This is a book on comets – visually stunning dirty space snowballs of mostly ice and dust that formed during the birth of the solar system 4.6 billion years ago. Most comets have stable orbits in the outer reaches of the solar system past the planet Neptune, or in the still-more remote Oort Cloud. But if destabilized by gravitational forces, they may move into the inner solar system – with some risk of collision with the Earth, and some justification for the dread, fear, and awe they have inspired in many different cultures and societies around the world and throughout time. The depiction of comets in popular culture is firmly rooted in the long Western tradition of seeing comets as harbingers of doom and as omens of world-altering change. Comets are thus horrifying and exhilarating, at the same time, with a profound impact on earthly societies and cultures.