It'd make a good follow-up to Terry Pratchett's book, The Fifth Elephant.

I still like Zapp Brannigan's application of the slingshot effect: "Ah, yes, comets: the icebergs of the sky. By jackknifing off one after another at breakneck speed, we can create a gravity boost, or something." You just know that's what Geordi LaForge was always thinking.