Radiothermal generators already power satellites and space probes. Nuclear fuel has the highest energy density of any fuel currently available to us. Wind and

Radiothermal generators already power satellites and space probes. Nuclear fuel has the highest energy density of any fuel currently available to us. Wind and water power is unavailable when there is no atmosphere or weather cycle. Solar power requires more surface area as you get further from the sun. Solar panels and storage batteries have mass, too. Combustion fuels have mass, and the rocket equation hates superfluous mass in objects launched to orbit. A good engine for low-gravity propulsion would be a nuclear reactor that heats propellant to a very high temperature and then releases it at high velocity. That's not the sort of engine that could get you to orbit, but it does provide a lot of delta-V once you are up there. Propellant can be found outside of gravity wells, and a refuel mission carrying nuclear fuel is cheap to launch on a dollar per joule basis. Nuclear waste does not endanger the environment if there is no environment to endanger. No one is likely to stumble across it accidentally. Criticality accidents would only kill the crew, if there even is a crew. Habitats that are buried deeply to protect against radiation and to contain a bio-friendly environment may not be able to rely on solar alone. They would likely need a large mass of batteries to make it through storms and nights, and an RTG or a full-blown reactor might provide more reliable power for less mass.