The final minutes of “Mars Dead or Alive” promise to be as gripping as a movie cliffhanger and the best reality TV going. The PBS “NOVA” special details the building and launch of a Mars-bound rover set to land on the red planet Saturday one day before the program airs. The ending, at this moment, is literally a blank. The film’s final 2 1/2-minute sequence will be hastily added when the fate of Spirit is known. The title is no exaggeration: Mars is a “graveyard” for half the spacecraft sent there, we are told.