Anti-Alien Invader Cannon (with possible applications against asteroids)

Everyone has heard of the famous nuclear test where scientists accidentally shot a drain cover out of the solar system, but few have considered the possible practical applications. This is much cheaper and faster than any ground mounted railguns, giant cannon, or even miles long acceleators, but it will certainly kill any people, or even electronics, in the projectile, and emit harmful fallout for those on the ground. What it is good for, however, is shooting alien invaders (or asteroids) who would do far more damage than the fallout from a single nuclear test. The applications of a solar escape-velocity hubcap are obvious against an alien enemy, but you may ask what good it would do against a kilometers wide asteroid. The solution: Don't shoot a drain cover. Instead, use a barely subcritical mass of plutonium surrounded with a much thicker layer of steel. No detonator is necessary: when it hits the alien spacecraft (or asteroid) the sheer impact performs the same function as the explosives in an ordinary plutonium bomb. this would act as a nudge to push it out of earths way. Because of the simplicity and inexpensiveness of the launch aparatus, many nukes could be shot at it, each with only a few kilotons of power, to avoid splitting it into multiple city-killers.

Advantages: proven concept, cheap, could be mobilised from our current state to shooting at aliens in a few days if need be.

Disadvantages: inaccurate until the first few shots are sufficiently modeled and learned from, fallout, name does not make cool acronym yet.