This idea stems from the Pascal B nuclear test, which placed a 900kg steel plate over a shaft in where a nuclear explosive was detonated: http://en.wikipedia.org/wiki/Operation_Plumbbob#Propulsion_of_steel_plate_cap The plate shot up at an estimated 66km/s, or 6x escape velocity. Say you could shoot blocks of weapons grade Uranium from planet to planet, moon to planet, or vice versa. If I understand correctly, the compression when the block lands on the planet's surface would be enough to cause a fission reaction right? These blocks would be small enough to be almost undetectable before it's too late, but big enough to cause serious damage on landing. Unlike the Pascal B test, the launch would be less of an abrupt blast, and more like a slow acceleration so the Uranium wouldn't blow up inside the shaft/gun barrel from compression. Instead of one block, you could shoot a shotgun blast of blocks to maximize the chances of hitting a target. So this could be used as a weapon, or possibly a means of terraforming a planet. If you could shoot fission/fusion devices accurately to, say, the ice cap of Mars, you could cheaply sublimate enough CO2 to cause a positive feedback loop and heat up the planet. Also, a means of redirecting asteroids on a collision course with Earth. Would this work? I don't know much about nuclear physics beyond the wiki page.