A malfunction caused this Soviet robotic lunar lander to miss the Moon, but engineers used the spacecraft as a practice run to test procedures for a future landing.

Nation Union of Soviet Socialist Republics (USSR)
Objective(s) Lunar Soft-Landing
Spacecraft Ye-6 No. 7
Spacecraft Mass 3,180 pounds (1,442 kilograms)
Mission Design and Management Experimental Design Bureau 1 (OKB-1)
Launch Vehicle Molniya-M + Blok L (8K78M no. U103-31, also U15000-33)
Launch Date and Time June 8, 1965 / 07:40 UT
Launch Site Baikonur Cosmodrome / Site 1/5
Scientific Instruments 1. Imaging System
2. SBM-1 Radiation Detector

Results

On this ninth Soviet attempt at a lunar soft-landing, the mission proceeded as planned until a major mid-course correction late on June 9, 1965. Although the main retro-rocket engine (the S5.5A) ignited on time, it failed to cut off and continued to fire until propellant supply was exhausted.

An investigation later indicated that the problem had been due to human error. A command had been mistakenly sent to the timer that ordered the main engine to shut down.

Although the spacecraft was sent on a completely off-nominal trajectory, ground controllers put the spacecraft through a series of steps to practice an actual landing (such as inflating the airbags, separating the lander, etc.), all of which were satisfactorily accomplished.

Luna 6 passed by the Moon late on June 11, 1965 at a range of 100,000 miles (161,000 kilometers) and eventually entered heliocentric orbit. Contact was maintained to a distance of 373,000 miles (600,000 kilometers) from Earth.

Source

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

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