It was mentioned by someone a while back but The ships are upgradeable. Someone raised the point that maybe this means you can't get to certain destinations e.g. mars, until you've earned it i.e. upgraded your ship. This would fit in with Bungie repeatedly saying you own and earn all your gear etc in Destiny.

Space travel is not being enhanced in with any sort of play mechanic, so ship speed shouldn't matter, as all guardians will travel at the same speed. And I wouldn't discount new galaxies to explore, though count on it after Bungie has put everything they've got into developing our own galaxy.