EXPLORATION OF THE MOON BY SOVIET SPACECRAFT

Launches of Soviet spacecraft to the Moon started on 2 January 1959, only 15 months after the launch of the first Sputnik. The full list of missions to the Moon is given in Table 1. It includes all officially declared Soviet lunar missions. According to some sources there were also several failed Soviet attempts to send spacecraft to the Moon, when for various reasons, spacecraft stayed in Earth orbit or failed even earlier (1). They have never been officially recognized by the Soviet Union as lunar missions, and this is why they are not mentioned here. As a source for Table 1, we used the Soviet topic of Cosmonautics (2). In total, there were 29 Soviet missions to the Moon. Of them, 20 were successful, one was partly successful, and eight missions failed. Twenty-four missions were named Luna (the Moon, in Russian); five missions were named Zond (probe, in Russian). All Soviet missions to the Moon were robotic, although there was a program of Soviet manned flights to the Moon, which was canceled in the late 1960s because of failures of the heavy booster rocket necessary for those flights (3).
It is necessary to mention that in the 1960s-1970s, when the Soviet Union sent spacecraft to the Moon, the space activity of both the Soviet Union and the United States was strongly politically motivated. The Soviet Union, by sending numerous lunar missions, wanted to demonstrate the superiority of its political system. The Apollo mission to the Moon, the backbone of the U.S. space program of that time, was a response to the challenge of Soviet space activity.
The first Soviet mission to the Moon, Luna 1, although it missed by 6000 km, was partly successful. It made geophysical measurements in near-Moon space and showed that the Moon does not have a significant magnetic field. Eight months later, Luna 2 reached the Moon and made scientific measurements until the spacecraft hit the surface. Data taken by magnetometer showed that the magnetic moment of the Moon could not be larger than 1/10,000 of the magnetic moment of Earth (4). The next Soviet lunar mission, Luna 3, October 1959, flew by the Moon sending back to Earth images of the far (invisible from the Earth) hemisphere of the Moon. Although the image quality was rather poor, it was discovered that the lunar farside, in comparison to the nearside, has much less dark plains, called maria by astronomers. Now, this is, like the absence of a lunar magnetic field, common knowledge.


Luna 1 Probe 2 Jan 1959
First attempt to reach the Moon. Missed it by 6000 km. Showed absence of lunar magnetic field. Sped up by lunar gravity and became the first artificial satellite of the Sun.
Luna 2 Probe 12 Sept 1959
Successfully reached the Moon at 1° W, 30° N (435 km from the Moon’s visible center), confirmed absence of lunar magnetic field.
Luna 3 Probe 4 Oct 1959
Flew by the Moon and made first images of the farside of the Moon, invisible from Earth. The pictures were developed onboard and then sent back to Earth. Deficit of maria on lunar farside was discovered.
Luna 4 Lander ? 2 Apr 1963
Mission failed: spacecraft missed the Moon by 8500 km.
Luna 5 Lander 9 May 1965
Mission failed: spacecraft reached the Moon but due to a retrorocket failure crashed in Mare Nubium.
Luna 6 Lander 8 Jun 1965
Mission failed: due to midcourse correction error, spacecraft missed the Moon by 160,000 km.
Zond 3 Probe 18 Jul 1965
Flew by the Moon and made images of the farside and partly nearside of the Moon. The pictures were developed onboard and then sent back to Earth. Deficit of maria on lunar farside was confirmed.
Luna 7 Lander 4 Oct 1965
Mission failed: spacecraft reached the Moon but due to a retrorocket failure crashed in Oceanus Procellarum.
Luna 8 Lander 3 Dec 1965
Mission failed: spacecraft reached the Moon but due to a retrorocket failure crashed in Oceanus Procellarum.
Luna 9 Lander 31 Jan 1966
First soft landing on the Moon (Oceanus Procellarum, 7.13°N, 64.37°W). First TV panoramas with close-up view of lunar surface were sent back to Earth.
Luna 10 Orbiter 31 Mar 1966
First lunar satellite. Solar plasma sensors, magnetometer, micrometeorite sensors, gamma-ray spectrometer to measure surface composition of lunar surface.
Luna 11 Orbiter 24 Aug 1966
Micrometeorite sensors, plasma sensors, gamma- and X-ray sensors to determine the Moon’s chemical composition, tracking to measure lunar gravity field.

