Lunar eclipse observations reveal anomalous thermal performance of Apollo reflectors

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. Corner-cube reflectors (CCRs) were placed on the Moon by the Apollo astronauts during the Apollo 11, Apollo 14, and Apollo 15 landings. Each reflector consists of an array of solid, circularly-cut fused silica CCRs 3.8 cm in diameter, installed for the purpose of lunar laser ranging (LLR) operations that could test gravitational physics, elucidate details of the lunar interior, and improve knowledge of Earth orientation and precession (Murphy, 2013). Soon after commencing LLR observations with the Apache Point Observatory Lunar Laser-ranging Operation (APOLLO: Murphy et al., 2008) in 2006, two problems became evident. First, the signal strength returning from the lunar reflectors is diminished by approximately a factor of ten compared to carefully calculated theoretical expectations (Murphy et al., 2007). Second, the reflector arrays suffer an additional order-of-magnitude signal reduction when the lunar phase is within about 20 of full moon (Murphy et al., 2010). Historical data indicate that the full-moon deficit condition slowly developed during the first decade after placement on the lunar surface. The combined effect of the two facets of signal reduction is that signal strength is never greater than about 10% of expectations at any lunar phase, reducing to $1% near full moon—schematically depicted by the dash-dot line in Fig. 1. The Apollo CCR arrays were designed and built in an impressive 6-month period by Arthur D. Little, Inc., including a substantial effort dedicated to thermal design in order to minimize thermal gradients within the solid prisms. It is well-understood that thermal gradients within an optical device impose variations in the refrac-tive index, leading to thermal lensing effects. The central intensity of the far-field diffraction pattern (FFDP) emerging from the CCR is severely diminished when differences of even a few degrees Kelvin exist across the corner cube (Goodrow and Murphy, …