A comet is a relatively small body in the solar system composed mostly of ice. They travel around the Sun in typically very wide orbits. When it travels close to the Sun the ice can begin to melt, releasing gases and dust in a process known as outgassing. Comets are generally a few kilometres across or less, the size of a small town. The main section of a comet is called the nucleus. There are currently approximately 3700 comets known with several million predicted in the Kuiper belt and Oort cloud.