Planet of the Damned

the long version

Man always expands his horizons, when Earth is full, man will venture out to space and colonize other worlds. Governments fail and contact with colonies is lost - leaving them to fend for themselves. When government is stabilized after many centuries and exploration along with searches for the old colonies is made. Mutations are likely in such circumstances and this is one of the themes of this story - with unexpected twists, turns and the likely destruction of a planet ruled by madmen because they are not only a threat to themselves, but to others as they have cobalt bombs and every intention of using them on a neighboring world. Fast paced and full of action makes this a very good read.