Bruce Murray Space Image LibraryThree half-megameter moons Three half-megameter moons Large objects in the solar system are round; small objects are lumpy. Where does round transition to lumpy? In the outer solar system, the transition happens at roughly 500 kilometers, give or take. Enceladus (512 kilometers) is geologically active and very round, while Miranda (472 kilometers) and Mimas (397 kilometers) have noticeable departures from roundness. NASA / JPL-Caltech / SSI / processing by Gordan Ugarkovic (Enceladus, Mimas) and Ted Stryk (Miranda). Montage by Emily Lakdawalla.