Medium-future didactic novel about the terraforming of Mars and the human impulse to violence, from one of sf's longest-serving and most distinguished professionals. Cold, arid, airless Mars can be terraformed--for a hefty price--by bombarding its surface with comets captured from the Oort Cloud, a huge collection of dirty snowballs orbiting far beyond Pluto. Young Martian Dekker DeWoe longs to become an Oort ``miner'' as his father once was. Yet when his dream comes true and he travels to Earth to begin training, Dekker learns that the price Mars has to pay for Earthly finance may be decades of near-slavery while its product sales are undercut by new Earth-orbiting habitats. Schooled in nonviolence and cooperation, Dekker finds himself increasingly at odds with current Earth attitudes: armies have been banished; social-awareness training is required; but full-sensory ``virts'' offering intimate vicarious experiences are freely available--including those featuring violence and war. But just as it seems that the Oort project will be canceled outright, Dekker's training schedule is accelerated; soon he and his classmates join a Mars-orbit station whence they direct incoming comets. The proffered explanation for the speed-up doesn't wash, and gradually Dekker uncovers a conspiracy among his colleagues: Their monstrous purpose is to smack a small comet into Japan, whose financial dealings (coupled with their own racism) they blame for jeopardizing the Oort project. Pohl's message--that humans must be tutored in nonviolence; simply abolishing war is not enough--meshes flawlessly with the story. Otherwise, a professional recycling of mostly standard notions, with above-average characters and a rather thin plot.