Mining the Oort by Frederik Pohl

The story of an idealistic young man from Mars who becomes an Oort miner. A project is started to give Mars an atmosphere by crashing comets from the Oort cloud onto its surface. The miners are the people who find the comets, attaches engines to them, and direct them to land on Mars. The young man becomes a little wiser during his training, and in the end thwarts an evil plot.