NASA 's Voyager 2 probe made a quick flyby of the planet with its closest encounter on August 25 , 1989 , and was the last planet to have been visited by at least one spacecraft . Voyager 2s closest approach to Neptune occurred on August 25 , 1989 . Since this was the last major planet the spacecraft could visit , it was decided to make a close flyby of the moon Triton , regardless of the consequences to the trajectory , similarly to what was done for Voyager 1s encounter with Saturn and its moon Titan .