  In this paper, we study the long-term (time scale of several years) orbital
evolution of lunar satellites under the sole action of natural forces. In
particular, we focus on secular resonances, caused either by the influence of
the multipole moments of the lunar potential and/or by the Earth's and Sun's
third-body effect on the satellite's long-term orbital evolution. Our study is
based on a simplified secular model obtained in `closed form' using the same
methodology proposed in the recently published report on the semi-analytical
propagator of lunar satellite orbits, SELENA. Contrary to the case of
artificial Earth satellites, in which many secular resonances compete in
dynamical impact, we give numerical evidence that for lunar satellites only the
2 g resonance affects significantly the orbits at secular timescales. We
interpret this as a consequence of the strong effect of lunar mascons. We show
that the lifetime of lunar satellites is, in particular, nearly exclusively
dictated by the 2 g resonance. By deriving a simple analytic model, we propose
a theoretical framework which allows for both qualitative and quantitative
interpretation of the structures seen in numerically obtained lifetime maps.
This involves explaining the main mechanisms driving eccentricity growth in the
orbits of lunar satellites. In fact, we argue that the re-entry process for
lunar satellites is not necessarily a chaotic process (as is the case for Earth
satellites), but rather due to a sequence of bifurcations leading to a dramatic
variation in the structure of the separatrices in the 2 g resonance's phase
portrait, as we move from the lowest to the highest limit in inclination (at
each altitude) where the 2 g resonance is manifested.
