Rhea is the second-largest natural satellite of Saturn and the ninth-largest moon in the Solar System, with a diameter of 1,528 kilometres (949 mi). Rhea is the smallest body in the Solar System that is confirmed to be in hydrostatic equilibrium. It has a nearly circular orbit around Saturn, but it is also tidally locked, like Saturn's other major moons. It rotates with the same period it revolves or orbits. Thus, one hemisphere always faces towards the planet.