The mention of the “dark side of the Moon” as a synonym for the lunar “far side” on any online forum inevitably leads to a torrent of negative comments objecting to the term. The typical argument is that the lunar far side, which is not observable from the Earth due to the Moon’s synchronous rotation, experiences the same month-long day-night cycle as the more familiar near side and therefor is not perpetually “dark”. But this argument is specious because of a misunderstanding of the original meaning of the term “dark side of the Moon” which dates back to at least the early 19th century.
The use of the word “dark” to mean hidden, obscured or unexplored dates back to at least Shakespeare’s play, All’s Well That Ends Well. (SCETI)
Before the Space Age, the far side of the Moon was called the “dark side” not because of some misconception that it was perpetually unlit, it was “dark” in the sense that it was hidden, obscured or unexplored – an alternate, figurative meaning of the word “dark” that dates back at least to Shakespeare in his play, All’s Well That Ends Well. This meaning of “dark” is implied in the common idiom “kept in the dark” (i.e. keeping knowledge away from someone) or terms like the “Dark Age” (i.e. a historical period which has few or no records) or “Darkest Africa” used in the late 19th century (i.e. the interior of the continent which was uncharted by European explorers at that time).
The term “dark side of the Moon” became obsolete as a synonym for the far side in October of 1959 when, what Soviet authorities at the time dubbed the “Automatic Interplanetary Station” (or, as it is known today, Luna 3), photographed the previously unseen lunar far side shedding light (in a metaphorical sense!) on this region for the first time. These historic photographs, taken just two years into the Space Age, were the first images acquired by a spacecraft of another world opening up the exploration of the previously unexplored landscapes of our solar system which continues to this day.
Designing the Probe
Viewing the far side of the Moon had long been a dream of astronomers. Throughout the mid-1950s Soviet planners examined a number of possible missions that could secure images of this unseen expanse. When Chief Designer Sergei Korolev of OKB-1 (Experimental Design Bureau 1) formally submitted his plans for lunar exploration in December 1957 after the successful launch of their first Sputnik satellites, lunar far side photography came second only to the easier goal of a lunar impact (see “The First Race to the Moon: Getting Off the Ground”). The coauthor of the lunar plan, Mikhail Tikhonravov, ran the group at OKB-1 that would design and build the first series of lunar probes.
Tikhonravov (left) and Korolev in 1947 at a celebration of the 90th anniversary of the birth of Tsiolkovsky. (TASS)
According to Korolev’s plan, lunar far side photography would be performed by the E-2 and E-3 spacecraft after the E-1 impact missions. Unlike previous Soviet satellites, these probes would need an attitude control system to accurately point its payload of cameras at the Moon. In 1955 Academician Mtsilav Keldysh – Vice President of the Soviet Academy of Science and head of MIAN (The Mathematical Institute of the Academy of Sciences) – authorized a group of engineers and scientists led by Boris Raushenbakh of NII-1 (Scientific Research Institute 1) to examine the question of spacecraft attitude control. The exploratory work they finished in late-1956 would find direct application in the E-2 and E-3 probes.
Korolev (left) with Igor Kurchatov (the “father” of the Soviet H-bomb) and Mstislav Keldysh in 1956. (Keldysh Museum)
Because of the required alignments of the Earth, Moon, and Sun, a far side photography mission could only be launched in the early fall or early spring. Throughout 1958 a team at OPM MIAN (the Department of Applied Mathematics of the V.A. Steklov Mathematic Institute) directed by Keldysh calculated detailed trajectories for these missions to find the best options. As 1958 began, Korolev hoped for the first E-2 or E-3 launch in October or November of 1958. It was quickly apparent that this was too ambitious a development schedule for such a complex mission.
The E2/3 program was assigned to a team at OKB-1 under Gleb Maksimov. While they worried about the basic design, Maksimov subcontracted the construction of key subsystems to other design bureaus. The imaging and radio systems for the E-3 were the responsibility OKB-MEI (Experimental Design Bureau of the Moscow Energetics Institute) headed by Chief Designer Aleksey Bogomolov. The OKB-MEI team envisioned that Maksimov’s spacecraft would be able to place their camera so that the Moon would be within 30° of the line of sight. The photo-television system’s single 750-mm focal length lens would then automatically acquire and photograph the Moon for later transmission to Earth.
