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On the 10th of January 1956⁠—about a decade into the Cold War and about a year into the Space Race⁠—the United States Air Force launched the first vehicle in its top secret Genetrix program. The vehicle was a balloon⁠—an enormous, 200-foot-tall, 100-foot-wide helium balloon⁠—the first of hundreds that the US would ultimately launch from sites in Scotland, Norway, Germany, and Turkey. Upon release, each balloon ascended into the stratosphere, where the winter jet stream was perfectly situated to carry it over and across the interior of the USSR. A coffin-sized gondola dangled from the bottom of each balloon, housing a set of downward-facing high-resolution cameras. Whenever an onboard photocell detected that the surface below was illuminated by daylight, these cameras snapped periodic photographs. The Genetrix balloons were some of the original high-altitude spy cameras⁠—precursors to spy planes and satellites.

Whenever a balloon cleared Soviet airspace, the US Air Force sent an encoded radio signal that would detonate a small explosive charge on the gondola’s attachment line. If all went according to plan, a specially equipped C-119 airplane would be loitering in the nearby airspace, ready to snag the parachuting payload of photographic film in mid-air. Once retrieved, the film was sent back to the states to be developed and analyzed.

The Genetrix balloons were designed to be practically invisible to radar, using very thin balloon film and a gondola much smaller than a typical aircraft. And this might have worked were it not for the fact that one of the steel rods in the balloon rigging was 91 centimeters long. US Air Force engineers didn’t realize it at the time, but 91 centimeters happened to correspond to one of the frequencies used by Soviet early-warning radar. This caused the otherwise inconsequential rod to resonate and glint like a mirror on Soviet radar screens.

Soviet leaders were understandably annoyed when their military pilots reported back regarding the nature of these radar reflections. US officials replied that these were innocuous weather balloons for the study of cloud formations, a claim which was roundly ridiculed. During the day, there was little the Soviets could do about it apart from political posturing⁠—the balloons cruised at 55,000 feet, which was higher than Soviet weapons could reach. But MiG fighter pilots soon discovered they could shoot the balloons out of the sky at sunrise. The chill of the night robbed the balloons of some of their buoyancy, and they dipped down into weapons range.

The Genetrix program lasted only 27 days. It had originally been planned to continue indefinitely, but president Eisenhower cancelled any further spy balloon launches due to the Soviets’ strenuous diplomatic protests. Of the 500 or so spy balloons that were launched, only about 50 camera gondolas were successfully recovered by the US Air Force. These provided over 10,000 reconnaissance photos of inland Soviet Union and China, including first peeks at nuclear and radar facilities.

The Soviets recovered a number of the gondolas themselves, and engineers began to dissect them, seeking useful information. To their surprise, they found something inside that happened to solve a little problem they had been having with one of their upcoming space missions: temperature-resistant and radiation-hardened photographic film.

The specialized film had been necessary in the Genetrix balloons due to the high altitudes involved⁠—up to 100,000 feet. Soviet engineers still didn’t know how the Americans made the film, but that didn’t stop them from repurposing it for their own spacecraft. On the 4th of October 1959, forty frames of this film were nestled inside a space probe nestled atop a rocket at the Soviet Baikonur Cosmodrome launch site in Kazakhstan. This space probe⁠—or Automatic Interplanetary Station⁠—would later come to be known as Lunik 3. It was two years to the day since the Soviets had launched Sputnik 1, history’s first artificial satellite. And less than a month prior, the Soviets had celebrated the first spacecraft to actually come into contact with the moon. That spacecraft, Lunik 2, had deliberately crashed into the moon, peppering the crash site with patriotic Soviet pennants. It would be another decade before any human set foot on the moon.

The modified SS-6 Sapwood rocket carried its payload up and over the Earth’s north pole. It flung Lunik 3 on a trajectory intended to intercept the moon in two days. The probe was a small cylinder about four feet (1.22 meters) long and wide, bristling with antennae and laminated with solar panels. It was designed to be directly radio-controlled from Soviet stations on Earth. There were no rocket motors for acceleration or major course corrections, just a few small gas jets for attitude control.

As Lunik 3 crossed the expanse of space to meet the moon, ground operators struggled to establish reliable communications with the probe. It was not responding to commands, and its telemetry data was garbled, preventing scientists from verifying the spacecraft was on the correct course. Radio specialists were rushed out to the remote communications sites to identify the problem. After some troubleshooting they discovered that the operators at separate sites were unknowingly sending conflicting commands. Overseers successfully installed some cooperation, and the probe returned a burst of solid telemetry indicating a good flight path and speed toward the moon’s southern pole. On the 6th of October, Lunik 3 dipped under the pole, blocking all of its communications with Earth and leaving the probe to its automatic systems.

