Asteroids

Asteroids are any of numerous small celestial bodies composed of rock and metal that move around the sun, mainly between the orbits of Mars and Jupiter. They are also known as 'Minor Planets' or 'Planetoids' as their small size and large numbers are relative to the major Planets. They are smaller than Planets but larger than Meteoroids. Asteroids or their fragments regularly strike Earth, entering the atmosphere as Meteors to reach its surface. 16 Asteroids have a diameter of 240 km or greater. According to a study, if the estimated total mass of all Asteroids was gathered into a single object, the object would be less than 1,500 kilometres (932 miles) across - less than half the diameter of Earth's Moon. There are about hundreds of thousands of Asteroids which are too small to be seen from the Earth. There also a few "Asteroids" (designated as "Centaurs") in the outer solar system: 2060 Chiron (aka 95 P/Chiron) orbits between Saturn and Uranus; the orbit of 5335 Damocles ranges from near Mars  to beyond Uranus; 5145 Pholus orbits from Saturn to past Neptune. A newly discovered Asteroid is given a provisional designation (such as 2002 AT4) consisting of the year of discovery and an alphanumeric code indicating the half-month of discovery and the sequence within that half-month. Once an Asteroid's 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.

Classification of Asteroids according to their Spectra and Albedo

Asteroids are classified according to their brightness (albedo) and colour (spectra).  Albedo, is a measurement of how well the object reflects light. Albedo ranges from 0 (perfectly black) to 1 (perfectly reflecting).

1.  C-Type Asteroids - The C-type Asteroids are the most common Asteroids and constitute about 75% of all the known Asteroids. They are also known as Carbonaceous Asteroids as their composition is similar to carbonaceous chondrite meteorites. They are very dark in appearance having Albedos (reflective power) of 0.03 to 0.09. Their chemical composition is similar to the Sun, but lack hydrogen, helium, and other volatiles. They probably consist of clay and silicate rocks and are dark in appearance. C-type Asteroids are most commonly found in the outer regions of the main Asteroid belt. C-type Asteroids are among the most ancient objects in our solar system.

2.  S-Type Asteroids - The S-type Asteroids constitute about 17% of all the known Asteroids. They are also known as Silicaceous Asteroids as they are composed of silicate (stony) materials and nickel-iron. They have fairly bright Albedos ranging from 0.10 to 0.22.  Moving in orbits within the inner regions of the main Asteroid belt, S-type Asteroids are likely equivalent to the ordinary Chondrite Meteorites, composed of rocky materials and a small amount of metallic iron.

3.  M-Type Asteroids - The M-type Asteroids are made up of nickel-iron and therefore are also known as Metallic Asteroids. These are found in the mid-region main belt. These are relatively bright, with Albedos ranging from 0.10 to 0.18.  

Classification of Asteroids according to their location in the Solar System

1.  Main Belt Asteroids - Main Belt is a region in the Solar System located between Mars and Jupiter roughly 2 - 4 AU (astronomical unit) from the Sun. Asteroids found in this region of the Solar System are known as Main Belt Asteroids. In 2005, scientists discovered that the largest main belt Asteroid, Ceres, must contain a lot of water ice. All of these formed in the same places as the Asteroids but have retained liquid water over the age of the solar system. Many main belt Asteroids may in fact contain a lot of ice, especially in the outer part of the belt.

Main Belt Asteroids are further divided into subgroups:

Hungarias.
Floras.
Phocaea.
Koronis.
Eos.
Themis
Cybeles.
Hildas.

2.  Near-Earth Asteroids - Near-Earth Asteroids are those Asteroids which closely approach the Earth an whose with orbit brings them within 1.3 AU of the Sun. They are also known as NEA's which is short term for Near-Earth Asteroids. Those Asteroids that can cross the orbit of Mars but that have perihelion distances greater than 1.3 AU—are known as Mars crossers. This class is further subdivided into two: shallow Mars crossers (perihelion distances no less than 1.58 AU but less than 1.67 AU) and deep Mars crossers (perihelion distances greater than 1.3 AU but less than 1.58 AU). According to some scientist these Asteroids are the nuclei of dead, short-period comets that have burned off all their gases, while some scientists believe that these Asteroids are objects which have been pushed away from the Main Asteroid Belt. This pushing may have occurred due to collisions between objects, the gravitational tug of the Giant Planets, or a subtle force known as the Yarkovsky Effect. The Yarkovsky Effect occurs when a rotating Asteroid’s sun-warmed surface re-radiates its heat on its afternoon side.  The photons departing the surface of the Asteroid create a tiny change in the Asteroid’s momentum over time.

