  The forces acting on solar Coronal Mass Ejections (CMEs) in the
interplanetary medium have been evaluated so far in terms of an empirical drag
coefficient $C_{\rm D} \sim 1$ that quantifies the role of the aerodynamic drag
experienced by a typical CME due to its interaction with the ambient solar
wind. We use a microphysical prescription for viscosity in the turbulent solar
wind to obtain an analytical model for the drag coefficient $C_{\rm D}$. This
is the first physical characterization of the aerodynamic drag experienced by
CMEs. We use this physically motivated prescription for $C_{\rm D}$ in a
simple, 1D model for CME propagation to obtain velocity profiles and travel
times that agree well with observations of deceleration experienced by fast
CMEs.
