Comets have played an interesting role in the history of astronomy. Since antiquity, many cultures saw them as omens or spirits, portending good or bad news for kings, queens and emperors. Over the past few hundred years, however, astronomers have studied them intently to understand the science behind these visitors to the inner solar system. Today, we know that these ghostly apparitions in the sky are dirty balls of ice and rock blasting through space, scattering dust and gases as they go.