Billions of years ago, the moon rotated around its axis faster than it does today. But early in the moon’s history, the drag of Earth’s gravity caused the moon’s rotation to slow to what appears to be a halt from an Earth perspective. One hemisphere of the moon always faces the Earth, and the other always faces away, with Earth’s gravity acting as a tether. In other words, the moon revolves once around the Earth in the same time it takes to rotate on its own axis⁠—a phenomenon known as tidal locking. Because of this, in 1959, although humankind had been staring up at the moon for our entire existence, we had only ever seen about half of the moon’s surface. Owing to various view angles from different parts of the Earth and a slight wobble in the moon’s orbit, the most ambitious jet-setting astronomer could have seen a maximum of 59 percent of the surface of our planetary companion. The other 41 percent was a complete mystery, hence the phrase “the dark side of the moon.”

Isolated in the radio shadow on the opposite side of the moon, Lunik 3 began to operate on its simple automatic systems as the moon’s gravity bent the probe’s trajectory northward. Back on Earth, only a sliver of moon was visible, which meant the far side of the moon would be in almost full daylight. When a photocell onboard Lunik 3 detected that the moon’s illuminated surface was in the line of sight, a pair of hinged camera doors opened like eyelids, and this primitive Soviet space robot became the first Earthling to glimpse the far side of the moon.

Lunik 3 began snapping an automatic sequence of photos with its wide-angle and zoom lenses, advancing the strip of film from its lead-lined cartridge. The camera was fixed to the spacecraft, so in order to capture images from various angles, Lunik 3 rotated itself up and down and side to side between photos using small gas jets. This went on for 40 minutes, exposing 29 squares of Genetrix film, and imaging 70 percent of the far side of the moon.

Unfortunately, unlike the Genetrix balloons from which the special film had been salvaged, the Soviets had not yet discovered a means to physically return the exposures from space to a development lab on Earth. Their solution was to process film in the same chemical-bath-and-dry method used on Earth, but inside a miniature, automatic, zero-gravity darkroom. Inside its pressurized hull, Lunik 3’s Yenisey photographic system slid the strip of exposed film between two rubber seals into a reservoir of thick, single-stage developing fluid, then out another sealed slit into the film dryer. Considering the quality of the cameras and film, the resulting black-and-white photographs must have been spectacular.

Just as the Soviet engineers had planned, the moon’s gravity curved Lunik 3’s path such that it emerged from behind the moon’s north pole heading back in the direction of Earth. This trajectory had been necessary to ensure that the probe would return to within radio range of the Soviet Union’s territory in the northern hemisphere. Calculating this trajectory had required the then-considerable computing power of the new Strela-1 electric computer at the Steklov Institute of Mathematics. The trajectory the mathematicians designed was the first applied use of a gravity assist maneuver, the most complex space maneuver ever attempted at that time.

Lunik 3 reestablished communication with Soviet operators on the 8th of October on its way back toward Earth. The operator on Earth directed Lunik 3 to send its first image. The probe’s internal mechanisms oriented the first frame of film in front of a bright bulb inside, projecting a small portion of the image onto a photomultiplier, a light-sensitive vacuum tube. Lunik 3 then transmitted the lightness and darkness information line-by-line via frequency-modulated analog signal⁠—in essence, a fax sent over radio. This enabled Soviet scientists to retrieve one photographic frame every 30 minutes or so. Due to the distance and weak signal, the first images received contained nothing but static. In subsequent attempts in the following few days, an indistinct, blotchy white disc began to resolve on the thermal paper printouts at Soviet listening stations.

On the 18th of October⁠—eleven days after the photos were taken⁠—the noisy image returned from Lunik 3 finally revealed some details and contours of the far side of the moon. Operators instructed Lunik 3 to shuffle through the developed images and send each one. The probe successfully scanned and faxed either 12 or 17 (reports vary) wide angle and close-up photos before Lunik 3 stopped responding on the 22nd of October.

These first images, though grainy, revealed some surprises. It had been generally assumed that the far side of the moon would be quite similar to the near side⁠—with relatively bright highlands mixed with the darker regions known as “maria” or “seas,” so named by early astronomers who presumed that they were actual seas. Later astronomers realized that the maria are plains of basalt rock caused by ancient lava flows. Contrary to expectations, the Lunik 3 images revealed that very little of the far side of the moon is covered in the darker maria⁠—just one percent compared to the near side’s 31 percent⁠—meaning that the phrase “the dark side of the moon” was completely at odds with reality. It didn’t even look like the same moon. The photos also showed that the far side has a much greater number of impact craters, mostly because there were so few maria to erase older impact craters.

