A Small Geodesy Surface Package for Future Lunar Robotic Missions

Abstract

We propose to deploy small (2-3 kg) Lunar geodesy packages on the Moon's surface, consisting of an optical Laser receiver, a small retroreflector, as well as a radio beacon. The optical receiver will maintain Earth pointing through the Lunar libration cycles and record arrival times of Laser shots from Earth. Judging from the photon budget for a 50 mJ pulse Laser, most of the existing more than 30 ILRS (International Laser Ranging Service) stations could participate in the experiment and produce large numbers of range measurements at high accuracy and unbiased temporal coverage. The light-weight 0.5 sqm retroreflector will be adequate for direct ranging measurements using the classic Lunar Laser Ranging stations. The simultaneous acquisition of data from the Laser receiver on the Moon and reflected Laser shots on Earth will allow us to calibrate the onboard clock. A small radio source shall be deployed within the package, for tracking by VLBI stations. The experience from the past 38 years of Apollo Lunar Laser Ranging suggests that there is enormous science potential in ranging data to further our understanding of the Moon's internal structure, the dynamics of the Earth-Moon system and fundamental physics. For example, from the Moon's tidal response, inferences can be made on a solid or liquid Lunar core and its size and oblateness. In addition, parameters from gravitational physics, e.g., the time-stability of the gravitational "constant", or the strong equivalence principle (Nordtvedt-effect) could be modeled with vastly improved accuracy. While the position of the Laser receiver will define an important anchor point in the lunar-fixed coordinate system, the radio transmitter will firmly tie the dynamical reference frame of the Lunar orbit into the quasi-inertial kinematic reference frame of Quasar coordinates for insights into the Moon's orbital behavior to as yet unknown frontiers.