 Fifty-five years ago today, Neil Armstrong and Buzz Aldrin landed on the moon. NPR's Nell Greenfield-Boyce reports on NASA's efforts to return astronauts to the lunar surface. It was July 20, 1969, when Armstrong piloted the lunar module Eagle down to a site called the Sea of Tranquility. Tranquility Base here. The Eagle has landed. Roger, Tranquility. We copy you on the ground.