This inconsistency of maria coverage on the two hemispheres⁠—known to astrophysicists as the Lunar Farside Highlands Problem⁠—is still a subject of scientific debate. One thing we do know is that the difference is not due to the pull of Earth’s gravity. As the moon spins around the Earth, its far side experiences the exact same amount of outward acceleration due to centrifugal force (and, before the science nerds jump in and claim there’s no such thing as “centrifugal force,” the force does exist in a rotating reference frame). Subsequent surveys of the moon using orbiting gamma-ray spectrometers found that there is a higher concentration of heat-producing elements on the hemisphere closer to Earth, but this suggestion cannot fully account for the differences. Another proposed explanation is that Earth once had a second, smaller moon, and that the two collided a few million years after their formation. The merged moon would have had a thicker crust around the impact point, reducing the likelihood of maria-producing volcanoes from reaching the surface in those areas.

More recent research points to an idea which hinges on the most likely explanation for the moon’s origin⁠—the Giant Impact Hypothesis. According to this hypothesis, shortly after the solar system began to form, while the planets were still young, malleable, molten globes, Earth shared its orbit with a Mars-sized planet dubbed Theia. Theia would have been about 60 degrees ahead of or behind Earth in its orbit around the sun, in one of Earth’s gravitationally-friendly Lagrange Points. Over time, the gravity of planetary neighbors nudged the smaller Theia from the stability of the Lagrange point, and set the planet on a collision course with Earth.

About four and a half billion years ago, Theia crashed into the Earth with a glancing blow at about four kilometers per second, vaporizing rock and splashing magma from both planets out into orbit. As gravity crushed the two planets into one, it also gradually assembled the orbiting debris into what would become the moon. The moon became tidally locked with the Earth almost immediately, and began to cool. But the Earth remained extremely hot⁠—more than 2,500 degrees Celsius⁠—due to its much larger size. This tremendous heat radiated out into space, broiling the exposed side of the moon for millions of years.

The photos from Lunik 3 also hinted at an enormous crater near the south pole on the far side of the moon, one which later surveys confirmed. This crater is now known as the South Pole–Aitken basin, one of the largest impact craters in the solar system. It is 2,600 kilometers (1,616 miles) wide, covering an area equivalent to over half the size of the United States. It is twice as deep as Mount Everest is tall, and it is the only place on the moon where the lower crust lies exposed.

Moscow shared the photos of the far side with the world on Monday, the 26th of October 1959, throwing yet more fuel on the increasingly lively space race.

In 1960 the Soviet Academy of Sciences published the first-ever atlas of the far side of the moon, outlining the major features they had discovered. It would not be for another eight years⁠—on the Apollo 8 mission in 1968⁠—that human eyes would look upon the far side of the moon directly.

Even after losing contact with Lunik 3, the probe continued to wander the Earth-Moon system under the combined gravitational influences of both bodies. Sometime in the early 1960s⁠—no one is certain of the exact date⁠—the probe entered Earth’s atmosphere and burned up as a falling star.

Astrophysicists today look at the far side of the moon as an excellent future location for radio telescopes to explore the cosmos. This is due to the one sense where the far side of the moon can be accurately described as the “dark” side: in the radio spectrum. Since the far side of the moon always points away from the Earth, the moon provides natural, ready-made shade from the glare of Earth’s radio transmissions. Furthermore, many of the plentiful, small, bowl-shaped craters would make natural, ready-made dishes to build stationary telescopes similar to the massive Arecibo observatory in Puerto Rico.

Its excellence as a radio telescope site notwithstanding, humans are unlikely to be installing any facilities on the far side of the moon anytime soon. In the history of space travel we have sent just twelve men to the moon to inspect it in person. And of those missions, every landing was on the near side of the moon. So although we have photographs of the far side, and we’ve seen it from far above, it remains a strange, vast place where no human being has ever set foot.

Update 02 January 2018: China has reached a milestone in space exploration, landing a vehicle on the far side of the moon for the first time in history, the country’s space agency announced.

An earlier revision of this article misstated that Lunik 3 contained “an array of 1,000 rows of light-sensitive vacuum tubes.” This was due to a misreading of the source material, the text has been corrected. Oopsy and/or daisy.

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