Investigation of the lunar interior with a microwave interferometer

Lunar Laser Ranging, by providing routine measurements of the distance to four retroreflector packages on the lunar surface with centimetric accuracy, has allowed very accurate investigations of its orbital dynamics, including relativistic corrections. The lunar rotational degrees of freedom and its rotational deformations have also been investigated, providing new valuable information about the fluid core and the physical properties of the interior (J. G. Williams, Adv. Space Res. 37, 67-71, 2006). A new laser ranging station is now operating with millimetric precision (T. W. Murphy, Jr. et al., Pub. Astron. Soc. Pac. 120, 20-37, 2008). However, this is still limited by the measurement accuracies. The normal n to the orbital plane, in its precession around the fixed ecliptic pole N, drags along the spin axis e; ideally, the three vectors remain coplanar in a Cassini state. The fact that e stays ahead of this plane by 0.23", is evidence of strong dissipative processes at work; they could be due, in particular, to tides and librations, or to the presence of a fluid core, with a deformation different from the mantle. Accurate measurements of the lunar attitude would complement radio science investigations of the gravity field and signal a possible dynamo and a magnetic dipole in the geological past. The angular accuracy required to unravel the different effects is about 10-4 ", corresponding, for a baseline of 1000 km, to an accuracy in differences between distances of 0.5 mm. The present work is a development of an earlier proposal (P. Bender, Adv. Space Res. 14, 233-242, 1994) of a microwave interferometer: a network of up to three widely separated and low-gain Ka-band transponders on the lunar surface is tracked coherently and simultaneously by a single antenna on the ground. The uplink signal will be separately transponded back by the transponders; measurements of the relative phases will provide the differences between the three distances and will determine changes in the attitude of the Moon, with little sensitivity to the atmospheric delay. The present work concentrates on the radio system; more work will be needed on the whole space mission, in particular the landing on the Moon, and its scientific aspects. The expertise gained in the design, the construction and the use (e.g., in the Cassini mission) of Ka-band transponders and Ka-band radio science has been used, with the goal of attaining in each measurement, lasting a few minutes, an accuracy of λ/100 = 0.1 mm in the differences between distances. Among the critical points, we mention the energy source, and the stability and orientation of the transponders in the harsh lunar environment. The present study is funded by the Italian Space Agency.