UNN railgun platform

Orbital weapons hold station over Earth: crewless platforms, each one carrying a railgun, a reactor, a control unit and a magazine. A railgun is a mass driver. Electroconductive rails accelerate a dense metal slug to enormous speed, and mass and velocity do the work a warhead would otherwise do, punching clean through a target rather than exploding against it. Firing takes a large charge and a moment to build it. Thruster modules at the rear absorb the recoil and hold or shift the platform's orbit, while reaction control thrusters across the chassis turn and aim it. Firing solutions are fed in from a command station elsewhere. These are the top of the United Nations Navy's planetary defenses, built to shoot first as readily as to shoot back.