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  Asteroids and Their Main Belt:  


Also known as “minor planets” or “planetoids,” asteroids are medium-sized rocky objects which orbit the Sun. They’re smaller than a planet, but bigger than a meteoroid. A good concentration of them is found in the Asteroid Belt – a doughnut-shaped region between the orbits of Mars and Jupiter.

Thousands of Asteroids
In this “belt” are some 40,000 asteroids, ranging in size from one mile to nearly 600 miles in diameter. The largest of these is indeed 1 Ceres, which measures 578 miles across and along with 4 Vesta, 2 Pallas, and 10 Hygiea, account for more than half of the belt’s surface.

Discovered on January 1, 1801, 1 Ceres was the first asteroid to be identified, named, and catalogued. Since, there have been over 2,000 other asteroids in the belt successfully numbered.
  How Did the Belt Form:
  There are several theories – just as there are for the formation of the Solar System itself – but the most widely accepted is the nebular theory. According to theory, the Solar System was at one time shapeless, aimless, until an explosion occurred that set everything in motion.

Post-explosion, as the Solar System’s eight planets formed from planetesimals, something happened between Mars and Jupiter that prevented the Asteroid Belt from turning into its own major planet.

Likely, it was the gravitational pull from sizeable Jupiter that disrupted the planetesimals of the potential new (Asteroid Belt) planet. Instead of sticking together and accreting, these planetesimals crashed into each other and broke apart – making them too small to form a planet.

Instead, the asteroids formed a belt.

Note: The Asteroid Belt is also known as the “main belt,” as a way of identifying it from similar concentrations in the Solar System, such as the Kuiper Belt.