story identification - Novel about a comet hitting the moon

I am trying to track down a novel (or novels) about a comet that is heading towards earth. The hero and heroine smash an asteroid into said comet and it ends up hitting the moon instead. The end of the novel (or second novel in the series) have the main characters attempting to get off the moon before the comet hits.

It would have been published sometime in the 80's or early 90's and was before the whole Armageddon/Deep Impact comet movies. I think I remember reading that the author at some point had worked for NASA but I'm unsure about that.