A new approach to the study of the dynamical evolution of planetary ri
ngs and similar objects is developed. A Boltzmann equation with a coll
isional term is used as an initial approximation. The case of relative
ly rare collisions is examined. It is shown that the system tends towa
rd either a fundamentally three-dimensional or a perfectly flat config
uration, depending on the coefficient of inelasticity of the collision
s.