We could park a missile in orbit around Pluto. This missile would be equipped with traditional 100 MT warhead and a cobalt-60 dirty bomb.

If aliens were to attack us, this missile would be launched towards their home planet. Over decades, it would accelerate to few percent of light speed, giving it immense kinetic energy. It would be equipped with stealth technology and it would emit no radio signals as to stay hidden for as long as possible.

After a while, it would impact their home planet with radiation and explosive power to wipe out all life.

Tell my why we shouldn't construct such a deterrent?