July 20, 1976, will be forever remembered by space enthusiasts. On that day, Viking 1 became the first U.S. spacecraft to land on another planet – in this case, Mars (the USSR Venera 9 spacecraft landed on Venus in 1975). That lander, and Viking 2 which followed it Sept. 3, 1976, paved the way for more complex missions later on, which would begin to finally unlock some of the secrets of the mysterious Red Planet. The two Viking landers, and their counterpart orbiters, were genuine trailblazers, opening up the vast Martian landscape to robotic and human eyes for the first time.
Read MoreViking remembered: Celebrating the 40th anniversary of the first search for life on Mars