  We study the kinetic equilibration of gluons produced in the very early
stages of a high energy heavy ion collision in a ``self-consistent'' relaxation
time approximation. We compare two scenarios describing the initial state of
the gluon system, namely the saturation and the minijet scenarios, both at RHIC
and LHC energies. We argue that, in order to characterize kinetic
equilibration, it is relevant to test the isotropy of various observables. As a
consequence, we find in particular that in both scenarios elastic processes are
not sufficient for the system to reach kinetic equilibrium at RHIC energies.
More generally, we show that, contrary to what is often assumed in the
literature, elastic collisions alone are not sufficient to rapidly achieve
kinetic equilibration. Because of longitudinal expansion at early times, the
actual equilibration time is at least of the order of a few fermis.
