So how do we explain the vast distance between Mars and Jupiter? Some astronomers have suggested that a separate planet or protoplanet actually formed between the two planets, but the impact of a high-speed comet broke up and scattered the newly formed body to create what we now know as the main asteroid belt.
While it's possible that comets and other large objects were flying around the solar system and breaking up material during the early stages, most scientists accept a much simpler theory -- asteroids are leftover matter from the solar system's formation that never successfully came together as one planet. But how come nothing came together?
If you look at Jupiter's mass, you'll notice it's extremely large. People refer to it as a gas giant for good reason -- while the Earth's mass is about 6x10^24 kilograms, Jupiter's mass is estimated to be 2x10^27 kilograms. It's a much closer relative to our sun than to rocky planets like Earth or Mars.
Jupiter's massive size would be enough to disturb the rocky matter that fell in between it and Mars -- its strong gravitational pull would cause any potential protoplanets to collide and break apart into smaller bits. We're then left with a large, spread-out collection of asteroids that orbits around the sun in the same direction as Earth -- the main asteroid belt. With its center around 2.7 AU from the sun, the belt separates Mars and the other rocky planets from the massive, cold gas giants like Jupiter and Saturn.
For a closer look at asteroids within the belt, see the next page.