Until October 1959, nobody had seen the far side of the Moon. The Moon turns once on its axis for every trip around Earth, so the same half always faces us. The other half was blank on every map.
The Soviet Union launched a probe to change that on 4 October 1959, two years to the day after Sputnik. The press called it the Automatic Interplanetary Station; the name Luna 3 came after 1963. NASA's history of deep space missions, written by the historian Asif Siddiqi, gives its mass as 278.5 kilograms and calls it the first spacecraft powered entirely by solar panels.
On 6 October the probe swung past the Moon's south pole about 7,900 kilometers away and climbed above it. At 03:30 UT on 7 October, about 65,200 kilometers from the Moon, its camera started shooting. It had two lenses side by side, a 200 millimeter one for the whole disk and a 500 millimeter one for detail. In about 40 minutes it took 29 pictures.
The film could never come home, so the probe carried a small automatic darkroom that developed, fixed and dried the 35 millimeter film. A beam of light then scanned each frame in up to 1,000 lines, and the signal went to Earth by radio, much like a fax. After a few failed tries, ground stations received the pictures. Seventeen were usable, and together they covered about 70 percent of the far side.
The hidden half looked different. It had far fewer of the dark plains, called maria or seas, that make the face we know, and scientists had to rethink how the Moon formed.
The film had a story of its own. After the Cold War it came out that the Soviets had taken it, unused, from an American reconnaissance balloon of Project Genetrix that had drifted into Soviet territory. The film was made to resist heat and radiation, and it had been stored at a military academy in Leningrad when the camera makers found it. Controllers later lost contact with Luna 3, and it probably burned up in Earth's atmosphere in early 1960.
