In this article we analyze the motion of a test particle of a planar, circular, restricted three-body problem in resonance, using the Kustaanheimo-Stiefel formalism. We show that a good qualitative description of the motion can be reduced to three simple equations for semi-major axis, eccentricity and resonance angle. Studying these equations reveals the onset of chaos, and sheds a new light on its weak nature. The 7:4 resonance is used as an example.
 J. Henrard and N. D. Caranicolas, “Motion near the 3/1 Resonance of the Planar Elliptic Restricted Three Body Problem,” Celestial Mechanics and Dynamical Astronomy, Vol. 47, No. 2, 1989, pp. 99-121. doi:10.1007/BF00051201
 A. Celletti, A. Chessa, J. Hadjidemetriou and G. B. Valsecchi, “A Systematic Study of the Stability of Symmetric Periodic Orbits in the Planar, Circular, Restricted Three-Body Problem,” Celestial Mechanics and Dynamical Astronomy, Vol. 83, No. 1-4, 2002, pp. 239-255. doi:10.1023/A:1020111621542