Poncelet property of planar elliptic integrable Kepler billiards

We consider the integrable dynamics of a Kepler billiard in the plane bounded by a branch of a conic section focused at the Kepler center. We show that in this case, for non-zero-energy orbits, the lines of consecutive second orbital foci along a billiard trajectory are all tangent to a fixed circle. Based on this observation we analyse in details the integrable dynamics of a planar Kepler billiard inside or outside an elliptic reflection wall, with the Kepler center occupying one of its foci. We identify the associated elliptic curve on which the dynamics is linearized, and the shift defined thereon. We also discuss explicit conditions on $n$-periodicity using Cayley’s criteria.