Complex sentence: At just 470 km in diameter, Miranda is one of the smallest objects in the Solar System known to be in hydrostatic equilibrium (spherical under its own gravity)--only Saturn's moon Mimas is smaller.
Split into simpler sentences: At just 470 km in diameter, Miranda is one of the smallest objects in the Solar System known to be spherical under its own gravity. Of the bodies that are known to be in hydrostatic equilibrium, only Saturn's moon Mimas is smaller.