The corresponding systems of the E-2 would be designed and built independently of the E-3 effort. The radio system would be taken care of by a team led by Mikhail Ryazansky at NII-885 then under Deputy Chief Designer Yevgeniy Boguslavskiy. The contract for the photo-television package was given to NII-380 in Leningrad (which reverted to its original name of St. Petersburg in 1991) directed by Igor Rosselevich in July 1958. By October 1958, NII-380 presented the first working prototype of their automated imaging system called Yenisey-1. It was a 35 mm film camera system capable of taking 40 photographs using 200 mm and 500 mm lenses. Upon command from the Earth, the system would take its photographs and process the film for later transmission to Earth.
Diagram showing the 8K72 launch vehicle equipped with a Blok E stage on the right. Used to launch the E-1 probes, it would be pressed into service for the E-2A mission. Click on image to enlarge. (RKK Energia)
As impressive as the quick development of Yenisey-1 was, troubles in other areas of the program were leading to lengthy delays. Originally the probes were to be launched on an 8K73 launch vehicle. Essentially an enlarged version of the three-stage 8K72 that sent the E-1 impact probes to the Moon (which itself was a modified R-7 ICBM with an escape stage designated “Blok E” added), development of the 8K73 escape stage’s RD-109 engine at OKB-456 was falling hopelessly behind schedule just as Korolev had originally feared. Fortunately, by the time development of the 8K73 was cancelled in early 1959, improvements in the performance of the 8K72 allowed it to launch the 300-kilogram photographic probes on a direct ascent trajectory to the Moon with payload margin to spare. But delays elsewhere ultimately led to pushing the first flight back to the fall of 1959. With more time on hand, development of the E-2 was cancelled in the summer of 1958 and work on an improved E-2A design immediately began. In August, NII-380 commenced work on an upgraded Yenisey-2 imaging system that E-2A would carry.
The E-2A spacecraft designed to photograph the far side of the Moon. (RSA via NASA)
But as work on the E-2A pushed ahead, problems with the E-3 design continued throughout 1958. To avoid missing the April 1960 launch window for this mission, Korolev authorized the development of yet another E-2 variant in late 1958 called E-2F which would carry an improved Yenisey-3 photographic system. With its development schedule continuing to falter, the E-3 was finally cancelled in early 1959. Around the same time development of an E-5 lunar orbiter meant to beat NASA’s new Pioneer orbiter to the Moon was started with a launch in the October to December 1959 timeframe (see “NASA’s Forgotten Lunar Program”). In the meantime, all attention was focused on getting the E-2A off the ground in October 1959.
Getting the Photos
The E-2A was by far the most advanced spacecraft built by the Soviet Union up to that time. It was a cylinder capped by a pair of hemispheres with a total length of 1.3 meters and a diameter of 1.19 meters. The exterior of the 278.5-kilogram probe was covered with banks of solar cells to recharge its batteries – the first Soviet spacecraft to do so. During its cruise, the probe was spin stabilized which also provided some measure of thermal control by spreading the solar heat load more evenly across the probe. Additional control was provided by rectangular thermal shutters between the banks of solar cells on the cylinder’s exterior. The interior was pressurized to 230 millibars and circulating fans helped to keep interior temperatures in the 25° to 30° C range.
Soviet diagram showing the major components of the E-2A spacecraft: 1) Viewport for Yenisey-2 camera, 2) attitude control jets, 3) Sun sensor, 4) solar cells, 5) thermal regulation shutters, 6) thermal screens, 7) antennas and 8) scientific instruments. Click on image to enlarge.
The development of the E-2A attitude control system started in earnest in early 1959 by the team under Raushenbakh at NII-1. They were responsible not only for the control jets, by the sensors designed to lock onto the Sun and Moon and feed commands to those jets. At the beginning the photography session, the probe’s spin would first be negated. Next the E-2A would turn to find and lock onto the Sun. The spacecraft would turn to within 0.5° to 0.7° of the Moon so that the Yenisey-2 camera could photograph the far side. After the photos were secured, the probe would turn once more and set itself spinning again.
Soviet diagram showing the orientation sequence for the E-2A photographic session with solar illumination coming from the right. Click on image to enlarge.
The Yenisey-2 photography package was a marvel of engineering simplicity. It carried a total of 40 exposures of radiation-resistant 35 mm isochrome film to be used by a 200 mm, f/5.6 and a 500 mm, f/9.5 lens. Since the Soviets did not have film with the needed radiation resistance available, they instead scavenged film from downed American reconnaissance balloons flown secretly as part of WS-119L “Project Genetrix”. Once exposed, the film would be automatically processed on board using a one-step developing process and then scanned by upon command for transmission back to Earth. The images could be transmitted at either 1.25 or 50 lines per second and could be scanned with a maximum resolution of 1000 lines – roughly equivalent to today’s HD video standard.
