In 1964, Mariner 4 revealed something about a planet that had till then seemed to be a dead and uninteresting world. It showed a cratered Martian surface reminiscent of the moon.
By 1971, Mariner 9 showed a much more complex surface, including what appeared to be dry river beds. Water was a necessary ingredient for life, and this raised speculation about past life on Mars.
In 1976, two Viking spacecraft landed on the surface of Mars and began a series of experiments that yielded a huge amount of information about the Red Planet.
Two decades later, Pathfinder and its Sojurner rover provided spectacular images from the surface of the Red Planet, and the Mars Global Surveyor and Mars Odyssey returned scientific data and images from orbit, including evidence of recent water activity.
As is the case with the moon, there is no doubt that these robotic spacecraft will lead the way for human exploration of Mars -- it is only a matter of when.
Image: A photograph taken from Phoenix's Robotic Arm shows the Martian terrain.
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