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What did the Ranger spacecraft missions reveal about the lunar surface?

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near and far sides of Earth's Moon — Moon (Left) Near side of Earth's Moon, photographed by the Galileo spacecraft on its way to Jupiter. (Right) Far side of the Moon with some of the near side visible (upper right), photographed by the Apollo 16 spacecraft.

The Ranger program, conducted by NASA between 1961 and 1965, represented the United States' initial efforts to study the Moon through robotic exploration. While early missions faced significant technical failures, the final three probes—Ranger 7, 8, and 9—successfully transmitted more than 17,000 high-resolution television images of the lunar surface before impacting as designed.123

These close-up photographs provided scientists with unprecedented views of lunar features that were too small to be observed from Earth. A major scientific discovery resulting from these images was the realization that even small-scale lunar features have been largely subdued or smoothed over by the effects of incessant meteorite impacts. The data gathered by Ranger 7 was so significant that its impact site was named Mare Cognitum, meaning "the sea that has become known," to commemorate the new understanding of the lunar surface gained from the mission.23