Comet. Illustration depicting the core of a comet. Comets are a loose aggregate of ice and dust, often referred to as a 'dirty snowball'. They have extended orbits, and develop their characteristic tails when they make a close approach to the Sun. The heat from the Sun causes the ice to evaporate or sublimate, creating jets of gas on the sunward side (left). This blasts more ice and dust from the comet, and the solar wind and radiation pressure draws this material out to form the visible tail. Comets are thought to originate either in the Kuiper Belt beyond Neptune, or from the Oort Cloud beyond the outer solar system.