> The great majority of Jupiter's 69 known moons travel in retrograde orbits, meaning they travel in the direction opposite the planet's spin.<p>Sketchy! I don't suppose we have a prediction for the next anticipated lunar collision? Surely this system isn't stable.<p>If two moons in opposite orbits collide, then there would presumably be an even more spectacular event when the majority of the combined mass of those moonlets fall straight down into Jupiter. Imagine a ~1km ball of magma zipping past the Roche limit and punching a hole right through the Jovian atmosphere.