The asteroid belt is a region of the Solar System located roughly between the orbits of the planets Mars and Jupiter. It is occupied by numerous irregularly shaped bodies called asteroids or minor planets. More than half the mass of the belt is contained in the four largest objects: Ceres, 4 Vesta, 2 Pallas, and 10 Hygiea. These have mean diameters of more than 400 km, while the remaining bodies range down to the size of a dust particle. Individual asteroids within the main belt are categorized by their spectra, with most falling into three basic groups: carbonaceous (C-type), silicate (S-type), and metal-rich (M-type). The asteroid belt formed from the primordial solar nebula as a group of planetesimals, which in turn formed protoplanets. Between Mars and Jupiter, gravitational perturbations from the giant planet imbued the protoplanets with too much orbital energy for them to accrete into a planet. Collisions became too violent and, instead of sticking together, the planetesimals and most of the protoplanets shattered. Asteroid orbits continue to be appreciably perturbed whenever their period of revolution about the Sun forms an orbital resonance with Jupiter. Other regions of small solar system bodies include the centaurs, the Kuiper belt and scattered disk, and the Oort cloud. Read the rest of this article: <http://en.wikipedia.org/wiki/Asteroid_belt> _______________________________ Today's selected anniversaries: 879: Pope John VIII became the first to officially recognise Croatia as a nation-state, and Branimir as its Duke. <http://en.wikipedia.org/wiki/Branimir_of_Croatia> 1674: John III Sobieski , elected by the szlachta, became the King of the Polish–Lithuanian Commonwealth. <http://en.wikipedia.org/wiki/John_III_Sobieski> 1881: Clara Barton founded the American Red Cross. <http://en.wikipedia.org/wiki/Clara_Barton> 1911: Mexican President Porfirio Díaz and the revolutionary Francisco Madero signed the Treaty of Ciudad Juárez to put an end to the fighting between the forces of both men, and thus concluding the initial phase of the Mexican Revolution. <http://en.wikipedia.org/wiki/Treaty_of_Ciudad_Ju%C3%A1rez> 1946: Manhattan Project physicist Louis Slotin accidentally triggered a fission reaction at the Los Alamos National Laboratory and gave himself a lethal dose of hard radiation, making him the second victim of a criticality accident in history. <http://en.wikipedia.org/wiki/Louis_Slotin> 1998: Indonesian President Suharto resigned following the collapse of support for his three-decade-long reign. <http://en.wikipedia.org/wiki/Suharto> _____________________________ Wiktionary's word of the day: don't shoot the messenger (proverb): The bearer of bad news should not be held accountable for the bad news <http://en.wiktionary.org/wiki/don%27t_shoot_the_messenger> ___________________________ Wikiquote quote of the day: All seems Infected that th' Infected spy, As all looks yellow to the Jaundic'd Eye. --Alexander Pope <http://en.wikiquote.org/wiki/Alexander_Pope>