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On 17 November 1970, the Soviet Union’s Luna 17 mission touched down on the surface of the Moon. The lander carried a huge rover the size of a small car, called Lunokhod-1, which trundled off into lunar landscape. Over the next year or so, Lunokhod-1 travelled some 10 km over the lunar surface, sending back tens of thousands of images and carrying out soil analyses at over 500 sites.

Even after it stopped working, Soviet and French scientists continued to bounce laser light off an array of French-built mirrors on the rover’s back. However, the last recorded glimpse of the rover was in May 1974 and the exact details of its position have since been lost.

That’s been a constant source of irritation for Tom Murphy at the University of California, San Diego, and pals who regularly bounce laser light off the other known reflectors on the lunar surface left by Apollos 11, 14 and 15 and Lunokhod-2.

More reflectors allow them to more accurately measure the position of the Moon. Getting the reflectors up there is not easy, so to have lost one is particularly galling.

The situation changed in March this year when a team analysing images from NASA’s Lunar Reconnaissance Orbiter spotted the Luna 17 landing site and the rover’s tracks. This pinpointed the rover’s position to within a few hundred meters.

On 22 April, Murphy and his crew fired their laser in that direction and immediately detected a return. Not only was Lunokhod-1’s retroreflector still working, but it turns out to be in significantly better condition that its sister craft’s. “The initial return was surprisingly bright, far surpassing the best-ever return signal from the twin reflector on Lunokhod 2,” say the team.

That’s good news. The rediscovery of the reflector could have an important impact in several areas of science that depend on accurately measuring the position and orbit of the Moon. Laser rangefinding currently provides the most precise tests of many aspects of gravity, including the strong equivalence principle, the constancy of Newton’s constant, geodetic precession, gravitomagnetism and the inverse square law.

“Ranging to this reflector will significantly advance the precision of Lunar laser rangefinding and the resulting gravitational and lunar science,” say Murphy and the team.

Nice to see a happy ending. Like being reunited with a long lost friend.

Ref: arxiv.org/abs/1009.5720: Laser Ranging to the Lost Lunokhod 1 Reflector

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