The Peacekeepers. Book 21. Moon Base.

The crew of the Damroyal detects an incoming comet, Wormwood, and begins efforts to divert it using a moon base and rail gun, but they face a dilemma when their plans threaten to cause a disastrous impact on their home planet. Meanwhile, enemies of the peacekeepers exploit the crisis to seize a spacecraft and build a formidable pirate fleet, planning a future confrontation. The peacekeepers must navigate these threats, protect their world, and ultimately find a way to prevent catastrophe.