Exploring the Moon with retroreflectors

Abstract:

Lunar surface retroreflectors deployed by the Apollo and Lunokhod missions have enabled the lunar laser ranging (LLR) experiment. The analysis of the two-way range data to these reflectors collected by ground observatories impact multidisciplinary science, such as lunar and planetary science, Earth science, fundamental physics, celestial mechanics, and ephemerides. LLR data continue to be acquired and now cover more than 50 years. The last two decades saw a dramatic improvement in data precision – currently at a few mm in range (averaged over a few minutes of observation). The use of infrared (IR) wavelength lasers enables the expansion of LLR-capable ground stations to the wider satellite laser ranging (SLR) network. The Commercial Lunar Payload Services (CLPS) program will deliver the next generation of retroreflectors in the upcoming years to expand the historical retro-reflector network on the Moon’s surface.

The locations of the retroreflectors play an important role in defining and determining the observable quantities of interest. Model parameters and their sensitivities can be mapped out based on the present and extended retroreflector networks, as expected from the landing sites of the upcoming CLPS landers. Differential measurements to two or more well-separated reflectors will enhance the detection of weaker signals of interest by common-mode suppression. Non-uniformities in the distribution of the historical LLR data from station locations, observation times, lunar phases, etc. are identified and their impact on the data analysis is evaluated. A coordinated observation strategy may help reduce some of its impacts and will promote the effective use of valuable observation time.