Same beam interferometry as a tool for the investigation of the lunar interior

Lunar Laser Ranging (LLR), by providing routine measurements of the distance to retro-reflector arrays on the lunar surface with centimeter accuracy, has allowed very accurate investigations of the Moon's orbital dynamics, including relativistic corrections. The lunar rotation, physical librations, and tidal deformation have also been investigated, providing new insight on the fluid core and the physical properties of the interior (Williams et al., 2006). A new laser ranging station is now operating with a precision of a few millimeters (Murphy et al., 2008). However, even with this improved accuracy, important details of the Moon's internal structure and physical properties may be missed. There are strong indications that significant dissipative processes are at play in the lunar interior; they could be due, in particular, to tides and to the presence of a fluid core, which rotates about a different axis than the mantle (Williams et al., 2006). Accurate measurements of the lunar orientation, together with radio science determinations of the gravity field, may reveal also interesting details about the Moon's geological past. The angular accuracy required to unravel the different effects is about 10(-10) rad, corresponding, for a baseline of 1000 km, to accuracy in differential distance of 0.1 mm. The present work is a follow-up of an earlier proposal (Bender, 1994) of a microwave interferometer: a network of three widely separated landers coherently and simultaneously tracked at Ka-band by a single antenna on the ground. The uplink signal will be separately sent back to the Earth by phase-stable transponders hosted by each lander; measurements of the differential phases will provide the differences between the three distances and will determine changes in the attitude and the tides of the Moon, with little sensitivity to the atmospheric delays. We present a technological assessment of this experimental configuration, known as Same Beam Interferometry (SBI), with emphasis on the technological challenges, weighted against the science benefits. Among the critical points of SBI operations with lunar landers, we mention the energy sources, and the issues related to the permanence of the transponders in the harsh lunar environment. Some operational scenarios are also analyzed. The recent advancement in the design of Ka-band transponders (e.g., in the BepiColombo mission) enables very accurate and stable phase measurements. The error budget presented in this paper shows that a single measurement provides an accuracy in the order of lambda/100=0.1 mm in the differential range of any lander pair with just 60 s integration time.