation of an asteroid (and of space travel in general) is deeply unrealistic.
Ben Bova's novel series The Asteroid Wars (2001-2004) focuses on a war over the mining of the asteroid belt.
- An episode of the political television drama, The West Wing entitled "Impact Winter" included a subplot in which the White House staff prepared for a possible asteroid strike on the Earth. (First broadcast on December 15, 2004).