This robotic Soviet sample return mission raced the U.S. crew of Apollo 11 to the Moon, but fell silent during its descent. Engineers believe it crashed into the side of a mountain due to slight error in its descent angle.

Nation Union of Soviet Socialists Republic (USSR)
Objective(s) Lunar Sample Return
Spacecraft Ye-8-5 (no. 401)
Spacecraft Mass 12,500 pounds (5,667 kilograms)
Mission Design and Management GSMZ imeni Lavochkina
Launch Vehicle Proton-K + Blok D (8K82K no. 242-01 + 11S824 no. 402L)
Launch Date and Time July 13, 1969 / 02:54:42 UT
Launch Site Baikonur Cosmogrome, Kazahkstan (NIIP-5 / Site 81/24)
Scientific Instruments 1. Stereo Imaging System
2. Remote Arm for Sample Collection
3. Radiation Detector

Results

Luna 15, launched only three days before the historic Apollo 11 mission to the Moon, was the second Soviet attempt to recover and bring lunar soil back to Earth.

In a race to reach the Moon and return to Earth, the parallel missions of Luna 15 and Apollo 11 were, in some ways, the culmination of the Moon race that defined the space programs of both the United States and the Soviet Union in the 1960s.

Prior to launch, due to mass constraints, designers removed one of two 3-pound (1.28-kilogram) radio transmitters from the Rye-85 ascent stage, leaving only one for the entire return part of the mission.

On the way to the Moon, controllers detected abnormally high temperatures in the propellant tanks feeding the S5.61 engine (to be used for takeoff from the Moon and return to Earth). By carefully keeping the tank in the Sun’s shadow, controllers were able to reduce the temperature and to avoid the risk of an explosion en route.

After a midcourse correction the day after launch, Luna 15 entered lunar orbit at 10:00 UT on July 17, 1969. Originally, plans were to carry out two orbital corrections, on July 18 and 19 respectively, to put the vehicle on its landing track, but the ruggedness of the lunar terrain prompted a delay. Instead, controllers spent nearly four days studying data (over 20 communication sessions) to map out a plan of action to account for the rough geography.

The two delayed corrections were eventually carried out on July 19 (at 13:08 UT) and July 20 (at 14:16 UT), putting Luna 15 into its planned orbit of about 68 x 10 miles (110 × 16 kilometers) at a retrograde inclination of 127 degrees. Less than 6 hours after the second correction, Apollo 11 began its descent to the Moon, landing at 20:17 UT on July 20, 1969.

The original plan was for Luna 15 to embark on the Moon less than 2 hours after Apollo 11, but it was not to be. Unsure of the terrain below, controllers delayed the landing by another 18 hours. During this critical period, Apollo 11 astronauts Neil A. Armstrong and Edwin E. “Buzz” Aldrin walked on the Moon.

Finally, at 15:46:43 UT on July 21, 1969, a little more than 2 hours prior to the Apollo 11 liftoff from the Moon, Luna 15, now on its 52nd orbit around the Moon, began its descent to the surface. Transmissions, however, abruptly ceased after four minutes instead of nearly five. According to the original plan, the main engine was to fire for 267.3 seconds and bring the vehicle down to about 1.6 miles (2.5 kilometers) altitude. During the descent, transmissions from the vehicle abruptly and suddenly ended 237 seconds into the engine firing at 15:50:40 UT. The data seemed to show that the spacecraft was about 2 miles (3 kilometers) above the lunar surface.

Later analysis indicated that Luna 15 had probably crashed onto the side of a mountain (at something like 298 miles per hour or 480 kilometers per hour) as a result of incorrect attitude of the vehicle at the time of ignition of the descent engine -- in other words, the spacecraft was probably descending not directly towards the surface, but at a slight angle. Luna 15 crashed about 9 miles (15 kilometers) laterally away and 28 miles (45 kilometers) ahead of its assumed location. Impact was roughly at 17 degrees north latitude and 60 degrees east longitude in Mare Crisium.

Source

Siddiqi, Asif A. Beyond Earth: A Chronicle of Deep Space Exploration, 1958-2016. NASA History Program Office, 2018.

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