When seen through a telescope, an asteroid looks like a faint star The largest known asteroid, Ceres, was discovered in 1801. Still, the mass of all the asteroids combined is less than the mass of the smallest planet, Pluto.
Most asteroids lie between Mars and Jupiter, in a vast ring called the Asteroid Belt.
But others inhabit different regions of the solar system. The Apollo asteroids, for example, cross or come near Earth's orbit. An Apollo asteroid may have caused the extinction of the dinosaurs when it collided with Earth 65 million years ago. Billions of years ago, when the planets were forming, many large asteroids may have inhabited the asteroid belt. Instead, over time, the asteroids crashed into one another and split apart into smaller and smaller fragments.