The word ‘‘terraforming’’ conjures up many exotic images and p- haps even wild emotions, but at its core it encapsulates the idea that worlds can be changed by direct human action. The ultimate aim of terraforming is to alter a hostile planetary environment into one that is Earth-like, and eventually upon the surface of the new and vibrant world that you or I could walk freely about and explore. It is not entirely clear that this high goal of terraforming can ever be achieved, however, and consequently throughout much of this book the terraforming ideas that are discussed will apply to the goal of making just some fraction of a world habitable. In other cases, the terraforming described might be aimed at making a world habitable not for humans but for some potential food source that, of course, could be consumed by humans. The many icy moons that reside within the Solar System, for example, may never be ideal locations for human habitation, but they present the great potential for conversion into enormous hydroponic food-producing centers. The idea of transforming alien worlds has long been a literary backdrop for science fiction writers, and many a make-believe planet has succumbed to the actions of direct manipulation and the indomitable grinding of colossal machines. Indeed, there is something both liberating and humbling about the notion of tra- forming another world; it is the quintessential eucatastrophy espoused by J. R. R. Tolkien, the catastrophe that ultimately brings about a better world. When oxygen was first copiously produced by cyanobacterial activity on the Earth some three billion years ago, it was an act of extreme chemical pollution and a eucatastrophy. The original life-nurturing atmosphere was (eventually) changed f- ever, but an atmosphere that could support advanced life forms came about.