Something always bothered me and maybe someone can answer this

When Rocky and Grace are trying to B-line it out of Adrian’s atmosphere and gravitational pull - why do they stop? Seriously, I mean why don’t they just keep going and make sure they’re clear? They seem to want to hit the Astrophage burners and stop them ASAP. Am I missing something? It would have of course completely stripped the story of its most devastating and dramatic scene, but in an astrophysics sense it’s making no sense to me.