Space is again in the headlines. E-billionaires Jeff Bezos and Elon Musk are planning to colonize Mars. President Trump wants a "Space Force" to achieve "space dominance" with expensive high-tech weapons. The space and nuclear arms control regimes are threadbare and disintegrating. Would-be asteroid collision diverters, space solar energy collectors, asteroid miners, and space geo-engineers insistently promote their Earth-changing mega-projects. Given our many looming planetary catastrophes (from extreme climate change to runaway artificial superintelligence), looking beyond the earth for solutions might seem like a sound strategy for humanity. And indeed, bolstered by a global network of fervent space advocates-and seemingly rendered plausible, even inevitable, by oceans of science fiction and the wizardly of modern cinema-space beckons as a fully hopeful path for human survival and flourishing, a positive future in increasingly dark times. But despite even basic questions of...