Wonder why the slingshot mechanism can't be used to propel us back to earth from Mars, cf. Mangalyaan. Come to think of it, why shouldn't the trip to Mars also be accomplished that way, lesser payload, would mean greater reserve fuel for the journey back. Also in the remote chance that we discover a black hole lurking in our own backyard, then we could plan a trip to it using Elon's rockets. Think Interstellar.