NEAs are grouped into 3 main categories, named for each category’s most famous member: 1221 Amor, 1862 Apollo, and 2062 Aten.

1. Amors - These Asteroids cross Mars' orbit but do not quite cross Earth's orbit. Amors have perihelion distances between 1.017 and 1.3 AU. Amor, the namesake of the group, are part-time Earth crossers. Only Asteroids that cross the orbits of Planets—i.e., Earth-approaching Asteroids and idiosyncratic objects such as (944). Hidalgo and Chir Eros, which was visited by the Near Earth Asteroid Rendezvous (NEAR) spacecraft, is an Amor. More than 1,500 Amors are known.
2. Apollos - These Asteroids are Earth orbit-crossing Asteroids with orbit periods that are longer than one year. These Asteroids have semi-major axes greater than 1.0 AU and perihelion distances less than 1.017 AUAsteroid Geographos is an Apollo. More than 1,700 Apollos are known.
3. Atens - These Asteroids cross Earth's orbit with an orbital period less than 1 year. These have semi-major axes less than 1.0 AU and aphelion distances greater than 0.983 AU. Roughly 300 Atens are known.

3.  Trojans - Trojan Asteroids are those objects which are located near Jupiter's Lagrange points (60 degrees ahead and behind Jupiter in its orbit). Two "clouds" of icy Asteroids 60 degrees ahead and behind Jupiter (and at or near Jupiter's orbital distance from the sun) are called 'Jupiter Trojans', while two similar objects in Mars orbit are called 'Martian Trojans'. Several hundred of Trojan Asteroids are now known, while thousands may actually exist. There are also some Asteroids known to orbit in the Lagrange points for Mars and Neptune. These may well have the compositions of comets; a rare binary Trojan, 617 Patroclus, is composed of ice.

4.  Centaurs - Centaur Asteroids are objects which orbit far from the Sun in the outer solar system. Because of their location, they are also known as Kuiper belt objects.  Their Planet orbit-crossing paths are unstable and are frequently perturbed by Planets and other objects. The composition of these objects is probably more like that of comets or Kuiper Belt objects, rather than that of ordinary Asteroids. Example, 2060 Chiron orbits between Saturn and Uranus; the orbit of 5335 Damocles ranges from near Mars to beyond Uranus; and 5145 Pholus orbits from Saturn to past Neptune. There are probably many Centaurs; more than 50 are currently named and tracked.

Characteristics and Physical Features of Asteroids

• Small Size - The vast majority of Asteroids are small in size. There are probably more than a million Asteroids in the 1 km range. They are considered to be either surviving Planetesimals or fragments of larger bodies. There are 26 known Asteroids larger than 200 km in diameter. The largest Asteroid by far is 1 Ceres. It is 974 km in diameter and contains about 25% of the mass of all the Asteroids combined. The next largest are 2 Pallas, 4 Vesta and 10 Hygiea which are between 400 and 525 km in diameter. All other known Asteroids are less than 340 km across.

• Varied Physical Composition - The physical composition of Asteroids is varied. The Asteroids' compositional differences are related to how far from the Sun they formed. Some experienced high temperatures after they formed and partly melted, with iron sinking to the centre and forcing basaltic (volcanic) lava to the surface. One such Asteroid, Vesta, survives to this day. Ceres appears to be composed of a rocky core covered by an icy mantle, where Vesta is thought to have a nickel-iron core, olivine mantle, and basaltic crust. 10 Hygiea appears to have a uniformly primitive composition of carbonaceous chondrite. Many, perhaps most, of the Smaller Asteroids are piles of rubble held together loosely by gravity. Some Have Moons or are co-orbiting binary Asteroids. The rubble piles, Moons, binaries, and scattered Asteroid families are believed to be the results of collisions that disrupted a parent Asteroid.

• Asteroid Rotation - In the early years of the 21st century, rotation periods were known for more than 2,300 Asteroids. They range from 42.7 seconds to 50 days, but more than 70% lie between 4 and 24 hours. In some cases, periods longer than a few days may actually be due to precession (a smooth slow circling of the rotation axis) caused by an unseen satellite of the Asteroid. Periods on the order of minutes are observed only for very small objects (those with diameters less than about 150 metres). The largest Asteroids, those with diameters greater than about 200 km, have a mean rotation period close to 8 hours; the value increases to 13 hours for Asteroids with diameters of about 100 km and then decreases to about 6 hours for those with diameters of about 10 km. The largest Asteroids may have preserved the rotation rates they had when they were formed, but the smaller ones almost certainly have had theirs modified by subsequent collisions  and, in the case of the very smallest, perhaps also by radiation effects. The difference in rotation periods between 200-km-class and 100-km-class Asteroids is believed to stem from the fact that large Asteroids retain all of the collision debris from minor collisions, whereas smaller Asteroids retain more of the debris ejected in the direction opposite to that of their spins, causing a loss of angular momentum and thus a reduction in speed of rotation.

