Comet, illustration. Comets are bodies of ice and dust that enter the inner solar system from the outer solar system. As they approach the Sun, the heat boils the ice, producing a coma around the nucleus, along with a tail stretching out behind it. The direction of the tail indicates the direction of the Sun. Comets passing near the Earth can be a spectacular sight, and have been seen as omens of doom. Comets have elongated orbits and can have a short or long period, ranging from less than 200 years to more than tens of thousands of years. They can vary in size from hundreds of metres to many kilometres across.