The possibility of measuring the inclination of Mercury's spin axis and the amplitude of its 88-day forced libration in longitude by a lander-orbiter measurement system is investigated. The experiment would involve ranging to a small lander placed at a high latitude on Mercury from a satellite in a near-circular polar orbit, which is accurately tracked from the Earth. These measurements can be used to determine the polar moment of inertia of the anticipated fluid outer core plus solid inner core, and thus to obtain information on the outer radius of the fluid core. Covariance analyses are carried out to assess the performance of a simulated data set consisting of 40 separate arcs with orbiter to lander ranging of about 1-m accuracy and high-accuracy Earth to orbiter ranging and Doppler tracking data. The effects of the uncertainties in a number of unestimated parameters are also considered. The force model includes the gravity field up to degree and order 14, solar tidal distortion of Mercury, and a fairly complex formulation for solar and Mercury radiation pressure on the spacecraft body and on the antenna. The total uncertainties for the estimated libration amplitude and the spin axis inclination are shown to be 0.26 to 0.03 arc seconds, respectively, for the model employed. The resulting uncertainty for the fractional moment of inertia of the core is 0.0045. Moreover, the Love number k2 can be determined with an accuracy of 0.01. Thus information on the elastic properties of Mercury's mantle also can be obtained. ¿ American Geophysical Union 1995