This lunar orbiter, launched on Oct. 22, 1966, included a test of the prototype electric motor for the Soviet Union's upcoming lunar rover, Lunokhod.
Luna 17
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The Moon's first remote-controlled rover, the 765-kg Lunokhod 1, rolled out to the Sea of Rains on November 17, 1970. It had been delivered by the 5,800-kg Luna 17 spacecraft.
Soviet scientists designed the rover for a 90-day journey. However, it kept on running for 11 months, the Soviet Union's greatest technical success in lunar exploration. A five-person team in the Deep Space Center near Moscow drove the rover using slow-scan TV to monitor its progress. Remote-control steering was a challenge because a new still TV frame arrived only every three to 20 seconds. In addition, any commands from Earth take a second and a half to reach the Moon.
In case Earth commands arrived too late to avoid an obstacle, the rover was equipped with its own intelligence to avoid dangerous movements. It automatically braked in front of obstructions and whenever the slope of the ground became steep enough to threaten it with overturning.
Luna 21
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The 840-kg Lunokhod 2 rover arrived on the Moon on January 16, 1973. Luna 21 delivered it to the east edge of the Moon's Sea of Serenity, inside the Le Monnier crater, about 180 km north of the Apollo 17 landing site. It traversed 37 km, under remote control from the Soviet Union, before expiring in early May. The rover was built with a hinged top lid that flipped open during the day to let solar cells soak up power, and closed at night to help keep the instruments warm.
The rover's last work was investigating a 16-km rift in the Moon's floor. The machine's photographs and probes established that the rift was a tectonic fault, caused when Moon quakes split a solid basaltic rock.
