It's not one of the historic space missions we celebrate year after year, but on July 20, 1965, Zond 3 took some of the first high-quality images of the far side of the moon as it flew by on its way to Mars.
The moon is tidally locked with the Earth, which means the same side faces us all the time. So ever since Earthly beings have looked skyward at our only natural satellite they've seen the same face we see nearly every day. No one knew what was on the other side.
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On Oct. 4, two years to the day after the Soviet Union launched Sputnik, it launched Luna 3. The third mission in the Luna program, the spacecraft reached the moon on Oct. 7. Its onboard cameras sprung to life and took 29 pictures of the lunar far side, covering about 70 percent of the unseen face. The spacecraft managed to send 17 noisy pictures back to its Soviet handlers on Earth before contact was lost on Oct. 22.
The pictures from Luna 3 weren't great – surface features were hard to distinguish from the static bands running over the image. But it was still a picture of the moon's far side, a first for the Soviet Union and humanity.
Zond 3 would add to our scant knowledge of the hidden face of the moon, though this was a secondary goal. The mission was one in a small series of Zond flights designed to prove the technology that would enable the Soviets' Mars and Venera probes. Zond 3 was a repeat of a mission that had failed in November of 1963, one designed to prove the Soviets could communicate with a spacecraft across the Earth-Mars distance. On this distance test, Zond 3 was outfitted with two cameras, infrared and ultraviolet spectrometers, a magnetometer, a cosmic ray detector, a solar particle detector, and a meteoroid detector.
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Following a fast outward trajectory to Mars, Zond 3 reached the moon in just 33 hours after launching on July 18, 1965. Its camera sprung to life as it passed by the far side. Focusing on the 30 percent that Luna 3 hadn't seen with cameras aligned with lenses able to measure infrared and ultraviolet light, Zond 3 took one picture every two minutes and 15 seconds for 68 minutes.
Immediately afterwards, the pictures were sent to the onboard scanner for development. From there they were transmitted back to Earth nine days after the encounter. The first transfer in mid-August was at a fast, low quality rate that took just two minutes and 15 seconds for each picture to make it back to Earth. A second, higher quality transfer in September took 34 minutes per picture. The images were transmitted a third time in October at an even higher rate.
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And they were fantastic. For the first time, we could distinguish features on the unseen face of the moon – mountain ranges, mare, and craters.
The last communications from Zond 3 came on Sept. 16, 1966 when the spacecraft was just over 95 million miles away from the Earth. The mission never made it to Mars - as it missed a critical launch window - but Zond 3 was an important spacecraft test with the mission highlight of imaging the lunar far side.
Image: One of the Zond 3 observations of the lunar far side. Credit: Public domain