The Yenisey-2 photographic system used on the E-2A spacecraft.
In addition to the Yenisey-2, the E-2A also carried instruments to detect micrometeoroids and cosmic radiation. As with the earlier E-1 impact probes, the E-2A did not take full advantage of the 8K72 lifting capability (see “The First Race to the Moon: Reaching our Neighbor”). The Blok-E escape stage carried an extra 156.5 kilograms of instrumentation. The escape stage was also equipped a radio command system that would shut down the RO-5 engine once the required velocity had been reached. Since the E-2A and its sisters had no course correction capability, high launch vehicle accuracy was essential to target the probe accurately for its mission.
The Mission
The first E-2A spacecraft was delivered to the NIIP-5 launch site in Kazakhstan (later to be known as the Baikonur Cosomodrome) in August of 1959 for its final assembly and testing. With the successful launch of Luna 2 on September 12 (which successfully impacted the Moon two days later), the way was clear for the lunar photography mission. After its hasty ground tests, the E-2A No. 1 was mounted on its 8K72 launch vehicle serial number I1-8 and prepared for launch. At 03:43:40 Moscow Time (00:43:40 GMT) on October 4, 1959, the E-2A probe now officially designated Automatic Interplanetary Station (and dubbed Lunik 3 in the West but today known as Luna 3) was successfully launched towards the Moon.
Launch of an 8K72 rocket towards the Moon. (RKK Energia)
When contact with Luna 3 was established with the tracking station in the Crimea, trouble was already evident with the radio signal being only about half of the expected strength. More worrisome still was the probe’s steadily rising interior temperature which threatened to spoil the film. While low signal strength would continue to hamper the entire mission, a solution to the overheating problem was found and implemented by the evening of October 6. The orientation of the spacecraft’s spin axis was changed with respect to the Sun and various pieces of equipment were shut off until needed to reduce the heat load. These steps allowed the interior temperature to drop from a high of 40° C to a safer 27° to 30° C.
After passing within 6,200 kilometers of the lunar south pole at 14:16 GMT on October 6, Luna 3 swung behind the Moon and into an extended Earth orbit with an initial apogee of 480,000 kilometers and a perigee of 47,500 kilometers, On October 7 when Luna 3 was 65,200 kilometers above the illuminated hemisphere of the Moon – a view that included 30% of the familiar nearside and 70% of the unseen far side. The overhead lighting with its short shadows was not ideal for spotting small topographic details but it would suffice for an initial far side survey. Upon command, Luna 3 stopped spinning and proceeded with its automated sequence to photograph the Moon starting at 03:30 GMT. A total of 29 photographs were exposed over 40 minutes before the mechanical shutter jammed at a distance of 66,700 kilometers from the Moon. With its photography session ended prematurely, the probe resumed its spinning and immediately began to develop its precious cargo of film.
Soviet diagram illustrating the trajectory of Luna 3 past the Moon and back towards the Earth. Click on image to enlarge.
As intended, the close lunar encounter also changed the inclination of the probe’s orbit so that it would rise steadily higher in the sky as it returned towards the Earth. Eventually the probe would become circumpolar allowing for continuous communications from Soviet tracking stations. During the first four transmission attempts starting October 8, no usable images were returned because of low signal strength as Luna 3 approached apogee on October 10. Decreasing distance and efforts to minimize radio interference in the area of the tracking station finally allowed the first two usable photos to be transmitted on the fifth attempt. Transmissions continued as the probe approached its October 18 perigee with a total of 17 usable photographs being received. But even the best of these were still heavily smeared with noise. On October 22 all contact with AIS was finally lost. Five days later, Soviet authorities publicly released a three-photo composite showing the far side of the Moon to the world for the first time. The orbit of Luna 3 was perturbed over time and the silent probe reentered Earth’s atmosphere in April 1960.
The first clear image of the far side of the Moon returned by Luna 3. (RSA via NASA)
While these photos were crude, they did reveal the nature of the lunar far side. Most notable was the near absence of dark mare that dominate the appearance of the familiar nearside. While some in the West would claim the photographs were fake, subsequent images returned years later by American and Soviet probes confirmed what Luna 3 first saw. Despite its problems, the Automatic Interplanetary Station was an outstanding success and the term “dark side of the Moon” to mean the far side became obsolete.
