Asteroids formed from small pieces of rock and metal.small particles run into each other and if they don't hit too hard, they'll stick together.
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Asteroids are leftovers from the formation of our solar system about 4.6 billion years ago. Early on, the birth of Jupiter prevented any planetary bodies from forming in the gap between Mars and Jupiter, causing the small objects that were there to collide with each other and fragment into the asteroids seen today.