Seeing the far side of the Moon for the first time

Credit: NASA APOD

The launch of Sputnik was a turning point for our species. When the small satellite went into orbit around the Earth on October 4, 1957, it marked the beginning of the space age and a new human adventure with the Soviet Union decisively in the lead. For most Soviet citizens, Sputnik was a source of pride, and for some Air Force pilots, it brought about the dream of flying beyond the atmosphere. But not Yuri Gagarin. Sputnik didn't capture his imagination; he was happy flying in the atmosphere. However, when Luna 3 launched two years to the day after Sputnik, it made the young Air Force pilot think about flying beyond Earth.

Luna 3 was one of the first probes ever sent the Moon. And even though it launched on the heels of Sputnik, which certainly changed the way people around the world thought about space, Luna 3 brought another wave of change, as it was the first spacecraft to return images of the far side of the Moon.

A modern picture of the near side we're familiar with, and the unseen far side.

The Earliest Exploration of the Moon

The jump from Sputnik to Luna was significant. In two years, the Soviet Union went from launching a small, 182-pound satellite into a low Earth orbit to sending a nearly 800 pound spacecraft a quarter of a million miles away.

The program kicked off with the launch of Luna 1. The spherical spacecraft decorated with protruding antennae and science instruments launched from Tyuratam on January 2, 1959. Radio equipment, a tracking transmitter, and a telemetering system enabled scientists on the ground to follow the spacecraft’s translunar progress while five science instruments (including a magnetometer, a Geiger counter, a scintillation counter, and a micrometeorite detector) gathered data about interplanetary space along the way.

Luna 1 was supposed to impact the Moon, but it never reached its target. Thirty-four hours after launch, on January 4, the spacecraft came within 3,725 miles of the lunar surface before settling into a heliocentric orbit somewhere between the orbits of Earth and Mars. Its failure to impact the Moon was a small loss, and Luna 1 was the first spacecraft in history to leave the Earth’s gravitational influence. It was a major first for the Soviet Union in the earliest days of the Space Race.

The Soviets followed this success with another spectacular mission just eight months later. On September 12 (September 13 in Moscow), Luna 2 rocketed out of Earth’s atmosphere, escaping the influence of our planet’s gravity on a path towards the Moon. This spacecraft was a near copy of its predecessor: similarly spherical loaded with telemetry systems and science instruments to measure interplanetary space. As it approached the Moon, Luna 2 found that the Moon has no appreciable magnetic field and is without a radiation belt. It also released a bright orange cloud of sodium gas, which served a dual purpose: it was both a tracking aid and a way for scientists to study how gases behaved in the vacuum of space.

The Luna 2 spacecraft, which was very similar to Luna 1.

Thirty-three and a half hours after launch,Luna 2abruptly fell silent. But this wasn't cause for concern. It was cause for celebration, a clear indication that the spacecraft had reached its target and impacted the lunar surface. The exact spot has never been confirmed, but telemetry data puts Luna 2’s impact site in the Palus Putredinus region of the Moon, an area roughly 0 degrees longitude and 29.1 degrees latitude. With this crash landing, the Soviet Union secured another first: the first spacecraft to land (in a manner of speaking) on another body in the solar system.

Luna 3 followed in short order. As was typical of the Soviet Union’s approach to space at the time, this mission didn't simply duplicate a previous success, it scored another major first. This spacecraft was armed with cameras and an automatic system that would allow it to photograph the unseen far side of the Moon.

Luna 3

From the Earth, we only see one side of the Moon. Through all lunar phases throughout the year, the face we see never changes. And while images of the lunar far side have been widely published in recent years, in 1959 no one knew what the other side of the Moon looked like. We see one side of the Moon because of the curious relationship between its rotation around its own axis, and its orbit around the Earth. The Moon rotates around its axis exactly once in the time it makes a full orbit around the Earth, leaving one face “locked” towards us while the other always faces into space.

Luna 3 was designed to shed light on this mysterious far side. Luna 3 spacecraft differed from its predecessors slightly; it was cylindrical rather than spherical, shaped like a canister with hemispherical ends and a wide flange near the top. But it was still small, just 51 inches long and 47 inches in diameter at its widest point. The bulk of its body was just 37 inches around.

