The critical eccentricity at which ﹩{\tau }_{\mathrm{res}}=1﹩ and the onset of chaos occurs as a function of ﹩\mu ^{\prime} {\left(\displaystyle \frac{a^{\prime} }{a^{\prime} -a}\right)}^{4}﹩. The solid red line shows the critical eccentricity computed by numerically solving Equation (10) for ﹩{\tau }_{\mathrm{res}}=1﹩. The gray line shows the approximation Equation (14) valid for ﹩\mu ^{\prime} {\left(\displaystyle \frac{a^{\prime} }{a^{\prime} -a}\right)}^{4}\gtrsim 0.03﹩. The dashed red line shows our numerically fit approximation, Equation (15).