THE MAIN ASTEROID BELT

THE MAIN ASTEROID BELT

The Main Asteroid Belt is the region of our solar system where the greatest concentration of asteroids can be found, between the planetary orbits of Mars and Jupiter. The Main Belt formed early in the formation of our solar system, over 4.5 billion years ago. Dust and gas orbiting the Sun were gradually pulled together by gravity into planetesimals, then protoplanets, and finally into planets. The Main Belt once contained enough material to form a planet nearly four times as large as Earth. The gravity of Jupiter, the largest of the planets in our solar system, disrupted the formation process for the planetesimals nearby and prevented them from coalescing into larger forms; Jupiter’s gravity also swept a lot of this material clear.

The total mass of the Main Belt is estimated to be 3.0 to 3.6×1021 kilograms; this is about 4 percent of the Earth's Moon. About 220 of the asteroids within the asteroid belt have a diameter larger than 100 kilometres. Vesta, Pallas, and Hygiea are more than 400 kilometres in diameter. The largest of the objects in the asteroid belt and responsible for a third of the total mass is Ceres. Ceres is categorised as a dwarf planet and is about 1000 kilometres in diameter. Ceres, Vesta, Pallas, and Hygiea between them contain about half the mass of the belt. None of the asteroids are large enough to maintain an atmosphere but there is evidence that some asteroids contain water. The temperature of the Main Belt varies: for dust particles within the belt, temperatures range from 200 K (−73 °C) at 2.2 AU down to 165 K (−108 °C) at 3.2 AU. The surface temperature of an asteroid can vary depending on rotation, as the sides are alternately exposed to solar radiation and then to the stellar background.

This large population of asteroids means the belt is a very active environment, where collisions occur regularly (in astronomical terms). These collisions can fragment the asteroids into smaller pieces or can meld the two asteroids together if the collision occurs at low speeds. Tens of thousands of the asteroids within the belt are currently known and it is likely that the total number of asteroids is in the billions, possibly even the trillions. Despite these collisions, the asteroid material within the belt is very thinly distributed; many unmanned spacecraft have travelled through it without incident.

In the 18th century astronomer Johann Titius noted that there was a mathematical pattern in the layout of the planets. He used this pattern to predict the existence of a planet between the orbits of Mars and Jupiter. Astronomers searched the skies for years for evidence of this mystery planet; in 1800 Baron Franz Xaver von Zach recruited 24 other astronomers to form the Vereinigte Astronomische Gesellschaft ("United Astronomical Society"), otherwise known as the Celestial Police. Members of the group included Herschel, the British Astronomer Royal Nevil Maskelyne, Charles Messier, and Heinrich Olbers. Each member was assigned their own 15 degrees of the zodiac to search for the mystery planet. It was not a member of the Celestial Police that discovered the first planetary body in this region however; that honour went to Italian astronomer Giuseppe Piazzi who named the object Ceres. Pallas was found a little over a year later by Heinrich Wilhelm Olbers.

Because of the small size of these objects and their rapid movement being the only way to distinguish them from stars, William Herschel suggested in 1802 that they be placed into a new category called asteroids, after the Greek asteroeides, meaning "star-like". It remained the common practice for several decades however to refer to them as ‘planets’. The discovery rate of these objects increased: by 1807 two new objects in the region, Juno and Vesta had been discovered; in 1845 astronomers detected Astraea; and shortly after this time period objects were found at an accelerated rate. Including these objects among the planets became increasingly cumbersome. By the early 1850’s they were referred to as ‘asteroids’.

The Japanese astronomer Kiyotsugu Hirayama noticed in 1918 that the orbits of some of the asteroids had similar parameters, and essentially formed families or groups. About a third of the asteroids in the Main Belt are within an asteroid family. They share similar orbital elements, such as semi-major axis, eccentricity, and orbital inclination as well as similar spectral features; these indicate they share a common origin in the breakup of a larger body. So far, there are about 20–30 associations that are most likely to be asteroid families.

Asteroids within the Main Belt are categorised by their spectra, and most fall into three basic groups: carbonaceous (C-type), silicate (S-type), and metal-rich (M-type).
-Carbonaceous asteroids are carbon-rich and dominate the belt’s outer regions; they represent over 75% of the visible asteroids. These asteroids have a more reddish hue than other asteroids and have a very low albedo (http://on.fb.me/134cw4D). Their spectra match the primordial composition of the early solar system.
-Silicate-rich asteroids are more common in the inner region of the belt and contain no significant carbonaceous compounds. They have a relatively high albedo and represent about 17% of the asteroid population. Their spectra show the presence of silica and some metals; they have likely been significantly modified from their primordial composition.
-Metal-rich asteroids represent about 10% of the total population. Their spectra show they are of iron-nickel composition. Some are thought to have formed from metallic cores of planetary bodies that were impacted by other asteroids.
-Basaltic asteroids, aka V-type, are very rare. Until 2001, most of the basaltic bodies discovered in the Main Belt were thought to have originated from Vesta. The asteroid 1459 Magnya however showed a slightly different chemical composition from the other basaltic asteroids discovered beforehand which suggested a different origin. In 2007 two asteroids in the outer belt, 7472 Kumakiri and (10537) 1991 RY, were discovered to have differing basaltic composition that could not have originated from Vesta.

Pioneer 10 was the first spacecraft to journey through the asteroid belt, in 1972. There have been a total of 12 missions that have made it through the belt. In 2007, NASA's Dawn probe was launched; its aim to explore and study Vesta and Ceres. The probe entered orbit around Vesta on July 16, 2011 and left its orbit on September 5, 2012, on a course for Ceres; it is scheduled to reach Ceres in February 2015.

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http://www.sciencedaily.com/articles/a/asteroid_belt.htm
http://www.space.com/16105-asteroid-belt.html
http://www.universetoday.com/32856/asteroid-belt/#ixzz2LREBMBvB
Image shows the location of the asteroid belt between Mars and Jupiter. © Sophie DesRosiers, Planétarium de Montréal