Emily Lakdawalla for The Planetary Society using image data from NASA / JPL-Caltech / SSI / MPS / DLR / IDA / ESO / Vernazza et al., processed by Björn Jónsson, Emily Lakdawalla, Ted Stryk, and Gordan Ugarkovic
On the Edge of Round: 500-Kilometer Worlds
There are 7 worlds in the solar system inside the Kuiper belt with diameters of approximately 500 kilometers: 3 asteroids (Vesta, Pallas, and Hygiea) and 4 moons (Mimas, Enceladus, Miranda, and Proteus). At this size, self-gravity and the strength of the component materials are very closely matched, so the shapes very from smoothly round (Enceladus) to dramatically lumpy (Vesta). The shape of each world depends on what kind of material it's made of as well as its geologic history. Enceladus' and Mimas' rounder shapes arise from their present and past warming, while Vesta's and Pallas' lumpier shapes reflect histories of major impacts.