Table 1. (Continued)

Luna 12 Orbiter 22 Oct 1966
Onboard TV camera.
Luna 13 Lander 21 Dec 1966
Soft landing on the Moon (Oceanus Procellarum, 18.87°N, 62.05°W); TV panoramas sent to Earth; soil mechanics measured.
Luna 14 Orbiter 7 Apr 1968
Plasma and particle sensors; tracking to measure lunar gravity field.
Zond 5 Flyby 14 Sept 1968
First lunar flyby with Earth return test; flight for Soviet manned expedition to the Moon, later canceled; a biological payload was included in the flight.
Zond 6 Flyby 10 Nov 1968
Lunar flyby with Earth return; continuation of test flight for Soviet manned expedition, micrometeorite and cosmic ray measurements, in-orbit photography, and biological payload.
Luna 15 Lander 13 Jul 1969
Attempt of sample return from Mare Crisium; soft landing failed.
Zond 7 Flyby 7 Aug 1969
Lunar flyby with Earth return; in-orbit photography of the Moon.
Luna 16 Lander 12 Sep 1970
First successful robotic sample return; spacecraft landed in Mare Fecunditatis (0.68°S, 56.3°E).
Zond 8 Flyby 20 Oct 1970
Lunar flyby with Earth return; in-orbit photography of the Moon.
Luna 17/Lunokhod 1 Lander 10 Nov 1970
Soft landed in Mare Imbrium (38.17°N, 35°W); deployed the first robotic lunar rover which functioned for almost a year.
Luna 18 Lander 2 Sep 1971
Failed attempt of robotic sample return from lunar highland region in between Mare Fecunditatis and Mare Crisium.
Luna 19 Orbiter 28 Sep 1971
Micrometeorite, plasma and particle sensors, magnetometer, TV camera, tracking to measure lunar gravity field.
Luna 20 Lander 14 Feb 1972
Robotic sample return from lunar highland region in between Mare Fecunditatis and Mare Crisium (3.53°N, 56.55°E).
Luna 21/Lunokhod 2 Lander 8 Jan 1973
Soft landed in crater Le Monier at the eastern edge of Mare Serenitatis (25.85°N, 30.45°E); deployed robotic lunar rover which functioned for 4 months.

Table 1. (Continued)

Luna 22 Orbiter 29 May 1974
Micrometeorite, plasma and particle sensors, magnetometer, TV camera, tracking to measure lunar gravity field.
Luna 23 Lander 28 Oct 1974
Failed attempt of robotic sample return from Mare Crisium; the spacecraft was damaged on landing and could not function properly.
Luna 24 Lander 9 Aug 1976
Robotic sample return from Mare Crisium (12.75°N, 62.2°E).

