Laser ranging measurements during the total lunar eclipse on 2010 December 21 verify previously suspected thermal lensing in the retroreflectors left on the lunar surface by the Apollo astronauts. Signal levels during the eclipse far exceeded those historically seen at full moon, and varied over an order of magnitude as the eclipse progressed. These variations can be understood via a straightforward thermal scenario involving solar absorption by a ~50% covering of dust that has accumulated on the front surfaces of the reflectors. The same mechanism can explain the long-term degradation of signal from the reflectors as well as the acute signal deficit observed near full moon.
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