How to solve both problems? Mark Fraser, Ganymedean colonist, hijacks an as yet unflown ship designed to land on Jupiter. Chased by a missile, he descends into the Jovian atmosphere and bounces along a denser atmospheric layer that destroys the missile. Then he simply lands on the invaders.
Next, he returns to Ganymede, pretending to surrender, and exits the hijacked ship, having set the autopilot to fly the ship, its hold now full of dense and explosive Jovian atmosphere, straight at the battleship. Solution to second problem and end of novel. As I say, neat.
Ironically, Mark had thought, "Life isn't a story book...There are no happy endings. It just goes on." (p. 136) For the reader, the story does end happily. For Mark, life goes on. He returns to his wife and family while the woman with whom he has shared the Jovian adventure feels it is better that she return to Earth.