Forty years ago today the Lunokhod-1 lunar rover landed on the Moon. This Soviet mission was the first remote controlled robot to operate outside Earth and it spent an impressive 11 months pottering around up there exploring the Mare Imbrium, even though, as you can see from the picture at the top of this post, it looked a lot like a tin bath with wheels!
The rover was also carrying a French built lunar retro-reflector for lunar laser ranging experiments, similar to the ones installed by the Apollo missions. Laser light is beamed at these reflectors from Earth and, by measuring the time it takes to receive the returning signal, the Earth-Moon distance can be measured very precisely (I’ve blogged about this before).
Soviet and French teams both reported that they’d successfully ranged to the Lunokhod retroreflector in 1970 and 1971 but then stopped reporting their results. An American team at the McDonald Observatory tried to do the same thing but without much success, as they didn’t know its location accurately enough. Over the past four decades the Apache Point Observatory Lunar Laser-ranging Operation (APOLLO) has occasionally attempted to find it but always unsuccessfully.
The launch of the Lunar Reconnaissance Orbiter last year provided a new search opportunity however. It imaged the Lunokhod region in March and hey presto, there it was:
Now that the rover and its retroreflector had been found the APOLLO team had a much better idea of where to aim their laser and in April they managed to successfully receive a signal reflected from it. A surprisingly strong signal in fact as they’d expected it would have degraded as much as its sister reflector on the Lunokhod-2 rover, which arrived in 1973. Why it hasn’t remains a mystery.
Rediscovering this rover and its retroreflector will improve the precision of the laser ranging experiments, which in turn help with tests of general relativity because of the predictions it makes about the Earth-Moon distance. All that aside however, I just think it’s nice that we can see the little guy again.
T. W. Murphy Jr, E. G. Adelberger, J. B. R. Battat, C. D. Hoyle, N. H. Johnson, R. J. McMillan, E. L. Michelsen, C. W. Stubbs, & H. E. Swanson (2010). Laser Ranging to the Lost Lunokhod~1 Reflector submitted to Icarus arXiv: 1009.5720v2