Lunar Laser Retroreflectors for ESA-ASI’s Moonlight, Farside & South Pole

ABSTRACT: Since 1969 Apollo and Luna missions deployed Laser Retroreflector Arrays (LRAs) of Cube Corner Retroreflectors (CCRs) on the Moon. These LRAs reflect the incoming incident light back to the emit-ter. Thanks to a technique known as Lunar Laser Ranging (LLR), high precision distance measure-ments of the Moon have been performed, by firing short laser pulses from Earth ground stations to these LRAs and measuring the two-way time of flight (ToF) of the light. LLR outputs include accurate tests of General Relativity (GR), information on the internal structure of the Moon, its ephemerides and geocentric positions and motions of Earth ground stations. Over the past 55 years, Earth ground sta-tions LLR capabilities have significantly improved, and nowadays the lunar CCRs represent the main limitation for achieving more accurate/precise meas-urements of ToFs. The main problem affecting the Apollo and Lunokhod LRAs is represented by the lunar librations, resulting from the eccentricity and inclination of the Moon orbit around the Earth [1,2]. For this reason, the Moon Laser Instrumentation for General relativity High-accuracy Tests (MoonLIGHT) instrument was envisaged at the Istituto Nazionale di Fisica Nucleare - Laboratori Nazionali di Frascati (INFN-LNF), aiming at designing, prototyping, manufacturing and qualifying the next-generation of lunar laser retroreflectors, moving from a multi (small) CCR LRA geometry to a single (large, 100 mm) CCR, unaffected by lunar librations [1,2]. The MoonLIGHT field of view is a cone with an opening angle of about 30˚ (with apex at the CCR vertex) and it must be pointed accurately to the Earth within 3˚. Since landers do not guarantee this accurate pointing, INFN-LNF proposed the MoonLIGHT Pointing Actuator (MPAc) hardware to ESA in 2018. MPAc was then selected by ESA for development. In 2021 ESA signed with NASA an MoU to launch MPAc to the Reiner Gamma swirl on the Moon, with a lander of the Commercial Lunar Payload Services (CLPS) program, the 3rd mission granted by NASA to Intuitive Machines (IM). This IM-3 launch is currently foreseen by NASA for 2025 [3], delayed due to recent failures / partial successes of the first two NASA returns to the Moon. Chang’E-6 landed OK.

MPAc will perform two perpendicular rotations to accurately point MoonLIGHT to Earth, operating in Ultra High Vacuum and in a wide temperature range [1,2]. MoonLIGHT+MPAc was integrated, successfully qualified and delivered in 2023. The instrument was accepted by ESA in November 2023, by NASA and IM in December 2023, and now it is storage in Houston, ready for lunch on IM-3.

The research products of MoonLIGHT are [1,2]:

Astrophysical Sciences. Deployment of Moon-LIGHTs will support, on the LLR space segment, an improvement up to a factor 100 of several tests of GR and relativistic gravity. In fact, LLR currently provides the best, or among the best, constraints on:

• Weak Equivalence Principle (WEP) at 10-13.

• Strong Equivalence Principle (SEP) at 4 x 10-4.

• Time-rate-of-change of Newton’s gravitational constant, G, to better than10-15 per year.

• Geodetic precession at 0.1%.

• Yukawa deviations from 1/r2 gravity at 10-10 times the strength of Newronian gravity.

• In addition, LLR currently allows to set strin-gent constraints on the following new theories of fundamental gravity:

o Spacetime torsion [4].

o f(R) gravity [5].

o Non-minimally coupled gravity [6].

o Lorentz-invariance violations.

Lunar Science. Moments of Inertia, Elastic Tides, Tidal Dissipation, Dissipation at the CMB (Core Mantle Boundary), Fluid Core Oblateness, Inner Core, Free Librations.

For LLR data analysis we use the PEP (Planetary Ephemeris Program) SW developed and maintained since the 1960s by the Harvard-Smithsonian Center for Astrophysics (CfA), MA, USA.