We could make a missile few tens of meters in length, hundreds of tons in mass, equip it with Cobalt-60 and park it into orbit around Pluto.

If aliens were to attack us, this missile would be launched towards their home planet. Over decades it would accelerate to relativistic speeds, lets say 5-10% light speed.

Ideally it would emit no radio or electromagnetic signals of any sort, so it could creep as close as possible before being detected. It would be as small as possible as well. It would impact the planet with the strength of hundreds of megatons + the effect of Cobalt-60 radiation, completely destroying all life present on the planet.

Is such a weapon possible and should we construct one as deterrence?