system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:Why are there no nuclear-powered spaceships?
gpt
It is technically possible and most space vessels already have a small nuclear reactor called Radioisotope Thermoelectric Generator (RTG).

The reasons why we are not using them exclusively for propulsion are mostly political. Project Orion did research a nuclear powered spacecraft and was canceled due to the Limited Test Ban Treaty (LTBT) of 1963 which prohibits any nuclear tests that are not underground.