Mars is the most intensively explored planet other than our own. 44 missions have been sent to explore the red planet and 17 have been reasonably successful. The US has been triumphant with the twin Viking Landers in the 1970s, the Mars Pathfinder and Sojourner Rover in the 90s, the twin rovers Spirit and Curiosity in the 2000s and of course Curiosity Rover which is currently exploring Gale Crater. But there is one mission that, for me at least, really stands out as the big science payoff