Photo of Comet Hale-Bopp taken in March 1997 near Pazin, Croatia. Image: P. Salzgeber/Wikipedia
A comet is a small object orbiting the Sun that is composed of a mixture of dust and ice. When it passes relatively close to the Sun it heats up and begins to outgas, producing a visible “coma”. Due to the influence of solar radiation the coma is elongated in a direction away from the Sun producing a tail; a feature that is characteristic of comets as viewed from Earth. Cometry material is of great interest to scientists as it is thought to be essentially unchanged since the birth of the Solar System. A significant amount of new information about comets has been learnt as a result of the NASA Stardust and European Space Agency Rosetta missions.