Study of the Lunar Core by VLBI Observations of Artificial Radio Sources on the Moon

Differential VLBI technique can measure the variation in the angular distance between two artificial radio transmitters on the Moon with an accuracy of a few centimeters. We can observe the physical librations of the Moon through this technique since rotation of the Moon changes the angular distance. The gravitational field of the Moon can also be determined by observations of the angular distance between the radio source on the Moon and that on a lunar orbiter. Calculations show that this technique can determine the amplitude of the physical librations with an accuracy of 10-5 which are one or two orders higher than those obtained by using the Apollo Lunar Surface Experiments Package in 1970's. We can estimate the density and the radius of the lunar core through accurate amplitude and period of the librations together with seismic observations. The accuracy of determination of the lunar gravity harmonics of degree 2 derivable from the VLBI observations are in the range from 10-4 to 10-6. We are developing radio transmitters which will be put on the lunar surface by hard landing from the lunar orbiter.