Poul Anderson 
Three Worlds to Conquer 

A dozen years had passed since Mark Fraser and his family had fled to Ganymede, hoping to find the peace and freedom which had eluded them on Earth. Now violence and terror had pursued them to their new-found home: Captain Swayne and his battleship Vega had made landfall on Ganymede and were using its resources to build missiles with which to hold Planet Earth to ransom. For the second time, Mark Fraser was a marked man, running for his life. His only chance of escape was to a third planet: Jupiter. But as Fraser knew only too well, Jupiter itself was the scene of conflict and carnage. And so far no human being had ever breathed its atmosphere and lived to tell the tale!