The Solar System:
The main asteroid belt
The inner Solar System, from the Sun to Jupiter, including the true relative positions of objects (as seen on 14 Aug 2006) in the main asteroid belt (white), the roughly triangular distribution of Hilda asteroids, which are caught in an orbital resonance with Jupiter (orange ), the Jupiter trojan asteroids that occupy the regions of stable gravitational equilibrium 60 degrees ahead (the Greeks) and behind (the Trojans) in the orbit of Jupiter (green), and the near-Earth asteroids (inside the orbit of Mars).Credit (image and some text): Mdf (Wikimedia Commons) [link]
Masses of the asteroids
Comparison of the masses of the twelve most massive asteroids known, as fractions of the total mass of the main asteroid belt. Ceres alone constitutes about 30% of the total mass in the asteroid belt, and the four most massive asteroids amount to almost 50% of the total mass.Credit: Kwamikagami (Wikimedia Commons), CC BY SA 3.0 [link]
Background information: interior structure of planets
Earth's interior layers, showing differentiation. The core of the Earth is composed of nickel-iron, while the mantle layers are less dense metals and rocks, and the crust is even lower density rocks.Credit: United States Geological Survey [link]
Structure and composition of asteroids
Cut and polished piece of the Cumberland Falls stony achondrite meteorite that fell in Whitley County, Kentucky, in 1919. Most stony achondrite meteorites that fall on Earth are believed to come from the crust of the asteroid Vesta.Credit: Claire H. (Wikimedia Commons), CC BY SA 2.0 [link]
The main asteroid belt is (relatively) crowded.
Remember: space is big!
When asteroid collisions happen, some fragments are ejected from the asteroid belt and some of these later collide with Earth. We see them as meteors streaking though the sky (being ablated by their passage through the atmosphere).