What is the main difference between asteroids and comets?

The primary distinction between asteroids and comets lies in their composition. Asteroids are predominantly made up of rock and metal, and they typically exhibit a more solid, non-volatile structure, which is reflective of their formation within the warmer inner regions of the solar system. Conversely, comets are composed mainly of ice, dust, and rocky material, giving them a "dirty snowball" structure. This icy composition allows comets to develop a bright coma and tail when they approach the Sun, as the heat causes the ices to vaporize.

The focus on composition is crucial because it influences not only how these celestial bodies form and evolve but also how they behave within the solar system. For instance, the difference in materials is responsible for the distinct appearances of asteroids and comets when they come close to the Sun.