paradigm [@Springel2008b]. Their physical association seems beyond doubt, given their relative proximity, correlated kinematics, and abundant evidence of past interaction [for a recent review, see, e.g., @Donghia2015].

The path of the Clouds around the Galaxy is well constrained by precise estimates of their distances, positions, radial velocities and proper motions, which indicate a nearly-polar orbit on a plane closely aligned with the Magellanic Stream [@Kallivayalil2006]. The Clouds are just past pericenter, since their Galactocentric radial velocities are positive and much smaller than their tangential velocities [$V_t\sim 314$, $V_r\sim +64$ km/s for the LMC, see, e.g., @Kallivayalil2013]. Their orbit must also have a fairly large apocentric radius, since their total speed ($|V_{\rm LMC}|\sim 321$ km/s) exceeds the circular velocity of the Milky Way ($\sim 220$ km/s) by a substantial amount. A large apocenter implies a long orbital period, which has led to the suggestion that the Clouds might be on their first pericentric passage.

This conclusion depends on the total mass assumed for the Milky Way halo, as well as on its assumed outer radial profile [@Besla2007], but it would explain naturally why the LMC and SMC are still so tightly bound. Indeed, if the Clouds were at first pericenter then 