Planet-encounter relative velocity ($u^2=3-T$)

In units where a circular planet has orbital radius and speed one, the particle's relative speed $u$ outside a close encounter satisfies $u^2=3-T$, to leading order in the planet-to-star mass ratio. This follows from the vis-viva equation and $u^2=v^2+1-2h_z$ at the encounter radius. A short conservative encounter changes the direction of this relative velocity while preserving its magnitude.