Space exploration and off-world commercial activity engage the attention of both enthusiasts and skeptics. Despite differing opinions, what does seem clear is that such activity has increased and is set to expand further--and dramatically so--during the present century. This book explores some of the ethical issues of the emerging space frontier and evaluates the prospects for the medium-range future: Can terraforming of other worlds succeed? Would it be defensible? Should there be limits to mining in space? Do lifeless planets have an integrity that ought to be respected? Could indigenous microbacteria have intrinsic value? Do we have a duty to extend human life? The ethics of sending generation ships on interstellar journeys and the risks associated with seeding other worlds with rudimentary forms of life are also discussed. As exploration is as much about humanity as it is about space, the book concludes with a study of the connection between the sharing of a home planet and membership of a single moral community.