• Asteroid Albedo - Albedo is the ratio between the amount of light actually reflected and that which would be reflected by a uniformly scattering disk of the same size, both observed at opposition. Snow has an albedo of approximately 1 and coal an albedo of about 0.05. An Asteroid’s apparent brightness depends on both its albedo and diameter as well as on its distance. For example, if Ceres and Vesta could both be observed at the same distance, Vesta would be the brighter of the two by about 15 percent, even though Vesta’s diameter is only a little more than half that of Ceres. Vesta would appear brighter because its albedo is about 0.40, compared with 0.10 for Ceres.

Asteroid Albedos range from about 0.02 to more than 0.5 and may be divided into 4 groups:

1. Low - 0.02–0.07. After corrections are added for the fact that the brighter and nearer Asteroids are favoured for discovery, about 78% of known Asteroids larger than about 25 km in diameter are found to be Low-Albedo objects. Most of these are located in the outer half of the main Asteroid belt and among the outer-belt populations. More than 95% of outer-belt Asteroids belong to this group.
2. Intermediate - 0.08–0.12. The intermediate and High-Albedo Asteroid groups make up the remaining 4% of the population.
3. Moderate - 0.13–0.28. Roughly 18% of known Asteroids belong to the Moderate-Albedo Group, the vast majority of which are found in the inner half of the main belt.
4. High - Greater than 0.28. They occupy the same part of the main belt as the Moderate-Albedo Objects.

• Orbit within the main Asteroid Belt - The majority of known Asteroids orbit within the main Asteroid belt between the orbits of Mars and Jupiter, generally in relatively low-eccentricity  (i.e., not very elongated) orbits. This belt is considered to contain between 1.1 and 1.9 million Asteroids larger than 1 km (0.6 mi) in diameter,and millions of smaller ones. All Asteroids orbit the Sun in elliptical orbits and move in the same direction as the major Planets. Most of these orbits, in turn, have semi-major axes, or mean distances from the Sun, between 2.06 and 3.28 AU, a region called the main belt. The mean distances are not uniformly distributed but exhibit population depletions, or “gaps.” These so-called Kirkwood gaps are due to mean-motion resonances with Jupiter’s orbital period.

• Irregularly Shaped -  Nearly all Asteroids are irregularly shaped, though a few are nearly spherical, and are often pitted or cratered. As they revolve around the Sun in elliptical orbits, the Asteroids also rotate, sometimes quite erratically, tumbling as they go.

• Absence of Atmosphere - Asteroids do not have Atmospheres. They do not have any layer surrounding it.

Size - Asteroids range in size from Ceres, which has a diameter of about 1000 km, down to the size of pebbles.

Some of the Major Asteroids

Ceres.
Pallas.
Vesta.
Toutatis.
Castalia.
Geographos.
Annefrank.
Braille.
Castalia.
Eros.
Gaspra.
Geographos.
Ida & Dactyl.
Lutetia.
Mathilde.

Origin and Evolution of Asteroids

It is believed that first Asteroids formed from rocky fragments left over from the formation of the solar system about 4.6 billion years ago. Early in the history of the solar system, the formation of Jupiter brought an end to the formation of planetary bodies in the gap between Mars and Jupiter and caused the small bodies that occupied this region to collide with one another, fragmenting them into the Asteroids of modern times. This region, called the Asteroid belt or simply the main belt, may contain millions of Asteroids. Because Asteroids have remained mostly unchanged for billions of years, studies of them can provide vital information about the early solar system. More likely, Asteroids are material that never coalesced into a Planet. It is believed that planetesimals in the main Asteroid belt evolved much like the rest of the Solar Nebula until Jupiter neared its current mass, at which point excitation from orbital resonances  with Jupiter ejected over 99% of planetesimals in the belt. Simulations and a discontinuity in spin rate and spectral properties suggest that Asteroids larger than approximately 120 km (75 mi) in diameter accreted during that early era, whereas smaller bodies are fragments from collisions between Asteroids during or after the Jovian (related to jupiter) disruption.