The spacecraft’s main section, lined with solar cells to power the batteries, was hermetically sealed and pressurized at 0.23 atmospheres to protect the instruments, which included radio equipment, propulsion systems, batteries, and gyroscopic units for attitude control. Temperature regulation inside the spacecraft was also a concern, but the Soviets employed a clever solution. If the internal temperature rose above 77 degrees Fahrenheit, panels would pivot open to expose a radiating surface to the frigid vacuum of space.

The Luna 3 probe, a change from its predecessors.

Imaging the Unseeable

Luna 3’s main cargo was housed in the upper portion of the spacecraft under a protective cover. Called Yenisey-2, Luna 3’s imaging system consisted of a dual lens camera: one a 200mm, f/5.6 lens and the other a 500mm, f/9.5 lens. Also included was an automatic film processing unit and a scanner. The scanner's memory bank was analogue and limited; the camera had just 40 frames of temperature and radiation-resistant 35mm isochrome film. But it was enough. Luna 3 was designed to photograph the Moon from a high enough altitude that the 200mm lens could image the full disk of the Moon and the 500 mm could take more detailed images of specific regions on the surface.

The mission got off to a good start after launch on October 4, 1959, but things soon turned rocky. Telemetry from the spacecraft revealed its signal to be about half as strong as expected, and the interior temperature increased to over 100 degrees Fahrenheit. The problems didn't last: the spacecraft’s spin axis was reoriented and some equipment shut down, two measures that served to lower the internal temperature to less than 80 degrees.

When Luna 3 was about 40,000 miles from the Moon, the spacecraft’s orientation system was switched back on, and its rotation was stopped. Its lower end was left pointing at the Moon, the far side of which was illuminated by light from the Sun. With the spacecraft in position, the automated photographing sequence began. A photocell on the spacecraft was designed to detect the Moon, causing it to spin and point its upper end (the end with the camera) towards the lunar surface. Only then would the cover open to give the camera a clear shot.

On October 7, the camera sprang to life. The photocell on the upper end of the spacecraft had detected the sunlit far side of the Moon, and the photography sequence started. The first image was taken at an altitude of just under 40,000 miles from the lunar surface, and by the time the last picture was taken 40 minutes later, the spacecraft had put 2,000 more miles between itself and the Moon. In all, Luna 3 snapped 29 pictures of the Moon covering 70 percent of its far side.

The second image Luna 3 took of the far side of the Moon.

Seeing the Unseen

Once Luna 3 had completed its imaging goal, the spacecraft resumed its spin, passed over the Moon’s north pole, and followed this trajectory back towards the Earth. But there was work to be done still.

Following commands from Earth, the exposed film was moved to Luna 3’s on-board processor where it was developed, fixed, and dried. Its next stop was the scanner, where a bright spot produced by a cathode ray tube projected through the film onto a photelectric multiplier. As the spot scanned across the film, the photomultiplier converted the light’s intensity into an electric signal. It was in this form that the pictures were designed to be sent back to Earth, since Luna 3 wasn't designed to return home in one piece. One image frame could be scanned with a resolution of 1000 lines. Transmission could be done at a slow rate for large distances from Earth, and a faster rate at closer range.

The first attempt to transmit the photographs to Earth was unsuccessful owing to low signal strength. But things got better as Luna 3 got closer to Earth. Eventually, 17 noisy photographs were successfully transmitted by October 18. Four days later, the Soviets lost contact with the spacecraft. It’s believed to have reentered the atmosphere and burned up in the spring of 1960, but there’s a chance it actually survived in orbit until sometime in 1962.

While the first images of the Moon’s far side might not have had the effect of the famous Earthrise picture taken by the Apollo 8 crew in 1968, these grainy black and white images are incredibly important ones. Not only had the space age begun with man-made object flying through unreachable realms, humans were seeing things that no one in history had ever seen before.

Further Reading: Zond Missions page, the Lunar and Planetary Institute.

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