We provide an analytical approximation to the dynamics in each of the three most important low order secondary resonances (1:1, 2:1, and 3:1) bifurcating from the synchronous primary resonance in the gravitational spin-orbit problem. To this end we extend the perturbative approach introduced in [10], based on normal form series computations. This allows to recover analytically all non-trivial features of the phase space topology and bifurcations associated with these resonances. Applications include the characterization of spin states of irregular planetary satellites or double systems of minor bodies with irregular shapes. The key ingredients of our method are: i) the use of a detuning parameter measuring the distance from the exact resonance, and ii) an efficient scheme to 'book-keep' the series terms, which allows to simultaneously treat all small parameters entering the problem. Explicit formulas are provided for each secondary resonance, yielding i) the time evolution of the spin state, ii) the form of phase portraits, iii) initial conditions and stability for periodic solutions, and iv) bifurcation diagrams associated with the periodic orbits. We give also error estimates of the method, based on analyzing the asymptotic behavior of the remainder of the normal form series.Keywords. Normal form -primary and secondary resonances -spin-orbit problem.
IntroductionThe study of resonant configurations is of primary importance in many astronomical problems. One of the most frequently observed commensurabilities in our Solar system is that between the orbital and the rotational period of natural satellites. Our Moon, for example, is locked in a synchronous (1:1) spin-orbit resonance and this is probably the case also for all large planetary satellites. In a simple spin-orbit coupling model, the dynamics about the synchronous resonance can be described with a pendulum approximation. The phase-space is separated by a separatrix into a rotation and a libration domain. The frequency of the librations is determined to a first-order approximation by the shape of the satellite. For particular values of the asphericity parameter, used to measure the divergence of the real shape from a sphere, this frequency can become resonant with the orbital frequency. This situation, known as a secondary resonance,