The Next Generation Retroreflector (NGR) will greatly extend the science that has already been achieved with the Apollo retroreflectors and as well as open up new areas of science (i.e., addressing the theories that describe Dark Matter and Dark Energy) [Currie 2013, Currie 2015]. The thermal challenges implied for the operation of the NGR over the full lunation has been the greatest challenge. In particular, the greatest challenge occurs shortly after lunar dawn, when the housing is at 300oK and the CCR is at 70oK. At this time, one must maintain temperature gradients within the CCR to be less than 0.2oK to assure proper return signal levels. To address these thermal issues, a series of programs have been developed to define the design and thermal coatings that are required to satisfy these requirements. The key aspects of these programs will be reviewed. They incorporate the wave-length dependent absorption of solar radiation within the CCR, the radiation exchanges between the CCR and the housing, between the housing and space and the regolith etc., etc. etc.
Critical to satisfying these requirements is the ability to deploy thermal coatings that have: 1) high emissivity in the infrared wavelengths and low absorption in the visible wavelengths and a different coating that has 2) very low emissivity and low absorption. Concerning the former, specialized white paints (e. g. AZW/LA-II by AZ Technologies) have been a traditional approach. However, we wish the NGRs to maintain their performance over the next few decades, so the effect of the degradation of the thermal properties due to exposure to solar ultraviolet radiation and particle flux must be considered. Another approach is a new type of coating (e. g., Interferential CERMET by Consorzio C.R.E.O.) that is very interesting to investigate. It has been certified for the Bepi-Colombo mission by ESA, but the behavior at the low temperatures of the lunar night are unknown. These two approaches will be compared, for the emissivity, absorption, and performance under ultraviolet and particle radiation as well as the behavior in the various thermal regimes. This review will include the results of tests of applicability of Interferential CERMET to electro-deposited gold and of experimental thermal tests conducted at the University of Maryland, College Park.
Another effect that is critical to effective operation of the NGRs is the behavior of Total Internal Reflection (TIR) and, for operation over many decades, is the effect of ultraviolet radiation and particle flux on the transparency of the fused silica. W.r.t. the former behavior of TIR, the breakthrough of solar radiation is more extensive than is popularly believed. W.r.t. the latter, the recovery of the ISEE-3 spacecraft provides information that is available from no other source.