= Damped Binet equation
{title2=$u^{\prime\prime}+\frac{k}{h}u^{\prime}+u=\frac{p}{h^2}$}
An attractive <inverse-square force> together with radial drag $-k\dot r/r^2$ preserves <angular momentum> $h=r^2\dot\theta$ but modifies the <Binet equation> for $u=1/r$ by a first derivative. For $k/|h|<2$ and $h\ne0$, the reciprocal radius is a decaying oscillation about $p/h^2$. Forward angular distance $\operatorname{sgn}(h)(\theta-\theta_0)$ makes its damping sign explicit for either orbital orientation.
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