3 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.

3.1 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.

3.2 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.

• A-type asteroids
• B-type asteroids
• D-type asteroids
• E-type asteroids
• F-type asteroids
• G-type asteroids
• P-type asteroids
• Q-type asteroids
• R-type asteroids
• T-type asteroids
• V-type asteroids

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.

3.2.1 Problems with spectral classification

Originally, spectral designations were based on inferences of an asteroid's composition:

• C - Carbonaceous
• S - Silicaceous
• M - Metallic

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.

4 Asteroid discovery

Asteroid discovery proceeded in several phases.

In the last years of the 18th century, Franz Xaver, Baron 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. Three others ( 2 Pallas, 3 Juno, 4 Vesta) followed 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".

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. 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, then 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.

When the orbit of an asteroid is confirmed, it is given a number, and later it may also be given a name (e.g. 1 Ceres). The first few are named after figures from Graeco-Roman mythology, but as such names started to run out, others were also used —famous people, the names of the discoverer's wives, even television characters. A few groups have names with a common theme —for instance Centaurs, asteroids orbiting between Saturn and Neptune, are all named after legendary Centaurs, and Trojans after heroes from the Trojan War. For a full list, see meanings of asteroid names.

Since 1998, a large majority of the asteroids have been discovered with automated systems that comprise Charge-Coupled Device ( CCD) cameras and computers directly connected to telescopes. A list of teams using such automated systems include [1]:

• 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
• The Campo Imperatore Near-Earth Objects Survey (CINEOS) team
• The Japanese Spaceguard Association
• The Asiago-DLR Asteroid Survey

The LINEAR system alone has discovered over 40,500 asteroids as of May, 2004 [2]. Between all of the automated systems, 2701 near-Earth asteroids have been discovered [3] including over 600 more than 1 km in diameter.

In 2003, a new class of asteroids, the Apoheles, was discovered when 2003 CP₂₀ was found to have an orbit that always keeps it closer to the Sun than the Earth.

5 Asteroid deflection

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 programs such as LINEAR, NEAT and LONEOS, which have drastically increased the rate of asteroid detection.

6 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.

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).

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 2006, which will orbit both 1 Ceres and 4 Vesta in 2010- 2014.

7 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.
• 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: [4]. The movie inspired the classic board game Awful Green Things from Outer Space .
• 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 ( 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.
• 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 unscientific.
• Ben Bova's novel series The Asteroid Wars ( 2001- 2004) focuses on a war over the mining of the asteroid belt.
• In the LucasArts game The Dig and its novelization, the impact-threatening asteroid Attila turns out to be an alien probe.

8 See also

• List of asteroids
• Meanings of asteroid names
• Minor Planet Center
• Asteroid_groups_and_families
• Asteroids

9 External links

• Known Asteroid Impacts & Their Effects
• Alphabetical list of minor planet names
• Near Earth Objects Dynamic Site
• Asteroids Dynamic Site

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