Asteroid

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 due to excessive gravitational perturbations by Jupiter. Some asteroids have moons. The vast majority of the asteroids are within the main asteroid belt, with elliptical orbits between those of Mars and Jupiter.

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 February 23, 2006, from a total of 325,627 registered minor planets, 120,437 have orbits known well enough to be given permanent official numbers. Of these, 12890 have official names (trivia: at least 610 of these names require diacritics). The lowest-numbered but unnamed minor planet is (3360) 1981 VA; the highest-numbered named minor planet is 117506 Wildberg [1].

Current estimates put the total number of asteroids in the solar system at several 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 2.3x10²¹ kg, or about 3% of the mass of our moon. Of this, 1 Ceres comprises 940 to 950x10¹⁸ kg, some 40% of the total. Adding in the next three most massive asteroids, 4 Vesta (12%), 2 Pallas (9%), and 10 Hygiea (4%), bring this figure up 66%; while the three after that, 511 Davida (1.6%), 704 Interamnia (1.4%), and 3 Juno (1.2%), only add another 4% 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.

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

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 - 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

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 [2].

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 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

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 [3]:

• 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 62,283 asteroids as of December 14, 2005 [4]. Between all of the automated systems, 3868 near-Earth asteroids have been discovered [5] including over 600 more than 1 km in diameter.

Naming asteroids

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. This has been addressed in some data fields 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. The highest number 120437 thus is cross-referenced as C0437 on some lists. Also, the fictional asteroid of The Little Prince, B612, now could be connected with the real (110612) 2001 TA₁₄₂ which is listed as (B0612) 2001 TA₁₄₂ 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.

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

2 Pallas

3 Juno

4 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.

For details, see James L. Hilton, 2001, When Did the Asteroids Become Minor Planets?.

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.

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 may launch the Dawn Mission in 2007 to orbit both 1 Ceres and 4 Vesta in 2010- 2014.

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 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.

• Professor Moriarty, Sherlock Holmes' arch-enemy, "is the celebrated author of "The Dynamics of an Asteroid", a book which ascends to such rarefied heights of pure mathematics that it is said that there was no man in the scientific press capable of criticizing it" ( The Valley of Fear, 1914, set in 1888).
• In The Little Prince, a 1943 novel by Antoine de Saint-Exupéry, the title character lives on an asteroid named "B-6-12". The asteroid moon Petit-Prince was named after the character, and 46610 Bésixdouze after his asteroid.
• ' Catch that Rabbit', one of the short stories in Isaac Asimov's collection I, Robot ( 1950), takes place on an asteroid, while Marooned Off Vesta, Asimov's first published story, concerns the plight of a group of astronauts stranded in orbit around the asteroid 4 Vesta.
• 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: [6]. 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 a binary asteroid whilst en route to Jupiter. The scene simply cuts briefly to two lone rocks passing by the ship, with tens of thousands of kilometres to spare.
• In James P. Hogan's Inherit the Stars ( 1977), first book of the Gentle Giants series, Minerva was a planet that exploded to form the asteroid belt 50,000 years ago.
• The disaster movie Meteor ( 1979) depicts an asteroid named Orpheus hurtling toward Earth after its orbit is deflected by a comet.
• Atari released the arcade game Asteroids in 1979.
• In The Empire Strikes Back ( 1980), Han Solo enters an asteroid field to flee from the Imperial fleet, and C-3PO thinks it is a bad idea. Han then hides his ship, the Millennium Falcon inside a giant asteroid; The ship is then attacked by a vast monster that lives (inexplicably) within the asteroid in the vacuum of space.
• In Orson Scott Card's Ender's Game ( 1985), a school on 433 Eros is dedicated to children learning to become fleet commanders.
• 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.
• L. Neil Smith's novel Pallas ( Tor Books, 1993) depicts a modernized hunting based life on the terraformed asteroid Pallas and introduces Emerson Ngu. The book was partly insired by the 1987 article "The Worst Mistake in the History of the Human Race" written by Jared Diamond. The book also includes a brief description of a way to encapsulate the entire surface of a small body such as an asteroid to enable creating an Earthlike environment.
• 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.
• In the BBC drama documentary Space Odyssey: Voyage to the Planets ( 2004), the Pegasus encounters a binary asteroid from much closer than expected, and dubs the rocks "Hubris" and "Catastrophe" as a result.
• 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).