While conspiracy nuts debate the reality of the Apollo landings, scientists must deal with some practical consequences of what astronauts put on the Moon. For the new Lunar Orbiter Laser Altimeter (LOLA), this means dodging the retro reflective mirrors mounted by astronauts at some of the landing sites.

The Apollo astronauts installed arrays of prisms at some landing sites, designed to bounce laser light directly back in the direction it came from. They reflect a signal, proportional to 1/R**2 instead of 1/R**4, where R is distance. These devices are still in use today, to monitor exact motion of the Moon and test physical theories.

The LRO carries a highly accurate laser altimeter, similar to the one installed on recent American Mars orbiters. It occured to me that this instrument might also verify the existance of the Apollo landing sites (as the LRO cameras have already done), but then it also occured to me that it might return a laser signal so strong that it could damage the instrument.

I wrote to David E. Smith at Goddard, the principal investigator for LOLA, to ask about this. He replied that this was most definitely a problem, and the LOLA instrument switches off when the orbiter passes over Apollo sites. If by small chance, the beam did strike the retro reflector, the light bounced back would be 1000 times the detector damage threshold!

The Russians have also been helpful in giving the LOLA team the best known locations for the two Lunokhod rovers, which also have laser retro reflectors mounted on them. Lunokhod-2 has been located precisely and is routinely probed by lasers from Earth. Lunokhod-1 has never been found by laser, and it is not known for certain if its reflector is deployed. Seen in the photo above, we can see the relfector (extended to the left) bouncing back the light from the camera’s flash.

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