A composite image of the far side of the Moon created using several Luna 3 images to help suppress the noise. (RSA via NASA)
The Final Curtain
While the achievement of Luna 3 was impressive, Korolev and his engineers knew they could do better for the next launch window in April 1960. But by February 1960 it was feared that the E-2F and its Yenisey-3 camera would not be ready in time. Instead, a pair of modified E-2A probes were constructed. They would have a more powerful communication system than Luna 3 and take their images at much closer range during the approach to the Moon. Their trajectories would allow the remaining 30% of the far side to be photographed.
The first of the improved E-2A probes (now assigned the defunct “E-3” designation and designated E-3 No. 1) was launched on 8K72 serial number I1-7 at 18:06:44 Moscow Time (15:06:44 GMT) on April 15, 1960. While at first it appeared to be a success, tracking quickly showed that the engine on the Blok-E escape stage had shut down too early because its fuel tank had been underfilled. As a result, the E-3 No. 1 reached a peak altitude of about 200,000 km before plunging back to Earth and a destructive reentry about four days later. Like all previous Soviet lunar mission launch failures, this was kept secret from the outside world and would not be revealed for another three decades.
With only a single chance left, Korolev decided to immediately launch E-3 No. 2 on launch vehicle I1-7b. At 19:07:43 Moscow Time (16:07:43 GMT) on April 19, E-3 No. 2 was launched but immediately ran into trouble. The RD-107 engine on one of the four strap-on boosters failed to ignite properly. After the 8K72 struggled upwards for 150 to 200 meters, the rocket began to tumble with all four boosters breaking free of the core. Two of the boosters crashed and exploded near the pad. Another passed a mere 30 to 40 meters over the heads of spectators scrambling for cover 1½ kilometers away. The booster detonated on impact near the MIK assembly building shattering its windows. The core with its Blok-E escape stage and payload crashed 800 meters from the pad near a small salt lake. The accident caused much damage to the pad and other facilities at Area 1.
The remaining 30% of the lunar far side unobserved by Luna 3 would not be photographed until July 1965 during the Zond 3 test flight of the 3MV spacecraft using a far superior photo-television system. (Sternberg Astronomical Institute)
This would mark the end of the first chapter in Soviet lunar exploration. While another mission could theoretically be launched in October 1960, Korolev already had plans to launch the first of his new interplanetary probes that month (see “The First Mars Mission Attempt”). Limited resources at OKB-1 were tightened further as the Vostok program approached its first test flights as 1960 progressed (“Korabl-Sputnik & The Origin of the Soviet Vostok Program“). With the hastily conceived E-5 cancelled, Soviet lunar planners turned to the E-6 lunar lander and E-7 orbiter missions planned for launch in 1961 (see “The Mission of Luna 5”). The remaining unseen part of the Moon’s far side would not be photographed until July 1965 by the Soviet Zond 3 mission (see “The Soviet Zond Missions of 1963-65: Planetary Probe Test Flights”). As NASA’s new Pioneer lunar program stumbled through 1960 (“NASA’s Forgotten Lunar Program“), it looked as though the Soviets would maintain their lead in the great Moon race.
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Related Reading
“The First Race to the Moon: Getting Off the Ground”, Drew Ex Machina, November 8, 2018 [Post]
“The First Race to the Moon: Reaching Our Neighbor”, Drew Ex Machina, September 14, 2019 [Post]
General References
James Harford, Korolev, John Wiley & Sons, 1997
Brian Harvey, Soviet and Russian Lunar Exploration, Springer-Praxis, 2007
Wesley T. Huntress and Mikhail Ya. Marov, Soviet Robots in the Solar System: Mission Technologies and Discoveries, Springer-Praxis, 2011
Nicholas L. Johnson, Handbook of Soviet Lunar and Planetary Exploration, Univelt, 1979
Asif A. Siddiqi, “First to the Moon”, Journal of the British Interplanetary Society, Vol. 51, No. 6, pp. 231-238, June 1998
Timothy Varfolomeyev, “Soviet Rocketry that Conquered Space Part 2: Space Rockets for Lunar Probes”, Spaceflight, Vol. 38, No. 2, pp. 49-52, February 1996
Timothy Varfolomeyev, “First to the Moon: Additional Comments”, Journal of the British Interplanetary Society, Vol. 52, No. 4, pp. 157-160, April 1999