Then, the 3-year period of mission failures (Luna 4 to 8) in 1963 started, when the Soviet Union tried to soft land spacecraft on the Moon. This period was partly interrupted by the success of Zond 3, which flew by the Moon and sent back to Earth higher quality images of the lunar farside and part of the nearside, confirming the farside deficit in maria and providing knowledge of the photometric properties of the farside (5,6). Finally, in January 1966, Luna 9, successfully made a soft landing, 4 months before it was done by the U.S. Surveyor 1, and sent back to Earth TV panoramas of the close vicinity of the landing point (Fig. 1). This mission provided the first close-up view of the lunar surface (centimeter-size features were clearly seen). The successful landing had proved that the lunar surface is strong enough to withstand a load of this and future landers, a conclusion very important for exploration of the Moon (7).
Next, still in 1966, there was a series of three orbiters: Luna 10, 11, and 12. Luna 10 and 11 measured the micrometeorite and plasma environment in near-lunar space as well as gamma-ray radiation from the lunar surface. The latter is indicative of the chemical composition of the surface material. It was found that the surface material resembles basaltic lavas of Earth (8). At that time, these were the only measurements of the chemistry of the lunar surface. Luna 12 provided TV images of several areas of the lunar surface, some with a resolution as high as 15-20 m per pixel. Photogeologic analysis of these images led to a morphological classification of small (<1km) lunar craters later used in the analysis of other images of the lunar surface. This series of successes was continued with the soft landing of Luna 13, December 1966, and the orbiter of Luna 14, April 1968. Luna 13, in its general design and presence of a panoramic TV camera was a repetition of Luna 9, but in addition, it had a mechanical penetro-meter, dynamograph, and radiation densitometer to study lunar soil mechanics. It was found, in particular, that the surface material (at least 5 cm thick) consists of fine grains and its bulk density is ~0.8 g/cm3 (9). Luna 14 was making measurements of the micrometeorite and plasma environment, and its radio tracking (as well as tracking of Luna 10 to 12) was used to study the structure of the lunar gravity field that was required for better ballistic control of future lunar missions.
Then there was the flight of Zond 5, and a month later, Zond 6. Zond 5 was launched on 14 September 1968. Four days later it flew by the Moon and safely returned to Earth on 21 September. These were test flights of circumlunar modules of the Soviet manned mission to the Moon (at that time still in preparation). The science payload included micrometeorite and cosmic ray measurements, important for the safety of future crews, as well as in-orbit photography and a biological payload. The latter included turtles, wine flies, and other creatures that were the first living earthlings brought close to the Moon.
Luna 9 lander after opening its petals; 1-petal antennae; 2-rod antennae; 3-photometric standard; 4-two-edge mirrors. The lander mass 99 kg. A similar petal design, which guarantees that despite spacecraft orientation on landing it would take the correct (antennae up) orientation, was used in the U.S. Mars Pathfinder, which successfully landed 30 years later than Luna 9.
Figure 1. Luna 9 lander after opening its petals; 1-petal antennae; 2-rod antennae; 3-photometric standard; 4-two-edge mirrors. The lander mass 99 kg. A similar petal design, which guarantees that despite spacecraft orientation on landing it would take the correct (antennae up) orientation, was used in the U.S. Mars Pathfinder, which successfully landed 30 years later than Luna 9.
On 13 July, Luna 15 was launched, its mission was the first robotic return of lunar samples to Earth. It was a race with the Apollo 11 mission, which started on 16 July and landed American astronauts on the Moon on 20 July. Unfortunately, on 21 July, trying to land, Luna 15 crashed. The first successful robotic sample return was accomplished in September 1970, when Luna 16 landed in Mare Fecunditatis and brought back to Earth 101 g of lunar soil. In a year, after another failed attempt (Luna 18), Luna 20 successfully landed in the highland area between Mare Fecunditatis and Mare Crisium and brought back to Earth 55 g of lunar soil (Fig. 2). Then, again after a failed attempt (Luna 23), the last Soviet mission to the Moon, Luna 24, August 1976, successfully landed, drilled deep into the surface and brought the sample (a 1.7-m long drill core with 170 g total mass) back to Earth.
The returned lunar samples were thoroughly studied in Soviet and many other laboratories of the world and were partly exchanged with the samples brought by six Apollo expeditions. Thus, the Luna 16, 20, and 24 samples, despite their relatively small mass, became an essential part of the research bank of lunar materials available for international scientific research that resulted in many publications (see e.g., (10-13)). Study of the Luna samples helped to construct a more complete picture of the distribution of different types of lunar materials along the visible part of the Moon. It was found, in particular, that the three Luna sites, all at the eastern equatorial part of the Moon, form a geologic province for which the presence of basalts, rich in aluminum and depleted in titanium and alkalies, was typical. It is necessary to add that studies of the Luna samples (and the Apollo samples as well) are still ongoing and produce more and more details in our understanding of our natural satellite.
Returned capsule of Luna 20 with lunar samples inside landed in snows of Western Siberia. The two extended features in the top part of the capsule are inflated antennae sending signals to the recovery teams.
Figure 2. Returned capsule of Luna 20 with lunar samples inside landed in snows of Western Siberia. The two extended features in the top part of the capsule are inflated antennae sending signals to the recovery teams.
