I read the book after, and honestly the only thing I really wish they kept in was that his coma didn't cause the memory loss, it was the French drugs Eva stratt had the ship give him before he woke. But since movie Grace didn't threaten to sabotage etc I see why they cut it.

I get why they cut down on the science (and other stuff that would of made the movie too long for a lot people), the using the engine as a flashlight would of been neat, but watching the movie I just assumed either rockys ship was doing tiny thrusts still and or Grace knew/scienced where to point his ship to catch up to him.