In 1965, work started on the third generation of the Soviet lunar probes. These were designed to support manned lunar landings, but they ended up as a perfect substitute for them.
Three different versions of the new probes, similar in scope to the US Prospector probes of the Sixties were designed. All versions shared the same spacecraft bus, having a dry mass of 1,100 kg and housing four 88 cm diameter tanks for hydrazine and nitric acid, the main engine, used for course corrections during the flight from the Earth to the Moon, for orbit insertion and manoeuvres and for landing in lander versions.
To this module four large cylindrical tanks were attached, rising the total propellant mass to 3,900. These tanks separated once the probe was in lunar orbit. On top of this bus was mounted the mission specific hardware.
The orbital version of the probe was called E-8LS (Lunni Sputnik, lunar satellite) and carried a simplified version of the Lunkhod rover instrumental module. This version carried less fuel for the main engine, as no landing was envisaged but carried more nitrogen gas for attitude control.
Due to the very high mass of the probes (up to 5,600 kg at launch) they could only be carried by the most powerful Soviet launcher, the Chelomei 8K82K Proton.
The initial plan called for the launch of an E-8LS in order to carry out a detailed orbital reconnaissance of the possible landing sites for manned missions. In the end, the first orbiter was launched on September 28, 1971, at which time the Soviet manned lunar mission plans were collapsing.
The probe, called Luna-19, reached lunar orbit on October 3, entering a 140 km high orbit having an inclination of 40 degrees and a period of two hours. This orbit was later lowered to a 127 km high circular one, from which the first part of the scientific mission was carried out. During this time the probe mapped the lunar gravitaty field of the nearside, measured the micrometeorite flux and the magnetic field of the Moon and of the interplanetary space, in cooperation with Mars-2 and -3, then orbiting Mars, Venera-7 and -8 in solar orbit and Prognoz-1 and -2 in Earth orbit.
During the month of November 1971 the probe's periastron was lowered to 77 km to carry out a photographic reconnaissance of some areas of particular interest. Among the very few shots taken by the probe and published by the Soviets were images of the highland area between 30 and 60 degrees South, and 20 and 30 degrees East, but other pictures showed the crater Eratostenes and Mare Vaporum.
Luna-19 also studied the Sun, observing no less than ten flares and was used in fifteen attempts to study the cislunar plasma by radiooccultation. Only seven attempts yielded useful data. During the first three attempts, made during three consecutive orbits on May 8, 1972, no trace of plasma was found but on June 11, a slight electron concentration was noted, having a maximum some 10 km above the lunar surface. The difference between the two attempts resided in the different Earth-Moon-Sun angle, which on June 11 was close to zero (i.e. the moon was new). No plasma was detected over the shadowed emisphere on other occasions.
Finally, measurements were taken of the soil composition using a gamma ray spectrometer, an altimetric reconnaissance was carried out using the "Vega" radar altimeter, mascons (mass concentrations) were studied, as were the electrical characteristics of the regolith.
The Luna-19 mission lasted more than a year, up to October 1972, during which the probe orbited the Moon no less than four thousand times.

The second and last E-8LS probe lifted off as Luna-22 from the Baykonur/Tyuratam cosmodrome on May 29, 1974 and entered a 220 km high lunar orbit on June 2. This orbit had an inclination of 19 degrees and a period of 130 minutes. At the start of the mission, a four days photographic reconnaissance was carried out, by lowering the periastron to as little as 25 km. From this orbit the probe also took readings of lunar topography using a laser altimeter and of lunar soil composition using a gamma ray spectrometer.
During the next five months Luna-22 was kept in a circular orbit to study the gravitational field and mascons, to help design the last three sample collection missions, of which only two were carried out. During this phase 23 impacts of micrometeorites were recorded, three times less than during the initial photographic phase. In November the probe was switched to a very eccentric orbit of 171 km periastron and 1,437 km apoastron. This orbit enables non focused observations of the whole lunar environment. On August 24, 1975 the probe's periastron was once again lowered to 30 km and the cameras, dormant for 14 months were switched on. A single image, of rather good quality was taken. On September 2 the probe, then in a 100 x 1,286 km by 21 degrees orbit, ran out of attitude control gas but contacts were maintained sporadically for two more months.
Like Luna-19, Luna-22 made twenty radiooccultation experiments to detect cislunar plasma. The occultations of August 18, 19 and 21, 1974 proved particularly useful as the Moon was then between novilune and first quarter. The model emerging from the occultation experiments showed that a tens of kilometers thick layer of ionized gas forms above the sunlit emisphere, having a maximum electron concentration some 8 km above the lunar surface.
No image taken by (or of) the probe has ever been published.
Luna-22 was to be the last Soviet lunar orbiter. However, the Soviets announced in 1978 that new lunar missions were on the pipeline, including a new E-8LS orbiter, to be placed in a 100 km high orbit for a detailed mapping mission. The scientific payload of the mission was overwhelming: a camera, gamma ray, X-ray and neutron spectrometers, a spectrophotometer, an infrared photometer, a laser altimeter, a mass spectrometer, a plasma sensor, a micrometeorite detector, a magnetometer and other instruments.
This mission was never carried out, although it was still announced as imminent as late as 1987, this time using a Fobos bus.

BIBLIOGRAPHY

Harvey, B.: The New Russian Space Programme, Chichester, Wiley
Kotelnikov. M.A., Savich, N.A., Yakovlev, O.I.: Spacecraft Radiophysical Investigations of the Sun and Planets. In: Kotelnikov, V. A. (editor): Problems of Modern Radio Engineering and Electronics; Moscow, Nauka
Oberg, J.: Soviet Lunar Exploration Past and Future. In: Lunar Bases and Space Activities in the 21st Century. Houston, LPI. (This article is available on line on NASA Astrophysic Data System)
Wilson, A.: Solar System Log, London, Jane's

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