By the time Neil Armstrong and Buzz Aldrin stepped out onto the Sea of Tranquility in July 1969, the moon was well-explored territory. For 10 years, robotic spacecraft from the Soviet Union and United States had been photographing, prob- ing, and mapping it from lunar orbit and from the surface. Tranquility alone had already been scouted twice-by Ranger 8 in 1965 and by the Surveyor 5 lander in 1967.
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