BSL Astronomy Glossary - comet - definition
Definition: Comets come from the icy Oort Cloud at the far edges of our solar system. Similar to large dirty snowballs, they are made up of ice and rocks. Some of them leave the Oort Cloud and travel across our solar system, orbiting the sun. As their trajectory approaches the sun, heat from solar winds starts to evaporate the ice in the comets, creating their long, gaseous tails, which can be thousands of kilometres long and always radiate out, away from the sun. When they are far from the sun, no heat reaches them and they have no tails at all, making them very hard to spot.