| − | A comet is a ball of ice, typically having a diameter of about five kilometers. Comets typically orbit stars and can develop a tails made of ice, gas and dust which can be millions of kilometers in length. | + | A comet is a ball of ice consisting of frozen gases as well as water, typically having a diameter of less than five kilometers. Comets can develop a tails made of ice, gas and dust which can be millions of kilometers in length. |