Four decades ago, the Soviet Union put a reflector on the moon able to bounce laser signals back to the Earth. There was just one problem: They lost it.

But now the marooned reflector has been found, thanks to the determined hunting of University of California, San Diego researchers. NASA’s Lunar Reconnaissance Orbiter, in orbit around the moon, photographed the landing area where the USSR’s Luna 17 mission dropped off the missing reflector, Lunokhod 1, in 1970. The photos turned up a faint reflective dot, and the team thought that was it.

With an idea now where to point their own laser, the researchers received a stronger signal back from Lunokhod 1 than they ever had in years of studying its sister craft, Lunokhod 2. “The best signal we’ve seen from Lunokhod 2 in several years of effort is 750 return photons, but we got about 2,000 photons from Lunokhod 1 on our first try,” said Murphy. “It’s got a lot to say after almost 40 years of silence” [UPI].

After landing near the Mare Imbrium, Lunokhod 1 stayed in touch with Soviet ground controllers for no less than 11 months, prowling the moon even as the US astronauts of Apollo 14 and 15 were driving about elsewhere in their manned moon buggies. However the robot crawler eventually ceased communications, and the project was officially terminated on October 4, 1971 [The Register]. The Soviet scientists lost the location of the reflector, and because it doesn’t reflect enough light from the sun for us to see it from Earth, they never found it again. Firing the laser to look for a signal only works if you know the reflector’s general location, and thus wasn’t possible until the LRO spotted Lunokhod 1 this year.

The American team had used Lunokhod 2 along with three reflectors left behind by Apollo missions to keeps tabs on our natural satellite and track its position and orbit as it ever-so-slowly moves away from us. And the researchers say that the re-discovered Russian reflector is particularly useful for studying the moon’s liquid core and testing ideas about gravity [Scientific American].

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Image: NASA