Luna 24

Luna 24 was the Soviet Union's third successful robotic mission to bring back Moon rocks, following Luna 16 and Luna 20. It touched down in the Mare Crisium, or Sea of Crises. The spacecraft returned 170.1 grams of lunar material to Earth on 22 August 1976.

The probe used the Ye-8-5 design, with a descent stage and an ascent stage stacked on top. When empty of fuel and on the ground, it weighed 1,880 kilograms and stood 3.96 meters tall; fully fueled for launch, its mass ranged from 5,600 to 5,750 kilograms. The descent stage was based on the lower section of the Lunokhod rover vehicles—a cylindrical body with four landing legs, fuel tanks, a radar altimeter, and two engines. The main KTDU-417 engine could produce up to 1,920 kilograms of thrust and was throttleable, handling mid-course corrections, orbital insertion, and the braking descent. After the main engine cut off, smaller vernier jets (210 and 350 kilograms of thrust) finished the landing. This stage also served as a launch pad for the ascent stage and carried a television camera, radiation and temperature monitors, communications gear, and a 90-centimeter extendable arm with a drill for collecting soil. It communicated via a conical antenna on a boom, using downlink frequencies of 768 and 922 MHz and an uplink of 115 MHz.

The ascent stage was a smaller cylinder with a spherical top, replacing the Lunokhod rover housing. It contained a sealed soil sample container inside a spherical re-entry capsule, mounted on a KRD-61 rocket engine producing 1,920 kilograms of thrust. The ascent stage weighed 520 kilograms total, including 245 kilograms of nitric acid and UDMH propellant, and stood about 2 meters tall. The sample return cabin was 50 centimeters in diameter and weighed 39 kilograms. The KRD-61 could fire only once, for 53 seconds, to put the capsule on a free-return trajectory to Earth, imparting a velocity of 2,600 to 2,700 meters per second with a specific impulse of 313 seconds.

Luna 24 was the third attempt to retrieve a sample from Mare Crisium, a region with a large lunar mascon, following Luna 23 and a launch failure in October 1975. After a trajectory correction on 11 August 1976, it entered lunar orbit three days later, initially at 115 by 115 kilometers with a 120-degree inclination. After orbital adjustments, it landed safely at 06:36 UTC on 18 August 1976 at 12°45' North, 62°12' East, near Luna 23's landing site. The precise coordinates (12.7145° N, 62.2097° E) were later determined by the Lunar Reconnaissance Orbiter in 2012; the adjacent crater was named Lev by the IAU.

On command from Earth, the lander extended its sample arm and drilled about two meters into the soil. The sample was sealed in the return capsule, and after nearly a day on the Moon, Luna 24 lifted off at 05:25 UTC on 19 August 1976. The return trip was uneventful, and the capsule entered Earth's atmosphere and parachuted to a landing at 05:55 UTC on 22 August 1976, about 200 kilometers southeast of Surgut in western Siberia. Analysis of the 170.1 grams of soil showed a laminated structure, suggesting successive deposits. In December 1976, the Soviet Union exchanged one gram of the sample for a lunar sample from NASA. Luna 24 was the last Soviet lunar spacecraft and the last soft landing on the Moon until China's Chang'e 3 on 14 December 2013, 37 years later. It also remained the last lunar sample return mission until China's Chang'e 5 in December 2020, over 40 years later.

In February 1978, Soviet scientists M. Akhmanova, B. Dement'ev, and M. Markov of the Vernadsky Institute published a paper claiming a detection of water in the returned samples. Their study indicated about 0.1% water by mass, based on infrared absorption spectroscopy at roughly 3 micrometers wavelength, with a signal about ten times above the detection threshold. However, as Crotts noted, the authors were not willing to state definitively that terrestrial contamination had been completely ruled out. This would have been the first detection of water on the lunar surface, but the result has not been confirmed by other researchers.