The series of robotic sample returns was interrupted from time to time by other missions. The last two Zond missions were in August 1969 and October 1970 (7,8). They continued in-orbit photography of the Moon, covering, in particular, some areas of the farside not covered in necessary detail by the U.S. Lunar Orbiter and Apollo missions (14). There were also Luna 19 and 22 missions whose major goal was the study of the structure of the lunar gravity field. And finally, there were two more successful landings, Luna 17 and Luna 21, which brought to the lunar surface the research rovers, Lunokhod (in Russian: Moonwalker) 1 and 2.
Luna 17 brought Lunokhod 1 to the northwest part of Mare Imbrium. It had traveled 10,540 m, sent to Earth more than 50,000 pictures from the navigation TV cameras and more than 200 TV panoramas, conducted more than 500 lunar soil tests, and made numerous measurements of the chemical composition of the soil by the X-ray fluorescence technique. Lunokhod 1 also had a French-made laser retro reflector for high-precision measurements of the distances between the Moon and Earth. Luna 21 brought Lunokhod 2 to the mare-like surface of the floor of the large crater Le Monier at the eastern edge of Mare Serenitatis (Fig. 3). It had traveled 37,450 m, partly along the mare-like surface, partly intruding into hilly terrain of the highland type, and studying the edges of a 15-km long linear trough named Fossa Recta. It sent to Earth more than 80,000 pictures from the navigation TV cameras and 86 TV panoramas and conducted more than 150 lunar soil tests and numerous chemical analyses. In addition to that of Lunokhod 1, the Lunokhod 2 payload included a magnetometer, a photometric standard in the field of view of the panoramic TV cameras, and a special up-looking photometer to study the brightness of the night sky of the Moon.
Lunokhod 2. General scheme; 1-magnetometer; 2-low-gain antenna; 3-high-gain antenna; 4-mechanism of antenna steering; 5-solar battery; 6-the cap (closed at nights); 7-panoramic TV cameras for vertical and horizontal scanning; 8-radioisotope heater; 9-instrument for soil-mechanics measurements; 10-rod antenna; 11-wheel with motor; 12-sealed instrument container; 13-X-ray fluorescence spectrometer; 14-stereo-scopic navigation cameras; 15-laser retroreflector; 16-upper navigation camera.
Figure 3. Lunokhod 2. General scheme; 1-magnetometer; 2-low-gain antenna; 3-high-gain antenna; 4-mechanism of antenna steering; 5-solar battery; 6-the cap (closed at nights); 7-panoramic TV cameras for vertical and horizontal scanning; 8-radioisotope heater; 9-instrument for soil-mechanics measurements; 10-rod antenna; 11-wheel with motor; 12-sealed instrument container; 13-X-ray fluorescence spectrometer; 14-stereo-scopic navigation cameras; 15-laser retroreflector; 16-upper navigation camera.
The analysis of TV images taken by Lunokhod 1 and 2 led to a better understanding of the geologic processes responsible for the formation of the lunar soil and small-scale (centimeters to hundred meters) features of lunar topography (Fig. 4). In particular, it was found that surface regardening by meteorite impacts was accompanied by a variety of down-slope gravity controlled mass-wasting phenomena (15-17). Joint consideration of local geology, the along the route measurements of the soil chemical composition (18) and soil optical reflectivity (19) led to conclusions on the mechanisms and scales of lateral and vertical mixing of lunar mare and highland materials. Measurements of the magnetic fields along the route and on the observation stations led to the discovery of local sites of residual magnetization associated with small impact craters and probably formed by the impacts (20,21). Analysis of variations of the interplanetary magnetic field registered by the Lunokhod 2 magnetometer led to estimates of the large-scale (hundreds of kilometers) structure of the Moon’s interior (22). Lunokhod 1 and 2 measurements formed a very large database of lunar soil mechanics (23). Its analysis showed, in particular, that the soil is more cohesive on horizontal surfaces and less cohesive on steep slopes. Photometric observations of the lunar sky led to the discovery of a significant concentration of dust particles above the lunar surface (24).
Fragment of Lunokhod 2 panorama showing rocky surface of the western edge of Fossa Recta trough and its opposite slope.
Figure 4. Fragment of Lunokhod 2 panorama showing rocky surface of the western edge of Fossa Recta trough and its opposite slope.
As mentioned before, both Soviet and American lunar space programs were politically motivated. For about a decade, from January 1959 until December 1968, in the Soviet-American race to the Moon, there was approximate parity and several cases when ”firsts” in key observations and discoveries were made by the Soviet Union. Since the Apollo program started its flights to the Moon, the U.S. role became dominant and some important achievements made by Soviet spacecraft lost importance because similar things had been done earlier and better by Apollo missions. The situation started to change after the last Apollo mission was completed (Apollo 17, 7-19 December, 1972). Meanwhile the Soviet studies of the Moon continued (Luna 21 to Luna 24), and it became obvious that the robotic approach to lunar studies could be very effective. The experience gained showed that Lunokhod type rovers could effectively study the Moon with a variety of instruments. Using the robotic arm, they could collect samples in places geologically interesting, but risky for landing, and reload them into the returning spacecraft of the Luna 16-24 type at sites safe for landing. The obvious advantage of this approach was that, compared to manned flights, it was much cheaper and could accept a much higher probability of risk. The Soviet lunar science community and space industry were actively discussing these possibilities, and even the Lunokhod 3 vehicle had been manufactured. But the Soviet leadership, which lost political interest in lunar studies, canceled all of these plans, and that was the end of Soviet missions to the Moon.

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