whatA rubble moon -- hundreds of pebbles holding one another by their own gravity, resting on each other by contact -- orbits a planet while a declared whisper of drag (megayears of tidal decay compressed into a minute) walks the orbit inward. The dashed circle is Edouard Roche's 1848 arithmetic, computed from this very moon's measured bulk density before anything happens.

whyThe claim: at the circle, tides beat self-gravity. Outside it the pile holds; as the orbit crosses the line, the planet's tide across the moon's width exceeds the moon's own pull -- the pile stretches into a lozenge, shears out along the orbit, and becomes a ring. Nothing is scripted: the same pairwise gravity that held the moon is simply outmatched, where the prediction says. Saturn's rings sit inside this line; its moons sit outside.

howFull pairwise self-gravity (softened) plus inelastic contact handled as position projection; leapfrog otherwise. The HUD tracks the largest bound fragment by union-find over proximity and the angular arc of the forming ring; the drag stops once the moon is gone, so the ring keeps its orbit. Drag the moon radially to force the question. One integer seeds the pile.